DEFECTS IN HEART AND LUNG DEVELOPMENT IN COMPOUND HETEROZYGOTES FOR 2DIFFERENT TARGETED MUTATIONS AT THE N-MYC LOCUS

Two types of mutant allele, one leaky and one null, have been generate d by gene targeting at the N-myc locus in embryonic stem cells and the phenotypes of mice homozygous for these mutations have been described . These mutations have shown that N-myc has a number of functions duri ng development, including a role in branching morphogenesis in the lun g, which manifests itself at birth in mice homozygous for the leaky al lele, and roles in the development of the mesonephric tubules, the neu roepithelium, the sensory ganglia, the gut and the heart, which become evident at midgestation in embryos homozygous for the null allele. In an attempt to define roles for N-myc at other stages of development, we have combined the two types of N-myc mutant allele in a compound he terozygote that as a result contains approximately 15% of normal level s of N-Myc protein. Compound heterozygotes died during gestation at a time intermediate to the times of death of embryos homozygous for eith er mutation individually, and their death appeared to result from card iac failure stemming from hypoplasia of the compact subepicardial laye r of the myocardium. Investigation of the expression pattern of N-myc and various markers of differentiation in wild-type and compound heter ozygote mutant hearts has suggested that N-myc may function in maintai ning the proliferation and/or preventing the differentiation of compac t layer myocytes. This study illustrates the importance of generating different mutations at a given locus to elucidate fully the function o f a particular gene during development.