Correcting the PiZ defect in the α₁-antitrypsin gene of human cells by targeted homologous recombination

BACKGROUND: Approximately 1% of people in North America carry the PiZ defect in the gene encoding the serum protein α₁-antitrypsin (AAT). Homozygotes are subject to early onset emphysema and liver disease. Here we demonstrate a means of correcting this gene defect, a point mutation in chromosome 14, by targeted homologous recombination. EXPERIMENTAL DESIGN: To target recombination to the AAT locus in human cells we use a normal complementary DNA cleaved once approximately 400 base pairs upstream of the PiZ mutation site in exon V. After transfection, targeting is monitored using a polymerase chain reaction assay devised to uniquely amplify successful homologous recombinants. RESULTS: When exercised on human PiZZ GM2522 fibroblasts, targeting replaces exon V of the endogenous gene, containing the PiZ mutation, with the exon V counterpart of a normal complementary DNA, simultaneously displacing the mutated exon V away from its promoter. By reversing the targeting strategy and using human PiM hepatoma cells, we have been able to supplant a normal copy of the AAT gene with a mutated PiZ version. This has allowed us to culture human HepG2 cells expressing the pathologic phenotype of the PiZ defect. Immunocytologic staining demonstrates the presence of inclusions in the liver cells due to aggregation of the PiZ isoform encoded by the recombined AAT gene under control of the endogenous gene promoter. CONCLUSIONS: (a) Targeting to the AAT locus is observed, using the methods we have developed, at rates that render feasible the selection of targeted cells and isolation of clones bearing a corrected AAT gene. (b) Correction of the exon V PiZ mutation alone, regardless of an accompanying exon III mutation, is sufficient to prevent aggregation of the PiZ protein in human liver cells. (c) Targeted homologous recombination offers advantages in potential gene therapy to correct AAT defects.