As the digestive system develops, a complex network of nerves forms around it, creating a “second brain”—the enteric nervous system (ENS)—that controls the movement of food and waste through the gut. But a combination of changes in the molecular letters that make up certain genetic instructions can prevent these nerves from developing properly, leading to Hirschsprung’s disease (HSCR), a painful and often dangerous condition in which babies develop intestinal obstruction and are unable to have bowel movements.
A study led by NYU Langone Health researchers reveals a new strategy for studying this disorder in mice that better mimics how the disease manifests itself in humans. Previous animal models of HSCR looked only at the role that individual genes played in causing the disease, but the new approach is based on how interactions between multiple genes control the condition.
We now have a much more realistic and accurate way to model Hirschsprung disease that will help us understand the disease in a way we couldn’t before. Our study shows for the first time how some of the most well-known mutations, altering the DNA code, in Hirschsprung disease work together to block the development of the enteric nervous system.”
Ryan Fine, PhD, first author of the study, postdoctoral fellow, Center for Human Genetics and Genomics, NYU Grossman School of Medicine
The work was led by Aravinda Chakravarti, PhD, the Muriel G. and George W. Singer Professor of Neuroscience and Physiology in the Department of Neuroscience at the NYU Grossman School of Medicine and director of the School’s Center for Human Genetics and Genomics. Chakravarti has studied HSCR for more than 30 years and helped identify the two main genes associated with HSCR: rearrangement during transfection (RET) and endothelial receptor type B (EDNRB)..
In previous animal studies of HSCR, researchers “cut out” either RET or EDNRB, that is, they mutated the gene so that its function was completely destroyed. While this prevented the ENS from forming properly and mimicked some aspects of the human disease, other HSCR features were missing in the mouse models. For example, in humans, the disease is four times more common in men and tends to affect only the lower colon. But in the knockout mice, the incidence of disease is similar between males and females, and the enteric nervous system is defective throughout the colon and small intestine.
Published online October 21 in the journal PNASthe new study describes how weaker mutations in both RETs combine and EDNRB creates a more realistic model of HSCR in mice. Instead of completely knocking out any gene, the researchers made different combinations of mice in which one or both genes either still partially functioned or in which only one copy of the gene had been deleted.
In the combination that best reproduced the symptoms of the human disease, only one copy of RET knocked out both EDNRB copies were partially functional. These mice had normal nervous system development in their small intestine, and male mice were more likely to be affected than females.
The researchers were then able to work out the molecular details of how the combined genetic mutations caused the disease. HSCR is thought to be caused by a complete lack of nerve cells in the gut, so the researchers were surprised to find that during development, HSCR mice had many immature nerve cells (progenitor cells) in their gut—in fact, they had more than healthy mice.
To understand what might explain the discrepancy between the abundance of immature ENS cells and the complete absence of mature ones, the researchers analyzed which genes were different in the HSCR mice. SOAK and EDNRB control the activity levels of many different genes, but the researchers found a particularly large increase in levels SOX2OTa gene that controls how neural progenitor cells mature and become part of a complete nervous system. This observation led them to hypothesize that without fully functioning RET and EDNRB to control it, SOX2OT it could affect how the progenitor cells matured and prevent the ENS from fully developing.
Chakravarti says his team plans to use this mouse model to answer other difficult questions about HSCR, but the approach is not limited to this condition. The strategy of studying multiple mutations simultaneously has been used in the past in cancer studies, he says, but not so much for developmental disorders.
“I think this is a model for many other complex human disorders,” Chakravarti said. “By studying the complex disease as it actually occurs in people—as a result of smaller mutations in multiple genes rather than the complete loss of a single gene—we can better understand the fine details of the condition and move closer to life-saving treatments.”
Funding for the study was provided by National Institutes of Health grant HD028088.
Other NYU Langone researchers involved in the study include Rebecca Chubaryov, Mingzhou Fu and Gabriel Grullon.
