Intrauterine Growth Restriction (IUGR) and imprinted gene expression in the placenta: Role of PLAGL1 and analysis of the 6q24.2 Region

Abstract

BACKGROUND: Fetal growth is a complex process which depends on nutrient and oxygen availability and transport from the mother to the fetus across the placenta. This involves hormones and growth factors as well as maternal and fetal genes. The failure of the fetus to reach his or her full potential for growth is called Intrauterine Growth Restriction (IUGR) and implies risks for adverse short‐ and long‐ term outcomes. Imprinted genes are a specific subset of genes that display, in mammals and flowering plants, monoallelic expression depending on the parental origin of the allele. The regulation of imprinted expression depends on epigenetic mechanisms, a subset of heritable marks that have the ability to regulate DNA functions without altering its sequence. Imprinted genes tend to cluster in the genome due to coordinated regulation through Imprinting Control Centers, usually in the form of Differentially Methylated Regions between the paternally and maternally inherited alleles. Studies in both animals and humans as well as imprinting syndromes have uncovered a role for this group of genes in prenatal growth. Two imprinted genes (PLAGL1 and HYMAI) have been described in the 6q24 locus. Genetic and epigenetic defects in this region relate to the Transient Neonatal Diabetes Mellitus 1 phenotype, including severe growth restriction. We aimed to study the involvement of this region in non‐syndromic IUGR.

PARTICIPANTS AND METHODS: One hundred placental samples from a cohort of healthy term singletons, fetal tissues from fifty‐four first trimester terminations and one hundred placental samples from healthy and complicated pregnancies of different gestational ages were used to analyze the role of the 6q24 region in normal fetal growth and IUGR, respectively. Relevant clinical data was obtained after informed consent. The methylation status of the 6q24 CpG islands was studied by array technology and bisulfite sequencing in normal term placenta and in first trimester fetal tissues. Methylation levels in the PLAGL1 DMR in healthy and IUGR placentas were compared by pyrosequencing. Allelic origin of expression was assessed by heterozygous DNA/cDNA SNP analysis. Levels of expression of imprinted transcripts were analyzed by qRT‐PCR.

RESULTS: PLAGL1 P1, HYMAI and two newly described PLAGL1 isoforms (P3 and P4) were the only transcripts subjected to genomic imprinting in the investigated 6q24 region. Correspondingly, the CpG island associated to the P1 promoter was the only differentially methylated region. There was no correlation between PLAGL1 expression in the placenta and fetal size in uneventful pregnancies. In placentas from IUGR gestations, expression of HYMAI was significantly higher than in those from normally grown fetuses. Levels of expression of PLAGL1 were lower in IUGR and correlated positively and significantly with the presence of IUGR in placentas from girls, but not boys. These changes in expression were not mediated by Loss of Imprinting or abnormalities in the levels of methylation of the promoter‐associated DMR, but possibly by a change in regulatory posttranscriptional mechanisms, as suggested by the loss of correlation of PLAGL1 P1 and HYMAI expression in IUGR.

CONCLUSIONS: Imprinted expression in the 6q24 region is limited to the PLAGL1/HYMAI locus, maybe due to demarcation of this region by CTCF boundaries. Intrauterine Growth Restriction is associated to abnormalities in expression of PLAGL1 and HYMAI in the placenta, which are not due to LOI or methylation changes.