136 Espinosa and Slack: The Role of MicroRNAs in Cancer miRNA SIGNATURES AND 5p and miR-20a, target the transcription fac- DETECTION METHODS tor E2F1. E2F1 regulates the transition from G1 to S phase of the cell cycle by regulating The multiple lines of evidence indicat- genes that are involved in DNA replication, ing that microRNAs are differentially ex- cell division, and apoptosis [64]. Notably, pressed in normal and in tumor samples E2F1 is known to function in a reciprocal suggest that miRNAs could serve as useful positive-feedback loop with MYC. Consis- tumor profiling tools. Indeed, profiling stud- tent with this, it was found that inhibition of ies have shown that each cancer type pos- miR-17-5 and miR-20a in a cervical cancer- sesses a distinct miRNA expression derived cell line resulted in a concomitant signature, and this signature can provide increase in the expression level of E2F1. useful information about the malignancy. Likewise, mutations of the sites complemen- The adaptation of high-throughput technolo- tary to miR-17-5p and miR-20a in the gies have facilitated the study of the expres- 3’UTR of E2F1 resulted in increased re- sion of multiple miRNAs in a given sample, porter activity, compared with that of con- making it possible to profile substantial sam- structs harboring the wild-type sequence of ple numbers with relative ease. Among the E2F1’s 3’UTR. These results suggest a most important methods for miRNA profil- model in which MYC can induce the tran- ing is the use of oligonucleotide miRNA mi- scription of both the miR-17-92 cluster and croarray analysis, which allows the E2F1; in turn, the miRNAs of the cluster researcher to simultaneously determine the negatively regulate the transcription of expression levels of hundreds of miRNAs in E2F1. One consequence of this model is that a given sample [65-67]. Multiple variations miRNAs of the miR-17-92 cluster directly of this technique have been developed, un- regulate the effector of MYC-mediated cell derscoring its position as the central miRNA growth. profiling tool [68-71]. Other important ap- An apparent paradox, however, also proaches to the problem are also available, emerges from these results. In the presence of such as quantitative PCR for precursor MYC, as noted above, the miR-17-92 cluster miRNAs [72] or mature miRNAs [73,74]; would function as a tumor suppressor, curbing genome-wide approaches with serial analy- the cellular growth mediated by E2F1, which is sis of gene expression (SAGE), such as mi- in contradiction with the results of He et al. that RAGE [75]; and bead-based flow implicated miR-17-92 as an oncogene. These cytrometric techniques [76]. results can perhaps be reconciled by consider- Taking advantage of these tools, several ing that while E2F1 can promote cellular pro- groups have studied the miRNA expression liferation, it has been reported that its presence of several types of cancer. Michael et al. [49] in excess levels can induce apoptosis [64]. As and Cummins et al. [75] have studied the ex- such, the negative regulation of E2F1 by the pression profile of the known miRNAs in miRNAs of the miR-17-92 cluster might in fact colorectal cancer samples. These studies in- prevent E2F1 levels from reaching the thresh- dependently found a general down-regula- old at which apoptosis is triggered, thus acting tion of miRNA levels in tumor cells as an oncogene by supporting MYC-mediated compared to normal colonic epithelium, proliferation. while also observing a significant decrease It is clear that the interaction of miRNAs in the levels of miR-143 and miR-145 be- with their targets is not as straightforward as yond this general effect. Cummins et al. initial studies might have suggested and intri- found a signature of 50 differentially ex- cate regulatory networks are in place for any pressed miRNAs in malignant vs. normal one cellular system. There exists the possi- colonic epithelium, of which 32 miRNAs bility that the precise role of miR-17-92 in a had reduced levels in tumor cells. Yanaihara given cell depends on what other miRNAs et al. studied over 100 patient-matched pairs are present, which in turn may be affected by of primary malignant and normal adjacent spatial and temporal considerations. lung tissue and found that the expression of