Modulation of mismatch repair and genomic stability by miR-155

MicroRNAs and DNA damage response: implications for cancer therapy

The DNA damage response (DDR) pathways play critical roles in protecting the genome from DNA damage. Abrogation of DDR often results in elevated genomic instability and cellular sensitivity to DNA damaging agents. Many proteins involved in DDR are subjected to precise regulation at multiple levels, such as transcriptional control and posttranslational modifications, in response to DNA damage. MicroRNAs (miRNAs) are a class of small non-coding RNAs that negatively regulate gene expression at the post-transcriptional level. The expression levels of some miRNAs change in response to DNA damage. Some miRNAs, such as miR-24, 138, 96 and 182, have been implicated in DDR and/or DNA repair and affect cellular sensitivity to DNA damaging agents. In this review, we summarize recent findings related to the emerging roles of miRNAs in regulating DDR and DNA repair and discuss their potential in cancer therapy.

Multiple microRNAs may regulate the DNA repair enzyme uracil-DNA glycosylase

Human nuclear uracil-DNA glycosylase UNG2 is essential for post-replicative repair of uracil in DNA, and UNG2 protein and mRNA levels rapidly decline in G2/M phase. Previous work has demonstrated regulation of UNG2 at the transcriptional level, as well as by protein phosphorylation and ubiquitylation. UNG2 mRNA, encoded by the UNG gene, contains a long 3'untranslated region (3'UTR) of previously unknown function. Here, we demonstrate that several conserved regions in the 3'UTR are potential seed sites for microRNAs (miRNAs), such as miR-16, miR-34c, and miR-199a. Our results show that these miRNAs down-regulate UNG activity, UNG mRNA, and UNG protein levels. Down-regulation was dependent on the 3'UTR, indicating that the miRNAs directly target the conserved seed sites in the 3'UTR. These results add miRNAs as a new modality to UNG's increasing list of complex regulatory mechanisms.

Epigenetics of colorectal cancer

In the early years of the molecular biology revolution, cancer research was mainly focused on genetic changes (ie, those that altered DNA sequences). Although this has been extremely useful as our understanding of the pathogenesis and biology of cancer has grown and matured, there is another realm in tumor development that does not involve changing the sequence of cellular DNA. This field is called "epigenetics" and broadly encompasses changes in the methylation of cytosines in DNA, changes in histone and chromatin structure, and alterations in the expression of microRNAs, which control the stability of many messenger RNAs and serve as "master regulators" of gene expression. This review focuses on the epigenetics of colorectal cancer and illustrates the impact epigenetics has had on this field.

DNA damage and repair in epithelium after allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) in humans, following hematoablative treatment, results in biological chimeras. In this case, the transplanted hematopoietic, immune cells and their derivatives can be considered the donor genotype, while the other tissues are the recipient genotype. The first sequel, which has been recognized in the development of chimerical organisms after allo-HSCT, is the graft versus host (GvH) reaction, in which the new developed immune cells from the graft recognize the host’s epithelial cells as foreign and mount an inflammatory response to kill them. There is now accumulating evidence that this chronic inflammatory tissue stress may contribute to clinical consequences in the transplant recipient. It has been recently reported that host epithelial tissue acquire genomic alterations and display a mutator phenotype that may be linked to the occurrence of a GvH reaction. The current review discusses existing data on this recently discovered phenomenon and focuses on the possible pathogenesis, clinical significance and therapeutic implications.

The role of microRNAs in colorectal cancer

During the last decade, it has become clear that aberrant microRNA expression has a functional role in the initiation and progression of colorectal cancer (CRC). Specific microRNAs can act as either tumor suppressors or oncogenes depending on the cellular environment in which they are expressed. The expression of microRNAs is reproducibly altered in CRC, and their expression patterns are associated with diagnosis, prognosis, and therapeutic outcome in CRC. Studies have begun to examine the association of microRNA-related polymorphisms and their association with CRC incidence and prognosis as well as the possibility of using circulating microRNAs or fecal microRNA expression as noninvasive early detection biomarkers. These data suggest that microRNAs may be potential molecular classifiers, early detection biomarkers, and therapeutic targets for CRC. Here, we will review the evidence demonstrating a role of microRNAs in CRC.

The role of microRNAs in colorectal cancer

During the last decade, it has become clear that aberrant microRNA expression has a functional role in the initiation and progression of colorectal cancer (CRC). Specific microRNAs can act as either tumor suppressors or oncogenes depending on the cellular environment in which they are expressed. The expression of microRNAs is reproducibly altered in CRC, and their expression patterns are associated with diagnosis, prognosis, and therapeutic outcome in CRC. Studies have begun to examine the association of microRNA-related polymorphisms and their association with CRC incidence and prognosis as well as the possibility of using circulating microRNAs or fecal microRNA expression as noninvasive early detection biomarkers. These data suggest that microRNAs may be potential molecular classifiers, early detection biomarkers, and therapeutic targets for CRC. Here, we will review the evidence demonstrating a role of microRNAs in CRC.

Expression of miRNAs and PTEN in endometrial specimens ranging from histologically normal to hyperplasia and endometrial adenocarcinoma

We investigated the relationship between frequently deregulated microRNAs (miRNAs) and enodometrial pathology in an attempt to find the most dependable miRNA or combination of miRNAs to identify normal, hyperplastic and malignant endometrial tissues. We also investigated the association between those miRNAs and PTEN status. We measured the expression of six miRNAs (miR-21, 182, 183, 200a, 200c and 205) in 75 formalin-fixed, paraffin-embedded normal, hyperplastic, and malignant endometrial tissue blocks using Taqman-based real-time PCR assays. PTEN loss of expression was assessed in the same endometrial tissues by immunohistochemistry. Expression of five miRNAs (miR-182, 183, 200a, 200c and 205) was significantly higher in endometrial carcinoma (CA) when compared with complex atypical hyperplasia (CAH), simple hyperplasia (SH) and normal endometrial tissue (P<0.05, respectively). Considering the likelihood ratio and number of parameters, the composite panel of six miRNAs was the best marker, revealing a sensitivity of 91% and a specificity of 94% in differentiating endometrial CA from endometrial hyperplasia or normal endometrium while the individual miRNAs exhibited 64-77% sensitivity and 66-91% specificity. Interestingly, in distinguishing endometrial CA from CAH, the composite panel of four miRNAs (miR-182, 183, 200a, 200c) was the best marker, producing 95% sensitivity and 91% specificity. The percentage of PTEN loss was significantly higher in endometrial CA compared with SH (68% vs 24%, P<0.05), and it was also higher in CAH compared with SH (71% vs 24%, P<005). Aberrant expression of miRNAs and loss of PTEN expression are common in endometrial hyperplasia and CA. They might serve to increase the diagnostic reproducibility and improve discrimination, especially, between CAH and CA by miRNA expression profiles and between simple and complex hyperplasia through PTEN expression patterns. Those expression profiles of biomarkers also might be used to predict the potential for progression from endometrial hyperplasia to invasive CA.

MicroRNAs as Novel Biomarkers in Colorectal Cancer

MicroRNAs (miRNAs) play an important role in various physiologic and developmental processes and in the initiation and progression of cancer. This class of small, non-coding RNAs critically regulate gene expression at the post-transcriptional level and evidence suggests that they may function as both oncogenes and tumor suppressors. Colorectal cancer (CRC) is a major healthcare concern worldwide and in order to reduce CRC related deaths, research is aimed into the search for some novel screening approaches. In this sense, miRNAs are rapidly emerging as a novel class of biomarkers, with good potential as diagnostic and therapeutic targets. This review summarizes the recent findings of the clinicopathological relevance that miRNAs have in CRC initiation, development, and progress, highlighting their potential diagnostic, prognostic, and therapeutic use in CRC, focusing on the group of microsatellite instable and the group of hypermethylated CRCs, as well as discussing future prospects.

miR-21 induces cell cycle at S phase and modulates cell proliferation by down-regulating hMSH2 in lung cancer

PURPOSE

MicroRNAs regulate critical genes associated with lung cancer. Human mutS homolog 2 (hMSH2), one of the core mismatch repair genes, is affected in lung cancer development. The aim of this study is to investigate the role of miR-21 in hMSH2 gene expression and the effect of miR-21 on cell proliferation and cell cycle in lung cancer.

METHODS

The targets of miR-21 were predicted by a bioinformatics tool, and hMSH2 was validated as a direct target of miR-21 by luciferase activity assay. MiRNA mimics or inhibitors were used to stimulate or attenuate the effect of endogenous miR-21 on hMSH2 expression. MiR-21 and hMSH2 expressions were assessed with real-time RT-PCR and Western blotting. Cell cycle was determined by flow cytometry, and cell growth was analyzed by MTT assay and real-time cell analysis system.

RESULTS

MiR-21 expression was inversely correlated with hMSH2 expression in human lung cancer cell lines. Further validation showed hMSH2 was directly regulated by miR-21. The up-regulation of miR-21 significantly promoted cell proliferation and revealed a higher proportion of cells at S phase. However, knockdown of miR-21 expression resulted in cell cycle arrest at G2/M phase and inhibited cell proliferation.

CONCLUSIONS

These data suggest miR-21 is a key regulator of hMSH2 and modulates cell cycle and proliferation by targeting hMSH2 in human lung cancer.

MicroRNAs in common human diseases

MicroRNAs (miRNAs) are a class of short non-coding RNA molecules that have attracted tremendous attention from the biological and biomedical research communities over the past decade. With over 1900 miRNAs discovered in humans to date, many of them have already been implicated in common human disorders. Facilitated by high-throughput genomics and bioinformatics in conjunction with traditional molecular biology techniques and animal models, miRNA research is now positioned to make the transition from laboratories to clinics to deliver profound benefits to public health. Herein, we overview the progress of miRNA research related to human diseases, as well as the potential for miRNA to becoming the next generation of diagnostics and therapeutics.

Dysregulation of microRNAs in colonic field carcinogenesis: implications for screening

Colorectal cancer (CRC) screening tests often have a trade-off between efficacy and patient acceptability/cost. Fecal tests (occult blood, methylation) engender excellent patient compliance but lack requisite performance underscoring the need for better population screening tests. We assessed the utility of microRNAs (miRNAs) as markers of field carcinogenesis and their potential role for CRC screening using the azoxymethane (AOM)-treated rat model. We found that 63 miRNAs were upregulated and miR-122, miR-296-5p and miR-503# were downregulated in the uninvolved colonic mucosa of AOM rats. We monitored the expression of selected miRNAs in colonic biopsies of AOM rats at 16 weeks and correlated it with tumor development. We noted that the tumor bearing rats had significantly greater miRNA modulation compared to those without tumors. The miRNAs showed good diagnostic performance with an area under the receiver operator curve (AUROC) of >0.7. We also noted that the miRNA induction in the colonic mucosa was mirrorred in the mucus layer fecal colonocytes isolated from AOM rat stool and the degree of miRNA induction was greater in the tumor bearing rats compared to those without tumors. Lastly, we also noted significant miRNA modulation in the Pirc rats- the genetic model of colon carcinogenesis, both in the uninvolved colonic mucosa and the fecal colonocytes. We thus demonstrate that miRNAs are excellent markers of field carcinogenesis and could accurately predict future neoplasia. Based on our results, we propose an accurate, inexpensive, non-invasive miRNA test for CRC risk stratification based on rectal brushings or from abraded fecal colonocytes.

Inflammation-associated cancer development in digestive organs: mechanisms and roles for genetic and epigenetic modulation

Chronic inflammation, regardless of infectious agents, plays important roles in the development of various cancers, particularly in digestive organs, including Helicobacter pylori-associated gastric cancer, hepatitis C virus-positive hepatocellular carcinoma, and colitis-associated colon cancers. Cancer development is characterized by stepwise accumulation of genetic and epigenetic alterations of various proto-oncogenes and tumor-suppressor genes. During chronic inflammation, infectious agents such as H pylori and hepatitis C virus as well as intrinsic mediators of inflammatory responses, including proinflammatory cytokines and reactive oxygen and nitrogen species, can induce genetic and epigenetic changes, including point mutations, deletions, duplications, recombinations, and methylation of various tumor-related genes through various mechanisms. Furthermore, inflammation also modulates the expressions of microRNAs that influence the production of several tumor-related messenger RNAs or proteins. These molecular events induced by chronic inflammation work in concert to alter important pathways involved in normal cellular function, and hence accelerate inflammation-associated cancer development. Among these, recent studies highlighted an important role of activation-induced cytidine deaminase, a nucleotide-editing enzyme essential for somatic hypermutation and class-switch recombination of the immunoglobulin gene, as a genomic modulator in inflammation-associated cancer development.

Inhibition of proliferation of human lung cancer cells by green tea catechins is mediated by upregulation of let-7

Green tea catechins are known to function as anticancer agents via inhibition of carcinogenesis during the initiation, promotion and progression stages. Many potential mechanisms have been proposed, yet the precise mechanism of lung cancer prevention by green tea catechins remains unclear. microRNAs (miRs) are a class of 21-24 nucleotide small non-coding RNAs and play critical roles throughout cellular development and regulation. Emerging evidence demonstrates that tea catechins influence the expression of miRs in human cancer cells to inhibit tumorigenesis. Both let-7a-1 and let-7g were detected in the human lung cancer cells treated with tea catechins. The cell viability and cell cycle were analyzed after tea catechins treatment. In the present study, we found that tea catechins upregulated the tumor-suppressor miRs, let-7a-1 and let-7g, in lung cancer cell lines. The upregulation of let-7a/7g repressed the expression of their targets, C-MYC and the regulatory protein of LIN-28, at the mRNA and protein levels. Moreover, the cell growth assay indicated that tea catechins significantly inhibited cell proliferation, and the flow cytometric analysis revealed an increase in the number of cells in the G2/M phase and a decrease in the number of cells in the S phase after treatment with tea catechins. These observations suggest that green tea catechins mediate the inhibition of proliferation of lung cancer cells through the let-7 signaling pathway.

Lynch or not Lynch? Is that always a question?

The familial cancer syndrome referred to as Lynch I and II was renamed hereditary nonpolyposis colorectal cancer (HNPCC) only to revert later to Lynch syndrome (LS). LS is the most frequent human predisposition for the development of colorectal cancer (CRC), and probably also for endometrial and gastric cancers, although it has yet to acquire a consensus name. Its estimated prevalence ranges widely from 2% to 7% of all CRCs due to the fact that tumors from patients with LS are difficult to recognize at both the clinical and molecular level. This review is based on two assumptions. First, all LS patients inherit a predisposition to develop CRC (without polyposis) and/or other tumors from the Lynch spectrum. Second, all LS patients have a germline defect in one of the DNA mismatch repair (MMR) genes. When a somatic second hit inactivates the relevant MMR gene, the consequence is instability of DNA repeat sequences such as microsatellites and the tumors are referred to as having the microsatellite instability (MSI) phenotype. However, some of the inherited predisposition to develop CRC without concurrent polyposis, termed HNPCC, is found in non-LS patients, while not all MSI tumors are from LS cases. LS tumors are therefore at the junction of inherited and MSI cases. We describe here the defining characteristics of LS tumors that differentiate them from inherited non-MSI tumors and from non-inherited MSI tumors.

The hMSH2(M688R) Lynch syndrome mutation may function as a dominant negative

The hMSH2(M688R) mismatch repair (MMR) gene mutation has been found in five large families from Tenerife, Spain, suggesting it is a Lynch syndrome or hereditary non-polyposis colorectal cancer (LS/HNPCC) founder mutation. In addition to classical LS/HNPCC tumors, these families present with a high incidence of central nervous system (CNS) tumors normally associated with Turcot or constitutional mismatch repair deficiency (CMMR-D) syndromes. Turcot and CMMR-D mutations may be biallelic, knocking out both copies of the MMR gene. The hMSH2(M688R) mutation is located in the ATP hydrolysis (ATPase) domain. We show that the hMSH2(M688R)-hMSH6 heterodimer binds to mismatched nucleotides but lacks normal ATP functions and inhibits MMR in vitro when mixed with the wild-type (WT) heterodimer. Another alteration that has been associated with LS/HNPCC, hMSH2(M688I)-hMSH6, displays no identifiable differences with the WT heterodimer. Interestingly, some extracolonic tumors from hMSH2(M688R) carriers may express hMSH2-hMSH6, yet display microsatellite instability (MSI). The functional analysis along with variability in tumor expression and the high incidence of CNS tumors suggests that hMSH2(M688R) may act as a dominant negative in some tissues, while the hMSH2(M688I) is most likely a benign polymorphism.

microRNA regulation of cell viability and drug sensitivity in lung cancer

INTRODUCTION

microRNAs (miRNAs) are 19 - 23 nucleotide long RNAs found in multiple organisms that regulate gene expression and have been shown to play important roles in tumorigenesis. In the context of lung cancer, numerous studies have shown that tumor suppressor genes and oncogenes that play crucial roles in lung tumor development and progression are targets of miRNA regulation. Manipulation of miRNA levels that modulate lung cancer cell survival and drug sensitivity can therefore provide novel therapeutic targets and agents.

AREAS COVERED

Here, the authors review the published in vitro, in vivo and preclinical studies on the functional role of miRNAs in modulating lung cancer cell viability and drug response, and discuss the limitations and promise of translating current findings into miRNA-based therapeutic and diagnostic strategies.

EXPERT OPINION

Although many miRNAs have been identified as potent regulators of cell viability and drug sensitivity in lung cancer, most of them have not been characterized for potential clinical application. Further study is warranted to evaluate translation of the current findings to the clinic to improve the diagnosis and treatment of lung cancer. In addition, most studies have focused on non-small cell lung cancer (NSCLC). It is therefore important to raise interest in investigating miRNAs in small cell lung cancer (SCLC) as well as in comparative studies of miRNA expression and function in different histological subtypes of lung cancer.

Humans accumulate microsatellite instability with acquired loss of MLH1 protein in hematopoietic stem and progenitor cells as a function of age

Hematopoietic stem and progenitor cells (HPCs) are necessary for long-term survival. Genomic instability and persistent DNA damage may cause loss of adult stem cell function. The mismatch repair (MMR) pathway increases replication fidelity and defects have been implicated in malignant hematopoietic diseases. Little, however, is known about the role MMR pathway failure plays in the aging process of human HPCs. We hypothesized that loss of MMR occurs in HPCs as a process of human aging. We examined microsatellite instability and expression of the MMR genes MutL homologue 1 (MLH1) and MutS homologue 2 (MSH2) in HPCs and colony-forming cell-derived clones (CFCs) from human donors aged 0 to 86 years. CFCs from donors > 45 years had a greater frequency of microsatellite instability and CD34(+) progenitors lacking MLH1 expression and protein than individuals ≤ 45 years. Loss of MSH2 did not correlate with age. Thus, a potentially early event in the normal human aging process is microsatellite instability accumulation in normal human HPCs associated with the loss of MLH1 protein expression.

Interrelationship between microsatellite instability and microRNA in gastrointestinal cancer

There is an increasing understanding of the roles that microsatellite instability (MSI) plays in Lynch syndrome (by mutations) and sporadic (by mainly epigenetic changes) gastrointestinal (GI) and other cancers. Deficient DNA mismatch repair (MMR) results in the strong mutator phenotype known as MSI, which is the hallmark of cancers arising within Lynch syndrome. MSI is characterized by length alterations within simple repeated sequences called microsatellites. Lynch syndrome occurs primarily because of germline mutations in one of the MMR genes, mainly MLH1 or MSH2, less frequently MSH6, and rarely PMS2. MSI is also observed in about 15% of sporadic colorectal, gastric, and endometrial cancers and in lower frequencies in a minority of other cancers where it is often associated with the hypermethylation of the MLH1 gene. miRNAs are small noncoding RNAs that regulate gene expression at the posttranscriptional level and are critical in many biological processes and cellular pathways. There is accumulating evidence to support the notion that the interrelationship between MSI and miRNA plays a key role in the pathogenesis of GI cancer. As a possible new mechanism underlying MSI, overexpression of miR-155 has been shown to downregulate expression of MLH1, MSH2, and MSH6. Thus, a subset of MSI-positive (MSI+) cancers without known MMR defects may result from miR-155 overexpression. Target genes of frameshift mutation for MSI are involved in various cellular functions, such as DNA repair, cell signaling, and apoptosis. A novel class of target genes that included not only epigenetic modifier genes, such as HDAC2, but also miRNA processing machinery genes, including TARBP2 and XPO5, were found to be mutated in MSI+ GI cancers. Thus, a subset of MSI+ colorectal cancers (CRCs) has been proposed to exhibit a mutated miRNA machinery phenotype. Genetic, epigenetic, and transcriptomic differences exist between MSI+ and MSI- cancers. Molecular signatures of miRNA expression apparently have the potential to distinguish between MSI+ and MSI- CRCs. In this review, we summarize recent advances in the MSI pathogenesis of GI cancer, with the focus on its relationship with miRNA as well as on the potential to use MSI and related alterations as biomarkers and novel therapeutic targets.

MicroRNA-101 (miR-101) post-transcriptionally regulates the expression of EP4 receptor in colon cancers

PURPOSE

Expression of the PGE2 receptor, EP4, is up-regulated during colorectal carcinogenesis. However the mechanism leading to deregulation of the EP4 receptor is not known. The present study was conducted to investigate the regulation of EP4 receptor by miRNAs.

EXPERIMENTAL DESIGN

We analyzed 26 colon cancers (i.e. 15 adenocarcinomas and 9 adenomas) and 16 normal colon specimens for EP4 receptor expression by immunohistochemistry. A bioinformatics approached identified putative microRNA binding sites with the 3'-UTR of the EP4 receptor. Both colon cancer cell lines and tumor specimens were analyzed for miR-101 and EP4 expression by qRT-PCR and Western analysis respectively and simultaneously in situ hybridizations was used to confirm our results. In vitro and in vivo assays were used to confirm our clinical findings.

RESULTS

We observed an inverse correlation between the levels of miR-101 and EP4 receptor protein. Transfection of LS174T cells with miR-101 significantly suppressed a luciferase reporter containing the EP4 receptor-3'-UTR. In contrast, a mutant EP4 receptor-3'-UTR construct was unaffected. Ectopic expression of miR-101 markedly reduced cell proliferation and motility. Co-transfection of EP4 receptor could rescue colon cancer cells from the tumor suppressive effects of miR-101. Moreover, the pharmacologic inhibition of EP4 receptor signaling or silencing of EP4 receptor phenocopied the effect of miR-101. This is the first study to show that the EP4 receptor is negatively regulated by miR-101.

CONCLUSIONS

These data provide new insights in the modulation of EP-4 receptor expression at the post-transcriptional level by miR-101 and suggests therapeutic strategies against miR-101 targets may be warranted.

Aberrant expression of miR-196a in gastric cancers and correlation with recurrence

MicroRNAs (miRNAs) are short noncoding RNAs (~22 nt) that play important roles in the pathogenesis of human diseases by negatively regulating gene expression. Here, we examined the relationship between miR-196a and gastric cancer.By the analysis of 72 gastric cancer samples, we found that the expression level of miR-196a microRNA significantly increased in primary gastric cancer tissues versus adjacent normal tissues. In addition, extracellular miR-196a detected in conditioned medium was strongly correlated with its cellular expression status and increased circulating miR-196a in patient serum was associated with gastric cancer disease status and relapse. Furthermore, ectopic expression of miR-196a microRNA promoted the epithelial-mesenchymal transition and migration/invasion capabilities of transfected cells, suggesting its oncogenic potential in gastric cancer progression. Altogether, our data demonstrate that miR-196a exerts an oncogenic role in gastric cancer and miR-196a may be a novel biomarker for detecting gastric cancer and for monitoring disease recurrence.

Nanoparticle-based therapy in an in vivo microRNA-155 (miR-155)-dependent mouse model of lymphoma

MicroRNA-155 (miR-155) is an oncogenic microRNA that regulates several pathways involved in cell division and immunoregulation. It is overexpressed in numerous cancers, is often correlated with poor prognosis, and is thus a key target for future therapies. In this work we show that overexpression of miR-155 in lymphoid tissues results in disseminated lymphoma characterized by a clonal, transplantable pre-B-cell population of neoplastic lymphocytes. Withdrawal of miR-155 in mice with established disease results in rapid regression of lymphadenopathy, in part because of apoptosis of the malignant lymphocytes, demonstrating that these tumors are dependent on miR-155 expression. We show that systemic delivery of antisense peptide nucleic acids encapsulated in unique polymer nanoparticles inhibits miR-155 and slows the growth of these "addicted" pre-B-cell tumors in vivo, suggesting a promising therapeutic option for lymphoma/leukemia.

Deficient expression of DNA repair enzymes in early progression to sporadic colon cancer

Background

Cancers often arise within an area of cells (e.g. an epithelial patch) that is predisposed to the development of cancer, i.e. a "field of cancerization" or "field defect." Sporadic colon cancer is characterized by an elevated mutation rate and genomic instability. If a field defect were deficient in DNA repair, DNA damages would tend to escape repair and give rise to carcinogenic mutations.

Purpose

To determine whether reduced expression of DNA repair proteins Pms2, Ercc1 and Xpf (pairing partner of Ercc1) are early steps in progression to colon cancer.

Results

Tissue biopsies were taken during colonoscopies of 77 patients at 4 different risk levels for colon cancer, including 19 patients who had never had colonic neoplasia (who served as controls). In addition, 158 tissue samples were taken from tissues near or within colon cancers removed by resection and 16 tissue samples were taken near tubulovillous adenomas (TVAs) removed by resection. 568 triplicate tissue sections (a total of 1,704 tissue sections) from these tissue samples were evaluated by immunohistochemistry for 4 DNA repair proteins. Substantially reduced protein expression of Pms2, Ercc1 and Xpf occurred in field defects of up to 10 cm longitudinally distant from colon cancers or TVAs and within colon cancers. Expression of another DNA repair protein, Ku86, was infrequently reduced in these areas. When Pms2, Ercc1 or Xpf were reduced in protein expression, then either one or both of the other two proteins most often had reduced protein expression as well. The mean inner colon circumferences, from 32 resections, of the ascending, transverse and descending/sigmoid areas were measured as 6.6 cm, 5.8 cm and 6.3 cm, respectively. When combined with other measurements in the literature, this indicates the approximate mean number of colonic crypts in humans is 10 million.

Conclusions

The substantial deficiencies in protein expression of DNA repair proteins Pms2, Ercc1 and Xpf in about 1 million crypts near cancers and TVAs suggests that the tumors arose in field defects that were deficient in DNA repair and that deficiencies in Pms2, Ercc1 and Xpf are early steps, often occurring together, in progression to colon cancer.

MiR-155 is a liposarcoma oncogene that targets casein kinase-1α and enhances β-catenin signaling

Liposarcoma can be an aggressive, debilitating, and fatal malignancy. In this study, we identifed miRNAs associated with the differentiation status of liposarcoma to gain insight into the basis for its progression. miRNA expression profiles determined in human tumors and normal fat specimens identified a dedifferentiated tumor expression signature consisting of 35 miRNAs. Deregulated miRNA expression was confirmed in a second independent sample cohort. The miR-155 was the most overexpressed miRNA and functional investigations assigned an important role in the growth of dedifferentiated liposarcoma cell lines. Transient or stable knockdown of miR-155 retarded tumor cell growth, decreased colony formation, and induced G(1)-S cell-cycle arrest in vitro and blocked tumor growth in murine xenografts in vivo. We identified casein kinase 1α (CK1α) as a direct target of miR-155 control which enhanced β-catenin signaling and cyclin D1 expression, promoting tumor cell growth. In summary, our results point to important functions for miR-155 and β-catenin signaling in progression of liposarcoma, revealing mechanistic vulnerabilities that might be exploited for both prognostic and therapeutic purposes.

miRNA signatures associate with pathogenesis and progression of osteosarcoma

Osteosarcoma remains a leading cause of cancer death in adolescents. Treatment paradigms and survival rates have not improved in two decades. Driving the lack of therapeutic inroads, the molecular etiology of osteosarcoma remains elusive. MicroRNAs (miRNAs) have demonstrated far-reaching effects on the cellular biology of development and cancer. Their role in osteosarcomagenesis remains largely unexplored. Here we identify for the first time an miRNA signature reflecting the pathogenesis of osteosarcoma from surgically procured samples from human patients. The signature includes high expression of miR-181a,miR-181b, and miR-181c as well as reduced expression of miR-16, miR-29b, and miR-142-5p. We also demonstrate that miR-181b and miR-29b exhibit restricted expression to distinct cell populations in the tumor tissue. Further, higher expression of miR-27a and miR-181c* in pre-treatment biopsy samples characterized patients who developed clinical metastatic disease. In addition, higher expression of miR-451 and miR-15b in pre-treatment samples correlated with subsequent positive response to chemotherapy. In vitro and in vivo functional validation in osteosarcoma cell lines confirmed the tumor suppressive role of miR-16 and the pro-metastatic role of miR-27a. Furthermore, predicted target genes for miR-16 and miR-27a were confirmed as down-regulated by real-time PCR. Affymetrix array profiling of cDNAs from the osteosarcoma specimens and controls were interrogated according to predicted targets of miR-16, miR142-5p, miR-29b, miR-181a/b, and miR-27a. This analysis revealed positive and negative correlations highlighting pathways of known importance to osteosarcoma, as well as novel genes. Thus, our findings establish a miRNA signature associated with pathogenesis of osteosarcoma as well as critical pre-treatment biomarkers of metastasis and responsiveness to therapy.

Molecular pathways: microsatellite instability in colorectal cancer: prognostic, predictive, and therapeutic implications

Microsatellite instability (MSI) is the molecular fingerprint of the deficient mismatch repair (MMR) system, which characterizes ∼15% of colorectal cancers. MSI develops as a result of germline mutations in MMR genes or, more commonly, from epigenetic silencing of MLH1 in sporadic tumors occurring in a background of methylation of CpG islands in gene promoter regions and in tumors that frequently show hotspot mutations in the BRAF oncogene. MSI tumors have distinct phenotypic features and have been consistently associated with a better stage-adjusted prognosis compared with microsatellite stable tumors. MSI negatively predicts response to 5-fluorouracil and may also determine responsiveness to other drugs used for treatment of colorectal cancers. Recent data have expanded the molecular heterogeneity of MSI tumors and may contribute to our understanding of differential chemosensitivity. The ability to identify deficient MMR has important implications for patient management, and it holds promise for therapeutic exploitation and for the development of novel therapeutics.

Modulation of microRNA processing by mismatch repair protein MutLα

MicroRNAs (miRNAs) are critical post-transcriptional regulators and are derived from hairpin-shaped primary transcripts via a series of processing steps. However, how the production of individual miRNAs is regulated remains largely unknown. Similarly, loss or overexpression of the key mismatch repair protein MutLα (MLH1-PMS2 heterodimer) leads to genome instability and tumorigenesis, but the mechanisms controlling MutLα expression are unknown. Here we demonstrate in vitro and in vivo that MLH1 and miR-422a participate in a feedback loop that regulates the level of both molecules. Using a defined in-vitro miRNA processing system, we show that MutLα stimulates the conversion of pri-miR-422a to pre-miR-422a, as well as the processing of other miRNAs tested, implicating MutLα as a general stimulating factor for miRNA biogenesis. This newly identified MutLα function requires its ATPase and pri-miRNA binding activities. In contrast, miR-422a downregulates MutLα levels by suppressing MLH1 expression through base pairing with the MLH1 3'-untranslated region. A model depicting this feedback mechanism is discussed.

Balancing repair and tolerance of DNA damage caused by alkylating agents

Alkylating agents constitute a major class of frontline chemotherapeutic drugs that inflict cytotoxic DNA damage as their main mode of action, in addition to collateral mutagenic damage. Numerous cellular pathways, including direct DNA damage reversal, base excision repair (BER) and mismatch repair (MMR), respond to alkylation damage to defend against alkylation-induced cell death or mutation. However, maintaining a proper balance of activity both within and between these pathways is crucial for a favourable response of an organism to alkylating agents. Furthermore, the response of an individual to alkylating agents can vary considerably from tissue to tissue and from person to person, pointing to genetic and epigenetic mechanisms that modulate alkylating agent toxicity.

MicroRNAs and inflammation in the pathogenesis and progression of colon cancer

There is a strong connection between inflammation, altered microRNA (miRNA) expression and colon cancer. Longstanding inflammatory bowel diseases-related colitis leads to increased risk for the development of colorectal cancer (CRC), while sporadic CRC is in part driven by the inflammatory microenvironment. This supports a causative role for inflammation in colon carcinogenesis. miRNAs are a class of small noncoding RNAs that have recently emerged as key players in both inflammation and cancer. Some miRNAs act as inflammatory mediators, others can act as either oncogenes or tumor suppressors depending on the cellular environment in which they are expressed. In particular, miR-21 is an oncogenic miRNA that has been implicated as an inflammatory mediator and may promote inflammation-associated colon carcinogenesis. miRNAs have potential as biomarkers and therapeutic targets in CRC. They are currently being evaluated as early detection biomarkers and prognostic classifiers. Polymorphisms in miRNAs and miRNA-binding sites may alter one's risk of CRC. This review will focus on the role of inflammation and miRNAs in colon carcinogenesis and discuss the potential for miRNAs and inflammatory genes to be used as biomarkers and therapeutic targets of CRC.

Genomic instability measured by inter-(simple sequence repeat) PCR and high-resolution microsatellite instability are prognostic of colorectal carcinoma survival after surgical resection

BACKGROUND

During the multiyear progression to colorectal cancer, numerous genomic alterations arise in events ranging from single base mutations to gains or losses of entire chromosomes. A single genetic change might not stand out as an independent predictor of outcome. The goal of this study was to determine if more comprehensive measurements of genomic instability provide clinically relevant prognostic information.

METHODS

Our study included 65 sporadic colorectal cancer patients diagnosed from 1987 to 1991 with last follow-up ascertained in 2006. We estimated an overall tally of alterations using the genome-wide sampling technique of inter-(simple sequence repeat [SSR]) polymerase chain reaction (PCR), and evaluated its relationship with all-cause survival. We also extended and sensitized the Bethesda criteria for microsatellite instability (MSI), by analyzing 348 microsatellite markers instead of the normal five. We expanded the MSI categories into four levels: MSI stable (MSS), very low-level MSI, moderately low-level MSI, and classical high-level MSI.

RESULTS

Tumors with genomic instability above the median value of 2.6% as measured by inter-SSR PCR, were associated with far greater risk of death compared to tumors with lower levels of genomic instability. Adverse outcome was most pronounced for patients presenting with stage 3 disease. A gradient of increased survival was observed across increasing MSI levels but did not reach statistical significance.

CONCLUSION

Our findings suggest genomic instabilities quantified by inter-SSR PCR and increased precision in MSI values may be clinically useful tools for estimating prognosis in colorectal cancer.

MicroRNAs in mutagenesis, genomic instability, and DNA repair

MicroRNAs (miRNAs) are aiding our understanding of cancer biology, and are now coming close to therapeutic use as well. Here, we focus specifically on the interaction between miRNAs and genomic instability. MiRNA regulation is essential to many cellular processes, and escape from this regulatory network seems to be a common characteristic of malignant transformation. Genomic instability may preferentially target miRNAs either because of selective pressure or because of inherent vulnerability related to their location near fragile sites. Furthermore, disruption of miRNA processing elements affords a more global release from miRNA regulation. Finally, we review how miRNAs function as both effectors and modulators of the DNA damage response, intricately weaved with traditional elements such as ATM, P53, and MMR. Thus, miRNAs are important substrates for genomic instability and play a crucial role in cellular DNA sensing and repair mechanisms.

Up-regulation of miR-1245 by c-myc targets BRCA2 and impairs DNA repair

BRCA2, a clinical prognostic factor, is significantly up-regulated in mRNA level, while its protein expression is often decreased in sporadic breast cancer. However, how BRCA2 protein expressions are suppressed in these tumors remains unknown. In this study, we demonstrated that miR-1245 directly suppressed BRCA2 3'-UTR and translation, impaired homologous recombination (HR)-mediated repair, reduced DNA damage-induced Rad51 nuclear foci, and rendered cells hypersensitive to γ-irradiation (IR), ultimately inducing high chromosomal abnormalities in normal breast cells and breast cancer cells. Conversely, inhibiting miR-1245 in breast cancer cells enhanced BRCA2 levels and induced resistance to IR. Furthermore, we demonstrated that c-myc up-regulated miR-1245 expression via direct binding to the miR-1245 promoter, which led to down-regulation of BRCA2 and reduction in HR efficiency. Significantly, miR-1245 levels in primary breast tumors correlated with c-myc overexpression and BRCA2 suppression. These findings uncover a BRCA2 regulatory and signaling pathway in sporadic breast cancer and support a functionally and clinically relevant epigenetic mechanism in cancer pathogenesis.

Alterations of microRNAs contribute to colon carcinogenesis

MicroRNAs are being evaluated as biomarkers and therapeutic targets for colon cancer. MicroRNAs have a functional role in the initiation and progression of colon cancer. Altered microRNA expression is found in tumors and their expression patterns may serve as useful cancer biomarkers. Polymorphisms in microRNAs or microRNA binding sites may modify the risk of developing cancer. As we continue to improve our understanding of the role for microRNAs in the initiation and progression of cancer, one goal is to gain insights that will allow for the development of microRNAs as biomarkers and therapeutic targets for cancer. This review provides a current understanding of the connection between microRNAs and colon cancer. We will cover evidence that global microRNA expression patterns are altered in colon tumors, that specific microRNAs have a functional role in colon carcinogenesis, that polymorphisms in microRNAs may be associated with risk of colon cancer, and the potential for using circulating microRNAs as a noninvasive biomarker for the detection of cancer.

MicroRNAs in the pathogenesis of cancer

MicroRNAs (miRs) are small (19-25 nucleotides) non-protein-coding RNAs involved in development, differentiation, and aging; they act by inducing messenger RNA (mRNA) silencing through degradation, and post-transcriptional or decoy activity. miR profiles of human solid and hematologic malignancies have highlighted their potential value as tumor markers in cancer patient management. Different experimental lines of evidence have confirmed that deregulation of miRs not only results as consequence of cancer progression but also directly promotes tumor initiation and progression in a cause-effect manner. These findings reveal a potential and appealing role for miRs as cancer therapeutic targets. This review focuses on the causes and consequences of miR deregulation in carcinogenesis and tumor progression. The work aims at providing the molecular bases for the understanding of the potential role of miRs in the translational and clinical setting.

Inhibition of hypoxia-induced miR-155 radiosensitizes hypoxic lung cancer cells

miR-155 is a prominent microRNA (miRNA) that regulates genes involved in immunity and cancer-related pathways. miR-155 is overexpressed in lung cancer, which correlates with poor patient prognosis. It is unclear how miR-155 becomes increased in lung cancers and how this increase contributes to reduced patient survival. Here, we show that hypoxic conditions induce miR-155 expression in lung cancer cells and trigger a corresponding decrease in a validated target, FOXO3A. Furthermore, we find that increased levels of miR-155 radioprotects lung cancer cells, while inhibition of miR-155 radiosensitizes these cells. Moreover, we reveal a therapeutically important link between miR-155 expression, hypoxia, and irradiation by demonstrating that anti-miR-155 molecules also sensitize hypoxic lung cancer cells to irradiation. Our study helps explain how miR-155 becomes elevated in lung cancers, which contain extensive hypoxic microenvironments, and demonstrates that inhibition of miR-155 may have important therapeutic potential as a means to radiosensitize hypoxic lung cancer cells.

Aberrant expression of miR-638 contributes to benzo(a)pyrene-induced human cell transformation

Identification of aberrant microRNA (miRNA) expression during chemical carcinogen-induced cell transformation will lead to a better understanding of the substantial role of miRNAs in cancer development. To explore whether aberrant miRNAs expression can be used as biomarkers of chemical exposure in risk assessment of chemical carcinogenesis, we analyzed miRNA expression profiles of human bronchial epithelial cells expressing an oncogenic allele of H-Ras (HBER) at different stages of transformation induced by benzo(a)pyrene (BaP) by miRNA array. It revealed 12 miRNAs differentially expressed in HBER cells at both pretransformed and transformed stages. Differentially expressed miRNAs were confirmed in transformed cells and examined in 50 pairs of primary human non-small-cell lung cancer (NSCLC) tissues using real-time PCR. Among these miRNAs, downregulation of miR-638 was found in 68% (34/50) of NSCLC tissues. However, the expression of miR-638 in HBER cells increased upon treatment of BaP in a dose-dependent manner. The expression of miR-638 was also examined in peripheral lymphocytes from 86 polycyclic aromatic hydrocarbons (PAHs)-exposed (PE) workers. We found that the average expression level of miR-638 in peripheral lymphocytes from 86 PE workers increased by 72% compared with control group. The levels of miR-638 were correlated with the concentration of urinary 1-hydroxypyrene (1-OHP) and external levels of PAHs. Overexpression of miR-638 aggravated cell DNA damage induced by BaP, which might be mediated by suppression of breast cancer 1 (BRCA1), one of the target genes of miR-638. In summary, we suggest that miR-638 is involved in the BaP-induced carcinogenesis by targeting BRCA1.

New players in ovarian cancer

Ovarian cancer is the leading cause of gynecologic cancer mortality worldwide. The aim of this review is to highlight the most recent studies regarding ovarian cancer pathogenesis and the new therapeutic approaches against this insidious disease. We focus on the relevance of some cell cycle genes, transcription factors, and microRNAs in the carcinogenesis of ovarian cancer as well as on a new hypothesis for therapy using histone deacetylase inhibitors. We also report recent studies regarding some mechanisms of chemoresistance, a major obstacle in the treatment of ovarian cancer. Together these studies can improve our knowledge of ovarian cancer tumorigenesis and diagnosis providing new tools to hopefully defeat this deadly disease.

Noncoding RNAs and cancer

The study of miRNAs and other noncoding RNAs has revolutionised our understanding of gene expression regulation during cancer development and progression, creating one of the fastest-growing research fields in cancer with realistic therapeutic potential. The 2011 Non-coding RNAs and Cancer Symposium hosted by the University College London Cancer Institute focused on the function and regulation of noncoding RNAs during oncogenesis.

Somatic deletions of genes regulating MSH2 protein stability cause DNA mismatch repair deficiency and drug resistance in human leukemia cells

DNA mismatch repair enzymes (e.g., MSH2) maintain genomic integrity, and their deficiency predisposes to several human cancers and to drug resistance. We found that leukemia cells from a substantial proportion of patients (~11%) with newly diagnosed acute lymphoblastic leukemia (ALL) have low or undetectable MSH2 protein levels (MSH2-L), despite abundant wild-type MSH2 mRNA. MSH2-L leukemia cells contained partial or complete somatic deletions of 1–4 genes that regulate MSH2 degradation (FRAP1, HERC1, PRKCZ, PIK3C2B); these deletions were also found in adult ALL (16%) and sporadic colorectal cancer (13.5%). Knockdown of these genes in human leukemia cells recapitulated the MSH2 protein deficiency by enhancing MSH2-degradation, leading to significant reduction in DNA mismatch repair (MMR) and increased resistance to thiopurines. These findings reveal a previously unrecognized mechanism whereby somatic deletions of genes regulating MSH2 degradation result in undetectable levels of MSH2 protein in leukemia cells, MMR deficiency and drug resistance.

Resveratrol, MicroRNAs, Inflammation, and Cancer

MicroRNAs are short noncoding RNAs that regulate the expression of many target genes posttranscriptionally and are thus implicated in a wide array of cellular and developmental processes. The expression of miR-155 or miR-21 is upregulated during the course of the inflammatory response, but these microRNAs are also considered oncogenes due to their upregulation of expression in several types of tumors. Furthermore, it is now well established that inflammation is associated with the induction or the aggravation of nearly 25% of cancers. Therefore, the above microRNAs are thought to link inflammation and cancer. Recently, resveratrol (trans-3,4′,5-trihydroxystilbene), a natural polyphenol with antioxidant, anti-inflammatory, and anticancer properties, currently at the stage of preclinical studies for human cancer prevention, has been shown to induce the expression of miR-663, a tumor-suppressor and anti-inflammatory microRNA, while downregulating miR-155 and miR-21. In this paper we will discuss how the use of resveratrol in therapeutics may benefit from the preanalyses on the status of expression of miR-155 or miR-21 as well as of TGFβ1. In addition, we will discuss how resveratrol activity might possibly be enhanced by simultaneously manipulating the levels of its key target microRNAs, such as miR-663.

MicroRNA-155 promotes resolution of hypoxia-inducible factor 1alpha activity during prolonged hypoxia

The hypoxia-inducible factor (HIF) is a key regulator of the transcriptional response to hypoxia. While the mechanism underpinning HIF activation is well understood, little is known about its resolution. Both the protein and the mRNA levels of HIF-1α (but not HIF-2α) were decreased in intestinal epithelial cells exposed to prolonged hypoxia. Coincident with this, microRNA (miRNA) array analysis revealed multiple hypoxia-inducible miRNAs. Among these was miRNA-155 (miR-155), which is predicted to target HIF-1α mRNA. We confirmed the hypoxic upregulation of miR-155 in cultured cells and intestinal tissue from mice exposed to hypoxia. Furthermore, a role for HIF-1α in the induction of miR-155 in hypoxia was suggested by the identification of hypoxia response elements in the miR-155 promoter and confirmed experimentally. Application of miR-155 decreased the HIF-1α mRNA, protein, and transcriptional activity in hypoxia, and neutralization of endogenous miR-155 reversed the resolution of HIF-1α stabilization and activity. Based on these data and a mathematical model of HIF-1α suppression by miR-155, we propose that miR-155 induction contributes to an isoform-specific negative-feedback loop for the resolution of HIF-1α activity in cells exposed to prolonged hypoxia, leading to oscillatory behavior of HIF-1α-dependent transcription.

Systematic exploration of cancer-associated microRNA through functional screening assays

MicroRNA (miRNA), non-coding RNA of approximately 22 nucleotides, post-transcriptionally represses expression of its target genes. miRNA regulates a variety of biological processes such as cell proliferation, cell death, development, stemness and genomic stability, not only in physiological conditions but also in various pathological conditions such as cancers. More than 1000 mature miRNA have been experimentally identified in humans and mice, yet the functions of a vast majority of miRNA remain to be elucidated. Identification of novel cancer-associated miRNA seems promising considering their possible application in the development of novel cancer therapies and biomarkers. Currently, there are two major approaches to identify miRNA that are associated with cancer: expression profiling study and functional screening assay. The former approach is widely used, and a large number of studies have shown aberrant miRNA expression profiles in cancer tissues compared with their non-cancer counterparts. Although aberrantly expressed miRNA are potentially good biomarkers, in most cases a majority of them do not play causal roles in cancers when functional assays are performed. In contrast, the latter approach allows screening of 'driver' miRNA with cancer-associated phenotypes, such as cell proliferation and cell invasion. Thus, this approach might be suitable in finding crucial targets of novel cancer therapy. The combination of both types of approaches will contribute to further elucidation of the cancer pathophysiology and to the development of a novel class of cancer therapies and biomarkers.

Tissue and serum microRNAs in the Kras(G12D) transgenic animal model and in patients with pancreatic cancer

microRNAs (miRs) modulate the expression levels of mRNAs and proteins and can thus contribute to cancer initiation and progression. In addition to their intracelluar function, miRs are released from cells and shed into the circulation. We postulated that circulating miRs could provide insight into pathways altered during cancer progression and may indicate responses to treatment. Here we focus on pancreatic cancer malignant progression. We report that changes in miR expression patterns during progression of normal tissues to invasive pancreatic adenocarcinoma in the p48-Cre/LSL-Kras^G12D mouse model mirrors the miR changes observed in human pancreatic cancer tissues. miR-148a/b and miR-375 expression were found decreased whereas miR-10, miR-21, miR-100 and miR-155 were increased when comparing normal tissues, premalignant lesions and invasive carcinoma in the mouse model. Predicted target mRNAs FGFR1 (miR-10) and MLH1 (miR-155) were found downregulated. Quantitation of nine microRNAs in plasma samples from patients distinguished pancreatic cancers from other cancers as well as non-cancerous pancreatic disease. Finally, gemcitabine treatment of control animals and p48-Cre/LSL-Kras^G12D animals with pancreatic cancer caused distinct and up to 60-fold changes in circulating miRs that indicate differential drug effects on normal and cancer tissues. These findings support the significance of detecting miRs in the circulation and suggests that circulating miRs could serve as indicators of drug response.

miR-429 modulates the expression of c-myc in human gastric carcinoma cells

AIM

MicroRNAs (miRNAs) are a recently discovered class of small non-coding RNAs that regulate gene expression and may contribute to the development and progression of many cancers. In this study, our goal was to investigate the regulation of miR-429 in gastric cancer and explored the mechanism/s by which it influenced pathogenesis of gastric cancer.

METHODS

We used real-time reverse transcriptase-polymerase chain reaction to quantify the expression level of miR-429 in 52 gastric cancer tissues and their paracancerous tissues. Bioinformatics was used to predict downstream target genes of miR-429. SGC-7901 gastric cancer cells were transfected with miR-429 mimics and endogenous c-myc expression was detected by western blots. We performed functional assays using the 3'UTR of the c-myc gene as a miR-429 target in a luciferase reporter assay system.

RESULTS

We showed that miR-429 was downregulated in human gastric carcinoma tissue and in SGC-7901 cells. Cell viability, proliferation and attachment were inhibited in miR-429-transfected cells. miR-429 significantly downregulated endogenous c-myc expression in SGC-7901 cells. Action of miR/429 on c-myc 3'UTR was confirmed. The levels of miR-429 in tumour tissue of patients with lymph node metastasis were significantly lower than in those without lymph node metastasis.

CONCLUSIONS

Our results suggested that miR-429 played a role in the pathogenesis of gastric carcinoma and may function as a recessive cancer gene. c-myc is an important miR-429 target gene.

Minireview: The roles of small RNA pathways in reproductive medicine

The discovery of small noncoding RNA, including P-element-induced wimpy testis-interacting RNA, small interfering RNA, and microRNA, has energized research in reproductive medicine. In the two decades since the identification of small RNA, first in Caenorhabditis elegans and then in other animals, scientists in many disciplines have made significant progress in elucidating their biology. A powerful battery of tools, including knockout mice and small RNA mimics and antagonists, has facilitated investigation into the functional roles and therapeutic potential of these small RNA pathways. Current data indicate that small RNA play significant roles in normal development and physiology and pathological conditions of the reproductive tracts of females and males. Biologically plausible mRNA targets for these microRNA are aggressively being discovered. The next phase of research will focus on elucidating the clinical utility of small RNA-selective agonists and antagonists.

MicroRNA-155 targets the SKI gene in human melanoma cell lines

The SKI protein is a transcriptional coregulator over-expressed in melanoma. Experimentally induced down-regulation of SKI inhibits melanoma cell growth in vitro and in vivo. MicroRNAs (miRNAs) negatively modulate gene expression and have been implicated in oncogenesis. We previously showed that microRNA-155 (miR-155) is down-regulated in melanoma cells as compared with normal melanocytes and that its ectopic expression impairs proliferation and induces apoptosis. Here, we investigated whether miR-155 could mediate melanoma growth inhibition via SKI gene silencing. Luciferase reporter assays demonstrated that miR-155 interacted with SKI 3'UTR and impaired gene expression. Transfection of melanoma cells with miR-155 reduced SKI levels, while inhibition of endogenous miR-155 up-regulated SKI expression. Specifically designed small interfering RNAs reduced SKI expression and inhibited proliferation. However, melanoma cells over-expressing a 3'UTR-deleted SKI were still susceptible to the antiproliferative effect of miR-155. Our data demonstrate for the first time that SKI is a target of miR-155 in melanoma. However, impairment of SKI expression is not the leading mechanism involved in the growth-suppressive effect of miR-155 found in this malignancy.

Colon cancer-related microRNAs: implications for translational research

Colon carcinogenesis is a stepwise progression from polyps to adenocarcinomas and distant metastasis. These pathologic changes are contributed by aberrant activation or inactivation of protein-coding proto-oncogenes and tumor suppressor genes. However, recent discoveries in microRNA research have reshaped our understanding of the role of non-protein-coding genes in carcinogenesis. In this regard, a remarkable number of microRNAs exhibit differential expression in colon cancer tissues. These microRNAs alter cell proliferation, apoptosis and metastasis through their interactions with intracellular signaling networks. From a clinical perspective, polymorphisms within microRNA-binding sites are associated with the risk for colon cancer while microRNAs isolated from feces or blood may serve as biomarkers for early diagnosis. Altered expression of microRNAs or polymorphisms in microRNA-related genes have also been shown to correlate with patient survival or treatment outcome. Further insights into microRNA dysregulation in colon cancer and the advancement of RNA delivery technology will make it very likely to develop novel microRNA-based therapeutics.

MicroRNAs as therapeutic targets in cancer

Cancer remains a worldwide epidemic. An improved understanding of the underlying molecular mechanisms and development of effective targeted therapies are still required for many deadly cancers. The discovery of microRNAs (miRNAs or miRs) nearly 20 years ago introduced a new layer of complexity to gene regulation, but it also afforded us the opportunity to further our understanding of the molecular pathogenesis of cancers. Dysregulation of miRNAs is fundamental to the pathogenesis of many cancers based on their involvement in basic cellular functions. In addition, these previously underrecognized, noncoding RNAs have the capacity to target tens to hundreds of genes simultaneously. Thus, they are attractive candidates as prognostic biomarkers and therapeutic targets in cancer. However, several challenges remain in translating our current understanding of miRNAs to clinical therapies. Herein, we provide a review of the current knowledge of miRNAs in both solid and hematological malignancies with a focus on their potential application as therapeutic targets in cancer.

Mutator activity induced by microRNA-155 (miR-155) links inflammation and cancer

Infection-driven inflammation has been implicated in the pathogenesis of ~15-20% of human tumors. Expression of microRNA-155 (miR-155) is elevated during innate immune response and autoimmune disorders as well as in various malignancies. However, the molecular mechanisms providing miR-155 with its oncogenic properties remain unclear. We examined the effects of miR-155 overexpression and proinflammatory environment on the frequency of spontaneous hypoxanthine phosphoribosyltransferase (HPRT) mutations that can be detected based on the resistance to 6-thioguanine. Both miR-155 overexpression and inflammatory environment increased the frequency of HPRT mutations and down-regulated WEE1 (WEE1 homolog-S. pombe), a kinase that blocks cell-cycle progression. The increased frequency of HPRT mutation was only modestly attributable to defects in mismatch repair machinery. This result suggests that miR-155 enhances the mutation rate by simultaneously targeting different genes that suppress mutations and decreasing the efficiency of DNA safeguard mechanisms by targeting of cell-cycle regulators such as WEE1. By simultaneously targeting tumor suppressor genes and inducing a mutator phenotype, miR-155 may allow the selection of gene alterations required for tumor development and progression. Hence, we anticipate that the development of drugs reducing endogenous miR-155 levels might be key in the treatment of inflammation-related cancers.

KRAS signaling pathway alterations in microsatellite unstable gastrointestinal cancers

Microsatellite instability (MSI) occurs in about 15% of gastrointestinal cancers and it is associated with specific clinic, pathologic, and molecular features of the tumors. MSI-high (MSI-H) carcinomas also follow specific tumor development pathways. This review is focused on the molecular profile of alterations in members of the KRAS signaling pathway (EGFR, KRAS, BRAF, PIK3CA, RASSF1A, and MLK3 genes) in MSI gastrointestinal carcinomas. Alterations in these genes characterize more than half of gastrointestinal cancers and frequently occur simultaneously in the same tumor, pinpointing the KRAS signaling pathway as one of the most frequently altered pathways in this subset of cancers. Nowadays, many and novel inhibitors targeting molecules of this signaling pathway are being described; therefore, it is worthwhile to test their efficacy in MSI gastrointestinal cancers in order to develop new and more directed targeted therapies for patients affected by this disease.