Expression of PiM- and PiZ-mutated forms of the human α₁-antitrypsin gene in transfected monkey COS1 cells

The PiZ mutation of the gene coding for α₁-antitrypsin results in a serum deficiency of this protein leading to early onset emphysema and liver disease. The PiZ gene has a Z-specific point mutation in exon V together with a point mutation in exon III which is also present in some normal (PiM) individuals. There has thus far been no system to study the effects of PiZ point mutations in tissue culture. We constructed plasmids containing α₁-antitrypsin cDNA synthetically altered at either exon III or exon V mutation sites and linked to simian virus 40 promoter sequences. Such constructs with the exon V mutation were transfected into monkey COS1 cells followed by analysis of expression of α₁-antitrypsin gene products. COS1 cells normally synthesize virtually no α₁-antitrypsin mRNA or protein, α₁-antitrypsin mRNA is transcribed at high levels in cells transfected with either M or Z plasmids. Immunologic staining of COS1 cells within 48 h of transfection localizes α₁-antitrypsin protein to specific regions of the cytoplasm. This extranuclear localization is also observed with human HepG2 hepatoma cells, which synthesize α₁-antitrypsin at high levels, and with human SK-Hep1 hepatoma cells transfected with an M plasmid. The cloned synthetically altered α₁-antitrypsin genes provide a system for dissecting contributions of distinct point mutations to the pathological effects of the PiZ protein.