18 The D19H variant confers OR of 2–3 and 7 for heterozygous R788 order and homozygous carriers, respectively, and therefore 8–11% of the total gallstone risk can be attributed to this variant.36 The molecular modeling of the ABCG8 D19H
variant polymorphism, which leads to the substitution of a positively charged amino acid (aspartic acid) with a negatively charged amino acid (histidine), showed that the overall RMS deviation was negligible between the wild-type and polymorphic ABCG8 protein structures, and the D19H SNP was predicted by PolyPhen to be benign. It appears that the effect of this SNP is mainly the result of the change in charge instead of the three dimensional structure of the protein. It is well established Pritelivir in vivo that ABCG8 plays an important role in the secretion of cholesterol into intestinal lumen in enterocytes and the secretion of cholesterol into bile in hepatocytes.17,37 The protein encoded by ABCG8 functions along with ABCG5 as a heterodimer.12,38 An aspartic acid at amino acid 19 in ABCG8 is highly conserved from plants to vertebrates, and its substitution to histidine results in the loss of negative charge. The variant H allele of the ABCG8 D19H polymorphism is suggested to increase the expression or function of the ABCG8 transporter, resulting in a positive
correlation with high biliary cholesterol secretion and the accumulation in the gallbladder, which serves as an initial step in gallstone formation.17,39,40 Mice overexpressing Carbohydrate human Abcg8 show reduced intestinal cholesterol absorption and hepatic cholesterol synthesis and biliary cholesterol secretion, thus leading to the supersaturation
of bile with cholesterol.41,42 Biliary cholesterol level has also been directly related to gene copy number of the transgene ABCG8, and vice versa, in Abcg8 knockout mice.43,44 Establishing the role of the ABCG8 19H variant might be clinically relevant, because it would be possible to predict if HMG-CoA reductase inhibitors could be particularly effective in lowering biliary cholesterol levels in patients carrying the ABCG8 risk allele. The importance of our study lies in the fact that there is no data available exploring the role of the ABCG8 D19H variant in the high-risk population of northern India. To the best of our knowledge, this is the first report implicating the role of the ABCG8 gene polymorphism with gallstone risk in the north Indian population. The present study suggests that ABCG8 19H plays a significant role in gallstone susceptibility, which might result in imbalance in bile component due to the increased expression of ABCG8. Furthermore, similar studies from other populations should be promoted to identify the genetic factors that define populations at risk and they might lead to the establishment of new preventive and therapeutic strategies. The study was supported by research and fellowship grants from the Indian Council of Medical Research New Delhi.