Diverse dystonin gene mutations cause distinct patterns of Dst isoform deficiency and phenotypic heterogeneity in D ystonia musculorum mice.

Loss-of-function mutations in dystonin (DST) can cause hereditary sensory and autonomic neuropathy type 6 (HSAN-VI) or epidermolysis bullosa simplex (EBS). Recently, DST-related diseases were recognized to be more complex than previously thought because a patient exhibited both neurological and skin manifestations, whereas others display only one or the other. A single DST locus produces at least three major DST isoforms: DST-a (neuronal isoforrn), DST-b (muscular isoform) and DST-e (epithelial isoform). Dystonia musculorum (dt) mice, which have mutations in Dst, were originally identified as spontaneous mutants displaying neurological phenotypes. To reveal the mechanisms underlying the phenotypic heterogeneity of DST-related diseases, we investigated two mutant strains with different mutations: a spontaneous Dst mutant (DSt(dt-23Rbrc) mice) and a gene-trap mutant (Dst(Gt) mice). The DSt(dt-23Rbrc) allele possesses a nonsense mutation in an exon shared by all Dst isoforms. The Dst(Gt) allele is predicted to inactivate Dst-a and Dst-b isoforms but not Dst-e. There was a decrease in the levels of Dst-a mRNA in the neural tissue of both DSt(dt-23Rbrc )and Dst(Gt) homozygotes. Loss of sensory and autonomic nerve ends in the skin was observed in both DSt(dt-23Rbrc) and Dst(Gt) mice at postnatal stages. In contrast, Dst(Gt) mRNA expression was reduced in the skin of DSt(dt-23Rbrc) mice but not in Dst(Gt) mice. Expression levels of Dst proteins in neural and cutaneous tissues correlated with Dst mRNAs. Because Dst-e encodes a structural protein in hemidesmosomes (HDs), we performed transmission electron microscopy. Lack of inner plaques and loss of keratin filament invasions underneath the HDs were observed in the basal keratinocytes of DSt(dt-23Rbrc) mice but not in those of Dst(Gt) mice; thus, the distinct phenotype of the skin of DSt(dt-23Rbrc) mice could be because of failure of Dst-e expression. These results indicate that distinct mutations within the Dst locus can cause different loss-offunction patterns among Dst isoforms, which accounts for the heterogeneous neural and skin phenotypes in dt mice and DST-related diseases.