Microhomology-mediated end joining

Microhomology-mediated end joining (MMEJ), also known as alternative nonhomologous end-joining (Alt-NHEJ) is one of the pathways for repairing double-strand breaks in DNA. As reviewed by McVey and Lee,[1] the foremost distinguishing property of MMEJ is the use of microhomologous sequences during the alignment of broken ends before joining, thereby resulting in deletions flanking the original break. MMEJ is frequently associated with chromosome abnormalities such as deletions, translocations, inversions and other complex rearrangements.

There are multiple pathways for repairing double strand breaks, mainly non-homologous end joining (NHEJ), homologous recombination (HR), and MMEJ. NHEJ directly joins both ends of the double strand break and is relatively accurate, although small (usually less than a few nucleotides) insertions or deletions sometimes occur. HR is highly accurate and uses the sister chromatid as a template for accurate repair of the DSB. MMEJ is distinguished from these other repair mechanisms by its use of microhomologous sequences to align the broken strands. This results in frequent deletions and occasionally insertions which are much larger than those produced by NHEJ [citation needed]. MMEJ is completely independent from classical NHEJ and does not rely on NHEJ core factors such as Ku protein, DNA-PK, or Ligase IV.[2]

In MMEJ, repair of the DSB is initiated by end resection by the MRE nuclease, leaving single stranded overhangs.[3] These single stranded overhangs anneal at microhomologies, which are short regions of complementarity, often 5–25 base pairs, between the two strands. A specialized form of MMEJ, called polymerase theta-mediated end-joining (TMEJ), is able to repair breaks using ≥1 bp of homology.[4][5] The helicase domain of DNA polymerase theta possesses ATP-dependent single-strand annealing activity and may promote annealing of microhomologies.[6] Following annealing, any overhanging bases (flaps) are removed by nucleases such as Fen1 and gaps are filled in by DNA polymerase theta.[7] This gap-filling ability of polymerase theta helps to stabilize the annealing of ends with minimal complementarity. Besides microhomology footprints, polymerase theta's mutational signature also consists of (infrequent) templated inserts, which are thought to be the result of aborted template-dependent extension, followed by re-annealing at secondary homologous sequences.[5]

  1. ^ McVey M, Lee SE (November 2008). "MMEJ repair of double-strand breaks (director's cut): deleted sequences and alternative endings". Trends in Genetics. 24 (11): 529–538. doi:10.1016/j.tig.2008.08.007. PMC 5303623. PMID 18809224.
  2. ^ Simsek D, Jasin M (April 2010). "Alternative end-joining is suppressed by the canonical NHEJ component Xrcc4-ligase IV during chromosomal translocation formation". Nature Structural & Molecular Biology. 17 (4): 410–416. doi:10.1038/nsmb.1773. PMC 3893185. PMID 20208544.
  3. ^ Truong LN, Li Y, Shi LZ, Hwang PY, He J, Wang H, et al. (May 2013). "Microhomology-mediated End Joining and Homologous Recombination share the initial end resection step to repair DNA double-strand breaks in mammalian cells". Proceedings of the National Academy of Sciences of the United States of America. 110 (19): 7720–7725. Bibcode:2013PNAS..110.7720T. doi:10.1073/pnas.1213431110. PMC 3651503. PMID 23610439.
  4. ^ Roerink SF, van Schendel R, Tijsterman M (June 2014). "Polymerase theta-mediated end joining of replication-associated DNA breaks in C. elegans". Genome Research. 24 (6): 954–962. doi:10.1101/gr.170431.113. PMC 4032859. PMID 24614976.
  5. ^ a b Schimmel J, van Schendel R, den Dunnen JT, Tijsterman M (September 2019). "Templated Insertions: A Smoking Gun for Polymerase Theta-Mediated End Joining". Trends in Genetics. 35 (9): 632–644. doi:10.1016/j.tig.2019.06.001. PMID 31296341. S2CID 195892718.
  6. ^ Mateos-Gomez PA, Kent T, Deng SK, McDevitt S, Kashkina E, Hoang TM, et al. (December 2017). "The helicase domain of Polθ counteracts RPA to promote alt-NHEJ". Nature Structural & Molecular Biology. 24 (12): 1116–1123. doi:10.1038/nsmb.3494. PMC 6047744. PMID 29058711.
  7. ^ Sfeir A, Symington LS (November 2015). "Microhomology-Mediated End Joining: A Back-up Survival Mechanism or Dedicated Pathway?". Trends in Biochemical Sciences. 40 (11): 701–714. doi:10.1016/j.tibs.2015.08.006. PMC 4638128. PMID 26439531.