Analysis of the breast cancer methylome using formalin-fixed paraffin-embedded tumour

Purpose: Aberrant DNA methylation occurs frequently in breast carcinogenesis. Tools for translational epigenetic studies of breast cancer involving formalin-fixed paraffin-embedded (FFPE) human tissues have now been developed. Few studies have measured genome-wide methylation in DNA derived from paraffin-embedded tumour tissues and compared the DNA methylation in corresponding adjacent non-tumour ductal epithelium (ADJ_NT). These studies are technically challenging due to the spectrum of breast cancer pathologies, the variable suitability of DNA extracted from FFPE material and the difficulties in identifying ADJ_NT. We assessed the suitability of FFPE breast cancer material for genome-wide DNA methylation assessment of tumour and ADJ_NT. Methods: Twenty-one archival breast tumour tissues with paired ADJ_NT obtained from separate blocks and at least 2 cm from the tumour were sourced from The Melbourne Collaborative Cohort Study (MCCS). DNA was prepared from macrodissected tissue samples and assessed for genome-wide methylation using the Infinium HumanMethylation450 Beadchip (HM450K) array. Results: The 1000 most differentially methylated probes between tumour and ADJ_NT in this FFPE-derived dataset differentiated tumour and ADJ_NT in The Cancer Genome Atlas Network data (TCGA; derived from high molecular weight DNA using the same HM450K array). Conclusions: Large-scale studies of genome-wide DNA methylation using FFPE breast cancer specimens offer the opportunity to further refine the pathological classification of tumours, to include subtypes that are underrepresented in the TCGA data and provide the capacity to further explore intra-tumoural heterogeneity.