Many RNA species have been identified as important players in the development of chronic diseases, including cancer. Over the past decade, numerous studies have highlighted how regulatory RNAs such as microRNAs (miRNAs) and long noncoding RNAs (lncRNAs) play crucial roles in the development of a disease state. It is clear that the aberrant expression of miRNAs promotes tumor initiation and progression, is linked with cardiac dysfunction, allows for the improper physiological response in maintaining glucose and insulin levels, and can prevent the appropriate integration of neuronal networks, resulting in neurodegenerative disorders. Because of this, there has been a major effort to therapeutically target these noncoding RNAs. In just the past 5 years, over 100 antisense oligonucleotide–based therapies have been tested in phase I clinical trials, a quarter of which have reached phase II/III. Most notable are fomivirsen and mipomersen, which have received FDA approval to treat cytomegalovirus retinitis and high blood cholesterol, respectively. The continued improvement of innovative RNA modifications and delivery entities, such as nanoparticles, will aid in the development of future RNA-based therapeutics for a broader range of chronic diseases. Here we summarize the latest promises and challenges of targeting noncoding RNAs in disease.

Micro-RNAs are small noncoding RNAs, which diminish the stability and/or translation of mRNAs. This study examined whether miR-206, previously shown to be elevated in estrogen receptor (ER)α-negative breast cancer, regulates the expression of ERα. Two putative miR-206 sites, (hERα1 and hERα2), were found in silico within the 3′-untranslated region of human ERα mRNA. Transfection of MCF-7 cells with pre-miR-206 or 2′-O-methyl antagomiR-206 specifically decreased or increased, respectively, ERα mRNA levels. Overexpression of pre-miR-206 reduced ERα and β-actin protein levels, with no effect on ERβ, E-cadherin, or glyceraldehyde-3-phosphate dehydrogenase. Reporter constructs containing the hERα1 or hERα2 binding sites inserted into the 3′-untranslated region of the luciferase mRNA conferred a 1.6- and 2.2-fold repression of luciferase activity, respectively, in HeLa cells. Both miR-206 sites responded accordingly to exogenous hsa-pre-miR-206 and 2′-O-methyl antagomiR-206, and both sites were rendered inactive by mutations that disrupted hybridization to the 5′-seed of miR-206. A C→T single nucleotide polymorphism in the hERα1 site increased repression of luciferase activity to approximately 3.3-fold in HeLa cells. MiR-206 levels were higher in ERα-negative MB-MDA-231 cells than ERα-positive MCF-7 cells, but only the ERα1 site mediated significantly more repression in reporter constructs. MiR-206 expression was strongly inhibited by ERα agonists, but not by an ERβ agonist or progesterone, indicating a mutually inhibitory feedback loop. These findings provide the first evidence for the posttranscriptional regulation of ERα by a micro-RNA in the context of breast cancer.

Immature self-reactive B cells undergo central tolerance as they emerge from the bone marrow. Xiao and colleagues show that B cell tolerance requires downregulation of the microRNA miR-148a, as aberrant expression of miR-148a increases survival of autoreactive B cells and contributes to autoimmunity. Autoreactive B cells have critical roles in a large diversity of autoimmune diseases, but the molecular pathways that control these cells remain poorly understood. We performed an in vivo functional screen of a lymphocyte-expressed microRNA library and identified miR-148a as a potent regulator of B cell tolerance. Elevated miR-148a expression impaired B cell tolerance by promoting the survival of immature B cells after engagement of the B cell antigen receptor by suppressing the expression of the autoimmune suppressor Gadd45α, the tumor suppressor PTEN and the pro-apoptotic protein Bim. Furthermore, increased expression of miR-148a, which occurs frequently in patients with lupus and lupus-prone mice, facilitated the development of lethal autoimmune disease in a mouse model of lupus. Our studies demonstrate a function for miR-148a as a regulator of B cell tolerance and autoimmunity.

Triple-negative breast cancer (TNBC) is an aggressive subtype with no clinically proven biologically targeted treatment options. The molecular heterogeneity of TNBC and lack of high frequency driver mutations other than TP53 have hindered the development of new and effective therapies that significantly improve patient outcomes. miRNAs, global regulators of survival and proliferation pathways important in tumor development and maintenance, are becoming promising therapeutic agents. We performed miRNA-profiling studies in different TNBC subtypes to identify miRNAs that significantly contribute to disease progression. We found that miR-34a was lost in TNBC, specifically within mesenchymal and mesenchymal stem cell–like subtypes, whereas expression of miR-34a targets was significantly enriched. Furthermore, restoration of miR-34a in cell lines representing these subtypes inhibited proliferation and invasion, activated senescence, and promoted sensitivity to dasatinib by targeting the proto-oncogene c-SRC. Notably, SRC depletion in TNBC cell lines phenocopied the effects of miR-34a reintroduction, whereas SRC overexpression rescued the antitumorigenic properties mediated by miR-34a. miR-34a levels also increased when cells were treated with c-SRC inhibitors, suggesting a negative feedback exists between miR-34a and c-SRC. Moreover, miR-34a administration significantly delayed tumor growth of subcutaneously and orthotopically implanted tumors in nude mice, and was accompanied by c-SRC downregulation. Finally, we found that miR-34a and SRC levels were inversely correlated in human tumor specimens. Together, our results demonstrate that miR-34a exerts potent antitumorigenic effects in vitro and in vivo and suggests that miR-34a replacement therapy, which is currently being tested in human clinical trials, represents a promising therapeutic strategy for TNBC. Cancer Res; 76(4); 927–39. ©2015 AACR.