Grey, a novel mutation in the murine Lyst gene, causes the beige phenotype by skipping of exon 25

Abstract

The murine beige mutant phenotype and the human Chediak-Higashi syndrome are caused by mutations in the murine Lyst (lysosomal trafficking regulator) gene and the human CHS gene, respectively. In this report we have analyzed a novel murine mutant Lyst allele, called Lyst^bg-grey, that had been found in an ENU mutation screen and named grey because of the grey coat color of affected mice. The phenotype caused by the Lyst^bg-grey mutation was inherited in a recessive fashion. Melanosomes of melanocytes associated with hair follicles and the choroid layer of the eye, as well as melanosomes in the neural tube-derived pigment epithelium of the retina, were larger and irregularly shaped in homozygous mutants compared with those of wild-type controls. Secretory vesicles in dermal mast cells of the mutant skin were enlarged as well. Test crosses with beige homozygous mutant mice (Lyst^bg) showed that double heterozygotes (Lyst^bg/Lyst^bg-grey) were phenotypically indistinguishable from either homozygous parent, demonstrating that the ENU mutation was an allele of the murine Lyst gene. RT-PCR analyses revealed the skipping of exon 25 in Lyst^bg-grey mutants, which is predicted to cause a missense D2399E mutation and the loss of the following 77 amino acids encoded by exon 25 but leave the C-terminal end of the protein intact. Analysis of the genomic Lyst locus around exon 25 showed that the splice donor at the end of exon 25 showed a T-to-C transition point mutation. Western blot analysis suggests that the Lyst^bg-grey mutation causes instability of the LYST protein. Because the phenotype of Lyst^bg and Lyst^bg-grey mutants is indistinguishable, at least with respect to melanosomes and secretory granules in mast cells, the Lyst^bg-grey mutation defines a critical region for the stability of the murine LYST protein.