Dmd(mdx-beta geo): a new allele for the mouse dystrophin gene.

During a gene trap screen, an insertion of the gene trap vector into the dystrophin gene, creating a new allele for the Dmd gene, has been discovered. Because the ROSA beta geo vector was used, the new allele is called Dmd(mdx-beta geo). The insertion occurred 3' of exon 63 of the dystrophin gene, resulting in a mutation that affects all presently known dystrophin isoforms. In contrast to spontaneous or ENU-induced alleles, Dmd(mdx-beta geo) can be used to follow dystrophin expression by staining for beta-galactosidase activity. The high sensitivity of this method revealed additional and earlier expression of dystrophin during embryogenesis than that seen previously with other methods. Dystrophin promoters are active predominantly in the dermamyotome, limb buds, telencephalon, floor plate, eye, liver, pancreas anlagen, and cardiovascular system. Adult Dmd(mdx-beta geo) mice show reporter gene expression in brain, eye, liver, pancreas, and lung. In skeletal and heart muscle, beta-galactosidase activity is not detectable, confirming Western blot data that indicate the absence of the mutant full-length protein in these tissues. Hemizygous Dmd(mdx-beta geo) mice show muscular dystrophy with degenerating muscle fibers, cellular infiltration, and regenerated muscle fibers that have centrally located nuclei. Some mutant animals develop a dilated esophagus, probably due to constriction by the hypertrophic crura of the diaphragm.