Simultaneous deletion of the methylcytosine oxidases Tet1 and Tet3 increases transcriptome variability in early embryogenesis

Significance Development of preimplantation embryos entails global DNA demethylation on the zygotic genome. The original thought was that TET-deficient embryos would be unlikely to survive early embryogenesis because they would be unable to mediate genome-wide demethylation in the zygote and preimplantation embryo. However, mice lacking the individual TET proteins Tet1, Tet2, or Tet3 have survived until birth and beyond, suggesting redundancy among TET proteins in the early embryogenesis. Here we report that preimplantation embryos doubly disrupted for Tet1 and Tet3 show abnormal embryonic phenotypes, whose incomplete penetrance correlates with a high variability of transcriptional profiles and DNA methylation status. Our data suggest that in addition to facilitating DNA demethylation, TET proteins and oxidized methylcytosines may regulate the consistency of gene transcription during embryogenesis. Dioxygenases of the TET (Ten-Eleven Translocation) family produce oxidized methylcytosines, intermediates in DNA demethylation, as well as new epigenetic marks. Here we show data suggesting that TET proteins maintain the consistency of gene transcription. Embryos lacking Tet1 and Tet3 (Tet1/3 DKO) displayed a strong loss of 5-hydroxymethylcytosine (5hmC) and a concurrent increase in 5-methylcytosine (5mC) at the eight-cell stage. Single cells from eight-cell embryos and individual embryonic day 3.5 blastocysts showed unexpectedly variable gene expression compared with controls, and this variability correlated in blastocysts with variably increased 5mC/5hmC in gene bodies and repetitive elements. Despite the variability, genes encoding regulators of cholesterol biosynthesis were reproducibly down-regulated in Tet1/3 DKO blastocysts, resulting in a characteristic phenotype of holoprosencephaly in the few embryos that survived to later stages. Thus, TET enzymes and DNA cytosine modifications could directly or indirectly modulate transcriptional noise, resulting in the selective susceptibility of certain intracellular pathways to regulation by TET proteins.