A genomewide screening of N-ethyl-N-nitrosourea-mutagenized mice for musculoskeletal phenotypes

Chemical mutagenesis followed by screening for abnormal phenotypes in the mouse holds much promise as a method for revealing gene function. We describe a mouse N-ethyl-N-nitrosourea (ENU) mutagenesis program incorporating a genomewide screen of dominant as well as recessive mutations affecting musculoskeletal disorders in C3H/HeJ mice. In a primary screen, progeny of one-generation dominant mutations (F(1)) and three-generation recessive (F(3)) mutations were screened at 10 weeks of age for musculoskeletal disorders using dual-energy X-ray absorptiometery (DEXA) and biochemical markers affecting bone metabolism, such as osteocalcin, type I collagen breakdown product, skeletal alkaline phosphatase, and insulin-like growth factor I (IGF-I). Abnormal phenotypes were identified as +/-3SD units different from baseline data collected from age- and sex-matched nonmutagenized control mice. A secondary screen at 16 weeks of age, which included peripheral quantitative computed tomography (pQCT) in addition to those parameters described in our primary screen, was used to confirm the abnormal phenotypes observed in the primary screen. The phenodeviant or outlier mice were progeny tested to determine whether their abnormality segregates bimodally in their offspring with the expected 1:1 or 1:3 Mendelian ratio, in dominant and recessive screens, respectively. With the above screening strategy, we were able to identify several mice with quantitative abnormalities in BMD, BMC, bone size, and bone metabolism. We have progeny tested and confirmed four outliers with low BMD, low bone size, and growth-related abnormality. Our results indicate that the magnitude of change in quantitative phenotypes in the ENU-mutagenized progeny was between 10 and 15%, and hence, the yield of outliers was dependent on the precision of the methods. So far, this ENU mutagenesis program has identified four outliers that can undergo positional cloning.