Pms2 Suppresses Large Expansions of the (GAA·TTC)n Sequence in Neuronal Tissues

Abstract

Expanded trinucleotide repeat sequences are the cause of several inherited neurodegenerative diseases. Disease pathogenesis is correlated with several features of somatic instability of these sequences, including further large expansions in postmitotic tissues. The presence of somatic expansions in postmitotic tissues is consistent with DNA repair being a major determinant of somatic instability. Indeed, proteins in the mismatch repair (MMR) pathway are required for instability of the expanded (CAG·CTG)n sequence, likely via recognition of intrastrand hairpins by MutSβ. It is not clear if or how MMR would affect instability of disease-causing expanded trinucleotide repeat sequences that adopt secondary structures other than hairpins, such as the triplex/R-loop forming (GAA·TTC)n sequence that causes Friedreich ataxia. We analyzed somatic instability in transgenic mice that carry an expanded (GAA·TTC)n sequence in the context of the human FXN locus and lack the individual MMR proteins Msh2, Msh6 or Pms2. The absence of Msh2 or Msh6 resulted in a dramatic reduction in somatic mutations, indicating that mammalian MMR promotes instability of the (GAA·TTC)n sequence via MutSα. The absence of Pms2 resulted in increased accumulation of large expansions in the nervous system (cerebellum, cerebrum, and dorsal root ganglia) but not in non-neuronal tissues (heart and kidney), without affecting the prevalence of contractions. Pms2 suppressed large expansions specifically in tissues showing MutSα-dependent somatic instability, suggesting that they may act on the same lesion or structure associated with the expanded (GAA·TTC)n sequence. We conclude that Pms2 specifically suppresses large expansions of a pathogenic trinucleotide repeat sequence in neuronal tissues, possibly acting independently of the canonical MMR pathway.

Repeat instability during DNA repair: Insights from model systems

“…Some differences are seen between specific models that may reflect the effect of sequence context, since cis -acting sequences affect expansion (Brock et al ., 1999; Cleary et al ., 2002; Libby et al ., 2003), or genetic background, since mouse strains differ significantly in the levels of expression of key MMR proteins (Pinto et al ., 2013; Tome et al ., 2013). Even in the case of FRDA in which none of the MutS components seem to play a role in intergenerational expansion in the transgenic mouse model (Ezzatizadeh et al ., 2012), loss of either of the MutSα components reduces somatic expansions (Bourn et al ., 2012). Furthermore, knockdown of MutSα reduces expansions of the endogenous FRDA allele in patient derived induced pluripotent stem cells (iPSCs) (Du et al ., 2012; Ku et al ., 2010) and expression of MutSβ promotes GAA expansions in a human kidney cell line (Halabi et al ., 2012).…”

“…However, in a DM1 mouse model, PMS2, the binding partner of Mlh1 in MutLα, is required for 50% of somatic expansions (Gomes-Pereira et al ., 2004). Despite the lack of a role for MutSα or MutSβ in intergenerationally transmitted expansions in the FRDA mouse (Bourn et al ., 2012; Ezzatizadeh et al ., 2012), Mlh1 is required for both germ line and somatic expansions (Ezzatizadeh et al ., 2014). Since PMS2 (MutLα), protects against expansions in this model (Bourn et al ., 2012) this suggests that there is also a role for MutLγ in FRDA expansions.…”

Tissue specificity in DNA repair: lessons from trinucleotide repeat instability

“…The mismatch repair proteins MSH2, MSH3, MLH1, MLH3, and PMS2 have all been shown to modulate TNR instability in mouse models with CAG, CTG, GAA, and CGG expansions [21,[29][30][31][32][33][34][35][36][37][38][39][50][51][52][53]. In addition, MSH6 is involved in GAA repeat instability but appears to have little effect on CAG/CTG instability [35,38,[51][52][53].…”

“…In addition, MSH6 is involved in GAA repeat instability but appears to have little effect on CAG/CTG instability [35,38,[51][52][53]. These results prompted a systematic investigation of the levels of MSH2, MSH3 and MSH6 in several murine tissues [54], but no obvious correlation was found between instability phenotypes and protein levels.…”

Trinucleotide Repeat Instability is Modulated by DNA Base Lesions and DNA Base Excision Repair

“…In a transgenic HD mouse model, CAG expansion was found to be dependent on MSH2/MSH3, and expansion was completely abolished in the MSH2 or MSH3 knockout mice in both the somatic and germ cells (177,178,(181)(182)(183)(184). MSH2/MSH3 has also been found to be required for expansion of CTG repeats in a mouse model for MD (179,180,(185)(186)(187), the GAA repeats in the FXN gene associated with FRDA (105,(188)(189)(190)(191)(192), and the CGG repeats associated with FXS (193). The MSH3 protein was found to be particularly important for the progressive, incremental (as small as 1 repeat unit) expansions that promote the transition into the pathogenic range of TNR length, while knockout of MSH3 resulted in more stable repeats in a yeast system (194).…”