Reproductive Technology

Tissue-Specific Elevated Genomic Cytosine Methylation Levels Are Associated with an Overgrowth Phenotype of Bovine Fetuses Derived by In Vitro Techniques

Abstract
Epigenetic perturbations are assumed to be responsible for abnormalities observed in fetuses and offspring derived by in vitro techniques. We have designed an experiment with bovine day 80 fetuses generated by somatic cell nuclear transfer (SCNT), in vitro fertilization (IVF), and artificial insemination (AI) to determine the relationship between fetal phenotype and genome-wide 5-methylcytosine (5mC) content. When compared with AI controls, SCNT and IVF fetuses displayed significantly increased body weight (61%/28%), liver weight (100%/36%) and thorax circumference (20%/11%). A reduced crown rump length : thorax circumference ratio (1.175±0.017 in SCNT and 1.292±0.018 in IVF vs. 1.390±0.018 in AI, P<0.001 and P<0.002) was the external hallmark of this disproportionate overgrowth phenotype. The SCNT fetuses showed significant hypermethylation of liver DNA in comparison with AI controls (3.46±0.08% vs. 3.17±0.09% 5mC, P<0.03), and the cytosine methylation levels for IVF fetuses (3.34±0.09%) were, as observed for phenotypic parameters, intermediate to the other groups. Regressions of fetal body and liver weight, and thorax circumference on 5mC-content of liver DNA were positive (P<0.073-0.079). Furthermore, a significant negative regression (P<0.021) of the crown rump length : thorax circumference ratio on liver 5mC was observed. The 5mC-content of placental cotyledon DNA was 46% lower than in liver DNA (P<0.001), but did not differ among groups. These data are in striking contrast to the recently reported hypomethylation of DNA from SCNT fetuses and indicate that hypermethylation of fetal tissue, but not placenta, is linked to the overgrowth phenotype in bovine SCNT and IVF fetuses.

Key words: Embryo • Assisted Reproductive Technology • Conceptus • Developmental biology • Placenta

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