Mapping Modifiers Affecting Muscularity of the Myostatin Mutant (Mstn) Compact Mouse

The hypermuscular Compact phenotype was first noted in a line of mice selected for high body weight and protein content. A new line, based on mice showing the Compact phenotype, was formed and selected for maximum expression of the Compact phenotype. Previously we mapped and identified a 12-bp deletion in the myostatin gene, denoted Mstn, which can be considered as a major gene responsible for the hypermuscular phenotype. Genetic analysis revealed that full expression of the hypermuscular phenotype requires the action of modifier loci in addition to Mstn. To map these modifier loci, an interspecific F2 population was generated between Comp9, an inbred line homozygous for Mstn, and CAST/Ei, an inbred line generated from Mus musculus castaneus. Selective DNA pooling and genotyping, separately by gender, was carried out within a subpopulation of the F2 consisting of individuals homozygous for Mstn. Significant association with hypermuscularity at a false discovery rate (FDR) of 0.05 was found for markers on chromosomes 3, 5, 7, 11, 16, and X. In all cases, the marker allele derived from the Comp9 parent showed a higher frequency in the hypermuscular group and the CAST/Ei allele in the normal group. The modifier loci apparently exerted their effects on muscularity only in the presence of Mstn. THE Compact “hypermuscular” mutation arose durregion of Mstn and shown to be responsible for the double-muscled phenotype in these breeds (Grobet et ing a selection experiment in Germany (Bunger et al. 2001). A “Compact” subline was formed and selected al. 1997; Kambadur et al. 1997; McPherron and Lee 1997). MstnCmpt-dl1Abc, however, was found in the propepfor maximum hypermuscularity. Genetic analysis of the tide region of the myostatin precursor (Szabó et al. Compact line indicated that a major gene, denoted 1998). In analogy with TGF1, we hypothesized that Cmpt, and one or more modifier genes were involved the propeptide region might play an important role in in determining the degree of expression of the Compact the proper folding, efficient secretion, processing, or phenotype. The putative major gene (Cmpt) was mapped targeting of the mature growth factor domain of myoto an 8.2-cM region on mouse chromosome 1 (Varga statin. Although some of these steps may be seriously et al. 1997). After the discovery of the myostatin gene impaired by the MstnCmpt-dl1Abc mutation, the activity of ma(McPherron et al. 1997), we identified the MstnCmpt-dl1Abc ture myostatin is not zero in homozygous Cmpt/Cmpt deletion in the propeptide region of the gene and mapped mutants. Modifier genes may therefore have a significant the mouse myostatin gene to a point 27.7 cM from the influence on the phenotype (Szabó et al. 1998). centromere on chromosome 1 (Szabó et al. 1998). In the current study we report the mapping of modifier Myostatin, belonging to the transforming growth difloci affecting the expression of the hypermuscularity in ferentiation factor(TGF) superfamily, is a negative the Compact mouse. Our basic assumption is that due regulator of muscle growth. Myostatin was first identito selection for hypermuscularity, the Compact line, in fied when null-mutant knockout mice exhibited a large addition to achieving homozygosity for the MstnCmpt-dl1Abc and widespread increase in skeletal muscle mass due to mutation, also accumulated modifier alleles that inan increase of muscle fiber number (hyperplasia) and creased the expression of hypermuscularity in the Comthickness (hypertrophy; McPherron et al. 1997). Subsepact mouse. To map these modifier genes, a new interquently, an 11-bp myostatin deletion in the Belgian Blue subspecific cross (Cross4) was initiated, which produced and a missense myostatin mutation in the Piedmontese a large F2 population. A genome scan was carried out cattle breeds were identified in the C-terminal coding by selective DNA pooling within the F2 progeny group homozygous for the MstnCmpt-dl1Abc deletion. Markers on several chromosomes showed linkage with the putative 1Corresponding author: Institute for Animal Biology, Agricultural Biomodifiers, and the strongest association was found for technology Center, P.O.B. 411, H-2101 Gödöllő, Hungary. E-mail: [email protected] markers on chromosomes 16 and X. Mapping of the Genetics 165: 257–267 (September 2003)