HBA2

Inheritance Pattern: AD

Disease ID:MONDO:0054802

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AR

Disease ID:MONDO:0011399

Submitter: Natera

Inheritance Pattern: AD

Disease ID:MONDO:0020835

Submitter: ClinGen

Hemoglobin M disease was first reported by Horlein and Weber in 1948 (PMID: 18105244) in a family with hereditary cyanosis due to methemoglobinemia. Chemical characterizations of hemoglobin M (Hb M) were described in the late 1950s and early 1960s by Gerald (PMIDs: 13572451, 13897827), who reported that Hbs M are poorly, or not at all, separable electrophoretically from normal adult hemoglobin unless the hemolysate has been treated with oxidizing agents and the isolated methemoglobins M differ in the spectral absorption curves. These chemical differences result in the methemoglobinemia known as Hemoglobin M disease, in which more than 1% of hemoglobin is oxidized to methemoglobin, a type of hemoglobin that contains the ferric form of iron, and patients are cyanotic but otherwise asymptomatic. The first variant specifically attributed in *HBA2* was HbM-Iwate, which was genetically confirmed in 1995 (PMID: 7745837, 31327183) in relation to autosomal dominant Hemoglobin M disease. The two predominant missense variants occur in conserved proximal or distal histidine residues, which are replaced by tyrosine. These amino acid substitutions in the heme binding pocket cause structural alterations which stabilize the ferric state, resulting in extremely low oxygen affinity and resistance to reduction. Three missense variants have been reported in 6 probands in 6 publications (PMIDs: 7745837, 31327183, 25031065, 25725047, 31269924, 21637412) and are included in this curation. Hemoglobin M variants have altered function, suggesting a neomorphic mechanism of pathogenicity, due to increased stabilization of the ferric state. This gene-disease relationship is also supported by experimental evidence, the high specificity of expression within the whole blood (PMID: 23715323) and its well-established function in carrying oxygen, as part of the hemoglobin structure (PMID: 11747442). In Hemoglobin M disease, the function is altered due to differences in the heme structure (PMID: 4521212). In summary, there is limited evidence supporting the relationship between *HBA2* and autosomal dominant methemoglobineamia, alpha type. Although more evidence is needed to support a causal role, no convincing evidence has emerged that contradicts the gene-disease relationship. The classification was approved by the ClinGen General GCEP on the meeting date July, 26, 2023 (SOP version 9). Of note, *HBA2* is also related to alpha-thalassemia, erythrocytosis, Heinz body anemia, and Hemoglobin H disease, however per criteria outlined by the ClinGen Lumping and Splitting Working group we found differences in that the molecular mechanism (missense variants which stabilize the ferric state) and phenotype (benign cyanosis) were unique to the Hemoglobin M disease entity, which is therefore curated separately from the additional disease entities.

Inheritance Pattern: AR

Disease ID:MONDO:0011399

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AD

Disease ID:MONDO:0018023

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0013512

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0015579

Submitter: Orphanet

Inheritance Pattern: AD

Disease ID:MONDO:0054802

Submitter: ClinGen

*HBA2* was first reported in relation to autosomal dominant erythrocytosis 7 in 1980 (Honig et al. 1980; PMID: 7213661). Erythrocytosis 7 is a disorder of the blood characterized by a high concentration of erythrocytes in the blood and an increased oxygen affinity of hemoglobin. Per criteria outlined by the ClinGen Lumping & Splitting Working Group, we found differences in molecular mechanisms, inheritance pattern, and phenotypic variability in erythrocytosis 7 relative to other disease entities associated with *HBA2*. Therefore, the following disease entities have been split into multiple curations: AD erythrocytosis, AD and AR alpha-thalassemia, unstable hemoglobin disease, and methemoglobinemia. This curation addresses only AD erythrocytosis. This curation includes at least 10 missense variants reported in 10 probands with AD erythrocytosis in 10 publications (PMIDs: 7213661, 1634363, 1634355, 11186265, 18818920, 19205974, 22959973, 25818820, 27240426, 33217156). This gene disease association is also supported by experimental evidence including expression, biochemical function, and protein-protein interactions (PMIDs: 11747442, 6644819). In summary, there is limited evidence to support this gene-disease relationship. Although more evidence is needed to support a causal role, no convincing evidence has emerged that contradicts the gene-disease relationship. This classification was approved by the ClinGen General Gene Curation Expert Panel on 10/25/2023 (SOP Version 9).

Inheritance Pattern: AD

Disease ID:MONDO:0020459

Submitter: ClinGen

*HBA2* was first reported in relation to autosomal dominant unstable hemoglobin disease in 1968 (Kleihauer et al. 1968, PMID: 5639009). Unstable hemoglobin disease results from the presence of a structurally abnormal hemoglobin variant which leads to Heinz bodies or basophilic inclusions in the red blood cells. The disease is characterized by hemolytic anemia, heat-labile hemoglobin, and is sometimes accompanied by splenomegaly and jaundice. The clinical severity and presentation of the unstable hemoglobinopathies vary widely. Per criteria outlined by the ClinGen Lumping and Splitting Working Group, we found differences in molecular mechanisms, inheritance pattern, and phenotypic variability in unstable hemoglobin disease and other disease entities associated with *HBA2*. Therefore, the following disease entities have been split into multiple disease entities: AD familial erythrocytosis 7, AD and AR alpha-thalassemia, AD hemoglobin M disease, and AD unstable hemoglobin disease. This curation addresses *HBA2* in relation to AD unstable hemoglobin disease. At least 8 (7 missense and 1 deletion) variants that have been reported in 8 probands in 8 publications (PMIDs: 5639009, 5780195, 2606724, 7717382, 9255611, 15481883, 19815833, 31145010) are included in this curation. Variants in this gene segregated with disease in 9 additional family members (PMID: 7558871). This gene disease association is also supported by experimental evidence including expression pattern of HBA2 across human tissues which is highly specific within whole blood (PMID: 23715323), biochemical studies indicating the well-established function of *HBA2* in oxygen binding (PMID: 11747442), and its protein-protein interaction with the beta hemoglobin chain encoded by the *HBB* gene (PMID: 6644819), which harbors variants associated with unstable hemoglobin disease. In summary, there is limited evidence to support this gene-disease relationship. Although more evidence is needed to support a causal role, no convincing evidence has emerged that contradicts the gene-disease relationship. This classification was approved by the ClinGen General Gene Curation Expert Panel on 2/28/2024 (SOP Version 10).

Inheritance Pattern: AD

Disease ID:MONDO:0007705

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AR

Disease ID:MONDO:0100562

Submitter: ClinGen

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 spectrum (MONDO:0100562). 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).

Inheritance Pattern: AD

Disease ID:MONDO:0054802

Submitter: Genomics England PanelApp

Inheritance Pattern: AD

Disease ID:MONDO:0007705

Submitter: PanelApp Australia

Inheritance Pattern: AD

Disease ID:MONDO:0054802

Submitter: PanelApp Australia

Inheritance Pattern: AR

Disease ID:MONDO:0011399

Submitter: PanelApp Australia