Clinical manifestations, pathologic features, and diagnosis of acute promyelocytic leukemia in adults

INTRODUCTION — Acute myeloid leukemia (AML) refers to a group of hematopoietic neoplasms involving cells committed to the myeloid lineage. Acute promyelocytic leukemia (APL) is a biologically and clinically distinct variant of AML. APL was classified as AML-M3 in the older French-American-British (FAB) classification system and is currently classified as acute promyelocytic leukemia with t(15;17)(q24.1;q21.2);PML::RARA [1,2]. (See "Acute myeloid leukemia: Classification".)

APL represents a medical emergency with a high rate of early mortality, often due to hemorrhage from a characteristic coagulopathy. It is critical to start treatment with a differentiation agent (eg, all-trans retinoic acid) without delay as soon as the diagnosis is suspected based on cytologic criteria, and even before definitive cytogenetic or molecular confirmation of the diagnosis has been made.

The clinical features, diagnosis, and prognosis of APL in adults will be reviewed here. The following other topics are covered separately:

●Molecular biology of APL (see "Molecular biology of acute promyelocytic leukemia")

●Treatment of APL (see "Initial treatment of acute promyelocytic leukemia in adults" and "Treatment of relapsed or refractory acute promyelocytic leukemia in adults")

●The clinical features, diagnosis, and prognosis of AML in adults (see "Clinical manifestations, pathologic features, and diagnosis of acute myeloid leukemia" and "Acute myeloid leukemia: Risk factors and prognosis" and "Acute myeloid leukemia: Cytogenetic abnormalities")

EPIDEMIOLOGY — Depending on geographic variations, APL accounts for 5 to 20 percent of cases of acute myeloid leukemia (AML); there appear to be approximately 600 to 800 new cases per year in the United States [3,4]. With respect to the incidence of APL among ethnic groups, contradictory data regarding a presumed higher incidence of APL in persons from Mexico, Central and South America, Italy, and Spain have been reported in the literature [3-8]. In the United States, Hispanic Americans were not found to have greater lifetime incidence rates of APL than White Americans, although the incidence rates for APL were higher among Hispanic children and young adults [5]. Black Americans had lower lifetime incidence rates than non-Hispanic White, Hispanic, and Asian Americans [3-7]. Various studies have reported a modest increase in the incidence of APL in the last two decades [7,9]. The cause for this increased incidence remains unknown.

The age distribution of patients with APL differs from other forms of AML. APL is uncommon in the first decade of life; its incidence increases during the second decade to reach a plateau during early adulthood, and then remains constant until it decreases after age 60 years [10]. Incidence does not vary by sex. Increased risk for APL has been associated with elevated body mass index (BMI) [11].

APL can sometimes occur after cytotoxic therapy for another disease (eg, breast cancer, lymphoma, other solid tumors), especially in association with the use of topoisomerase-II inhibitors such as etoposide and doxorubicin, or after radiation therapy [12]. APL has been observed in patients with multiple sclerosis who have been treated with mitoxantrone, another topoisomerase II inhibitor. Therapy-related AML is presented separately. (See "Therapy-related myeloid neoplasms: Epidemiology, causes, evaluation, and diagnosis" and "Acute myeloid leukemia: Cytogenetic abnormalities", section on 'Therapy-related myeloid neoplasms'.)

CLINICAL FEATURES

General features — Patients with acute myeloid leukemia (AML) in general, and APL in particular, typically present with symptoms related to complications of pancytopenia (ie, anemia, neutropenia, and thrombocytopenia), including weakness and easy fatigability, infections of variable severity, and/or hemorrhagic findings such as gingival bleeding, ecchymoses, epistaxis, or menorrhagia. Combinations of these symptoms are common. Unique to APL is a presentation with bleeding secondary to disseminated intravascular coagulation, which is discussed below. (See "Clinical manifestations, pathologic features, and diagnosis of acute myeloid leukemia", section on 'Clinical presentation'.)

Patients with APL, particularly the hypergranular variant, can have a low white cell count and only rare leukemic cells in the peripheral blood. Although APL is not usually a rapidly proliferative acute leukemia, by the time that patients present with symptoms, the situation has often become a life-threatening emergency because of the risk of catastrophic bleeding. How long APL is typically present in its preclinical phase is not known, but at diagnosis the marrow is often nearly 100 percent replaced by malignant promyelocytes, leading to severe anemia, thrombocytopenia, and neutropenia. Only a small number of APL cells may be present in the blood. More importantly, this mass of malignant cells in the marrow provokes a severe coagulopathy with both disseminated intravascular thrombosis and primary fibrinolysis. The variant of APL characterized by microgranular APL cells does not bear any additional independent prognostic value, but tends to present with higher peripheral blast counts and perhaps more rapid proliferation. A white blood cell count >10,000/microL (>10 x 10⁹/L) at diagnosis identifies a high-risk subset of APL, due in part to a higher early death rate.

Coagulopathy and APL — Coagulopathy associated with APL is complex and involves both disseminated intravascular coagulation (DIC) and primary hyperfibrinolysis and is either present at diagnosis or occurs soon after the initiation of cytotoxic chemotherapy [13]. This complication constitutes a medical emergency, because, if left untreated, it can cause pulmonary or cerebrovascular hemorrhage in up to 40 percent of patients and a 10 to 20 percent incidence of early hemorrhagic death [14-18]. Thrombotic complications are less common. (See 'Microgranular variant' below and "Evaluation and management of disseminated intravascular coagulation (DIC) in adults".)

●Mechanism – The mechanism of the complex coagulopathy in APL is incompletely understood. However, the following factors may be of primary importance [13,15]. (See "Cancer-associated hypercoagulable state: Causes and mechanisms".)

•Tissue factor (TF), which forms a complex with factor VII to activate factors X and IX. The rearranged RARA in APL activates the TF promoter and increases its expression in the leukemic cells resulting in a procoagulant state. TF expression can also be upregulated by cells undergoing apoptosis.

•Death of APL cells by ETosis (a death pathway distinct from apoptosis and necrosis) releases extracellular chromatin and phosphatidylserine, which contribute to a hypercoagulable state by increasing thrombin generation and fibrin formation, damaging endothelial cells and converting them to a procoagulant phenotype, and increasing plasmin generation [19].

•Primary hyperfibrinolysis is a result of expression of annexin II, tissue and urokinase plasminogen activator as well as an acquired deficiency of alpha-2 antiplasmin and plasminogen activator inhibitor-1. Annexin II expression is increased on the surface of the leukemic promyelocytes [20]. Annexin II binds plasminogen and its activator, tissue plasminogen activator, increasing plasmin formation by a factor of 60.

The induction of tumor cell differentiation with retinoic acid, plus appropriate supportive therapy, can lead to rapid improvement in the coagulopathy primarily by reducing the hypercoagulable state but with little effect on the hyperfibrinolytic pathway. (See "Initial treatment of acute promyelocytic leukemia in adults", section on 'Control of coagulopathy'.)

PATHOLOGIC FEATURES

Morphology — A significant number of patients will present with low white cell count at the time of diagnosis with only rare leukemic cells in the peripheral blood. In addition, the microgranular variant can be easily confused with myeloid leukemia with monocytic differentiation. A careful evaluation of the peripheral blood smear and recognition of the classic and variant morphology is therefore critical in making a presumptive diagnosis of APL before the molecular/cytogenetic data become available.

APL is characterized by the presence of atypical promyelocytes in the bone marrow and peripheral blood. Promyelocytes are large (usually >20 microns in diameter) myeloid precursors with variable morphology. Often there is a high nucleus to cytoplasmic ratio, fine chromatin, and prominent nucleoli. The cardinal feature of the promyelocyte is the presence of many violet granules in the cytoplasm with either a dense or coarse pattern, often obscuring the nucleus. The cells of APL differ morphologically from those of normal promyelocytes in that they are larger and typically have creased, folded, bilobed, kidney-shaped, or dumb-bell shaped nuclei (picture 1) [21].

There are two morphologic variants of APL: the hypergranular form and the microgranular form. Clinicians should be aware of these morphologic variants to allow for the identification of potential cases and the rapid initiation of treatment when the diagnosis is suspected. (See "Initial treatment of acute promyelocytic leukemia in adults".)

Hypergranular form — The hypergranular, or "typical," form of APL is the most common variant, accounting for approximately 75 percent of cases [21]. On Wright-stained smears, the cytoplasm of these promyelocytes typically contains densely-packed bright pink, reddish-blue, or dark purple granules (picture 1 and picture 2). Neoplastic promyelocytes frequently contain Auer rods or occasionally bundles of Auer rods (picture 3 and picture 4).

Microgranular variant — The microgranular (hypogranular) variant accounts for approximately 25 percent of cases [22]. The cells in this variant typically have a bilobed nucleus and no apparent granules on light microscopy (picture 5). Primary granules are present, however, and are easily demonstrated using electron microscopy [23]. The hypogranular promyelocytes can sometimes be mistakenly identified as being of monocytic origin. However, in contrast to monocytes, the myeloperoxidase reaction is strongly positive, and the non-specific esterase reaction is negative or only weakly positive.

In a typical case of APL, the malignant promyelocytes usually account for 30 percent or more of the myeloid cells in the bone marrow and are considered equivalent to blasts. Sometimes there are also increased blasts, but there are very few cells that mature past the promyelocyte stage of differentiation. The abnormal promyelocytes are usually intensely myeloperoxidase reaction positive. The non-specific esterase reaction is weakly positive in a quarter of cases but should not be construed as evidence of monocyte differentiation. Monocytes show bright non-specific esterase reactivity.

Immunophenotype — Typically, APL cells share certain immunophenotypic features with their normal promyelocytic counterparts [6,21]. The hypergranular variants have a high side scatter; express bright cytoplasmic myeloperoxidase, CD13 and CD33; are partial, weak, or negative for CD34; and do not express or only dimly express HLA-DR and CD11b. In contrast to normal promyelocytes, APL cells express abnormally low levels of CD15 and weak or variable CD117.

The hypogranular variant of APL shows a similar phenotype with a comparably bright myeloperoxidase expression and frequent coexpression of CD2, and can sometimes express CD34. Some cases can also show CD56, and this has been reported to be associated with worse prognosis [24]. Thus, with the caveats noted above, the APL phenotype is considered CD34 negative/partial or weak positive, HLA-DR negative, CD13 positive, CD33 positive, CD11b negative, CD15 weak or negative, CD117 weak/variable, and sometimes CD2 positive and CD56 positive.

Immunostaining with anti-PML monoclonal antibodies is a somewhat specific test that can be performed in as little as two to four hours [25]. APL demonstrates a microspeckled nuclear staining pattern with nucleolar exclusion that differs from the discrete larger and fewer nuclear dots seen in normal cells and other forms of acute myeloid leukemia (AML) (picture 6). Sometimes, however, the staining pattern of non-APL cases can be difficult to interpret.

Furthermore, additional accompanying acquired mutations in particular genes such as FLT3-ITD (fms-like tyrosine kinase-internal tandem duplications) may be present as well although their prognostic significance for the time being is not certain and may differ by age [26,27]. (See "Acute myeloid leukemia: Risk factors and prognosis".)

Genetic features — APL with t(15;17)(q24.1;q21.2);PML::RARA is defined by the presence of a reciprocal translocation between the long arms of chromosomes 15 and 17, with the creation of a fusion gene, PML::RARA which links the retinoic acid receptor alpha (RARA) gene on chromosome 17 with the promyelocytic leukemia (PML) gene on chromosome 15 [28-30]. (See "Molecular biology of acute promyelocytic leukemia".)

Of importance, some differences exist regarding the precise location of the translocation breakpoints depending on whether banded chromosomes are studied by light microscopy or the genetic loci by molecular studies. By chromosomal banding the translocation has been noted as t(15;17)(q22;q12), but based on the human genome sequence, the translocation is now believed to be more precisely written as t(15;17)(q24.1;q21.2). Rare cases of APL are associated with variant translocations/gene rearrangements. (See 'Variant RARA translocations' below.)

The genetic abnormality may be detected using the following techniques, each of which has advantages and disadvantages [6]. Bone marrow samples are preferred for each technique:

Conventional karyotype — This method is highly specific, and an essential part of the standard workup, but it is time-consuming, usually taking approximately two days for results. Cryptic rearrangements leading to PML::RARA fusion will be missed (false negatives). However, rare molecular subtypes of APL, such as t(11;17), t(5;17), and other additional coexistent cytogenetic abnormalities can be detected.

Fluorescence in situ hybridization — Fluorescence in situ hybridization (FISH) for the PML::RARA fusion is less expensive and quicker than conventional cytogenetics. The sensitivity varies depending on the probe used. FISH does not provide information about the PML::RARA isoform detected and therefore cannot be used for the molecular monitoring of residual disease. FISH can be performed usually within 24 hours or, in some instances, in an even shorter time.

Reverse transcriptase polymerase chain reaction — Reverse transcriptase polymerase chain reaction (RT-PCR) for PML::RARA RNA is considered by many to be the current "gold standard" method for confirming the diagnosis of APL. RT-PCR also provides information on the PML breakpoint location, which can be used for monitoring measurable residual disease (MRD; also referred to as minimal residual disease). However, RT-PCR can have both false-positives (contamination artifacts) and false-negatives (due to poor RNA yield). (See "Acute myeloid leukemia: Induction therapy in medically fit adults", section on 'Introduction'.)

Variant RARA translocations — Multiple variant translocations involving the RARA gene on chromosome 17 have been described. These cases can have morphologic features that overlap with APL. However, because of variation in biology as well as differences in response to pharmacologic doses of ATRA, these cases have been reclassified in the current World Health Organization system as "APL with a variant RARA translocation" [31]. (See "Molecular biology of acute promyelocytic leukemia", section on 'Variant translocations'.)

●t(11;17)(q23;q21) – ZBTB16::RARA (previously known as PLZF::RARA), often CD13+, CD56+

●t(5;17)(q35;q21) – NPM1::RARA, often CD13-negative, CD56-negative

●t(11;17)(q13;q21) – NuMA::RARA

●t(17;17)(q21;q21) – STAT5b::RARA

Cells with the variant translocation t(11;17) tend to have nuclei that are more regular in outline, with an increased number of maturing neutrophils with hypolobulation (ie, acquired Pelger-Huet anomaly). These cases have features intermediate between AML with maturation and APL, with sparser granules, lack of "faggot" cells, and absence of the typical bilobed nucleus seen in the typical forms of APL [21,32,33]. The ZBTB16::RARA fusion protein that is produced in this chromosomal rearrangement shows reduced sensitivity to retinoic acid that cannot be overcome by pharmacologic doses of ATRA [28,34].

DIAGNOSIS — APL represents a medical emergency with a high rate of early mortality, often due to hemorrhage from a characteristic coagulopathy. It is strongly recommended to start treatment with a differentiation agent (eg, tretinoin, also known as all-trans retinoic acid or ATRA) without delay as soon as the diagnosis is suspected based on cytologic and clinical criteria, and before definitive genetic, cytogenetic, or immunostaining confirmation of the diagnosis has been made. If the diagnosis is not confirmed, ATRA can be discontinued and treatment changed to that used for other types of acute myeloid leukemia (AML). (See "Initial treatment of acute promyelocytic leukemia in adults".)

The diagnosis of APL is suspected by the characteristic morphology of the leukemic cells, immunophenotype, or the presence of severe coagulopathy as described above. Another diagnostic clue is that the non-microgranular form of APL often presents with leukopenia. The diagnosis is confirmed by the identification of the PML::RARA fusion gene and/or the associated chromosomal translocation (or one of the rare variants described above). While all available diagnostic tools should be utilized, a request for an expedited molecular/FISH analysis looking for PML::RARA generally provides the fastest diagnostic result. This genetic confirmation serves to differentiate APL from other forms of acute leukemia.

PROGNOSIS — Without treatment, APL has a very short median survival of less than one month resulting from uncontrolled bleeding. However, with modern therapy, APL is associated with the highest proportion of patients who are presumably cured of their disease. Despite the overall good prognosis for this group of patients, there is heterogeneity in this population. Patients younger than age 30 years and those who present with a white blood cell (WBC) count less than 10,000/microL (10 x 10⁹/L) have superior event-free survival [35]. In contrast to other types of acute myeloid leukemia (AML), age over 60 years is not a predictor of poor results in APL [36]. (See "Pretreatment evaluation and prognosis of acute myeloid leukemia in older adults", section on 'Prognosis'.)

In a multivariate analysis of 217 patients with newly diagnosed PML::RARA-positive APL treated in the Italian GIMEMA and the Spanish PETHEMA trials, adverse risk factors were a total WBC count greater than 10,000/microL and a platelet count less than or equal to 40,000/microL [37]. Using these two parameters, three prognostic categories could be distinguished, with the following estimated probabilities of three-year relapse-free survival (RFS):

●Low risk – WBC ≤10,000/microL and platelets >40,000/microL; RFS 98 percent

●Intermediate – WBC ≤10,000/microL and platelets ≤40,000/microL; RFS 89 percent

●High risk – WBC >10,000/microL; RFS 70 percent

These three risk groups originally proposed by the PETHEMA group can be collapsed into two groups based entirely on the WBC count (≤10,000 or >10,000/microL at diagnosis) [38,39]. That is, the platelet count is not necessary to distinguish a low/intermediate risk group when arsenic trioxide (ATO) is used early in therapy. Studies have demonstrated that patients in this risk group have superior outcome when treated with ATRA plus ATO in comparison to ATRA plus anthracycline-based chemotherapy [40].

Up to 40 percent of patients will have chromosomal abnormalities in addition to t(15;17). There is no evidence that additional cytogenetic abnormalities or molecular aberrations (eg, FLT3-ITD [Fms-like tyrosine kinase-3 receptor] gene mutations) have an adverse effect on prognosis [41-44]. In two large studies of patients treated with an anthracycline plus cytarabine or with all-trans retinoic acid (ATRA) and chemotherapy, additional chromosomal abnormalities had no impact on prognosis [43,44].

Internal tandem duplications in the FLT3-ITD gene are frequently seen in APL. They are associated with an increased WBC count at diagnosis but do not add unambiguous independent prognostic value. (See "Acute myeloid leukemia: Risk factors and prognosis".)

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: Acute promyelocytic leukemia".)

SUMMARY

●Description – Acute promyelocytic leukemia (APL) is a biologically and clinically distinct category of acute myeloid leukemia (AML) in which abnormal promyelocytes predominate. APL represents a medical emergency with a high rate of early mortality that requires initiation of treatment as soon as the diagnosis is suspected (ie, even before genetic confirmation of the diagnosis).

●Epidemiology – APL accounts for 5 to 20 percent of AML. It is uncommon in the first decade of life, the incidence increases during early adulthood and it remains at a plateau until it decreases after age 60 years. (See 'Epidemiology' above.)

●Clinical presentation – APL most often presents with weakness/fatigue from anemia, infections due to neutropenia, and/or hemorrhage due to thrombocytopenia or disseminated intravascular coagulation (DIC). (See 'Clinical features' above.)  

DIC, which is often present at diagnosis, requires emergency therapy. (See 'Coagulopathy and APL' above and "Initial treatment of acute promyelocytic leukemia in adults", section on 'Control of coagulopathy'.)

●Pathology – Characteristic malignant promyelocytes are present in blood and marrow.

•Blood – Atypical promyelocytes, which are large myeloid precursors with variable morphology, are seen on blood smear.

There are two categories of APL:

-Hypergranular APL – The cardinal feature is numerous violet cytoplasmic granules with a dense or coarse pattern that often obscures the nucleus (picture 1 and picture 2); malignant promyelocytes often contain Auer rods (picture 3 and picture 4).

-Microgranular variant – One-quarter of cases correspond to the microgranular variant of APL, in which malignant promyelocytes have a bilobed nucleus and no apparent granules on light microscopy (picture 5); the promyelocytes may be resemble monocytic cells.

Patients with APL, particularly the hypergranular variant, can have a low white cell (WBC) count at diagnosis.

•Bone marrow – Infiltration of marrow by malignant promyelocytes.

●Genetics – Most (≥95 percent) cases of APL have the t(15;17)(q24.1;q21.2) chromosomal translocation, which is associated with PML::RARA fusion gene that links retinoic acid receptor alpha (RARA) on chromosome 17 with the promyelocytic leukemia (PML) gene on chromosome 15. (See 'Genetic features' above.)

Rare APL variants include: t(11;17)/ZBTB16::RARA, t(5;17)/NPM1::RARA, t(11;17)/NuMA::RARA, and t(17;17)/STAT5b::RARA.

●Immunophenotype – Malignant cells typically express CD13 and CD33, are usually negative or weakly positive for CD34, and do not express or only dimly express CD15, CD117, HLA-DR, and CD11b. (See 'Immunophenotype' above.)

●Diagnosis – The diagnosis of APL is confirmed by detection of PML::RARA by reverse transcriptase polymerase chain reaction (RT-PCR) or the associated translocation t(15;17) chromosomal translocation by conventional cytogenetics or fluorescence in situ hybridization (FISH). Some cases have cytogenetic or molecular findings associated with a rare variant of APL. (See 'Genetic features' above and 'Diagnosis' above.)

Treatment with all-trans retinoic acid (ATRA) should be initiated when the diagnosis of APL is suspected by the clinical presentation, morphology, cytochemistry, and/or flow cytometry, even before genetic confirmation of the diagnosis. (See "Initial treatment of acute promyelocytic leukemia in adults".)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges John Anastasi, MD, who contributed to earlier versions of this topic review.