Espinosa and Slack: The Role of MicroRNAs in Cancer 137 43 miRNAs was significantly different in the tumor tissues compared to the normal adjacent tissues. Of these miRNAs, 28 were down-regulated and 15 were up-regulated in the malignant tissue [46]. Similar studies have been carried out in CLL [77], breast cancer [50], glioblastoma [78], pancreatic cancer [79], hepatocellular carcinoma [80], and thyroid papillary carcinoma [58]. The ability to profile the miRNA ex- pression of a tumor with accuracy and repro- ducibility in a clinical setting would constitute a valuable medical tool. Indeed, miRNA profiles have been shown to be highly informative. Using a bead-based ap- proach, Lu et al. attempted to classify 17 poorly differentiated tumors with non-diag- nostic histological appearance based on their miRNA expression profiles. A clinical diag- nosis on these tumors was established by anatomical context, whether directly (a pri- mary tumor observed in a given organ) or indirectly (a metastasis of a previously iden- tified primary tumor). They were able to cor- rectly diagnose 12 of the 17 samples, against 1/17 correct diagnoses for an analysis using their mRNA profile [76]. While the number of transcripts analyzed was much smaller for miRNAs than for mRNAs (~200 against > 15,000), it is possible that the miRNA-based diagnoses are more accurate because of the regulatory role of these molecules, while most mRNAs will not be regulatory in na- ture. Further studies, however, are necessary to ascertain the value of miRNAs as diag- nostic tools, and their potential value as prognostic markers, by correlating miRNA expression with type-specific parameters such as metastatic potential, proliferative index, and response to existing treatments. CLOSING REMARKS: miRNAS AS TREATMENT TARGETS While the field of miRNAs — and par- ticularly the study of the roles of miRNAs in cancer — is at an early stage, their poten- tial as targeted therapeutic tools has not gone unnoticed. A promising approach is to target oncogenic miRNAs with oligonucleotides complementary for either their mature or precursor sequence, deemed anti-miRNA oligonucleotides (AMOs). A number of chemical modifications for these oligonu- cleotides are available, with the purpose of making them more stable and less toxic. 2’- O-methylated AMOs are perhaps the most common form of modified RNA, conferring limited nuclease resistance, and enhancing the binding affinity to RNA. Inhibition of let-7 function with 31-nucleotide 2’-O-Me AMOs has been demonstrated in HeLa cells and C. elegans larvae [81], and 2’-O-Me AMOs have been reported to abro- gate miRNA function in cellular assays [82,83], providing proof-of-principle on the blockage of miRNA function by extraneous oligomers. Another common modification to the structure of AMOs is that of the locked nucleic acid (LNA)-modified oligomers. In these RNAs, the 2’ oxygen atom is linked to the 4’ position through a methylene bridge, forming a rigid bicyclic structure locked into a C3’-endo (RNA) sugar conformation [84]. This modification results in greatly en- hanced affinity for RNA and the formation of exceedingly strong duplexes with excel- lent mismatch discrimination properties [85- 87]. This makes them prime candidates for AMO-based therapy, and results in rodents have been encouraging [88]. While the use of AMOs could be useful to inhibit oncogenic miRNAs, another ther- apeutic possibility concerns the cases in which certain miRNAs are under-expressed, such as let-7 in lung cancer, or miR-15a and mir-16-1 in CLL. In this case, synthetic miRNAs — likely modified as discussed above — in either their mature or precursor form could be delivered to the target cells to counteract the deficiency and halt malignant growth. While this approach has been at- tempted in cell culture [89], further testing in animal models is required. A pressing challenge to the use of miRNAs as therapeutic tools concerns find- ing methods to deliver synthetic miRNAs or AMOs to the desired tissues in a targeted and effective manner. Given that these mol- ecules cannot discriminate between healthy and malignant cells, side effects of treatment remain a concern. Likewise, since these