ABSTRACT
Aneuploidy, an abnormal chromosome number, is a hallmark of cancer. We recently showed that depletion of microtubule-associated protein ATIP3 (AT2 receptor-interacting protein 3) induces aneuploidy and sensitizes breast cancer cells to taxanes. Combining taxane treatment with ATIP3 depletion cooperates to reach a detrimental level of aneuploidy.
KEYWORDS: Breast cancer, MTUS1, ATIP3, predictive biomarker, taxanes, multipolar spindle, centrosome amplification, chromosome instability
Aneuploidy, an abnormal chromosome number, is a hallmark of cancer. Aneuploidy can arise by several mechanism but frequently occurs as a result of aberrant mitotic division. In particular, checkpoint signaling defects, improper attachment of chromosomes to spindle microtubules, or multipolar division frequently give rise to aneuploid cells.1 The consequence of aneuploidy is still a matter of debate. In cancer cells, while aneuploidy is associated with tumorigenesis and poor prognosis, it has also been shown to reduce tumor growth and to promote cell death.2 Several studies aimed at understanding this paradox and led to the notion of threshold. Thus, considering the rate of chromosome missegregation and aneuploidy, it was demonstrated that induction of aneuploidy confers cancer cell advantages and promotes tumorigenesis, while excessive aneuploidy is deleterious and leads to cell death and tumor suppression.3
Of interest, aneuploidy and chromosomal instability (CIN), a high frequency of genomic alterations, are induced by cytotoxic drugs used in chemotherapy. In breast cancer, chemotherapy regimen includes DNA targeting agents (anthracyclines) and microtubule poisons (taxanes). By inducing either DNA damage or multipolar spindles, these drugs impair chromosome segregation and promote cell death.4 However, this treatment only benefits to 10–20% of breast cancer patients. The identification of biomarkers able to predict sensitivity to chemotherapy is an important issue to select patients who may benefit from chemotherapy and thus to avoid unnecessary treatment and side effects for those who remain resistant. Whether aneuploidy may be considered as a predictive biomarker for tumor response to chemotherapy is under investigation.
In a recent study comparing transcriptional and clinical data from three independent cohorts of breast cancer patients treated in neoadjuvant setting with taxane-based chemotherapy, we identified the microtubule-associated protein ATIP3 (AT2 receptor-interacting protein 3) as a new predictive biomarker of tumor response to chemotherapy. We showed (Figure 1) that low level of ATIP3 in breast tumors is associated with higher sensitivity to chemotherapy and can predict response to treatment with a good accuracy.5 ATIP3 is encoded by microtubule-associated tumor suppressor gene (known as MTUS1). Our previous studies have shown that ATIP3 is significantly down-regulated in 47% of invasive breast carcinomas and 85% of triple-negative breast tumors.6 ATIP3 represents a prognostic biomarker of breast cancer patients’ survival.7 At the molecular level, ATIP3 closely associates with the microtubule cytoskeleton in interphase and with the mitotic spindle during mitosis.6 ATIP3 is a potent microtubule-stabilizing protein, its depletion being associated with increased microtubule dynamic instability that accounts for increasing breast cancer cell growth, polarity and migration.6,7
Figure 1.
Effects of taxanes on breast cancers expressing or not ATIP3.
At the tumor level, only 1% of patients with AT2 receptor-interacting protein 3 (ATIP3)-positive tumors achieve pathological Complete Response (pCR) – a marker of chemosensitivity – upon taxane-based chemotherapy, compared to 30–40% of ATIP3-negative tumors. In ATIP3-positive cells, taxanes induce mitotic defects such as multipolar spindles (MPS) that lead to aneuploidy. Silencing of ATIP3 also induces multipolar spindles and aneuploidy, a phenotype strongly exacerbated by taxane treatment. Taxane treatment of ATIP3-positive tumors gives rise to abnormal mitosis. In ATIP3-negative tumors, that are aneuploid, taxane treatment further increases aneuploidy over a threshold (red dashed line) which is deleterious and promotes cell death.
In our recent study,5 we showed that loss of ATIP3 not only predicts the response to taxane-based chemotherapy but also enhances sensitivity to treatment in breast patient-derived xenografts and in multicellular spheroids models. Interestingly, depletion of ATIP3 increases cell death upon taxane treatment without affecting sensitivity to anthracyclines. As taxanes stabilize the microtubule network and impair cell division, we performed immunofluorescence and time-lapse experiments to evaluate the consequences of ATIP3 depletion on cell division upon taxane treatment. Our studies demonstrated that silencing of ATIP3 induces mitotic defects including centrosome amplification and multipolar spindles. These defects are associated with chromosome missegregation and increased aneuploidy, as measured by chromosome number after metaphase chromosome spreading. After treatment with low doses of taxanes, all mitotic abnormalities induced by ATIP3 silencing were exacerbated, further increasing the percentage of aneuploid cells and cell death. Thus, combining ATIP3 silencing with taxane treatment led to high rate of multipolar spindles, over amplification of centrosomes, increased aneuploidy and cell death, indicating that taxane treatment cooperates with ATIP3 depletion to reach a detrimental level of chromosomal instability (Figure 1). In line with these results, several studies have demonstrated that increasing aneuploidy in breast cancer cells either by inducing DNA damage, or by altering mitotic regulators or centrosome numbers, potentiates the anti-tumoral effect of taxanes.8–10
Interestingly, using a breast cancer patient’s cohort in which aneuploidy has been evaluated, we found that 65% of low-ATIP3 expressing breast tumors are aneuploid as compared to 42% of high-ATIP3 tumors. The genomic alterations were also evaluated in breast tumors using a 25-genes signature of chromosomal instability (CIN). Our study showed that 60% of low-ATIP3 tumors are associated with high CIN compared to 40% of high-ATIP3 counterparts.5 Taken together, these results support the notion that the frequency of aneuploidy in a tumor may determine the response to taxane-based chemotherapy, aneuploid tumors being more chemosensitive than near-diploid tumors. This point is under investigation in a clinical study that evaluates whether breast cancers with high chromosomal instability (CIN) may respond better to taxane-based chemotherapy than low-CIN cancers (NCT03096418).
Strategies to target aneuploidy in breast tumors may represent an interesting therapeutic option to improve sensitivity to taxane-based chemotherapy. As aneuploidy is very rare in normal cells, compounds able to impair chromosome segregation and to increase aneuploidy may show high specificity to cancer cells and few side effects.
Funding Statement
This work has been funded by Gustave Roussy, CNRS, INSERM, the Comité Ile-de-France of the Ligue Nationale contre le Cancer, the Ligue contre le Cancer 94/Val-de-Marne, the Fondation ARC, the Fondation Janssen Horizon, the Fonds de Dotation Agnès b., the association Odyssea and Prolific.
Disclosure of potential conflict of interest
The authors report no conflict of interest
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