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, ␣-spectrin Bughill or ␣ 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 ␣II domain peptides at 46 kD and 35 kD. A point mutation at codon 970 of the ␣-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 ␣ 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 ␣ BH variant appeared to be homozygous for the ␣ 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 ␣ BH allele and a second, presumably abnormal, ␣-spectrin gene. These results suggest that, in these 6 patients, the second ␣-spectrin allele is in fact associated with one or more genetic defect(s), causing decreased accumulation of ␣-spectrin. The pattern of transmission of the ␣ BH allele in certain families suggests that the ␣ 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 ␣-spectrin gene defect that itself is a cause of ndHS. Am.

Coinheritance of two α-spectrin gene defects in a recessive spherocytosis family

“…Further conclusions were drawn on examining the proband and his mother, both of whom being heterozygous for the Sp‐α BH allele. Findings obtained from this family, together with those reported by Tse et al . (1997) and Dhermy (2000), have identified a constitutive alternative acceptor splice site in intron 30 of the α‐Sp gene which is 70 nucleotides upstream from the major regular splice site.…”

“…The relative amount of the alternative isoform was low in the mother whereas it was significantly higher in the proband and his father. To complete the characterization of the α LEPRA allele, we searched for the presence of the Sp variant α‐Bughill ( Tse et al . 1997 ) in cis to the α LEPRA allele.…”

“…The α BH allele has so far always been found in cis to the α LEPRA allele ( Wichterle et al . 1996 ; Tse et al . 1997 ; Dhermy 2000) and, as shown on Figure 2C, this paradigm was confirmed in family H since the GCT→GAT mutation in codon 970 (which creates a Fok I restriction site) was found in the father and the affected children.…”

“…In agreement with previous reports on αIIa peptide expression ( Jarolim et al . 1996 ; Tse et al . 1997 ; and Dhermy 2000), in family H, the α‐Sp allele inherited in trans to the α LEPRA allele was highly defective.…”

“…The αIIa polymorphism was found to be related to a point mutation in the Sp α‐chain (A970D). Based on this molecular characterization, this Sp variant was designated as α‐Sp Bughill (α BH ) ( Tse et al . 1997 ), which is a polymorphism with a frequency of 7.6% in the normal population ( Boivin et al .…”