FUNCTIONAL-ANALYSIS OF AN UNUSUAL LENGTH POLYMORPHISM IN THE HUMAN ANTITHROMBIN-III (AT3) GENE PROMOTER

The prevalence of the alternative alleles of an unusual length polymor phism in the promoter of the human antithrombin III (AT3) gene was det ermined in a sample of 155 unrelated individuals from the Northern Iri sh population. The 108bp L allele and the 32bp S allele occurred at fr equencies of 0.21 and 0.79 respectively. Some homology was noted betwe en the L-specific sequence and the region immediately downstream. Resi dual homology was also evident between the L and S sequences, suggesti ng that the S allele was derived from the L allele during evolution by partial deletion followed by sequence divergence. The functional sign ificance of the polymorphism was investigated by transient transfectio n of AT3 promoter/luciferase reporter gene constructs into two human h epatoma cell, lines in vitro. The promoter strength of the L allele wa s found to be 1.6-fold higher than the S allele in HepG2 cells whereas in Hep3B cells, the strength of the S allele was 1.7-fold higher than that of the L allele. In order to evaluate the phenotypic consequence s of the AT3 promoter polymorphism in vivo, plasma samples from the 15 5 control individuals were assayed for antithrombin III (ATIII) activi ty. Mean activities of the different promoter polymorphism genotypes ( SS, LL, SL) were not significantly different. These results suggest th at the AT3 promoter polymorphism does not contribute to the variation in plasma ATIII activity that occurs in the general population.