Non-canonical imprinting, manifesting as post-fertilization placenta-specific parent-of-origin dependent methylation, is not conserved in humans

Genomic imprinting is the parent-of-origin dependent monoallelic expression of genes often associated with regions of germline-derived DNA methylation that are maintained as differentially methylated regions (gDMRs) in somatic tissues. This form of epigenetic regulation is highly conserved in mammals and is thought to have co-evolved with placentation. Tissue-specific gDMRs have been identified in human placenta, suggesting that species-specific imprinting dependent on unorthodox epigenetic establishment or maintenance may be more widespread than previously anticipated. Non-canonical imprinting, reliant on differential allelic H3K27me3 enrichment, has been reported in mouse and rat pre-implantation embryos, often overlapping long terminal repeat (LTR)-derived promoters. These non-canonical imprints lose parental allele-specific H3K27me3 specificity, subsequently gaining DNA methylation on the same allele in extra-embryonic tissues resulting in placenta-specific, somatically acquired maternal DMRs. To determine if similar non-canonical imprinting is present in the human placenta, we interrogated allelic DNA methylation for a selected number of loci, including (i) the human orthologues of non-canonical imprinted regions in mouse and rat, (ii) promoters of human LTR-derived transcripts, and (iii) CpG islands with intermediate placenta-specific methylation that are unmethylated in gametes and pre-implantation embryos. We failed to identify any non-canonical imprints in the human placenta whole villi samples. Furthermore, the assayed genes were shown to be biallelically expressed in human pre-implantation embryos, indicating they are not imprinted at earlier time points. Together, our work reiterates the continued evolution of placenta-specific imprinting in mammals, which we suggest is linked to epigenetic differences during the maternal-to-embryo transition and species-specific integration of retrotransposable elements.