Characterization of a human alpha 1-antitrypsin null allele involving aberrant mRNA splicing.

alpha 1-Antitrypsin (alpha 1AT) is a major protease inhibitor present in high concentrations in the plasma. Inheritance of alpha 1AT deficiency or null alleles (alleles associated with no detectable serum alpha 1AT) is associated with an increased risk for emphysema. In contrast to beta zero-thalassemia variants in which RNA splicing and promoter mutations constitute more than 40% of beta zero-thalassemia variants, all nine alpha 1AT null variants identified are the result of mutations involving the protein coding region of the alpha 1AT gene. During routine screening of individuals applying for enrollment in the USA alpha 1AT Deficiency Registry we identified an individual with emphysema and a Protease Inhibitor (PI*) type heterozygous for a novel alpha 1AT null allele. Direct DNA sequencing of this individual's alpha 1AT alleles demonstrated one normal and one novel allele, designated PI*QOwest, characterized by a single G-->T base substitution at position 1 of intron II, a highly conserved nucleotide position in vertebrate splice donor sites. Metabolic labeling of NIH-3T3 cells transfected with a plasmid vector containing an alpha 1AT minigene with the QOwest mutation demonstrated an absence of detectable immunoprecipitable alpha 1AT confirming that the G-->T mutation is responsible for the observed null phenotype. QOwest alpha 1AT minigene transfected cells expressed 25-100 fold less alpha 1AT mRNA than a normal control. DNA sequencing of polymerase chain reaction amplified mRNA obtained from transfected cells demonstrated the use of a cryptic splice site 84 bases upstream from the normal splice site.(ABSTRACT TRUNCATED AT 250 WORDS)