The Site-Specific Influence of Gene-By-Diet Interactions on Trabecular Bone in Male Mice

Osteoporosis and fractures are debilitating skeletal problems. Accumulating the highest peak bone mass in both cortical and trabecular bone (Tb) as well as developing strong Tb microarchitecture play an integral role in preventing bone loss and osteoporotic fractures later in life. Because Tb is modulated by genetics (G) and environment (e.g. diet, D), my dissertation research focuses on the influence of dietary calcium (Ca) intake, genetics as well as GxD interaction controlling Tb phenotypes in two clinically relevant skeletal sites, i.e. the femur and the L5 vertebra. Male mice from 11 in bred lines and 51 BXD recombinant inbred (RI) lines were fed either adequate (Basal, 0.5%) or low (0.25%) Ca diets from 4-12 weeks of age. We used microcomputed tomography (CT) to measure Tb mass and microarchitecture phenotypes. We systematically proved that there are site-specific effects of diet, genetic, and GxD interactions influencing Tb phenotypes. This indicates that there are unique genetic effects modulating Tb at each bone site. Therefore, we conducted a genetic mapping experiment using the 51 BXD RI lines separately for each bone site. We coupled genetic mapping analysis with bioinformatics analysis to identify novel genetic variation and candidate genes accounting for the variation in each phenotypes. The findings from this work serve as a foundation for future research to identify novel pathways and genes underlying the development of Tb as well as an adaptation to Ca insufficiency

Diet X Gene Interactions Control Femoral Bone Adaptation to Low Dietary Calcium

Genetics and dietary calcium (Ca) are each critical regulators of peak bone mass but it is unclear how genetics alters the physiologic response of bone to dietary Ca restriction (RCR). Here, we conducted genetic mapping in C57BL/6J × DBA/2J (BXD) recombinant inbred mouse lines to identify environmentally sensitive loci controlling whole-bone mass (bone mineral density [BMD], bone mineral content [BMC]), distal trabecular bone, and cortical bone midshaft of the femur. Mice were fed adequate (basal) or low Ca diets from 4-12 weeks of age. Femurs were then examined by dual-energy X-ray absorptiometry (DXA) and micro-computed tomography (μCT). Body size-corrected residuals were used for statistical analysis, genetic mapping, and to estimate narrow sense heritability (h2). Genetics had a strong impact on femoral traits (eg, bone volume fraction [BV/TV] basal Ca, h2 = 0.60) as well as their RCR (eg, BV/TV, h2 = 0.32). Quantitative trait locus (QTL) mapping identified up to six loci affecting each bone trait. A subset of loci was detected in both diet groups, providing replication of environmentally robust genetic effects. Several loci control multiple bone phenotypes suggesting the existence of genetic pleiotropy. QTL controlling the bone RCR did not overlap with basal diet QTL, demonstrating genetic independence of those traits. Candidate genes underlying select multi-trait loci were prioritized by protein coding effects or gene expression differences in bone cells. These include candidate alleles in Rictor (chromosome [chr] 15) and Egfl7 (chr 2) at loci affecting bone in the basal or low Ca groups and in Msr1 (chr 8), Apc, and Camk4 (chr 18) at loci affecting RCR. By carefully controlling dietary Ca and measuring traits in age-matched mice we identified novel genetic loci determining bone mass/microarchitecture of the distal femur as well as their physiologic adaptation to inadequate dietary Ca intake