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Polλ promotes microhomology-mediated end-joining.

Gurushankar Chandramouly1, Joonas Jamsen2, Nikita Borisonnik1

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|December 19, 2022
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Summary
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DNA polymerase lambda (Polλ) drives a distinct microhomology-mediated end-joining (MMEJ) DNA repair pathway, separate from Polθ and NHEJ factors. This discovery reveals a new mechanism for double-strand break repair.

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Area of Science:

  • Molecular Biology
  • DNA Repair Mechanisms
  • Structural Biology

Background:

  • Microhomology-mediated end-joining (MMEJ) is a DNA double-strand break (DSB) repair pathway.
  • MMEJ is traditionally considered dependent on DNA polymerase theta (Polθ) and independent of nonhomologous end-joining (NHEJ) factors.
  • The existence of alternative MMEJ pathways remained an open question.

Purpose of the Study:

  • To investigate whether MMEJ is exclusively mediated by Polθ or if other pathways exist.
  • To explore the role of DNA polymerase lambda (Polλ) in MMEJ.
  • To elucidate the mechanism and structural basis of Polλ-mediated MMEJ.

Main Methods:

  • Cellular assays in mammalian cells to assess MMEJ activity.
  • Genetic analysis involving NHEJ factors (LIG4/XRCC4) and Polθ.
  • X-ray crystallography of Polλ at 2.0 Å resolution using in situ photo-induced DSB formation.

Main Results:

  • Human Polλ exhibits robust MMEJ activity, similar to Polθ.
  • Polλ promotes MMEJ independently of essential NHEJ factors (LIG4/XRCC4) and Polθ, indicating a distinct Polλ-dependent MMEJ mechanism.
  • X-ray crystallography captured Polλ stabilizing a microhomology DNA synapse and performing replication across it.
  • Polλ was found to be semisynthetic lethal with BRCA1 and BRCA2.

Conclusions:

  • DNA polymerase lambda (Polλ) mediates a distinct MMEJ pathway for DNA double-strand break repair.
  • This Polλ-dependent MMEJ mechanism operates independently of canonical NHEJ factors and Polθ.
  • The structural insights reveal how Polλ functions in joining DNA ends via microhomology recognition and replication.
  • Polλ represents a potential therapeutic target in cancer, given its synthetic lethality with BRCA1/2.