Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex

Abstract

Huntington's disease (HD) is an age-dependent neurodegenerative disease. DNA repair pathways have recently been implicated as the most predominant modifiers of age of onset in HD patients. We report that endogenous huntingtin protein directly participates in oxidative DNA damage repair. 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. 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. In HD patient fibroblasts bearing typical clinical HD allele lengths, we demonstrate that there is deficient oxidative DNA damage repair. 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.

Is Huntingtin Dispensable in the Adult Brain?

“…Recently, it was demonstrated in a cell culture model that N17-phosphorylated HTT is found at sites of DNA damage in the nucleus, where it co-localizes with ATM to participate in the base excision repair pathway, and this co-localization is dependent on ATM kinase activity [50]. Furthermore, HTT also exhibits an oxidative stress-dependent interaction with other DNA repair proteins such as XRCC1, Flap structure-specific endonuclease 1 (FEN1), APE1, and high mobility group box 1 (HMGB1) [50]. …”

“…When ROS stress is relieved, de-phosphorylation of S13 and S16 occurs, and HTT translocates back into the cytoplasm by a CRM1-dependent nuclear export mechanism [56]. Oxidative stress and ROS cause DNA damage, and the nuclear entry of N17-phosphorylated HTT allows it to participate with ATM in DNA base excision repair [50]. …”

Structure/Function Analysis of the Huntingtin N-terminus Encoded by Htt Exon 1 Using Knock-In Mouse Models

“…I note that a recent study investigating the consequence of Htt inactivation in the brains of adult mice also did not detect significant changes in LC3 or p62 protein levels [147]. Because stress can induce selective macroautophagy [57], and recent in vitro studies have identified N17 domain-dependent roles for HTT in cellular stress and DNA damage responses [60,82,114,116], it is possible that the contribution of the HTT N-terminus in these pathways may not be apparent until homeostasis is perturbed, such as under chronic stress. For example, the expression of 140Q-Htt was needed to elicit more robust autophagy phenotypes in Htt 140Q/ΔQ mice [110].…”

“…HTT is a scaffolding protein that is involved in multiple cellular functions [65,81,82]. The generation and characterization of Htt knock-out mice revealed that Htt has an essential function during early embryonic development [83][84][85].…”

“…During conditions of oxidative stress, the sulfur side group on methionine 8 in the N17 domain is oxidized and leads to Htt dissociation from the ER membrane, phosphorylation of serine 13 and 16, and accumulation of Htt within the nucleus [117]. Subsequent work showed that upon entering the nucleus, Htt can also localize to sites of oxidative DNA damage and may act as a scaffold for DNA base excision repair machinery [82].…”