RESUMO
BACKGROUND: Genomic DNA sequences in cell-free plasma are biomarkers of cancer prognosis, where characteristic changes in methylation of tumour suppressor or oncogene DNA regions are indicative of changes in gene activity. Also, cell-free fetal DNA can be distinguished, by its methylation status, from the maternal DNA in the plasma of pregnant women, hence providing DNA biomarkers for the proposed minimally-invasive diagnosis of fetal aneuploidies, including Down's syndrome. However, the production and clearance of cell-free DNA from plasma in relation to its methylation status, are poorly understood processes. METHODS: We studied the methylation status of DNA derived from the imprinted GNAS1 locus, in cell-free plasma DNA of healthy adults. Heterozygotes were identified that carried the SNP rs1800905 in the imprinted region. The parent-of-origin-dependent DNA methylation was analysed by bisulfite conversion, followed by cloning and sequencing. RESULTS: Genomic DNA molecules derived from both the methylated, maternal, allele and the unmethylated, paternal, allele were found in plasma. Methylated and unmethylated DNA molecules were present in equal numbers. CONCLUSIONS: Our data indicate that the methylation status of a DNA sequence has no effect on its steady state concentration in the cell-free DNA component of plasma, in healthy adults.