A recent proposal by Eisman and coworkers in Australia has raised an important hypothesis regarding the genetic basis of osteoporosis. The Australian group has identified intronic polymorphisms within the gene encoding the vitamin D receptor (VDR) and described a strong correlation between variation and bone dihydroxyvitamin D3 (1,25D), in the low BMD subjects compared to the high BMD subjects and that this resistance was attributable to the allelic variation in the VDR gene. However, the basis for 1,25D resistance is unknown. At present, this hypothesis has not yet been confirmed by an independent group. Because of its potentially great importance, we have decided to submit a comprehensive proposal to evaluate the hypothesis and examine the possible underlying mechanisms by which VDR polymorphisms might affect BMD. Our grant proposal has three specific aims.
Aim I will employ three ongoing longitudinal epidemiological studies in which data on BMD and other markers of bone turnover have been accumulated. We will determine the genotype of these subjects and correlate existing data with allelic variation. The studies (P.I. indicated) are: i. The PEPI Trial (Marcus) where post menopausal Caucasian women have received estrogen and/or progesterone intervention. Six centers in addition to the Stanford center have agreed to collaborate in this effort. ii. Mexican-American Study (Kelsey) where elderly Hispanic women are followed longitudinally. iii. Adolescent Study (Bachrach) where young Caucasian, African-American, Asian and Hispanic subjects are followed longitudinally. Cumulatively, these studies will provide die patient material to examine the relationship of VDR genotype to peak bone mass, rate of bone acquisition and rate of bone loss in various ethnic groups.
Aim II will examine the responses of individuals of each genotype to a 1,25D challenge in studies conducted on the Aging Study Unit of the Palo Alto V.A. Hospital. The responses of the subjects to incremental doses of 1,25D will be measured and will include intestinal calcium absorption, serum 1,25D and osteocalcin levels, as well as blood and urine determinations of markers of bone turnover.
Aim III will investigate the mechanism of the polymorphism effect by obtaining cultured denial fibroblasts from skin biopsies of subjects with each genotype. These cultured cells will be studied; for their VDR abundance and responses to 1,25D in an attempt to unravel the basic cellular changes that lead to 1,25D resistance. In case the genotype hypothesis is disproven, we have provided a contingency plan for the continuation of our efforts. We postulate that, unrelated to genotype, there may be subtle exonic mutations in the VDR gene that cause mild 1,25D resistance and which eventually lead to increased risk of osteoporosis. This alternate hypothesis will be evaluated using the same methods and population studies as the genotype hypothesis except that subjects will be selected on the basis of 1,25D resistance and not genotype. In summary, our grant proposal represents a comprehensive plan including epidemiologic, physiologic, cellular and molecular elements. The goal is to validate the genotype hypothesis and to determine its physiological and molecular mechanism.
