Genetic diversity from a limited repertoire of mutations on different common allelic backgrounds: alpha 1-antitrypsin deficiency variant Pduarte.

alpha 1-Antitrypsin (alpha 1AT) is one of the most polymorphic gene loci in the human genome. alpha 1AT variants are typically identified by their migration position in an isoelectric focusing gel at pH 4-5. Heterogeneity of the isoelectric point of alpha 1AT variants, hence variant migration, most often results from amino acid substitutions which alter the net charge of the molecule. We identified an individual heterozygous for an alpha 1AT variant migrating in the "P" variant region which differs from other known "P" variants. Using isoelectric focusing on an immobilized pH gradient at pH 4.50-4.85 the novel P allele, Pduarte, migrates between Pst. albans and Plowell. Densitometric analysis of normal "M" type alpha 1AT and the deficiency variant Plowell major bands separated by isoelectric focusing demonstrates that Pduarte contributes approximately 41% as much alpha 1AT to the total serum alpha 1AT concentration as the normal "M" alpha 1AT, similar to Plowell. Direct DNA sequencing of the proband's genomic DNA demonstrates that the Pduarte allele differs from the normal M1 (V213) allele by two amino acid substitutions, R101 (CGT)-->H(CAT) and D256 (GAT)-->V(GTT). Individually, these amino acid substitutions characterize the normal M4 allele (R101-->H) and the deficient Plowell allele (D256-->V). Thus the Pduarte allele differs from the Plowell allele only by the normal allelic background in which the V256 mutation occurs. Comparison of amino acid sequences among several alpha 1AT variants demonstrates that Pduarte is an example of a more general observation regarding diversity within the PI (protease inhibitor) system.(ABSTRACT TRUNCATED AT 250 WORDS)