Discovered in 1993 from nematode Caenorhabditis elegans, miRNA are small, non-coding RNAs composed of 18–24 nucleotides. They act by negatively regulating gene expression at the post-transcriptional level through modulation of mRNA expression. Lee et al. 1 found that lin-4, an important gene that is important for post-embryonic development of C. elegans, does not code for a protein. Instead, it is transcribed into a 22-nucleotide RNA molecule. Seven years later, a second miRNA, let-7, also emerged from C. elegans genetic screening. The discovery of many new miRNAs has continued there after, with evidence emerging to suggest these small RNAs play a crucial role in gene regulation. This role has been confirmed by the identification of hundreds of conserved miRNA sequences in nematodes, flies, and mammals 2.

To date, more than 3% of the genes in humans have been found to encode for miRNAs. Anywhere from 40% to 90% of the human protein encoding genes are under miRNA-mediated gene regulation 3. MiRNAs exert their effect by negatively regulating gene expression through one of two mechanisms, i.e., mRNA degradation or translational suppression. The mechanism by which the miRNA proceeds is dependent on the complementarity between the miRNA and its mRNA target 4. A high level of complementarity corresponds to the degradation of the target mRNA via the RNA-mediated interference (RNAi) pathway. When there is inadequate complementarity, miRNAs bind to the untranslated region of the target mRNAs and translational suppression occurs through a RNA-induced silencing complex (RISC). Plants more commonly utilize the first mechanism while animals employ the second 2.

MiRNAs play an important role in different biological process such as cell proliferation during development, cell growth, and organization as well as apoptosis, biogenesis, transcription, signal transduction, cell cycle, neurogenesis, and fat metabolism. The deregulation of miRNA function can lead to human diseases as the target genes of miRNA are involved in several metabolic disorders such as various forms of cancers, lymphomas, diabetes and neurological disorders such as Parkinson's and Alzheimer's diseases. Recently, there has been considerable interest in understanding the mechanism of the RNA regulatory phenomena and how miRNA functions in carcinogenesis. Specifically, determining how the regulation of miRNA may be targeted to develop cancer therapies has been a focus. In this review, we discuss recent discoveries related to miRNA in different cancer diagnoses and, in particular, treatment related to ovarian cancer (OVCA).

Like proteins, miRNAs are transcribed from DNA into a primary (Pri) RNA transcript. In the first step, formation of Pri-miRNA inside the nucleus occurs from the transcription of the miRNA gene by RNA polII. The Pri-miRNA is capped, polyadenylated, and then processed by RNAase III endonuclease Drosha and the double stranded (ds) RNA-binding protein Pasha. This processing results in the formation of a 60 to 70 nucleotide stem loop precursor (Pre) called Pre-miRNA. The Pre-miRNA is then transported to the cytoplasm by the Ras-related nuclear protein-guanosine-5'-triphosphate (RAN-GTP)-dependent export receptor Exportin-5. Next, the Pre-miRNA is cleaved at the base of the loop by a second RNase III endonuclease, a cytoplasmic Dicer 5. This forms a transient, double stranded miRNA (dsmiRNA) of 22 nucleotides in length. The dsmiRNA complex then enters the miRNA-associated multiprotein RISC (miRISC), which contains the Argonuate 2 protein, and the mature miRNA is retained in the miRISC complex 2. Argonuate proteins are active RNase enzymes and extremely conserved in the RISC. The mature miRNA is now capable of regulating its target mRNAs (See Fig. 1). Deregulation of this processing results in perturbation in miRNA genesis and may have oncogenic consequences.

Gene regulation by miRNA can affect a wide variety of cell functions, such as regulation of cell differentiation, proliferation, and apoptosis. MiRNAs are important for normal cell functioning; dysfunction of the miRNA regulation system results in disruption of the normal cellular activity leading to induction of a disease.

Recent studies show that several miRNAs are differentially expressed in several human cancers, which indicate that miRNAs may have a role in the carcinogenic processes (See Table 1). In 2002, Calin et al. 25 correlated aberrant miRNA expression with cancer. The first study that was reported regarding the abnormalities in miRNA expression was in miR-15a and miR-16-1 in B-cell chronic lymphocytic leukemia (CLL); 25. The study showed that miRNA 15 and miRNA 16 genes are located in a deleted region present in more than half of B-cell CLL cases and that both of the genes are deleted/mutated in more than half of the in CLL cases. In addition, miR-17-92, which is present as a cluster of miRNAs on chromosome 13, was reported to be associated with B cell lymphoma 26. Other miRNAs that have been discovered to be abnormally expressed in cancerous tissues include: miR-145, which is down regulated in breast cancer 27 ovarian cancer 4; miR-10b, which is found to be mostly down regulated in breast cancer 27; and miR-145, which has reduced expression in colorectal cancer 28.

MiRNAs participate in various biological processes including cell differentiation, proliferation, apoptosis, stress resistance, and fat metabolism 29 and often have dual functions. For instance, in some types of cancers, they possess oncogenic activity, while in others they can suppress tumor development. Michael et al. 28 showed that miR-143 and mi-R-145 have been associated with the reduction of some malignancies, suggesting their potential tumor suppressing activities. Additional studies demonstrated that miRNAs were found to suppress BCL2, an anti-apoptotic gene 30; supporting the idea that miRNA can also have oncogenic activity. Furthermore, miR-372 and miR-373 cooperate with oncogenic Ras in the cellular transformation of testicular germ cell tumors 31. Additionally, in the Let-7 family, Let 7a-2 and Let 7a-3 are down regulated in lung cancer and Ras is frequently mutated 32. Let-7b is involved in Ras regulation and inhibits cell cycle progression in melanoma 3334.

MiR-17-92 has been shown to function in embryogenesis 35 and is overexpressed in lung cancer, especially in small cell lung cancer, which is characterized by neuroendocrine differentiation. In addition to being overexpressed, there are an increased number of gene copies. This suggests they have a role in the development of lung cancer 36. MiR-34a, which is absent in pancreatic cells, was directly trans-activated by the p53 gene and proved to be associated with modulation of gene expression initiated by p53 37. C-MYC, a proto-oncogene, encodes for a transcription factor that is responsible for cell proliferation and growth. A mutation in this gene results in most common abnormalities associated with human cancers. Finally, using microarray-based large scale profiling, Volinia et al. 38 analyzed the miRNA expression pattern in 540 normal and tumor samples from six types of tissues (lung, breast, stomach, prostate, colon, and pancreas). They proved that the abnormal expression pattern of many miRNAs is connected to carcinogenesis.

For additional information regarding differential expression of miRNAs in various cancers including glioblastomas, colorectal neoplasia, hepatocellular carcinoma, papillary thyroid carcinoma etc. ' [see Additional file 1]'.

OVCA is the most common of the gynecologic malignancies, but little is known about the molecular genetics of its initiation and progression. Epithelial OVCA (EOC), which accounts for approximately 90% of OVCAs, continues to be the leading cause of death resulting from gynecological malignancies 39. Histologically, OVCA is divided into different subgroups such as serous, mucinous, endometrioid, clear cell, Brenner, and undifferentiated carcinomas 40. The high mortality rate of this disease is due to late stage diagnosis for more than 70% of affected patients (Chemoresistance). Trans-vaginal ultrasound and serum CA-125 remain poor tests for diagnosing the disease in its early stage and, as a result, most cases are still more likely to be detected at the advanced stage 41.

New, more effective diagnostic tests must be developed to identify OVCA in its early stage. If OVCA is diagnosed in the early stage, current treatments will be more successful, which in turn will significantly reduce the high mortality rate. The discovery that miRNAs are expressed at different levels in normal versus malignant tissues offers the possibility of identifying new methods of diagnosing early stage incidence and discovering new therapies for the treatment of OVCA.

With the advent of new miRNA technology, it is possible to plan appropriate treatments and predict chemotherapy outcomes. Studies have shown that miRNAs, which are noncoding regulatory RNA products, are aberrantly expressed in many cancer types including breast 27, pancreatic 42, and lung cancer 4. In addition to these forms, a study conducted by Iorio et al. in 2007, 4 confirmed that miRNAs are also abnormally expressed in OVCA (See Table 2). It was concluded that in the cancerous tissues, i.e., miR-141, miR-200a, miR-200b, and miR-200c, had the highest level of up-regulation while miR-125b, miR-140, miR-145, and miR-199a were the most down-regulated. Moreover, this study demonstrated that the levels of expression between the normal and malignant ovaries were so drastic that expression levels alone could distinctly distinguish between tissue types. This finding is extremely significant because it points to the possibility that miRNAs may have oncogenic or tumor suppressing activities.

In addition to the difference in expression levels between normal and cancerous tissues, it has been shown that miRNAs can also be histotype-specific. In endometrioid tumors, miR-21, miR-182, and miR-205 are considerably overexpressed while miR-144, miR-222, and miR-302a have reduced expression compared to normal. However, in serous and clear cell tumors, these miRNAs are normally expressed 4. These findings suggest that while all types of OVCA are currently treated with the same standard therapies, a more effective course of treatment could be targeted towards specific histotypes of various cancers.

The above tests are invasive procedures because they require a tissue sample to determine the expression levels of miRNAs for diagnostic purposes. However, a new non-invasive approach has recently been discovered that only requires a serum sample from the patient. In the serum of OVCA patients, miRNAs 21, 29a, 92, 93, and 126 were overexpressed while miRNAs 127, 155, and 99b were under expressed as compared to controls. This indicates that a patient's serum miRNA level could be used to potentially diagnose OVCA. Additionally, miRNAs 21, 92, and 93 were also overexpressed in patients with normal CA125 levels. This suggests that earlier diagnosis of OVCA may be possible through the identification of these miRNAs even though patients may present with normal CA-125 levels 43.

Another new non-invasive method with the potential for diagnosing OVCA involves isolating tumor-derived exosomes. Exosomes are saucer shaped nanovesicles (30–100 nm) that are normally released by multiple mammalian cell types and have a significant role in cell-to-cell communication. In addition to exosomes released by normal proliferating cells, tumor cells also release exosomes. Tumor cells release exosomes at a much higher level than normal cells and the exosomes also display numerous tumor antigens that mirror the primary tumor cells. Therefore, it is proposed that these tumor-derived exosomes could be used as a diagnostic biomarker. In addition to the antigens, circulating tumor-derived exosomes have been shown to contain miRNAs that parallel those of the original tumor 44. This demonstrates that miRNA profiling can be executed and accurately represent the originating tumor through a blood sample rather than requiring the more invasive tissue sample. It has been shown that eight particular miRNAs are of diagnostic significance in OVCA. MiRNA 21, 141, 200a, 200b, 200c, 203, 205, and 214 are all overexpressed in tumor cells and tumor-derived exosomes. The levels are different enough to clearly distinguish between benign and malignant tumors as well 44.

The current treatment utilized for patients with OVCA consists of primary cytoreductive surgery followed by platinum and taxane combination chemotherapy treatments. This method of therapy is initially successful; however, most of the patients become resistant to the platinum-based chemotherapy 45. Recent studies indicate that there are significant differences in the level of miRNA expression between chemotherapeutic sensitive and resistant OVCA cell lines and tissues. Boren et al. 45 found that 27 miRNAs were connected to OVCA cell line sensitivity to platinum-based chemotherapeutic agents and the differential expression of these miRNAs leads to chemoresistance. In the cells resistant to the drugs, 18 miRNAs were up-regulated and 9 were down-regulated. It was determined that increased expression of miR-181a, miR-181b, and miR-213 in OVCA cell lines results in resistance to doxorubicin and gemcitabine and increased expression of miR-99b and miR-14 leads to docetaxel and paclitaxel resistance. Eitan et al. 46 also discovered several miRNAs that were differentially expressed in stage 3 ovarian tumors. It was established that increased expression of miR-23a, miR-27a, miR-30c, let-7g, and miR-199a-3p corresponds to resistance to platinum-based chemotherapy while reduced expression of miR-378 and miR-625 relates to resistance.

The difference in miRNA expression levels between chemotherapy-sensitive and resistant cells is clinically significant. New treatments could exploit these differences if the mechanism behind regulating miRNA expression is determined. Then, for instance, miR-378, which is down-regulated in malignant ovarian tissue, could be induced to over express to a normal level, possibly making the cells more sensitive to platinum-based chemotherapy. Moreover, identification of specific miRNAs between types of cancers (lung, ovarian, pancreatic) and even within types (endometrioid, clear cell, serious in ovarian) could allow for more personalized treatments, which certainly present the chance for a better prognosis.

While there is not much information currently available on the mechanisms responsible for inducing aberrant miRNA expression, there is a new possible explanation for their overexpression in OVCA. Iorio et al. 4 recently proved that treating OVCAR3 cells with 5-aza-2' deoxycytidine, a demethylating treatment, results in the overexpression of miR-21, miR-203, and miR-205. These miRNAs are also up-regulated in ovarian malignant tumors. This indicates that the mechanism causing their up-regulation in vivo could be DNA hypomethylation.

The discovery of aberrant miRNA expression between control patients and those with OVCA holds great promise for improved patient care and more positive outcomes. Thus, identification of altered miRNA expression, as well as their targets, provides new opportunities for therapeutic strategies. MiRNA-based gene therapy targeting deregulated miRNAs will be the future tool for cancer treatment. Using data from miRNA profiling and knowledge of familial history of a certain diseases will help in developing miRNA-based personalized therapy. Detailed understanding of the characteristic miRNA abnormalities could contribute to novel approaches in early diagnosis and treatment of different diseases including cancer and can serve as a biomarker for cancer. miRNA expression profiling eventually assist physicians in providing patients with accurate diagnosis, personalized therapy, and treatment outcome.

(miRNA): MicroRNA; (mRNA): messenger RNA; (RNAi): RNA-mediated interference; (RISC): RNA-induced silencing complex; (Pri): primary; (Pre): precursor; (RAN): Ras-related nuclear protein; (GTP): guanosine-5'-triphosphate; (ds): double stranded; (dsmiRNA): double stranded miRNA; (miRISC): miRNA-associated multiprotein RISC; (SMA): Spinal muscularatrophy; (SMN): survival of motor neuron; (miRNP): miRNA-containingribonucleoprotein particle; (ban): bantam; (polyQ): polyglutamine; (SCA 3): spinocerebellar ataxia type 3; (MHC): major histocompatibility complex; (TR): thyroid-hormone-receptor; (THRAP1): thyroid-hormone-receptor co-regulator TR-associated protein 1; (CLL): chronic lymphocytic leukemia; (EOC): Epithelial ovarian cancer; (OVCA): ovarian cancer.

The authors declare that they have no competing interests.

PPS drafted the manuscript. LEH contributed in writing part of the manuscript. SSK conceptualized, edited, and revised the manuscript. All authors have read and approved the final manuscript.