Abstract
To identify genes affecting bone strength, we studied how genetic variants regulate components of a phenotypic covariation network that was previously shown to accurately characterize the compensatory trait interactions involved in functional adaptation during growth. Quantitative trait loci (QTLs) regulating femoral robustness, morphologic compensation, and mineralization (tissue quality) were mapped at three ages during growth using AXB/BXA Recombinant Inbred (RI) mouse strains and adult B6-iA Chromosome Substitution Strains (CSS). QTLs for robustness were identified on chromosomes 8, 12, 18, and 19 and confirmed at all three ages, indicating that genetic variants established robustness postnatally without further modification. A QTL for morphologic compensation, which was measured as the relationship between cortical area and body weight, was identified on chromosome 8. This QTL limited the amount of bone formed during growth and thus acted as a setpoint for diaphyseal bone mass. Additional QTLs were identified from the CSS analysis. QTLs for robustness and morphologic compensation regulated bone structure independently (ie, in a nonpleiotropic manner), indicating that each trait may be targeted separately to individualize treatments aiming to improve strength. Multiple regression analyses showed that variation in morphologic compensation and tissue quality, not bone size, determined femoral strength relative to body weight. Thus an individual inheriting slender bones will not necessarily inherit weak bones unless the individual also inherits a gene that impairs compensation. This systems genetic analysis showed that genetically determined phenotype covariation networks control bone strength, suggesting that incorporating functional adaptation into genetic analyses will advance our understanding of the genetic basis of bone strength. © 2010 American Society for Bone and Mineral Research.
Highlights
A major challenge with studying the genetic basis of fracture susceptibility is that bone strength and clinically useful surrogate measures of strength such as bone mineral density (BMD) depend on multiple genetically defined structural and tissue-quality traits that change over time
The functional adaptation process that establishes bone stiffness and strength during growth involved coordinated changes among morphologic and compositional traits.[10]. We identified a pattern to this coordination that was consistent with engineering principles and that may be useful in genetic analyses
To identify genetic variants affecting bone strength, we studied how genetic variants regulate specific aspects of the phenotypic covariation network that defines mechanical function.[7]. We tested whether the growth processes specifying robustness and those specifying mechanical compensation are regulated by the same genes
Summary
A major challenge with studying the genetic basis of fracture susceptibility is that bone strength and clinically useful surrogate measures of strength such as bone mineral density (BMD) depend on multiple genetically defined structural and tissue-quality traits that change over time. This complexity is problematic for genetic analyses because it means that individuals within a population are at risk of fracture for different genetic reasons at different times and after different life histories. Functional adaptation determines adult bone strength and is important to study for understanding fracture susceptibility later in life
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