al-Antitrypsin Deficiency Caused by the al-Antitrypsin NUIlMattaWa Gene An Insertion Mutation Rendering the al-Antitrypsin Gene Incapable of Producing al-Antitrypsin

al-Antitrypsin (alAT) deficiency is a hereditary disorder associated with reduced serum alAT levels and the development of pulmonary emphysema. An alAT gene is defined as "Null" when no alAT in serum is attributed to that alAT gene. Although all alAT Null genes have identical phenotypic consequences (i.e. no detectable alAT in the serum), different genotypic mechanisms can cause the Null state. This study defines the molecular basis for the alAT gene Nullf,,,,, identified and cloned from genomic DNA of an individual with the Null-Null phenotype and emphysema resulting from the heterozygous inheritance of the Nullm.,w, and Nullb.Iuigm genes. Sequencing of exons Ic-V and all exon-intron junctions of the Null",,Vt. gene demonstrated it was identical to the common normal Ml(Va1213) alAT gene except for the insertion of a single nucleotide within the coding region of exon V, causing a 3 ' frameshift with generation of a premature stop signal. Family analysis using oligonucleotide probes specific for the Nullmt,. sequence demonstrated the gene was inherited in an autosomal fashion. Examination of blood monocytes demonstrated that a normal-sized, 1.8-kb alAT mRNA transcript is associated with the Nul1tf,1,. gene and in vitro translation of mRNA with the Null.,,,, mutation showed it translated at a normal rate but produced a truncated alAT protein. Additionally, retroviral transfer of the alAT Nulls wa cDNA to murine fibroblasts demonstrated no detectable intracellular or secreted alAT, despite the presence of alAT Null,,twa mRNA transcripts. These findings are consistent with the concept that the molecular pathophysiology of Nullf,,wa is likely manifested at a posttranslational level. The identification of the Null~t.,,, gene supports the concept that Null a1AT alleles represent a heterogenous group in which very different mechanisms cause the identical phenotypic state