VITAMIN-D-RECEPTOR GENE POLYMORPHISM IS ASSOCIATED WITH URINARY CALCIUM EXCRETION BUT NOT WITH BONE-MINERAL DENSITY IN POSTMENOPAUSAL WOMEN

Polymorphism of vitamin D receptor (VDR) gene has been found to be ass ociated with serum osteocalcin (OC) levels and bone mineral density (B MD) in Caucasian identical twins and unrelated postmenopausal women. B eing ethnically different and living in a geographic area with adequat e vitamin D status due to abundant sunshine exposure, it is unclear wh ether VDR gene polymorphism will affect bone mass in Thai population. In the present study, we investigated the association between VDR gene polymorphism and bone metabolism in Thai postmenopausal women. Subjec ts consisted of 84 postmenopausal women, Bsm I, Taq I and Apa I polymo rphisms of VDR gene were determined by PCR-RFLP. B, T and A represent the absence of the corresponding restriction sites while b, t and a in dicate the presence of the restriction sites. Data were expressed as m ean +/- SE. Sixty-six subjects (78.6%) had bb genotype while 18 (21.4% ) had Bb genotype. None of the subjects was found to have BB genotype. Taq I restriction site was in linkage disequilibrium to the Bsm I sit e. For Apa I polymorphism, 33 (39.3%), 42 (50.0%) and 9 (10.7%) of the subjects had aa, Aa and AA genotypes, respectively, There was no sign ificant difference in serum intact OC levels and BMD at various skelet al sites among subjects with different genotypes. Despite the lack of difference in BMD and intact OC levels, subjects with bb genotype had higher 24-hour urinary calcium excretion than those with Bb genotype ( bb, 6.1+/-0.3 mmol/day; Bb, 4.4+/-0.6 mmol/day; p<0.05). The effect of Bsm I VDR genotype was still significant (p<0.05) after dietary calci um intake was controlled using analysis of covariance. Despite the dif ference in urinary calcium levels, there was no significant difference in fractional excretion of calcium among subjects with different Bsm I-related genotypes, suggesting that the effect of the VDR gene polymo rphism on urinary calcium excretion is more likely due to the effect o n intestinal calcium absorption rather than renal tubular calcium reab sorption. We conclude that VDR genotype distributions in Thai postmeno pausal women are different from those reported in Caucasians. VDR gene polymorphism does not appear to be associated with BMD or bone turnov er in Thai postmenopausal women. However, Bsm I VDR polymorphism may h ave physiologic role in calcium homeostatasis by modulating intestinal calcium absorption. (C) 1997, Editrice Kurtis.