Molecular characterization of a new alpha-1-antitrypsin M variant allele, M-whitstable: Implications for DNA-based diagnosis

The mother and second child from a family, already with one PI ZZ child, we re typed PI MZ by isoelectric focusing and unexpectedly as PI ZZ using a co mmercial alpha-1-antitrypsin genotyping kit. Both methods typed the father and first child as PI MZ and PI ZZ, respectively. DNA sequence analysis ide ntified a 26-base pair (bp) deletion and 2-bp insertion in intron IV of the normal PI*M allele from both the mother and second child. The majority of the binding site for an amplification primer of the genotyping kit was abse nt in the variant deletion-insertion allele. The apparent PI*Z/PI*Z genotyp e of the mother and second child therefore arose from amplification of the PI*Z allele alone. Two hundred random DNA samples were subsequently examine d and 5 of these were found to be heterozygous for the same deletion-insert ion allele. The authors have designated the previously undescribed PI*M all ele that harbors this benign polymorphism PI*M-whitstable. The genotyping k it has been redesigned and revalidated, and its performance is not affected by the presence of the PI*M-whitstable allele. The Gen Bank accession numb er for the nucleotide sequence described is AF159454.