Human MutLγ, the MLH1-MLH3 heterodimer, is an endonuclease that promotes DNA expansion

Proc Natl Acad Sci U S A. 2020 Feb 18;117(7):3535-3542. doi: 10.1073/pnas.1914718117. Epub 2020 Feb 3.


MutL proteins are ubiquitous and play important roles in DNA metabolism. MutLγ (MLH1-MLH3 heterodimer) is a poorly understood member of the eukaryotic family of MutL proteins that has been implicated in triplet repeat expansion, but its action in this deleterious process has remained unknown. In humans, triplet repeat expansion is the molecular basis for ∼40 neurological disorders. In addition to MutLγ, triplet repeat expansion involves the mismatch recognition factor MutSβ (MSH2-MSH3 heterodimer). We show here that human MutLγ is an endonuclease that nicks DNA. Strikingly, incision of covalently closed, relaxed loop-containing DNA by human MutLγ is promoted by MutSβ and targeted to the strand opposite the loop. The resulting strand break licenses downstream events that lead to a DNA expansion event in human cell extracts. Our data imply that the mammalian MutLγ is a unique endonuclease that can initiate triplet repeat DNA expansions.

Keywords: DNA repair; MLH3; endonuclease; genome instability; triplet repeat expansion diseases.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • DNA Mismatch Repair
  • Dimerization
  • Endonucleases / chemistry
  • Endonucleases / genetics
  • Endonucleases / metabolism
  • Humans
  • MutL Protein Homolog 1 / chemistry
  • MutL Protein Homolog 1 / genetics
  • MutL Protein Homolog 1 / metabolism*
  • MutL Proteins / chemistry
  • MutL Proteins / genetics
  • MutL Proteins / metabolism*
  • Trinucleotide Repeat Expansion


  • MLH1 protein, human
  • MLH3 protein, human
  • Endonucleases
  • MutL Protein Homolog 1
  • MutL Proteins