Submission Details

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).