A targeted mutation reveals a role for N-myc in branching morphogenesis in the embryonic mouse lung.

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Abstract
The N-myc proto-oncogene encodes a putative transcription factor that has been postulated to be involved in the control of differentiation in a number of lineages at various stages during mammalian embryogenesis. We have generated a leaky mutation in N-myc by gene targeting in embryonic stem cells. In this allele, the neo(r) gene was inserted into the first intron of N-myc, in such a way that alternative splicing around this insertion could result in the generation of a normal N-myc transcript in addition to a mutant transcript. Mice homozygous for this mutation died immediately after birth owing to an inability to oxygenate their blood. Histological examination revealed a marked underdevelopment in the lung airway epithelium, resulting in a decreased respiratory surface area. Analysis of N-myc expression in wild-type and homozygous mutant embryonic lungs suggests that N-myc is required for the proliferation of the lung epithelium in response to local inductive signals emanating from the lung mesenchyme. Homozygous mutant embryos were slightly smaller than normal and also had a marked reduction in spleen size, whereas other tissues that normally express N-myc appeared to be unaffected by the mutation. Molecular analysis revealed that normal N-myc transcripts were found in tissues from homozygous mutant embryos. Different tissues expressed the normal N-myc transcript at different levels relative to those observed in wild-type embryos, with the lowest levels being observed in the lungs. These results illustrate one way in which gene targeting can be used to generate partial loss-of-function mutations and support the importance of generating a series of alleles at a given locus to elucidate the various different functions of a gene during development.