Msh2 Acts in Medium-Spiny Striatal Neurons as an Enhancer of CAG Instability and Mutant Huntingtin Phenotypes in Huntington’s Disease Knock-In Mice

PLOS ONE, Dec 2019

The CAG trinucleotide repeat mutation in the Huntington’s disease gene (HTT) exhibits age-dependent tissue-specific expansion that correlates with disease onset in patients, implicating somatic expansion as a disease modifier and potential therapeutic target. Somatic HTT CAG expansion is critically dependent on proteins in the mismatch repair (MMR) pathway. To gain further insight into mechanisms of somatic expansion and the relationship of somatic expansion to the disease process in selectively vulnerable MSNs we have crossed HTT CAG knock-in mice (HdhQ111) with mice carrying a conditional (floxed) Msh2 allele and D9-Cre transgenic mice, in which Cre recombinase is expressed specifically in MSNs within the striatum. Deletion of Msh2 in MSNs eliminated Msh2 protein in those neurons. We demonstrate that MSN-specific deletion of Msh2 was sufficient to eliminate the vast majority of striatal HTT CAG expansions in HdhQ111 mice. Furthermore, MSN-specific deletion of Msh2 modified two mutant huntingtin phenotypes: the early nuclear localization of diffusely immunostaining mutant huntingtin was slowed; and the later development of intranuclear huntingtin inclusions was dramatically inhibited. Therefore, Msh2 acts within MSNs as a genetic enhancer both of somatic HTT CAG expansions and of HTT CAG-dependent phenotypes in mice. These data suggest that the selective vulnerability of MSNs may be at least in part contributed by the propensity for somatic expansion in these neurons, and imply that intervening in the expansion process is likely to have therapeutic benefit.

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Msh2 Acts in Medium-Spiny Striatal Neurons as an Enhancer of CAG Instability and Mutant Huntingtin Phenotypes in Huntington’s Disease Knock-In Mice

et al. (2012) Msh2 Acts in Medium-Spiny Striatal Neurons as an Enhancer of CAG Instability and Mutant Huntingtin Phenotypes in Huntington's Disease Knock-In Mice. PLoS ONE 7(9): e44273. doi:10.1371/journal.pone.0044273 Msh2 Acts in Medium-Spiny Striatal Neurons as an Enhancer of CAG Instability and Mutant Huntingtin Phenotypes in Huntington's Disease Knock-In Mice Marina Kovalenko 0 Ella Dragileva 0 Jason St. Claire 0 Tammy Gillis 0 Jolene R. Guide 0 Jaclyn New 0 Hualing Dong 0 Raju Kucherlapati 0 Melanie H. Kucherlapati 0 Michelle E. Ehrlich 0 Jong-Min Lee 0 Vanessa C. Wheeler 0 Kari Hoyt, Ohio State University, United States of America 0 1 Center for Human Genetic Research, Massachusetts General Hospital , Boston , Massachusetts, United States of America, 2 Department of Medicine, Brigham and Women's Hospital , Boston , Massachusetts, United States of America, 3 Farber Institute for Neurosciences, Thomas Jefferson University College of Medicine , Philadelphia, Pennsylvania , United States of America The CAG trinucleotide repeat mutation in the Huntington's disease gene (HTT) exhibits age-dependent tissue-specific expansion that correlates with disease onset in patients, implicating somatic expansion as a disease modifier and potential therapeutic target. Somatic HTT CAG expansion is critically dependent on proteins in the mismatch repair (MMR) pathway. To gain further insight into mechanisms of somatic expansion and the relationship of somatic expansion to the disease process in selectively vulnerable MSNs we have crossed HTT CAG knock-in mice (HdhQ111) with mice carrying a conditional (floxed) Msh2 allele and D9-Cre transgenic mice, in which Cre recombinase is expressed specifically in MSNs within the striatum. Deletion of Msh2 in MSNs eliminated Msh2 protein in those neurons. We demonstrate that MSN-specific deletion of Msh2 was sufficient to eliminate the vast majority of striatal HTT CAG expansions in HdhQ111 mice. Furthermore, MSNspecific deletion of Msh2 modified two mutant huntingtin phenotypes: the early nuclear localization of diffusely immunostaining mutant huntingtin was slowed; and the later development of intranuclear huntingtin inclusions was dramatically inhibited. Therefore, Msh2 acts within MSNs as a genetic enhancer both of somatic HTT CAG expansions and of HTT CAG-dependent phenotypes in mice. These data suggest that the selective vulnerability of MSNs may be at least in part contributed by the propensity for somatic expansion in these neurons, and imply that intervening in the expansion process is likely to have therapeutic benefit. - Funding: This work was supported by the National Institutes of Health [NS049206] and the CHDI Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. . These authors contributed equally to this work. Huntingtons disease (HD) is a dominantly inherited neurodegenerative disorder characterized by motor, cognitive and psychiatric symptoms [1]. The underlying cause is the expansion .35 repeats of a polymorphic CAG repeat within HTT gene that lengthens a glutamine tract in the huntingtin protein [2]. Stringent statistical analyses in a large HD patient data set indicate that the CAG expansion determines onset age in a fully dominant fashion with no evidence for a major role of either the wild-type allele or a second mutant allele [3]. While mutant huntingtin exerts its toxic effects in many brain regions as well as peripheral tissues over the course of the disease, medium-spiny GABA-ergic projection neurons (MSNs) in the striatum are the most vulnerable [46]. Therefore, the factors that contribute to this neuronal susceptibility are likely to provide clues to pathogenesis. Despite being caused by a single gene defect the disease is clearly complex, with a multitude of cellular pathways disrupted in response to mutant huntingtin [7]. Discerning those events that are critical to pathogenesis in order to design rational therapeutics remains a challenge. An alternative to targeting downstream pathways that are disrupted during the course of disease is to target the CAG repeat mutation itself. Given that onset age and disease severity are highly correlated with the length of the expanded CAG repeat [3,8], one would predict that reducing CAG length, even within the disease range, would have a beneficial effect. Notably, the mutant HTT CAG repeat exhibits both intergenerational and somatic instability [817]. The latter is highly biased towards expansions and is tissue-specific, with the greatest expansions seen in the striatum [13]. The striatum appears to be particularly susceptible to expansion in several trinucleotide repeat diseases [1820], consistent with findings that expansion reflects an intrinsic property of this tissue rather than being a consequence of ongoing pathogenesis [21]. However, (...truncated)


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Marina Kovalenko, Ella Dragileva, Jason St. Claire, Tammy Gillis, Jolene R. Guide, Jaclyn New, Hualing Dong, Raju Kucherlapati, Melanie H. Kucherlapati, Michelle E. Ehrlich, Jong-Min Lee, Vanessa C. Wheeler. Msh2 Acts in Medium-Spiny Striatal Neurons as an Enhancer of CAG Instability and Mutant Huntingtin Phenotypes in Huntington’s Disease Knock-In Mice, PLOS ONE, 2012, Volume 7, Issue 9, DOI: 10.1371/journal.pone.0044273