Elucidation of molecular mechanism of neuronal cell death in spinal and bulbar muscular atrophy

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Spinal and bulbar muscular atrophy (SBMA) is an adult-onset motor neuron disease affecting only males. The molecular basis of SBMA is the expansion of a trinucleotide CAG repeat in the first exon of the androgen receptor (AR) gene, which encodes the polyglutamine (polyQ) tract. The length of CAG repeat correlates with the severity of SBMA. Immunohistochemical analysis of autopsied specimen demonstrates that diffuse accumulation of mutant AR in the nuclei of spinal motor neurons is a fundamental neuropathological feature of SBMA, extent of which closely relates to CAG repeat length. Thus, diffuse nuclear accumulation of mutant AR is likely crucial pathogenic step in SBMA. We generated two mouse models of SBMA. Among these, a transgenic mouse model carrying full-length AR containing 97 CAGs driven by a chicken (3-actin promoter shows progressive motor impairment and nuclear accumulation of mutant AR. These neurological phenotypes were devastating in male mice, but not observed or far less severe in the females. Surgical castration dramatically prevented the phenotypic expression in the male Tg mice by diminishing the nuclear accumulation of mutant AR. In contrast, the female Tg mice demonstrated striking deterioration of symptoms by testosterone administration. Leuprorelin, an LHRH agonist that reduces testosterone release from the testis, suppressed nuclear accumulation of mutant AR, leading to rescue of motor dysfunction in the male SBMA mice. Our studies have also indicated several candidates of therapeutics for SBMA. Genetic overexpression of heat shock protein (HSP) ameliorates neurodegeneration by improving conformational alteration of mutant AR in the mouse model of SBMA. Oral administration of sodium butyrate, a histone deacetylase inhibitor, resulted in improvement of neurological dysfunction in the SBMA mouse model, al though its therapeutic effects were seen in a narrow dose range.