Gene repression. H3K27me and PRC2 transmit a memory of repression across generations and during development

Abstract

For proper development, cells must retain patterns of gene expression and repression through cell division. Repression via methylation of histone H3 on Lys27 (H3K27me) by Polycomb repressive complex 2 (PRC2) is conserved, but its transmission is not well understood. Our studies suggest that PRC2 represses the X chromosomes in Caenorhabditis elegans germ cells, and this repression is transmitted to embryos by both sperm and oocytes. By generating embryos containing some chromosomes with and some without H3K27me, we show that, without PRC2, H3K27me is transmitted to daughter chromatids through several rounds of cell division. In embryos with PRC2, a mosaic H3K27me pattern persists through embryogenesis. These results demonstrate that H3K27me and PRC2 each contribute to epigenetically transmitting the memory of repression across generations and during development.

Fig. 1. XO males with a sperm-inherited X do not require H3K27me and rely on H3K9me as an alternative mechanism of X repression

Analysis of germline proliferation, presence of sperm, fertility, and germline repression of an X-linked transgene in males with an oocyte-inherited X (X^oo) or sperm-inherited X (X^sp): wild type (WT), lacking H3K27me (mes-3), lacking H3K9me (met-2 set-25), or lacking both. Statistically significant differences between genotypes, *P < 0.01 Mann-Whitney U test. Also see figs. S1 and S2.

Fig. 2. Repressive H3K27me3 is transmitted to embryos on sperm chromosomes and without PRC2 is transmitted through cell divisions

(A) Diagram of oocyte and sperm union to generate M−P+ embryos with H3K27me3 inherited from the sperm (P+) but not the oocyte (M−) and lacking maternal and paternal HMT. (B) Images of the two pronuclei in a one-cell embryo, and a diploid nucleus in a two-cell embryo. Merge panels show DNA (red) and H3K27me3 (green). The polar bodies [shown in (A)] identify the pronuclei as oocyte-derived (oo) or sperm-derived (sp). (C) Perdurance of paternally inherited H3K27me3 on a subset of chromosomes until the ~24-cell stage. Also see fig. S5, and fig. S6 for wild type.

Fig. 3. PRC2 maintains the memory of repression on gamete-of-origin chromosomes

(A) Diagram of oocyte and sperm union to generate M+P− embryos with H3K27me3 inherited from the oocyte (M+) but not the sperm (P−) and containing maternal HMT. (B) Images (as described for Fig. 2B) of M+P− embryos with a III-X-IV fusion chromosome in the oocyte (M+) chromosome set (arrow). See fig. S6 for wild type.

Fig. 4. The memory of repression is maintained by PRC2 during embryogenesis and is newly established during germ cell development in larvae

Images of worms generated as in Fig. 3. M+P− ~100-cell embryo (one nucleus circled), L1 larva (germ nuclei circled), and L3 and adult germ lines. See fig. S6 for wild type.