Abstract P4-09-08: miR-135a is associated with a metastatic phenotype in invasive lobular carcinoma

Abstract

Listen

Translate article

Abstract Invasive lobular carcinoma (ILC) is the second most common type of breast cancer. Classic type ILC is generally regarded as indolent in nature with its favourable biologic characteristics such as low grade, ER positivity and luminal A subtype. Despite this, patients with ILC can develop significant distant recurrence or metastases. Thus the ability to identify those patients at highest risk of recurrence or metastasis, and identification of novel therapies for ILC are urgently required. To this end, we recently profiled the miRNA expression in primary surgical ILC specimens from patients who went on to have metastatic disease compared to those who remained tumor free long term. As miRNA are stable in formalin fixed paraffin embedded tissues1, we speculated they could be robust biomarkers. RNA was isolated from laser capture microdissected ILC tumor epithelium, and subjected to miRNome analysis using a PCR-based amplification method. Many differentially expressed miRNAs were identified, and we initially focused further validation on those which had been previously linked to metastasis. One of these, miR-135a, was elevated in tumors from ILC patients who developed metastases compared to those that did not. We utilized two representative ILC cell lines which differ in their invasive ability, MDA-MB-134VI (non-invasive) and MDA-MB-330 (invasive), to test whether miR-135a regulated ILC invasion. We found that levels of miR-135a correlated with the invasive potential of ILC cell lines and was elevated in the invasive MDA-MB-330 cells compared to less invasive MDA-MB-134VI cells. We also found that decreasing miR-135a expression using specific hairpin inhibitors in MDA-MB-330 cells resulted in decreased cell invasion. As miR-135a has been shown to regulate invasion via targeting metastasis suppressor 1 (MTSS1) mRNA for degradation2, we examined whether MTSS1 levels were inversely associated with miR-135a levels in ILC cells. As predicted, in MDA-MB-330 cells where miR-135a levels were significantly higher, levels of MTSS1 were the lowest while MTSS1 levels were higher in parallel with decreased levels of miR-135a in the non-invasive MDA-MB-134VI cells. Overexpressing miR-135a using miRNA mimics in normal mammary epithelial cells (HMEC) where miR-135a is normally low, reduced levels of MTSS1 supporting suggestions it is a direct target of miR-135a. We also confirmed reduced mRNA levels of MTSS1 in surgical specimens from ILC patients who developed metastases compared to those that did not. Taken together, our results suggest that high levels of miR-135a, and low levels of MTSS1 may be useful prognostic information to assess risk of metastasis in ILC.