UHRF1

UHRF1 has five conserved domains; a ubiquitin-like domain (UBL), a tandem tudor domain (TTD), a plant homeodomain (PHD), a SET and RING associated (SRA) domain, and a Really Interesting New Gene (RING) domain. The tandem tudor-like domains have distinct affinities for specific histone marks: they bind preferentially to histone H3 that lacks methylation at arginine 2 (H3R2me0), whereas the adjacent PHD zinc finger recognizes histone H3 bearing the trimethyl mark at lysine 9 (H3K9me3). Together, these tudor-like domains can engage H3K9me3 through an aromatic pocket in the first subdomain, while simultaneously accommodating unmethylated H3K4 (H3K4me0). The linker region contributes structurally by shaping a binding cavity for histone H3, formed jointly by the tudor-like modules and the PHD finger. It stabilizes this interface through extended contacts with the tandem tudor-like domains. The SRA domain (also named YDG domain) is specialized for recognizing hemimethylated CpG sites at replication forks. This domain contains a pocket that encloses the flipped-out methylated cytosine, while two loops penetrate the gap left in the duplex from both the major and minor grooves to inspect the remaining three bases of the CpG pair. The loop contacting the major groove establishes CpG specificity and ensures discrimination against methylation occurring on the opposite DNA strand. In addition to 5-methylcytosine, the SRA domain can also bind 5-hydroxymethylcytosine (5hmC). Finally, the RING finger domain is essential for the protein’s ubiquitin ligase activity. == Subcellular location ==

UHRF1 is mainly localized in the nucleus, where it associates with chromatin and replication sites, although cytoplasmic localization has been observed in certain cellular and pathological conditions. == Function ==

UHRF1 acts as an organizational hub within the cell, interacting with a wide range of proteins to assemble regulatory complexes involved in signaling pathways, transcriptional control, and additional cellular activities. == Post-translational modifications for UHRF1 gene ==

Several post-translational modifications regulate the stability and activity of UHRF1. • Phosphorylation at serine 298 in the linker region decreases UHRF1’s ability to bind histone H3 carrying the H3K9me3 mark. • Phosphorylation at serine 639 by CDK1 during the M phase disrupts the interaction with USP7, preventing deubiquitination and promoting proteasomal degradation. • UHRF1 undergoes ubiquitination, which targets the protein for degradation by the proteasome. UHRF1 is capable of self-ubiquitination; binding to USP7 removes ubiquitin chains and stabilizes UHRF1, preventing degradation. During mitosis, phosphorylation at Ser-639 blocks the association with USP7, resulting in increased ubiquitination and subsequent degradation. DNA damage has been reported to enhance polyubiquitination of UHRF1. • Identified ubiquitination sites include Lys31, Lys50, Lys399, Lys408, Lys500, Lys525, Lys570, Lys600, and Lys692. • One O-linked glycosylation site has been reported in GlyGen for UHRF1 (Q96T88). Therefore, the expression levels and biological functions of UHRF1 are controlled not only by transcriptional mechanisms but also by a set of post-translational modifications. Earlier studies have shown that ubiquitination regulates UHRF1 protein stability as well as its biological activities. Moreover, phosphorylation influences UHRF1’s interactions with other proteins and affects various cellular signaling pathways. Acetylation contributes to chromatin remodeling and modulates the interplay between different PTMs occurring on UHRF1. Methylation also affects UHRF1 by altering its protein–protein interactions and its subcellular localization. Multiple post-translational modifications (PTMs) contribute to the regulation of UHRF1 stability, localization, and activity. Several domains of the protein appear to accommodate specific types of modifications, including the RING, PHD, and tandem Tudor domains, where methylation and acetylation enzymes such as SET7, SET8, PCAF, and p300 have been reported to interact with UHRF1. Other PTMs, including glycosylation, SUMOylation, and lipidation, have also been detected, although their structural and functional implications remain less well characterized. Glycosylation on serine, threonine, or lysine residues may affect UHRF1 turnover and subcellular distribution, while SUMOylation and lipidation have been associated with chromatin remodeling, DNA methylation dynamics, and transcriptional regulation. The combination and interplay of different PTMs adds another layer of complexity to UHRF1 regulation. Individual modifications such as lysine acetylation have been linked to changes in DNA methylation, whereas phosphorylation of serine residues has been associated with enhanced chromatin binding and protein stability. Methylation on serine, lysine, or arginine residues may further influence how UHRF1 interacts with chromatin or with its protein partners. These observations suggest that the biological properties of UHRF1 are shaped by a coordinated network of PTMs that modulate its role in epigenetic maintenance. Altered PTM patterns of UHRF1 have been associated with processes relevant to tumor development, including cell proliferation, metastasis, and treatment resistance. Several modifications appear to influence interactions with DNA methyltransferases, histone-modifying enzymes, p53, and cell-cycle regulators, highlighting their relevance in cancer biology. Most of the available data on UHRF1 PTMs have been obtained from in vitro studies, and additional in vivo work may help clarify their physiological and pathological significance. These regulatory layers highlight the complexity of UHRF1 biology and emphasize its importance as a central coordinator of epigenetic information. == Clinical significance ==

Role in cancer UHRF1 has recently been identified as a novel oncogene in hepatocellular carcinoma, the primary type of liver cancer. Defects in UHRF1 may be a cause of cancers. Indeed, UHRF1 is overexpressed in several kinds of human cancers, such as bladder, breast, cervical, colorectal and prostate cancers, but also pancreatic adenocarcinomas, rhabdomyosarcomas and gliomas. It contributes to linking histone modifications with gene silencing during cancer development. Elevated UHRF1 expression is observed in multiple cancer types and is associated with poorer patient outcomes, indicating that this protein may play a role in promoting tumor progression. Research suggests that UHRF1 may contribute to the maintenance of DNA methylation in cancer cells through mechanisms that extend beyond its classical association with DNMT1. Experimental depletion of UHRF1 has been reported to cause a stronger loss of DNA methylation than depletion of DNMT1 alone, an effect that does not appear to result from passive demethylation linked to cell division. Findings from the same study indicate that UHRF1 may enhance the activity of the de novo methyltransferases DNMT3A and DNMT3B while limiting TET2-mediated demethylation. These observations support the hypothesis that UHRF1 acts as a central regulator of the balance between methylation and demethylation in cancer cells, although further research is needed to fully establish the extent and physiological relevance of these non-canonical functions. UHRF1 as a therapeutic target in cancer Research in cancer epigenetics indicates that the abnormal methylation patterns characteristic of tumor cells may offer opportunities for therapeutic intervention. DNA methylation has long been considered a relevant target, and inhibitors of DNMT1, such as 5-aza-cytidine, are already used clinically in disorders like myelodysplastic syndromes and acute myeloid leukemia. However, these drugs present several limitations, including toxicity, instability, and the development of resistance. Recently, more selective DNMT1 inhibitors have been developed and may reduce some of these drawbacks, although they still induce DNMT1 degradation, which could have unintended consequences. Recent findings suggest that targeting UHRF1 might represent an alternative therapeutic strategy. UHRF1 is frequently overexpressed in tumors, which could potentially offer a therapeutic window in which cancer cells are more vulnerable than healthy tissues. Ongoing efforts in drug discovery are exploring ways to inhibit UHRF1 function, although, as with any essential protein, defining an appropriate dosage or delivery method remains a major challenge. The study proposing this approach highlights possible non-canonical functions of UHRF1 and suggests that further investigation may clarify its relevance as a therapeutic target in both normal physiology and disease. == References ==