Molecular defect of truncated beta-spectrin associated with hereditary elliptocytosis. Beta-spectrin Gottingen

A large German family with autosomal dominantly inherited elliptocytosis, associated with truncated beta-spectrin missing the phosphorylated C-terminal peptide, has been described (Eber, S.W., Morris, S. A., Schroeter, W., and Gratzer, W. B. (1988) J. Clin. Invest. 81, 523-530). We have attempted to delineate the molecular defect of this abnormality at the gene level. Southern blot analyses revealed no evidence of a partial gene deletion or rearrangement. We used polymerase chain reaction (PCR) and amplified several relevant portions of the beta-spectrin gene, using genomic DNA from two different heterozygous patients. No abnormality was found in the last four exons of the beta-spectrin gene produced by PCR. Examination of the introns connecting the last four exons revealed a T to A substitution in the 5' splice site following the exon X in four of eight clones prepared from two affected individuals, but not from a normal subject of this family. To examine the effect of the T to A substitution in these patients, we made cDNA from the reticulocyte mRNA of the patient and examined its composition by PCR. Two distinct PCR fragments produced from the patient's beta-spectrin cDNA were found. One matched the predicted size for normal spectrin, while the other was 197 base pairs shorter. The mutant cDNA sequence revealed that the entire exon preceding the T to A substitution was spliced out, while the two terminal exons were preserved. The deletion of this exon resulted in a frameshift giving a different terminal amino acid (serine instead of leucine) as well as a new termination codon causing a deletion of 129 amino acids including potentially phosphorylated residues.