GENETIC DIVERSITY FROM A LIMITED REPERTOIRE OF MUTATIONS ON DIFFERENTCOMMON ALLELIC BACKGROUNDS - ALPHA(1)-ANTITRYPSIN DEFICIENCY VARIANT-P(DUARTE)

Alpha1-Antitrypsin (alpha1AT) is one of the most polymorphic gene loci in the human genome. Alpha1AT variants are typically identified by th eir migration position in an isoelectric focusing gel at pH 4-5. Heter ogeneity of the isoelectric point of alpha1AT variants, hence variant migration, most often results from amino acid substitutions which alte r the net charge of the molecule. We identified an individual heterozy gous for an alpha1AT variant migrating in the ''P'' variant region whi ch differs from other known ''P'' variants. Using isoelectric focusing on an immobilized pH gradient at pH 4.50-4.85 the novel P allele, P(d uarte), migrates between P(st. albans) and P(lowell). Densitometric an alysis of normal ''M'' type alpha1AT and the deficiency variant P(lowe ll) major bands separated by isoelectric focusing demonstrates that P( duarte) con, tributes approximately 41% as much alpha1AT to the total serum alpha1AT concentration as the normal ''M'' alpha1AT, similar to P(lowell). Direct DNA sequencing of the proband's genomic DNA demonstr ates that the P(duarte) allele differs from the normal M1(V213) allele by two amino acid substitutions, R101 (CGT underbar)--> H(CAT underba r) and D256 (GAT underbar)-V (GTT underbar). Individually, these amino acid substitutions characterize the normal M4 allele (R101--> H) and the deficient P(lowell) allele (D256--> V). Thus the P(duarte) allele differs from the P(lowell) allele only by the normal allelic backgroun d in which the V256 mutation occurs. Comparison of amino acid sequence s among several alpha1AT variants demonstrates that Pd(duarte) is an e xample of a more general observation regarding diversity within the PI (protease inhibitor) system. It is apparent that the heterogeneity ob served among alpha1AT variants is due in part to combinations of a lim ited repertoire of amino acid substitutions. These combinations may be the product of mutational hot spots and/or recombination on normal al lelic backgrounds. (C) 1993 Wiley-Liss, Inc.(dagger)