Isolated short CTG/CAG DNA slip-outs are repaired efficiently by hMutS , but clustered slip-outs are poorly repaired (original) (raw)
Related papers
Journal of Biological Chemistry, 2012
Background: Slipped-DNAs are mutagenic intermediates in disease-causing trinucleotide repeat instability; their processing is not well understood. Results: MutL␣ is required to repair single short slip-outs and enhances repair of clustered slip-outs. Conclusion: Aberrant mismatch repair attempts on clustered slip-outs may cause repeat instability. Significance: This work has determined one of the proteins involved in slipped-DNA repair, which is useful for understanding disease-causing repeat instability. Mismatch repair (MMR) is required for proper maintenance of the genome by protecting against mutations. The mismatch repair system has also been implicated as a driver of certain mutations, including disease-associated trinucleotide repeat instability. We recently revealed a requirement of hMutS in the repair of short slip-outs containing a single CTG repeat unit (1). The involvement of other MMR proteins in short trinucleotide repeat slip-out repair is unknown. Here we show that hMutL␣ is required for the highly efficient in vitro repair of single CTG repeat slip-outs, to the same degree as hMutS. HEK293T cell extracts, deficient in hMLH1, are unable to process single-repeat slip-outs, but are functional when complemented with hMutL␣. The MMR-deficient hMLH1 mutant, T117M, which has a point mutation proximal to the ATP-binding domain, is defective in slip-out repair, further supporting a requirement for hMLH1 in the processing of short slip-outs and possibly the involvement of hMHL1 ATPase activity. Extracts of hPMS2-deficient HEC-1-A cells, which express hMLH1, hMLH3, and hPMS1, are only functional when complemented with hMutL␣, indicating that neither hMutL nor hMutL␥ is sufficient to repair short slip-outs. The resolution of clustered short slip-outs, which are poorly repaired, was partially dependent upon a functional hMutL␣. The joint involvement of hMutS and hMutL␣ suggests that repeat instability may be the result of aberrant outcomes of repair attempts. Numerous hereditary neurological, neurodegenerative, and neuromuscular diseases including myotonic dystrophy type 1
Slipped (CTG)•(CAG) repeats can be correctly repaired, escape repair or undergo error-prone repair
Nature Structural & Molecular Biology, 2005
Expansion of (CTG)(CAG) repeats, the cause of 14 or more diseases, is presumed to arise through escaped repair of slipped DNAs. We report the fidelity of slipped-DNA repair using human cell extracts and DNAs with slip-outs of (CAG) 20 or (CTG) 20. Three outcomes occurred: correct repair, escaped repair and error-prone repair. The choice of repair path depended on nick location and slip-out composition (CAG or CTG). A new form of error-prone repair was detected whereby excess repeats were incompletely excised, constituting a previously unknown path to generate expansions but not deletions. Neuron-like cell extracts yielded each of the three repair outcomes, supporting a role for these processes in (CTG)(CAG) instability in patient post-mitotic brain cells. Mismatch repair (MMR) and nucleotide excision repair (NER) proteins hMSH2, hMSH3, hMLH1, XPF, XPG or polymerase b were not required-indicating that their role in instability may precede that of slip-out processing. Differential processing of slipped repeats may explain the differences in mutation patterns between various disease loci or tissues.
DNA Repair, 2008
Mammalian cells have evolved sophisticated DNA repair systems to correct mispaired or damaged bases and extrahelical loops. Emerging evidence suggests that, in some cases, the normal DNA repair machinery is "highjacked" to become a causative factor in mutation and disease, rather than act as a safeguard of genomic integrity. In this review, we consider two cases in which active MMR leads to mutation or to cell death. There may be similar mechanisms by which uncoupling of normal MMR recognition from downstream repair allows triplet expansions underlying human neurodegenerative disease, or cell death in response to chemical lesion.
DNA Triplet Repeat Expansion and Mismatch Repair
Annual Review of Biochemistry, 2015
DNA mismatch repair is a conserved antimutagenic pathway that maintains genomic stability through rectification of DNA replication errors and attenuation of chromosomal rearrangements. Paradoxically, mutagenic action of mismatch repair has been implicated as a cause of triplet repeat expansions that cause neurological diseases such as Huntington disease and myotonic dystrophy. This mutagenic process requires the mismatch recognition factor MutSβ and the MutLα (and/or possibly MutLγ) endonuclease, and is thought to be triggered by the transient formation of unusual DNA structures within the expanded triplet repeat element. This review summarizes the current knowledge of DNA mismatch repair involvement in triplet repeat expansion, which encompasses in vitro biochemical findings, cellular studies, and various in vivo transgenic animal model experiments. We present current mechanistic hypotheses regarding mismatch repair protein function in mediating triplet repeat expansions and discus...
CTG repeat instability and size variation timing in DNA repair-deficient mice
The EMBO Journal, 2003
Type 1 myotonic dystrophy is caused by the expansion of an unstable CTG repeat in the DMPK gene. We have investigated the molecular mechanisms underlying the CTG repeat instability by crossing transgenic mice carrying >300 unstable CTG repeats in their human chromatin environment with mice knockout for genes involved in various DNA repair pathways: Msh2 (mismatch repair), Rad52 and Rad54 (homologous recombination) and DNA-PKcs (nonhomologous end-joining). Genes of the non-homologous end-joining and homologous recombination pathways did not seem to affect repeat instability. Only lack of Rad52 led to a slight decrease in expansion range. Unexpectedly, the absence of Msh2 did not result in stabilization of the CTG repeats in our model. Instead, it shifted the instability towards contractions rather than expansions, both in tissues and through generations. Furthermore, we carefully analyzed repeat transmissions with different Msh2 genotypes to determine the timing of intergenerational instability. We found that instability over generations depends not only on parental germinal instability, but also on a second event taking place after fertilization.
Biochemistry, 2013
Expansions of (CTG)•(CAG) repeated DNAs are the mutagenic cause of 14 neurological diseases, likely arising through the formation and processing of slipped-strand DNAs. These transient intermediates of repeat length mutations are formed by out-of-register mispairing of repeat units on complementary strands. The three-way slipped-DNA junction, at which the excess repeats slip out from the duplex, is a poorly understood feature common to these mutagenic intermediates. Here, we reveal that slipped junctions can assume a surprising number of interconverting conformations where the strand opposite the slip-out either is fully base paired or has one or two unpaired nucleotides. These unpaired nucleotides can also arise opposite either of the nonslipped junction arms. Junction conformation can affect binding by various structure-specific DNA repair proteins and can also alter correct nick-directed repair levels. Junctions that have the potential to contain unpaired nucleotides are repaired with a significantly higher efficiency than constrained fully paired junctions. Surprisingly, certain junction conformations are aberrantly repaired to expansion mutations: misdirection of repair to the nonnicked strand opposite the slip-out leads to integration of the excess slipped-out repeats rather than their excision. Thus, slippedjunction structure can determine whether repair attempts lead to correction or expansion mutations.
PLoS ONE, 2013
Trinucleotide repeat (TNR) expansions and deletions are associated with human neurodegeneration and cancer. However, their underlying mechanisms remain to be elucidated. Recent studies have demonstrated that CAG repeat expansions can be initiated by oxidative DNA base damage and fulfilled by base excision repair (BER), suggesting active roles for oxidative DNA damage and BER in TNR instability. Here, we provide the first evidence that oxidative DNA damage can induce CTG repeat deletions along with limited expansions in human cells. Biochemical characterization of BER in the context of (CTG) 20 repeats further revealed that repeat instability correlated with the position of a base lesion in the repeat tract. A lesion located at the 59-end of CTG repeats resulted in expansion, whereas a lesion located either in the middle or the 39-end of the repeats led to deletions only. The positioning effects appeared to be determined by the formation of hairpins at various locations on the template and the damaged strands that were bypassed by DNA polymerase b and processed by flap endonuclease 1 with different efficiency. Our study indicates that the position of a DNA base lesion governs whether TNR is expanded or deleted through BER.
DNA Repair, 2016
-CAG/CTG triplet repeats block replication in both orientations on a yeast chromosome -Replication fork stalling depends on mismatch repair integrity -Msh2p is enriched at CAG/CTG triplet repeats in both orientations -MSH2 overexpression favors or stabilizes the formation of heteroduplex molecules Abstract Trinucleotide repeat expansions are responsible for at least two dozen neurological disorders.
PLoS Genetics, 2013
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 strainspecific 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 MutLc (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 MutSb (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.
Molecular and General Genetics MGG, 1999
Trinucleotide repeats undergo contractions and expansions in humans, leading in some cases to fatal neurological disorders. The mechanism responsible for these large size variations is unknown, but replicationslippage events are often suggested as a possible source of instability. We constructed a genetic screen that allowed us to detect spontaneous expansions/contractions of a short trinucleotide repeat in yeast. We show that deletion of RAD27, a gene involved in the processing of Okazaki fragments, increases the frequency of contractions tenfold. Repair of a chromosomal double-strand break (DSB) using a trinucleotide repeat-containing template induces rearrangements of the repeat with a frequency 60 times higher than the natural rate of instability of the same repeat. Our data suggest that both gene conversion and single-strand annealing are major sources of trinucleotide repeat rearrangements.