HDAC8 suppresses the epithelial phenotype and promotes EMT in chemotherapy-treated basal-like breast cancer

Background Basal-like breast cancer (BLBC) is one of the most aggressive malignant diseases in women with an increased metastatic behavior and poor prognosis compared to other molecular subtypes of breast cancer. Resistance to chemotherapy is the main cause of treatment failure in BLBC. Therefore, novel therapeutic strategies counteracting the gain of aggressiveness underlying therapy resistance are urgently needed. The epithelial-to-mesenchymal transition (EMT) has been established as one central process stimulating cancer cell migratory capacity but also acquisition of chemotherapy-resistant properties. In this study, we aimed to uncover epigenetic factors involved in the EMT-transcriptional program occurring in BLBC cells surviving conventional chemotherapy. Results Using whole transcriptome data from a murine mammary carcinoma cell line (pG-2), we identified upregulation of Hdac4, 7 and 8 in tumor cells surviving conventional chemotherapy. Subsequent analyses of human BLBC patient datasets and cell lines established HDAC8 as the most promising factor sustaining tumor cell viability. ChIP-sequencing data analysis identified a pronounced loss of H3K27ac at regulatory regions of master transcription factors (TFs) of epithelial phenotype like Gata3, Elf5, Rora and Grhl2 upon chemotherapy. Interestingly, impairment of HDAC8 activity reverted epithelial-TFs levels. Furthermore, loss of HDAC8 activity sensitized tumor cells to chemotherapeutic treatments, even at low doses. Conclusion The current study reveals a previously unknown transcriptional repressive function of HDAC8 exerted on a panel of transcription factors involved in the maintenance of epithelial cell phenotype, thereby supporting BLBC cell survival to conventional chemotherapy. Our data establish HDAC8 as an attractive therapeutically targetable epigenetic factor to increase the efficiency of chemotherapeutics. Graphical abstract

Elevated SOX11 expression distinguishes basal-like human breast cancer.

Patients diagnosed with basal-like breast cancer face a more aggressive disease course and more dismal prognosis than patients diagnosed with luminal A and luminal B breast cancer molecular subtypes (1-4). We mined published microarray data (5, 6) to understand in an unbiased fashion the most distinguishing transcriptional features of tumors from patients with basal or basal-like subtype breast cancer. We observed transcriptome-wide differential expression of SRY-box 11, SOX11, when comparing tumors of patients with basal-like breast cancer with that of other PAM50 molecular subtypes. SOX11 mRNA was present at significantly higher quantities in the tumors of patients with basal-like breast cancer. Analysis of patient survival data revealed that SOX11 primary tumor expression was correlated with overall survival, with higher SOX11 associated with inferior outcomes - in basal-like patients but not in luminal A, luminal B, HER2+, or normal-like patients. Elevated SOX11 expression appears to distinguish basal-like human breast cancer from the other molecular subtypes.

High VEGF-A expression distinguishes basal-like human breast cancer.

Patients diagnosed with basal-like breast cancer face a more aggressive disease course and more dismal prognosis than patients diagnosed with luminal A and luminal B breast cancer molecular subtypes (1-4). We mined published microarray data (5, 6) to understand in an unbiased fashion the most distinguishing transcriptional features of tumors from patients with basal or basal-like subtype breast cancer. We observed transcriptome-wide differential expression of the vascular endothelial growth factor A, VEGFA, when comparing tumors of patients with basal-like breast cancer with that of other PAM50 molecular subtypes. VEGF-A mRNA was present at significantly higher quantities in the tumors of patients with basal-like breast cancer. Analysis of patient survival data revealed that VEGF-A primary tumor expression was correlated with recurrence-free survival, with higher VEGF-A associated with inferior outcomes - in basal-like patients but not in luminal A, luminal B, HER2+, or normal-like patients. High VEGF-A expression appears to distinguish basal-like human breast cancer from the other molecular subtypes.

Low GATA3 expression distinguishes basal-like human breast cancer.

Patients diagnosed with basal-like breast cancer face a more aggressive disease course and more dismal prognosis than patients diagnosed with luminal A and luminal B breast cancer molecular subtypes (1-4). We mined published microarray data (5, 6) to understand in an unbiased fashion the most distinguishing transcriptional features of tumors from patients with basal or basal-like subtype breast cancer. We observed transcriptome-wide differential expression of the transcription factor GATA3 when comparing tumors of patients with basal-like breast cancer with that of other PAM50 molecular subtypes. GATA3 mRNA was present at significantly reduced quantities in the tumors of patients with basal-like breast cancer. Analysis of patient survival data revealed that GATA3 primary tumor expression was correlated with distant metastasis-free survival, with low GATA3 expression correlated with inferior survival outcomes. Low GATA3 expression appears to distinguish basal-like human breast cancer from the other molecular subtypes.

A single nucleotide variant on chromosome 10 residing within nebulette (C10orf113) distinguishes patients with basal-like human breast cancer.

Patients diagnosed with basal-like breast cancer face a more aggressive disease course and more dismal prognosis than patients diagnosed with luminal A and luminal B breast cancer molecular subtypes (1-4). We mined published microarray data (5, 6) to understand in an unbiased fashion the most distinguishing genetic and transcriptional features of tumors from patients with basal or basal-like subtype breast cancer. We identified a single nucleotide polymorphism (rs625223) residing within NEBL (C10orf113) as among the most significant genetic differences in the tumors of patients with basal-like breast cancer. In a separate cohort of patients with basal subtype breast cancer, we observed transcriptome-wide differential tumor expression of a C10orf113 transcript. Analysis of patient survival data revealed that C10orf113 primary tumor expression was correlated with recurrence-free survival and distant metastasis-free survival in patients with basal-like breast cancer, but in a contrary manner. Thus, single nucleotide variants on both chromosome 5 (7) and chromosome 10 are fundamental differences that define the genetic composition of basal-like breast cancer in humans.

An allele on chromosome 5 residing within the ZSWIM6 region distinguishes patients with basal-like human breast cancer.

Patients diagnosed with basal-like (also known as basal) subtype breast cancer face a more aggressive disease course and more dismal prognosis than patients diagnosed with luminal A and luminal B breast cancer molecular subtypes (1-4). We mined published microarray data (5, 6) to understand in an unbiased fashion the most distinguishing genetic and transcriptional features of tumors from patients with basal or basal-like subtype breast cancer. We identified a single nucleotide polymorphism, rs6449531 in ZSWIM6 as among the most significant genetic differences in the tumors of patients with basal-like breast cancer. In a separate cohort of patients with basal subtype breast cancer, we observed transcriptome-wide differential expression of a ZSWIM6 transcript. Analysis of patient survival data revealed that ZSWIM6 primary tumor expression was correlated with overall survival in patients with basal subtype breast cancer. Sequence variation in the ZSWIM6 gene may be important in understanding differences in genetic background that favor development of basal subtype human breast cancer.

Tumor Suppressor PLK2 May Serve as a Biomarker in Triple-Negative Breast Cancer for Improved Response to PLK1 Therapeutics

Polo-like kinase (PLK) family members play important roles in cell-cycle regulation. The founding member PLK1 is oncogenic and preclinically validated as a cancer therapeutic target. Paradoxically, frequent loss of chromosome 5q11–35, which includes PLK2, is observed in basal-like breast cancer. In this study, we found that PLK2 was tumor suppressive in breast cancer, preferentially in basal-like and triple-negative breast cancer (TNBC) subtypes. Knockdown of PLK1 rescued phenotypes induced by PLK2 loss both in vitro and in vivo. We also demonstrated that PLK2 directly interacted with PLK1 at prometaphase through the kinase but not the polo-box domains of PLK2, suggesting PLK2 functioned at least partially through the interaction with PLK1. Furthermore, an improved treatment response was seen in both Plk2-deleted/low mouse preclinical and patient-derived xenograft (PDX) TNBC models using the PLK1 inhibitor volasertib alone or in combination with carboplatin. Reexpression of PLK2 in an inducible PLK2-null mouse model reduced the therapeutic efficacy of volasertib. In summary, this study delineates the effects of chromosome 5q loss in TNBC that includes PLK2, the relationship between PLK2 and PLK1, and how this may render PLK2-deleted/low tumors more sensitive to PLK1 inhibition in combination with chemotherapy. Significance: The tumor-suppressive role of PLK2, and its relationship with oncogene PLK1, provide a mechanistic rationalization to use PLK1 inhibitors in combination with chemotherapy to treat PLK2-low/deleted tumors. TNBC, and other cancers with low PLK2 expression, are such candidates to leverage precision medicine to identify patients who might benefit from treatment with these inhibitors.

Decreased PRC2 activity supports the survival of basal-like breast cancer cells to cytotoxic treatments

Breast cancer (BC) is the most common cancer occurring in women but also rarely develops in men. Recent advances in early diagnosis and development of targeted therapies have greatly improved the survival rate of BC patients. However, the basal-like BC subtype (BLBC), largely overlapping with the triple-negative BC subtype (TNBC), lacks such drug targets and conventional cytotoxic chemotherapies often remain the only treatment option. Thus, the development of resistance to cytotoxic therapies has fatal consequences. To assess the involvement of epigenetic mechanisms and their therapeutic potential increasing cytotoxic drug efficiency, we combined high-throughput RNA- and ChIP-sequencing analyses in BLBC cells. Tumor cells surviving chemotherapy upregulated transcriptional programs of epithelial-to-mesenchymal transition (EMT) and stemness. To our surprise, the same cells showed a pronounced reduction of polycomb repressive complex 2 (PRC2) activity via downregulation of its subunits Ezh2, Suz12, Rbbp7 and Mtf2. Mechanistically, loss of PRC2 activity leads to the de-repression of a set of genes through an epigenetic switch from repressive H3K27me3 to activating H3K27ac mark at regulatory regions. We identified Nfatc1 as an upregulated gene upon loss of PRC2 activity and directly implicated in the transcriptional changes happening upon survival to chemotherapy. Blocking NFATc1 activation reduced epithelial-to-mesenchymal transition, aggressiveness, and therapy resistance of BLBC cells. Our data demonstrate a previously unknown function of PRC2 maintaining low Nfatc1 expression levels and thereby repressing aggressiveness and therapy resistance in BLBC.