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Mismatch Repair Genes Mlh1 and Mlh3 Modify CAG Instability in Huntington's Disease Mice: Genome-Wide and Candidate Approaches
Open accessAbstract
The Huntington's disease gene (HTT) CAG repeat mutation undergoes somatic expansion that correlates with pathogenesis. Modifiers of somatic expansion may therefore provide routes for therapies targeting the underlying mutation, an approach that is likely applicable to other trinucleotide repeat diseases. Huntington's disease Hdh(Q111) mice exhibit higher levels of somatic HTT CAG expansion on a C57BL/6 genetic background (B6.Hdh(Q111) ) than on a 129 background (129.Hdh(Q111) ). Linkage mapping in (B6x129).Hdh(Q111) F2 intercross animals identified a single quantitative trait locus underlying the strain-specific difference in expansion in the striatum, implicating mismatch repair (MMR) gene Mlh1 as the most likely candidate modifier. Crossing B6.Hdh(Q111) mice onto an Mlh1 null background demonstrated that Mlh1 is essential for somatic CAG expansions and that it is an enhancer of nuclear huntingtin accumulation in striatal neurons. Hdh(Q111) somatic expansion was also abolished in mice deficient in the Mlh3 gene, implicating MutLγ (MLH1-MLH3) complex as a key driver of somatic expansion. Strikingly, Mlh1 and Mlh3 genes encoding MMR effector proteins were as critical to somatic expansion as Msh2 and Msh3 genes encoding DNA mismatch recognition complex MutSβ (MSH2-MSH3). The Mlh1 locus is highly polymorphic between B6 and 129 strains. While we were unable to detect any difference in base-base mismatch or short slipped-repeat repair activity between B6 and 129 MLH1 variants, repair efficiency was MLH1 dose-dependent. MLH1 mRNA and protein levels were significantly decreased in 129 mice compared to B6 mice, consistent with a dose-sensitive MLH1-dependent DNA repair mechanism underlying the somatic expansion difference between these strains. Together, these data identify Mlh1 and Mlh3 as novel critical genetic modifiers of HTT CAG instability, point to Mlh1 genetic variation as the likely source of the instability difference in B6 and 129 strains and suggest that MLH1 protein levels play an important role in driving of the efficiency of somatic expansions.
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![Figure 1 Somatic HTT CAG instability differs between B6.HdhQ111 /+ and 129.HdhQ111 /+ mice. (A) Representative GeneMapper profiles of HTT CAG repeat size distributions in the tail, striatum and liver of 10-week-old B6.HdhQ111/+ and 129.HdhQ111/+ mice, highlighting the altered contribution of B6 and 129 genetic background to somatic HTT CAG repeat expansion, as previously described [17]. Tail and striatum: B6.HdhQ111/+, CAG116; 129.HdhQ111/+, CAG112. Liver: B6.HdhQ111/+, CAG113; 129.HdhQ111/+, CAG111 (B) Quantification of CAG instability index reveals a statistically significant decrease in somatic HTT CAG instability in the striatum and liver of 129.HdhQ111 /+ mice compared to B6.HdhQ111 /+ mice. B6.HdhQ111/+ striatum, n = 10, CAG116.9±1.2SD; B6.HdhQ111/+ liver, n = 10, CAG114.3±1.2SD; 129.HdhQ111/+ striatum, n = 12, CAG110.9±1.2SD; 129.HdhQ111/+ liver, n = 9, CAG109.5±1.4SD; Bar graphs represent mean ±SD; ****, p<0.0001.](/paper-figures/e33007d2d8ff415bb226e95f198df88b/mismatch-repair-genes-mlh1-and-mlh3-modify-cag-instability-in-huntington-s-figure-1.jpg)

![Figure 3 Identification of a quantitative trait locus (QTL) associated with striatal HTT CAG instability. Linkage analysis in 10-week-old (B6x129).HdhQ111/+ F2 mice (n = 69) identified a single QTL on chromosome 9, with a maximum LOD score of approximately 14 and a 2-LOD-dropoff interval of 5 Mb (chr9:107,982,655–113,057,967; GRCm38/mm10) (Figure S6). Note that the 2 markers positioned within the Mlh1 gene (dbSNP rs30131926 and rs30174694) define the QTL peak. The red dashed line represents the threshold (LOD = 4.3) considered for the identification of significant QTLs [85]. The coordinates (cM) of the 147 genetic markers used are represented by open triangles.](/paper-figures/c8f2e16588f640bc96f8f0f80e196abc/mismatch-repair-genes-mlh1-and-mlh3-modify-cag-instability-in-huntington-s-figure-3.jpg)



![Figure 7 B6-129 MLH1 protein polymorphisms. (A) Schematic representation of the murine MLH1 protein showing the location of B6-129 nonsynonymous polymorphisms identified (purple triangles) and their positions relative to conserved ATP binding motifs and ATP binding domain (dark and light red boxes, respectively) [46], as well as proposed MMR protein interaction domains (blue boxes) [41]. (B) Cross-species alignment of B6 and 129 MLH1 proteins in regions encompassing the polymorphic sites between the two strains. Protein sequence alignment was performed using Clustal Omega [96] and visualized in Jalview [97] with BLOSUM62 color scheme: white, residue does not match the consensus residue at that position; light blue, residue does not match the consensus residue but the two residues have a positive BLOSUM62 score; dark blue, residue matches consensus sequence.](/paper-figures/9a8880f9f71d40c5b885a60ed3f15cdd/mismatch-repair-genes-mlh1-and-mlh3-modify-cag-instability-in-huntington-s-figure-7.jpg)



![Figure 11 Proposed model of MutS and MutL-dependent events leading to CAG•CTG somatic instability. CAG•CTG repeat structures are initially recognized by the MutSβ (MSH2-MSH3) complex [25], [98]. The loop in the CAG•CTG repeat tract represents a short slip-out, previously identified as the main substrate for MMR protein-dependent repair of CAG•CTG structures in cell free systems [50], [51]. However, the nature of the putative CAG•CTG structure(s) that leads to MutS and MutL-dependent somatic instability in vivo is unknown. Following ATP hydrolysis by DNA-bound MutSβ [27], a MutLγ (MLH1–MLH3) heterodimer is preferentially recruited to the complex (thick arrow) over the MutLα (MLH1-PMS2) heterodimer (thin arrow). The total absence of HTT CAG expansion in Mlh3 −/− mice suggests that PMS2 plays no role at all in this process. However, PMS2 has been shown to play a role in the expansion of CTG repeats in a DM1 mouse model [24], suggesting that these events may be genetic locus and/or mouse strain dependent. Following MutLγ binding, various pathways, e.g. canonical mismatch repair (MMR), noncanonical mismatch repair (ncMMR) and/or other DNA repair processes may be engaged and process the repeats such that they ultimately undergo expansion. Other members of alternative DNA repair pathways, namely OGG1, XPA and NEIL1 have been directly implicated in CAG/CTG somatic instability in mice [76]–[78], however, how these proteins intersect with MMR protein-dependent pathways has yet to be demonstrated.](/paper-figures/3439e6e518d040d6baf663abeb39710a/mismatch-repair-genes-mlh1-and-mlh3-modify-cag-instability-in-huntington-s-figure-11.jpg)
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“Notably, CAG expansion has been found to be dependent upon DNA repair proteins such as OGG1, MLH1, and MSH2, whose detection of oxidized or mismatched bases in these repetitive elements could potentially elicit erroneous repair, leading to CAG expansion (Kovtun et al, 2007; Bettencourt et al, 2016; Pinto et al, 2013; Manley et al, 1999; Lee et al, 2017, 1).”
From Mechanisms of DNA damage-mediated neurotoxicity in neurodegenerative disease (2022) · cites this paper as Pinto et al, 2013
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Citations
Cites
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“(B) Distribution of polymorphisms identified between B6, 129, FVB and DBA mouse strains at a 16 kb genomic region containing the Trex1 and Atrip genes (chr9:109,057,932–109,074,124; GRCm38/ mm10) using information from the Mouse Genomes Project [44,45].”
BackgroundMethodInfluential
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Cited by
Sorted by citations
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Identification of Genetic Factors that Modify Clinical Onset of Huntington’s Disease
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“coli DNA mismatch repair gene mutL), whose mouse homolog, Mlh1, was discovered in a genome-wide genetic screen to modify somatic instability of the CAG repeat and the timing of CAG length-dependent phenotypes in the striatum of genetic HD replica CAG knockin mice (Pinto et al., 2013).”
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Tabrizi, S. J., Flower, M. D., Ross, C. A., & Wild, E. J. (2020) · Nature Reviews Neurology · doi:10.1038/s41582-020-0389-4 · Cited by 568
CAG Repeat Not Polyglutamine Length Determines Timing of Huntington’s Disease Onset
Lee, J., Correia, K., Loupe, J. M., Kim, K., Barker, D., Hong, E. P., Chao, M. J., Long, J. D., Lucente, D. E., Vonsattel, J. P., et al. (2019) · Cell · doi:10.1016/j.cell.2019.06.036 · Cited by 543
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Metadata
- Authors
- Pinto, R. M., Dragileva, E., Kirby, A., Lloret, A., Lopez, E., St. Claire, J., Panigrahi, G. B., Hou, C., Holloway, K., Gillis, T., Guide, J. R., Cohen, P. E., Li, G.-M., Pearson, C. E., Daly, M. J., & Wheeler, V. C.
- Affiliations
- Massachusetts General Hospital (US)
- Broad Institute (US)
- University of Toronto (CA)
- University of Kentucky (US)
- Cornell University (US)
- Corresponding author
- Wheeler, Vanessa C.
- ORCID
- Venue
- PLoS Genetics
- Publisher
- Public Library of Science (PLoS)
- Published
- 2013-10-31
- Volume
- 9
- Issue
- 10
- Pages
- e1003930
- ISSN
- 1553-7390 (ISSN-L)
- 1553-7404
- PMC
- PMC3814320
- OpenAlex
- W2107416433
- Cited by
- 250
- References
- 100
- Citations by year
- 2026: 20 · 2025: 24 · 2024: 32 · 2023: 26 · 2022: 12 · 2021: 30 · 2020: 27 · 2019: 8 · 2018: 19 · 2017: 16 · 2016: 17 · 2015: 11 · 2014: 7
- FWCI
- 5.66
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Locations
- PLoS Genetics (publishedVersion, cc-by)
- Europe PMC (PubMed Central) (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- UKnowledge (University of Kentucky) (submittedVersion, cc-by)
- Digital Access to Scholarship at Harvard (DASH) (Harvard University) (submittedVersion, cc-by)
- DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
- Figshare (submittedVersion, cc-by)
- Funders
- National Institutes of Health
- National Institute of General Medical Sciences (R01GM089684, GM089684, R01GM097263)
- National Institute of Neurological Disorders and Stroke (NS049206, R01NS049206)
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (R00HD065870)
- Topics
- Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Genetic Neurodegenerative Diseases
- Health Sciences › Medicine › Pathology and Forensic Medicine › Genetic factors in colorectal cancer
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › DNA Repair Mechanisms
- Keywords
- Huntington's disease, CAG repeat instability, somatic repeat expansion, DNA mismatch repair, MLH1, MLH3, MSH2, MSH3, genetic modifiers, strain differences, striatum, DNA repair
- MeSH terms
- MutL Proteins, MutL Protein Homolog 1, Animals, Carrier Proteins, Disease Models, Animal, Humans, Huntington Disease, Nuclear Proteins, RNA, Messenger, Trinucleotide Repeat Expansion, Genomic Instability, Adaptor Proteins, Signal Transducing, Mice, Genome-Wide Association Study, Genetic Association Studies
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