Identification of Candidate Gene Regions in the Rat by Co-Localization of QTLs for Bone Density, Size, Structure and Strength

Sofia Lagerholm,Fiona Mcguigan,Kristina Åkesson,Holger Luthman,Marc Grynpas,Hee-Bok Park,Maria Swanberg

doi:10.1371/journal.pone.0022462

Abstract

Susceptibility to osteoporotic fracture is influenced by genetic factors that can be dissected by whole-genome linkage analysis in experimental animal crosses. The aim of this study was to characterize quantitative trait loci (QTLs) for biomechanical and two-dimensional dual-energy X-ray absorptiometry (DXA) phenotypes in reciprocal F2 crosses between diabetic GK and normo-glycemic F344 rat strains and to identify possible co-localization with previously reported QTLs for bone size and structure. The biomechanical measurements of rat tibia included ultimate force, stiffness and work to failure while DXA was used to characterize tibial area, bone mineral content (BMC) and areal bone mineral density (aBMD). F2 progeny (108 males, 98 females) were genotyped with 192 genome-wide markers followed by sex- and reciprocal cross-separated whole-genome QTL analyses. Significant QTLs were identified on chromosome 8 (tibial area; logarithm of odds (LOD) = 4.7 and BMC; LOD = 4.1) in males and on chromosome 1 (stiffness; LOD = 5.5) in females. No QTLs showed significant sex-specific interactions. In contrast, significant cross-specific interactions were identified on chromosome 2 (aBMD; LOD = 4.7) and chromosome 6 (BMC; LOD = 4.8) for males carrying F344mtDNA, and on chromosome 15 (ultimate force; LOD = 3.9) for males carrying GKmtDNA, confirming the effect of reciprocal cross on osteoporosis-related phenotypes. By combining identified QTLs for biomechanical-, size- and qualitative phenotypes (pQCT and 3D CT) from the same population, overlapping regions were detected on chromosomes 1, 3, 4, 6, 8 and 10. These are strong candidate regions in the search for genetic risk factors for osteoporosis.

Highlights

Osteoporosis is a common multifactorial disorder characterized by reduced bone mineral density (BMD) and compromised bone quality, with micro-architectural deterioration leading to an increased susceptibility to fracture [1]
This QTL met the criteria for genome-wide cross-specificity at the suggestive level (DLODcross$2.4), and the likelihood ratio test confirmed a cross-specific interaction for this locus (LR = 10.4, p = 0.005) (Fig. 1B)
We identified multiple QTLs for biomechanical and two-dimensional dual-energy X-ray absorptiometry (DXA) phenotypes in F2 progeny of GK and F344 rats and confirmed bone QTL interactions with reciprocal cross

Summary

Introduction

Osteoporosis is a common multifactorial disorder characterized by reduced bone mineral density (BMD) and compromised bone quality, with micro-architectural deterioration leading to an increased susceptibility to fracture [1]. Identification of genes and pathways regulating these phenotypes and thereby underlying bone strength could provide valuable insight regarding susceptibility to osteoporosis and fracture risk. Translational genetics starting with experimental models is a valuable tool to dissect complex genetic traits. By using inbred strains and crosses between strains, genetic heterogeneity is dramatically reduced and experimental tests, such as destructive testing of bone, can be performed. One example of successful translational genetics is the identification of the arachidonate lipoxygenase 15 (Alox15) gene encoding an enzyme that modifies polyunsaturated fatty acids, as a candidate for osteoporosis. Mice deficient in Alox were found to have increased bone mass [5] and subsequent association analyses in humans have shown association with BMD for SNPs in the human orthologues ALOX12 and ALOX15 [6,7]

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