cancer of the oral cavity and oropharynx ( occ ) remains among the top ten malignancies 
 in the united states and worldwide . these cancers account for 4% of malignancies in 
 men and 2% of malignancies in women . in the united states despite current advances 
 in treatment one human life every hour is lost to this cancer . according to american 
 cancer society while overall new head and neck / oral cavity cancer cases increased 
 about 8% during the past 5 years , the new cases for oral cancer increased about 21% 
 ( table 1 ) . in the 2010 cancer statistics 
 published by the american cancer society there are 36,540 estimated new cases of 
 oral cavity / oropharynx cancers and 7,880 deaths from these cancers . an alarming fact is the recent increase of 
 these cancers worldwide among younger individuals often without risk factors or not 
 engaged in known high risk social habits such as smoking and alcohol consumption 
 . despite current advances in treatment the reported overall five year 
 disease free survival worldwide remains largely unchanged over the past several 
 decades for all races ( range between 45 - 65% ) [ 1,4 - 7 ] . failure to cure occ despite optimal treatment is a reality and these 
 cancers remain an undertreated and poorly understood disease process that represents 
 a major health problem . improvement of survival and treatment outcomes require a 
 better understanding of the disease progression so that these cancers can be 
 diagnosed early and most importantly targeted therapy can be initiated 
 accordingly . the changes of head and neck / oral cavity cancer ( hnocc ) incidence and 
 death for the past 5 years the initiation and progression of occ is a highly complex multistep process that 
 entails progressive acquisition of genetic and epigenetic alterations and dynamic 
 changes in the genome . thus far , the majority of the studies on the cancer genome 
 have focused primarily on the protein coding genes and their alterations . the impact 
 of non coding sequences in disease initiation and progression including cancers 
 remain largely unknown [ 8 - 10 ] . although at least 65% of the genome is 
 transcribed , protein - coding transcripts are derived from less than 2% of the human 
 genome . the mammalian genome harbours 
 genes that although they do not encode proteins have an important role in normal 
 cell development and disease process and these non protein transcripts may make up 
 at least half of all transcripts in mammals . rna interference ( rnai ) in a variety of organisms is a known process of sequence 
 specific post - transcriptional gene silencing initiated by double - stranded rna . rnai was first discovered in 1998 in the neomatome ceanorhabditis elegans , but is 
 conserved in variety of organisms and is considered a major regulatory mechanism in 
 eukaryotic gene expression . in mammalian 
 cells rnai regulation of endogenous genes occurs by the production of short 
 double - stranded rna molecules or microrna . micrornas mediate gene expression at the 
 post - transcriptional level by degrading or repressing target messenger rnas ( mrna ) 
 or by translational inhibition of target genes . since the initial discovery of the 
 founding members of the microrna family , lin-4 and let-7 several hundreds have been 
 identified in all species by combination of molecular cloning and bioinformatics . it 
 is now estimated that 1,000 micrornas exist in the human genome [ 16 - 24 ] . the purpose of this article was to review the literature related to microrna 
 deregulation in the head and neck / oral cavity cancers . a comprehensive review of the available literature from 2000 to 2011 relevant to 
 microrna deregulation in oral cancer was undertaken using pubmed , medline , scholar 
 google and scopus . keywords for the search were : microrna and oral cancer , microrna 
 and squamous cell carcinoma , microrna deregulation . micrornas are encoded by genes located either in non coding regions or in introns of 
 protein coding genes and require a complex set of proteins for their formation 
 . thus primary microrna transcription may be by an independent 
 promoter or by a promoter of the proximal coding gene . most micrornas are 
 transcribed by the rna polymerase ii to primary micrornas that are longer nucleotide 
 sequences ( hundreds to even thousands of nucleotides ) . these are then spliced 
 and capped with a 5 ' 7-methylguanosine cap ( g ) and poly - adenylated at 
 the 3 ' end . the 
 primary micrornas form specific hairpin - shaped stem loop secondary structures prior 
 to be processed by a microprocessor complex ( 500 - 650 kda ) into pre - micrornas . the 
 complex , consistent of drosha ( rnase iii endonuclease ) and the essential cofactor 
 dgcr8/pasha , processes the primary mircornas into 60- to 70- nucleotide long 
 pre - microrna with a 5 ' phosphate and a 3 ' nucleotide overhang . exportin 5 , a member 
 of the ran transport receptor family , transports the pre - microrna to the cytoplasm . 
 in the cytoplasm further processing to short double strand microrna / microrna * occurs 
 by dicer , a second rnase iii endonuclease , prior to unwind of the duplex by a 
 helicase to reveal the final mature microrna and microrna * , which is quickly 
 degraded . the average ratio of microrna 
 to microrna * is approximately 100 to 1 but can be much lower in cases of both 
 strands are functional and incorporated into risc that is shown to occur . the mature microrna product is noncoding , regulatory rna molecules 22 nucleotides 
 long that can be asymmetrically incorporated into rna - induced silencing complexes 
 ( risc ) that are then guided to the target mrna . microrna physiologic functions it is well established that micrornas are involved in diverse physiologic processes 
 . studies with mouse embryos and zebra fish dicer - null phenotype 
 revealed that microrna pathway is not generally required for cellular viability but 
 plays a prominent role in various tissue specific cell types and morphogenesis of 
 embryonic structures . such examples are impact of micrornas on t - cell 
 development / differentiation as well as morphogenesis of lung , limp and skin as well 
 as maintenance of hair follicles . the ability of micrornas to dramatically 
 influence tissue(s ) specific generation and behaviour is another important function 
 of these molecules . this is demonstrated in studies on microrna-181 , the first 
 mammalian microrna to be carefully studied as well as microrna-1 the most highly 
 conserved microrna . microrna-181 's ectopic expression in hematopoietic progenitor 
 cells skews their differentiation towards the b - cell lineage while the same microrna 
 is up - regulated during differentiation and regeneration of muscle cells . microrna-1 conversely demonstrates skeletal and cardiac muscle specific expression 
 and is shown to be critical in development of normal muscle . over - expression or inhibition of 
 microrna-1 promotes or inhibits respectively mammalian muscle cell differentiation 
 in vitro . in addition microrna-1 has been shown to also play an important 
 role in muscle physiology . the involvement of microrna-138 , that is also frequently deregulated in oscc as will 
 be discussed later , in differentiation of human adipose tissue derived mesenchymal 
 stem cells is another recent discovery . during adipose differentiation microrna-138 
 was found to be significantly down - regulated , while it 's over expression effectively 
 reduced lipid droplets accumulation and inhibit expression of key adipogenic 
 transcription factors . these findings could provide insights into the pathogenesis 
 of a number of diseases such as obesity and diabetes and potentially broaden the 
 spectrum of stem cell based therapy for these conditions . these findings stress the fact that a single microrna can 
 participate and impact on distinct pathways in various tissues and have the ability 
 to influence the generation and behaviour of tissue - specific cell types . it is now accepted that micrornas are important in establishing and/or maintaining 
 gene expression patterns that are characteristic of specific tissues . it has been shown that many micrornas and 
 their predicted target are reciprocally expressed . lastly , several 
 cell - autonomous functions , not related to development or differentiation , have been 
 shown to be controlled by micrornas . such examples include insulin regulation and 
 specific expression of microrna-375 in pancreatic islet beta cells and cholesterol 
 homeostasis by liver specific microrna-122 . the brain and nervous system is perhaps 
 the most extensively studied in reference to microrna and regulation of function in 
 vertebras . for example neuronal differentiation and synaptic function have been 
 found to be controlled by microrna-9 and microrna-124 while neuronal outgrowth and 
 dendritic morphogenesis by microrna-134 , microrna-132 . another interesting 
 implication of micrornas in function has originated from identification of 
 microrna-138 involvement in dendritic spine size morphogenesis , via synaptic protein 
 synthesis , that is associated with formation of long lasting memories . in addition to key roles in the nervous system development and function micrrna-138 
 has been implicated in cardiac patterning - compartmentalization during embryonic 
 development . it is evident from the 
 current and growing literature that micrornas have global participation and impact 
 on normal physiologic processes . a logical extension to this conclusion is that any 
 alteration or abnormalities in their function would influence disease phenotypes in 
 all organisms . additional 
 information on microrna functions and micrornas in physiology and disease process 
 can be found in the excellent reviews on the subject by bushati et al . and microrna and cancer of the oral cavity and oropharynx ( occ ) since their initial discovery the microrna gene family is continuously growing with 
 novel members discovered in association with several disease processing . once 
 sufficient information on microrna was made available several commercially available 
 microrna array platforms were developed and subsequently employed successfully in 
 identification of microrna deregulation in head and neck / oral cancers . the currently 
 available technology has necessitated and facilitated the establishment of several 
 online databases for tracking and to accommodate this constantly growing list . identification of potentially " cancerous " micrornas has been based mainly in their 
 differential expression in cancers compared with controls . interestingly enough 
 studies have suggested that microrna signatures could be used to classify malignancy 
 based on their tissue of origin . jiang et al . in 2005 employed real time 
 quantitative pcr - based methods to successfully identify microrna deregulations in 
 thirty two cancer cell lines including five from the head and neck / oral cavity 
 . clustering analysis based on the 
 expression values of these microrna precursors enabled most of the cancer cell lines 
 to be clustered based on the tissue of origin . this is suggestive of the presence of microrna expression 
 signatures / profiles of cancers based on the specific tissue of origin . this is in 
 line with the previously discussed identification of organ / tissue specific micrornas 
 and their link to physiologic function and disease process . using microarray 
 profiled for 261 micrornas using 9 cell lines ( from hypopharynx , base of tongue , 
 oral tongue , tonsil and larynx ) identified 33 up - regulated and 22 down - regulated 
 micrornas , several of which are known to be involved in carcinogenesis . this study remains the first to provide 
 such a large genome - wide survey of mature microrna in head and neck cancer . hebert 
 et al . in 2007 studied microrna expression patterns in squamous cell cancer cells 
 from the head and neck that were cultured under hypoxia conditions . cells from 3 cell lines were grown under 
 normoxic conditions or hypoxia ( 5% and 1% oxygen ) and profiling was carried using 
 human_v7.1c_051017 microrna array ( lc sciences ) . interestingly twenty micrornas were 
 up - regulated including microrna-572 , microrna-214 , microrna-563 , microrna-15a , 
 microrna-200a , microrna-7 , let-7a , let-7 g , let-7i . among the 16 down - regulated 
 micrornas under these conditions were microrna-122a , microrna-565 , microrna-195 , 
 microrna-30e-5p , microrna-374 , microrna-19a , microrna-22 . hypoxia is important in 
 progression and treatment as it has been implicated in development of 
 chemoresistance in head and neck / oral cancers . the results reflected profiling 
 differences in the cancer cell lines and no nonmalignant controls were used , but the 
 study associates hypoxia conditions with microrna deregulation and potential 
 development of resistance to chemotherapy . identification of microrna alterations in 
 these conditions could facilitate our understanding of this adaptation by the cancer 
 cell and guide targeted therapy . an 
 array containing 646 mature and pre - microrna , genoexplorertm from genosensor 
 corporation ( tempe , az , usa ) was used to screen for altered microrna expression by 
 chang et al . in 2008 . the study , that 
 included head and neck squamous cell carcinoma cell lines , primary tissue samples 
 and normal tissue controls , identified eight micrornas to be up - regulated and one 
 down - regulated in the cancer samples compared to controls . microrna-21 , microrna-18 , 
 microrna-19 , microrna-29c , microrna-142 , microrna-3p , microrna-155 , microrna-146b 
 and let-7 that known to be involved in tumorigenesis were in the unregulated group 
 . profiling of squamous cell 
 carcinoma of the tongue was carried in two studies by wong et al . in 2008 . laser dissected cells from four tongue cancers and paired normal tissue were used to 
 examine expression levels of 156 human mature micrornas using qrt - pcr ( tan man 
 microrna assays ; human panel ) . twenty four micrornas , including microrna-184 and 
 microrna-21 , were up - regulated and 13 were down - regulated , including microrna-100 , 
 microrna-125 , microrna-133a and microrna-133b . a 3-fold expression difference was 
 the cut - off level used . in their attempt to identify a microrna 
 signature specific to oral cavity squamous cell carcinoma , kozaki et al . in 2008 
 examined the expression profile of 148 micrornas in 18 cancer cell lines and 
 immortalized oral keratinocyte line rt7 that served as control . the cancer cell lines originated from ten 
 stage 2 ( t2 ) and one stage 4 ( t4 ) frozen primary samples . the expression levels of 
 the microrna genes were examined using the tan man microrna assay ( applied 
 biosystems ) . eleven micro - rnas were up - regulated by at least 1.5-fold expression or 
 higher and 54 were down - regulated by less than 0.5-fold expression in the cancer 
 cell lines compared to rt7 . the 
 mirvana mirna bioarray system from ambion ( austin , tx , usa ) was used in two studies 
 aiming to provide a microrna expression profile for head and neck cancers including 
 oral cavity tumours . this microarray platform was used by avissar et al . in 2009 to 
 determine microrna expression of 662 micrornas in 16 fresh - frozen hnscc tumours , 5 
 nondiseased head and neck epithelial tissues , and 2 individual hnscc cell lines in 
 one study . eleven micrornas , including 
 microrna-21 , were up - regulated and one was down - regulated , ( microrna-375 ) . this 
 study demonstrated a significant variation of microrna expression between tissue 
 samples and cell lines putting emphasis on the possibility that cultured cell lines 
 maybe not appropriate for microrna profiling of cancer . in the second study employing the same microrna microarray platform five tumour samples 
 from four subsites , that included the tongue ( 2 out of 5 ) and floor of the mouth ( 1 
 out of 5 ) were compared to normal tissue harvested from adjacent to the tumour 
 . in addition to identifying 16 
 up - regulated micrornas ( including microrna-21 ) and 4 down - regulated micrornas the 
 study demonstrated potential for stratification of tumour versus adjacent normal 
 tissue based on the differential expression of microrna and their targeted genes . a 
 comprehensive list of the studies demonstrating microrna deregulation in head and 
 neck / oral cancer is provided in table 2 . studies demonstrating microrna deregulation in head and neck / oral cavity 
 cancer ( hnocc ) most recently , clague et al . in 2009 examined the potential role of microrna 
 polymorphism in identifying patients with oral premalignant lesions ( opl ) that maybe 
 at high risk for progression into cancer . they concluded that individual and combined genotypes of 
 microrna - related variants could potentially be used to predict risk for progression . the potential role of micrornas in tumour progression was investigated by scapoli et 
 al . using microarray analysis the group examined 15 oral cancers ( 8 without evidence 
 of metastasis and 7 with nodal involvement ) and among the 19 deregulated micrornas 
 they identified three ( let-7i , microrna-155 and microrna-146a ) to be associated with 
 disease progression , signified by nodal involvement . identified differences in microrna expression 
 patterns of normal epithelia and squamous cell carcinoma of the oral cavity and 
 developed a molecular classifier that included 61 micrornas and provided 93% 
 accuracy . in addition the group 
 concluded that hpv ( human papilloma virus ) infected tumours may have a distinct 
 clinical behaviour potentially due to influence of hpv on microrna . microrna deregulation in oral cancer and prognosis in addition to exploring microrna deregulation some studies have attempted to 
 demonstrate an association between microrna expression in head and neck including 
 oral cavity cancer and survival . microrna expression profiles from 64 squamous cell 
 carcinomas , that included 31 oral cavity tumours , carried in fresh specimens and 
 adjacent normal tissue identified micrornas let-7 and microrna-205 as poor 
 prognosticators in survival . using a custom microrna microarray representing a total 
 of 236 human microrna genes deregulation was identified in 49 . an average of 2-fold 
 lower expression was demonstrated for 43 , while at least a 2-fold higher expression 
 in tumours versus controls was found for 6 micrornas . five microrna genes 
 ( microrna-21 , microrna-1 , microrna-133 , microrna-205 , and let-7d ) were selected for 
 quantification based on existing evidence of their deregulation in head and neck 
 cancers from previous studies . findings were consistent with previous studies : 
 higher expression levels of microra-21 in tumours versus controls and lower 
 expression levels of microrna-205 and let-7d in tumours versus controls . when 
 investigating microrna expression and clinical outcomes the reduced expression of 
 let-7d and microrna-205 combined were significant predictors of cancer progression 
 independent of site . another interesting population - based case - control study that included 513 cancers 
 ( 283 oral cancer , 132 pharyngeal and 98 laryngeal cancers ) and 597 controls ( matched 
 to cases by gender , age and town of residency ) examined the let-7 microrna - binding 
 site polymorphism in the kras 3 " utr that arises in the let-7 complementary site . this leads to a kras - lcs6 variant allele that alters the expression of kras and 
 levels of let-7 . the interest in this phenomenon was driven by observations that 
 when this variant allele was identified in lung cancers it was associated with poor 
 outcome . lung , pancreas and colon 
 adenocarcinomas activation of kras proto - oncogene via mutation is a well documented 
 phenomenon . although kras mutations are 
 rare in cancers of the head and neck , amplifications of kras have been reported in 
 squamous cell carcinomas originating from this site . in this study two important observations were made : kras - lcs6 variant allele was significantly 
 associated with poor prognosis and the prognosis was worse in cancers originating in 
 the oral cavity . metastasis is the major distinctive event in malignancy progression and severely 
 impacts on prognosis . using three pairs of cancer cell lines from the head and neck 
 with differences in migration and invasion liu et al . in 2009 identified several 
 micrornas to be deregulated many of them previously implicated in tumorigenesis and 
 metastasis . these included let-7 family 
 members , microrna-7 , microrna-16 , microrna-21 , microrna-27 family , microrna-98 , 
 microrna-99b , microrna-101 , microrna-106b , microrna-125 , microrna-138 , 
 microrna-193 , microrna-200a , microrna-203 , and microrna-224 . among the identied microrna 
 deregulations , reduced expression of microrna-138 was consistently observed in the 
 highly invasive cell lines . down - regulation of microrna-138 has been previously observed in scc of the tongue , 
 thyroid carcinoma , lung cancer in never smokers and has been implicated in multidrug 
 resistance of leukaemia cells . the majority of these studies especially early on utilized cell lines originating 
 from different sub - sites of the head and neck such as hypopharynx , oral and base of 
 tongue or larynx . the substantial differences in behaviour , response to treatment 
 and pathogenesis among these tumours is well known and may have contributed to some 
 contradicting findings . furthermore , cancer cells lines were not compared to 
 controls or tissue from tumours in some of the studies . this may as well have 
 contributed to some extend to the confusing reports regarding some micrornas . additional limitation of the studies using cancer lines that prevents definitive 
 conclusions is that the cultured cells may not demonstrate the same profiles as the 
 cancers of origin . it has been suggested actually that the in vitro conditions , 
 inclusion of synthetic medium and addition of fetal bovine serum may affect the 
 expression profiles of micrornas . significant progress has been made in identification of specific micrornas that are 
 consistently deregulated in oral cavity cancers . from the available studies , it can 
 be concluded that microrna-21 and let-7 are among the micrornas found to be always 
 up - regulated in cancers of the oral cavity and oropharynx versus controls while 
 microrna-133a and microrna-133b are consistently down - regulated . among the 
 deregulated micrornas , several have been functionally validated and their potential 
 target genes have been identified ( table 3 ) . several other micrornas known to be involved in carcinogenesis are reported with 
 some conflicting findings potentially due to the study limitations mentioned 
 earlier . additional evidence exists on specific roles of these molecules and their 
 involvement in pathways , known to be altered in cancer of the oral cavity and 
 oropharynx as shown in table 3 . the expanding 
 knowledge of specific roles of certain micrornas is further contributing to our 
 understanding of the complexity of tumour progression and behaviour . consideration 
 of this information and incorporation into treatment modalities through targeted 
 therapy could potentially enhance our abilities to improve outcome especially when 
 other established therapies have failed . commonly deregulated micrornas in head and neck / oral cavity cancer 
 ( hnocc ) and their proposed target genes
abstractobjectivesoral cancer is the sixth most common malignancy worldwide . 
 cancer development 
 and progression requires inactivation of tumour suppressor genes and 
 activation of proto - oncogenes . 
 expression of these genes is in part 
 dependant on rna and microrna based mechanisms . 
 micrornas are essential 
 regulators of diverse cellular processes including proliferation , 
 differentiation , apoptosis , survival , motility , invasion and morphogenesis . 
 several micrornas have been found to be aberrantly expressed in various 
 cancers including oral cancer . 
 the purpose of this article was to review the 
 literature related to microrna deregulation in the head and neck / oral cavity 
 cancers.material and methodsa comprehensive review of the available literature from 2000 to 2011 relevant 
 to microrna deregulation in oral cancer was undertaken using pubmed , 
 medline , scholar google and scopus . 
 keywords for the search were : microrna 
 and oral cancer , microrna and squamous cell carcinoma , microrna 
 deregulation . 
 only full length articles in the english language were 
 included . 
 strengths and limitations of each study are presented in this 
 review.resultsseveral studies were identified that investigated microrna alternations in 
 the head and neck / oral cavity cancers . 
 significant progress has been made in 
 identification of microrna deregulation in these cancers . 
 it has been 
 evident that several micrornas were found to be deregulated specifically in 
 oral cavity cancers . among these 
 , several micrornas have been functionally 
 validated and their potential target genes have been identified.conclusionsthese findings on microrna deregulation in cancer further enhance our 
 understanding of the disease progression , response to treatment and may 
 assist with future development of targeted therapy .