RHINO restricts MMEJ activity to mitosis

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• DNA double-strand breaks (DSBs) are critical DNA lesions impacting genome stability.
• Non-homologous end-joining (NHEJ) and homologous recombination (HR) are primary DSB repair pathways in G1 and S/G2 phases, respectively.
• Microhomology-mediated end-joining (MMEJ) is an error-prone backup pathway, essential when NHEJ and HR are impaired.

Approach:

• Utilized CRISPR/Cas9-based synthetic lethal screens to identify key factors in MMEJ.
• Investigated the roles of the 9-1-1 complex (RAD9A-HUS1-RAD1) and RHINO in DSB repair.
• Performed mechanistic studies to elucidate the function of 9-1-1 and RHINO in M phase DNA repair.

Key Points:

• Identified MMEJ as the predominant DSB repair pathway during mitosis (M phase).
• Uncovered subunits of the 9-1-1 complex and RHINO as crucial MMEJ factors.
• Demonstrated RHINO's novel role in recruiting Polymerase theta (Polθ) to mitotic DSBs, independent of ATR signaling.
• Showed that mitotic MMEJ repairs S-phase DNA damage unresolved by HR, explaining synthetic lethality with POLQ and BRCA1/2.

Conclusions:

• Microhomology-mediated end-joining (MMEJ) is the major pathway for repairing DNA double-strand breaks during cell division (mitosis).
• RHINO plays a critical, previously unrecognized role in directing mutagenic MMEJ to M phase by interacting with Polymerase theta (Polθ).
• Findings provide insights into the synthetic lethality observed between POLQ and BRCA1/2 and the combined effects of Polθ and PARP inhibitors.