PolyQ disease: misfiring of a developmental cell death program?

Polyglutamine (polyQ) repeat diseases are neurodegenerative ailments elicited by glutamine-encoding CAG nucleotide expansions within endogenous human genes. Despite efforts to understand the basis of these diseases, the precise mechanism of cell death remains stubbornly unclear. Much of the data seem to be consistent with a model in which toxicity is an inherent property of the polyQ repeat, whereas host protein sequences surrounding the polyQ expansion modulate severity, age of onset, and cell specificity. Recently, a gene, pqn-41, encoding a glutamine-rich protein, was found to promote normally occurring non-apoptotic cell death in Caenorhabditis elegans. Here we review evidence for toxic and modulatory roles for polyQ repeats and their host proteins, respectively, and suggest similarities with pqn-41 function. We explore the hypothesis that toxicity mediated by glutamine-rich motifs may be important not only in pathology, but also in normal development.