الفهرس 
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Abstract
MicroRNAs are involved in multiple biological processes and metabolic regulation including cell proliferation, differentiation, programmed cell death (apoptosis) and haematopoiesis. Most recently, studies have supported the concept that microRNAs expressed in cell and tissue specific manner may contribute to the establishment and maintenance of cellular identity. More than 100 microRNAs have been cloned from hematopoietic cells and found to be regulated differently along the various hematopoietic cell lineages. This suggests that microRNAs may control cell fate determination during haematopoiesis. MicroRNAs have been shown to be aberrantly expressed in AML. Furthermore, specific miRNA expression patterns were found to be associated with certain genetic and cytogenetic alterations in this disease. Also microRNA have a role in diagnosis of AML, for example the expression levels of miR-181a were elevated in AML samples with M1 or M2 morphology compared with M4 or M5 AML. In addition, microRNA have a prognostic value in AML as the levels of miR-181a and miR-181b were negatively and those of miR-124, miR-128-1, miR-194, miR-219-5p, miR-220 a, and miR-320 positively associated with the risk of an event. Also over expression of miR-191 and miR-199 adversely affect overall and disease-free survival. MiRNAs regulate MM pathogenesis providing important functional insights regarding the role of miRNAs with distinctive signature in MM and MGUS patients. A MM-specific microRNA signature characterized by multiple up-regulated miRNAs including miR-32, miR-21, miR-17-92, miR-106-25, and miR-181 a and b, mir-222, mir-221and mir-382 and down-regulation of mir-15a, mir-16 was identified. While miR-106-25, miR-181a and b, and miR-21 were up-regulated also in MGUS patients with respect to normal PCs, miR-32 and the miR-17-92 cluster (in particular miR-19a and b) were highly expressed only in MM patients suggesting that these miRNAs are MM-specific. A distinctive signature of twenty-five microRNAs were differentially expressed between cHL and RLNs and thirty-six microRNAs were differentially expressed between nodular sclerosis and mixed cellularity subtypes. In addition, a subset of ten host miRNAs whose expression was influenced by the presence of EBV. Both classic and variant translocations involving MYC activation are associated with PVT1 oncogene, which encodes seven miRNAs including hsa-miR-1204 responsible for c-myc activation . Two of the miRNAs down regulated in BL cell lines, EBV transformed B-cell lines, CLL and B cell lymphomas are miR-143 and miR-145, expression levels of which are inversely associated with cell proliferation. Specific miRNA signature is associated with MDS and with 5q- phenotype. Especially, miR-34a that is highly up-regulated in 5q- syndrome patients. In addition, the up-regulation of several miRNAs (miR-10a/b, miR-126, miR-99b, and miR-130a) implicated in the regulation of HOX genes. The analysis of miRNA expressions and predicted targets showed that aberrantly expressed miRNAs might be involved in the pathogenesis of MDS by the modulation of their target genes. Finally, the discovery of miRNAs and their functions in tumorigenesis opens up a number of opportunities for translational bridging of the latest research into clinical gain amongst these is the ability of miRNAs to allow diagnosis and prognostication. Since the discovery in 1993, miRNAs have been implicated as integral players in the biology of normal cells as well as diseased processes. Researches over the last few years have allowed for the identification of various mechanisms and targets involving miRNAs, significantly improving our knowledge of biology of haematological malignancies. Additionally, it has been determined that miRNAs may be extended to the clinic due to their potential value in diagnosis, prognosis and therapy. Although these researches are very promising, it is important to remember that it does not come without obstacles. Further researches will most likely solve these types of problems and identify more functional roles of miRNAs that could be applied for haematological malignancies prevention and therapy. Finally, we believe that the future holds a great promise for the miRNA revolution in the fight against cancer and other health problems.