Genetic engineering in the mouse: from functional genomics to zootechnical applications (2007)

Muscular hypertrophy (MH) in cattle is a condition occuring in various breeds and often referred to as “double muscling”. Affected individuals are characterized by a mean 20% increase of skeletal muscle mass. This trait has been selected for in some breeds, especially in the Belgian White and Blue breed, and has attracted considerable attention from geneticists. The mode of inheritance has been characterized as autosomal and recessive (Hanset & Michaux, 1985 a et b). A positional cloning approach has localized MH to the centromeric tip of bovine chromosome 2 (Charlier et al., 1995). Inactivation of the Growth and Differentiation Factor 8 (now referred to as Myostatin - MSTN) in the mouse (McPherron et al., 1997) resulted in a dramatic increase of skeletal muscle mass as a result of combined muscular hyperplasia and hypertrophy. Using a positional candidate approach, Grobet et al. (1997, 1998) demonstrated that MH in cattle is due to loss of function alleles of MSTN. MSTN is a member of the TGF- superfamily of growth and differentiation factors (McPherron et al., 1997). As in cattle, naturally occurring loss of MSTN function in mice (Szabo et al., 1998), sheep (Clop et al., 2006), dogs (Mosher et al., 2007) and humans (Schuelke et al., 2004) result in a spectacular muscular hypertrophy. Consequently, MSTN has become a major research topic in muscle biology either for fundamental research or for more applied purposes. For unravelling some aspects of its biology, for exploring new avenues for palliating muscle wasting conditions or for developing new zootechnical strategies, MSTN over-expression as well as inhibition has been performed in various mouse models (Bogdanovitch et al., 2002; Zimmers et al., 2002; Lee, 2007). Engineering of MSTN in the mouse was carried out in our unit for achieving either its post-natal or male-specific muscular inactivation. Post-natal inactivation has been performed using a conditional knock-out approach (Grobet et al., 2003). The third exon of MSTN, mainly coding for the bioactive domain of the protein, has been flanked by two loxP sites in the same orientation. Post-natal and muscle-specific genomic inactivation was achieved by use of a trangene expressing the Cre recombinase under the MCK promoter. This resulted in a skeletal muscle mass increase of the same magnitude as found in the constitutive knock-out. This demonstrated clearly that post-natal manipulation of MSTN can still have a major effect on muscle development. For modelizing a male-specific muscular hypertrophy, the MSTN latency associated propeptide (previously demonstrated as having MSTN dominant-negative properties; Lee & McPherron, 2001) was inserted on the murine Y chromosome under a muscle-specific promoter (Pirottin et al., 2005). This first successful targeting of the Y chromosome in live mice allowed the expression of a male-specific muscular hypertrophy. This model demonstrated the feasibility of a novel production system in cattle, combining superior beef ability in bulls without impeding dairy production in cows of the same lineage. The MSTN story depicted here is a clear example of the contribution of mouse genetic engineering to fundamental research as well as to more applied model development.