Serum α1-antitrypsin deficiency associated with the common S-type (Glu²⁶⁴ → Val) mutation results from intracellular degradation of α1-antitrypsin prior to secretion

The S-type α1-antitrypsin (α1AT) deficiency allele differs from the normal M1(Val²¹³) allele by a single amino acid substitution (Glu²⁶⁴ → Val). To evaluate the molecular pathophysiology responsible for the reduced serum levels of α1AT associated with the S-type allele, α1AT gene expression was examined in blood monocytes, cells which normally produce α1AT, as well as murine fibroblasts modified by retroviral gene transfer to express the S-type and normal M-type human α1AT geens. Northern analysis and S1 protection analysis demonstrated that monocytes of M and S homozygotes both express 1.8-kilobase α1AT mRNA transcripts in comparable levels and similar in structure. Pulse-chase labeling studies demonstrated that both M and S monocytes synthesized and secreted a 52-kDa protein, but the S monocytes secreted significantly less. The cellular lysates of both M and S monocytes contained a newly synthesized 50-kDa precursor form of α1AT, but the S monocytes contained reduced amounts. Pulse-chase labeling in the presence of tunicamycin, an inhibitor of core oligosaccharide addition, demonstrated that S monocytes exhibited a selective inhibition of secretion of 45-kDa nonglycosylated α1AT not observed in M monocytes. Consistent with these observations, murine fibroblasts modified by retroviral gene transfer to contain as integrated human S-type α1AT cDNA demonstrated reduced secretion of α1AT compared with fibroblasts containing an integrated human M-type α1AT cDNA and also reproduced the abnormality of α1AT biosynthesis observed with S-type monocytes. Furthermore, in the presence of leupeptin, an inhibitor of cellular proteinases, the S-type modified fibroblasts demonstrated a selective augmentation of human α1AT secretion not observed for the M-type. Together, these observations are consistent with the concept that the single A → T mutation of the S-type α1AT gene results in reduced cellular secretion of α1AT because the newly synthesized S-type α1AT protein is degraded intracellularly prior to secretion.