title: Altered expression of MGMT in high-grade gliomas results from the combined effect of epigenetic and genetic aberrations.
abstract: MGMT downregulation in high-grade gliomas (HGG) has been mostly attributed to aberrant promoter methylation and is associated with increased sensitivity to alkylating agent-based chemotherapy. However, HGG harboring 10q deletions also benefit from treatment with alkylating agents. Because the MGMT gene is mapped at 10q26, we hypothesized that both epigenetic and genetic alterations might affect its expression and predict response to chemotherapy. To test this hypothesis, promoter methylation and mRNA levels of MGMT were determined by quantitative methylation-specific PCR (qMSP) or methylation-specific multiplex ligation dependent probe amplification (MS-MLPA) and quantitative RT-PCR, respectively, in a retrospective series of 61 HGG. MGMT/chromosome 10 copy number variations were determined by FISH or MS-MLPA analysis. Molecular findings were correlated with clinical parameters to assess their predictive value. Overall, MGMT methylation ratios assessed by qMSP and MS-MLPA were inversely correlated with mRNA expression levels (best coefficient value obtained with MS-MLPA). By FISH analysis in 68.3% of the cases there was loss of 10q26.1 and in 15% of the cases polysomy was demonstrated; the latter displayed the highest levels of transcript. When genetic and epigenetic data were combined, cases with MGMT promoter methylation and MGMT loss depicted the lowest transcript levels, although an impact in response to alkylating agent chemotherapy was not apparent. Cooperation between epigenetic (promoter methylation) and genetic (monosomy, locus deletion) changes affecting MGMT in HGG is required for effective MGMT silencing. Hence, evaluation of copy number alterations might add relevant prognostic and predictive information concerning response to alkylating agent-based chemotherapy.
authors.affiliation: Cancer Epigenetics Group, Research Center of the Portuguese Oncology Institute-Porto, Porto, Portugal.
authors.forename: João
authors.initials: J
authors.lastname: Ramalho-Carvalho
authors.forename: Malini
authors.initials: M
authors.lastname: Pires
authors.forename: Susana
authors.initials: S
authors.lastname: Lisboa
authors.forename: Inês
authors.initials: I
authors.lastname: Graça
authors.forename: Patrícia
authors.initials: P
authors.lastname: Rocha
authors.forename: João Diogo
authors.initials: JD
authors.lastname: Barros-Silva
authors.forename: Joana
authors.initials: J
authors.lastname: Savva-Bordalo
authors.forename: Joaquina
authors.initials: J
authors.lastname: Maurício
authors.forename: Mário
authors.initials: M
authors.lastname: Resende
authors.forename: Manuel R
authors.initials: MR
authors.lastname: Teixeira
authors.forename: Mrinalini
authors.initials: M
authors.lastname: Honavar
authors.forename: Rui
authors.initials: R
authors.lastname: Henrique
authors.forename: Carmen
authors.initials: C
authors.lastname: Jerónimo
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002869:Chromosome Aberrations; D056915:DNA Copy Number Variations; D019175:DNA Methylation; D015254:DNA Modification Methylases / Q000235:genetics*; D045643:DNA Repair Enzymes / Q000235:genetics*; D044127:Epigenesis, Genetic; D015972:Gene Expression Regulation, Neoplastic; D005910:Glioma / Q000235:genetics* / Q000401:mortality / Q000473:pathology*; D006801:Humans; D008875:Middle Aged; D060787:Neoplasm Grading; D011379:Prognosis; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000235:genetics; D025521:Tumor Suppressor Proteins / Q000235:genetics*; D055815:Young Adult
chemical_list: D012333:RNA, Messenger; D025521:Tumor Suppressor Proteins; D015254:DNA Modification Methylases; C515491:MGMT protein, human; D045643:DNA Repair Enzymes