O6-methylguanine-DNA methyltransferase in equine sarcoids: molecular and epigenetic analysis
© Altamura et al.; licensee BioMed Central Ltd. 2012
Received: 1 August 2012
Accepted: 6 November 2012
Published: 10 November 2012
Bovine papillomaviruses (BPVs) types 1 and 2 are the only known papillomaviruses able to jump the species. In fact, BPVs 1/2 induce neoplasia in their natural bovine host but infection is also associated to neoplastic skin lesions in equids termed sarcoids. The equine sarcoid is considered to be the most common equine cutaneous tumour worldwide for which no effective therapy is available. Very little is known about the molecular mechanisms underlying tumourigenesis, although genes contributing to sarcoid development have been identified. Several studies associate the development of cancer to the loss of function of a number of oncosuppressor genes. In this study the putative role of O6-methylguanine-DNA methyltrasferase (MGMT) was investigated for sarcoids. The expression of the oncosuppressor protein was assessed in normal and sarcoid cells and tissues. In addition, the DNA methylation profile was analysed to assess the role of epigenetic mechanism in regulation of MGMT expression.
A group of 15 equine sarcoids and two primary sarcoid cell lines (fibroblasts) were analyzed for the expression of MGMT protein by immunohistochemistry, immunofluorescence and Western blotting techniques. The sarcoid cell line EqSO4b and the tumour samples showed a reduction or absence of MGMT expression. To investigate the causes of deregulated MGMT expression, ten samples were analyzed for the DNA methylation profile of the CpG island associated to the MGMT promoter. The analysis of 73 CpGs encompassing the region of interest showed in 1 out of 10 (10%) sarcoids a pronouncedly altered methylation profile when compared to the control epidermal sample. Similarily the EqSO4b cell line showed an altered MGMT methylation pattern in comparison to normal fibroblasts.
As previously demonstrated for the oncosuppressor gene FHIT, analysis of MGMT expression in sarcoid tissues and a sarcoid-derived fibroblast cell line further suggests that oncosuppressor silencing may be also involved in BPV-induced equine tumours. Abnormal DNA methylation seems to be one of the possible molecular mechanisms involved in the alteration of MGMT expression. Further studies are required to address other basic molecular mechanisms involved in reduced MGMT expression. This study underlines the possible role of DNA methylation in oncosuppressor inactivation in equine sarcoids.
KeywordsBPV Equine sarcoid MGMT
Sarcoids are locally invasive skin tumours of equids and are considered to be the most common equine cutaneous neoplasm worldwide . Macroscopically, sarcoids vary and six different clinical types are recognized: occult, verrucous, nodular, fibroblastic, mixed and malevolent . They are histologically characterized by disorganized dermal proliferation of spindle-shaped fibroblasts that form whorls and by epidermal hyperplasia, hyperkeratosis, and rete peg formation . The tumours can occur anywhere on animal’s body; however they often arise in areas of previous injury or scarring; additionally, they very rarely regress, more often persist and can be locally aggressive. Currently, there is no effective therapy available for sarcoids .
Many reports in the literature have shown that BPV type 1 and less commonly type 2 are involved in the pathogenesis of sarcoids. Recent studies have highlighted the role of the BPV oncogenes E5 and E7 in the carcinogenesis [5–7]. However, little is known regarding the molecular mechanisms underlying sarcoid tumourigenesis and only few genes contributing to the development of this neoplastic disease have been identified so far .
Several studies associate the development of cancer to the loss of function of a number of oncosuppressor genes. In a recent work (Strazzullo et al., 2012) we have provided evidence for an altered expression of the FHIT protein in sarcoid tissues and derived cell lines. FHIT is the first oncosuppressor gene whose aberrant expression has been associated to equine sarcoids whereas it is frequently altered in human tumours associated to papillomavirus infection.
MGMT is another oncosuppressor gene often inactivated in human PV-induced tumours. This gene, also called O6-alkylguanine-DNA alkyltransferase, encodes for a DNA repair protein that removes mutagenic and cytotoxic adducts from O6-guanine in DNA . Alkylation of DNA at the O6 position of guanine is an important step in the formation of mutations in cancer, primarily due to the tendency of the O6–methylguanine to pair with thymine during replication, resulting in the conversion of guanine-cytosine to adenine-thymine pairs in DNA . Moreover, the O6 –methylguanine adducts crosslink with the opposite cytosine residues, blocking DNA replication . MGMT is able to protect cells from such biological processes, resulting in the direct restoration of the normal guanine structure . The alkylated MGMT becomes detached from DNA and is degraded through ubiquitination-dependent proteolysis .
The expression of the MGMT protein is decreased in a wide spectrum of human tumours due to several genetic mechanisms and as a consequence of the hypermethylation of the promoter region . In some human cancers, such as gliomas, inactivation of MGMT gene correlates with a better prognosis as this condition allows a better responsiveness to therapy with alkylating agents .
MGMT is a well-known target for methylation inactivation in HPV-induced cancer  but its role in BPV-induced cancer has not been investigated so far. DNA methylation is a frequent epigenetic event in many human cancers. Many studies have pointed out that promoter methylation of tumour suppressor genes is linked with HPV-induced cervical carcinogenesis [16, 17].
Whilst alterations in MGMT expression have been reported for several human cancers comprising those induced by HPV, a putative role of this gene and of epigenetic alteration, to the best of our knowledge, has never been investigated in veterinary oncology [14, 15, 18].
In order to gain new insights into possible mechanisms underlying BPV-mediated carcinogenesis, we investigated the status of MGMT protein expression in equine sarcoid cell lines and sarcoid tumours. The samples showing significantly reduced MGMT protein expression were further analyzed for the DNA methylation status of equine MGMT CpG island spanning the predicted 5′UTR.
MGMT protein expression in sarcoids and cell lines
Normal fibroblasts derived from healthy horses displayed nuclear and cytoplasmic staining pattern for the MGMT protein. Additionally, cells from the different skin layers also stained positive (Figure 1C). Human tonsil tissue section used as positive control showed positive immunostaining (Figure 1D).
Equine MGMT CpG island and DNA methylation analysis in sarcoid tissues and cell lines
The 10 sarcoids that stained biochemically or immunohistochemically negative or very weakly positive for MGMT protein expression, were analyzed for the potential involvement of epigenetic mechanisms in the abnormal expression of this oncosuppressor. To this aim, DNA methylation profile of the CpG island associated to the putative regulatory region was analyzed. A similar analysis was performed also for sarcoid-derived cell lines EqSO1a and EqSO4b.
The DNA extracted from the sarcoid samples and from a BPV negative normal skin sample, was converted by sodium bisulfite. Figure 4B on the top, shows the CpG island prediction obtained through the MethPrimer algorithm (http://www.urogene.org/methprimer/index1.html) and the position of the specific primers obtained using the same algorithm. The region of interest was amplified, cloned and sequenced. For each samples at least 10 clones were sequenced. One sarcoid sample showed a clear difference in the DNA methylation content compared to the normal control. A similar analysis was performed also for the sarcoid cell lines, EqSO1a and EqSO4b compared to the E-Derm fibroblast cell lines. Also the EqSO4b showed a different methylation pattern when compared to the normal skin derived cell line E-Derm, similarly to that observed in the positive sarcoid sample. In the lower part of Figure 4B, the position of methylated and unmethylated cytosines in the different clones sequenced is shown for normal and sarcoid tissue and for the E-Derm cell line and sarcoid cell line EqSO4b.
DNA methylation is a frequent epigenetic event in many human cancers [19, 20]. The transcriptional silencing by the hypermethylation of CpG islands in the promoter region, is a well-known common mechanism for the inactivation of tumour suppressor genes in human malignancies [21, 22]. In an ongoing study aiming at investigating the possible epigenetic changes of oncosuppressor genes involved in sarcoid carcinogenesis, we have recently demonstrated a noticeable alteration of expression of the FHIT protein (Strazzullo et al., 2012), which encouraged the investigation of the role of other oncosuppressor genes and of their regulative mechanisms in sarcoid molecular pathology. Since many lines of evidence support the association between HPV infection and MGMT expression alteration [23, 24], we analyzed the role of the MGMT gene.
In this study we investigated the expression of the MGMT protein within a normal fibroblast cell line (E-Derm) and the fully transformed sarcoid fibroblast lines explanted from an equine sarcoid tumour (EqSO1a and EqSO4b), as well as the pattern of expression in normal skin and sarcoid tissues. Our data indicate, for the first time, a noticeable reduction and or absence of MGMT protein in naturally occurring sarcoid tumours as well as in sarcoid derived cell lines.
Sarcoids may exist as six different clinical types , we have not found a correlation between down-regulation of MGMT expression and clinical appearance, suggesting the existence of a common mechanism underlying the reduction of protein expression that acts early during the development of equine sarcoids. Our results are in agreement with several previous studies indicating that the loss and or reduced expression of MGMT is frequent in a variety of tumours , particularly in HPV-induced cervical lesions [26–28]. It is possible that decrease of MGMT protein expression is a common mechanism of cancer development among different species.
It has been suggested that the activity of MGMT increases as the severity of neoplasia and its clinical stage also increases [29, 30]. Sarcoid is considered as a “benign” non metastatising tumour. In this regard, it is reasonable to speculate that the reduction and/or absence of MGMT in sarcoids may be correlated to their relative benign biological behavior [1, 4].
The presence of a functional MGMT gene is essential to avoid occurring mutations in other important genes involved in cancerogenic processeses. The silencing of MGMT gene induces a mutator pathway affecting some oncogenes such as Ras . It is worth noting that sarcoid tumours and sarcoid derived cell lines show an up-regulation of Ras activity (Altamura et al., manuscript in preparation). It is therefore possible to hypothesize a link between MGMT down-regulation and Ras constitutive activation. However, further investigations are needed to confirm this, which is at moment only a matter of speculation.
As previously observed for the FHIT protein, EqSO4b cells revealed lower MGMT expression in comparison to EqSO1a. MGMT reduced expression in vitro has been linked to its binding to HPV E6 oncoprotein which is able to enhance the ubiquitin-dependent proteolysis of this oncosuppressor . Additionally, HPV-16 E7 is able to modulate the DNA methylation activity . Since the EqSO4b cell line contains higher copy numbers of viral genome and oncoprotein transcripts than the EqSO1a , it is possible that the observed decreased level of MGMT protein found may directly correlate with BPV oncogene expression levels and be due to a similar mechanisms induced by BPV E5/E7 oncoproteins. Further studies are needed to elucidate the molecular mechanisms underlying this peculiar aspect.
Promoter methylation is the primary epigenetic alteration associated with transcriptional silencing of tumour suppressor genes during tumourigenesis [34–36]. To determine if DNA methylation was involved in altered MGMT expression in sarcoid tumours, we performed the sodium bisulfite sequencing analysis of the CpG island associated to the 5′ untranslated region of MGMT . Our data indicated that in MGMT gene promoter heavy hypermethylation occurred in 1 out of the 10 analyzed sarcoid samples and in EqSO4b cells. The sarcoid sample and the EqSO4b cell line showed an overall similar hypermethylation pattern. Differences may be explained by the presence of some polymorphisms, a fortiori considering that the analysis was carried out for a not translated region. EqSO4b is a primary culture cell line and this may introduce some difference too. Moreover it’s to be considered that the region is very complex in its sequence structure and this may promote polymerase slippage.
Similar frequency of MGMT hypermethylation is observed in HPV induced cervical cancer , thus supporting the hypothesis that MGMT promoter methylation is not a common feature of PVs induced tumours.
Our findings indicate an association between BPV infection and MGMT protein expression alteration suggesting the possibility of a mechanistic role for this gene as a cofactor triggering the development of equine sarcoid tumour in concert with BPV.
The biochemical, immunohistochemical and immunofluorescence analyses suggest that MGMT protein is reduced or absent in sarcoid tissues and sarcoid derived cell lines. The epigenetic analysis suggests DNA methylation as one of the possible causative mechanisms for the loss of expression of MGMT in this tumour. Further studies will be useful to confirm these findings and to clarify the other mechanisms underlying MGMT alteration in sarcoid carcinogenesis, but this study represents the first example in veterinary oncology of the epigenetic inactivation of the MGMT oncosuppressor.
Samples of equine sarcoid and normal skin from healthy horse (N°15) were derived from the archives of the Department of Pathology and Animal Health, University of Naples “Federico II”. Sections taken from paraffin blocks were stained by haematoxylin and eosin (HE) and re-evaluated to confirm the diagnosis. Histologically, the samples were characterized by epidermal hyperplasia with rete peg invading the dermal tissue beneath. A diffuse proliferation of dermal fibroblasts arranged in whorls and or bundles was seen. A storiform pattern was also recorded. All the tumour samples were diagnosed as equine sarcoids.
The samples were derived from different animals. All sarcoids were known to be positive for BPV DNA (Borzacchiello et al., 2008).
Five out of 15 sarcoids were immediately frozen at −80°C and available for biochemical analysis.
E-Derm fibroblast cell lines, derived from horse dermis, were obtained from the American Type Culture Collection. EqSO1a and EqSO4b have been described previously (Yuan et al., 2008). All cells were maintained in culture in DMEM (Dulbecco's modified eagle medium) supplemented with 10% FBS (Gibco) in a 37°C humidified atmosphere of 5% CO2 in air.
Ten sarcoid samples and 1 skin sample were stained. A section from a normal human tonsil was included as positive control . Briefly, paraffin sections were deparaffinized, and blocked for endogenous peroxidase in 0.3% H2O2 in methanol for 20 min. Antigen enhancement was performed by pretreating with microwave heating (twice for 5 min each at 525 W). The anti-MGMT antibody (Resnova, Rome, Italy) was applied at 1:50 dilution overnight at room temperature in a humified chamber. The slides were washed three times with phosphate-buffered saline (PBS), then incubated for 30 min with the appropriate biotinylated secondary antibody (labelledstreptavidin–biotin (LSAB) Kit; DakoCytomation, Denmark) as previously reported (Borzacchiello et al., 2006 Oncogene). Sections were washed three times with PBS and then incubated with streptavidin-conjugated to horseradish peroxidase (LSAB Kit; DakoCytomation, Denmark). Colour development was obtained by treatment with diaminobenzidine (DakoCytomation, Denmark) for 5 min. Sections were counterstained with Mayer’s haematoxylin. In the corresponding negative control section, the primary antibody was either omitted or replaced with appropriate normal serum.
The scoring of the immunoreactivity was determined in a ‘blind’ study by two observers (GB and AC). The intensity of labelling in each specimen was scored from absent to very strong immunosignal.
Immunofluorescence and confocal laser-scanning microscopy
E-Derm, EqSO1a and EqSO4b cell lines were grown for 2 days on coverslips, washed with PBS, fixed in 4% paraformaldehyde for 20 min, permeabilized with 0,1% triton X-100 in PBS 5 min. The slides were blocked with 2% BSA for 30 min. The anti-MGMT primary antibody was applied O/N at 4°C in a humified chamber at 1:50 dilution and after washing with PBS, incubated with Alexa Fluor 488 goat anti-rabbit 30 min at RT (Molecular Probes. Leiden, The Netherlands).
Finally, after washing with PBS, the slides were mounted in aqueous medium PBS: Glycerol 1:1 (Sigma, Milan, Italy). For scanning and photography, a confocal laser-scanning microscope LSM-510 (Zeiss, Gottingen, Germany) was used. The lambda of the argon ion laser was set at 488 nm, and fluorescence emission was revealed by BP 505–530 nm band pass filter.
Protein extraction and SDS PAGE/western blotting
Five sarcoids (S1;S2;S3;S4;S5) and one sample of normal skin (N) were available for molecular analysis. These were snap frozen in liquid nitrogen and homogenized in ice-cold lysis buffer (50mM Tris pH7.5; 150mM NaCl; 1mM EDTA; 0.25% Deoxicolic acid, 1% Triton X100) added with 20 mM sodium pyrophosphate, 0.1 mg/ml aprotinin, 2 mM phenylmethylsulphony fluoride (PMSF), 10 mM sodium orthovanadate (Na2VO3), and 50 mM NaF.
Cell lines were grown for 2 days in 60-mm dishes, washed with ice-cold phosphate saline buffer two times and lysed for 20 minutes in ice-cold lysis buffer. Tissue homogenates and cell lysates were clarified by centrifugation, and the quantity of proteins was determined by use of a protein assay kit (Bio-Rad Laboratories, Milan, Italy). 50 μg of total protein were boiled and fractionated in 15% SDS-PAGE gel. The proteins were blotted from the gel onto nitrocellulose membranes. The membranes were blocked with 5% non-fat dry milk in TBS buffer at room temperature, washed with TBS–0.1% Tween and incubated with anti-MGMT antibody at dilution 1: 500. After appropriate washing steps, peroxidase-conjugated anti-rabbit IgG (Amersham Pharmacia Biotech) were applied for 1 hour at 1:5,000. After washing, bound antibody was visualized on ECL film (Amersham Pharmacia Biotech). The blots were stripped and reprobed against mouse anti-actin antibody (Calbiochem) at 1:5,000 to confirm equal loading of proteins in each lane.
Bioinformatic web tools for sequence analyses and primer design
NCBI Genbank (http://www.ncbi.nlm.nih.gov/sites/entrez) to obtain gene specific sequences; UCSC BLAT (http://genome.ucsc.edu/cgi-bin/hgBlat) to obtain the sequence of non-coding regions and to recovery and compare the sequences of different species. BLAST algorithm (http://blast.ncbi.nlm.nih.gov/Blast.cgi) to identify the more informative sequences, to compare the horse specific nucleotidic sequences with that of other species; Primer3plus (http://www.bioinformatics.nl/cgi-bin/primer3plus/primer3plus.cgi) and NCBI primer-blast (http://www.ncbi.nlm.nih.gov/tools/primer-blast/index.cgi) to design PCR primers; EMBOSS CpGPlot/CpGReport/Isochore (http://www.ebi.ac.uk/Tools/emboss/cpgplot/) for the analysis of the CpG content.
Cell line DNA extraction and CpG island gap sequencing
Genomic DNA was extracted from one 60-mm dish respectively of E-Derm, EqSO1a and EqSO4b cell lines according to the Wizard Genomic DNA Purification Kit protocol (Promega, corporation, USA). The purified DNA was resuspended in TE.
The lacking sequence of the equine specific MGMT CpG island was obtained by PCR amplification using as template E-Derm genomic DNA. The primers were designed referring to the 5′ MGMT sequence annotated in UCSC sequence. The primer sequences are: MGMTgapF 5′-GAGGCAACCCAGACACTCAC-3′ and MGMTgapR 5′-CAGGACAGCCAGCGAGAC-3′. The purified PCR product (535 bp) was directly sequenced by the dye terminator method (PRIMM Facility, Naples, Italy). The sequence analysis showed that the gap was of 359 bp. This sequence was submitted to GenBank JX390728].
DNA extraction from paraffin-embedded sarcoid samples, sodium bisulfite conversion and sequencing
Genomic DNA was isolated from paraffin-embedded sections using standard procedures (Strazzullo et al., 2003) and was precipitated with 250 mM NaCl and isopropanol. DNA extracted from paraffin embedded samples and cell lines (0.25 – 1 μg) was sodium bisulfite-converted using the EpiTect Bisulfite Kit (Qiagen, Germany), according to the protocol relative to the paraffin embedded samples. Samples were eluted in a final volume of 20 μl of elution buffer. To study the differential methylation status of MGMT putative promoter, bisulfite sequencing was performed. Briefly, 1 μl of sodium bisulfite-converted DNA was amplified using a double step PCR (semi-nested PCR) carried out using Gradient PCR Express (Hybaid, Middlesex, UK). The step 1 primer sequences are: MGMTbisF 5′-GAGGGAGGTAATTTAGATATTTAT-3′ and MGMTbisR2 5′-AAAAAACCTACAATAACAACAAC-3′; the step 2 primer sequences are: MGMTbisF and MGMTbisR2 5′-CACAAACAAACAAAAAAACCC-3′. The second amplification was carried out using 1 μl of first amplification product as template. The thermal cycling conditions were: 95°C for 3 min, followed by 95°C for 30 sec, 52°C for 45 sec and 72°C for 45 sec for 25 (first amplification) or 35 (second amplification) cycles. The product of the first amplification was of 628 bp; the second product was of 572 bp. After amplification, PCR products were recovered from agarose gel using QIAquick Gel Extraction Kit (Qiagen), following manufacturer’s instructions. Purified PCR products were cloned in the pGEMTeasy cloning vector (Promega corporation, USA). After cloning, DNA fragments were sequenced by the dye terminator method (PRIMM Facility, Naples, Italy).
The authors thank Dr. Roberto Di Vaia for his help in collecting tumour samples. The study was funded by Ministero dell’Istruzione, Università e Ricerca scientifica PRIN Anno 2008 - prot.2008LTY389 and Legge regionale n° 5 by Regione Campania.
- Nasir L, Campo MS: Bovine papillomaviruses: their role in the aetiology of cutaneous tumours of bovids and equids. Vet Dermatol. 2008, 19: 243-254. 10.1111/j.1365-3164.2008.00683.x.View ArticlePubMed
- Pilsworth RC, Knottenbelt D: Equine sarcoid. Equine Vet Educ. 2007, 19: 260-262. 10.2746/095777307X209194.View Article
- Martens A, De Moor A, Demeulemeester J, Ducatelle R: Histopathological characteristics of five clinical types of equine sarcoid. Res Vet Sci. 2000, 69: 295-300. 10.1053/rvsc.2000.0432.View ArticlePubMed
- Borzacchiello G, Corteggio A: Equine Sarcoid: State of the Art. Ippologia. 2009, 20: 7-14.
- Corteggio A, Di Geronimo O, Roperto S, Roperto F, Borzacchiello G: Bovine papillomavirus E7 oncoprotein binds to p600 in naturally occurring equine sarcoids. J Gen Virol. 2011, 92: 378-382. 10.1099/vir.0.025866-0.View ArticlePubMed
- Borzacchiello G, Russo V, Della Salda L, Roperto S, Roperto F: Expression of platelet-derived growth factor-beta receptor and bovine papillomavirus E5 and E7 oncoproteins in equine sarcoid. J Comp Pathol. 2008, 139: 231-237. 10.1016/j.jcpa.2008.07.006.View ArticlePubMed
- Yuan Z, Gault EA, Campo MS, Nasir L: Different contribution of bovine papillomavirus type 1 oncoproteins to the transformation of equine fibroblasts. J General Virol. 2011, 92: 773-783. 10.1099/vir.0.028191-0.View Article
- Nixon C, Chambers G, Ellsmore V, Campo MS, Burr P, Argyle DJ, Reid SW, Nasir L: Expression of cell cycle associated proteins cyclin A, CDK-2, p27kip1 and p53 in equine sarcoids. Cancer Lett. 2005, 221: 237-245. 10.1016/j.canlet.2004.08.039.View ArticlePubMed
- Pegg AE, Dolan ME, Moschel RC: Structure, function, and inhibition of O6-alkylguanine-DNA alkyltransferase. Prog Nucleic Acid Res Mol Biol. 1995, 51: 167-223.View ArticlePubMed
- Coulondre C, Miller JH: Genetic studies of the lac repressor. IV. Mutagenic specificity in the lacI gene of Escherichia coli. J Mol Biol. 1977, 117: 577-606. 10.1016/0022-2836(77)90059-6.View ArticlePubMed
- Erickson LC, Laurent G, Sharkey NA, Kohn KW: DNA cross-linking and monoadduct repair in nitrosourea-treated human tumour cells. Nature. 1980, 288: 727-729. 10.1038/288727a0.View ArticlePubMed
- Pegg AE: Mammalian O6-alkylguanine-DNA alkyltransferase: regulation and importance in response to alkylating carcinogenic and therapeutic agents. Cancer Res. 1990, 50: 6119-6129.PubMed
- Srivenugopal KS, Ali-Osman F: The DNA repair protein, O(6)-methylguanine-DNA methyltransferase is a proteolytic target for the E6 human papillomavirus oncoprotein. Oncogene. 2002, 21: 5940-5945. 10.1038/sj.onc.1205762.View ArticlePubMed
- Esteller M, Hamilton SR, Burger PC, Baylin SB, Herman JG: Inactivation of the DNA repair gene O6-methylguanine-DNA methyltransferase by promoter hypermethylation is a common event in primary human neoplasia. Cancer Res. 1999, 59: 793-797.PubMed
- Esteller M, Toyota M, Sanchez-Cespedes M, Capella G, Peinado MA, Watkins DN, Issa JP, Sidransky D, Baylin SB, Herman JG: Inactivation of the DNA repair gene O6-methylguanine-DNA methyltransferase by promoter hypermethylation is associated with G to A mutations in K-ras in colorectal tumorigenesis. Cancer Res. 2000, 60: 2368-2371.PubMed
- Henken FE, Wilting SM, Overmeer RM, van Rietschoten JG, Nygren AO, Errami A, Schouten JP, Meijer CJ, Snijders PJ, Steenbergen RD: Sequential gene promoter methylation during HPV-induced cervical carcinogenesis. Br J Cancer. 2007, 97: 1457-1464. 10.1038/sj.bjc.6604055.PubMed CentralView ArticlePubMed
- Virmani AK, Muller C, Rathi A, Zoechbauer-Mueller S, Mathis M, Gazdar AF: Aberrant methylation during cervical carcinogenesis. Clin Cancer Res. 2001, 7: 584-589.PubMed
- Herfarth KK, Brent TP, Danam RP, Remack JS, Kodner IJ, Wells SA, Goodfellow PJ: A specific CpG methylation pattern of the MGMT promoter region associated with reduced MGMT expression in primary colorectal cancers. Mol Carcinog. 1999, 24: 90-98. 10.1002/(SICI)1098-2744(199902)24:2<90::AID-MC3>3.0.CO;2-B.View ArticlePubMed
- Baylin SB, Herman JG, Graff JR, Vertino PM, Issa JP: Alterations in DNA methylation: a fundamental aspect of neoplasia. Adv Cancer Res. 1998, 72: 141-196.View ArticlePubMed
- Costello JF, Fruhwald MC, Smiraglia DJ, Rush LJ, Robertson GP, Gao X, Wright FA, Feramisco JD, Peltomaki P, Lang JC, et al: Aberrant CpG-island methylation has non-random and tumour-type-specific patterns. Nat Genet. 2000, 24: 132-138. 10.1038/72785.View ArticlePubMed
- Jones PA, Laird PW: Cancer epigenetics comes of age. Nat Genet. 1999, 21: 163-167. 10.1038/5947.View ArticlePubMed
- Baylin SB, Herman JG: DNA hypermethylation in tumorigenesis: epigenetics joins genetics. Trends Genet. 2000, 16: 168-174. 10.1016/S0168-9525(99)01971-X.View ArticlePubMed
- Sun LL, Cao DY, Yang JX, Li H, Zhou XR, Song ZQ, Cheng XM, Chen J, Shen K: Population-based case–control study on DAPK1, RAR-beta2 and MGMT methylation in liquid-based cytology. Arch Gynecol Obstet. 2012, 285: 1433-1439. 10.1007/s00404-011-2149-6.View ArticlePubMed
- Spathis A, Aga E, Alepaki M, Chranioti A, Meristoudis C, Panayiotides I, Kassanos D, Karakitsos P: Promoter methylation of p16(INK4A), hMLH1, and MGMT in liquid-based cervical cytology samples compared with clinicopathological findings and HPV presence. Infect Dis Obstet Gynecol. 2011, 2011: 927861-PubMed CentralView ArticlePubMed
- Yanagawa N, Osakabe M, Hayashi M, Tamura G, Motoyama T: Detection of HPV-DNA, p53 alterations, and methylation in penile squamous cell carcinoma in Japanese men. Pathol Int. 2008, 58: 477-482. 10.1111/j.1440-1827.2008.02259.x.View ArticlePubMed
- Giaginis C, Michailidi C, Stolakis V, Alexandrou P, Tsourouflis G, Klijanienko J, Delladetsima I, Theocharis S: Expression of DNA repair proteins MSH2, MLH1 and MGMT in human benign and malignant thyroid lesions: an immunohistochemical study. Med Sci Monit. 2011, 17: BR81-BR90.PubMed CentralPubMed
- Theocharis S, Klijanienko J, Giaginis C, Rodriguez J, Jouffroy T, Girod A, Point D, Tsourouflis G, Sastre-Garau X: Expression of DNA repair proteins, MSH2, MLH1 and MGMT in mobile tongue squamous cell carcinoma: associations with clinicopathological parameters and patients’ survival. J Oral Pathol Med. 2011, 40: 218-226. 10.1111/j.1600-0714.2010.00945.x.View ArticlePubMed
- Neyaz MK, Kumar RS, Hussain S, Naqvi SH, Kohaar I, Thakur N, Kashyap V, Das BC, Husain SA, Bharadwaj M: Effect of aberrant promoter methylation of FHIT and RASSF1A genes on susceptibility to cervical cancer in a North Indian population. Biomarkers. 2008, 13: 597-606. 10.1080/13547500802078859.View ArticlePubMed
- Citron M, Schoenhaus M, Rothenberg H, Kostroff K, Wasserman P, Kahn L, White A, Burns G, Held D, Yarosh D: O6-methylguanine-DNA methyltransferase in normal and malignant tissue of the breast. Cancer Invest. 1994, 12: 605-610. 10.3109/07357909409023045.View ArticlePubMed
- Musarrat J, Wilson JA, Abou-Issa H, Wani AA: O(6)-alkylguanine DNA alkyltransferase activity levels in normal, benign and malignant human female breast. Biochem Biophys Res Commun. 1995, 208: 688-696. 10.1006/bbrc.1995.1393.View ArticlePubMed
- Esteller M, Herman JG: Generating mutations but providing chemosensitivity: the role of O6-methylguanine DNA methyltransferase in human cancer. Oncogene. 2004, 23: 1-8. 10.1038/sj.onc.1207316.View ArticlePubMed
- Burgers WA, Blanchon L, Pradhan S, de Launoit Y, Kouzarides T, Fuks F: Viral oncoproteins target the DNA methyltransferases. Oncogene. 2007, 26: 1650-1655. 10.1038/sj.onc.1209950.PubMed CentralView ArticlePubMed
- Yuan ZQ, Gault EA, Gobeil P, Nixon C, Campo MS, Nasir L: Establishment and characterization of equine fibroblast cell lines transformed in vivo and in vitro by BPV-1: model systems for equine sarcoids. Virology. 2008, 373: 352-361. 10.1016/j.virol.2007.11.037.View ArticlePubMed
- Strazzullo M, Cossu A, Baldinu P, Colombino M, Satta MP, Tanda F, De Bonis ML, Cerase A, D’Urso M, D’Esposito M, et al: High-resolution methylation analysis of the hMLH1 promoter in sporadic endometrial and colorectal carcinomas. Cancer. 2003, 98: 1540-1546. 10.1002/cncr.11651.View ArticlePubMed
- Schildhaus HU, Krockel I, Lippert H, Malfertheiner P, Roessner A, Schneider-Stock R: Promoter hypermethylation of p16INK4a, E-cadherin, O6-MGMT, DAPK and FHIT in adenocarcinomas of the esophagus, esophagogastric junction and proximal stomach. Int J Oncol. 2005, 26: 1493-1500.PubMed
- Scarano MI, Strazzullo M, Matarazzo MR, D’Esposito M: DNA methylation 40 years later: Its role in human health and disease. J Cell Physiol. 2005, 204: 21-35. 10.1002/jcp.20280.View ArticlePubMed
- Matarazzo MR, De Bonis ML, Gregory RI, Vacca M, Hansen RS, Mercadante G, D’Urso M, Feil R, D’Esposito M: Allelic inactivation of the pseudoautosomal gene SYBL1 is controlled by epigenetic mechanisms common to the X and Y chromosomes. Hum Mol Genet. 2002, 11: 3191-3198. 10.1093/hmg/11.25.3191.View ArticlePubMed
- Wentzensen N, Sherman ME, Schiffman M, Wang SS: Utility of methylation markers in cervical cancer early detection: appraisal of the state-of-the-science. Gynecol Oncol. 2009, 112: 293-299. 10.1016/j.ygyno.2008.10.012.PubMed CentralView ArticlePubMed
- Jacob R, Shafiei N, Nagel G, Welkoborsky HJ, Mann W, Kaina B: MGMT activity in mucosal epithelium and squamous cell carcinoma of the head and neck. Anticancer Res. 2010, 30: 2561-2566.PubMed
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