Characterization of the coding sequence of the normal M4 ∝1-antitrypsin gene
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The nucleotide sequences of the common normal "M" family of alpha 1-antitrypsin (alpha 1AT) variants are known, including M1(Val213), M1(Ala213), M2 and M3. Less common, but also migrating with the "M" family on isoelectric focusing gels, is the normal M4 allele. Being relatively rare, the M4 allele is usually found in heterozygous combination with another alpha 1AT allele making sequence characterization more difficult. To facilitate analysis of the coding exons of the alpha 1AT M4 allele, a method was developed to combine blood monocyte RNA extraction, reverse transcription of the alpha 1AT mRNA, amplification with the polymerase chain reaction and direct sequencing. This analysis demonstrated that the M4 allele differs from the M1(Val213) allele by a single nucleotide substitution G--greater than A, causing the amino acid substitution Arg101 CGT--greater than His101 CAT. This same mutation is also a part of the M2 gene suggesting that this region of the alpha 1AT gene may be one of increased mutational activity.