Cleavage of huntingtin by apopain, a proapoptotic cysteine protease, is modulated by the polyglutamine tract.

Apoptosis has recently been recognized as a mode of cell death in Huntington disease (HD). Apopain, a human counterpart of the nematode cysteine protease death-gene product, CED-3, has a key role in proteolytic events leading to apoptosis. Here we show that apoptotic extracts and apopain itself specifically cleave the HD gene product, huntingtin. The rate of cleavage increases with the length of the huntingtin polyglutamine tract, providing an explanation for the gain-of-function associated with CAG expansion. Our results show that huntingtin is cleaved by cysteine proteases and suggest that HD might be a disorder of inappropriate apoptosis.

Caspase cleavage of gene products associated with triplet expansion disorders generates truncated fragments containing the polyglutamine tract.

The neurodegenerative diseases Huntington disease, dentatorubropallidoluysian atrophy, spinocerebellar atrophy type 3, and spinal bulbar muscular atrophy are caused by expansion of a polyglutamine tract within their respective gene products. There is increasing evidence that generation of truncated proteins containing an expanded polyglutamine tract may be a key step in the pathogenesis of these disorders. We now report that, similar to huntingtin, atrophin-1, ataxin-3, and the androgen receptor are cleaved in apoptotic extracts. Furthermore, each of these proteins is cleaved by one or more purified caspases, cysteine proteases involved in apoptotic death. The CAG length does not modulate susceptibility to cleavage of any of the full-length proteins. Our results suggest that by generation of truncated polyglutamine-containing proteins, caspase cleavage may represent a common step in the pathogenesis of each of these neurodegenerative diseases.