RHINO directs MMEJ to repair DNA breaks in mitosis

Area of Science:

• Molecular Biology
• Genetics
• Cancer Research

Background:

• DNA double-strand breaks (DSBs) are primarily repaired by nonhomologous end-joining (NHEJ) and homologous recombination (HR) during interphase.
• Microhomology-mediated end-joining (MMEJ) was considered a secondary DNA repair pathway.
• The 9-1-1 complex (RAD9A-RAD1-HUS1) and RHINO are implicated in DNA damage response.

Purpose of the Study:

• To identify novel factors involved in microhomology-mediated end-joining (MMEJ) DNA repair.
• To elucidate the regulatory mechanisms controlling MMEJ during the cell cycle.
• To explore the therapeutic implications of MMEJ pathway targeting in cancer.

Main Methods:

• CRISPR-Cas9-based synthetic lethal screens were employed in cancer cells.
• Immunofluorescence and co-immunoprecipitation assays were used to study protein localization and interactions.
• Analysis of MMEJ activity in different cell cycle phases.

Main Results:

• Subunits of the 9-1-1 complex and RHINO were identified as essential MMEJ factors.
• RHINO restricts MMEJ to mitosis, accumulating and undergoing phosphorylation by PLK1 during M phase.
• RHINO facilitates the recruitment of polymerase θ (Polθ) to DSBs in mitosis, repairing persistent S-phase-induced DSBs.

Conclusions:

• RHINO plays a critical role in cell cycle-dependent regulation of MMEJ.
• MMEJ in mitosis repairs residual DSBs, highlighting its importance beyond a backup mechanism.
• Findings provide insights into synthetic lethality involving POLQ, BRCA1/2, and the efficacy of Polθ and PARP inhibitors.