Histone serotonylation in HCC: Decoding the impact of "happy" histones on liver cancer progression

Hepatocellular carcinoma (HCC) is a leading cause of cancer-related mortality worldwide, with its incidence on the rise, primarily driven by chronic liver diseases such as viral hepatitis, metabolic dysfunction–associated steatotic liver disease (MASLD), and alcohol-related liver disease (ALD). Despite advances in systemic therapies, the prognosis for patients with advanced HCC remains poor due to limited therapeutic options and high recurrence rates following treatment. 1 Understanding novel molecular pathways involved in HCC progression is essential to identify new therapeutic targets. In this issue of Journal of Hepatology, Dong et al. present a compelling study on the role of nuclear-localised transglutaminase 2 (TGM2) in histone serotonylation, a newly emerging epigenetic modification, and its impact on MYC signalling in HCC. This work adds to the growing body of evidence supporting the role of epigenetic regulation in liver carcinogenesis 2 and opens new avenues for targeted therapy development. Epigenetic modifications such as DNA methylation, histone modifications, and noncoding RNAs play critical roles in regulating gene expression in cancer (Fig. 1). 3,4 Among histone modifications, acetylation, methylation, and phosphorylation have been studied extensively. However, histone serotonylation, the covalent attachment of serotonin (5-hydroxytryptamine, 5HT) to histone 3 (H3), only gained attention in 2019, with Farrelly LA et al. highlighting its involvement in transcriptional regulation. 5 The study by Dong et al. identifies serotonylation of glutamine 5 in H3 (H3Q5ser) as a key modification in HCC, catalysed by TGM2. Their study emphasizes the role of this permissive epigenetic mark in promoting chromatin accessibility, ultimately leading to increased MYC transcriptional activity and tumour progression. Given MYC’s well-established role in oncogenesis, 6 understanding how histone serotonylation regulates its activity provides critical insights into HCC pathogenesis. TGM2, a multifunctional enzyme involved in protein crosslinking, cell adhesion, apoptosis, and signal transduction, has been implicated in various diseases, including cancer, where it contributes to tumour progression, metastasis, and drug resistance. 7 Previous studies have shown TGM2 to be upregulated in HCC and correlated with poor prognosis. 8,9 However, its role in epigenetic regulation remained unclear until now. Dong et al. show that TGM2-mediated H3Q5Ser enhances MYC signalling, promoting proliferation, survival, and metastasis of HCC cells. This suggests that targeting TGM2 could suppress MYC activity, providing a potential therapeutic strategy for MYC-driven HCC. The study also demonstrates that TGM2-mediated H3Q5Ser occurs at positions adjacent to trimethylated H3 lysine 4 (H3K4me3), a histone mark found at gene promoters correlating with active transcription. While these histone marks do not regulate each other directly, their coexistence (H3K4me3Q5Ser) seems to amplify gene expression. 5 Recent studies have identified tissuespecific enhancers and super-enhancers of MYC sequences that play a critical role in regulating MYC expression. 10,11 This raises additional questions: Does histone serotonylation promote the gain of enhancer activity? If so, could targeting histone serotonylation modify MYC’s enhancer landscape? To explore this, further investigation into TGM2’s potential to serotonylate enhancer regions marked by H3K4me1 and H3K27ac is needed. Additionally, the authors identify TRIM28, a protein overexpressed in HCC, 12 as an adaptor protein that facilitates the recruitment of TGM2 to the MYC promoter, inducing MYC transcription via H3Q5Ser. Understanding other transcription factors and mechanisms that recruit TGM2 to specific genetic loci could provide further insight into this See Article, pages 105–118 DOI of original article: https:// doi.org/10.1016/j.jhep.2024. 12.038 Histone serotonylation in HCC: Decoding the impact of "happy" histones on liver cancer progression Amaia Navarro-Corcuera 1 , María L. Martínez-Chantar 2, * * Corresponding author. Address: Liver Disease Laboratory, Building 801A, Technologic Park of Biscay, Derio (Biscay), Spain; Tel.: +34-944-061-318. E-mail address: [email protected] (M.L. Martínez-Chantar). Received 11 February 2025; received in revised form 14 February 2025; accepted 17 February 2025; available online 27 February 2025 https://doi.org/10.1016/j.jhep.2025.02.020 Journal of Hepatology, July 2025. vol. 83 j18–20 Editorial process and guide the development of novel therapeutic strategies. Targeting MYC directly in cancer therapy is challenging, prompting researchers to explore alternative approaches, such as modulating upstream regulators. The discovery that TGM2mediated serotonylation enhances MYC expression presents an exciting opportunity for drug development. Several smallmolecule inhibitors of transglutaminases are currently under preclinical development, primarily for neurodegenerative and inflammatory diseases, 13 and repurposing these inhibitors for HCC therapy warrants further investigation. Furthermore, targeting histone serotonylation as an epigenetic modification represents a novel direction in oncology. Epigenetic drugs, such as histone deacetylase inhibitors and DNA methyltransferase inhibitors, have already shown promise in various cancers. 2 Developing small molecules that specifically inhibit histone serotonylation or block TGM2 activity offers a tumourselective approach to disrupting MYC-driven oncogenesis in HCC. Histone serotonylation’s translational potential in HCC also lies in its potential as a biomarker for patient stratification. Given its key role in MYC-driven HCC, evaluating serotonylation levels as a predictive biomarker for disease progression or treatment response is essential. Identifying reliable biomarkers for clinical use would allow for precision medicine approaches, enabling targeted therapies for patients most likely to benefit from them. Developing assays to detect histone serotonylation in tumour biopsies or liquid biopsies could be a crucial step forward. Therapeutic target validation is another critical consideration. While Dong et al. provide compelling preclinical evidence, further studies are needed to determine if inhibiting TGM2 or histone serotonylation can effectively reduce tumour burden in animal models of HCC. Validating this approach in robust in vivo models will be essential before moving into clinical trials. Moreover, exploring whether serotonylation inhibitors affect other histone modifications or cellular processes will be crucial to ensuring specificity and minimizing off-target effects. Another key question is whether targeting histone serotonylation could be integrated into combination therapies. Given the limited efficacy of single-agent treatments in advanced HCC, combining serotonylation inhibitors with existing therapies, such as kinase inhibitors (e.g., sorafenib, lenvatinib) or immunotherapies, 14 could enhance therapeutic outcomes. Evaluating the synergy between serotonylationtargeted agents and current HCC treatments could provide insights into optimizing combination strategies for improved patient response. Understanding the interplay between serotonylation and other oncogenic pathways in HCC is also essential. MYC is not the sole driver of HCC; mutations in TP53,CTNNB1 (b-catenin), and aberrant WNT signalling also contribute to tumorigenesis. 14 Investigating how serotonylation interacts with these pathways may refine our understanding of the broader epigenetic landscape in HCC, potentially revealing additional therapeutic vulnerabilities. For example, exploring whether serotonylation influences b-catenin-driven tumorigenesis could yield new insights for combinatorial targeting strategies. Furthermore, the hepatic microenvironment plays a fundamental role in HCC progression, and serotonylation may influence key processes such as immune cell infiltration, angiogenesis, and tumour-stroma interactions. Serotonin has been shown to regulate hepatic stellate cell activation, contributing to fibrosis and tumorigenesis. 15,16 Investigating how serotonylation affects immune suppression, angiogenic signalling, and fibroblast activation could expand the therapeutic potential of targeting this pathway beyond direct effects on tumour cells. Histone serotonylation as a tumour-promoting epigenetic modification may extend beyond liver cancer. This finding suggests that other neurotransmitters, such as dopamine and histamine, might similarly be substrates for transglutaminases to modify chromatin and regulate gene expression in various malignancies. The intersection of metabolic cues, noncanonical neurotransmitter signalling, and epigenetics is an emerging area of cancer research that holds promise for novel therapeutic interventions. Moreover, exploring whether targeting histone serotonylation has potential applications in other cancers with MYC dysregulation, such as colorectal, breast, and lung cancers, is essential. Histone serotonylation might play significant roles in organs like the pancreas, which relies on transglutaminase activity for insulin secretion, 7 and the gastrointestinal tract, where most serotonin is produced and stored. 17 Identifying tumour types particularly susceptible to serotonylation-targeted therapies could broaden clinical applications. Histone modifications DNA methylation Long non-coding RNAs Global DNA hypomethylation and TSG hypermethylation result in aberrant transcription of target gene Target gene H3K9me3 Promoter TSG l nc RNA Methyl groups Target gene H3K27me3 Target gene H3K4me3 Target gene H3K27ac Methylation Acetylation Serotonylation MYC H3Q5Ser Epigenetic modifier Target gene H4K16ac Gene expression mRNA stability Protein complex formation Transcription mRNA l nc RNA Individual proteins Protein complex Histone modification Fig. 1. Epigenetic mechanisms in HCC. Epigenetic changes, including 1) histone modifications, 2) DNA methylation, and 3) lncRNAs, play crucial roles in regulating gene expression during liver cancer progression. 1) Post-translational modifications of histones, such as the methylation and acetylation of lysine residues on histone H3, modulate the transcription of TSGs and oncogenes, promoting HCC progression. The study discussed here identifies serotonylation of glutamine 5 on histone H3 (H3Q5Ser) as a novel histone modification that enhances MYC oncogene expression and promotes HCC progression. 2) Global DNA hypomethylation of gene bodies and hypermethylation of TSG promoters result in aberrant gene transcription, driving HCC. 3) LncRNAs, non-coding RNAs longer than 200 bp, are also key epigenetic regulators in HCC, performing various roles such as guiding, stabilizing, and scaffolding: lncRNAs regulate gene expression by mediating histone modifications through interactions with DNA and other epigenetic modifiers (left); bind mRNA, stabilizing it (middle); and act as scaffolds to facilitate protein-protein interactions, such as the formation of transcription factor-epigenetic modifier complexes (right). Ac, acetylation; HCC, hepatocellular carcinoma; H, histone; K, lysine; me, methylation; lncRNA, long noncoding RNA; Q, glutamine; Ser, serotonylation; TSG, tumour suppressor gene. Created in https://BioRender.com. Journal of Hepatology, July 2025. vol. 83 j18–20 19 Editorial In summary, Dong et al. show for the first time the functional significance of histone serotonylation in HCC, which represents a significant advance in our understanding of HCC epigenetics. Their findings emphasize the role of TGM2-mediated histone serotonylation in MYC activation and tumour progression, suggesting a direct crosstalk between bioactive monoamines (e.g. serotonin) and the epigenome. This offers a strong rationale for developing therapeutic strategies targeting this pathway. As we continue to unravel liver cancer’s epigenetic complexities, serotonylation-targeted interventions may emerge as a promising addition to the therapeutic landscape of HCC. Future studies to validate these findings in preclinical and clinical settings will be essential for translating this knowledge into effective therapies. Further investigation into the upstream regulators of TGM2, downstream effects of H3Q5Ser, and TGM2’s ability to induce enhancer reprogramming at protooncogene sequences, will advance our understanding. Another possibility is to elucidate potential mechanisms for inhibiting serotonin uptake and evaluate their consequences. The intersection of epigenetics, metabolism, and oncogenic signalling continues to be a fertile ground for discovery, and targeting histone serotonylation may represent an innovative step toward precision medicine in HCC. Affiliations 1 Division of Liver Diseases, Department of Medicine, Icahn School of Medicine at Mount Sinai, New York, NY 10029, USA; 2 Liver Disease Lab, CIC bioGUNE, Basque Research and Technology Alliance, BRTA, Derio 48160 Bizkaia, Spain; Centro de Investigación Biomédica en Red de Enfermedades Hepáticas y Digestivas (CIBERehd), Carlos III National Health Institute, 28029 Madrid, Spain Financial support No funding was received for this article. Conflict of interest The authors declare no conflicts of interest that pertain to this work. Please refer to the accompanying ICMJE disclosure forms for further details. Authors’contributions Both authors contributed equally. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/ j.jhep.2025.02.020. References Author names in bold designate shared co-first authorship [1] Singal AG,Kanwal F,Llovet JM. Global trends in hepatocellular carcinoma epidemiology: implications for screening, prevention and therapy. Nat Rev Clin Oncol 2023;20:864–884. [2] Fernández-Barrena MG, Arechederra M, Colyn L, et al. Epigenetics in hepatocellular carcinoma development and therapy: the tip of the iceberg. JHEP Rep 2020;2:100167. [3] Feinberg AP, Koldobskiy MA, Göndör A. Epigenetic modulators, modifiers and mediators in cancer aetiology and progression. Nat Rev Genet 2016;17:284–299. [4] Mattick JS, Amaral PP, Carninci P, et al. 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