All three mammalian MutL complexes are required for repeat expansion in a mouse cell model of the Fragile X-related disorders

Abstract

Expansion of a CGG-repeat tract in the 5' untranslated region of the FMR1 gene causes the fragile X-related disorders (FXDs; aka the FMR1 disorders). The expansion mechanism is likely shared by the 35+ other diseases resulting from expansion of a disease-specific microsatellite, but many steps in this process are unknown. We have shown previously that expansion is dependent upon functional mismatch repair proteins, including an absolute requirement for MutLγ, one of the three MutL heterodimeric complexes found in mammalian cells. We demonstrate here that both MutLα and MutLβ, the two other MutL complexes present in mammalian cells, are also required for most, if not all, expansions in a mouse embryonic stem cell model of the FXDs. A role for MutLα and MutLβ is consistent with human GWA studies implicating these complexes as modifiers of expansion risk in other Repeat Expansion Diseases. The requirement for all three complexes suggests a novel model in which these complexes cooperate to generate expansions. It also suggests that the PMS1 subunit of MutLβ may be a reasonable therapeutic target in those diseases in which somatic expansion is an important disease modifier.

DNA Mismatch Repair and its Role in Huntington’s Disease

“…The importance of MutL␣ in the MMR reaction is highlighted by the observation that loss of MLH1 or PMS2 not only results in elevation of mutation rate, but also increases the lifetime risk of tumorigenesis in both mice and humans [77]. Interestingly, both MLH1 and PMS2 have been identified as onset modifiers of HD, and loss of Mlh1 or Pms2 substantially attenuates somatic triplet repeat expansion in animal and cellular models of HD, myotonic dystrophy type 1 (DM1), and Fragile X-related disorders (FXDs) [78][79][80].…”

“…The diverse genome-stabilizing activities of MMR notwithstanding, this pathway has also been implicated in mutation production in the context of neurodegenerative disease. The most compelling corpus of data in this regard pertains to HD and will be considered here, although recent findings have also suggested a role for MMR in the repeat expansions underlying FXDs and Friedreich's ataxia [78,104,[144][145][146][147].…”

Modifiers of Somatic Repeat Instability in Mouse Models of Friedreich Ataxia and the Fragile X-Related Disorders: Implications for the Mechanism of Somatic Expansion in Huntington’s Disease

“…Given the importance of MLH1 in expansions, we can infer that one or more of the other MLH1-binding partners, PMS1 and/or MLH3, must play a role in expansions. In the case of the FXD mouse, all three MLH1-binding proteins, PMS1, PMS2 and MLH3, are required for expansion since the loss of any one of these proteins eliminates expansions either in vivo or in embryonic stem cells derived from these animals [59,70]. Furthermore, a point mutation (D1185N) in the nuclease domain of MLH3 also eliminates all expansions in FXD mouse embryonic stem cells [71], suggesting that the nuclease activity of MutL␥ is required.…”

Histone deacetylase knockouts modify transcription, CAG instability and nuclear pathology in Huntington disease mice

“…Overall, our results support the conclusion that Hdac2 and Hdac3 are moderate enhancers of CAG expansion in MSNs of Htt Q111/+ mice, with the impact of Hdac2 KO appearing to depend on the age of the mice. To explore a possible mechanism for instability modification, we investigated the RNA-seq data for expression levels of DNA repair genes that modify repeat instability in mice ( Kovalenko et al, 2012 ; Dragileva et al, 2009 ; Pinto et al, 2013 ; Kovtun et al, 2007 ; Mollersen et al, 2012 ; Zhao and Usdin, 2018 ; Miller et al, 2020 ). Of these, only Msh3 mRNA levels were modestly decreased in Htt Q111/+ Hdac2 ΚO striata relative to Htt Q111 /+ Hdac2 KO striata (nominal p<0.01; adjusted p=0.07) ( Figure 6—figure supplement 3A, B ).…”