DOT1L inhibition enhances pluripotency beyond acquisition of epithelial identity and without immediate suppression of the somatic transcriptome

Potential use of human pluripotency-related gene expression reporter cell line for screening small molecules to enhance induction of pluripotency

The reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) is a crucial development in regenerative medicine, providing patient-specific cells for therapeutic uses. Traditional methods often utilize viral vectors and transcription factors that pose tumorigenic risks, rendering them unsuitable for clinical applications. This study explored the use of chemicals as a non-tumorigenic alternative for cell reprogramming. Utilizing CRISPR/Cas9 technology, we previously created iPSCs expressing OCT4-EGFP and NANOG-tdTomato, and derived OCT4-EGFP and NANOG-tdTomato fibroblastic cells (ON-FCs). These cells were reprogrammed using episomal vectors, and their pluripotency was validated by fluorescence and FACS analyses. High-content screening was employed to assess small molecules that improve reprogramming efficiency, confirming the usefulness of ON-FCs as a dual reporter cell line for identifying small molecules effective in generating human iPSCs. This study underscores the utility of a dual reporter system and high-content screening in identifying effective reprogramming chemicals, establishing a scalable platform for high-throughput screening. Discovering new chemicals that can reprogram iPSCs would provide a non-tumorigenic method to advance the field of regenerative medicine. [BMB Reports 2025; 58(4): 183-189].

Chromatin environment-dependent effects of DOT1L on gene expression in male germ cells

The H3K79 methyltransferase DOT1L is essential for multiple aspects of mammalian development where it has been shown to regulate gene expression. Here, by producing and integrating epigenomic and spike-in RNA-seq data, we decipher the molecular role of DOT1L during mouse spermatogenesis and show that it has opposite effects on gene expression depending on chromatin environment. On one hand, DOT1L represses autosomal genes that are devoid of H3K79me2 at their bodies and located in H3K27me3-rich/H3K27ac-poor environments. On the other hand, it activates the expression of genes enriched in H3K79me2 and located in H3K27me3-poor/H3K27ac-rich environments, predominantly X chromosome-linked genes, after meiosis I. This coincides with a significant increase in DOT1L expression at this stage and a genome-wide acquisition of H3K79me2, particularly on the sex chromosomes. Taken together, our results show that H3K79me2 positively correlates with male germ cell genetic program throughout spermatogenesis, with DOT1L predominantly inhibiting rather than activating gene expression. Interestingly, while DOT1L appears to directly regulate the (re)activation of X genes following meiotic sex chromosome inactivation, it also controls the timely expression of (autosomal) differentiation genes during spermatogenesis.

Chromatin-based memory as a self-stabilizing influence on cell identity

Cell types are traditionally thought to be specified and stabilized by gene regulatory networks. Here, we explore how chromatin memory contributes to the specification and stabilization of cell states. Through pervasive, local, feedback loops, chromatin memory enables cell states that were initially unstable to become stable. Deeper appreciation of this self-stabilizing role for chromatin broadens our perspective of Waddington's epigenetic landscape from a static surface with islands of stability shaped by evolution, to a plasticine surface molded by experience. With implications for the evolution of cell types, stabilization of resistant states in cancer, and the widespread plasticity of complex life.

Retrospective identification of cell-intrinsic factors that mark pluripotency potential in rare somatic cells

Pluripotency can be induced in somatic cells by the expression of OCT4, KLF4, SOX2, and MYC. Usually only a rare subset of cells reprogram, and the molecular characteristics of this subset remain unknown. We apply retrospective clone tracing to identify and characterize the rare human fibroblasts primed for reprogramming. These fibroblasts showed markers of increased cell cycle speed and decreased fibroblast activation. Knockdown of a fibroblast activation factor identified by our analysis increased the reprogramming efficiency. We provide evidence for a unified model in which cells can move into and out of the primed state over time, explaining how reprogramming appears deterministic at short timescales and stochastic at long timescales. Furthermore, inhibiting the activity of LSD1 enlarged the pool of cells that were primed for reprogramming. Thus, even homogeneous cell populations can exhibit heritable molecular variability that can dictate whether individual rare cells will reprogram or not.

DOT1L stimulates MYC/Mondo transcription factor activity by promoting its degradation cycle on chromatin

The proto-oncogene c-MYC is a key representative of the MYC transcription factor network regulating growth and metabolism. MML-1 (Myc- and Mondo-like) is its homolog in C. elegans. The functional and molecular cooperation between c-MYC and H3 lysine 79 methyltransferase DOT1L was demonstrated in several human cancer types, and we have earlier discovered the connection between C. elegans MML-1 and DOT-1.1. Here, we demonstrate the critical role of DOT1L/DOT-1.1 in regulating c-MYC/MML-1 target genes genome-wide by ensuring the removal of "spent" transcription factors from chromatin by the nuclear proteasome. Moreover, we uncover a previously unrecognized proteolytic activity of DOT1L, which may facilitate c-MYC turnover. This new mechanism of c-MYC regulation by DOT1L may lead to the development of new approaches for cancer treatment.

KDM3A and KDM3B Maintain Naïve Pluripotency Through the Regulation of Alternative Splicing

Histone modifying enzymes play a central role in maintaining cell identity by establishing a conducive chromatin environment for lineage specific transcription factor activity. Pluripotent embryonic stem cell (ESC) identity is characterized by a lower abundance of gene repression associated histone modifications that enables rapid response to differentiation cues. The KDM3 family of histone demethylases removes the repressive histone H3 lysine 9 dimethylation (H3K9me2). Here we uncover a surprising role for the KDM3 proteins in the maintenance of the pluripotent state through post-transcriptional regulation. We find that KDM3A and KDM3B interact with RNA processing factors such as EFTUD2 and PRMT5. Acute selective degradation of the endogenous KDM3A and KDM3B proteins resulted in altered splicing independent of H3K9me2 status or catalytic activity. These splicing changes partially resemble the splicing pattern of the more blastocyst-like ground state of pluripotency and occurred in important chromatin and transcription factors such as Dnmt3b, Tbx3 and Tcf12. Our findings reveal non-canonical roles of histone demethylating enzymes in splicing to regulate cell identity.

DOT1L interaction partner AF10 controls patterning of H3K79 methylation and RNA polymerase II to maintain cell identity

Histone 3 lysine 79 methylation (H3K79me) is enriched on gene bodies proportional to gene expression levels and serves as a strong barrier for the reprogramming of somatic cells to induced pluripotent stem cells (iPSCs). DOT1L is the sole histone methyltransferase that deposits all three orders-mono (me1), di (me2), and tri (me3) methylation-at H3K79. Here, we leverage genetic and chemical approaches to parse the specific functions of orders of H3K79me in maintaining cell identity. DOT1L interacts with AF10 (Mllt10), which recognizes unmodified H3K27 and boosts H3K79me2/3 methylation. AF10 deletion evicts H3K79me2/3 and reorganizes H3K79me1 to the transcription start site to facilitate iPSC formation in the absence of steady-state transcriptional changes. Instead, AF10 loss redistributes RNA polymerase II to a uniquely pluripotent pattern at highly expressed, rapidly transcribed housekeeping genes. Taken together, we reveal a specific mechanism for H3K79me2/3 located at the gene body in reinforcing cell identity.

DOT1L is a barrier to histone acetylation during reprogramming to pluripotency

Embryonic stem cells (ESCs) have transcriptionally permissive chromatin enriched for gene activation-associated histone modifications. A striking exception is DOT1L-mediated H3K79 dimethylation (H3K79me2) that is considered a positive regulator of transcription. We find that ESCs are depleted for H3K79me2 at shared locations of enrichment with somatic cells, which are highly and ubiquitously expressed housekeeping genes, and have lower RNA polymerase II (RNAPII) at the transcription start site (TSS) despite greater nascent transcription. Inhibiting DOT1L increases the efficiency of reprogramming of somatic to induced pluripotent stem cells, enables an ESC-like RNAPII pattern at the TSS, and functionally compensates for enforced RNAPII pausing. DOT1L inhibition increases H3K27 methylation and RNAPII elongation-enhancing histone acetylation without changing the expression of the causal histone-modifying enzymes. Only the maintenance of elevated histone acetylation is essential for enhanced reprogramming and occurs at loci that are depleted for H3K79me2. Thus, DOT1L inhibition promotes the hyperacetylation and hypertranscription pluripotent properties.

Nucleophosmin 1 cooperates with the methyltransferase DOT1L to preserve peri-nucleolar heterochromatin organization by regulating H3K27me3 levels and DNA repeats expression

BACKGROUND

NPM1 is a phosphoprotein highly abundant in the nucleolus. However, additional nuclear functions have been attributed to NPM1, probably through interaction with other nuclear factors. DOT1L is one interaction partner of NPM1 that catalyzes methylation of histone H3 at lysine 79 (H3K79). DOT1L, playing functional roles in several biological processes, is known for its capability to organize and regulate chromatin. For example, DOT1L modulates DNA repeats expression within peri-nucleolar heterochromatin. NPM1 also affects peri-nucleolar heterochromatin spatial organization. However, it is unclear as of yet whether NPM1 and DOT1L functionally synergize to preserve nucleoli organization and genome stability, and generally, which molecular mechanisms would be involved.

RESULTS

We characterized the nuclear function of NPM1 on peri-nucleolar heterochromatin organization. We show that (i) monomeric NPM1 interacts preferentially with DOT1L in the nucleus; (ii) NPM1 acts in concert with DOT1L to maintain each other's protein homeostasis; (iii) NPM1 depletion results in H3K79me2 upregulation and differential enrichment at chromatin binding genes including Ezh2; (iv) NPM1 and DOT1L modulate DNA repeats expression and peri-nucleolar heterochromatin organization via epigenetic mechanisms dependent on H3K27me3.

CONCLUSIONS

Our findings give insights into molecular mechanisms employed by NPM1 and DOT1L to regulate heterochromatin activity and structural organization around the nucleoli and shed light on one aspect of the complex role of both proteins in chromatin dynamics.

Retrospective identification of intrinsic factors that mark pluripotency potential in rare somatic cells

Pluripotency can be induced in somatic cells by the expression of the four "Yamanaka" factors OCT4, KLF4, SOX2, and MYC. However, even in homogeneous conditions, usually only a rare subset of cells admit reprogramming, and the molecular characteristics of this subset remain unknown. Here, we apply retrospective clone tracing to identify and characterize the individual human fibroblast cells that are primed for reprogramming. These fibroblasts showed markers of increased cell cycle speed and decreased fibroblast activation. Knockdown of a fibroblast activation factor identified by our analysis led to increased reprogramming efficiency, identifying it as a barrier to reprogramming. Changing the frequency of reprogramming by inhibiting the activity of LSD1 led to an enlarging of the pool of cells that were primed for reprogramming. Our results show that even homogeneous cell populations can exhibit heritable molecular variability that can dictate whether individual rare cells will reprogram or not.

Connecting the DOTs on Cell Identity

DOT1-Like (DOT1L) is the sole methyltransferase of histone H3K79, a modification enriched mainly on the bodies of actively transcribing genes. DOT1L has been extensively studied in leukemia were some of the most frequent onco-fusion proteins contain portions of DOT1L associated factors that mislocalize H3K79 methylation and drive oncogenesis. However, the role of DOT1L in non-transformed, developmental contexts is less clear. Here we assess the known functional roles of DOT1L both in vitro cell culture and in vivo models of mammalian development. DOT1L is evicted during the 2-cell stage when cells are totipotent and massive epigenetic and transcriptional alterations occur. Embryonic stem cell lines that are derived from the blastocyst tolerate the loss of DOT1L, while the reduction of DOT1L protein levels or its catalytic activity greatly enhances somatic cell reprogramming to induced pluripotent stem cells. DOT1L knockout mice are embryonically lethal when organogenesis commences. We catalog the rapidly increasing studies of total and lineage specific knockout model systems that show that DOT1L is broadly required for differentiation. Reduced DOT1L activity is concomitant with increased developmental potential. Contrary to what would be expected of a modification that is associated with active transcription, loss of DOT1L activity results in more upregulated than downregulated genes. DOT1L also participates in various epigenetic networks that are both cell type and developmental stage specific. Taken together, the functions of DOT1L during development are pleiotropic and involve gene regulation at the locus specific and global levels.