Abstract
We compared global levels of DNA methylation as well as methylation of a specific locus (MyoD1) in ovarian cystadenomas, ovarian tumours of low malignant potential (LMP) and ovarian carcinomas to investigate the association between changes in DNA methylation and ovarian tumour development. As we realized that cystadenomas showed different methylation patterns from both LMP tumours and carcinomas, we verified their monoclonal origin as a means of confirming their true neoplastic nature. High-pressure liquid chromatographic (HPLC) analyses showed that global methylation levels in LMP tumours and carcinomas were 21% and 25% lower than in cystadenomas respectively (P = 0.0001 by one-way variance analysis). Changes in the methylation status of the MyoD1 locus were not seen in any of ten cystadenomas analysed but were present in five of ten LMP tumours and in five of ten carcinomas (P = 0.03). These findings suggest that alterations in DNA methylation are absent (or at least not as extensive) in ovarian cystadenomas, but are present in LMP tumours, the phenotypic features of which are intermediate between those of benign and malignant ovarian tumours. The results also emphasize the merit of distinguishing ovarian LMP tumours from cystadenomas, in spite of their similar clinical characteristics.
Highlights
We measured the global levels of methylation in DNA samples obtained from seven ovarian cystadenomas, eight low malignant potential (LMP) tumours and ten carcinomas in order to determine if these different tumour subtypes, which can be regarded as representing different degrees of neoplastic transformation, could be distinguished on the basis of such measurements
DNA isolated from the various ovarian tumours was digested to single nucleosides and analysed by High-pressure liquid chromatographic (HPLC) as explained in Materials and methods
We examined eight ovarian cystadenomas ranging in size from 3 to 12 cm and determined whether these tumours were of monoclonal origin or not
Summary
Tumour specimens were obtained fresh from the operating rooms of either the USC/Norris Comprehensive Cancer Center or the Women's Hospital of the Los Angeles County Medical Center and frozen immediately. Histological sections of all frozen carcinoma and LMP tumour samples were examined before DNA extraction in order to confirm the presence of tumour tissue and to rule out the presence of unacceptable amounts of non-neoplastic. 25- to 200-p1 aliquots of the supernatants were injected directly into a HPLC apparatus (purchased from Waters, Milford, MA, USA), equipped with a 440 Absorbance Detector and a 10 cm Brownlee RP-18 column guarded by a Brownlee Aquaphore ODS precolumn. The purity of each peak was verified by variations of mobile phase pH and methanol concentrations. Standard deoxynucleosides were obtained from Sigma (St Louis, MO, USA)
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