Gene test interpretation: HBA1 and HBA2 (alpha globin genes)

INTRODUCTION — This monograph discusses implications of a genetic test result for alpha globin genes (HBA1 and HBA2). Pathogenic variants (often deletions) in these genes cause alpha thalassemia.

Alpha globin gene testing cannot identify beta thalassemia or other conditions affecting beta globin such as sickle cell disease.

Indications for testing and clinical care of the tested individual are discussed separately in UpToDate [1].

How to read the report

●Accuracy – Confirm the test was performed in a Clinical Laboratory Improvement Amendments (CLIA)-certified (or other nationally certified) laboratory (table 1).

●Genotype – Determine whether testing involved alpha globin gene sequencing or a panel of variants. Some laboratories test for selected common variants, which may be sufficient in some cases and insufficient in others.

●Clinical correlation – Review (or obtain) a complete blood count (CBC). Review the red blood cell (RBC) indices, especially mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH). Determine whether protein-based hemoglobinopathy testing was performed. (See 'Laboratory/diagnostic confirmation' below.)

Determine why testing was done and whether results are sufficient for the indication.

•Reproductive counseling may require more definitive DNA-based diagnosis to determine the risk to offspring and the need for partner testing. (See 'Reproductive counseling and testing' below.)

•Consultation with a genetics expert may be required if severe disease is suspected or reproductive counseling is not straightforward. (See 'Settings for alpha globin testing' below and 'Locating a specialist' below.)

Alpha thalassemia genetics — Hemoglobin (Hb) is a tetramer of two alpha globin chains and two beta globin chains.

Alpha thalassemias are caused by pathogenic variants in the alpha globin genes, leading to reduced alpha globin production [2]. There are four functional alpha globin genes (two distinct, adjacent genes in the alpha globin gene cluster inherited from each parent) (figure 1). Any number of alpha globin genes from one to four may be affected. The more alpha chains affected, the more severe the alpha thalassemia syndrome. Although beta thalassemia may be more familiar, alpha thalassemia variants are more prevalent worldwide [2].

Alpha⁰ variants (deletional variants) abolish alpha globin production; alpha⁺ variants (non-deletional variants, sometimes designated a^ND) produce some alpha globin but less than normal [3]. While some alpha⁺ variants are mild, others can be as severe or worse than a deletional variant.

●Alpha⁰ variants – Often delete an entire alpha globin gene cluster.

●Alpha⁺ variants – Alter one or a few nucleotides. Hemoglobin Constant Spring (Hb CS) is a common non-deletional variant, caused by a mutation of the terminal codon in the alpha2 globin gene. (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb Constant Spring'.)

Alpha globin production begins in utero (figure 1). In the fetus, alpha globin pairs with gamma globin to make fetal hemoglobin (Hb F); beta globin production begins after birth. Thus, alpha globin variants manifest in utero, whereas beta globin variants manifest after six months of age. (See "Structure and function of normal hemoglobins".)

The ratio of alpha and beta chains must be closely matched. If one type of chain is in excess, it will form precipitates within RBCs or RBC precursors in the bone marrow. These precipitated Hbs include Hb Barts (tetramers of gamma globin) and Hb H (tetramers of beta globin). These abnormal Hbs raise the Hb level but are functionally useless because they do not transport oxygen. The precipitates promote hemolysis. (See 'Clinical features' below.)

CLINICAL

Settings for alpha globin testing — Genetic testing may be appropriate for:

●Diagnosis of alpha thalassemia. (See 'Alpha thalassemia genetics' above and 'Clinical features' below.)

●Reproductive testing (patient, partner, and/or fetus) and counseling. (See 'Reproductive counseling and testing' below.)

●Testing the first-degree relative(s) of an individual with alpha thalassemia or an alpha thalassemia carrier state. (See 'Relatives' below.)

Clinical features — Clinical features range from an asymptomatic carrier state to death in utero, depending on the number of alpha globin genes affected and whether the variants are deletional or non-deletional. (See 'Alpha thalassemia genetics' above.)

These are summarized in the table (table 2).

●One alpha thalassemia gene – Individuals with a single alpha gene variant/deletion (alpha thalassemia minima or silent carrier) are asymptomatic without any laboratory changes.

●Two alpha thalassemia genes – Individuals with two alpha gene variants/deletions (alpha thalassemia minor or alpha thalassemia trait) are asymptomatic. They may have mild microcytic anemia or microcytosis without anemia. Hemoglobin (Hb) is decreased by approximately 1 g/dL; it may decrease further during pregnancy. Two possible genotypes are heterozygosity for an alpha⁰ genotype (aa/--) or compound heterozygosity for an alpha⁺ genotype (a-/a-). These generally do not affect the individual's clinical status, but they may be important for reproductive counseling. (See 'Reproductive counseling and testing' below.)

●Three alpha thalassemia genes – Hb H disease refers to three alpha gene deletions (a-/--) or variants such as (a-/-a^CS). Hb H is a tetramer of beta globin. The Constant Spring (CS) variant is discussed above. (See 'Alpha thalassemia genetics' above.)

•Individuals with deletional Hb H disease have mild to moderate anemia and occasionally require transfusions. Splenomegaly and pigment gallstones are common. Anemia can be exacerbated by common viral diseases and exposure to oxidant drugs, with sudden transient transfusion requirement.

•Individuals with non-deletional Hb H disease such as (a-/-a^CS) have more severe anemia but less microcytosis. They can require transfusions in utero or at birth; some may require transfusions after birth. Some infants with Hb CS develop marked splenomegaly and severe symptoms requiring long-term chronic transfusions; up to 20 percent of patients with Hb CS require transfusions and splenectomy.

Individuals with three alpha thalassemia gene variants may have ineffective erythropoiesis and increased iron absorption and may develop transfusional or non-transfusional iron overload. (See "Management of thalassemia", section on 'Regular transfusions' and "Management of thalassemia", section on 'Assessment of iron stores and initiation of chelation therapy'.)

●Four alpha thalassemia genes – Variants/deletion of all four alpha globin genes (--/--) causes alpha thalassemia major, the most severe form of alpha thalassemia. Most of the circulating Hb is Hb Barts, which does not transport oxygen. These individuals develop hydrops fetalis and typically die in utero unless they receive intrauterine transfusions. After birth, they require chronic transfusions with chelation therapy. Allogeneic hematopoietic stem cell transplant (HSCT) is potentially curative. (See "Alpha thalassemia major: Prenatal and postnatal management".)

Laboratory/diagnostic confirmation — All individuals with suspected alpha thalassemia should have a complete blood count (CBC) and review of the red blood cell (RBC) indices. Hemolysis testing is appropriate if there is anemia or reticulocytosis (table 3).

●CBC – Mean corpuscular volume (MCV) <80 fL and/or mean corpuscular hemoglobin (MCH) <27 is consistent with alpha thalassemia, regardless of whether anemia is present, but they are not diagnostic, as they can also occur in iron deficiency and other disorders (algorithm 1). Typically, the RBC count is increased in thalassemia and decreased in iron deficiency. (See "Microcytosis/Microcytic anemia", section on 'Approach to the evaluation'.)

●Iron studies – Individuals being evaluated for thalassemia should have iron studies to exclude or diagnose iron deficiency and to evaluate iron overload.

•Iron deficiency causes microcytic anemia and can interfere with protein-based hemoglobin analysis (algorithm 1). (See "Causes and diagnosis of iron deficiency and iron deficiency anemia in adults", section on 'Iron studies (list of available tests)'.)

•Iron overload is common in thalassemia. (See "Diagnosis of thalassemia (adults and children)", section on 'Iron overload'.)

●Protein or DNA-based hemoglobin analysis – Diagnostic confirmation can be done by protein-based or molecular testing [3,4]:

•Protein-based testing such as high-performance liquid chromatography (HPLC) or electrophoresis (gel-based or capillary) can confirm alpha thalassemia if Hb H (or Hb Barts) is present. HPLC or capillary electrophoresis are preferred; they are more accurate and quantitative. Protein-based methods may detect Hb A2 and Hb F, which can be supportive but less specific. Hb A2 >4 percent is diagnostic of beta thalassemia; low Hb A2 is suggestive of alpha thalassemia or iron deficiency. (See "Methods for hemoglobin analysis and hemoglobinopathy testing", section on 'Protein chemistry methods'.)

•Molecular (DNA) testing provides the specific genotype. Alpha thalassemia minima (single alpha globin gene affected) requires molecular testing. (See "Methods for hemoglobin analysis and hemoglobinopathy testing", section on 'Molecular genetic (DNA-based) methods'.)

Hb analysis may be omitted in some cases, such as individuals with normal CBC who are not considering childbearing.

Interventions

●Folic acid – All individuals with chronic hemolysis are advised to take folic acid (typical dose, 1 mg daily; higher doses prior to conception and during pregnancy). This may be reasonably omitted in individuals with a folate-replete diet who are not attempting to conceive.

●Transfusions – Transfusions may be needed for severe anemia, transient aplastic crisis, or other bone marrow stressors.

●Cholecystectomy/splenectomy – Cholecystectomy may be necessary in Hb H disease due to pigment gallstones. Hb H disease with a non-deletional variant often requires splenectomy to improve Hb levels.

●Chelation – As Hb H patients age, they develop iron overload even without transfusions; they require monitoring and potentially iron chelation.

●PK activator – Mitapivat is a small molecule activator of pyruvate kinase (PK) that increased hemoglobin in a small study; further trials are ongoing. (See "Management of thalassemia", section on 'Mitapivat'.)

●Alpha thalassemia major – Alpha thalassemia major requires aggressive treatment, with transfusions in utero followed by lifelong transfusions and iron chelation. Hematopoietic stem cell transplantation (HSCT) can be curative; in utero fetal HSCT is under investigation. The possibility of HSCT after birth should be discussed. (See "Alpha thalassemia major: Prenatal and postnatal management".)

REPRODUCTIVE COUNSELING AND TESTING — Individuals with alpha thalassemia should be offered age-appropriate reproductive counseling. Those planning to conceive should be offered counseling and partner testing. Relevant information is available at the UCSF website [5].

Alpha thalassemia variants are prevalent in individuals with ancestry from Southern China, Malaysia, Thailand, and Africa. However, due to migration, alpha thalassemia variants can be present in individuals with any ancestry.

●Alpha thalassemia-1 trait (two alpha gene deletions in cis; aa/--) is more common in individuals with ancestry from Southern China, Malaysia, and Thailand; these individuals are at greater risk for having a pregnancy with hydrops fetalis (Hb Barts).

●Individuals of African ancestry typically carry the alpha thalassemia-2 trait (two alpha gene deletions in trans; a-/a-). The pathogenic variants are on opposite chromosomes, and regardless of the paternal variants, a fetus with four pathogenic variants (Hb Barts) is not possible. Hb H remains possible depending on the genotype of the partner.

Pregnancies with severe variants are at greater risk of having an affected fetus, and individuals who carry an alpha⁰ genotype (aa/--) are at risk for having a child with alpha thalassemia major. They may elect:

●Preimplantation genetic testing (PGT) on embryos created by in vitro fertilization (IVF). (See "Preimplantation genetic testing".)

●Donor oocytes or donor sperm. (See "In vitro fertilization: Overview of clinical issues and questions", section on 'Oocyte donation' and "Donor insemination".)

●Adoption. (See "Adoption".)

●Prenatal testing with intensive monitoring and in utero treatment if needed. (See "Alpha thalassemia major: Prenatal and postnatal management", section on 'Prenatal'.)

RELATIVES — Relatives may benefit from knowing about a thalassemia diagnosis (including carrier state) to avoid routine iron supplementation based on the mistaken impression that microcytosis is due to iron deficiency, and for reproductive counseling (algorithm 1).

RESOURCES

UpToDate topics

●Thalassemias – (See "Molecular genetics of the thalassemia syndromes" and "Pathophysiology of thalassemia" and "Diagnosis of thalassemia (adults and children)" and "Management of thalassemia" and "Alpha thalassemia major: Prenatal and postnatal management".)

●Other alpha globin disorders – (See "Unstable hemoglobin variants" and "Methemoglobinemia", section on 'Hemoglobin M disease and cytochrome b5 deficiency'.)

Locating a testing laboratory — The table lists hemoglobinopathy testing laboratories (table 4).

Locating a specialist

●Genetics professionals

•Clinical geneticists – American College of Medical Genetics and Genomics (ACMG)

•Genetic counselors – National Society of Genetic Counselors (NSGC)

●Alpha thalassemia experts – The Thalassemia International Federation provides a list of specialty centers worldwide. The Cooley's Anemia Foundation website lists selected thalassemia centers in the United States [6]. These organizations provide educational materials and other resources. Other experts are available outside these centers.