Differential DNA methylation between fetus and mother as a strategy for detecting fetal DNA in maternal plasma.

BACKGROUND: Fetal DNA has been detected in maternal plasma by the use of genetic differences between mother and fetus. We explore the possibility of using epigenetic markers for the specific detection of fetal DNA in maternal plasma.
METHODS: A differentially methylated region in the human IGF2-H19 locus and a single-nucleotide polymorphism in this region were chosen for the study. The methylation status in this region is maintained in such a way that the paternal allele is methylated and the maternal allele is unmethylated. The single-nucleotide polymorphism was typed by direct sequencing of PCR products. The methylation status of this region was ascertained by bisulfite conversion and methylation-specific PCR. Differentially methylated fetal alleles were detected in maternal plasma by direct sequencing and a primer-extension assay.
RESULTS: Women in the second (n = 21; 17-21 weeks) and third (n = 18; 37-42 weeks) trimesters of pregnancy were recruited. Among these 39 volunteers, the 16 who were heterozygous for the single-nucleotide polymorphism were chosen for further analysis. In 11 of these 16 cases, paternally inherited methylated fetal alleles were different from the methylated alleles of the respective mothers. Using direct sequencing, we detected paternally inherited methylated fetal DNA in 6 of 11 (55%) cases. In 8 of the 16 heterozygous cases, the fetuses possessed an unmethylated maternally inherited allele that was different from the unmethylated allele of the mother. Using a primer-extension assay, we detected fetal-derived maternally inherited alleles in maternal plasma of four of eight (50%) cases.
CONCLUSIONS: These results represent the first use of fetal epigenetic markers in noninvasive prenatal analysis. These data may also have implications for the investigation of other types of chimerism.