HBB

Inheritance Pattern: AD

Disease ID:MONDO:0011381

Submitter: ClinGen

*HBB* was first reported in relation to autosomal dominant (AD) beta thalassemia in 1973 by Weatheral et al. (PMID: 4351905). Patients with beta thalassemia are grouped into three categories based upon clinical severity. Patients with beta thalassemia major, the most severe form, present early in life with severe anemia requiring blood transfusions and if untreated often have phenotypes such as growth retardation, hepatosplenomegaly, and jaundice. Patients with beta thalassemia intermedia can present later in life with moderate anemia, in some patients requiring blood transfusions, while thalassemia minor patients range on a spectrum from mild anemia to asymptomatic. In the dominant form of the disorder over 44 unique variants have been reported in humans (PMID:23637309), the majority being frameshift, in frame insertion or deletion, or missense, but nonsense variants have also been reported. Many of the frameshift variants are found in the final exon and lead to elongated protein products. The mechanism of autosomal dominant beta thalassemia is dominant negative and is due to variants causing unstable beta globin, which lead to the buildup of beta globin chains in addition to alpha globin chains, resulting in ineffective erythropoiesis. Additionally, unlike recessive forms of the disease, which have been selected for in areas endemic to Plasmodium falciparum malaria, autosomal dominant beta thalassemia is not thought to confer selective advantage due in part to the affected status of heterozygous individuals. Evidence supporting this gene-disease relationship includes case-level data, segregation data, and experimental data. Eleven unique variants from 11 probands in 11 publications were curated (PMIDs:15977037, 24789613, 2070092, 1971109, 11300352, 1958498, 34323379, 31084366, 16628732, 16189162, 2001456). More evidence is available in the literature, but the maximum score for genetic evidence has been reached. Experimentally, this gene-disease relationship is supported by the biochemical function of beta globin (PMID:8978308) and GTEx bulk tissue expression showing *HBB* is overwhelmingly expressed in the blood (PMID:23715323). In summary *HBB* is definitively associated with autosomal dominant beta thalassemia. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time. This classification was approved by the ClinGen General Gene Curation Expert Panel on the meeting date 3/22/2023 (SOP Version 9).

Inheritance Pattern: AD

Disease ID:MONDO:0020459

Submitter: ClinGen

*HBB* was first reported in relation to autosomal dominant unstable hemoglobin disease in 1962 (Frick et al. 1962, PMID: 13895148). 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 *HBB*. 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, AD unstable hemoglobin disease, and AR Sickle cell disease. This curation addresses *HBB* in relation to AD unstable hemoglobin disease. At least 14 unique missense variants that have been reported in 15 probands in 14 publications (PMIDs: 13895148, 6771953, 6877904, 3781864, 8330973, 9730372, 11920235, 29573049, 25347256, 20730880, 30032685, 32029504, 33613322, 37457725) are included in this curation. Eight were reported as de novo occurrences. This gene disease association is also supported by experimental evidence including expression pattern of HBB across human tissues which is highly specific within whole blood (PMID: 23715323), biochemical studies indicating the well-established function of *HBB* in oxygen binding (PMID: 11747442), and its protein-protein interaction with the alpha hemoglobin chain encoded by *HBA1* and *HBA2* genes (PMID: 6644819), which harbors variants associated with unstable hemoglobin disease. Additionally, an ENU-induced mouse model replicated the human phenotype (PMIDs: 28304246). In summary, there is definitive evidence supporting the relationship between *HBB* and autosomal dominant unstable hemoglobin disease. This has been repeatedly demonstrated in both the research and clinical diagnostic settings and has been upheld over time. This classification was approved by the ClinGen General Gene Curation Expert Panel on 2/28/2024 (SOP Version 10).

Inheritance Pattern: AR

Disease ID:MONDO:0016487

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016486

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016243

Submitter: Orphanet

Inheritance Pattern: AD

Disease ID:MONDO:0018749

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0019402

Submitter: Myriad Women's Health

Inheritance Pattern: AR

Disease ID:MONDO:0016668

Submitter: Orphanet

Inheritance Pattern: AD

Disease ID:MONDO:0011381

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016491

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016490

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016489

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016672

Submitter: Orphanet

Inheritance Pattern: AD

Disease ID:MONDO:0007705

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AR

Disease ID:MONDO:0013517

Submitter: ClinGen

Beta-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. The inherited basis was first traced to partial or complete deletions in the *HBB* locus encoding the hemoglobin beta chain in 1975 (Ottolenghi et al., PMID: 49057), followed by the identification of specific HBB variants in two subsequent studies (PMID: 88735, PMID: 287080). Please note that earlier publications had previously identified the molecular basis of beta-thalassemia at the protein level through sequencing of the HBB gene product (PMID: 5863839, PMID: 6025643, PMID: 6038175). Individuals with biallelic *HBB* disruption are generally diagnosed with beta-thalassemia major or beta-thalassemia intermedia, depending on severity. Genotype is strongly correlated with phenotype, with higher severity associated with absence of detectable beta chains (beta-zero-thalassemia), and milder cases associated with the persistence of some detectable beta globin (beta-plus-thalassemia). Less severe forms of disease can also have milder anemia, due to moderately increased HbA2 hemoglobin and persistence of hemoglobin F, and reduced or no dependency on transfusions. The clinical and laboratory features of severely affected individuals include erythrocyte abnormalities such as hypochromic microcytic anemia, anisopoikilocytosis, elliptocytosis, decreased mean corpuscular volume, basophilic stippling, and dependence on blood transfusions with many disease phenotypes secondary to extramedullary hematopoiesis, e.g. splenomegaly and chronic transfusions resulting in iron overload. Characteristic abnormalities in hemoglobin itself include lack of beta-hemoglobin production, reduced hemoglobin A, increased HbA2 hemoglobin, and persistence of hemoglobin F later into childhood. Heterozygous carriers of causal variants are referred to as having beta-thalassemia minor or beta-thalassemia trait and can be mildly or sub-clinically affected. Per criteria outlined by the ClinGen Lumping & Splitting Working Group, the molecular mechanism has been found to be consistent between cases harboring biallelic and monoallelic loss-of-function variants in *HBB*. However, the dramatic difference in phenotypic severity and penetrance between the biallelic and monoallelic cases distinguishes them as two different conditions rather than a single spectrum of disease. Therefore, cases of beta-thalassemia major or beta-thalassemia intermedia caused by biallelic *HBB* variants have been separately curated here for beta-thalassemia HBB/LCRB (MONDO:0013517). Cases of beta-thalassemia caused by dominant *HBB* variants in the heterozygous state have been found to cause disease through a different mechanism, and have been evaluated for dominant beta thalassemia (MONDO:0011381, MIM #603902) in a separate curation. Ten suspected disease-causing variants have been scored as part of this curation (one missense, two nonsense, three frameshift, two disrupting splicing, one partial gene deletion, and one disrupting the first nucleotide of the 5’ UTR). These have been collectively reported in nine probands in four publications (PMID: 88735, PMID: 1581247, PMID: 287080, PMID: 3683554, PMID: 18196920, PMID: 12752111, PMID: 27263053). The mechanism of pathogenicity appears to be biallelic loss of *HBB* function conferred by null and/or hypomorphic variants. Six out of nine probands were homozygous for their respective variants, while three probands were compound heterozygous. It has been established that the phenotype can also result from variants that disrupt a critical enhancer upstream of the *HBB* coding region (PMID: 1689188, PMID: 93487), although some of these variants disrupt the *HBD* locus as well. Evidence that the phenotype co-segregates with the homozygous genotype in a large family also contributed to the scoring of the gene-disease relationship (PMID: 18196920). A large amount of additional case-level evidence is available in the literature but was not 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 *HBB* across human tissues is highly restricted to the blood (PMID: 23715323). Extensive biochemical studies indicate that *HBB* encodes the beta 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). HBB protein interacts with the alpha hemoglobin chain encoded by the *HBA1* and *HBA2* genes (PMID: 6644819), which can harbor variants associated with the closely related condition alpha-thalassemia. Mouse models of *Hbb* loss of function recapitulate features of human beta-thalassemia patients such as hypochromic microcytic anemia, reduced hemoglobin A, abnormal erythrocyte morphology, splenomegaly, reticulocytosis, Heinz bodies, anisopoikilocytosis, presence of target cells, bone marrow hypercellularity, and abnormal iron deposition (PMID: 7568113, PMID: 9490703). CRISPR/Cas9-based editing of the mouse mutation in one case resulted in significant rescue of anemia, low hemoglobin concentration, anisocytosis and poikilocytosis, bone marrow proliferation, iron accumulation, and splenomegaly (PMID: 34706494). In summary, *HBB* is definitively associated with autosomal recessive inheritance of beta-thalassemia. 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 April 26th, 2023 (SOP Version 9).

Inheritance Pattern: AR

Disease ID:MONDO:0017146

Submitter: ClinGen

The relationship between *HBB* and sickle cell disease and related disorders (SCD), inherited in the autosomal recessive pattern, has been evaluated using the ClinGen Clinical Validity Framework as of August, 2023. This association was made using case-level and experimental data. *HBB* encodes the hemoglobin beta subunit, which forms the functional hemoglobin protein complex upon interaction with the alpha subunit. Sickle cell disease is characterized by hemolytic anemia vaso-occlusive disease, painful crises, which lead to multi-organ damage including acute chest syndrome, functional splenomegaly, increased susceptibility to infections. A majority of SCD is caused by a single variant in the HBB gene, p.Glu7Val, often referred to as Glu6Val when numbered within the mature processed protein. The p.Glu7Val variant leads to exposure of a hydrophobic patch that causes the abnormal hemoglobin to polymerize and thus initiate the pathological process associated with SCD. Homozygosity for p.Glu7Val results in the classic, severe SCD. SCD also results from compound heterozygosity for Glu7Val and an additional variant in *HBB* in trans. Severity varies, but is influenced by the effect of the additional variant on the polymerization ability of p.Glu7Val. Individuals heterozygous for the p.Glu7Val variant have the sickle cell trait but do not show clinical symptoms related to SCD. Rare cases of SCD have been reported in the literature with a proposed autosomal dominant inheritance pattern with the p.Glu7Val variant in cis with another variant in *HBB*; these cases have not been included in this curation. The first clinical report of sickle cell disease dates to 1910 by Herrick (PMID: 11501714). The amino acid change in *HBB* resulting in SCD, p.Glu7Val, was first identified in 1957 using tryptic digests by Ingram (PMID: 13464827). The nucleotide change was identified from the cDNA sequence in 1977 and reported by Marotta et al (PMID: 68958). Summary of Case Level Data (12 points): The association is seen in many individuals with 7 probands in 7 publications curated for this assessment (PMID: 21329186, 18473247, 8112743, 8438884, 24814631, 17038017, 37113902). The p.Glu7Val variant and other common pathogenic variants are seen at a high frequency in population databases due to asymptomatic carriers being present in sequenced cohorts. The p.Glu7Val variant is reported in gnomAD v2.1.1 at the highest MAF of 0.0449 (4.5%; 1121/24964 African/African American alleles). According to the CDC data and statistics, the prevalence of sickle cell disease in the African/African American population is 0.27% and the carrier frequency is 7.7%. The allele is maintained at a high level in the African populations because, in the heterozygous state, it affords some protection against the malarial parasite, which replicates in red blood cells (PMID: 24396149). More case-level evidence is available in the literature, but the maximum score for genetic evidence (12 pts) has been reached. Five different haplotypes based on the presence or absence of certain restriction sites have been described and thought to represent independent origins of the p.Glu7Val variant. Haplotype analysis and statistical modeling in PMID: 29526279 inferred a single origin of the p.Glu7Val allele approximately 7300 years ago, in the Green Sahara or west-central Africa, during the Holocene Wet phase. Malaria preexisted in this environment that favored selection of the allele, and this predated the population split in Africa represented by the different haplotypes. The mechanism for disease is gain of function. Summary of Experimental Data (6 points): The p.Glu7Val variant in homozygosity or compound heterozygosity in *HBB* results in an abnormal hemoglobin protein, HbS. The normal function of Hb is to transport oxygen from the lung to various tissues. HbS has low oxygen affinity and in deoxygenated state polymerizes within erythrocytes. Polymerization leads to rigidity and sickling of erythrocytes. This also causes altered erythrocyte membrane properties, leading to cellular dehydration, abnormal adhesion to PBMCs and platelets and the endothelium, resulting in vaso-occlusion, hemolysis, anemia, and organ damage (PMID: 21131035). *HBB* is expressed exclusively in blood (PMID: 23715323). The beta hemoglobin encoded by *HBB* interacts with the alpha subunit encoded by *HBA1* and *HBA2* (PMID: 14634627, 27085422). Several mouse models that are generated from introducing the human SCD variant, p.Glu7Val, are reported in the literature. Mice solely expressing the human alpha and beta (with the p.Glu7Val variant) chains, with the murine hemoglobin chains knocked out, recapitulate the human phenotype. They show irreversibly sickled cells in-vitro and in-vivo as well as extensive organ damage (PMIDs: 9346488, 9346487, 2296310). Rescue of the SCD phenotype has been accomplished in patient iPSCs, mouse models and human patients via clinical trials, wherein the introduction of wild-type human adult hemoglobin results in correction of the disease (PMIDs: 21881051, 16638928, 21881051). In summary, the *HBB*-sickle cell disease gene-disease relationship is definitive. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time. This classification was approved by the ClinGen General GCEP on August 23, 2023 (SOP Version 9).

Inheritance Pattern: AR

Disease ID:MONDO:0011382

Submitter: Orphanet

Inheritance Pattern: AD

Disease ID:MONDO:0018023

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016242

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0013517

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AR

Disease ID:MONDO:0011382

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AD

Disease ID:MONDO:0011381

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AD

Disease ID:MONDO:0054801

Submitter: ClinGen

*HBB* was first reported in relation to autosomal dominant erythrocytosis 6 in 1968 (Stamatoyannopoulos et al., PMID: 17795074). Erythrocytosis 6 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 6 relative to other disease entities associated with *HBB*. Therefore, the following disease entities have been split into multiple curations; AD erythrocytosis, AD and AR beta-thalassemia, hemoglobin M disease, and unstable hemoglobin disease. This curation addresses only AD erythrocytosis. This curation includes at least 43 missense, 2 nonsense, and 1 indel variants reported in 46 probands with AD erythrocytosis in 25 publications (PMIDs: 17795074, 2363414, 2272838, 1814856, 12908805, 14649314, 15727901, 16282896, 16987804, 18818920, 21077764, 21250878, 23859443, 23859762, 24482100, 24200101, 27312559, 27651169, 27250824, 28332377, 31657650, 31132167, 32151172, 34167411, 35052472). This gene disease association is also supported by experimental evidence including expression, biochemical function, protein-protein interactions, functional alteration of patient cells, and animal models (PMIDs: 11747442, 6644819, 3997804, 22203977). In summary, *HBB* is definitively associated with autosomal dominant erythrocytosis 6. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time. This classification was approved by the ClinGen General Gene Curation Expert Panel on 9/27/2023 (SOP Version 10).

Inheritance Pattern: AD

Disease ID:MONDO:0018023

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 reported in *HBB* was HbM-Hyde Park, which was identified as substitution of the proximal Histidine for Tyrosine in the late 1960s (PMID: 5700302) and genetically confirmed in 1992 (PMID: 1398295) in relation to autosomal dominant Hemoglobin M disease. The majority of the variants occur in the E or F-helices, most commonly involving conserved proximal or distal histidine residues. 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. Six missense variants that have been reported in 28 probands in 21 publications (PMIDs: 1398295, 6311728, 7663000, 7713749, 8226097, 8294206, 9494043, 15929117, 19199228, 19727720, 19758826, 24744675, 25079170, 28096792, 31267164, 31431070, 33251782, 34092029, 34789072, 34789072, 30828177) 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 oxygen binding, as part of the hemoglobin structure (PMID: 11747442). In Hemoglobin M disease, the function is altered due to differences in the heme structure (PMID: 15222763) and in patient cells alterations in the light absorption pattern explain the formation of chocolate-brown blood (PMID: 1398295). In summary, there is definitive evidence supporting the relationship between *HBB* and autosomal dominant Hemoglobin M disease. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time. The classification was approved by the ClinGen General GCEP on the meeting date July, 26, 2023 (SOP version 9). Of note, *HBB* is also related to beta-thalassemia, sickle cell, erythrocytosis, and unstable hemoglobin 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: AD

Disease ID:MONDO:0020989

Submitter: PanelApp Australia

Inheritance Pattern: AD

Disease ID:MONDO:0018023

Submitter: PanelApp Australia

Inheritance Pattern: AD

Disease ID:MONDO:0054801

Submitter: PanelApp Australia

Inheritance Pattern: AD

Disease ID:MONDO:0007705

Submitter: PanelApp Australia

Inheritance Pattern: AD

Disease ID:MONDO:0016489

Submitter: PanelApp Australia

Inheritance Pattern: AR

Disease ID:MONDO:0016669

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016670

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0016671

Submitter: Orphanet

Inheritance Pattern: AD

Disease ID:MONDO:0011381

Submitter: PanelApp Australia

Inheritance Pattern: AD

Disease ID:MONDO:0054801

Submitter: Genomics England PanelApp

Inheritance Pattern: AR

Disease ID:MONDO:0013517

Submitter: PanelApp Australia

Inheritance Pattern: AR

Disease ID:MONDO:0011382

Submitter: PanelApp Australia

Inheritance Pattern: AR

Disease ID:MONDO:0017145

Submitter: Natera

Inheritance Pattern: AD

Disease ID:MONDO:0011381

Submitter: Natera

Inheritance Pattern: AR

Disease ID:MONDO:0011382

Submitter: Natera

Inheritance Pattern: AD

Disease ID:MONDO:0054801

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AD

Disease ID:MONDO:0018023

Submitter: Ambry Genetics

Inheritance Pattern: SD

Disease ID:MONDO:0013517

Submitter: Ambry Genetics