Rates of change of genetic parameters of body weight in selected mouse lines.

Summary A method based on the animal model is described which allows the estimation of continuous changes in variance components over time using restricted maximum likelihood (REML). The method was applied to the analysis of a selection experiment in which a foundation population formed from a cross between two inbred strains of mice (C57BL/6J and DBA/2J) was divergently selected for 6 week body weight over 20 generations. The analysis suggested that there was an increase in phenotypic variance of about 50 % in the low selected lines over the course of the experiment which was attributed to increases in the environmental and additive variance components. Variance changes in the High selected lines were generally smaller than in the Low lines, although there was an estimated 20 % increase in the environmental variance. Simple models to explain these effects involving dominance, linkage and epistasis were explored. Testing which of these was responsible for the variance changes noted in this experiment (if any) is difficult, although the epistasis and dominance models require less stringent conditions than the linkage model, and the dominance model is supported by evidence of heterosis in the F t

Clinical and genetic characterisation of dystrophin-deficient muscular dystrophy in a family of Miniature Poodle dogs

Four full-sibling intact male Miniature Poodles were evaluated at 4–19 months of age. One was clinically normal and three were affected. All affected dogs were reluctant to exercise and had generalised muscle atrophy, a stiff gait and a markedly elevated serum creatine kinase activity. Two affected dogs also showed poor development, learning difficulties and episodes of abnormal behaviour. In these two dogs, investigations into forebrain structural and metabolic diseases were unremarkable; electromyography demonstrated fibrillation potentials and complex repetitive discharges in the infraspinatus, supraspinatus and epaxial muscles. Histopathological, immunohistochemical and immunoblotting analyses of muscle biopsies were consistent with dystrophin-deficient muscular dystrophy. DNA samples were obtained from all four full-sibling male Poodles, a healthy female littermate and the dam, which was clinically normal. Whole genome sequencing of one affected dog revealed a >5 Mb deletion on the X chromosome, encompassing the entire DMD gene. The exact deletion breakpoints could not be experimentally ascertained, but we confirmed that this region was deleted in all affected males, but not in the unaffected dogs. Quantitative polymerase chain reaction confirmed all three affected males were hemizygous for the mutant X chromosome, while the wildtype chromosome was observed in the unaffected male littermate. The female littermate and the dam were both heterozygous for the mutant chromosome. Forty-four Miniature Poodles from the general population were screened for the mutation and were homozygous for the wildtype chromosome. The finding represents a naturally-occurring mutation causing dystrophin-deficient muscular dystrophy in the dog

Pax7 Is Necessary and Sufficient for the Myogenic Specification of CD45(+):Sca1(+) Stem Cells from Injured Muscle

CD45(+):Sca1(+) adult stem cells isolated from uninjured muscle do not display any myogenic potential, whereas those isolated from regenerating muscle give rise to myoblasts expressing the paired-box transcription factor Pax7 and the bHLH factors Myf5 and MyoD. By contrast, CD45(+):Sca1(+) isolated from injured Pax7 ( −/−) muscle were incapable of forming myoblasts. Infection of CD45(+):Sca1(+) cells from uninjured muscle with retrovirus expressing Pax7 efficiently activated the myogenic program. The resulting myoblasts expressed Myf5 and MyoD and differentiated into myotubes that expressed myogenin and myosin heavy chain. Infection of CD45(−):Sca1(−) cells from Pax7 ( −/−) muscle similarly gave rise to myoblasts. Notably, infection of Pax7-deficient muscle with adenoviral Pax7 resulted in the de novo formation of regenerated myofibers. Taken together, these results indicate that Pax7 is necessary and sufficient to induce the myogenic specification of CD45(+) stem cells resident in adult skeletal muscle. Moreover, these experiments suggest that viral transduction of Pax7 is a potential therapeutic approach for the treatment of neuromuscular degenerative diseases

Deletion of Murine SMN Exon 7 Directed to Skeletal Muscle Leads to Severe Muscular Dystrophy

Spinal muscular atrophy (SMA) is characterized by degeneration of motor neurons of the spinal cord associated with muscle paralysis and caused by mutations of the survival motor neuron gene (SMN). To determine whether SMN gene defect in skeletal muscle might have a role in SMA pathogenesis, deletion of murine SMN exon 7, the most frequent mutation found in SMA, has been restricted to skeletal muscle by using the Cre-loxP system. Mutant mice display ongoing muscle necrosis with a dystrophic phenotype leading to muscle paralysis and death. The dystrophic phenotype is associated with elevated levels of creatine kinase activity, Evans blue dye uptake into muscle fibers, reduced amount of dystrophin and upregulation of utrophin expression suggesting a destabilization of the sarcolemma components. The mutant mice will be a valuable model for elucidating the underlying mechanism. Moreover, our results suggest a primary involvement of skeletal muscle in human SMA, which may contribute to motor defect in addition to muscle denervation caused by the motor neuron degeneration. These data may have important implications for the development of therapeutic strategies in SMA

Genetics and livestock breeding in the UK: Co-constructing technologies and heterogeneous biosocial collectivities

Cattle and sheep breeders in the UK and elsewhere are increasingly being encouraged to use a variety of genetic technologies to help them make breeding decisions. The technology of particular interest here is ‘classical’ statistical genetics, which use a series of measurements taken from animals’ bodies to provide an estimate of their ‘genetic merit’ known as Estimated Breeding Values (EBVs). Drawing on empirical research with the representatives of national cattle breed societies and individual cattle breeders the paper explores the complex ways in which they are engaging with genetic breeding technologies. The concept of ‘heterogeneous biosocial collectivity’ is mobilised to inform an understanding of processes of co-construction of breeding technologies, livestock animals and humans. The paper presents case studies of livestock breeding collectivities at different scales, arguing that the ways in which the ‘life’ of livestock animals is problematised is specific to different scales, and varies too between different collectivities at the same scale. This conceptualisation problematises earlier models of innovation-adoption that view farmers as either ‘adopters’ or ‘non-adopters’ of technologies and in which individual attitudes alone are seen as determining the decision to adopt or not adopt. Instead, the paper emphasises the particularity and specificity of co-construction, and that the co-construction of collectivities and technologies is always in process

Myogenin Regulates Exercise Capacity but Is Dispensable for Skeletal Muscle Regeneration in Adult mdx Mice

Duchenne muscular dystrophy (DMD) is the most prevalent inherited childhood muscle disorder in humans. mdx mice exhibit a similar pathophysiology to the human disorder allowing for an in-depth investigation of DMD. Myogenin, a myogenic regulatory factor, is best known for its role in embryonic myogenesis, but its role in adult muscle maintenance and regeneration is still poorly understood. Here, we generated an mdx:Myogflox/flox mouse harboring a tamoxifen-inducible Cre recombinase transgene, which was used to conditionally delete Myog during adult life. After tamoxifen treatment, three groups of mice were created to study the effects of Myog deletion: mdx:Myogflox/flox mice (mdx), Myogflox/flox mice (wild-type), and mdx:MyogfloxΔ/floxΔ:Cre-ER mice (mdx:Myog-deleted). mdx:Myog-deleted mice exhibited no adverse phenotype and behaved normally. When run to exhaustion, mdx:Myog-deleted mice demonstrated an enhanced capacity for exercise compared to mdx mice, running nearly as far as wild-type mice. Moreover, these mice showed the same signature characteristics of muscle regeneration as mdx mice. Unexpectedly, we found that myogenin was dispensable for muscle regeneration. Factors associated with muscle fatigue, metabolism, and proteolysis were significantly altered in mdx:Myog-deleted mice, and this might contribute to their increased exercise capacity. Our results reveal novel functions for myogenin in adult muscle and suggest that reducing Myog expression in other muscle disease models may partially restore muscle function

A comparison of the bone and growth phenotype of mdx, mdx:cmah-/- and mdx:utrn+/- murine models with the C57BL10 wildtype mouse

The muscular dystrophy X-linked (mdx) mouse is commonly used as a mouse model of Duchenne muscular dystrophy (DMD). Its phenotype is, however, mild, and other mouse models have been explored. The mdx:Cmah−/− mouse carries a human-like mutation in the Cmah gene and has a severe muscle phenotype, but its growth and bone development are unknown. In this study, we compared male mdx, mdx:Utrn+/−, mdx:Cmah−/− and wild-type (WT) mice at 3, 5 and 7 weeks of age to determine the suitability of the mdx:Cmah−/− mouse as a model for assessing growth and skeletal development in DMD. The mdx:Cmah−/− mice were lighter than WT mice at 3 weeks, but heavier at 7 weeks, and showed an increased growth rate at 5 weeks. Cortical bone fraction as assessed by micro-computed tomography was greater in both mdx and mdx:Cmah−/− mice versus WT mice at 7 weeks. Tissue mineral density was also higher in mdx:Cmah−/− mice at 3 and 7 weeks. Gene profiling of mdx:Cmah−/− bone identified increased expression of Igf1, Igf1r and Vegfa. Both the mdx and mdx:Cmah−/− mice showed an increased proportion of regulated bone marrow adipose tissue (BMAT) but a reduction in constitutive BMAT. The mdx:Cmah−/− mice show evidence of catch-up growth and more rapid bone development. This pattern does not mimic the typical DMD growth trajectory and therefore the utility of the mdx:Cmah−/− mouse for studying growth and skeletal development in DMD is limited. Further studies of this model may, however, shed light on the phenomenon of catch-up growth