MicroRNAs (miRNAs) are small, endogenous, noncoding RNAs that act as posttranscriptional repressors of gene expression. MiRNAs act as repressors of target mRNAs by promoting their degradation, when their sequences are perfectly complementary, or by inhibiting translation, when their sequences contain mismatches.
MiRNAs are transcribed by RNA polymerase II (pol II) or RNA polymerase III (pol III; see Qi et al. (2006) Cellular & Molecular Immunology, Vol. 3:411-419) and arise from initial transcripts, termed primary miRNA transcripts (pri-miRNAs), that are generally several thousand bases long. Pri-miRNAs are processed in the nucleus by the RNase Drosha into about 70- to about 100-nucleotide hairpin-shaped precursors (pre-miRNAs). Following transport to the cytoplasm, the hairpin pre-miRNA is further processed by Dicer to produce a double-stranded miRNA. The mature miRNA strand is then incorporated into the RNA-induced silencing complex (RISC), where it associates with its target mRNAs by base-pair complementarity. In the relatively rare cases in which a miRNA base pairs perfectly with an mRNA target, it promotes mRNA degradation. More commonly, miRNAs form imperfect heteroduplexes with target mRNAs, affecting either mRNA stability or inhibiting mRNA translation.
MicroRNAs have been implicated in several diseases including cancer. For example, human miRNA genes miR15a and miR16-1 are deleted or down-regulated in approximately 60% of B-cell chronic lymphocytic leukemia (CLL) cases (Calin et al., Proc Natl Acad Sci, 2002; 99:15524-15529). Similarly, dysregulation of miR-155-5p has been linked to signaling events that are implicated in the pathogenesis of cutaneous T cell lymphoma (CTCL). It has been shown that malignant T-cells constitutively express an IL-2 receptor complex and associated Janus kinases (JAKs) that activate transcription via signal transducers and activators of transcription (STAT) proteins. Chromatin immuno-precipitation experiments showed that STAT-5 was associated with the promoter of MIR155HG, a host gene for miR-155-5p. This suggests that miR-155-5p may regulate the STAT-5 signaling pathway in CTCL malignant T-cells. Inhibition of the JAK/STAT pathway resulted in the down-regulation of miR-155-5p expression whereas treatment of cells with cytokines that activate STAT-5 resulted in increased miR-155-5p levels (Kopp et al. 2013). These results suggest that miR-155-5p may play a role in the pathogenesis of CTCL.
Currently there are no therapies that cure or prolong the survival of late-stage CTCL patients (Prince et al., 2009). Treatments for CTCL patients at an early-stage of disease are palliative and non-aggressive with careful physician monitoring. More advanced-stage CTCL patients are typically treated with systemic drugs, such as retinoids (bexarotene) or histone deacetylase inhibitors (vorinostat). Radiotherapy is typically the last line of defense and can result in partial disease regression but not full eradication. Many treatments have serious side effects or result in resistance over time. Thus, there remains an unmet medical need for new therapies to treat cutaneous T-cell lymphoma.