MutLα heterodimers modify the molecular phenotype of Friedreich ataxia

Abstract

Background: Friedreich ataxia (FRDA), the most common autosomal recessive ataxia disorder, is caused by a dynamic GAA repeat expansion mutation within intron 1 of FXN gene, resulting in down-regulation of frataxin expression. Studies of cell and mouse models have revealed a role for the mismatch repair (MMR) MutS-heterodimer complexes and the PMS2 component of the MutLα complex in the dynamics of intergenerational and somatic GAA repeat expansions: MSH2, MSH3 and MSH6 promote GAA repeat expansions, while PMS2 inhibits GAA repeat expansions.

Methodology/principal findings: To determine the potential role of the other component of the MutLα complex, MLH1, in GAA repeat instability in FRDA, we have analyzed intergenerational and somatic GAA repeat expansions from FXN transgenic mice that have been crossed with Mlh1 deficient mice. We find that loss of Mlh1 activity reduces both intergenerational and somatic GAA repeat expansions. However, we also find that loss of either Mlh1 or Pms2 reduces FXN transcription, suggesting different mechanisms of action for Mlh1 and Pms2 on GAA repeat expansion dynamics and regulation of FXN transcription.

Conclusions/significance: Both MutLα components, PMS2 and MLH1, have now been shown to modify the molecular phenotype of FRDA. We propose that upregulation of MLH1 or PMS2 could be potential FRDA therapeutic approaches to increase FXN transcription.

Figure 1. Intergenerational GAA repeats.

Representative example of the ethidium bromide-stained agarose gels used to determine the GAA repeat sizes, showing GAA PCR products obtained from a YG22 GAA⁺/Mlh1 ^+/+ parent and 10 GAA⁺ offspring. M = 100 bp DNA size marker.

Figure 2. Intergenerational GAA repeat frequencies.

Analysis of intergenerational transmission of GAA repeat expansion frequency based on the parental genotype (WT = GAA⁺/Mlh1 ^+/+, Het = GAA⁺/Mlh1 ^+/−). Frequencies of ‘GAA expansions’, ‘no change’ and ‘GAA repeat contractions’ transmitted to offspring are represented as percentages of total GAA repeat transmissions. Chi squared (x ²) statistical testing was applied to this expriment. WT n = 30 GAA PCR products from 10 mice; Het n = 63 GAA PCR products from 21 mice; *** = p<0.001.

Figure 3. Effect of Mlh1 on somatic GAA repeat dynamics.

Representative image of the ethidium bromide-stained agarose gels used to determine GAA repeat expansion dynamics from different tissues of 3–5 month-old mice in absence of presence of Mlh1. M = 100 bp size marker, B = brain, C = cerebellum, H = heart, L = liver. WT (Mlh1^+/+) n = 2, KO (Mlh ^−/−) n = 7.

Figure 4. Effect of MutLα components on somatic FXN transcription in vivo.

Relative RT-qPCR analyses of somatic FXN transcription level based on the MMR genotype (WT, Pms2 KO or Mlh1 KO) in (A) FXN ^GAA/MMR brain tissues (n = 2–4), and (B) FXN ^GAA/MMR cerebellum tissues (n = 2–4). Statistical analysis of the experiment was perform using the student's t test. Error bars = S.E.M, * = p<0.05, ** = p<0.01, *** = p<0.001.

Figure 5. Effect of the MutLα-complex on FXN transcription in vitro.

Relative RT-qPCR analyses of the mean FXN transcriptional level, isolated from MMR-proficient human cells, NCM-460, and MutLα heterodimer-deficient cells, HCT-116 (n = 3). Statistical analysis of the experiment was perform using the student's t test. Error bars = S.E.M, * = p<0.05, ** = p<0.01, *** = p<0.001.

Figure 6. Proposed mechanism of MMR action on intergenetional GAA repeat expansions.

Schematic images representing: (A) a small loop caused by triplex DNA structure; (B) recognition of the loop by the MutS complex, cleavage with endonuclease (ENDO), opening of the loop, recruitment of MutLα and synthesis of expanded DNA, and (C) end of repair by ligation of the further expanded strand and release of MMR proteins.

Figure 7. Proposed mechanism of MMR action on post-mitotic somatic GAA repeat expansions.

Images represent: (A) a small GAA loop is formed as part of the triplex DNA•RNA R-loop structure caused by transcription within GAA repeats; (B) recognition of the small GAA loop by MutS-heterodimers, cleavage of the CTT DNA strand with an endonuclease (ENDO) and recruitment of MutLα, and (C) release of the RNA, synthesis of expanded DNA and end of repair by ligation of the expanded strand and release of MMR proteins.

Figure 8. Proposed mechanism of MLH1 action on FXN transcription.

Images illustrate: (A) inhibition of transcription by triplex DNA•RNA R-loop formation; (B) binding of MLH1 and MSH6 to the R-loop, together with other unknown factors (?), and (C) release of bound premature mRNA to allow continuation of transcription.