Evidence for both the nucleus and cytoplasm as subcellular sites of pathogenesis in Huntington's disease in cell culture and in transgenic mice expressing mutant huntingtin

A unifying feature of the CAG expansion diseases is the formation of intrac ellular aggregates composed of the mutant polyglutamine-expanded protein. D espite the presence of aggregates in affected patients, the precise relatio nship between aggregates and disease pathogenesis is unresolved. Results fr om in vivo and in vitro studies of mutant huntingtin have led to the hypoth esis that nuclear localization of aggregates is critical for the pathology of Huntington's disease (HD). We tested this hypothesis using a 293T cell c ulture model system by comparing the frequency and toxicity of cytoplasmic and nuclear huntingtin aggregates. Insertion of nuclear import or export se quences into huntingtin fragments containing 548 or 151 amino acids was use d to reverse the normal localization of these proteins. Changing the subcel lular localization of the fragments did not influence their total aggregate frequency There were also no significant differences in toxicity associate d with the presence of nuclear compared with cytoplasmic aggregates. These studies, together with findings in transgenic mice, suggest two phases for the pathogenesis of HD, with the initial toxicity in the cytoplasm followed by proteolytic processing of huntingtin, nuclear translocation with increa sed nuclear concentration of N-terminal fragments, seeding of aggregates an d resultant apoptotic death. These findings support the nucleus and cytosol as subcellular sites for pathogenesis in HD.