Heterogeneous stock rat: a unique animal model for mapping genes influencing bone fragility

Abstract

Previously, we demonstrated that skeletal mass, structure and biomechanical properties vary considerably among 11 different inbred rat strains. Subsequently, we performed quantitative trait loci (QTL) analysis in four inbred rat strains (F344, LEW, COP and DA) for different bone phenotypes and identified several candidate genes influencing various bone traits. The standard approach to narrowing QTL intervals down to a few candidate genes typically employs the generation of congenic lines, which is time consuming and often not successful. A potential alternative approach is to use a highly genetically informative animal model resource capable of delivering very high resolution gene mapping such as Heterogeneous stock (HS) rat. HS rat was derived from eight inbred progenitors: ACI/N, BN/SsN, BUF/N, F344/N, M520/N, MR/N, WKY/N and WN/N. The genetic recombination pattern generated across 50 generations in these rats has been shown to deliver ultra-high even gene-level resolution for complex genetic studies. The purpose of this study is to investigate the usefulness of the HS rat model for fine mapping and identification of genes underlying bone fragility phenotypes. We compared bone geometry, density and strength phenotypes at multiple skeletal sites in HS rats with those obtained from five of the eight progenitor inbred strains. In addition, we estimated the heritability for different bone phenotypes in these rats and employed principal component analysis to explore relationships among bone phenotypes in the HS rats. Our study demonstrates that significant variability exists for different skeletal phenotypes in HS rats compared with their inbred progenitors. In addition, we estimated high heritability for several bone phenotypes and biologically interpretable factors explaining significant overall variability, suggesting that the HS rat model could be a unique genetic resource for rapid and efficient discovery of the genetic determinants of bone fragility.

Fig. 1

Schematic diagram of development of heterogeneous stock (HS) rats from 8 inbred strains of rats: Agouti (ACI/N), Brown Norway (BN/SsN), Buffalo (BUF/N), Fischer 344 (F344/N), M520/N, Maudsley Reactive (MR/N), Wistar-Kyoto (WKY/N) and Wistar-Nettleship (WN/N).

Fig. 2

Body weight for female inbred progenitors (ACI, BN, BUF, F344 and WKY) and HS strains of rats. Data presented are mean ± SD (n=6-7 for inbred rats and n=348 for HS rats). *p<0.05 HS vs BN and F344; BUF vs ACI; #p<0.005 HS vs ACI

Fig. 3

Femur length (A), head width (B), neck width (C) and axis length (D) for female inbred progenitors (ACI, BN, BUF, F344 and WKY) and HS strains of rats. Data presented are mean ± SD (n=6-7 for inbred rats and n=348 for HS rats). For femur length *p<0.05 BN vs ACI; F344 vs WKY; **p<0.005 ACI vs F344; BUF vs BN; HS vs WKY; #p<0.0001 BUF vs F344; HS vs BN and F344; head width *p<0.05 HS vs WKY; neck width *p<0.005 BUF vs BN, F344 and WKY; #p<0.0001 HS vs all others; axis length *p<0.005 HS vs all others except BN; #p<0.0001 HS vs BN

Fig. 4

Femur aBMD (A), femur BMC (B), lumbar aBMD (C) and lumbar BMC (D) for female inbred progenitors (ACI, BN, BUF, F344 and WKY) and HS strains of rats. Data presented are mean ± SD (n=6-7 for inbred rats and n=348 for HS rats). For femur aBMD *p<0.005 F344 vs ACI; BN vs WKY; #p<0.0001 HS vs all others; BUF vs all others; femur BMC *p<0.05 BN vs F344 and HS; F344 vs WKY; **p<0.005 ACI vs F344; HS vs WKY; #p<0.0001 BUF vs all others; HS vs BUF and F344; lumbar aBMD **p<0.005 ACI vs F344; #p<0.0001 BUF vs all others; HS vs all others; lumbar BMC *p<0.05 HS vs WKY; **p<0.005 F344 vs WKY; #p<0.0001 BUF vs all others; HS vs all others except WKY

Fig. 5

Femur UF (A), femur W (B), femur neck UF (C) and femur neck W (D) for female inbred progenitors (ACI, BN, BUF, F344 and WKY) and HS strains of rats. Data presented are mean ± SD (n=6-7 for inbred rats and n=348 for HS rats). For femur UF *p<0.05 BN vs ACI and F344; HS vs BUF; **p<0.005 BUF vs ACI and F344; femur W *p<0.05 HS vs BN and WKY; **p<0.005 F344 vs ACI and HS; BUF vs HS; #p<0.0001 F344 vs BN, BUF and WKY; femur neck UF *p<0.05 F344 vs ACI, BN and BUF; **p<0.005 BUF vs WKY and HS; HS vs ACI and BN; #p<0.0001 BUF vs ACI and BN; femur neck W *p<0.05 HS vs ACI; F344 vs BN; **p<0.005 ACI vs F344

Fig. 6

Geometry (A), bone mass (B) and bone strength (C) parameters for male and female HS strains of rats. Data presented are mean ± SD (n=319 for male and n=348 for female HS rats) *p<0.0001 between male and female