Scientists Find a Shared Weakness That Could Crack Triple-Negative Breast Cancer’s Drug Resistance

Triple-negative breast cancer is the most aggressive form of breast cancer, and it is also one of the hardest to treat. Lacking the three receptors that define other breast cancer subtypes, it offers clinicians few molecular handles for targeted therapy, leaving chemotherapy as the mainstay of treatment. Yet even chemotherapy often fails, because the tumors are remarkably adept at surviving DNA damage and at reshaping themselves into more resistant forms. A new study published in Cell Death Discovery by Kathrin Niedermayer, Sara Greco and colleagues at Ulm University, together with collaborators in Hamburg and Buenos Aires, suggests that this adaptability may itself be the tumor’s Achilles’ heel. By mapping how triple-negative breast cancer cells cope with damaged DNA as they shift between different biological states, the team identified two druggable enzymes that appear to be indispensable across all those states, and showed that blocking them can restore the effectiveness of an established cancer drug.

The research focused on a family of cellular survival strategies known collectively as DNA damage tolerance. When chemotherapeutic drugs such as anthracyclines and platinum compounds inflict lesions on DNA, they do their most damage during replication, the process by which a cell copies its entire genome before dividing. Stalled replication forks, the Y-shaped structures where the two DNA strands are prised apart and copied, are a lethal hazard. Cells respond with several alternative maneuvers: translesion synthesis, in which specialized DNA polymerases such as POLη, POLι and POLζ copy directly across the damaged stretch; fork reversal, in which DNA translocases such as ZRANB3, HLTF and SMARCAL1 remodel the fork so the lesion can be repaired; and repriming, in which the enzyme PRIMPOL simply restarts copying downstream of the block, leaving a gap that POLζ later fills in. Because these pathways allow cancer cells to tolerate the very damage that chemotherapy is designed to inflict, they have emerged as prime suspects behind chemoresistance.

What makes triple-negative breast cancer especially tricky is that its cells are not static targets. They frequently undergo epithelial-to-mesenchymal transition, a developmental program that makes them more invasive and more stem-like, and this plasticity is thought to change how they handle DNA damage. To capture that dynamic behavior, the researchers grew ten breast cancer cell lines, four triple-negative and six non-triple-negative, in two different configurations: flat two-dimensional monolayers and three-dimensional spheroids, which better mimic the architecture of a real tumor. They then measured a panel of markers, including the epithelial genes KRT18 and CDH1, the mesenchymal markers ZEB1 and Vimentin, and the stemness indicators CD44, CD24 and Nestin, both with and without exposure to the clinically relevant drugs Olaparib, Carboplatin and Doxorubicin.

The results were striking. Triple-negative cell lines, particularly in 3D culture, showed elevated Vimentin, reduced epithelial markers, a higher CD44-to-CD24 ratio and increased Nestin, hallmarks of a mesenchymal, stem-like state. More importantly, these phenotypic changes came with a measurable change in replication behavior. Using the DNA fiber spreading assay, a technique that labels newly synthesized DNA with two different nucleotide analogs and measures the length of the resulting tracks under a microscope, the team found that replication slowed dramatically and consistently in all four triple-negative lines when they were grown as spheroids, while the six non-triple-negative lines showed no such pattern. Adding N-acetylcysteine, a scavenger of reactive oxygen species, did not abolish the slowdown, and the ratio of long to short tracks revealed no excess fork stalling. In other words, the triple-negative cells were not failing to replicate; they were deliberately slowing down.

The molecular signature behind that slowdown pointed squarely at DNA damage tolerance. Western blotting revealed higher levels of monoubiquitinated PCNA, the sliding clamp that coordinates replication and whose ubiquitination marks perturbed forks, in the triple-negative lines across treatments and culture conditions. Immunofluorescence microscopy then showed that nuclear foci of the translocase ZRANB3, and of POLι foci colocalizing with it, accumulated more frequently in 3D than in 2D. Only in the 3D-cultured triple-negative cells did the combination of PCNA ubiquitination and ZRANB3 recruitment coincide with the shortened replication tracks, linking the stem-like state to a shift toward fork reversal as the dominant tolerance strategy.

To dissect which pathways each cell line actually used, the researchers silenced key components with siRNA and repeated the fiber assays in three triple-negative lines: MDA-MB-231, Hs578t and MDA-MB-468. The picture was heterogeneous but revealing. In 2D, all three lines relied on repriming driven by the checkpoint kinase CHK1, with contributions from translesion polymerases that varied by cell line. In 3D, the balance tipped toward fork reversal, mediated by ZRANB3 in MDA-MB-231, SMARCAL1 in Hs578t and MDA-MB-468, and HLTF in a context-dependent fashion, with HLTF’s dual roles as a translocase and as the ligase that polyubiquitinates PCNA producing opposite effects in the two culture formats. Despite the heterogeneity, the underlying dependence on fast, flexible lesion bypass was a common thread.

That common thread became a therapeutic opportunity. The team screened ten small-molecule inhibitors targeting different parts of the DNA damage response, from translesion polymerases and CHK1 to RAD51, RAD52, ATR, MRE11, DNA2 and the origin-firing kinase CDC7. Two stood out. The POLζ inhibitor JH-RE-06 showed half-maximal inhibitory concentrations of 7 micromolar or below in all three cell lines under both culture conditions, and the CHK1 inhibitor PF477736 followed at 20 micromolar or below. Mechanistically, both drugs target multiple tolerance routes at once: CHK1 supports both repriming and fork stability, while POLζ fills the single-stranded gaps left by repriming and extends translesion synthesis. Fiber assays confirmed that CHK1 inhibition slowed replication in every cell line regardless of culture format, nearly erasing the 2D-versus-3D difference, while POLζ inhibition decelerated forks in specific contexts. By contrast, inhibitors of homologous recombination and break-induced replication had little or no effect on viability, arguing that the cells’ survival hinged on tolerance rather than repair of broken forks.

The most clinically consequential finding concerned Olaparib, a PARP inhibitor approved for tumors with BRCA1 or BRCA2 defects. The triple-negative lines used in the study carry wild-type BRCA genes, and accordingly they responded poorly to Olaparib alone, becoming almost fully resistant in 3D culture. But when the researchers combined Olaparib with low doses of the POLζ, CHK1 or even RAD51 inhibitor, all three cell lines were resensitized in 2D, and two of the three, Hs578t and MDA-MB-468, were resensitized in 3D as well. The logic is a form of synthetic lethality: PARP inhibition blocks the restart of reversed forks, forcing cells onto the very translesion and repriming routes that the partner drug simultaneously disables. The lone exception was 3D-cultured MDA-MB-231, which resisted every combination. That line stood out for its heavy reliance on ZRANB3-mediated fork reversal, its high CD44 levels and its low burden of DNA damage markers, reinforcing the idea that stemness, ZRANB3-dependent fork protection and chemoresistance are mechanistically intertwined.

The study has limits the authors acknowledge: it examined ten cell lines with deep mechanistic work in only three, no direct inhibitors of PRIMPOL or the fork-remodeling enzymes were available, and no patient data were included. Even so, the translational momentum is real. CHK1 inhibitors such as Prexasertib and MK-8776 have already been tested in Phase I and II trials across solid tumors including triple-negative breast cancer, and the USP1 inhibitor KSQ-4279, which prevents PCNA de-ubiquitination, has entered Phase I testing with early signs of synergy with Olaparib. POLζ inhibitors have not yet reached the clinic, but their tolerability profile appears acceptable. If the pattern holds in patients, the message of this work is that the shifting disguises of triple-negative breast cancer may matter less than the replication machinery the cells can never abandon, and that attacking the common denominators of DNA damage tolerance, alone or alongside PARP inhibition, could finally turn the tumor’s adaptability against it.

Subject of Research: DNA damage tolerance pathway targeting as a therapeutic strategy against chemoresistance in triple-negative breast cancer

Article Title: Targeting common denominators of DNA damage tolerance mechanisms as therapeutic strategy to cope with dynamic phenotypic changes in triple-negative breast cancer cells

Article References: Niedermayer, K., Greco, S., Oechsle, A., Köhler, T., Pantel, K., Friedl, T. W. P., Janni, W., Rack, B., Pfister, K., Gottifredi, V., & Wiesmüller, L. (2026). Targeting common denominators of DNA damage tolerance mechanisms as therapeutic strategy to cope with dynamic phenotypic changes in triple-negative breast cancer cells. Cell Death Discovery, 12(1), Article 405. https://doi.org/10.1038/s41420-026-03377-4

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03377-4

Keywords: triple-negative breast cancer, DNA damage tolerance, translesion synthesis, fork reversal, PCNA ubiquitination, POLζ, CHK1, Olaparib, PARP inhibitor, chemoresistance, epithelial-to-mesenchymal transition, cancer stem cells