| DNMT3L (DNA methyltransferase 3‑like) is a catalytically inactive member of the DNMT3 family that functions as a regulatory cofactor for the de novo methyltransferases DNMT3A and DNMT3B, integrating histone H3 tail status and Polycomb occupancy with DNA methylation patterning in germ cells and embryonic stem cells. DNMT3L contains an N‑terminal cysteine-rich domain that binds the extreme N‑terminus of histone H3 and a C‑terminal methyltransferase‑like domain that interacts with the catalytic C‑terminal domains of DNMT3A and DNMT3B, forming complexes in which DNMT3L accelerates DNA and S‑adenosyl‑L‑methionine binding and stimulates methyltransferase activity without itself modifying cytosine. DNMT3L enhances DNMT3A and DNMT3B catalytic activity by 1.5–15‑fold, directly binds their catalytic domains via its own C‑terminus, lowers Km for DNA and AdoMet, and acts as a transient “substrate exchange factor” that opens the active site to promote substrate binding, then dissociates once DNA is bound, leaving DNMT3A or DNMT3B in a closed conformation with slow DNA release that favors processive methylation. DNMT3L also reads histone H3 lysine 4 methylation state: it binds specifically to H3 tails unmethylated at Lys4 (H3K4me0), and this interaction is strongly inhibited by H3K4 methylation, so DNMT3L–H3 binding identifies chromatin regions lacking active H3K4 marks and couples them to recruitment or activation of DNMT3A2 for de novo DNA methylation, a mechanism that underlies the establishment of methylation imprints and retrotransposon silencing in germ cells. DNMT3L predominantly associates with DNMT3A2, DNMT3B and core histones, supporting a model in which DNMT3L sits at nucleosomal H3K4me0 sites and nucleates DNMT3A/3B-containing complexes that install methylation on CpG-rich DNA within imprinted loci and transposable elements during gametogenesis. In embryonic stem cells, DNMT3L plays dual roles: it is a positive regulator of DNA methylation at the gene bodies of housekeeping genes, promoting methylation of constitutively expressed loci together with DNMT3A and DNMT3B, and simultaneously acts as a negative regulator of methylation at promoters of bivalent developmental genes marked by H3K4me3/H3K27me3. At these bivalent promoters, DNMT3L interacts with the PRC2 component EZH2 and, by competing with DNMT3A/3B for binding to PRC2/H3K27me3 regions, helps maintain low promoter methylation, preserving hypomethylated, Polycomb‑repressed but activation‑competent states; when DNMT3L is reduced, bivalent promoters become hypermethylated and developmental gene expression programs are perturbed. DNMT3L is also required for ESC differentiation into primordial germ cells by inhibiting DNA methylation at the promoter of the homeotic gene RHOX5, thereby activating its transcription and supporting germline specification, highlighting context-dependent use of DNMT3L as both methylation facilitator and antagonist at distinct genomic targets. Regulatory control of DNMT3L expression further ties it into the methylation machinery: its promoter is methylated by DNMT3A, DNMT3B and DNMT3L itself during embryonic development, forming an auto-regulatory circuit in which deficiencies in DNMT3B, as seen in ICF syndrome models, cause hypomethylation of the DNMT3L promoter, increased DNMT3L transcription and disturbed methylation balance, with implications for imprinting and genome stability. The methyltransferase‑like C‑terminal structure, H3K4me0-sensitive histone-binding domain, direct activation of DNMT3A/3B catalysis, interaction with PRC2 at bivalent promoters, and roles in imprint establishment, retrotransposon silencing, housekeeping-gene methylation and germ cell differentiation define DNMT3L as a mechanistically central regulator for understanding how chromatin cues and DNMT3 enzymes cooperate to sculpt de novo DNA methylation patterns in development, disease and related immune pathologies. |
