DOT1L, or Disruptor of Telomeric Silencing 1-like, is an enzyme present in human cells that performs a specific biological activity. DOT1L plays a part in the intricate processes that maintain cellular function.
The Role of DOT1L in Gene Regulation
DOT1L functions as a histone methyltransferase. Histones are proteins around which DNA is wound, forming nucleosomes, the basic units of chromatin. DOT1L’s specific target is lysine 79 (K79) on histone H3, a modification called H3K79 methylation.
Histone methylation is a type of epigenetic modification, meaning it alters gene expression without changing the underlying DNA sequence itself. This modification can influence how tightly DNA is packed, thereby affecting the accessibility of genes for transcription. DOT1L can add one, two, or three methyl groups to H3K79, leading to mono-, di-, or tri-methylation.
The level of H3K79 methylation correlates with gene activity, with mono- and di-methylation associated with active gene transcription. This methylation promotes an open chromatin structure, allowing cellular machinery, including RNA Polymerase II, to access and transcribe genes. This process is important for various cellular functions, including normal development, cell cycle progression, and DNA damage repair.
H3K79 methylation is found in actively transcribed gene bodies, rather than promoter regions, and its enrichment correlates with the level of gene expression. While H3K79 tri-methylation has been linked to gene repression, the overall role of DOT1L-mediated H3K79 methylation is associated with transcriptional activation.
DOT1L and Disease Development
When DOT1L’s normal function is disrupted, it can contribute to the development of various diseases, particularly certain types of cancer. It is associated with Mixed Lineage Leukemia (MLL-rearranged leukemia), characterized by chromosomal translocations involving the MLL gene at cytogenetic band 11q23.
These translocations result in the MLL gene fusing with various partner proteins, such as AF4, AF9, AF10, or ENL. While the MLL protein normally has its own histone modifying activity, this activity is lost in these fusion proteins. Instead, MLL fusion proteins aberrantly recruit DOT1L to specific gene targets.
The mis-targeting of DOT1L leads to excessive H3K79 methylation at these MLL-fusion target loci. This aberrant methylation drives the sustained expression of certain genes, such as HOXA9 and MEIS1, which promote leukemia development. This dysregulation of DOT1L’s activity contributes to the uncontrolled cell growth characteristic of MLL-rearranged leukemias.
The dependence of these leukemias on DOT1L’s aberrant activity suggests that interfering with this process could be a therapeutic strategy. The widespread epigenetic changes induced by MLL fusions, mediated by DOT1L, can explain why these leukemias have few other DNA sequence alterations. This makes DOT1L a compelling target for new therapeutic interventions, especially given the poor prognosis associated with MLL-rearranged leukemias.
Targeting DOT1L in Therapy
The involvement of DOT1L in driving MLL-rearranged leukemias has made it an appealing target for drug development. Researchers have focused on creating specific inhibitors that can block the enzyme’s activity. These inhibitors aim to reduce the aberrant H3K79 methylation that fuels the growth of these cancer cells.
One such DOT1L inhibitor that has progressed into clinical trials is pinometostat (EPZ-5676). Pinometostat is a small-molecule inhibitor designed to specifically block DOT1L’s methyltransferase activity. By inhibiting DOT1L, pinometostat reduces the levels of H3K79 methylation, which in turn suppresses the expression of the oncogenes that are overactive in MLL-rearranged leukemias.
In a Phase 1 clinical study involving adult patients with advanced acute leukemias, including those with MLL gene rearrangements, pinometostat was evaluated for its safety and effectiveness. The study demonstrated that pinometostat could reduce H3K79 methylation in patients’ peripheral blood mononuclear cells. While efficacy as a single agent was modest, two patients with a specific translocation, t(11;19), achieved complete remission at a dose of 54 mg/m² per day.
These results provide proof of concept that targeting DOT1L with a specific inhibitor can lead to clinically meaningful responses in MLL-rearranged leukemia patients. The safety profile of pinometostat was favorable, suggesting its potential for use in combination therapies. Ongoing research continues to explore DOT1L inhibitors, both as single agents and in combination with other treatments, to offer new avenues for patients with these leukemias.