Submission Details

Alpha-thalassemias have long been recognized as inherited abnormalities of hemoglobin structure that result in anemias from early childhood when fetal hemoglobin expression is diminished. These recessive disorders occur with high frequency among populations that have historically inhabited parts of the world where malaria is endemic. Early publications identified the molecular basis of alpha-thalassemia at the protein level through sequencing of the HBA2 gene product (PMID: 1062801, PMID: 1155453, PMID: 4422784), while the inherited basis was first traced to the HBA2 locus encoding the hemoglobin alpha chain in 1977 (PMID: 909779). Patients with homozygous deletion of HBA2 were first described in 1984 (Oppenheimer et al., PMID: 6142152), and subsequent studies reported diverse non-deletional HBA2 variants (PMID: 6725554, PMID: 6725554). The hemoglobin alpha chain is now known to be encoded by two closely linked loci on chromosome 16, HBA1 and HBA2, with alpha-thalassemia trait caused by deletion or inactivation of two of the four alleles. While many affected cases exhibit a digenic mode of inheritance, this curation of the HBA2 gene has focused on cases with biallelic disruption of HBA2 and absence of deleterious variants in HBA1. These cases generally exhibit mild to severe anemia, decreased volume of red blood cells, decreased hemoglobin content per red blood cell, reticulocytosis, the presence of abnormal hemoglobin tetramers of gamma-globin chains (Hemoglobin Barts) or beta-globin chains (Hemoglobin H), and/or the presence of erythrocyte inclusion bodies that contain denatured hemoglobin. Splenomegaly, hyperbilirubinemia, and/or history of jaundice can also be present. Per criteria outlined by the ClinGen Lumping & Splitting Working Group, these cases have been distinguished from other HBA2-related disease entities by their mode of inheritance (autosomal recessive), molecular mechanism (biallelic HBA2 loss-of-function), and clinical presentation (less severe than cases with inactivation of three or four hemoglobin alpha chains). Therefore, HBA2 has been separately curated for unstable hemoglobin disease, methemoglobinemia, and familial erythrocytosis, each with an autosomal dominant mode of inheritance. The HBA1 gene has also been curated separately.

Eight suspected disease-causing variants have been scored as part of this curation (one complete gene deletion, two stop-loss, three missense, one canonical splice site deletion, and one disrupting the polyadenylation signal within the 3’ UTR). These have been collectively reported in ten probands in eight publications (PMID: 2079430, PMID: 2430881, PMID: 6142152, PMID: 6725554, PMID: 8555062, PMID: 15768554, PMID: 20642339, PMID: 20854116). The mechanism of pathogenicity appears to be biallelic loss of HBA2 function conferred by null and/or hypomorphic variants that reduce alpha-globin accumulation. Eight out of ten probands were homozygous for their respective variants, while two probands were compound heterozygous. Large families with co-segregation of the phenotype with the homozygous genotype have not been found for inclusion in this gene curation. Additional case-level evidence is available in the literature but has not been included in this curation as the maximum score for this category of evidence had already been reached.

This gene-disease association is also supported by experimental evidence that the expression pattern of HBA2 across human tissues is highly restricted to the blood (PMID: 23715323). Biochemical studies indicate that HBA2 encodes the alpha chain of hemoglobin, which binds and releases oxygen as part of its function central to the role of red blood cells in carrying oxygen to the tissues of the body (PMID: 11747442). HBA2 protein interacts with the beta hemoglobin chain encoded by the HBB gene (PMID: 6644819), which can harbor variants associated with the closely related condition beta-thalassemia. Cells from an affected patient exhibit altered function of the variant HBA2 protein as it abnormally associates with the membrane skeleton and enhances the mechanical stability and rigidity of the membrane (PMID: 9057661). Patient cells also show destabilization of beta-globin chains and the presence of globin chain precipitates (PMID: 6725554). A mouse model of heterozygous Hba1 and Hba2 deletion recapitulates features of the human patients such as decreased volume of red blood cells, the presence of erythrocyte inclusion bodies, and reduced ratio of alpha-globin to beta-globin synthesis (PMID: 7550311). Defects of mice with homozygous Hba1 and Hba2 deletion can be rescued with the addition of a human HBA2 transgene (PMID: 7550311), confirming the relevance of the model to the human disease state.

In summary, HBA2 is definitively associated with autosomal recessive inheritance of alpha-thalassemia (MONDO:0011399). This has been repeatedly demonstrated in both research and diagnostic settings, and has been upheld over time without the emergence of contradictory evidence, leading to a Definitive classification. This classification was approved by the ClinGen General Gene Curation Expert Panel on June 28th, 2023 (SOP Version 9).