Clonality analysis in breast tumour pairs using DNA methylation data

Multiple tumours from the same patient were analysed for DNA methylation to assess tumour clonality. Seventy-four tumours corresponding to 37 patients were stratified into four groups based on the anatomic location of the multiple breast cancers (ipsilateral or bilateral) and time interval between the diagnoses (synchronous or metachronous). Ipsilateral was defined as tumours occurring in the same breast while bilateral was defined as the occurrence of tumours in both breasts. Metachronicity was defined as a time interval greater than six months between the diagnoses of the first and second tumours, while synchronicity specified that the two tumours occurred concurrently (BM: bilateral-metachronous; BS: bilateral-synchronous; IM: ipsilateral-metachronous; IS: ipsilateral-synchronous). A subset of 16 samples was randomly selected to represent each clinical group with four samples corresponding to two patients per group and analysed for DNA methylation using Illumina Infinium Human MethylationEPIC BeadChips. Genomic DNA was isolated from fresh-frozen tissue samples