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Use of gene expression patterns post neoadjuvant chemotherapy to identify a role for the MAPK phosphatase DUSP4 in therapeutic resistance and a stem-like phenotype in basal-like breast cancer (BLBC).

10509 Background: Neoadjuvant chemotherapy (NAC) leads to a pathological complete response in ~20% of patients with breast cancer. The remaining patients have residual disease and are more likely to develop metastasis. In post-NAC tumors, Ki67 levels, a marker of cell proliferation, have been shown to inform odds of recurrence and death. We hypothesized that post-NAC residual cancers with a high Ki67 harbor molecular alterations that are associated with drug resistance.


METHODS
We utilized Nanostring digital RNA quantification to measure the expression levels of 350 cancer-related genes in 49 post-NAC breast cancers. Gene expression data were tested for associations with post-NAC Ki67. The cohort was enriched for triple-negative breast cancer (24/49 tumors). The candidate gene DUSP4 was explored in in vitro models of breast cancer.


RESULTS
The post-NAC Ki67 (range 2.4 - 99%; median: 31.6%) was statistically different among the molecular subtypes of breast cancers enrolled, being highest in BLBC (p<0.0001). Gene expression signatures of KRAS activation were enriched in BLBC. Low levels of the MAPK phosphatase DUSP4 correlated with a high Ki67 in BLBCs (r=-0.54, p=8x10-4) and with Ras/MAPK pathway activation in human breast tumors and cell lines. Genome-wide studies identified preferential methylation of the DUSP4 promoter in BLBC. Among BLBC cell lines, low DUSP4 expression predicted for sensitivity to MEK inhibitors and resistance to docetaxel. Inhibition of MEK with AZD6244 or overexpression of DUSP4 in DUSP4-low BLBC cell lines improved sensitivity to docetaxel and inhibited mammosphere formation. RNAi-mediated knockdown of DUSP4 in BLBC cells enhanced ETS-1 phosphorylation, mammosphere formation, and MAPK-dependent secretion of the cancer stem cell-promoting cytokines IL6 and IL8.


CONCLUSIONS
Low levels of DUSP4 correlate with high tumor cell proliferation after NAC. Downregulation of DUSP4 may underlie activation of the MAPK pathway in the absence of KRAS mutations in BLBC while conferring resistance to chemotherapy and a stem cell-like phenotype that is therapeutically targetable with MAPK inhibitors.