Calcium sensing receptor gene polymorphism, circulating calcium concentrations and bone mineral density in healthy adolescent girls

Sports Medicine, Department of Surgical and Perioperative Sciences, Umea University, Sweden. Mattias.Lorentzon@idrott.umu.se

OBJECTIVE: Bone mineral density (BMD) in adolescence is under strong genetic control. The calcium sensing receptor (CASR) is involved in the regulation of calcium homeostasis and bone resorption. The A986S polymorphism of the CASR has recently been associated with serum calcium levels, in one hitherto unconfirmed report. We investigated whether this polymorphism was related to BMD, circulating calcium and parathyroid hormone (PTH) concentrations in girls. DESIGN: BMD, plasma calcium and serum PTH were measured in adolescent girls and compared with regard to CASR genotype. METHODS: In 97 healthy Caucasian girls (mean age 16.9+/-1.2 years (mean+/-s.d.)), the A and S alleles were determined using PCR with a mismatched primer and the restriction enzyme BsaHI. BMD (g/cm) of the total body, humerus, femoral neck and lumbar spine was measured using dual energy X-ray absorptiometry. RESULTS: The genotype frequencies were 71% AA, 26% AS and 3% SS. The genotypes were divided into presence (29%) or absence of S allele (71%). Subjects with the S allele had higher levels of plasma calcium, corrected for albumin (2.17+/-0.06>2.14+/-0.06; P < 0.05, using independent samples t-test), lower BMD at the lumbar spine (P=0.02) and total body (P=0.04), and were significantly less physically active (2.9+/-2.6 vs 4.3+/-2.6 h/week; P=0.01) than the subjects lacking the S allele. PTH levels were not significantly different between the two allelic groups. A multiple regression analysis, including age, height, weight and physical activity, revealed that the CASR allelic variants were not independent predictors of BMD at any site measured (beta=-0.03-0.09; P>0.05). Physical activity was an independent predictor of BMD, was significantly different between the CASR genotypes, and could therefore have a role in explaining the difference in BMD between the CASR genotypes. CONCLUSIONS: The CASR alleles are related to BMD, but it cannot be definitely concluded whether the CASR polymorphism has a direct influence on BMD, or whether the differences in BMD were mediated via an influence of the amount of physical activity.