Loss of DNMT1o Disrupts Imprinted X Chromosome Inactivation and Accentuates Placental Defects in Females

Abstract

The maintenance of key germline derived DNA methylation patterns during preimplantation development depends on stores of DNA cytosine methyltransferase-1o (DNMT1o) provided by the oocyte. Dnmt1omat−/− mouse embryos born to Dnmt1Δ1o/Δ1o female mice lack DNMT1o protein and have disrupted genomic imprinting and associated phenotypic abnormalities. Here, we describe additional female-specific morphological abnormalities and DNA hypomethylation defects outside imprinted loci, restricted to extraembryonic tissue. Compared to male offspring, the placentae of female offspring of Dnmt1Δ1o/Δ1o mothers displayed a higher incidence of genic and intergenic hypomethylation and more frequent and extreme placental dysmorphology. The majority of the affected loci were concentrated on the X chromosome and associated with aberrant biallelic expression, indicating that imprinted X-inactivation was perturbed. Hypomethylation of a key regulatory region of Xite within the X-inactivation center was present in female blastocysts shortly after the absence of methylation maintenance by DNMT1o at the 8-cell stage. The female preponderance of placental DNA hypomethylation associated with maternal DNMT1o deficiency provides evidence of additional roles beyond the maintenance of genomic imprints for DNA methylation events in the preimplantation embryo, including a role in imprinted X chromosome inactivation.

Developmental regulation of X-chromosome inactivation

“…DNA--methylation, which is essential for autosomal imprints, is not believed to play a major role in the maternal repressive imprint on Xist, as maternal deletion of the de novo DNA methyltransferase genes Dnmt3a and Dnmt3b does not interfere with imprinted XCI [49]. Nevertheless, depletion of the DNA--methyltransferase DNMT1o, which is deposited in the oocyte and needed for maintenance of autosomal imprints during preimplantation development, is also important for imprinted XCI in extraembryonic tissues [50]. Tsix and its enhancer Xite [51], which are normally expressed only from the Xm in placentae, are expressed both from the Xp and the Xm in the absence of DNMT1o, resulting in repression of Xist and biallelic expression of X--linked genes [50].…”

“…Nevertheless, depletion of the DNA--methyltransferase DNMT1o, which is deposited in the oocyte and needed for maintenance of autosomal imprints during preimplantation development, is also important for imprinted XCI in extraembryonic tissues [50]. Tsix and its enhancer Xite [51], which are normally expressed only from the Xm in placentae, are expressed both from the Xp and the Xm in the absence of DNMT1o, resulting in repression of Xist and biallelic expression of X--linked genes [50]. The major function of DNMT1o thereby seems to be to maintain a DNA--methylation imprint on Xite during preimplantation development, which is established during spermatogenesis (Fig.…”

“…How this could promote Xist expression later on during imprinted XCI is still unclear, as the molecular nature of this permissive Xist imprint still needs to be established. Potentially it could involve the repressive DNA--methylation imprint on the Xist--repressors Xite and Tsix mentioned above [50,52]. Alternatively, it could also have to do with the fact that Xist is one of few genes expressed in a heterochromatic environment and therefore the condensed state during meiosis could facilitate later Xist expression during embryogenesis [55].…”

Non-germ Line Restoration of Genomic Imprinting for a Small Subset of Imprinted Genes in Ubiquitin-like PHD and RING Finger Domain-Containing 1 (Uhrf1) Null Mouse Embryonic Stem Cells

“…In support of this notion, the de novo DNA methyltransferase Dnmt3a and its non-enzymatic partner Dnmt3L are critically important for establishment of imprinting in both male and female germ cells (9 -11). Once established, DNMT1 is required to maintain the methylation of imprinted genes following fertilization and throughout development (12,13). In fact, the methylation in imprinted genes is protected against the genome-wide DNA demethylation that occurs in pre-implantation embryos.…”

The Effects of Maternal Periconceptional Ethanol Exposure on the Development of the Pre-Implantation Embryo, Placenta, and the Influence of the Uterine Environment

“…Other mouse mutants have also begun to investigate sexually dimorphic phenotypes in extraembryonic trophoblast and the placenta, where they consider the influence of the maternal or paternal derived X chromosomes, and X chromosome inactivation. Some of these models include the ATP-dependent helicase Atrx (Garrick et al, 2006), DNA methyltransferase Dnmt1 (McGraw et al, 2013), X-linked G6pd (Longo et al, 2002), and stress response Ndrg1 (Larkin et al, 2014). Curiously, it is again the females which seem to show the most deleterious phenotypes in these models, often showing reduced placental weight, labyrinth and/or junctional zones, as 59 well as embryonic viability.…”