MicroRNAs (miRNAs) are small, non-coding RNA molecules that function as master regulators of the genome. They are encoded by their respective genes that produce precursor molecules with varying lengths. These precursors are further processed into approximately 21-nucleotide mature miRNAs that repress the expression of protein-encoding genes by interacting with messenger RNAs with sequences that are complementary to specific miRNAs. More than 1,500 human miRNA genes have been identified that collectively control an estimated >30% of all human genes. Each miRNA appears to regulate the expression of tens to hundreds of genes to efficiently coordinate multiple cellular pathways. By coordinating the expression of multiple genes, miRNAs are responsible for guiding embryonic development, immune and related inflammatory responses, as well as cellular growth and proliferation.
The growing awareness of the importance of miRNAs has generated intense activity in the biomedical research community. The first published report of a miRNA occurred in 1993 and resulted from a genetic screen in worms. The worm miRNA was the only known small RNA until 1999, when a second miRNA was discovered. The miRNAs appeared to be an oddity of worms until 2001, when the cloning and sequencing of more than 100 human and mouse miRNAs was described in a publication in the journal Science (Lagos-Quintana et al., 2001). Since then there has been an explosion in the number of publications related to miRNAs, with a substantial number of those describing the contributions of various miRNAs to cancer development.
miRNAs are intimately associated with normal cellular processes and therefore, deregulation of miRNAs contributes to a vast array of diseases including cancer. miRNAs are differentially expressed in cancer tissues compared to normal tissues and can play a causative role in tumorigenesis. Examples of oncogenic miRNAs that are frequently over-expressed in cancer tissues are miR-21, miR-155 and miR-17-92. In contrast, miR-15a, miR-16-1, miR-34, as well as miRNAs of the let-7 family
function as tumor suppressors and are frequently reduced in cancer tissues. A likely explanation for their direct role in cancer is the observation that miRNAs regulate many transcripts of proteins that are involved in the control of cellular proliferation and apoptosis. Regulated proteins include conventional proto-oncoproteins and tumor suppressors such as RAS,
MYC, BCL2, PTEN and p53. Aberrant expression of miRNAs therefore often has dire consequences and can therapeutically be corrected by either inhibiting oncogenic miRNAs or replacing the depleted tumor suppressor miRNA.
Increasing evidence indicates that the introduction of specific miRNAs into disease cells and tissues induces favorable therapeutic responses. The promise of miRNA therapy is perhaps greatest in cancer due to the apparent role of miRNAs as tumor suppressors and oncogenes. The rationale for miRNA-based therapeutics for cancer is supported by the following observations:
miRNAs are frequently mis-regulated and expressed at aberrant levels in diseased tissues when compared to normal tissues. Numerous laboratories have provided strong evidence showing that miRNAs are consistently altered in cancerous tissues relative to their corresponding normal tissues. Often, altered expression is the consequence of genetic mutations that lead to increased or reduced expression of particular miRNAs. Various diseases reveal unique miRNA expression signatures that can be exploited as diagnostic and prognostic markers. Mirna Therapeutics has generated a large collection of data describing miRNAs with differential expression in normal and tumor tissues.
Mis-regulated miRNAs contribute to cancer development by functioning as oncogenes or tumor suppressors.
Oncogenes are defined as genes whose over-expression or inappropriate activation
leads to oncogenesis. Tumor suppressors are genes that are required to keep
cells from becoming cancerous; the down-regulation or inactivation of tumor suppressors is a common inducer of cancer. Both types of genes represent preferred intervention points to specifically target the molecular basis for a given cancer. Examples of oncogenic miRNAs are miR-21 and miR-17-92; let-7 and miR-34 are examples of tumor suppressive miRNAs. Mirna Therapeutics has established a proprietary IP portfolio of miRNAs that
covers both types of miRNAs.
Administration of miRNA induces a therapeutic response by blocking or reducing tumor growth in pre-clinical animal studies. Several laboratories, including Mirna, have provided evidence demonstrating that restoring miRNA function can prevent or reduce the growth of cancer cells in vitro and in animal models. A well-characterized example is the anti-tumor activity of miR-34 that blocks tumor growth in mouse models of non-small cell lung cancer, prostate cancer, hepatocellular carcinoma and B-cell lymphoma (see Wiggins et al., Cancer Res, 2010.; Trang et al., Mol. Ther., 2011; Liu et al., Nat. Med., 2011; Craig et al., Leukemia, 2012). Mirna has established proof-of-concept for the systemic delivery of a synthetic miRNA in mouse models of cancer and is driving these lead candidates to the clinic. Mirna intends to file an IND for its lead program, MRX34, in 1Q 2013, followed by initiation of a Phase 1 trial in patients with advanced or metastatic cancers with liver involvement.
A given miRNA controls multiple cellular pathways and therefore may have superior therapeutic activity. Based on their biology, miRNAs act as “master switches” of the genome, regulating multiple gene products and coordinating multiple pathways. Genes regulated by miRNAs include genes that encode conventional oncogenes and tumor suppressors, many of which are individually pursued as drug targets by the pharmaceutical
and biotechnology industry. Thus, miRNA therapeutics may have superior activity by targeting multiple cancer-associated genes. Given the observation that mis-regulation of miRNAs is frequently an early event in the process of tumorigenesis, miRNA-based therapeutics, which replace missing miRNAs, may be the most appropriate therapy.
miRNAs are natural molecules and are therefore less likely to induce non-specific side-effects.
Millions of years of evolution helped to develop the regulatory network of miRNAs, fine-tuning the interaction of miRNAs with target messenger RNAs. Therefore, miRNAs and miRNA derivatives will have few if any sequence-specific “off-target” effects.