Congenic mice reveal sex-specific genetic regulation of femoral structure and strength.

Genetic linkage studies in C3H/HeJ (C3H) and C57BL/6J (B6) mice identified several chromosomal locations or quantitative trait loci (QTL) linked to femoral volumetric bone mineral density (vBMD). From QTL identified on chromosomes (chr) 1, 4, 6, 13, and 18, five congenic mouse strains were developed. In each of these mice, genomic DNA from the QTL region of the donor C3H strain was transferred into the recipient B6 strain. Here we report the effects of donated C3H QTL on femoral structure, cortical vBMD and bending strength. Femoral structure was quantified by the polar moment of inertia (Ip) at the mid-diaphysis, which reflects the bending or torsional rigidity of the femur. Although the C3H progenitor mice have a smaller Ip than B6 progenitor mice, the congenic mice carrying the C3H segment at Chr 4 had significantly increased Ip in both males and females, giving these mice stronger femora. In female mice from the congenic Chr 1 strain, Ip was increased whereas male mice from the Chr 1 strain had smaller femoral cross-sections and significantly reduced Ip. This sex-specific effect on femoral structure was seen to a lesser extent in Chr 18 congenic mice. In addition, cortical vBMD was measured using peripheral quantitative computed tomography. Cortical vBMD was similar among most congenic strains except in Chr 6 congenic mice, where cortical vBMD was significantly less in females, but not in males. We conclude that (1) chromosomal QTL from C3H mice, which are genetically linked to total femoral vBMD, also regulate femoral structure; (2) the QTL on Chr 4 improves femoral structure and strength; (3) QTL on Chr 1 and 18 impart sex-specific effects on femoral structure; and (4) the QTL on Chr 6 imparts a sex-specific effect on cortical vBMD and femoral strength