QTL analyses identified several chromosomal regions influencing skeletal phenotypes of the femur and tibia in BXD F 2 and BXD RI populations of mice. QTLs for skeletal traits co-located with each other and with correlated traits such as body weight and length, adipose mass, and serum alkaline phosphatase.Introduction: Past research has shown substantial genetic influence on bone quality, and the impact of reduced bone mass on our aging population has heightened the interest in skeletal genetic research. Materials and Methods: Quantitative trait loci (QTL) analyses were performed on morphologic measures and structural and material properties of the femur and tibia in 200-day-old C57BL/6J × DBA/2 (BXD) F 2 (second filial generation; n ‫ס‬ 400) and BXD recombinant inbred (RI; n ‫ס‬ 23 strains) populations of mice. Body weight, body length, adipose mass, and serum alkaline phosphatase were correlated phenotypes included in the analyses. Results: Skeletal QTLs for morphologic bone measures such as length, width, cortical thickness, and crosssectional area mapped to nearly every chromosome. QTLs for both structural properties (ultimate load, yield load, or stiffness) and material properties (stress and strain characteristics and elastic modulus) mapped to chromosomes 4, 6, 9, 12, 13, 15, and 18. QTLs that were specific to structural properties were identified on chromosomes 1, 2, 3, 7, 8, and 17, and QTLs that were specific to skeletal material properties were identified on chromosomes 5, 11, 16, and 19. QTLs for body size (body weight, body length, and adipose mass) often mapped to the same chromosomal regions as those identified for skeletal traits, suggesting that several QTLs identified as influencing bone could be mediated through body size. Conclusion: New QTLs, not previously reported in the literature, were identified for structural and material properties and morphological measures of the mouse femur and tibia. Body weight and length, adipose mass, and serum alkaline phosphatase were correlated phenotypes that mapped in close proximity of skeletal chromosomal loci. The more specific measures of bone quality included in this investigation enhance our understanding of the functional significance of previously identified QTLs.

The aim of this study was to compare three methods of adjusting skeletal data for body size and examine their use in QTL analyses. It was found that dividing skeletal phenotypes by body mass index induced erroneous QTL results. The preferred method of body size adjustment was multiple regression.Introduction: Many skeletal studies have reported strong correlations between phenotypes for muscle, bone, and body size, and these correlations add to the difficulty in identifying genetic influence on skeletal traits that are not mediated through overall body size. Quantitative trait loci (QTL) identified for skeletal phenotypes often map to the same chromosome regions as QTLs for body size. The actions of a QTL identified as influencing BMD could therefore be mediated through the generalized actions of growth on body size or muscle mass. Materials and Methods: Three methods of adjusting skeletal phenotypes to body size were performed on morphologic, structural, and compositional measurements of the femur and tibia in 200-day-old C57BL/6J × DBA/2 (BXD) second generation (F 2 ) mice (n ‫ס‬ 400). A common method of removing the size effect has been through the use of ratios. This technique and two alternative techniques using simple and multiple regression were performed on muscle and skeletal data before QTL analyses, and the differences in QTL results were examined. Results and Conclusions:The use of ratios to remove the size effect was shown to increase the size effect by inducing spurious correlations, thereby leading to inaccurate QTL results. Adjustments for body size using multiple regression eliminated these problems. Multiple regression should be used to remove the variance of co-factors related to skeletal phenotypes to allow for the study of genetic influence independent of correlated phenotypes. However, to better understand the genetic influence, adjusted and unadjusted skeletal QTL results should be compared. Additional insight can be gained by observing the difference in LOD score between the adjusted and nonadjusted phenotypes. Identifying QTLs that exert their effects on skeletal phenotypes through body size-related pathways as well as those having a more direct and independent influence on bone are equally important in deciphering the complex physiologic pathways responsible for the maintenance of bone health.

Several QTLs were coincident in males and females although the modest correlation between male and female median lifespans and the identification of sex specific QTLs provide evidence that the genetic architecture underlying longevity in the sexes may differ substantially. The identification of multiple QTLs for longevity will provide valuable resources for both reductionist and integrationist research into mechanisms of life span determination.

The aim of the study was to explore the genetic architecture influencing weight of fast- and slow-twitch skeletal muscles. The weights of the slow-twitch soleus, the mixed gastrocnemius, the fast-twitch tibialis anterior (TA), and extensor digitorum longus (EDL) muscles were 11-34% greater (P < 0.001) in 200-day-old C57BL/6J (B6) than in DBA/2J (D2) mice. Male muscles were 13-28% larger than female (P < 1 x 10(-5), no strain by sex interaction). The sex-related difference in muscle weight, however, varied significantly among the 23 derivative BXD recombinant inbred (RI) strains (strain by sex interaction for soleus, P < 0.01; TA, P < 1 x 10(-4); EDL, not significant; and gastrocnemius, P < 0.001). Quantitative trait loci (QTL) affecting muscle weight were mapped in an F2 intercross of B6 and D2 mice (B6D2F2) and BXD RIs. A total of 10 autosomal, muscle-specific, but not muscle-type-specific, QTL, explaining a total of 5.4, 7.7, 22.9, and 8.6% of phenotypic variance for soleus, TA, EDL, and gastrocnemius muscles, respectively, were found across chromosomes 1 (Chr 1), 2, 3 (female-specific), 5 (two), 6, 7, 8, and 9 in B6D2F2 mice. The QTL on Chr 8 for EDL and the female-specific QTL on Chr 3 for gastrocnemius muscles were statistically significant, but the remaining QTL were at the suggestive level of statistical significance. Ten QTL on Chr 1, 2, 4, 5, 7, 8, 14, 17 (two), and 19 were identified in BXD RIs. Half of the QTL in BXD RIs had pleiotropic effects and were at the suggestive level of significance (except for the significant QTL for gastrocnemius muscle on Chr 17). The B6D2F2 nominated QTL on Chr 8 for EDL weight was validated in BXD RIs (P < 0.03). Support intervals for the QTL on Chr 1 and 5 overlapped between B6D2F2 and BXD RIs. An epistatic interaction between markers on Chr 1 and 17 affected gastrocnemius weight in BXD RIs. The interaction was not, however, validated in the B6D2F2 population. Our results indicate that the differences in muscle weight in the B6 and D2 segregating populations were the outcome of a polygenic system, with each factor contributing a small amount to the phenotypic variance and the genetic architecture affecting muscle weight was muscle specific, but not muscle-type specific, and in some instances sex specific.

Genetic determinants of weight of fast-and slow-twitch skeletal muscle in 500-day-old mice of the C57BL/6J and DBA/2J lineage. Physiol Genomics 21: 184-192, 2005. First published February 1, 2005 doi;10.115210. / physiolgenomics.00209.2004 and DBA/2J (D2) strains and two derivative populations, BXD recombinant inbred strains (BXD RIs) and B6D2F 2, were used to explore genetic basis for variation in muscle weight at 500 days of age. In parallel with findings in 200-day-old mice (Lionikas A, Blizard DA, Vandenbergh DJ, Glover MG, Stout JT, Vogler GP, McClearn GE, and Larsson L. Physiol Genomics 16: 141-152, 2003), weight of slow-twitch soleus, mixed gastrocnemius, and fast-twitch tibialis anterior (TA) and extensor digitorum longus (EDL) muscles was 13-22% greater (P Ͻ 0.001) in B6 than in D2. Distribution of BXD RI strain means indicated that genetic influence on muscle weight (strain effect P Ͻ 0.001, all muscles) was of polygenic origin, and effect of genetic factors differed between males and females (strain-by-sex interaction: P Ͻ 0.01 for soleus, EDL; P Ͻ 0.05 for TA, gastrocnemius). Linkage analyses in B6D2F 2 population identified QTL affecting muscle weight on Chr 1, 2, 6, and 9. Pleiotropic influences were observed for QTL on Chr 1 (soleus, TA), 2 (TA, EDL, gastrocnemius), and 9 (soleus, TA, EDL) and were not related to muscle type (fast/slowtwitch) or function (flexor/extensor). Effect of QTL on Chr 9 on soleus muscle was male specific. QTL on Chr 2 and 6 were previously observed at 200 days of age, whereas QTL on Chr 1 and 9 are novel muscle weight QTL. In summary, muscle weight in B6/D2 lineage is affected by a polygenic system that has variable influences at different ages, between males and females, and across muscles in a manner independent of muscle type. muscle size; quantitative trait loci ASIDE FROM ITS primary power-generating function, skeletal muscle serves as a source of energy and amino acids during critical illness and starvation, contributes to thermoregulation, and also protects bones and viscera. Muscle size is of major importance for all of these functions. Skeletal muscle is characterized by an extraordinary capacity to adapt to different external influences. However, there is also a substantial genetic component determining its size. It has been estimated that in humans, heritability of muscle size ranged between 0.53 and 0.9 (9, 13, 17). The quest for specific genes causing alterations in muscle mass has identified several candidates. Myostatin has been recognized as a powerful suppressor of muscle mass growth in cattle and mouse (22, 23, 52) and recently in humans (41). Insulin-like growth factors-1 and -2 (IGF-1 and IGF-2) are also well-known modulators of muscle weight in rodents (1,10,34). Recent findings indicate that mutations of the intronic region of the IGF-2 gene induced a substantial increase in pig muscle size via imprinting mechanisms (25,49). A more complex influence on muscle weight, involving a single nucleotide polymorphism (SNP) in a long-range control element a...