DNA Repair Enzyme Pol ι Emerges as Key Player in Early Breast Cancer Risk in BRCA2 Carriers

Women who inherit a single faulty copy of the BRCA2 gene face a dramatically elevated lifetime risk of breast, ovarian, and prostate cancers, yet the earliest molecular events that push normal breast tissue toward malignancy have remained frustratingly obscure. A new study published in iScience now reveals that an unusual, error-prone DNA polymerase called polymerase iota, or Pol ι, steps into the breach when BRCA2 levels fall, reshaping how non-cancerous mammary epithelial cells cope with the chronic DNA replication stress that defines BRCA2 deficiency. The findings, from a team led by Kavya Vipparthi and Mihriban Karaayvaz at the Cleveland Clinic, suggest that this adaptive response may help explain how breast tissue from mutation carriers begins its slide toward cancer long before any tumor appears.

BRCA2 is best known as a guardian of the genome. The protein it encodes is essential for homologous recombination, the high-fidelity repair mechanism cells use to mend dangerous double-strand breaks in DNA. But BRCA2 does more than patch breaks. It also stabilizes stalled replication forks, the Y-shaped structures that unzip the double helix so each strand can be copied, and it suppresses the accumulation of single-stranded DNA gaps that arise when replication runs into trouble. When BRCA2 function drops, even partially, cells accumulate under-replicated DNA, and this chronic replication stress creates a permissive environment for cancer initiation. Crucially, breast epithelium from BRCA2 mutation carriers shows signs of functional haploinsufficiency, meaning that a single working copy of the gene is not enough to fully maintain genome stability.

The research team began by mining a publicly available single-cell RNA sequencing dataset of primary breast epithelial cells from BRCA2 mutation carriers and non-carrier controls. They examined the expression of four Y-family translesion synthesis polymerases, REV1, Pol κ, Pol η, and Pol ι, across the major epithelial compartments of the breast, including basal or myoepithelial cells, alveolar cells, and hormone-sensing luminal cells. Among all four enzymes, POLI, the gene encoding Pol ι, stood out with the most striking genotype-dependent difference. The proportion of POLI-positive hormone-sensing epithelial cells was selectively and significantly enriched in samples from BRCA2 mutation carriers, while expression of the other three polymerases remained essentially unchanged in that compartment.

To confirm that this induction was not merely a correlation, the researchers silenced BRCA2 with small interfering RNA in non-transformed MCF10A mammary epithelial cells, in primary human mammary epithelial cells, and in MCF7 breast cancer cells. In every model, BRCA2 depletion produced a marked increase in nuclear Pol ι protein, detected by immunofluorescence and quantified across hundreds of individual cells. Cell-cycle profiling showed that the increase was not confined to a single phase: Pol ι fluorescence intensity rose across G1, S, and G2/M after BRCA2 knockdown. However, cells with the highest Pol ι levels were significantly enriched in G2/M, the phase immediately preceding cell division, hinting at a role for the enzyme late in the cell cycle. Analysis of primary hormone-sensing cells from BRCA2 mutation carriers reinforced this picture, revealing a greater proportion of Pol ι-high cells in S and G2/M phases, and these Pol ι-high cells carried transcriptional signatures associated with mitosis.

That timing mattered, because it pointed the investigators toward a specialized salvage pathway known as mitotic DNA synthesis, or MiDAS. When replication stress leaves stretches of DNA uncopied by the end of S phase, some cells complete the job during mitosis itself, particularly at common fragile sites, genomic regions that are intrinsically difficult to replicate. The canonical mediator of this process is RAD52, a recombination protein. To test whether Pol ι also contributes, the team depleted BRCA2, POLI, or both in MCF10A cells, applied a mild replication stress with low-dose aphidicolin, a drug that slows the replicative polymerase, and measured DNA synthesis in mitotic cells using incorporation of the nucleotide analog EdU alongside FANCD2 foci, which mark under-replicated DNA. BRCA2 depletion sharply increased both EdU incorporation and FANCD2 foci in mitotic cells, and combined BRCA2 and POLI depletion pulled both readouts back down toward control levels. POLI depletion alone had little effect in BRCA2-proficient cells, and the reduction achieved by removing Pol ι was comparable to that achieved by removing RAD52, although RAD52 produced a somewhat stronger overall effect.

Then came the surprise. Post-S-phase DNA synthesis is generally regarded as protective, a last-ditch mechanism that prevents under-replicated DNA from being dragged into mitosis and causing chromosome breakage. If Pol ι were performing that protective function, removing it should have made BRCA2-deficient cells even more genomically unstable. Instead, the opposite happened. When the team quantified 53BP1 nuclear bodies, well-established markers of under-replicated DNA that persist into the next G1 phase, combined BRCA2 and POLI depletion did not increase their formation and in fact trended toward fewer nuclear bodies than BRCA2 depletion alone. Micronucleus assays, which detect chromosome fragments that fail to segregate properly, told the same story, and metaphase spreads revealed a modest but statistically significant reduction in chromosome gaps and breaks at fragile sites when POLI was removed from BRCA2-deficient cells. In primary mammary epithelial cells from a BRCA2 mutation carrier, POLI depletion under mild replication stress significantly reduced levels of γH2AX, a marker of DNA damage signaling, and combined depletion also lowered RPA32, a marker of single-stranded DNA accumulation.

The resolution to this paradox came from DNA fiber assays, which track the progress of individual replication forks in living cells. The researchers sequentially labeled newly synthesized DNA with two distinguishable nucleotide analogs, CIdU for a short pulse and IdU for an extended pulse, in the presence of aphidicolin, and measured the ratio of tract lengths as a readout of fork speed. BRCA2 depletion increased the IdU/CIdU ratio, and combined BRCA2 and POLI depletion pushed fork progression even higher, indicating that loss of Pol ι actually helps replication forks move faster through stressed BRCA2-deficient genomes. Parallel untreated controls confirmed the effect was specific to replication stress. In other words, removing Pol ι appears to partially alleviate the very stress it was recruited to tolerate, suggesting that Pol ι-dependent DNA synthesis may itself impose a burden, or that compensatory synthesis pathways take over when Pol ι is absent. The precise molecular mechanism linking fork dynamics to post-S-phase synthesis remains an open question the authors flag for future work.

The study also connected these replication-level changes to epithelial cell identity, a link with direct relevance to early tumor evolution. Preneoplastic breast tissue from BRCA mutation carriers is known to show expansion of luminal epithelial populations, and the team’s own single-cell analysis had highlighted hormone-sensing luminal cells as the site of POLI induction. Using flow cytometry for EpCAM, a pan-epithelial marker, and CD24, a marker associated with luminal identity, the researchers found that both BRCA2 depletion and POLI depletion expanded the EpCAM-high/CD24-high population in MCF10A cells. POLI depletion produced the larger expansion, and the two depletions were not additive, indicating that Pol ι-dependent and Pol ι-independent adaptive pathways both feed into this luminal-like cell state. Prior studies have associated CD24-high epithelial states with adaptive stress responses in several biological contexts, lending plausibility to the idea that replication stress processing and epithelial state remodeling are intertwined.

The authors are careful to delineate what their data do and do not establish. The functional experiments relied on transient siRNA silencing, which captures acute responses but cannot speak to the consequences of sustained POLI alteration over longer timescales. The EdU-based assay detects DNA synthesis across the G2/M boundary but cannot distinguish canonical mitotic DNA synthesis from persistent G2-phase synthesis or S-to-M runover, which is why the team refers collectively to post-S-phase DNA synthesis. And while POLI depletion was associated with both altered replication stress responses and expansion of a luminal-like population, the study does not prove a causal relationship between the two phenotypes. The upstream mechanisms that drive POLI upregulation in BRCA2-deficient cells, and the pathways through which Pol ι influences fork dynamics, also remain to be defined.

Even with those caveats, the work delivers a compelling conceptual advance. It positions Pol ι not as a universal replication factor but as a context-dependent regulator whose importance emerges specifically under BRCA2-associated replication stress, and it ties together three threads, post-S-phase DNA synthesis, fork progression, and epithelial cell state, into a single framework for the earliest stages of BRCA2-associated breast tumorigenesis. If the adaptive, Pol ι-coordinated response to chronic replication stress helps shape the preneoplastic epithelium of mutation carriers, then interfering with that response, or understanding which cells it favors, could eventually inform prevention strategies aimed at the window before cancer takes hold. For now, the study offers a vivid reminder that the road to cancer in BRCA2 carriers may begin not with a catastrophic break, but with the quiet recruitment of an error-prone polymerase to unfinished DNA.

Subject of Research: The role of polymerase iota in replication stress adaptation in BRCA2-deficient mammary epithelial cells

Article Title: Polymerase iota coordinates adaptation to replication stress in BRCA2-deficient mammary epithelial cells

Article References: Vipparthi, K., Bellare, A., Ozbilgic, R., Bazeley, P., Khanna, N., Wang, F., Bhushan, A., Xie, L., Al-Hilli, Z., Zhang, Y.-W., & Karaayvaz, M. (2026). Polymerase iota coordinates adaptation to replication stress in BRCA2-deficient mammary epithelial cells. iScience, 29(11), Article 117830. https://doi.org/10.1016/j.isci.2026.117830

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117830

Keywords: BRCA2, polymerase iota, replication stress, mitotic DNA synthesis, mammary epithelial cells, translesion synthesis, genome instability, common fragile sites, luminal progenitors, breast cancer, RAD52, DNA fiber assay