Fruit Fly Model Provides Insights into Pathogenesis of New Genetic Disorder

Research conducted in the laboratory of Dr Hugo J. Bellen, a Distinguished Service Professor at Baylor College of Medicine and a principal investigator at the Jan and Dan Duncan Neurological Research Institute (Duncan NRI) at Texas Children's Hospital, has identified a novel disorder linked to gain-of-function variants in the DOT1L gene.

Subsequent investigations unveiled that a significant portion of the patients' symptoms was surprisingly attributed to an elevation in the enzymatic activity of a histone methylase encoded by this gene. This study has been published in the American Journal of Human Genetics.

This project originated when the Undiagnosed Diseases Network, SickKids Complex Care Genomics project in Canada headed by Dr Gregory Costain, and a search of the GeneMatcher database identified nine unrelated individuals with intellectual disability, developmental delays, distinctive facial features, and other overlapping features carried variants in DOT1L gene. We found this intriguing because this gene had not been previously associated with a genetic or neurological disorder.”

Dr Hugo J. Bellen, Distinguished Service Professor, Baylor College of Medicine

DOT1L is a gene that is evolutionarily conserved and can be found in a wide range of species, from yeast to humans. This gene encodes an enzyme known as lysine methyltransferase (KMT), which is responsible for adding methyl groups to a specific amino acid, lysine 79 (K79), located on a particular histone, H3. Methylation of specific lysine residues within histones serves as a molecular switch to either activate or deactivate the expression of target genes.

Up to this point, it has been established that only partial loss-of-function variants in DOT1L, specifically in around half of the lysine methyltransferase (KMT) encoding genes (16 out of 34), have the capacity to induce dominant developmental disorders in humans.

“We found the variants in DOT1L cause a dominant disorder through a gain-of-function mechanism, which is different from other KMTs,” notes Dr Bellen.

While DOT1L is known to be engaged in several crucial cellular processes and its dysregulation has been linked to cancer, the mechanism by which variants in the DOT1L gene lead to congenital disorders was not well-understood before this study.

To address this question, Dr Zelha Nil, the first author and a Postdoctoral Associate in the Bellen lab, turned to fruit flies for investigation.

DOT1L and its fruit fly counterpart, grappa (gpp), share similar protein sequences and are likely to have overlapping functions. A significant number of disease-causing variants in the human DOT1L gene are situated in its enzymatic domain.

Additionally, gpp is expressed in a substantial subset of neurons and some glial cells. The research team generated a mutant fly lacking gpp, and these flies exhibited slow growth and did not survive beyond the larval stages.

Through the use of these mutant flies and flies in which gpp RNA was suppressed, compelling evidence was obtained, demonstrating that gpp is indispensable for the survival of the flies, plays a crucial role in proper development, and is essential for the development and function of the fly nervous system, as well as H3K79 methylation.

We attempted to suppress the lethality of gpp mutant flies by expressing the human DOT1L gene in flies. To our surprise, expressing normal or variant versions of the human DOT1L gene in tissues where it occurs naturally in flies was not sufficient for gpp mutants who had lost both copies of the gene to survive. Surprisingly, flies with a partial loss of gpp that expressed the human DOT1L variants were less viable and had more profound morphological defects than the normal DOT1L expressing flies, suggesting the human DOT1L expression in flies was toxic.”

Dr Zelha Nil, Study First Author and Postdoctoral Associate, Baylor College of Medicine

In line with this finding, both gpp mutant flies and cultured cells expressing altered versions of the human DOT1L gene showed markedly elevated levels of H3K79 methylation compared to normal DOT1L. This indicates that the heightened methylation levels likely serve as the molecular basis for the symptoms observed in the affected patients.

Based on our studies in flies, it appears that the variants result in excess enzymatic activity of DOT1L in these patients. While additional studies are needed to unravel the exact mechanism of disease pathogenesis, our study suggests that reducing DOT1L activity is a viable therapeutic strategy that can be developed in the future to treat this new genetic condition.”

Dr Hugo J. Bellen, Distinguished Service Professor, Baylor College of Medicine