Summary

We investigated the molecular mechanisms responsible for typeI congenital antithrombin (AT) deficiency in two unrelatedChinese pedigrees manifesting multiple site venous thrombosis.Phenotype analysis showed both probands had almost 50% ofnormal AT levels. Direct sequencing of amplified DNA revealed2757C>T in proband 1 and 13328G>A in proband 2, predictinga heterozygous Thr98Ile (T98I) and Ala404Thr (A404T), respectively.No proband had 20210A allele or factorV Leiden mutation.Transient expression of complementary DNA coding forthe mutations in COS-7 cells showed impaired secretion of themutant molecules.Real-time quantitative PCR indicated that themutant AT mRNA was transcribed at a similar or even higher level as that of wild-type (wt). Pulse-chase labeling studies suggestedboth AT variants did not accumulate, but degraded intracellularly.Immunohistochemical staining of the transfected cellsrevealed that CHO cells expressing the AT-I98 mutant werestained diffusely without perinuclear enhancement and cells expressingAT-T404 mutant mainly in the whole cytoplasm withweaker perinuclear enhancement. We conclude that the impairedsecretion of the mutant AT molecules, due to intracellulardegradation, is the molecular pathogenesis of AT deficiencycaused by T98I and A404T mutation for the two families,respectively.