Recently, we have identified the miR-424(322)/503 cluster as an important regulator of mammary epithelial homeostasis. The miR-424(322)/503 cluster was identified as one of the few miRNAs significantly upregulated during involution after pregnancy. By generating a knock-out mouse model, we found that regression of the mammary epithelium after pregnancy was compromised in the absence of miR-424(322)/503. Mechanistically, our studies unveiled that miR-424(322)/503 is induced by the canonical TGF-β-SMAD pathway, and that it orchestrates changes in the mammary epithelium by downregulating the expression of key components of signal transduction (IGF1R) and apoptosis (BCL2) Llobet et al. Genes&Development 2014).

Remarkably, our new studies have revealed that miR-424(322)/503-/- female mice develop hyperplasia and mammary tumors that are promoted by pregnancy. Thus, we investigated the status of this cluster in human breast cancers. For this we analyzed the METABRIC dataset. These studies revealed that the miR-424(322)503 cluster was heterozygously deleted in ∼16% of breast cancers and that its deletion correlates with lower expression levels of the mature miRNA forms. Importantly, miR-424(322)/503 is located on the X-chromosome and we have confirmed that is monoallelically expressed due to X-chromosome inactivation. Thus, the mutation of the active allele strongly impacts the expression of the miR-cluster. Deletions of the miR-424(322)/503 locus were more frequent in molecular subtypes with aggressive behavior (Luminal B, HER2+ and Basal) and both deletions and low expression of the cluster were associated with poor prognosis and reduced survival.

Some of the miR-424(322)/503 targets that we have previously validated (BCL2 and IGF1R) are involved in resistance to chemotherapy. Importantly, primary breast tumors presenting deletion of this miR-cluster locus presented significantly worse response to chemotherapy than the rest of tumors. Thus, we investigated in vivo, utilizing our knock-out mouse model, if loss of miR-424(322)/503 induces resistance to chemotherapeutic drugs. For this we crossed our miR-424(322)/503-/- animals with the HER2+ model FVB/N-Tg(MMTVneu)202Mul/J. Tumors emerging in HER2+/miR-424(322)/503-/- animals presented higher levels of BCL2 and hyperactivation of the IGF1R-AKT signaling compared to HER2+/miR-424(322)/503+/+ counterparts. Furthermore, these tumors were resistant to standard chemotherapy. Importantly, inhibition of BCL2 with ABT-199 and IGF1R with BMS-754807, two compounds currently in clinical trial, completely reverted chemotherapy resistance.

Overall, our data present evidence supporting a tumor suppressor role for miR-424(322)/503 cluster and its implication in resistance to chemotherapy.

Citation Format: David Llobet Navas, Francois Bertucci, Ruth Rodriguez-Barrueco, Jose Silva. The miR-424/503 cluster is a breast cancer tumor suppressor with a role in chemoresistance. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2666.