After an intrepid, decades-long search, Johns Hopkins Medicine researchers say they have found a new role for a pair of enzymes that regulate genome function and, when missing or mutated, are linked to diseases such as brain tumors, blood cancers and Kleefstra syndrome — a rare genetic, neurocognitive disorder.
The new findings, published November 21 in Epigenetics and chromatincould eventually help researchers understand diseases caused by disruptions of these enzymes and develop new treatments for them.
“Developing a better understanding of how enzymes affect the activity of our genomes provides valuable insights into biology and can help researchers design new therapeutic approaches for disease,” said Sean Taverna, Ph.D., associate professor of pharmacology and molecular sciences at Johns. Hopkins University School of Medicine.
The search began more than a decade ago, when Taverna was looking for factors that affect DNA activity in Tetrahymena thermophila – a single-celled organism that lives in fresh water. During the original study, the research team found a previously unknown signal that the single-celled creature uses to “tag” genes that it has turned off.
The location of the mark is on histone proteins, which act as coils that tightly coil DNA, often turning genes off and protecting DNA from damage. If Tetrahymena can’t add the marks — a process called methylation, which adds chemical tags to a section of histones called H3K23 — the DNA becomes damaged and the cells grow poorly.
In a follow-up study published in 2016, Taverna found that the H3K23 site is conserved between Tetrahymena and mammals, including humans. However, the enzymes that control how the chemical tags are placed on H3K23 differ between species.
Without the identity of these enzymatic H3K23 “authors” of methylation, researchers found it difficult to study the role of H3K23 in human biology and disease.
So, Taverna, recent Ph.D. academic David Vinson and Srinivasan Yegnasubramanian, MD, Ph.D., professor of oncology and pathology at the Johns Hopkins Kimmel Cancer Center, led a new study to search for the mammalian enzymes that add the chemical tags to H3K23.
After screening many enzymes that write methylation, Vinson found only a pair of enzymes, EHMT1/GLP and EHMT2/G9a, that placed chemical tags on the H3K23 histone site.
When the researchers used drug inhibitors and genetic mutations targeting the pair of enzymes in human brain cells (neurons) grown in the laboratory, the enzymes’ ability to place methylation tags on the H3K23 histone site was significantly reduced.
“With this initial precedent established in human neuronal cells, the door is now wide open to study the role of these enzymes and the H3K23 modification in many contexts of health and disease, including human cancer,” says Yegnasubramanian.
Now that the researchers know that EHMT1/GLP and EHMT2/G9a place chemical tags on the H3K23 histone site, they aim to understand the exact mechanism of how they do so and develop drugs that target this activity.
“We want to better understand why diseases occur when these enzymes don’t work properly, and what their connections are with H3K23,” says Taverna.
In addition to Taverna, Yegnasubramanian and Vinson, other researchers who contributed to the study are Kimberly Stephens of the University of Arkansas for Medical Sciences; Robert N. O’Meally, Shri Bhat, and Robert Cole of Johns Hopkins; and Blair Dancy of the Walter Reed Army Institute of Research.
Funding for the study was provided by the National Institutes of Health (R01GM118760, R01CA221306, and F31GM130114), the National Science Foundation, the Irving A. Hansen Memorial Foundation, and the Commonwealth Foundation.