Slipped (CTG)*(CAG) repeats can be correctly repaired, escape repair or undergo error-prone repair.
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 beta 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.
Panigrahi
Gagan B
GB
Program of Genetics & Genomic Biology, The Hospital for Sick Children, 555 University Avenue, Elm Wing 11-135, Toronto, Ontario M5G 1X8, Canada.
Lau
Rachel
R
Montgomery
S Erin
SE
Leonard
Michelle R
MR
Pearson
Christopher E
CE
D002457:Cell Extracts / Q000235:genetics*; D004260:DNA Repair / Q000235:genetics*; D045643:DNA Repair Enzymes / Q000378:metabolism*; D004259:DNA-Directed DNA Polymerase / Q000378:metabolism; D004586:Electrophoresis; D030342:Genetic Diseases, Inborn / Q000235:genetics; D006367:HeLa Cells; D006801:Humans; D008957:Models, Genetic; D009154:Mutation / Q000235:genetics; D009474:Neurons / Q000166:cytology; D018709:Statistics, Nonparametric; D019680:Trinucleotide Repeat Expansion / Q000235:genetics*
D002457:Cell Extracts; D004259:DNA-Directed DNA Polymerase; D045643:DNA Repair Enzymes