Amino-acid substitution in α-spectrin commonly coinherited with nondominant hereditary spherocytosis

Abstract

Nondominant hereditary spherocytosis (ndHS) is a disorder characterized in some patients by severe hemolytic anemia and marked deficiency of erythrocyte spectrin. This report describes the identification of a variant spectrin chain, α-spectrin Bughill or α(BH), that is associated with this disorder in a number of patients. Tryptic maps of spectrin from affected individuals revealed an acidic shift in isoelectric point of the αII domain peptides at 46 kD and 35 kD. A point mutation at codon 970 of the α-spectrin gene (GCT→GAT), that changes the encoded amino acid from an alanine to an aspartic acid, was identified in genomic DNA of affected patients. The α(BH) variant was present in 8 patients with ndHS from five different kindreds but was absent in 4 patients from two other kindreds. The 8 ndHS patients with the α(BH) variant appeared to be homozygous for the α(BH) variant by analysis of peptide maps of limited tryptic digests of erythrocyte spectrin. However, following genomic DNA analysis, only 2 of these patients were true homozygotes, whereas 6 were found to be doubly heterozygous for the α(BH) allele and a second, presumably abnormal, α-spectrin gene. These results suggest that, in these 6 patients, the second α-spectrin allele is in fact associated with one or more genetic defect(s), causing decreased accumulation of α-spectrin. The pattern of transmission of the α(BH) allele in certain families suggests that the α(BH) amino-acid substitution is not itself responsible for ndHS but is more likely a polymorphic variant that, in some but not all cases, is in linkage disequilibrium with another uncharacterized α-spectrin gene defect that itself is a cause of ndHS.