Identification and prioritization of NUAK1 and PPP1CC as positional candidate loci for skeletal muscle strength phenotypes.

Muscle strength is an important determinant in elite sports performance as well as in the activities of daily living. Muscle metabolism also plays a role in the genesis, and therefore prevention, of common pathological conditions and chronic diseases. Even though heritability estimates between 31 and 78% suggest a significant genetic component in muscle strength, only a limited number of genes influencing muscle strength have been identified. This study aimed to identify and prioritize positional candidate genes within a skeletal muscle strength quantitative trait locus on chromosome 12q22-23 for follow-up. A two-staged gene-centered fine-mapping approach using 122 single nucleotide polymorphisms (SNPs) in stage 1 identified a family-based association (n=500) between several tagSNPs located in the ATPase, Ca2+ transporting, cardiac muscle, slow twitch 2 (ATP2A2; rs3026468), the NUAK family, SNF1-like kinase, 1 (NUAK1; rs10861553 and rs3741886), and the protein phosphatase 1, catalytic subunit, gamma isoform (PPP1CC; rs1050587 and rs7901769) genes and knee torque production (P values up to 0.00092). In stage 2, family-based association tests on additional putatively functional SNPs (e.g., exonic SNPs, SNPs in transcription factor binding sites or in conserved regions) in an enlarged sample (n=536; 464 individuals overlap with stage 1) did not identify additional associations with muscle strength characteristics. Further in-depth analyses will be necessary to elucidate the exact role of ATP2A2, PPP1CC, and NUAK1 in muscle strength and to find out which functional polymorphisms are at the base of the interindividual strength differences.

Genome-wide linkage scan for contraction velocity characteristics of knee musculature in the Leuven Genes for Muscular Strength Study.

The torque-velocity relationship is known to be affected by ageing, decreasing its protective role in the prevention of falls. Interindividual variability in this torque-velocity relationship is partly determined by genetic factors (h(2): 44-67%). As a first attempt, this genome-wide linkage study aimed to identify chromosomal regions linked to the torque-velocity relationship of the knee flexors and extensors. A selection of 283 informative male siblings (17-36 yr), belonging to 105 families, was used to conduct a genome-wide SNP-based (Illumina Linkage IVb panel) multipoint linkage analysis for the torque-velocity relationship of the knee flexors and extensors. The strongest evidence for linkage was found at 15q23 for the torque-velocity slope of the knee extensors (TVSE). Other interesting linkage regions with LOD scores >2 were found at 7p12.3 [logarithm of the odds ratio (LOD) = 2.03, P = 0.0011] for the torque-velocity ratio of the knee flexors (TVRF), at 2q14.3 (LOD = 2.25, P = 0.0006) for TVSE, and at 4p14 and 18q23 for the torque-velocity ratio of the knee extensors TVRE (LOD = 2.23 and 2.08; P = 0.0007 and 0.001, respectively). We conclude that many small contributing genes are involved in causing variation in the torque-velocity relationship of the knee flexor and extensor muscles. Several earlier reported candidate genes for muscle strength and muscle mass and new candidates are harbored within or in close vicinity of the linkage regions reported in the present study.

Polymorphisms in the CNTF and CNTF receptor genes are associated with muscle strength in men and women.

Genotypic associations between polymorphisms in the ciliary neurotrophic factor (CNTF) and CNTF receptor (CNTFR) genes and muscular strength phenotypes in 154 middle-aged men (45-49 yr) and 138 women (38-44 yr) and 99 older men (60-78 yr) and 102 older women (60-80 yr) were tested to validate earlier association studies. Allelic interaction effects were hypothesized between alleles of CNTF and CNTFR. We performed analysis of covariance with age, height, and fat-free mass (FFM) as covariates. FFM was anthropometrically estimated by the equation of Durnin-Womersley. Isometric, concentric, and eccentric torques for the knee flexors (KF) and extensors (KE) were measured using Biodex dynamometry. In the older male group, T-allele carriers of the C-1703T polymorphism in CNTFR performed significantly better on all noncorrected KF torques, whereas only noncorrected KE isometric torque at 120 degrees and concentric torque at 240 degrees/s were higher than the C/C homozygotes (P < 0.05). When age, height, and FFM were used as covariates, T-allele carriers performed only better on KE and KF isometric torque at 120 degrees (P < 0.05). Concentric KF torque at 180 degrees/s was lower in middle-aged female A-allele carriers compared with the T/T subjects for the T1069A polymorphism in CNTFR. After correction for age, height, and FFM, middle-aged female A-allele carriers exhibited lower values on all concentric KF strength measures and isometric torque at 120 degrees . There was a lack of association with the CNTF G-6A polymorphism in men, with inconclusive results for a limited number of phenotypes in women. No significant CNTF/CNTFR allele interaction effects were found. Results indicate that CNTFR C-1703T and T1069A polymorphisms are significantly associated with muscle strength in humans.

Selection of genes and single nucleotide polymorphisms for fine mapping starting from a broad linkage region.

Fine mapping of linkage peaks is one of the great challenges facing researchers who try to identify genes and genetic variants responsible for the variation in a certain trait or complex disease. Once the trait is linked to a certain chromosomal region, most studies use a candidate gene approach followed by a selection of polymorphisms within these genes, either based on their possibility to be functional, or based on the linkage disequilibrium between adjacent markers. For both candidate gene selection and SNP selection, several approaches have been described, and different software tools are available. However, mastering all these information sources and choosing between the different approaches can be difficult and time-consuming. Therefore, this article lists several of these in silico procedures, and the authors describe an empirical two-step fine mapping approach, in which candidate genes are prioritized using a bioinformatics approach (ENDEAVOUR), and the top genes are chosen for further SNP selection with a linkage disequilibrium based method (Tagger). The authors present the different actions that were applied within this approach on two previously identified linkage regions for muscle strength. This resulted in the selection of 331 polymorphisms located in 112 different candidate genes out of an initial set of 23,300 SNPs.

Covariance of isometric and dynamic arm contractions: multivariate genetic analysis.

The purpose of the present study was to examine genetic and environmental contributions to individual differences in maximal isometric, concentric and eccentric muscle strength and muscle cross-sectional area (MCSA) of the elbow flexors. A generality versus specificity hypothesis was explored to test whether the 4 strength variables share a genetic component or common factors in the environment or whether the genetic/environmental factors are specific for each strength variable. The 4 variables under study were measured in 25 monozygotic and 16 dizygotic male Caucasian twin pairs (22.4 +/- 3.7 years). The multivariate genetic analyses showed that all 4 variables shared a genetic and environmental component, which accounted for 43% and 6% in MCSA (h2 = 81%), 47% and 20% in eccentric (h2 = 65%), 58% and 4% in isometric (h2 = 70%) and 32% and 1% in concentric strength (h2 = 32%) respectively. The remaining variation was accounted for by contraction type specific and muscle cross-sectional area specific genetic and environmental effects, which accounted for 38% and 14% in MCSA, 18% and 15% in eccentric, 12% and 26% in isometric and 0% and 67% in concentric strength respectively. This exploratory multivariate study suggests shared pleiotropic gene action for MCSA, eccentric, isometric and concentric strength, with a moderate to high genetic contribution to the variability of these characteristics.

Comprehensive fine mapping of chr12q12-14 and follow-up replication identify activin receptor 1B (ACVR1B) as a muscle strength gene.

Muscle strength is important in functional activities of daily living and the prevention of common pathologies. We describe the two-staged fine mapping of a previously identified linkage peak for knee strength on chr12q12-14. First, 209 tagSNPs in/around 74 prioritized genes were genotyped in 500 Caucasian brothers from the Leuven Genes for Muscular Strength study (LGfMS). Combined linkage and family-based association analyses identified activin receptor 1B (ACVR1B) and inhibin ß C (INHBC), part of the transforming growth factor ß pathway regulating myostatin - a negative regulator of muscle mass - signaling, for follow-up. Second, 33 SNPs, selected in these genes based on their likelihood to functionally affect gene expression/function, were genotyped in an extended sample of 536 LGfMS siblings. Strong associations between ACVR1B genotypes and knee muscle strength (P-values up to 0.00002) were present. Of particular interest was the association with rs2854464, located in a putative miR-24-binding site, as miR-24 was implicated in the inhibition of skeletal muscle differentiation. Rs2854464 AA individuals were ∼2% stronger than G-allele carriers. The strength increasing effect of the A-allele was also observed in an independent replication sample (n=266) selected from the Baltimore Longitudinal Study of Aging and a Flemish Policy Research Centre Sport, Physical Activity and Health study. However, no genotype-related difference in ACVR1B mRNA expression in quadriceps muscle was observed. In conclusion, we applied a two-stage fine mapping approach, and are the first to identify and partially replicate genetic variants in the ACVR1B gene that account for genetic variation in human muscle strength.

Protective role of alpha-actinin-3 in the response to an acute eccentric exercise bout.

The ACTN3 gene encodes for the alpha-actinin-3 protein, which has an important structural function in the Z line of the sarcomere in fast muscle fibers. A premature stop codon (R577X) polymorphism in the ACTN3 gene causes a complete loss of the protein in XX homozygotes. This study investigates a possible role for the alpha-actinin-3 protein in protecting the fast fiber from eccentric damage and studies repair mechanisms after a single eccentric exercise bout. Nineteen healthy young men (10 XX, 9 RR) performed 4 series of 20 maximal eccentric knee extensions with both legs. Blood (creatine kinase; CK) and muscle biopsy samples were taken to study differential expression of several anabolic (MyoD1, myogenin, MRF4, Myf5, IGF-1), catabolic (myostatin, MAFbx, and MURF-1), and contraction-induced muscle damage marker genes [cysteine- and glycine-rich protein 3 (CSRP3), CARP, HSP70, and IL-6] as well as a calcineurin signaling pathway marker (RCAN1). Baseline mRNA content of CSRP3 and MyoD1 was 49 + or - 12 and 67 + or - 25% higher in the XX compared with the RR group (P = 0.01-0.045). However, satellite cell number was not different between XX and RR individuals. After eccentric exercise, XX individuals tended to have higher serum CK activity (P = 0.10) and had higher pain scores than RR individuals. However, CSRP3 (P = 0.058) and MyoD1 (P = 0.08) mRNA expression tended to be higher after training in RR individuals compared with XX alpha-actinin-3-deficient subjects. This study suggests a protective role of alpha-actinin-3 protein in muscle damage after eccentric training and an improved stress-sensor signaling, although effects are small.