[Molecular-targeted therapy for spinal and bulbar muscular atrophy (SBMA)]

Abstract

Spinal and bulbar muscular atrophy (SBMA) is an adult-onset neurodegenerative disease characterized by slowly progressive muscle weakness and atrophy. The cause of SBMA is the expansion of a trinucleotide CAG repeat, which encodes the polyglutamine tract, within the first exon of the androgen receptor (AR) gene. SBMA exclusively occurs in males, whereas both heterozygous and homozygous females are usually asymptomatic. In a transgenic mouse model of SBMA, neuromuscular symptoms are markedly pronounced in the male mice, but far less severe in the female counterparts. Androgen deprivation through both surgical and chemical castration substantially suppresses nuclear accumulation of the pathogenic AR, and thereby improves symptoms in the male mice. Since the nuclear translocation of AR is ligand-dependent, testosterone appears to show toxic effects by accelerating nuclear translocation of the pathogenic AR. In a phase 2 clinical trial, 12-month treatment with leuprorelin significantly diminished the serum level of creatine kinase, and suppressed nuclear accumulation of the pathogenic AR. The ligand-dependent accumulation of the pathogenic AR, an initial step in the neurodegenerative process in SBMA, is followed by several downstream molecular events such as transcriptional dysregulation, axonal transport disruption, and mitochondrial insufficiency, indicating that both upstream and downstream molecular abnormalities should be corrected.