Understanding the role of the WNT signaling pathway in breast cancer dormancy, therapy resistance and cancer relapse.

Breast cancer is the most common and most deadly neoplastic disease among women worldwide. Triple-negative breast cancer is a subtype of breast cancer characterized by the lack of expression of hormone and growth factor receptors, highly aggressive clinical features, poor prognosis, and high relapse rates. The lack of hormone and growth factor receptors renders targeted therapies that are highly efficient in other breast cancer subtypes virtually useless in triple-negative breast neoplasms. Thus, chemotherapy remains the primary treatment option for triple-negative breast cancer patients.

Triple-negative breast cancer patients achieve higher pathological complete response rates to neoadjuvant chemotherapy more frequently than non-triple-negative patients. However, despite the better response rates, triple-negative breast tumors still present with paradoxically higher relapse rates.

The relapsing disease is a consequence of treatment failure and drug resistance. Drug resistance can be either intrinsic or acquired. Intrinsic resistance presupposes the existence of resistance-mediating features in cancer cell populations prior to treatment, ultimately causing insufficient treatment response. On the other hand, acquired resistance entails the adaptation of cancer cell populations to cytotoxic stimuli during treatment, ultimately acquiring new features that enable their survival.

In recent years, several studies have confirmed that triple-negative breast cancers adapt to treatment not by selecting pre-existing resistance-mediating genotypes but rather by transitioning between different cell states and transient transcriptional and epigenetic reprogramming. However, we still lack actionable targets to prevent chemo-adaptation and acquisition of resistance and disrupt established acquired chemotherapy-resistant states.

In this thesis, we sought to investigate which signaling pathways maintain stable acquired chemotherapy resistance in triple-negative breast cancers using in vitro and in vivo models of carboplatin resistance. Finally, we discovered that the primordial developmental Wnt signaling pathway maintained a stem-cell-like state in carboplatin-resistant isogenic cell lines and that disruption of Wnt ligand secretion reverted carboplatin resistance both in vitro and in vivo.

We then wanted to understand which signaling pathways are modulated during acute exposure to chemotherapy. Analysis of transcriptomic datasets of breast cancer patients' paired pre- and post- chemotherapy biopsies revealed enrichment of Wnt signaling after chemotherapy treatment. Transcriptomic analysis of triple-negative breast cancer cell lines exposed to chemotherapy demonstrated early Wnt signaling enrichment upon chemotherapy in vitro. Furthermore, we determined that enriched Wnt signaling activity was a consequence of de novo transcriptional activation of the pathway in cells with inactive Wnt signaling prior to treatment. Interestingly,chemotherapy-induced Wnt-active cells appear to be more resilient and less likely to induce apoptosis while maintaining higher expression of stem cell markers and in vitro tumorsphere formation capacity. Significantly, inhibition of Wnt ligand secretion disrupts chemotherapy-induced Wnt activation and sensitizes triple-negative breast cancer cell lines to treatment.

Altogether these results indicate that Wnt signaling activity contributes to stable acquired chemotherapy resistance in triple-negative breast cancers. Importantly, modulation of Wnt signaling seems to occur in the very early stages of exposure to chemotherapy. It is thus likely that chemo-adaptation and acquisition of resistance are facilitated by upregulation of Wnt signaling. Inhibiting this pathway both disarms established acquired resistance and enhances early response to treatment in vitro, suggesting this could be a potentially beneficial therapeutic strategy to enhance chemotherapeutic efficiency and prevent acquired resistance.