HBB p.Ala28Ser

The c.82G>T (p.Ala28Ser) variant is located in the HBB gene. This variant has been reported in 12 unrelated individuals displaying a hematological phenotype consistent with beta thalassemia trait (reduced MCV and MCH with normal or increased RBC count), giving a total score of 2.15 [PS4_M; PMID: 7104238; 3955238; The Hemoglobinopathies Laboratory, Department of Human and Clinical Genetics, Leiden University Medical Center; Department of Medical Genetics, National and Kapodistrian University of Athens]. This variant has been reported to segregate with a beta-thalassemia intermedia phenotype in 8 affected family members from 4 families. The total number of unaffected segregations is 8 with a LOD score of 5.81 [PP1_S; PMID:7104238; 3955238; 25332589; 30777047; 17949282]. It was detected in combination with a pathogenic variant in one affected individual, confirmed in trans by DNA studies, and found in a homozygous state in another, both presenting with a beta-thalassemia intermedia phenotype. Total PM3 points are 1.5 [PM3; PMID:3955238; 25332589; Variation ID: 15438]. Hemoglobin biosynthesis assay showed a change in biosynthetic ratio of globins in a heterozygous sample (β/α ratio 0.45-0.55), indicating that this variant impacts protein function. In addition, in vitro splicing assay showed that this variant leads to abnormal RNA processing by creating/utilizing a cryptic splice site in exon 1 of HBB [PS3_P; PMID:6733281; 7104238; 7173395]. The computational predictor SpliceAI gives a Δ score of 0.79 for donor gain in exon 3, which is above the VCEP threshold >0.3, predicting that this variant may affect splicing by disrupting the donor splice site of intron 3 in HBB [PP3]. The minor allele frequency in gnomAD v4.1 is 0.000002479 (4/1613406) alleles, which is lower than the ClinGen Hemoglobinopathy VCEP threshold <0.0001 for PM2_Supporting, and therefore meets this criterion [PM2_P]. In summary, this variant meets the criteria to be classified as pathogenic for beta thalassemia (MONDO:0013517) in an autosomal recessive manner based on the ACMG/AMP criteria applied, as specified by the ClinGen Hemoglobinopathy VCEP (specification version 1.0.0): PP1_S, PS4_M, PM3, PM2_P, PP3, PS3_P.