20 NUCLEAR TRANSFER ALTERS EXPRESSION AND HISTONE MODIFICATIONS OF THE IMPRINTED GENE PHLDA2 IN THE BOVINE PLACENTA

Abstract

Proper implantation and placental formation are crucial for the continuity of mammalian species. Embryonic and placental developments are under intense genetic and epigenetic control, such as the regulation of differentiation of pluripotent cells into highly specialised fetal and placental cells. In the present study the objectives were to evaluate expression and epigenetic control of the imprinted gene PHLDA2, a maternally expressed gene that appears to be a regulator of placental growth, in cotyledonary and inter-cotyledonary tissues of bovine placentas on Day 60 of pregnancies produced by embryo transfer (ET; n = 3), in vitro fertilization (IVF; n = 5), and nuclear transfer (NT; n = 6), by real time PCR (qPCR). In vitro culture of IVF and NT embryos was performed in SOF medium supplemented with 2.5% fetal bovine serum, at 39°C in a humidified atmosphere of 5% CO₂ and 5% O₂ for 7 days. For evaluation of gene expression, gene-specific standard curves were used, and results were analysed as a ratio to 2 separate housekeeping controls (GAPDH and β-actin). Chromatin immunoprecipitation followed by qPCR (ChIP-qPCR; precipitated/total input DNA) was also performed on the proximal promoter region of PHLDA2, with antibodies against H3K4me2 (permissive histone modification) and H3K9me2 (inhibitory histone modification) in these samples. Products of the ChIP-qPCR for PHLDA2 were digested with a restriction enzyme (AciI) that recognises a specific sequence of the maternal allele (Bos indicus), separating it visually on a gel, from the paternal allele (Bos taurus). Digestion products were separated on a 3% agarose gel, and ethidium bromide was used for visualisation. ImageJ (NIH, Bethesda, MD, USA) was used to analyse band intensity. Gene expression, ChIP, and digestion data were analysed using the least-squares ANOVA and the general linear model procedures (SAS Institute Inc., Cary, NC, USA). Further comparison of means was performed using Duncan's multiple range test (P < 0.05 was considered significant). Expression of the imprinted gene PHLDA2 was 11 times higher in samples produced by NT and, interestingly, also in samples produced by IVF (P < 0.05) compared with the samples produced by ET. ChIP-qPCR for the histone marks, followed by allelic analysis, showed a significant increase of the permissive mark H3K4me2, especially in the silenced paternal allele (P < 0.05), and a reduction of the inhibitory H3K9me2 mark, in the promoter region of the PHLDA2 gene, in clones. The differences observed for these 2 histone marks corroborated with the pattern of gene expression for these samples (elevated in TN placentas). In conclusion, the reproductive biotechnologies of nuclear transfer and in vitro fertilization induce changes in placental expression of the imprinted gene PHLDA2, and nuclear transfer also affects the pattern of histone marks on the proximal promoter region of the imprinted gene PHLDA2.