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Cantor Lab Uncovers New Vulnerability in BRCA1-deficient Cancers

Wednesday, October 01, 2025

New research from the lab of Sharon Cantor, PhD, has revealed an unexpected, targetable vulnerability in cells that are defective for BRCA1, a key gene mutated in many inherited breast and ovarian cancers. The discovery offers insight into potential therapeutic strategies for BRCA1-mutant cancers.

The study builds upon the lab’s prior work identifying single-stranded DNA replication gaps —or, more specifically, the failure to repair or suppress these gaps—as the core defect in BRCA-deficient cells that underlies their sensitivity to genotoxic chemotherapeutic agents, such as PARP inhibitors (PARPi). These findings have challenged the long-held model that chemosensitivity arises from an inability to repair double-stranded DNA breaks, upending a central dogma in the field.

Sequestered at the gaps: RAD51 in an unexpected role

Graphic abstract — Graphical abstract from Peng et al. illustrating how the relative levels of RAD51 chromatin enrichment in BRCA1-proficient and BRCA1-deficient cells impact overall RAD51 availability and, consequently, the formation of IR-induced RAD51 foci.
From: RAD51 is chromatin enriched and targetable in BRCA1-deficient cells. Min Peng, Silviana Lee, Hitha Gopalan Nair, Nathan MacGilvary, Ke Cong, Michitsura Kraemer, Rui Li, Jill McConnell, Christina Baer, Bin Deng, Lihua Zhu, Sharon B. Cantor. *Molecular Cell 85, 1–15 2025,* https://www.sciencedirect.com/science/article/pii/S1097276525007026
Creative Commons (CC)-BY 4.0

The new study, published in Molecular Cell, pushes this gap-centric model further by uncovering a surprising role for RAD51, a protein traditionally linked to double-stranded DNA break repair. Under conditions that induce double-stranded breaks, such as ionizing radiation (IR), DNA damage-sensing kinases phosphorylate histone H2AX to generate γH2AX, which acts as a beacon to recruit DNA repair proteins, including MDC1 and 53BP1. In BRCA1-proficient cells, BRCA1 recruitment promotes end resection, generating a single-stranded DNA substrate for RAD51, which binds to the DNA and forms a nucleoprotein filament that promotes strand invasion and homologous recombination (HR) repair. These RAD51 filaments can be visualized microscopically as nuclear foci.

BRCA-deficient tumors lack RAD51 foci, a phenomenon traditionally attributed to a failure to load RAD51 in the absence of BRCA1-mediated end resection. However, Dr. Cantor’s study reveals an additional mechanism restricting RAD51 HR activity: in BRCA1-deficient cells, RAD51 is hyper-engaged on chromatin at single-stranded DNA replication gaps, which are present genome-wide, effectively depleting the pool of RAD51 available to form IR-induced foci.

What is the basis of increased RAD51 chromatin enrichment in BRCA1-deficient cells? Dr. Cantor’s group showed that RAD51 enrichment requires H2AX, MDC1 and 53BP1, which are also chromatin-enriched in BRCA1-deficient cells. Loss of H2AX, MDC1, or 53BP1 reduces replication gaps, indicating these factors promote gap formation in BRCA1-deficient cells.

Their findings support an intriguing model. In BRCA1-proficient cells, BRCA1 represses the replisome recruitment of H2AX, MDC1, and 53BP1, which in turn limits RAD51 engagement with chromatin and preserves a free pool of RAD51 for recruitment to sites of DNA damage and the formation of IR-induced RAD51 foci. By contrast, in BRCA1-deficient cells, H2AX, MDC1, and 53BP1 accumulate at replisomes and promote replication gap formation, leading to RAD51 enrichment at these gaps. This sequestration exhausts the available pool of RAD51, limiting its ability to form IR-induced foci.

Therapeutic implications

Unexpectedly, the study revealed that RAD51 is essential for the viability of BRCA1-deficient cells, perhaps by protecting replication gaps from expansion, which would otherwise compromise genome integrity. Accordingly, they found that treatment of BRCA1-deficient cells with small molecule RAD51 inhibitors, which disrupt either filament formation or DNA binding, abrogates RAD51 association with chromatin and decreases cell survival. These findings have translational implications for the use of RAD51 inhibitors to treat BRCA1-mutant cancers.

The findings also underscore the need to re-evaluate the biomarkers currently used to predict patient response to PARPi therapy. RAD51 foci have been widely used as a surrogate marker of HR repair efficiency and predictor of PARPi sensitivity: their presence is typically interpreted as evidence of functional HR repair and thus resistance to PARPi monotherapy. However, Dr. Cantor’s group showed that RAD51 foci formation can be uncoupled from PARPi resistance. Specifically, they demonstrated that loss of MDC1 or H2AX confers resistance to PARPi without restoring RAD51 foci. Instead, they found that PARPi response correlates more accurately with RAD51 chromatin enrichment than with foci formation, suggesting that chromatin-bound RAD51 may serve as a more reliable biomarker for predicting patient response to PARPi therapy.

Read the open access article here.

Headshot of Sharon Cantor, PhD

These findings overturn the idea that BRCA1-mutant cancers lack RAD51 activity. Instead, RAD51 is diverted to gaps, creating both a vulnerability and a biomarker we can exploit.

—Sharon Cantor, PhD

From dogma to discovery: Additional related reading

Over the past few years, the Cantor Lab has steadily challenged the view that BRCA1-mutant cancers are vulnerable to PARPi because they fail to repair DNA double-stranded breaks, showing instead that the failure to repair or suppress single-stranded DNA replication gaps lies at the heart of BRCA1 deficiency and PARPi therapy response.

Interested in learning more? Click to view a selection of related papers.

Replication gaps underlie BRCA deficiency and therapy response. Panzarino NJ, Krais JJ, Cong K, Peng M, Mosqueda M, Nayak SU, Bond SM, Calvo JA, Doshi MB, Bere M, Ou J, Deng B, Zhu LJ, Johnson N, Cantor SB. Cancer Research 2021, 81: 1388.
This study introduced the gap-centric model that single-stranded DNA gaps underlie “BRCAness" and are fundamental to the mechanism-of-action of genotoxic chemotherapies. This ground-breaking paper was one of the five most-cited research articles published in 2021-2022 in the AACR journal Cancer Research.

Replication gaps are a key determinant of PARP inhibitor synthetic lethality with BRCA deficiency. Cong K, Peng M, Kousholt AN, Lee WTC, Lee S, Nayak S, Krais J, VanderVere-Carozza PS, Pawelczak KS, Calvo J, Panzarino NJ, Turchi JJ, Johnson N, Jonkers J, Rothenberg E, Cantor SB. Molecular Cell 2021, 81: 3128.
By comparing BRCA1- and FANCJ-deficient cells, which have common repair defects but distinct responses to PARP inhibitors, the Cantor Lab identified the presence of single-stranded DNA gaps as the distinguishing factor that sensitizes BRCA1-deficient cells to PARP inhibition, changing the paradigm for PARP inhibitor synthetic lethal interactions.

Inhibition of translesion synthesis polymerase REV1 exploits replication gaps as a cancer vulnerability. Nayak S, Calvo JA, Cong K, Peng M, Berthiaume E, Jackson J, Zaino AM, Vindigni A, Hadden MK, Cantor SB. Science Advances 2020, 6: eaaz7808.
In this study, the Cantor Lab demonstrated that translesion synthesis (TLS) drives chemoresistance and showed that a small-molecule inhibitor targeting the TLS factor REV1 not only disrupts DNA replication and cancer cell fitness but also synergizes with gap-inducing therapies such as inhibitors of ATR or Wee1. This work illuminated that gap suppression during replication is critical for cancer cell fitness and therefore a targetable vulnerability.

Targeting BRCA1-deficient PARP inhibitor-resistant cells with nickases reveals nick resection as a cancer vulnerability. Whalen JM, Earley J, Wisniewski C, Mercurio AM, Cantor SB. Nature Cancer 2025, 6: 278.
Using CRISPR-mediated genome engineering, the Cantor Lab introduced single-stranded DNA nicks into breast and ovarian cancer cells with and without BRCA mutations. They found that BRCA-deficient cells were sensitive to nicks and that in these cells, the nicks expand into large single-stranded DNA gaps (through the process of nick resection) and cause the cells to die—not because double-strand breaks occur, but because the gaps themselves are lethal.