Scientists Map the Earliest Steps of Polycomb Gene Silencing in Human Embryos

Every human being begins as a single cell carrying two complete genomes, one from each parent, and for the first days of life those genomes must be quietly reprogrammed before the embryo can build its first tissues. A study published in Nature Genetics by Xiaoyu Hu, Chuanxin Zhang, Bofeng Liu, Fangnong Lai, Shuiying Ma and colleagues, led by Wei Xie of Tsinghua University together with Keliang Wu, Han Zhao and Zi-Jiang Chen of Shandong University, now provides the most detailed view yet of how one of the cell’s principal gene-silencing systems is switched back on during that fragile window. The work focuses on the Polycomb group complexes, protein machines long known to keep developmental genes locked down, and reveals that a histone mark called H2AK119ub, or H2Aub, is the linchpin that allows Polycomb repression to be rebuilt from scratch in human embryos.

Polycomb repression operates through two cooperating complexes. PRC1 decorates histone H2A with a single ubiquitin molecule at lysine 119, producing the H2Aub mark, while PRC2 trimethylates histone H3 at lysine 27, producing H3K27me3. Together these marks silence genes that would otherwise trigger premature differentiation. In mouse embryos, previous work had shown that H2Aub and H3K27me3 can be decoupled during preimplantation development, and that maternal H3K27me3 can guide certain imprinted expression patterns. But human embryos posed a puzzle: H3K27me3 is largely erased across the genome after fertilization and only later restored. If the repressive mark disappears, how does the cell know where to put it back?

To answer that question, the team needed to measure H2Aub in samples that are extraordinarily scarce, human sperm, fully grown oocytes, two-cell embryos, eight-cell embryos, blastocysts and embryonic stem cells. They adapted a targeted profiling approach called Stacc-seq, an in situ method derived from CUT&Tag that can map chromatin marks from as few as a handful of cells. The authors validated the technique extensively, showing that profiles from 500, 50 or even 20 cultured cells closely matched conventional ChIP-seq, and confirming in engineered mouse and human stem cells that the signal vanished when PRC1 was acutely degraded. Immunofluorescence on human sperm independently confirmed that H2Aub is present in the paternal genome.

The resulting atlas shows that H2Aub does not simply follow H3K27me3. Instead, it emerges remarkably early, appearing in GC-rich regions of the genome as soon as the two-cell stage, right around the time the embryonic genome switches on. From there it progressively accumulates at both active and inactive genes, and by the blastocyst stage it concentrates strongly at canonical Polycomb target genes, the developmental regulators that must stay silent until the embryo commits to its first lineages. Notably, H2Aub in humans tracks GC content and CpG density far more closely than it does in mice, where the mark is largely confined to inactive genes, revealing a species difference in how Polycomb complexes are recruited to chromatin.

The relationship between the two Polycomb marks changes across development. In oocytes and two-cell embryos, H3K27me3 is only weakly correlated with H2Aub, consistent with the genome-wide erasure of the H3K27me3 landscape after fertilization. By the blastocyst stage, however, the two marks colocalize precisely at inactive Polycomb target genes that lack the active mark H3K4me3. This suggested a causal hypothesis: rather than inheriting H3K27me3 from the gametes, human embryos may rebuild it on a scaffold laid down by PRC1 through H2Aub.

To test that idea, the researchers removed H2Aub in human embryos by overexpressing PR-DUB, the BAP1-containing complex that erases the ubiquitin mark, using microinjection of three-pronuclear zygotes donated through IVF programs. Single-embryo RNA sequencing at day 3 and day 4 showed that losing H2Aub impaired embryonic development and caused moderate derepression of Polycomb target genes, particularly those that are normally silent. More striking was what happened to H3K27me3: its reestablishment at Polycomb targets was abolished, while the mark was instead deposited ectopically at inappropriate regions, including gene-poor, partially methylated domains of the genome. In other words, without the H2Aub scaffold, PRC2 not only failed to find its proper targets but painted the wrong parts of the genome.

The team extended the analysis to human embryonic stem cells using an inducible protein-degradation system that removes PRC1 within hours. Acute loss of H2Aub again collapsed H3K27me3 at Polycomb targets and generated ectopic H3K27me3 domains. The experiments also uncovered a second layer of control: DNA methylation. When the researchers inhibited the maintenance methyltransferase DNMT1, additional ectopic H3K27me3 appeared, and combining DNMT1 inhibition with PRC1 degradation produced the strongest mislocalization. H2Aub and DNA methylation therefore cooperate to keep H3K27me3 confined to its correct domains, a partnership that echoes the known interplay between Polycomb complexes and DNA methylation valleys in pluripotent cells.

The study also clarified how PRC1 itself is recruited in human cells. The variant PRC1 machinery, including components such as RYBP, KDM2B and CBX proteins, binds both the GC-rich active genes and the classical Polycomb targets. Knocking down RYBP in stem cells preferentially affected the active GC-rich genes, while catalytic loss of PRC1 hit both classes, with the strongest transcriptional consequences at silenced Polycomb targets. This dual role, dampening highly active GC-rich promoters while enforcing repression at developmental genes, helps explain why H2Aub accumulates at active genes in human embryos without silencing them outright.

Taken together, the findings sketch a stepwise model for the earliest establishment of Polycomb repression in humans. H2Aub appears first, seeded at GC-rich DNA in two-cell embryos, expands across active and inactive genes, and culminates at Polycomb targets in the blastocyst, where it guides PRC2 to redeposit H3K27me3 on a genome that had been wiped nearly clean. Because the experiments were performed in human gametes and embryos, the work fills a gap that mouse studies could not, and it carries practical weight for assisted reproduction, since errors in this reprogramming are thought to contribute to early developmental failure. The datasets have been deposited in public archives, and the authors suggest that the H2Aub-dependent mechanism they describe represents the foundational step by which human embryos rebuild one of their most important layers of gene control.

Subject of Research: H2Aub-dependent reestablishment of Polycomb repression in human preimplantation embryos

Article Title: Establishing Polycomb regulatory landscapes in human early development

Article References: Hu, X., Zhang, C., Liu, B., Lai, F., Ma, S., Hou, Z., Shao, Q., Liu, L., Lu, X., Yu, G., Liu, H., Yang, Z., Wang, N., Wu, X., Lin, Z., Wang, L., Shan, Y., Pan, G., Chen, D., … Xie, W. (2026). Establishing Polycomb regulatory landscapes in human early development. Nature Genetics. https://doi.org/10.1038/s41588-026-02764-y

Image Credits: AI Generated

DOI: 10.1038/s41588-026-02764-y

Keywords: Polycomb, H2AK119ub, H3K27me3, PRC1, PRC2, human embryos, epigenetics, blastocyst, histone modification, DNA methylation, embryonic stem cells, Stacc-seq