Genetic separation of Brca1 functions reveal mutation-dependent Polθ vulnerabilities

Area of Science:

• Genetics
• Molecular Biology
• Cancer Research

Background:

• Homologous recombination (HR) deficiency in cancer cells can lead to a dependency on DNA polymerase theta (Polθ) for survival.
• Polθ inhibitors (Polθi) are being developed as targeted cancer therapies, particularly for HR-deficient tumors.
• The precise mechanisms and determinants of synthetic lethality between HR defects and Polθ inhibition are not fully understood.

Purpose of the Study:

• To mechanistically define the synthetic lethality between BRCA1 functional defects and Polθ dependence using mouse models.
• To investigate the role of DNA end resection in mediating sensitivity to Polθ inhibitors in HR-deficient cells.
• To identify factors influencing patient selection for Polθ inhibitor-based cancer therapies.

Main Methods:

• Utilized mouse models with distinct Brca1 functional defects to study HR-deficiency and Polθ-dependence.
• Assessed cell viability, chromosomal instability, and DNA repair pathway activation (RPA foci) in response to Polθ inhibition.
• Compared Polθ inhibitor sensitivity in isogenic cell lines with mutations in BRCA1, PALB2, and BRCA2 to evaluate the role of DNA end resection.

Main Results:

• Homozygous Brca1 mutant, Polq^-/- cells were viable but exhibited slow growth and chromosomal instability.
• Brca1 mutant cells with proficient DNA end resection showed significantly higher dependence on Polθ for viability.
• Treatment with Polθ inhibitors in these cells led to persistent RPA foci throughout mitosis.
• PALB2 and BRCA2 mutant cells, unlike BRCA1 null cells, displayed active resection and greater sensitivity to Polθ inhibitors.

Conclusions:

• DNA end resection is a critical determinant of Polθ inhibitor sensitivity in HR-deficient cancer cells.
• The degree of DNA end resection influences the synthetic lethality between HR deficiency and Polθ inhibition.
• These findings highlight the importance of assessing DNA end resection status for patient stratification in clinical trials involving Polθ inhibitors.