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The normal development of human body is based on a series of precise controls on the timing of proliferation, growth, apoptosis of cells, although many elements, both epigenetic and genetic, might contribute to the process of the transformation of normal cells into cancer cells, the essence of the molecular mechanism of cancer development has been found to depend on he alteration of the complex network of oncogenes and proto-oncogenes regulated by a class of 21- to 24-nucleotides, noncoding, RNA molecules known as microRNAs recently, to further explore the relationship between human cancer and microRNAs, studied from many different levels of the control of gene expression with advanced detecting and analyzing technology of microRNAs and lead to the discovery of potential treatments and prognosis for human cancers that are most commonly seen.
In 2007, cancer caused about 13% of all human death worldwide (7.9 million). Such rates are still rising as the quick expansion of the worldwide human population, and the proportion of the aging group and the massive change of people's lifestyle as the world develops (Jemal A. et al., 2011). So far, there are over 200 different known cancers that afflict humans (Cancer research UK, 2012). The total number of new cases diagnosed in the United States, 2004 was 1,368,030, and total cancer deaths were 563,700, estimated by the American Cancer Society from data collected from the National Cancer Institute (NIC) for incidence and the National Center for Health Statistics (NCHS) for mortality (Sandra Ulrich et al.,2005), and each year those numbers goes up, which makes cancer to be among top three on the list of the "most deadly diseases" together with heart attack in the United States based on more recent statistics (Cancer Statistics, Ahmedin Jemal et al., 2009); The effort to combat cancer has led to many fundamental discoveries in cell biology, among which, mciroRNAs have been discovered to be intensely involved in the development of cancer due to the abnormalities in their function that leads to uncontrolled cell growth, increased division and decreased death of cells, which then became cancerous (Croce CM, 2009).
I. Molecular mechanism of cancer development
1. Basis of cancer development
Cancer is not one but many diseases that share similar characteristics, characterized by uncontrolled growth and spread of cells. Therefore, cancer is a disease of cells, the cancerous transformation of cells was believed to be caused by many factors: chemical agents (chemical carcinogens), e.g., hydrocarbons (oil, tar, and sugars, etc.); irradiation - X-rays, ultraviolet light (physical carcinogens); viruses (oncogenic viruses) (biological carcinogens) (Anand P. et al., 2001); Later studies show that the essence of such transformation lies in the unwanted change of gene expression on a molecular level: the activation of oncogenes (inherited or mutated from proto-oncogenes) or the inactivation of tumor suppressor genes. All those genes referred are responsible for the regulation of proliferation and growth of cells through their gene products (Chung et al., 2002).
2. Cancerous transformation and the control of cell development
(1) The cell cycle: control of cell proliferation
One increased mutation rate per cell could raise the probability of cancer, so any chance contributes to the increase of the number of proliferating cells available for mutation assist in the cancerous outbreak, e.g., people with obesity, will have a strongly increased risk of many types of cancer, comparing with people of normal weight clinically (Sánchez-Lara K et al., 2010), which implies that the uncontrolled proliferation of
Fig. 1 Cell Cycle and its four phases (Alberts et al., 2008) cells is very crucial to cancer development. During normal developmental process of either human or animal bodies, a large portion of somatic cells seized to divide after they finish differentiating into specialized cells. Such development is based on the proliferation and growth of cells at the right place and time. In order to continue divide, normal cells have to go through a special process in request of rigid control of timing as well, which is called the cell cycle. The eukaryotic cell cycle is traditionally divided into four sequential phrases: G1, S, G2 and M phase (Fig.1). For each different phase, cells going through division have to wait certain amount of time and complete the targeted job for each step, including the duplication of genetic material, increase of cell volume, accumulation of energy and proteins required for entering the next phase. Meanwhile, the two gap phases are more than simple time delays to allow cell growth, but also help provide time for cells to monitor the internal and external environment to ensure that conditions are favorable and get fully prepared for mitosis. Like a soldier waiting for further command to act, whether or not cells will enter the next phase is precisely restrained by signaling molecules resulted from selective activation or inactivation of certain genes which are responsible for the control of cell growth and proliferation, e.g., hormones like insulin.
Under normal circumstances, if cells never receive any further signal to allow it to divide, after G1 phase, they will enter a specialized resting state known as G0, in which it could remain for days, weeks before proliferation or even totally stops for the rest of the individual consists of them, and become differentiated, specialized for unique function, e.g., neurons, also known as nerve cells.
The cell-cycle control system operates much like a timer or oscillator that triggers the events of the cell cycle in a set sequence (Fig.2). It conducts those essential processes of the cell cycle such as DNA replication, mitosis, and cytokinesis, and when it reaches checkpoints, in most eukaryotic cells, e.g., human cells, checkpoints refer to the major three regulatory, phase-to-phase transitions between G1/S, G2/M, and M/G1 phases. At each checkpoint, the control system will block the progression through the next if it detects problems inside or outside of the cell, e.g., the damage of DNA sequences, in which case the division and proliferation of cells will be ceased in order to allow a checking system to go through along the DNA sequences, replace the false base pair, repair the breakage of chromosomes, and cut out the redundant part, also known as "DNA damage response".
Fig.2 cell cycle and checkpoints (Alberts et al., 2008 )
All different kinds of carcinogens work either directly or indirectly to prevent those signaling molecules required for the regulation of a functional cell-cycle control system will contribute to the cancerous transformation of cells, as well as others that would help cells bypass the checkpoints without arousing the DNA damage response and getting the damaged DNA repaired. In other words, cancer cells acquire such "advantage" to continue to multiply even with damaged DNA, and eventually, accumulation of the damaged genetic materials will lead to the final step of cancerous transformation of those cells.
(2) Programmed cell death
For eukaryotes, normal cells that undergoing division but encountered with sever DNA damage at the checkpoint will not attempt to go with the purpose of proliferation, a arrest of cell cycle will be generated to see if the damaged DNA shall be repaired or not, in case of not, a special mechanism would help prevent those cells from dividing and lead them to commit suicide by undergoing apoptosis, a process a known as the programmed cell death, which is most important in protecting us against cancer, and believed to be often in the state of inactivation in cancer cells by mutations in the genes that encode essential components of the checkpoint response.
So, one of the most important properties of many types of cancer cells is that they fail to undergo apoptosis when a normal cell would do so. And once such balance is jeopardized, things will go awry and allow those revolted cells to further invade surrounding tissues and colonize distant organs, i.e., become malignant, and will kill the organism that owns them without early awareness and proper treatment.
microRNAs (miRNAs), a class of 21- to 24-nucleotides (nt), noncoding,
RNA molecules, after its first discovery in 1993 by genetic screens in nematode Caenorhabditis elegans, transcribed by gene lin-4 (Lee RC et al., 1993), thousands of miRNAs have been identified in a wide variety of species up to date: worms, flies, fish, frogs, mammals and flowering plants. Among which, and over 10,883 miRNA sequences have been found been found in total. And according to statistics established and published in the miRBase database, at least 721 of them were from human sample (http://microma.sanger.ac.uk, Release 14: September 2009) while most recent number wasn't for sure yet, however, more and more researches have been focused on revealing many striking discoveries about the function of miRNAs in regulating gene expression in a more prevalent way than ever imagined. About 2% of the known human genes encode microRNAs, which is more than 400 different miRNAs (Ines Alvarez-Garcia et al., 2005), and it is estimated that more than 1/3 of all human protein-coding-genes are regulated by miRNAs, and there are still more which are under the modulation of miRNAs indirectly (Lagos-Quintana et al., 2001; Watanabe, T., et al., 2005). As key regulatory molecules, miRNAs have been increasingly recognized in almost all fields of biological and biomedical fields for its involvement with various types of human diseases and being able to control a wide variety of fundamental cellular processes, such as proliferation, death, differentiation, motility, invasiveness through base-pairing with specific mRNAs and regulate their stability and translation (Reinhart et al., 2000), which is now found to be crucial to the process of cancer development (Zhang et al., 2007) and the practical implication of miRNAs as biomarkers, drug targets and therapeutic tools for diagnosis, prognosis, and treatments of human cancers (Mengfeng Li et al., 2010).
1. miRNA biogenesis
In humans, miRNAs are transcribed and synthesized by RNA polymerase II as long primary transcripts, refer as pri-miRNAs generally of 1 kb in size (80 bases), which have a stem-loop structure consists of a capped 5'end and a 3'-poly-A tail that believed to unique for their function (Lee et al., 2003).
They then undergo a special type of processing, which is quite a complex procedure
as shown in Fig.3, involving with a shorter version of pri-miRNAs: pre-miRNAs being generated by Drosha/DGCR8/Pasha complex (microprocessor complex ~650 kDa in humans) (Han J,et al., 2006); an export strategy: Exportin 5/Ran-GTP dependent mechanism (Lund E, et al., 2994), to help pre-miRNAs exit the nuclear membrane and enter the cytoplasm, digested by a second dsRNA-specific ribonuclease (RNasIII ), also known as Dicer with the assistance of the trans-activator RNA-binding protein (TRBP) to get mature miRNAs as part of short RNA duplexes, which are unwound by helicase subsequently before binding with a complex called miRNA-associated RNA-induced silencing complex (miRISC). Only one strand of the miRNA, known as the guide strand, is integrated into miRISC, while the other strand, miRNA*, also known as the anti-guide or passenger strand, is degraded by the RISC (Schwarz et al. 2003, Du and Zamore, 2005).
Fig.3 Diagram of microRNA biogenesis
(Wei Wu, 2006).
2. miRNA functions in the regulation of gene expression
(1) Post-transcriptional regulation of the levels of mRNAs
The remained strand of the miRNA duplex will guide the RISC to its target mRNA by searching for complementary nucleotide sequence, facilitated by a special kind of protein known as Argonante protein, which is also part of RISC. Argonaute exposes the 5' region ("seed region") of the miRNA aming for the 3' untranslated region (3'UTR) of the target mRNA and provide it with the optimal binding position for the happening of base-pairing between the miRNA and mRNA (Ruby et al. 2006).
Once such base-pairing, the fate of the bound mRNA could vary differently which depends on if the base-pairing is perfect or not, in a sense of "perfect", a sequence complementarity between 2-8 nucleotides is critical for recognition (Lewis et al., 2003). If the binding contains at least 7 nucleotide pairs, then it would be recognized as perfect or extensive. In which case, the miRNA, the target mRNA will be cleaved(sliced) by the Argonaute protein through removing the 3'-poly-A tail, as mentioned before, and exposing it to exonucleases which caused the degradation of the target mRNA, a process called RNA interference (Hutvagner & Zamore, 2002; Martinez&Tuschl, 2004). Following cleavage of mRNA, RISC (with its associated
miRNA) is released, and it can seek out additional mRNAs. Thus, a single miRNA can act catalytically to destroy many complementary mRNAs. The miRNAs can be thought of as guide sequences that bring destructive nucleases into contact with specific mRNAs (Yekta et al. 2004).
Fig.4 miRNA processing and mechanism of action (Alberts et al., 2008)
If the complementarity is not perfect or extensive, a repression of translation instead of cleavage will occur either pre-initiation or post-initiation of translation, and the target mRNA will be sequestered from ribosomes and going through deadenylation and decapping, a process associated with the shortening of the 3'-poly-A tail and enclosure of the target mRNA into P-bodies, a dynamic, cytosolic structure composed of large assemblies of mRNAs and RNA-degrading enzymes, which is believed to be responsible for the final destruction of mRNAs that are not regulated by miRNAs in eukaryotic cells (Engels & Hutvagner, 2006; Liu et al., 2005), see Fig.4 (). Although details of this mechanism described here is under debate currently, the eventual outcome is the same, which leads to the repression and destabilization of "miRNA targeted" mRNA (Petersen et al., 2006; Pillai et al., 2005; Wang et al., 2006). In addition, most animals, including human, the complementarity between miRNA and the target mRNA is imperfect and not extensive.(Behm-Ansmant et al. 2006, Giraldez et al. 2006, Jackson & Standart 2007,Wu et al. 2006).
(2) Three levels of gene expression regulated by miRNAs
The biological functions of miRNAs could be lying in several distinct molecular mechanisms for large sets of mRNAs required at a particular developmental stage to be repressed (Lee P. Lim et al., 2005), which refer to the gene products of a special category: gene batteries, sets of functionally related effector genes from the gene regulatory networks, encoding proteins needed for cell proliferation (E. H. Davidson, 2006).
miRNAs could also control the expression of vital transcriptional regulators: transcription factors (TFs), transcription repressors, which are required in the regulation of those effector genes mentioned above (James C., et al., 2003). Other activities of miRNAs in regulating gene expression have been revealed by recent studies, in which miRNAs were found to target global regulators of alternative pre-mRNA and induce the splicing of them in order to change the degree of gene expression (P. L. Boutz, 2007).
The functions of miRNAs have been proved to be involved in many vital biological processes that are important for the normal development of cells and organisms, including humans, by controlling the development timing, cell differentiation, cell proliferation, cell death, migration, invasion, adhesion, etc. (Eugene V. Makeyev et al., 2008) (Table.1), which indicates that aberrant expression of miRNAs or the malfunction of miRNAs might lead to the cancerous transformation of cells by acquiring such ability to divide and proliferate at wrong timing and go beyond the control of normal growth signals, some of which might even become capable of migrating to other parts or regions of the human body that are restricted under normal condition and invading the normal tissues.
Table.1 Function of animal microRNAs in vivo
(Ines Alvarez-Garcia, et al., 2005)
3. miRNA and cancer
In regulating many different aspects of development, the expression of this important class of molecules is commonly correlated with many pathological conditions, among which cancer may be the most relevant and prominent disease related to the abnormal expression and functions of miRNAs. The significance of miRNA in human cancer was first recognized and discussed in 2002, due to the discovery of a small genomic region in chromosome 13q14 that is commonly deleted in chronic lymphocytic leukemia (CLL) contained miR-15a and miR-16-1 genes, suggesting a link of these miRNAs to CLL (Calin GA et al., 2002). Following this observation, more and more miRNAs have been found to be aberrantly expressed in various types of cancer cell lines and clinical tumor specimens. In addition to the identified abnormal levels of specific miRNAs in certain types of human cancers, biological evidence that suggests an important role of miRNAs in cancer development and progression was also experimentally demonstrated in animal models (Croce CM, et al., 2009).
In order to find out the relationship between miRNAs and cancer, a huge obstacle used to trouble scientists and researches, considering the size and abundance of miRNAs, it was very hard to measure the level of the expression of miRNAs in vivo, which could be the major reason for the discovery of the connection between the miRNAs and cancerous transformation. The recent development of deep-sequencing technologies (Lu, C. et al., 2005, Margulies, M. et al., 2005), and computational prediction methods (Nam, J. W. et al., 2005; Li, S. C. et al., 2006; Huang, T. H. et al., 2007), have accelerated the discovery of less abundant small RNAs. Currently, abnormal expression profiles of miRNAs have been found in both clinical tumor specimens and cancer cell lines when compared with chosen normal controls by microarray, å¹´northern blotting, or real-time RT-PCR analyses (Iorio MV, et al., 2008), analytical example would be given as Fig.5 shows, in which markers were selected to correlate with the normal versus tumor distinction.
Observation based on such comparison indicates that the expression of miRNAs was down-regulated in tumors, and hypothesis has been generated that global miRNA expression reflects the state of cellular differentiation (Ramaswamy S. et al., 2001), rapidly accumulating evidence has revealed that miRNAs are associated with cancer because of deregulation (Meltzer PS, 2005; Esquela-Kerscher A et al., 2006; Sevignani C et al., 2006). However, an up-regulation of miRNAs in cancer cells has also been observed frequently in several recent studies (Metzler M et al., 2004; Eis PS et al., 2005), but there's one thing for sure, the development of advanced profiling technology further helps determine the relationship between miRNAs and cancers, and future studies might be able to tell whether or not miRNAs could be used to distinguish tumors from normal tissues.
Fig.5 Comparison between normal and tumor samples reveals global changes in miRNA expression
(Jun Lun et al., 2005)
Up to date, a spectrum of cancer-associated miRNAs has been identified. While some miRNAs function as tumor suppressors and are down-regulated in cancer cells, other miRNAs act as oncogenes, inducing or promoting cancer development or progression (Aguda BD et al., 2008). Examples of the miRNAs that are aberrantly expressed in tissue-specific cancers are summarized in Table.2.
Table.2 miRNAs Aberrantly Expressed in Cancers (Mengfeng Li, et al., 2010)
The mechanisms that lie in the development of cancer caused by the abnormal change of the level of miRNAs in vivo vary from each other. Expression profiling studies indicate that most miRNA are under the control of development and/or tissue specific signals (Landgraf, P. et al., 2007). If the precise control of the level of miRNAs is vital to maintain normal cell function as mentioned before, then either the down-regulation or the up-regulation of the level of miRNAs might be associated with cancer diseases of the human body, and anything that serves to cause aberrant expression of miRNAs or disables the normal function of miRNAs will be considered to exhibit powerful capability in oncogenesis and be targeted as potential treatment focus. In which case, it could be a development signal going rogue, or a lose end during miRNA biogenesis, alter the function of miRNAs. For example, as a major part of regulation in miRNA biogenesis, the transcriptions of miRNAs from miRNA genes are controlled by a number of Pol-II associated transcription factors, abnormal activity with any of those TFs will lead to the unwanted result of
miRNA expressions (Rao et al., 2006); or an epigenetic factors that alter the crucial functional structure of miRNAs (Bueno, M. J. et al., 2008); and a couple of elements involve with the post-transcriptional regulation of miRNAs, i.e., mutated Drosha processing, mutated transport protein: Exportin 5 or Argonante protein, etc..
III. Molecular mechanism of cancer development regulated by miRNAs
1. MiRNA modulation of tumor suppressor and oncogenic pathways
According to which, a focus will be generated based on the most common cancers and several important cellular signaling pathways which are part of those regulatory networks that were frequently altered in cancer cells. First, an illustration would be given to show that the regulation of miRNAs and the regulatory networks in determining the cancerous transformation of cancers are quite complicated and are conducted through more than one kind of miRNA but instead, is the result of a team work from several different miRNAs on all levels. To offer an example, a very famous and well-known gene shall be brought up here: TP53 gene, and its gene product: p53 protein, also known as tumor protein 53, a tumor suppressor protein with multifaceted function in cell-cycle control, apoptosis (programmed cell death), and in maintenance of genetic stability, all aspects of the fundamental role in protecting human against the consequences of cell damage and risk of cancer (Isobe M et al., 1986), see Fig.7 (Alex N. Bullock et al., 2001).
Fig.7 p53 tumour suppression (Alex N. Bullock et al., 2001)
miR-29a, miR-29b, and miR-29c was found to be able to suppress p85, the regulatory subunit of PI3K, and CDC42, both negative regulators of p53, and induce p53-dependent cell apoptosis (Park SY et al., 2009). Such apoptosis could be further attenuated when the expression of miR-34a is quite low and in a state of inactivation in CLL (Raver-Shapira N et al., 2007), surprisingly, the miR-34 family members are among direct targets of p53, and refer as p53-induced miRNAs (Corney DC et al., 2007), in addition, miR-34a could also down-regulates SIRT1, leading to acetylation of p53 and increasing the level of p21 (Yamakuchi M at el, 2008). Another p53-induced miRNA, miR-145, has a potential p53 response element (p53RE) in its promoter region, and directly downregulates c-myc, linking the p53 pathway to the c-myc pathway, relating to oncogene Myc (Sachdeva M et al., 2009).
miRNAs also target other important regulatory networks that were frequently altered in cancer cells, e.g., in the retinoblastoma (pRb) pathway, pRb works as a tumor and inhibits a specific transcription factor family, the E2F family, causing cell cycle arrest. miR-106a , a kind of miRNA that is overexpressed in many different types of cancers, could associate with the 3â€²-UTR of the pRb gene and suppress it (Volinia S et al., 2006). Also, back to Myc, it induces transcription of miRNAs such as the miR-106b-25 cluster and the miR-17-92 cluster, which inhibit the expression of E2F, an important element for the activation of oncogenes, indicating a bidirectional effect of miRNA-involved oncogene regulation (O'Donnell KA et al., 2005).
In addition, miRNAs were found to play important roles in virus-induced human cancers and numerous types of human cancer-associated viruses have been found to express miRNAs. By searching the 3â€² UTR sequences of the human genome with the assistance of computer approaches, a large number of cellular transcripts were predicted as potential targets of viral miRNAs (Jin WB et al., 2007), among which, the Epstein-Barr virus (EBV), a herpesvirus relates to Burkitt's lymphoma and nasopharyngeal carcinoma, is capable of expressing multiple different kinds of miRNAs, and miR-BART5 is one of them and works to down-regulate the level of a specific protein that improves the survival of host cells and immunity against latent infection of the virus (Choy EY et al., 2008).
For various, different types of cancer diseases, the possible roles of miRNA in the regulatory network for the molecular development and progression of the cancerous transformation of cells based on the regulation of cancer-related genes: oncogenes and proto-oncogenes (transcribe tumor suppressors) are generalized in Fig.8
Fig. 8 Functions of miRNA in cancer development and progression
2. Important crosslink between cancer diseases
Lung cancer is the leading cause of cancer mortality worldwide (Jemal et al, 2009). Despite for technologies and newly developed chemo/targeted therapies that improve treatment responses, the overall 5-year survival for NSCLC patients remains low (15%) and the recurrence rate is high (Miller, 2005). For which reason, the mechanisms of the development of lung cancer have been the focus of scientific and clinical studies for the past few years, among many prominent discoveries, the genes of a cluster of miRNAs known as the let-7 family were found to map to different chromosomal regions that are frequently deleted in human lung cancer (Calin et al, 2004). The role of let-7 as tumor suppressor has aroused huge attention to studies focus on other cancer diseases like breast cancer, colon cancer, etc. and was found to be reduced in its expression in colon cancer cells (Yukihiro Akao et al., 2006). Such finding with the same genetic aberration of RAS and over-expression of c-myc are frequently involved in colon cancer is consistent with the role of let-7 family plays in the development of lung cancer: negatively regulates multiple oncogenes, including the RAS (Johnson et al, 2005), MYC (Kumar et al, 2007), and HMGA2 (Lee and Dutta, 2007), and cell-cycle progression regulators, such as CDC25A, CDK6, and cyclin D2 (Johnson et al, 2007).
IV. Future direction for treatment and prognosis
Similar mechanisms associate with breast cancer (Marilena V. Iorio et al., 2005), prostate cancer and kidney cancer (James W.F. Catto et al., 2011), suggesting great possibility for miRNAs to become novel drug targets or therapeutic tools for the development of novel strategies and the treatment of human cancers on different levels.
1. Potential treatment on transcriptional level
Plausible approaches could be through either down-regulating "oncogenic" miRNAs or up-regulating "tumor suppressor" To up-regulate the expression of miRNAs, gene therapy should be applicable if the depletion of normal amount of miRNAs are caused by mutated miRNA-encoding genes, or other functional genes that are responsible for the biogenesis of miRNAs, or products of oncogenes that inhibits the generation or function of miRNAs, in which case, replacements with bioengineered plasmid vectors or viruses contain normal, functional human genes might be a solution, e.g., in treatment of prostate cancer (PCa) in mouse model, which in lack of miRs-143/145 compared with normal ones (Zaman MS et al., 2010), engineered the herpes simplex virus-1 (HSV-1) to incorporate miRs-143/145 seed regions into an essential gene, and successfully reduced tumor volume by 80% (Lee et al.,2009 ). This kind of therapy requires several crucial conditions to be satisfied: the virus used as vectors must be harmless to the human body, and the replication of the virus has to be within control, the delivery of such virus-transferred miRNAs can be confined in most desired regions. To ensure such requirements, a nano-material based technology might of great help by producing artificial nano-vectors with greater efficiency and safety, containing physically engineering surface to guarantee target-oriented, tissue-specialized delivery (Rajni Sinhal et al., 2006).
2. Potential treatment on post-transcriptional level
For cancer diseases shows prominent unbalanced level of functional miRNAs, potential treatment might lie in either the up-regulation or down-regulation of miRNAs, there are two practical approaches that have been tested to prevent miRNAs from functioning which now could be applied as plausible solution: 1. miRNA antagomir or antimir, short oligonucleotides bind to miRNAs complementarily and silence miRNAs (Krutzfeldt J et al. 2005); 2. synthetic mRNAs (snRNAs) containing multiple binding sites for a specific miRNA, competitively sequestering the endogenous miRNA (Ebert MS et al., 2007). And to improve the efficiency of binding of miRNAs to the antagonists, except for the 2-O-methyl oligoribonucleotide, a conformational RNA analoque known as locked nucleic acid (LNA) could help generate unprecedented affinity and specificity, and are commercially available from various suppliers (Jan Stenvang et al., 2008).
3. Future direction on the prognosis of cancer diseases by miRNAs
However, based on the complexity of the regulations of miRNAs in the process of cancer development, and even if we have pretty good idea about which miRNA families are related to a specific cancer, simply manipulate the levels of those miRNAs might not be sufficient enough to be considered as potential treatment for cancer diseases. Thus, for future research, an inclination to the prognosis of cancer diseases based on the alternation of miRNA levels will be favored. For years, researchers have been looking through various methods to serve such purpose by using the miRNA-expression signatures to classify cancer diseases and to define miRNA biomarkers that might help predict preferable prognosis due to the fact that the lethality of cancers lies right above those characteristics: late disease presentation, current clinical diagnosis is still largely basing on the pathological manifestations of the patient and physical observation of the nidus to determine the canceration of certain tissues and organs, which are meanwhile very likely to possess heterogeneities from one histological subtype to another (P-Y Lin et al., 2010). The earliest method to conduct miRNA expression profiling is quite familiar to each and every biologist: Northen blotting, and is still widely used due to its capability to detect both the pre-miRNAs and the mature miRNAs simultaneously (Hammond SM, 2006). However, Northern blotting is not sensitive for it's not a quantitative method. The appearance of real-time PCR (RT-PCR) and microarray methods were brought into application to detect the expression of miRNAs, and became more and more high-throughput after years' exploration and modification (Schmittgen TD et al., 2004; Jiang J. et al., 2005). To date, 4 platforms of miRNA-arrays have been utilized to screen miRNA expression pattern in hematopoietic malignancies and solid tumors (Kim VN at el., 2006), including dotted cDNA arrays, oligonucleotide
arrays,bead-based flow cytometric miRNA expression profiling and locked nucleic acid(LAN)-modified oligonucleotide arrays(Lu J et al., 2005).
Valuable data has been generated from these accumulated, massive scientific work for the past few years: the analysis of samples consist of lung, breast, head,neck,stomach, prostatic, colorectal, pancreatic and hematopoietic cancers on the expression of miRNAs (Jiang et al,2005; Volinia S et al., 2006).Together, those miRNA expression profiles were expected to be utilized to define the expression pattern of diverse cancers and predict the prognosis, and to monitor response to therapy or toxicity, however, with all that much information, it's very hard for hospitals to actually adapt those scientific statistics to clinical application, in another word, it's next to impossible or doctors to make connections and prognosis by themselves and by going simply through the ocean of data full of miRNAs profiles, especially when their roles in cancer development haven't been understood thoroughly. A systemic method that could both integrate all the information collected from miRNA-profiling and build detailed connections to one another, and be able to make prediction based on calculation and deduction faster, and more accurate than humans is in urgent need for future cancer prognosis and treatment. Computational methods may serve to such purpose precisely (Li Li et al., 2010). A well-established computation model will possess such important functions: computational identification of miRNAs, computational identification of miRNA targets, and computational prediction of the combinatorial regulation by miRNAs. And only with the integrating computational approaches, could we find the final solution to achieve the goal of early cancer diagnosis and come up with the best therapeutic treatment for patients.