The huntingtin protein that is localized to sites of DNA damage is phosphorylated at the N17 domain, and its recruitment is sensitive to ATM kinase inhibition. Using model systems that interrogate endogenous human proteins and disease-relevant polyglutamine lengths in clinically isolated cells, we have found that huntingtin co-localizes with ATM at nuclear puncta and at sites of DNA damage.

Our work on the nuclear localization of huntingtin ( 2, 4, 10) led us to hypothesize that dysregulated huntingtin and ATM activities in the nucleus may contribute to DNA repair deficiency in HD. It is also activated by single-stranded breaks ( 25), chromatin reorganization ( 26), oxidative stress ( 27), and has an impaired response to manganese in human HD neuroprogenitor cells ( 28). Moreover, the DNA damage response protein ataxia-telangiectasia mutated (ATM) contributes to disease progression in the BACHD mouse model ( 24).ĪTM, a serine/threonine kinase rich in Huntingtin, Elongation Factor 3, Protein Phosphatase 2 A, m TOR (HEAT) domains, signals through a host of proteins in response to double-stranded DNA breaks. Damage to nuclear DNA in HD patient samples and mouse models, in the form of strand breaks and damaged bases, has been extensively reported ( 15–21), and more recent studies support the involvement of DNA repair pathways in HD age at onset ( 22, 23). ROS-induced oxidative stress is suggested to play a key role in selective neuronal loss associated with aging and neurodegeneration, including HD ( 12–14). While the accumulation of mutant huntingtin in the nucleus has been associated with pathogenesis in HD brains ( 11), it is likely that N17-phosphorylated huntingtin performs vital functions in the nucleus in response to ROS. We have recently discovered that a conserved methionine at position eight within N17 (M8) is a reactive oxygen species (ROS) sensor by sulfoxidation, which augments N17 alpha-helical structure and triggers the release of membrane-bound huntingtin from the endoplasmic reticulum, allowing N17 phosphorylation and thereby nuclear retention ( 4, 10). Mutant huntingtin is hypo-phosphorylated at serine residues S13 and S16 ( 2), and restoration of phosphorylation changes the conformation of the protein ( 2, 9). This domain also modifies the disease progression in animal models as shown by phospho-mimetic mutants ( 7) and chemical biology approaches ( 2, 8). The N17 domain, comprising the first 17 amino acids of huntingtin adjacent to the polyglutamine tract, regulates huntingtin localization, endoplasmic reticulum and the vesicle membrane binding, and aggregation ( 2–6). Huntington’s disease (HD) is a genetic neurodegenerative disease caused by a CAG-repeat expansion in the first exon of the HTT gene encoding the huntingtin protein, producing an expanded polyglutamine tract in the amino terminus ( 1). We propose that DNA damage in HD is caused by dysfunction of the mutant huntingtin protein in DNA repair, and accumulation of DNA oxidative lesions due to elevated reactive oxygen species may contribute to the onset of HD. In HD patient fibroblasts bearing typical clinical HD allele lengths, we demonstrate that there is deficient oxidative DNA damage repair. Super-resolution microscopy and biochemical assays revealed that huntingtin co-localizes with and scaffolds proteins of the DNA damage response pathway in response to oxidative stress. Using novel chromobodies to detect endogenous human huntingtin in live cells, we show that localization of huntingtin to DNA damage sites is dependent on the kinase activity of ataxia telangiectasia mutated (ATM) protein. We report that endogenous huntingtin protein directly participates in oxidative DNA damage repair. DNA repair pathways have recently been implicated as the most predominant modifiers of age of onset in HD patients. Huntington’s disease (HD) is an age-dependent neurodegenerative disease.

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