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Pyruvate kinase deficiency

Pyruvate kinase deficiency
Literature review current through: Jan 2024.
This topic last updated: Sep 29, 2023.

INTRODUCTION — Pyruvate kinase (PK) deficiency is an inherited (autosomal recessive) red blood cell (RBC) enzyme disorder that causes chronic hemolysis. It is the second most common RBC enzyme defect but is the commonest cause of chronic hemolytic anemia from an RBC enzyme deficiency.

This topic reviews the pathogenesis, clinical presentation, diagnosis, and treatment of PK deficiency. General approaches to evaluating the cause of hemolytic anemia are presented separately:

Children – (See "Overview of hemolytic anemias in children".)

Adults – (See "Diagnosis of hemolytic anemia in adults".)

PATHOPHYSIOLOGY

Genetics — PK deficiency is an autosomal recessive disorder; affected individuals are either homozygous for a single pathogenic mutation or compound heterozygous for two different pathogenic variants affecting the function of the PK enzyme in red blood cells (RBCs) and liver [1,2]. Individuals who are heterozygous for PK deficiency have intermediate enzyme levels and are not affected clinically.

PK enzymes consist of several isoforms. They are products of two distinct genes, PKLR and PKM, both encoding enzymes that catalyze the transphosphorylation of phosphoenolpyruvate (PEP) into pyruvate and ATP during the terminal part of the glycolysis pathway. Clinical PK deficiency with hemolytic anemia is limited to mutations of the PKLR gene:

PKLR – The PKLR gene encodes the L (liver) and R (RBC) isoenzymes. The R isoform, unique to RBCs, is 33 amino acids larger than the L isoform, which is unique to hepatocytes. Expression in RBCs versus liver is due to differential use of tissue-specific promoters, which drive expression as well as tissue-specific exon usage (use of exon 1 but not exon 2 in RBCs and exon 2 but not exon 1 in liver). The PKLR gene is located on chromosome 1q21.

PKM – The PKM gene encodes the M (muscle) enzyme. This form is expressed in muscle, brain, white blood cells (WBCs), and platelets. There are two isoforms, M1 and M2, which result from differential processing of a single transcript. The M2 isoform is dominant during fetal development and is overexpressed in many tumors. After birth, the M2 isoform persists in WBCs and platelets. In RBC progenitor cells, the M2 isoform is progressively replaced with the R form during fetal development. The PKM gene is located on chromosome 15q22.

Well over 260 pathogenic variants have been reported on the PKLR gene [3]. The variants include single nucleotide substitutions as well as intronic and exonic deletions and insertions (figure 1) [4-6]. Some variants are relatively common; as an example, the R510Q PKLR (also referred to as PKLR 1529A) mutation is found in approximately 40 percent of Northern European patients with PK deficiency [7]. The R486W PKLR mutation is found in approximately 25 percent of patients with PK deficiency in southern Europe [8]. There is relatively little predictive value with respect to the severity of the clinical course, and the phenotypic expression of identical mutations can be strikingly different [9,10].

Known variants are listed in a publicly available database [8,11].

The frequency of pathogenic PKLR gene mutations is far lower than that of the mutations affecting glucose-6-phosphate dehydrogenase (G6PD), which cause G6PD deficiency. However, unlike G6PD deficiency, which may only manifest hemolysis under certain circumstances such as exposure to oxidant drugs, in PK deficiency, the hemolysis is typically chronic, with occasional superimposed acute hemolysis during hematopoietic stresses such as infection, surgery, trauma, or pregnancy. Thus, PK deficiency is the most common RBC enzyme deficiency causing chronic congenital nonspherocytic hemolytic anemia.

Pathogenic variants in genes other than PKLR have been shown to reduce PK enzymatic activity, although these are rare. As an example, compound heterozygous disease variants in the gene that encodes Kruppel-like factor 1 (KLF1), the hematopoietic-specific transcription factor essential for induction of expression of adult beta globin and other erythroid genes, have been associated with severe transfusion-dependent hemolytic anemia and PK enzyme deficiency [12]. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Quantitative trait loci associated with HBG expression'.)

Genes that increase PK activity have also been postulated, although no specific genes causing PK hyperactivity have been described [13].

PK enzymatic function — RBCs use several enzyme systems to maintain their viability and function. Two of the major processes under enzymatic control are the production of energy, in the form of ATP, and protection from oxidative injury by compounds that act as reducing agents. The PK enzyme functions in the energy-producing glycolytic pathway, which metabolizes glucose to generate ATP for the cell. However, as PK is the terminal enzyme in glycolysis, the proximal glycolytic intermediates are increased [2,14]. This includes increases in 2,3-diphosphoglycerate (2,3-DPG, also called 2,3-BPG), with resultant three- to fourfold increase in 2,3-DPG:ATP ratios [15]. Levels of 2,3-DPG may be elevated up to two times normal in PK-deficient individuals, resulting in decreased hemoglobin oxygen affinity and improved oxygen delivery per unit of hemoglobin, with better tolerance of anemia than would be otherwise expected [16].

As shown in the figure, PK catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate by removal of a phosphate group (figure 2). The phosphate group from PEP is transferred to ADP to create one molecule of ATP.

The active PK enzyme is a homotetramer that contains four molecules of the PK protein. Thus, in an individual who is heterozygous for two different PKLR mutations, five different tetramer compositions will be present (eg, tetramers containing 0, 25, 50, 75, and 100 percent of each of the two abnormal protein subunits). The common R510Q and R486W PKLR mutations as well as some but not all other PKLR mutations, destabilize formation of PK R enzyme tetramers [17].

PK is an allosteric enzyme (an enzyme for which binding of an effector to one region of the enzyme results in altered conformation and/or altered enzymatic activity towards a substrate that binds to another region of the enzyme). The substrate for PK is PEP and the allosteric regulator is fructose 1,6-diphosphate (FDP); binding to FDP changes the conformational structure of PK and its enzymatic activity towards PEP.

As noted above, there are two different genes that encode different PK isoforms (see 'Genetics' above). Individuals with PK deficiency have mutations in the PKLR gene that cause reduced PK activity in RBCs. However, the PK activity in other cell types such as WBCs, platelets, and other tissues is normal because the enzyme expressed in these cells is encoded on a separate gene.

Pathogenic mutations in the PKLR gene may affect any of the following properties of the PK enzyme [7,17]:

Altered affinity for PEP (its substrate)

Altered affinity for FDP (its allosteric activator)

Altered protein stability

Altered stability of PK homo- or heterotetramers

Genotype-phenotype correlations have been examined for several of the common mutations [9]. The most severe clinical phenotypes are generally associated with mutations that cause premature stop codons, frameshifts, or large deletions; however, the genotype/phenotype association and predictions of their severity are not clearly foreseeable [1,10,18].

Mechanism of hemolysis — The mechanism for hemolysis in PK deficiency is not clear. Although the defect in ATP generation contributes to hemolysis, it is not a sufficient explanation; this conclusion is based on observations that ATP deficiency is difficult to demonstrate in some of the affected patients [19]. In addition, other disorders with more severe degrees of ATP deficiency are not associated with significant hemolysis [20].

Hemolysis in PK deficiency is mainly extravascular (ie, due to phagocytosis of cells by reticuloendothelial macrophages); however, if the hemolysis is severe, there may be spillover to intravascular hemolysis. Thus, many affected patients have normal lactate dehydrogenase (LDH) levels, but most have elevated bilirubin and clinical jaundice [18].

It has been proposed that the mechanism of hemolysis in PK deficiency is similar to the as yet unexplained destruction of young RBCs described in individuals descending from high altitude, a process termed "neocytolysis." It is possible that the process may involve impaired mitochondrial autophagy, resulting in increased mitochondrial mass, in turn leading to increased production of reactive oxygen species (ROS) and prolongation of mitochondrial clearance, along with prolonged reticulocyte survival; however, this remains speculative [21-23].

Patients with hemolytic anemia who undergo splenectomy, with a resultant decrease in the hemolytic process and improvement of anemia, have a higher number of reticulocytes than they did before the splenectomy. This perplexing observation indicates that knowledge of the regulation of erythropoiesis and reticulocyte kinetics remains incomplete. A significant interaction may occur between the spleen and PK-deficient, young RBCs, through an as yet unknown mechanism that influences premature splenic destruction of reticulocytes and young RBCs, especially in patients with more severe PK deficiency [24-28].

The metabolic disturbances in PK deficiency affect the survival of RBCs as well as the maturation of erythroid progenitor cells in the spleen, which results in their apoptosis (referred to as ineffective erythropoiesis). Ineffective erythropoiesis in the spleen has been demonstrated in a four-year-old PK-deficient patient who underwent splenectomy, as well as in a mouse model of PK deficiency [28,29]. However, it remains to be established whether apoptosis of erythroid progenitor cells in the bone marrow plays a role in the anemia of PK deficiency, as well as whether PK activity has a more general role in apoptotic pathways [9,30].

Increased oxygen delivery — PK-deficient RBCs show enhanced oxygen delivery for a given partial pressure of oxygen in the bloodstream. This is because the block in glycolysis in PK deficiency is downstream of the Rappaport-Luebering shunt, which results in formation of the metabolic intermediate 2,3 diphosphoglycerate (2,3-DPG, also called 2,3 bisphosphoglycerate [2,3 BPG]). An accumulation of 2,3-DPG has been noted in PK deficiency, in contrast to a low concentration in hexokinase deficiency, which is upstream from the shunt. This increased 2,3-DPG leads to a rightward shift of the oxygen dissociation curve for hemoglobin in patients with PK deficiency (figure 2), resulting in better oxygen delivery to the tissues. As a result, individuals with PK deficiency may tolerate anemia better than those with defects upstream to the Rappaport-Luebering shunt that do not cause a rightward shift in the hemoglobin oxygen curve, such as hexokinase deficiency [9,24]. (See "Structure and function of normal hemoglobins", section on '2,3-bisphosphoglycerate'.)

Mechanism of iron overload — As in any patient with ineffective erythropoiesis and hyperactive production of early RBC precursors, iron absorption is increased, and iron retention in macrophages is decreased, due to erythroferrone-mediated decreases in the levels of hepcidin. As expected, the iron overload is even more common in those patients requiring transfusions [18]. This pathway is discussed in more detail separately. (See "Regulation of iron balance".)

Possible protection from malaria — It is not clear whether PK deficiency protects against any form of malaria.

Evidence supporting a protective role – In vitro, RBCs from individuals with PK deficiency show reduced invasion by Plasmodium falciparum, and RBCs from individuals with PK deficiency as well as heterozygous carriers show a preferential macrophage clearance of ring-stage-infected RBCs [31]. In a mouse model, PK deficiency confers protection against malaria [32].

Evidence against a protective role – The geographic distribution of PK deficiency does not show any indication of a positive selection pressure from malaria as there is for other inherited RBC gene variants such as the sickle mutation, thalassemia mutations, the Duffy blood group system, and certain forms of hereditary elliptocytosis. (See "Protection against malaria by variants in red blood cell (RBC) genes".)

EPIDEMIOLOGY — PK deficiency is extremely rare; its true prevalence is unknown. Based on the gene frequency of the PKLR 1529A variant (encoding PK R510Q) in White people and its relative abundance in people with hemolytic anemia, a prevalence of approximately 51 cases per million has been estimated in White Europeans [33]. However, in the experience of this author, the true prevalence of PK deficiency encountered by hematologists to whom rare red blood cell (RBC) disorders are referred is much lower. This coincides with the published prevalence of diagnosed PK deficiency, suggesting that most people with PK deficiency remain undiagnosed [34,35].

PK deficiency has a worldwide distribution, but it is more common among people of northern European and perhaps Chinese ancestry [36].

As with any autosomal recessive disorder, PK deficiency can be more common in groups with a history of consanguinity. As an example, a high frequency of PK deficiency has been documented in Pennsylvanian Amish communities and in a fundamentalist branch of the Church of Jesus Christ of the Latter-day Saints (FLDS Church) at the Utah/Arizona border [37,38]. In such isolated populations, a "founder" effect can be implicated. In these populations, the frequency of heterozygosity may exceed 1 percent [39].

PK deficiency has also been described in an inbred mouse strain and in the Basenji dog [40,41].

TYPICAL PRESENTATION AND CLINICAL FEATURES

Overview of presenting findings — PK deficiency is a lifelong condition, but the age of presentation is not predictable and may vary widely due to significant heterogeneity in the severity of hemolysis and anemia, even among individuals who share the same genotype (figure 3). In some cases, the burden of disease can be high, especially in early childhood [42].

In most cases, members of a family with the same gene variant tend to have a similar degree of disease severity. The disease has been reported to be particularly severe among the Amish of Pennsylvania, with occasional lethal outcomes in children unless splenectomy is performed [37]. (See 'Genetics' above.)

There is no typical presentation. The rarity of the condition and variability of presentation make diagnostic delays common, and affected individuals may carry a diagnosis of unexplained anemia or may be misdiagnosed as having other causes of anemia.

In a report from 2018 that included 254 individuals in a PK registry, the median age at diagnosis was approximately five months, but the range was large (from birth to 60 years of age) [18]. Approximately one-third of the individuals in the registry were genetically related to other participants, which may explain the early age at diagnosis for some. In contrast, in a registry study from 2005 that included 61 individuals referred to a single center, the median age at diagnosis was 16 years (range, 1 day to 65 years) [43].

The frequency of findings from the larger cohort was as follows [18]:

Anemia – Most of the individuals; however, some have compensated hemolysis

Neonatal hyperbilirubinemia – 90 percent

Transfusions – 84 percent

Iron overload – 47 percent (including some individuals who never received a transfusion)

Gallstones – 45 percent

Perinatal complications (preterm birth, intrauterine transfusion, growth retardation, preterm labor) – 28 percent

Bone fractures – 17 percent

Other complications included leg ulcers, cirrhosis, and endocrine dysfunction; these may have been related to iron overload [44]. The high rate of neonatal jaundice has been observed in other studies, such as a series of 124 children, in whom 88 percent had neonatal hyperbilirubinemia [42].

The majority of patients (59 percent) had a splenectomy, usually to treat anemia and/or to decrease the transfusion burden (at a median age of 4.1 years); of those who did not undergo splenectomy, 35 percent had splenomegaly [18]. Cholecystectomy had been performed in 40 percent (median age, 15.1 years). In those who had a splenectomy without simultaneous cholecystectomy, 48 percent later required a cholecystectomy [18]. Hemolytic rates were improved after splenectomy but may still remain high even after splenectomy; therefore, cholecystectomy should be considered at the time of splenectomy, even in those without any evidence of gallbladder stones, to avoid future second surgery [45].

The severity of presenting findings is highly variable, from hydrops fetalis with death in utero or shortly after birth, to transfusion-dependent anemia, to a mild, compensated hemolysis that does not require transfusions [46]. Some children have severe, transfusion-dependent anemia at birth [42]. Some adults may present with liver failure, which is almost always associated with iron overload; however, a contribution of the PK L isoform remains a hypothetical possible contributing factor.

A 2020 review that compared clinical characteristics in childhood, adolescence and adulthood found that severe anemia requiring regular transfusions was present in 30 to 50 percent of children compared to 10 to 15 percent of adults [45]. Iron overload was equally distributed across ages, with a trend towards increased frequency in adults (42 percent in children and adolescents and 57 percent in adults). Gallstones were reported in 12 percent of children, 64 percent of adolescents, and 70 percent of adults.

Chronic hemolytic anemia from birth — An illustrative unpublished case involves a 19-year-old female who presented with anemia (hemoglobin 7.5 g/dL), profound reticulocytosis (46 percent), and progressive liver failure. She had kernicterus as a neonate and was found to have hemolytic anemia without any specific morphologic RBC abnormalities on the peripheral blood smear. She required weekly blood transfusions. A diagnosis of PK deficiency was made based on the typical presentation, absence of positive findings on testing for other inherited hemolytic anemias, and positive testing for PK deficiency on enzymatic testing performed by a reference laboratory. (See 'Biochemical testing' below.)

Management included splenectomy at the age of five months. No transfusions were administered after splenectomy; however, she was moderately anemic (hemoglobin 7.5 to 9 g/dL) and had laboratory evidence of iron overload (ferritin >3000 ng/mL, transferrin saturation [TSAT] close to 100 percent). She was treated with iron chelation and judicious phlebotomies (starting with volumes of approximately 50 mL and progressively increasing volumes up to 250 mL), and after two years of therapy, her liver function improved and normalized. During this time, her hemoglobin ranged from 8.2 to 9.4 g/dL and her iron markers improved (ferritin decreased to 420 ng/mL; transferrin saturation [TSAT] decreased to 62 percent).

This case illustrates the combination of clinical features that can develop, their contribution to overall health, and the importance of treating iron overload. (See 'Iron overload' below.)

Complications of chronic hemolysis — Some individuals with mild hemolysis due to PK deficiency may not be symptomatic. Those with more severe hemolysis may present with (or develop) one or more of the following [47]:

Splenomegaly of varying degree

Pigment (bilirubin) gallstones

Folate deficiency secondary to increased requirements

Skin ulcers

Osteoporosis

Pulmonary hypertension

Liver cirrhosis

Hemolysis may worsen during pregnancy and after use of oral contraceptives [48,49]; the mechanism is unknown.

Anemia may worsen with transient bone marrow aplasia caused by infections such as parvovirus, which might not cause significant anemia in individuals without chronic hemolysis.

Individuals with gallstones who require surgery should be evaluated for the need for splenectomy, as it may be possible and/or advantageous to perform both procedures at the same time in selected cases.

Iron overload — Iron overload is less common than neonatal jaundice or chronic anemia as a presenting finding that brings the patient to medical attention, but it becomes clinically more apparent during adulthood. Children can also develop iron overload; in a series of 124 children with PK deficiency, iron overload was present in nearly half of those who were evaluated by serum ferritin and/or liver magnetic resonance imaging (MRI) [42].

In a 2019 series that evaluated 242 patients using ferritin and MRI, iron overload was seen in 82 percent of patients who were not receiving regular transfusions [44]. Ferritin >1000 ng/mL had a sensitivity for liver iron concentration (LIC) >3 mg/g dry weight of only 53 percent, but the sensitivity increased to 90 percent using a ferritin threshold >500 ng/mL, suggesting ferritin >500 ng/mL is a better cut off. The high prevalence of iron overload is the rationale for routine surveillance and preventive strategies. (See 'Typical monitoring schedule' below and 'Prevention/treatment of iron overload' below.)

Causes of iron overload include ineffective erythropoiesis, leading to increased erythroferrone and decreased hepcidin, which in turn leads to increased intestinal iron absorption, as well as transfusional iron overload. Similar to thalassemia, severe iron overload may occur in nontransfused patients as well.

If not detected by surveillance, advanced iron overload may present in a number of ways depending on the organ most affected. Examples include heart failure, liver disease, and endocrine dysfunction; these may be similar to the findings in individuals with other forms of iron overload. (See "Approach to the patient with suspected iron overload", section on 'Typical clinical findings'.)

Laboratory findings — PK deficiency produces a chronic, Coombs-negative, nonspherocytic hemolytic anemia (table 1).

The following findings are seen:

Complete blood count (CBC)

Low hemoglobin and hematocrit

Normal to increased mean corpuscular volume (MCV; increases due to reticulocytosis)

Normal white blood cell (WBC) count, WBC differential

Platelet count may be increased (particularly in splenectomized individuals), normal, or decreased (in individuals with splenomegaly and hypersplenism)

Increased reticulocyte count (eg, >50 percent), especially if postsplenectomy; this degree of extreme reticulocytosis has not been observed in any other hemolytic anemia

The severity of hemolysis and degree of anemia is highly variable [9,50]. In a series of 61 individuals with PK deficiency who had not undergone splenectomy, the median hemoglobin was 9.8 g/dL (range 2.2 to 14.4 g/dL) [9,43].

Hemolysis is chronic (figure 3).

Peripheral blood smear

Normal red blood cell (RBC) morphology or nonspecific changes such as echinocytes (burr cells), anisocytes, or poikilocytes (table 2)

Possibly polychromasia related to reticulocytosis (picture 1)

Normal WBCs and platelet morphology and abundance

Testing consistent with non-immune hemolysis

Increased indirect bilirubin

Decreased haptoglobin (variable, may be normal)

Increased lactate dehydrogenase (LDH; variable and may be normal)

Negative direct antiglobulin (Coombs) test (DAT)

Negative indirect antiglobulin test

Increased ferritin

A rapid, non-invasive method to assess the degree of hemolysis based on measurements of exhaled carbon monoxide (end-tidal breath carbon monoxide [ETCO]) has been described and is clinically available. An assay of RBC survival using radioactive chromium is no longer available in the United States [51]. (See "Diagnosis of hemolytic anemia in adults", section on 'Diagnostic approach'.)

Echinocytes are variably present and are neither sensitive nor specific for PK deficiency [52,53]. Acanthocytes (spiculated RBCs) have also been reported on the peripheral blood smears of Basenji dogs with PK deficiency [54]. (See "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane", section on 'Pyruvate kinase deficiency'.)

Unlike congenital spherocytic hemolytic anemias, RBC osmotic fragility is normal in PK deficiency (figure 3). The autohemolysis test, in which hemolysis is evaluated in vitro in the presence or absence of added glucose, lacks physiologic relevance and should not be used [55].

DIAGNOSIS

Indications for testing — Delayed diagnosis of PK deficiency is common. (See 'Overview of presenting findings' above.)

Testing for PK deficiency is appropriate in the following settings (algorithm 1):

Any sibling of a patient with PK deficiency who has unexplained anemia or unexplained hemolysis. This may include prenatal diagnosis [56].

Any individual with suspected congenital hemolytic anemia who has a negative direct antiglobulin (Coombs) test (DAT) and absence of morphologic abnormalities suggestive of other conditions (eg, absence of profound microcytosis, sickle cells, spherocytes, basophilic stippling, Heinz bodies). These findings and the conditions they suggest are described briefly below (see 'Differential diagnosis' below) and in separate topic reviews.

Prenatal testing should be offered to parents who have had a child with hydrops fetalis or severe transfusion-dependent anemia not abrogated by splenectomy.

Initial evaluation — Prenatal diagnosis of anemia can be done using ultrasound and calculating middle cerebral artery velocity [56].

After birth, the first step in the evaluation of a person with possible PK deficiency is to establish if hemolysis is present. This is done by measuring the reticulocyte count and serum markers of hemolysis such as indirect bilirubin or haptoglobin. Hemolytic anemia is characterized by an increased reticulocyte count, increased indirect bilirubin, and possibly by increased LDH and decreased haptoglobin. (See 'Laboratory findings' above.)

Not all affected individuals have anemia, however, as some may have compensated hemolysis without anemia. In some cases, these individuals come to medical attention when they develop an aplastic crisis (eg, in the setting of parvovirus infection), when they present with complications of hemolysis such as pigment gallstones, or when they develop iron overload.

The next step is to determine whether the hemolysis is chronic and present since birth versus acquired, following a period of normal findings (absence of anemia and hemolysis). If hemolysis has been present throughout life, or if this information is inferred (eg, due to family history) or not verifiable, the peripheral blood smear is examined, with a focus on red blood cell (RBC) morphology (eg, schistocytes, elliptocytes, echinocytes, spherocytes, acanthocytes, sickle cells, RBC inclusions). This may allow elimination of other possible causes of inherited hemolytic anemia with highly characteristic findings on the blood smear (table 3), such as hemoglobinopathies (eg, sickle cell disease, unstable hemoglobin, or autoimmune hemolytic anemia) and membrane disorders (eg, hereditary spherocytosis, hereditary elliptocytosis). (See 'Differential diagnosis' below.)

PK deficiency should be considered after these obvious alternative diagnoses have been eliminated, as illustrated in the figure (algorithm 1).

In many cases, the presence of chronic lifelong anemia and characteristic RBC morphologies are strongly suggestive of specific inherited disorders of the RBC membrane or hemoglobinopathies, and specific diagnostic testing can be obtained. The pace of the evaluation and whether testing is ordered sequentially or simultaneously depends on the severity of the anemia, patient symptoms, and other considerations such as the ease of additional blood draws and follow-up testing.

If PK deficiency is suspected based on the family history and/or initial evaluation, subsequent testing can be done using a biochemical assay and/or genetic testing, as discussed below. (See 'PK-specific testing: Where and how to test' below.)

Additional details of routine diagnostic testing that may be appropriate prior to testing for PK activity or PKLR gene mutations are presented in separate topic reviews. (See "Diagnosis of hemolytic anemia in adults" and "Approach to the child with anemia".)

PK-specific testing: Where and how to test — Testing for PK deficiency can be done by measuring PK activity in RBCs (biochemical testing) and by identifying a pathogenic PKLR gene mutation (genetic testing) [55,57].

International consensus recommendations on the diagnosis of PK deficiency confirmed that biochemical testing of PK activity is the most direct evidence of the disease [57]. However, results should be interpreted with caution, as the following may cause falsely normal PK levels:

Increased number of reticulocytes.

Allogeneic RBCs in recently transfused patients.

Incomplete removal of platelets and white blood cells (WBCs), which may lead to compensatory expression of the PK M2 isoenzyme, encoded by the PKM gene.

Some kinetically abnormal PK enzymes may display normal features in vitro but are ineffective in vivo.

In addition, reduced activity may be observed in heterozygous carriers [57].

The recommendations advise genetic testing to confirm the diagnosis demonstrating biallelic pathogenic variants in PKLR (homozygous or compound heterozygous).

Advantages of genetic testing include [57]:

More straightforward handling and shipping of samples

Smaller sample volume required

Lack of interference from reticulocytes, platelets, WBCs, and transfused RBCs

Greater ease of screening first-degree relatives and prenatal testing

However, not every variant detected by DNA analysis can immediately be classified as pathogenic (disease-causing), and further investigation may be required that is only available in a few specialized laboratories (four in the United States and three in Europe). This is especially true for variants in KLF1, which may decrease PK enzyme activity. The KLF1 gene may not be included in next generation sequencing (NGS) panels.

Ultimately, enzyme analyses and DNA studies are complementary techniques for the diagnosis of PK deficiency. A detailed description of the two diagnostic approaches follows.

Biochemical testing — The gold standard test for diagnosing PK deficiency is testing of PK activity and estimation of enzyme kinetics from RBCs free of white blood cells (WBCs) and platelets and the allosteric activator of PK activity, fructose 1,6-diphosphate (FDP).

Removal of WBCs and platelets is important because these cells express PK from a different PK gene (PKM) that is unaffected by the causative PKLR gene mutations.

Removal of FDP is important to detect mutants with altered allosteric interaction with FDP.

Biochemical testing should optimally be deferred for two to three months following the last transfusion; this may not be possible in severely anemic, transfusion-requiring individuals.

WBCs and platelets are removed by filtration with cellulose chromatography. FDP is removed by dialyzing the hemolysate; the enzyme activity is measured in filtered, dialyzed hemolysate tested at different concentrations of the substrate phosphoenolpyruvate (PEP), with and without FDP [55,58]. Transfused RBCs cannot be removed, and biochemical testing may be affected (eg, falsely normalized) by recent transfusion. In a series of 61 patients with PK deficiency, the median PK activity was 35 percent of normal [43].

The abnormal PK enzyme may also be partially purified and analyzed via kinetic and electrophoretic studies, although this is not widely available [59].

An alternative to the gold standard biochemical assay is to use a rapid screening assay that is widely available in many commercial laboratories. In this assay, the PK activity is measured in an RBC hemolysate, without the initial steps for removing platelets, WBCs, and FDP. This rapid assay identifies most but not all PK-deficient patients, and if positive for PK deficiency, it is sufficient to confirm the diagnosis (see 'Diagnostic confirmation' below). However, a negative assay cannot be used to eliminate the possibility of PK deficiency, because the contribution of WBC and platelet PK may make the RBC PK activity appear normal when it is not. A similar issue arises in a patient who has recently been transfused.

In addition to performing a potentially less sensitive assay, routine commercial laboratories typically cannot perform the appropriate quantitative analyses with varying concentrations of the PEP substrate, which is helpful for screening for high Km (low affinity) mutants, or to remove FDP, to detect variant enzymes with altered FDP interaction [55].

Other biochemical testing such as measuring the level of the glycolytic intermediate 2,3-DPG is not used. Increased 2,3-DPG is a common occurrence in PK deficiency but is nonspecific and highly variable. It has been reported that the measurements of RBC 2,3-DPG:ATP ratios would be more specific, but this is not done in routine practice [15].

Genetic testing — Genetic testing for PK deficiency is increasingly available, including testing performed by commercial laboratories and advanced genomic sequencing methods, and it is recommended for diagnostic confirmation. (See "Next-generation DNA sequencing (NGS): Principles and clinical applications".)

Considerations with genetic testing include:

If the familial PKLR variants are not known, it may be necessary to sequence the entire gene (figure 1), including all exons, flanking regions, and the PKLR promoter [43,58].

PKLR may contain a number of variants for which the pathogenicity may be unclear (variants of uncertain significance [VUS] and benign variants).

Some patients have large deletions and intronic mutations at cryptic splice sites that may not be detected by routine sequencing methods [58].

It is important that the RBC PK gene (PKLR) and not the PKM gene is analyzed.

PKLR sequencing will not detect rare variants in genes other than PKLR that can reduce PK enzymatic activity. (See 'Genetics' above.)

In individuals with more than one PKLR variant (possible compound heterozygotes), it is also important that parental samples be obtained to determine whether the two variants are present in cis (from the same parent) or in trans (one from each parent) [58]. (See 'Genetics' above.)

Another benefit of genetic testing can facilitate testing of potentially affected relatives, prenatal testing, and genetic counseling [60]. Identification of the causative variants(s) in parents is essential for determining the specific method of prenatal diagnosis.

Diagnostic confirmation — The diagnosis of PK deficiency is confirmed in a patient with hemolytic anemia (or compensated hemolysis) who has laboratory evidence of reduced RBC PK enzymatic activity and genetic evidence of pathogenic variants in PKLR [55]:

Low RBC PK enzymatic activity, either on a rapid screening test or on a more sophisticated laboratory test.

Homozygosity or compound heterozygosity for pathogenic variants in PKLR.

As the severity of hemolysis may be variable even among relatives with the same PKLR variant, it is likely that some affected yet relatively asymptomatic patients are never diagnosed.

DIFFERENTIAL DIAGNOSIS — The differential diagnosis of PK deficiency includes other heritable red blood cell (RBC) disorders that present as congenital nonspherocytic hemolytic anemia (table 3), other heritable anemias (figure 3), and acquired hemolytic anemias.

RBC enzyme disorders – RBC enzyme disorders that may present similarly to PK deficiency include glucose-6-phospate dehydrogenase (G6PD) deficiency, which is relatively common but typically manifests with isolated episodes of hemolysis rather than chronic hemolytic anemia, and deficiencies of enzymes involved in anaerobic glycolysis, glutathione metabolism, and nucleotide salvage, which are rare (glutathione synthase deficiency and glutathione reductase associated with Heinz bodies); pyrimidine-5'-nucleotidase-1 deficiency, associated with basophilic stippling of RBCs; glucose-6-phosphate isomerase deficiency). The evaluation and diagnosis for these other conditions is discussed separately. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Rare RBC enzyme disorders" and "Overview of hemolytic anemias in children", section on 'Intrinsic hemolytic anemias'.)

Like PK deficiency, these are associated with chronic or intermittent non-immune hemolytic anemia.

Unlike PK deficiency, these have normal RBC PK activity and lack pathogenic variants in PKLR. Some have intermittent rather than chronic hemolysis.

Hemoglobinopathies or RBC membrane disorders – Hemoglobinopathies, thalassemia, unstable hemoglobins, or RBC membrane disorders such as hereditary spherocytosis (HS) or hereditary elliptocytosis (HE) cause chronic hemolytic anemia. (See "Unstable hemoglobin variants" and "Methods for hemoglobin analysis and hemoglobinopathy testing" and "Hereditary spherocytosis" and "Hereditary elliptocytosis and related disorders".)

Like PK deficiency, these cause chronic hemolysis, and in some cases, iron overload may develop.

Unlike PK deficiency, in these disorders, the RBCs have classic morphologic features on the peripheral blood smear that suggest the diagnosis (table 2); tests for unstable hemoglobins or other testing such as osmotic fragility show characteristic findings; and PK activity is normal.

Congenital dyserythropoietic anemias – The congenital dyserythropoietic anemias (CDAs) are rare inherited RBC disorders with abnormal RBC development in the bone marrow [61]. (See "Overview of causes of anemia in children due to decreased red blood cell production", section on 'Congenital dyserythropoietic anemia'.)

Like PK deficiency, these disorders cause chronic anemia, jaundice and splenomegaly, and, later in life, iron overload. The RBC morphology may be normal or may show nonspecific abnormalities.

Unlike PK deficiency, in the CDAs, the reticulocyte count is low and the bone marrow shows various abnormalities in developing RBC precursor cells [61].

Acquired hemolysis – The causes of acquired hemolysis or hemolytic anemia are numerous and include both intrinsic (intracorpuscular) RBC defects and extrinsic (extracorpuscular) hemolysis. (See "Overview of hemolytic anemias in children" and "Diagnosis of hemolytic anemia in adults".)

Like PK deficiency, the age of presentation and severity of anemia are variable, the reticulocyte count is increased, and the peripheral blood smear may be unrevealing; occasional spherocytes may be present.

Unlike PK deficiency, in acquired hemolysis and hemolytic anemia, a causative condition or medication can usually be identified from the medical history, medication list, or laboratory testing such as Coombs testing or flow cytometry, which may reveal antibodies to RBCs or other defects such as absence of glycosylphosphatidylinositol (GPI)-anchored proteins as found in paroxysmal nocturnal hemoglobinuria (PNH).

Some of these conditions and their site of RBC destruction (ie, intravascular or extravascular) are listed in the table (table 3).

TREATMENT

Overview of management — Treatment depends on the age when the disorder becomes evident.

Before birth – Fetal hydrops due to severe anemia may require intrauterine transfusion (IUT) [56]. (See "Intrauterine fetal transfusion of red blood cells".)

Neonatal period – Hyperbilirubinemia during the neonatal period may necessitate phototherapy or exchange transfusion. (See "Unconjugated hyperbilirubinemia in neonates: Risk factors, clinical manifestations, and neurologic complications" and "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management".)

Infancy through adulthood – Severe anemia in infants, children, and adults may require one or more of the following:

Red blood cell (RBC) transfusions (see "Red blood cell transfusion in infants and children: Indications" and "Indications and hemoglobin thresholds for RBC transfusion in adults")

Folic acid (see 'Folic acid' below)

Mitapivat, a small molecule that increases RBC PK activity (see 'Mitapivat for symptomatic anemia' below)

Splenectomy (see 'Splenectomy' below)

Iron chelation (see 'Prevention/treatment of iron overload' below)

Investigational therapies such as hematopoietic stem cell transplant, gene therapy, or gene editing (see 'Hematopoietic stem cell transplant and gene therapy (investigational)' below)

As with any hereditary hemolytic anemia, it is important to thoroughly evaluate the possibility of other causes of anemia for individuals who have a change in symptoms, hemoglobin level, or reticulocyte count, rather than attributing these changes to the underlying disorder. As examples, a decline in hemoglobin and reticulocytes may be a sign of parvovirus infection, new macrocytosis may be a sign of vitamin B12 or folate deficiency, and new microcytosis may be a sign of iron deficiency. At a minimum, these individuals warrant increased monitoring, and if the changes do not resolve in a reasonable period of time, additional testing may be needed, as discussed in separate topic reviews. (See "Approach to the child with anemia" and "Diagnostic approach to anemia in adults".)

As with any genetic condition, individuals of childbearing potential may benefit from prenatal genetic counseling. (See 'Genetic counseling, prenatal testing, and pregnancy' below.)

Glucocorticoids are of no value in PK deficiency.

Drugs with oxidant potential appear to be safe. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Inciting drugs, chemicals, foods, illnesses'.)

Typical monitoring schedule — Patients with PK deficiency are monitored during routine medical care for symptoms of anemia, and they require an evaluation for any changes in symptoms. The minimal evaluation includes:

Any history of new medications, changes in symptoms, or new medical conditions

Examination for appropriate growth and development, signs of anemia, and findings associated with other causes of anemia

Laboratory testing with complete blood count (CBC) and reticulocyte count

Additional testing may be indicated depending on the details of the presentation.

Iron studies (or ferritin levels) are monitored periodically with additional testing as appropriate (interval ranges from annually to more frequently for those with increasing ferritin). (See 'Prevention/treatment of iron overload' below.)

Individuals who have a dramatic decline in reticulocyte count may have a bone marrow insult such as parvovirus infection. In most such cases, detection of viral infection is sufficient without the need for bone marrow examination. If bone marrow examination is done, testing for parvovirus should be included. (See "Clinical manifestations and diagnosis of parvovirus B19 infection".)

Interventions for anemia — Supportive treatments for anemia may include transfusions, the PK activator mitapivat, and folic acid. Splenectomy is generally reserved for severe cases.

Transfusions and phototherapy — Phototherapy with or without exchange transfusion is indicated for severe hyperbilirubinemia during the neonatal period. (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management".)

Transfusions are generally given for symptoms. The level of hemoglobin should not be the sole criterion, and the need for transfusions should be individualized.

RBCs in PK deficiency have a right-shifted hemoglobin oxygen dissociation curve, which means that symptoms are generally less severe than in other individuals with a similar degree of anemia; this is the reason that the hemoglobin level alone should not be used to guide transfusions [43].

In a large natural history cohort that included 254 patients, 84 percent received at least one transfusion, with 30 to 50 percent of children on regular transfusions, compared to 10 to 15 percent of adults [18]. In a series of patients with PK deficiency referred to a single center, 38 of 59 (64 percent) received blood transfusions (median number, 15; range, 1 to >100), and 19 (32 percent) were transfusion-dependent during childhood or until they underwent splenectomy [43].

For those receiving chronic transfusions, chelation therapy should be instituted early, before iron overload develops. (See 'Prevention/treatment of iron overload' below.)

Mitapivat for symptomatic anemia — Mitapivat (previously called AG-348) is an oral small molecule that activates PK enzymatic function in RBCs. It was approved by the US Food and Drug Administration (FDA) in February 2022 for adults with PK deficiency [62].

Indications – It would be reasonable to try mitapivat in any adult with PK deficiency who requires transfusions. While some individuals with specific pathogenic variants in the PKLR gene may have a suboptimal response or no response, even a borderline response would be of significance in an individual who is transfusion-dependent. Mitapivat is very unlikely to be effective in individuals who are compound heterozygotes for the R479H variant or two non-missense mutations, and these individuals may reasonably choose not to take mitapivat. Mitapivat is a lifelong therapy. These considerations regarding efficacy and need for lifelong treatment should be discussed with patients at the time of therapy decision.

Mitapivat may also be reasonable in adults with symptomatic anemia who do not require transfusions, and even in individuals with compensated hemolysis who do not have overt anemia, as they may have mild symptoms that only become apparent when hemolysis is reduced (eg, fatigue of which they were unaware until it resolved). Further, amelioration of the severity of hemolysis is likely to reduce the accumulation of excess iron leading to iron overload. Although not approved for individuals <18 years, we would also use mitapivat in children and adolescents after informed discussion of the risks and benefits.

Safety data for pregnancy are not available, and we would not use mitapivat during pregnancy or in individuals who may become pregnant. Safety in children is under study.

Dosing – The starting dose is 5 mg orally twice per day. If the hemoglobin level does not increase after four weeks, it can be titrated by increasing the dose to 20 mg twice daily, and again after four weeks to a maximum of 50 mg twice daily if needed.

Adverse effects – Abrupt discontinuation could cause dramatic hemolysis; the dose should be tapered before discontinuation. Product information states that concomitant use with strong CYP3A inducers or inhibitors should be avoided.

Supporting evidence – The efficacy and safety of mitapivat has been demonstrated in two randomized trials.

A 2019 open-label trial randomly assigned 52 adults with anemia due to PK deficiency who were not receiving regular transfusions to one of two doses of mitapivat (50 or 300 mg twice daily) for 24 weeks [63]. The dose could be increased if the hemoglobin remained low or decreased for adverse events or excessive hemoglobin increases. Half of the patients had an increase in hemoglobin of >1 g/dL, with a mean increase of 3.4 g/dL at a median of 10 days. The higher dose did not result in greater response rates, suggesting that the lower dose was sufficient.

Responses were best in individuals with missense mutations in PKLR; responses did not occur (or were lower) in individuals who were homozygous for R479H or for two non-missense mutations [63]. Of the 20 individuals with a good response who continued therapy in an extension phase, 19 had a maintained increase in hemoglobin. Therapy was well-tolerated.

The most common adverse events were headache, insomnia, and nausea, which resolved within seven days after the initiation of treatment in most patients. Hypertriglyceridemia occurred in four individuals. Changes from baseline in sex hormone levels, the result of off-target aromatase inhibition, were observed in males; however, testosterone and estradiol remained within the normal range in most cases.

In 49 patients who were evaluated, there was no worsening of bone mineral density as determined by DXA (dual-energy x-ray absorptiometry) scans.

One individual who had the dose held for a rapid increase in hemoglobin level developed acute hemolysis; following that event, the remaining individuals who required a dose reduction had their dose tapered rather than stopped abruptly without incident.

A 2022 trial randomly assigned 80 adults with anemia due to PK deficiency who were not receiving regular transfusions to receive mitapivat or placebo for 24 weeks, with a dose escalation period during the first 12 weeks [64]. At least one of their PKLR pathogenic variants had to be a missense mutation; most had been previously treated with splenectomy and/or cholecystectomy. An increase in hemoglobin of 1.5 g/dL occurred in 16 of 40 in the mitapivat arm (response rate, 40 percent; mean hemoglobin increase, 3.5 g/dL); none of the 40 patients in the placebo arm had a response. Patient-reported outcomes were improved with mitapivat. Missing data precluded assessment of response in three patients in the mitapivat arm and five in the placebo arm. Improvement in the PKDD (pyruvate kinase daily diary) score was greater in the mitapivat arm. Therapy was well-tolerated, with similar rates of adverse events in both arms. The most common adverse events in the mitapivat group were nausea (18 percent) and headache (15 percent); adverse events of grade 3 or higher (hypertension and hypertriglyceridemia) occurred in 10 patients (25 percent) who received mitapivat versus 5 patients (13 percent) who received placebo.

In vitro testing studies using RBCs from patients with PK deficiency have demonstrated that PK activity can be increased by more than 10-fold over baseline [65].

Mechanism of actionMitapivat is a quinolone sulfonamide that was developed using biochemical assays for compounds that could increase PK activity in RBCs. The mechanism of action involves allosteric activation of the PK enzyme, similar to the mechanism of fructose 1,6-diphosphate (FDP) but with greater efficacy [17]. By making the dysfunctional PK enzyme function better, treatment with the drug reduces hemolysis. (See 'PK enzymatic function' above.)

Folic acid — Increased RBC turnover in PK deficiency may lead to folate deficiency in those with inadequate fruit and vegetable intake, but routine folate administration is not needed in those with adequate intake of fresh fruits and vegetables or a diet that includes folic acid-supplemented grains. Periodic serum folate testing is advised to avoid deficiency.

For individuals who place a high value on avoiding folate deficiency, which could cause worsening anemia, taking daily folic acid (typical dose, 1 to 2 mg daily) is safe and inexpensive, and there are essentially no side effects or contraindications.

Discontinuation of folic acid may be considered in individuals receiving mitapivat who no longer have ongoing hemolysis and who have adequate folic acid intake (eg, frequent intake of fruits and fresh vegetables).

Splenectomy — Splenectomy improves hemolytic anemia, but there are several risks, including surgical complications, the possibility of sepsis due to encapsulated organisms, and the increased risk of venous thromboembolism (VTE).

Indications and decision support – There is no reliable way to predict success of splenectomy.

Splenectomy is typically reserved for adults with severe transfusion-dependent anemia not responsive to mitapivat. However, in children with frequent transfusions, the benefits of splenectomy may outweigh its risks, even in neonates. Thus, the decision of whether to pursue splenectomy must be made on a case-by-case basis. Guidelines for splenectomy used in more common congenital hemolytic anemias such as hereditary spherocytosis may be helpful. (See "Hereditary spherocytosis", section on 'Splenectomy'.)

This decision to pursue splenectomy is particularly difficult in adolescents, where the efficacy and safety of mitapivat is under investigation. We generally raise the possibility of splenectomy in all individuals who require chronic transfusions despite mitapivat; we also evaluate the possibility of splenectomy in individuals who do not require chronic transfusions but who have a significant decrease in daily activities due to anemia despite mitapivat. Splenectomy is discouraged in patients >65 years and individuals with increased risk of thrombosis or infection.

For additional decision support, it is reasonable to assume that an individual is likely to have a similar benefit as other affected family members, but the severity of hemolysis is known to vary even among the family members carrying the same PKLR mutation.

Preoperative considerations – For those who elect to pursue splenectomy, especially children, we try to delay the procedure as long as possible (until after the age of three years or after age six years if possible). Young children who undergo splenectomy should be treated with penicillin until they reach the age of three years. Evidence to guide the optimal age of splenectomy comes mainly from observational data in other conditions. (See "Hereditary spherocytosis", section on 'Splenectomy'.)

We make sure to provide pre-splenectomy vaccinations against encapsulated organisms, similar to all individuals undergoing splenectomy for other hematologic conditions such as immune thrombocytopenia (ITP). (See "Second-line and subsequent therapies for immune thrombocytopenia (ITP) in adults", section on 'Pre-splenectomy considerations'.)

Patients must be educated about the potential risks of serious/life-threatening infections and VTE and the need to seek immediate medical attention for fever or symptoms of VTE, as discussed in detail separately. (See "Prevention of infection in patients with impaired splenic function" and "Clinical features, evaluation, and management of fever in patients with impaired splenic function" and "Second-line and subsequent therapies for immune thrombocytopenia (ITP) in adults", section on 'Splenectomy risks'.)

The skill of the surgeon is important in preventing surgical complications and allowing a laparoscopic technique, which appears to have lower rates of morbidity and mortality in other conditions such as ITP. In some individuals, it may be possible to perform partial splenectomy. A single report indicated failure of partial splenectomy (80 percent) to reduce the transfusion requirement of a four-year-old patient with PK deficiency [66]. Six months later, she successfully underwent total splenectomy and became transfusion independent. (See "Elective (diagnostic or therapeutic) splenectomy", section on 'Partial splenectomy'.)

Splenectomy and cholecystectomy can sometimes be done simultaneously using laparoscopic techniques. For individuals who require cholecystectomy for pigment gallstones, the option of concomitant splenectomy should be discussed, and if the patient is considering splenectomy, the option of combining the procedures should be strongly encouraged. Likewise, any splenectomy candidate should be evaluated for gallstones to consider concomitant cholecystectomy.

Supporting evidence – The beneficial effect of splenectomy on hemolysis has been well documented. Typically, hemolysis and anemia are ameliorated but not entirely abated. In severe cases, the transfusion requirement is generally, but not invariably, abolished.

In a series of patients with PK deficiency, 18 of 61 (30 percent) underwent splenectomy (11 before diagnosis and seven after diagnosis) [43]. The median hemoglobin level was 7.3 g/dL in those who underwent splenectomy; this increased by approximately 1.8 g/dL (range, 0.4 to 3.4 g/dL) post-splenectomy. In comparison, individuals not treated with splenectomy had a median hemoglobin level of 9.8 g/dL. Another study in 124 children with PK deficiency reported that most had increased hemoglobin and reduced transfusion burden, but complication rates were considerable (sepsis or infection in 12 percent and thrombosis in 1.3 percent) [42].

Prevention/treatment of iron overload — Individuals with PK deficiency are at risk of iron overload from frequent transfusions as well as from increased iron absorption due to ineffective erythropoiesis. Iron overload in turn can cause serious organ toxicity to the liver, heart, and endocrine organs.

Approximately 50 percent of children <18 years have iron overload. The risk of iron loading is lifelong and does not change by age. In children requiring regular transfusions, regular ferritin monitoring and magnetic resonance imaging (MRI) for iron assessment are advisable on an annual basis. Individuals not receiving regular transfusions should have an MRI once the patient is at an age at which the MRI can be conducted without sedation and/or if their ferritin level is >500 ng/mL [45].

Rarely, symptoms of these organ toxicities may be responsible for bringing the patient to medical attention and ultimately leading to the diagnosis of PK deficiency. (See 'Iron overload' above.)

In some cases, clinically significant iron overload can develop even if no transfusions have been administered; some of these individuals were found to have concomitant hereditary hemochromatosis [67-69].

An iron chelation program is started in those with early signs of iron overload or in those at greatest risk of iron overload (eg, those on a chronic transfusion program). (See 'Typical monitoring schedule' above.)

The details of iron chelation therapy including choice of chelating agent, dosing, monitoring of iron burden, and monitoring for adverse effects of chelating agents are discussed in detail separately. (See "Iron chelators: Choice of agent, dosing, and adverse effects".)

Hematopoietic stem cell transplant and gene therapy (investigational)

Hematopoietic stem cell transplant – Allogeneic hematopoietic stem cell transplant is a potential option for patients with extremely severe disease who continue to require chronic transfusions after mitapivat and splenectomy.

In a 2018 study of allogeneic transplantation for transfusion-dependent PK deficiency involving 16 individuals (median age 6.5 years) transplanted between 1996 and 2015, all in European or Asian centers, transplant-related mortality was 31 percent, with a cumulative two-year survival of 74 percent [70]. Infectious complications (mostly pneumonia) occurred in 10 individuals (62 percent) and grade 4 graft-versus-host disease (GVHD) in six (38 percent). A few additional case reports described successful allogenic transplantation in children with severe PK deficiency [71,72].

Gene therapy and gene editing – Monogenic disorders such as PK deficiency are amenable to gene therapy and gene editing approaches. In hematopoietic disorders, this is likely to require gene transduction or CRISPR-mediated correction of PKLR variants using autologous hematopoietic stem cells and followed by autologous transplant, similar to approaches that are being tested in sickle cell disease.

Ex vivo lentiviral-mediated gene therapy has been initiated, and preliminary results in two adults showed an increase in hemoglobin from 7.4 to 13.3 g/dL and from 7 to 14.8 g/dL at 12 months, and an improvement in hemolytic markers, with no severe adverse events reported [73-75]. A Phase I study (NCT04105166) with a gene therapy product containing autologous genetically modified CD34-positive hematopoietic cells with the corrected PKLR gene is currently ongoing. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Inherited single gene disorders'.)

Decisions regarding eligibility for gene therapy were addressed in a 2022 discussion that also proposed a classification for disease severity [75]. A severe disease candidate for gene therapy requires one major plus at least one additional criterion.

Major criteria:

Transfusion-dependence

Pre-splenectomy hemoglobin <7 g/dL

Post-splenectomy hemoglobin <8 g/dL

Additional criteria:

Fatigue

Extramedullary hematopoiesis

Nonhealing lower extremity ulcers despite appropriate anemia therapy

Nontraumatic bone fractures

Icterus limiting social interactions, educational or work activities, not due to other genetic variants

Severe iron overload despite appropriate chelation or intolerance of chelation therapy

Severe pathogenic variant(s)

Genetic counseling, prenatal testing, and pregnancy — As with all inherited conditions, individuals with PK deficiency may benefit from genetic counseling regarding the risk of a child being affected. This counseling can be provided by a specialist (hematologist, genetic counselor) who understands the risks and can obtain a thorough family history and assess possible consanguinity. Individuals with heterozygosity for a pathogenic variant in PKLR who are of childbearing potential may benefit from a discussion of testing their partner. (See "Genetic counseling: Family history interpretation and risk assessment".)

Individuals who have had a severely affected child may wish to pursue in vitro fertilization with preimplantation genetic testing or to use prenatal testing to determine whether a subsequent pregnancy is affected. The type of genetic analysis will depend on the specific PK variant. If desired, intrauterine transfusions of a severely affected hydropic fetus may be possible. (See "Preimplantation genetic testing".)

Management of pregnancy in an individual with PK deficiency is similar to other inherited hemolytic anemias, with close monitoring of hematologic status and transfusion when appropriate [76]. Pregnancy has been thought to precipitate hemolysis in patients with PK deficiency. In-utero blood transfusion may be required, as some fetuses and neonates with severe PK deficiency present with hydrops fetalis [46].

Guidance on fetal blood sampling and intrauterine transfusion is provided separately [77]. (See "Intrauterine fetal transfusion of red blood cells".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anemia in adults".)

SUMMARY AND RECOMMENDATIONS

Genetics and mechanisms of hemolysis – Pyruvate kinase (PK) deficiency is an autosomal recessive hemolytic anemia characterized by reduced activity of one of the two red blood cell (RBC) isoforms of the PK enzyme, which is encoded by the PKLR gene (figure 1). PK generates ATP during glycolysis, and reduced ATP may contribute to hemolysis. PK-deficient RBCs show enhanced oxygen delivery for a given partial pressure of oxygen; thus, anemia may be better tolerated than in other hemolytic anemias. (See 'Pathophysiology' above.)

Prevalence – PK deficiency is rare. It has a worldwide distribution, but it is more common among people of northern European and perhaps Chinese ancestry. (See 'Epidemiology' above.)

Clinical features – Symptoms are variable and diagnostic delays are common. Presentations include neonatal jaundice, chronic nonspherocytic hemolytic anemia (table 1), gallstones, and iron overload. (See 'Typical presentation and clinical features' above.)

Initial evaluation – Testing is appropriate in any sibling of a patient with PK deficiency who has unexplained anemia, or any individual with Coombs-negative hemolytic anemia and absence of morphologic abnormalities suggestive of other conditions (algorithm 1). Blood smear findings in other anemias are summarized in the table (table 2). Prenatal testing should be offered to parents of a child with hydrops fetalis or severe transfusion-dependent anemia not abrogated by splenectomy. (See 'Indications for testing' above.)

Specialized testing – PK deficiency is diagnosed by measuring reduced PK activity in RBCs and identifying homozygous or compound heterozygous pathogenic variants in PKLR. (See 'PK-specific testing: Where and how to test' above and 'Diagnostic confirmation' above.)

Differential diagnosis – The differential diagnosis of PK deficiency includes other congenital nonspherocytic hemolytic anemias (figure 3), certain other inherited anemias, and acquired hemolytic anemias (table 3). (See 'Differential diagnosis' above.)

Management

Transfusions for severe anemia – Individuals with severe anemia may require RBC transfusions (intermittent or chronic). Phototherapy may be indicated for neonatal hyperbilirubinemia. (See 'Transfusions and phototherapy' above.)

Mitapivat for symptomatic anemia – For adults with symptomatic anemia (transfusion-dependent or independent), we suggest mitapivat (Grade 2B). Mitapivat is also reasonable in children and adolescents with transfusion-dependent anemia and in individuals with mild anemia without symptoms or compensated hemolysis without anemia. Individuals with two non-missense mutations in the PKLR gene may reasonably choose not to take mitapivat as it may not be effective. (See 'Mitapivat for symptomatic anemia' above.)

Folic acid – For individuals with signs of ongoing hemolysis, we suggest folic acid for most (Grade 2C). A typical dose is 1 mg daily. (See 'Folic acid' above.)

Splenectomy – Splenectomy may be indicated in selected individuals with transfusion-dependent anemia not responsive to mitapivat (ideally, deferred until later childhood and preceded by indicated vaccinations). (See 'Splenectomy' above.)

Other interventions – Cholecystectomy may be required for pigment gallstones. Iron chelation may be needed to prevent or treat iron overload. Genetic counseling may be appropriate; preimplantation genetic testing or prenatal testing may be offered. (See 'Prevention/treatment of iron overload' above and 'Genetic counseling, prenatal testing, and pregnancy' above.)

ACKNOWLEDGMENTS — UpToDate gratefully acknowledges Stanley L Schrier, MD (deceased), who contributed as Section Editor on earlier versions of this topic review and was a founding Editor-in-Chief for UpToDate in Hematology.

The UpToDate editorial staff also acknowledges extensive contributions of Donald H Mahoney, Jr, MD, and Robert A Brodsky, MD, to earlier versions of this topic review.

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Topic 7129 Version 42.0

References

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