Microarray assessment of methylation in individual mouse blastocyst stage embryos shows that in vitro culture may have widespread genomic effects.

BACKGROUND: Although assisted reproductive technology (ART) is reported to result in abnormal genomic imprinting and/or altered genomic methylation, few if any studies have used high-throughput methods to analyze genomic methylation in ART embryos. We hypothesized that a microarray-based assessment of genomic methylation could be used to reveal differences between ART and normal preimplantation embryos.
METHODS: In this pilot study, we performed methylation-sensitive amplification of genomic DNA from preimplantation mouse blastocysts, obtained by natural mating and either maintained in vivo until E3.5 (n = 4) or cultured in vitro (n = 4) from E0.5 until E3.5. An oligonucleotide microarray was then used to perform comparative hybridization of amplified DNA, allowing us to assess relative methylation at ~16,000 loci on mouse chromosome 7.
RESULTS: We show that for in vivo derived embryos, the methylation/microarray results were strikingly consistent. In contrast, all four in vitro cultured embryos showed evidence of generalized hypermethylation as well as greater locus-to-locus variability, when compared with in vivo derived embryos. Genomic segments that overlapped exons and CpG islands were most likely to be hypomethylated in both normal and experimental blastocysts. Other sequence features, such as repetitive elements, were not associated with the presence of or the degree of methylation.
CONCLUSIONS: We conclude that a general assessment of genomic methylation in blastocyst stage embryos is technically feasible. Data from this small sample suggest that in vitro embryo culture is associated with generalized hypermethylation as well as increased locus-to-locus variability in methylation. However, it is premature to conclude that this is a general property of in vitro cultured blastocysts.