Uncovering the molecular basis of genetic diseases
By Wendy Bindeman
The laboratory and the doctor’s office can feel like two separate worlds. Doctors run tests and observe symptoms, doing their best to treat them, but this can be difficult if the cause of the disease isn’t known. Conversely, researchers identify mutations and study them extensively in model organisms but can’t always determine what the consequences of a mutation are in a patient.
Vanderbilt’s Program in the Molecular Basis of Genetic Diseases is one promising effort to bridge this so-called bench-to-bedside gap, focusing, as the name implies, on genetic diseases, those caused by an inherited mutation in the DNA. This initiative offers Vanderbilt researchers and clinicians a powerful opportunity for cross-disciplinary collaboration focused on investigating the links between genetic mutations of interest (the “genotype”) and the way a disease presents in a patient (the “phenotype”).
Hassane Mchaourab, professor of molecular physiology and biophysics and one of the founders of the MBGD program, got the idea from a collaborative research project focused on understanding a rare, non-inherited genetic mutation associated with autism spectrum disorder that was initially identified by Jim Sutcliffe, an associate professor in Mchaourab’s department. The project grew to include Mchaourab, whose specialty is protein biochemistry and biophysics, plus Aurelio Galli, formerly a professor of molecular physiology and biophysics; Jens Meiler, research professor of chemistry and associate professor of pharmacology; and several collaborators in the School of Engineering.
The labs pooled their expertise to investigate the impact of the identified mutation—an amino acid deletion—on the structure and function of the dopamine membrane transporter encoded by that gene. They found that although the transporter, whose role it is to transport the neurotransmitter dopamine across the cell membrane, was in fact produced and even correctly located at the membrane, the mutation rendered it dysfunctional. In fruit flies, a commonly used model in biomedical research, expression of the mutated protein produced behavioral phenotypes analogous to those observed in patients who carry the mutation. The collaboration resulted in a series of publications, including three in the prestigious journals Molecular Psychiatry and Proceedings of the National Academy of Sciences.
Mchaourab was particularly intrigued by the project because, he said, the mutation is an “experiment of nature.” This is a reference to the fact that scientists often use mutated versions of proteins to study them; disabling part of a protein is an excellent way to find out that part’s function. This naturally occurring mutation had the same effect: It “locks” the transporter in a certain conformation that is otherwise unobservable, thereby revealing a novel aspect of the molecular mechanism of the protein. This project therefore had a double benefit: identifying both a genotype-phenotype link that is relevant to the treatment of a disease and yielding a basic science discovery.
That moment of inspiration led Mchaourab, in collaboration with Meiler; Todd Edwards, an associate professor of medicine; and Tony Capra, a research associate professor of biological sciences, to propose the creation of the Molecular Basis of Genetic Diseases program. Their proposal was successful and the program was funded by the dean of the Vanderbilt University School of Medicine Basic Sciences, launching in January 2019.
A successful pilot
According to Mchaourab, the MBGD program aims to build a pipeline between geneticists, who identify rare genetic variants, and clinicians, who observe phenotypes in patients, through various collaborators who specialize in the “missing link,” the molecular and cellular studies needed to determine why a particular genotype produces a phenotype of interest. There are currently eight active collaborations in various stages of development.
The program recently completed its pilot phase and is now “open for business” and happy to collaborate with any Vanderbilt-affiliated lab or researcher. It provides collaborators access to four central types of facilities and expertise. The first is a collaboration with the Personalized Structural Biology initiative led by Meiler and Capra, which provides expertise in computational biology and determination of protein structure. The other three include facilities for protein expression and purification, biochemical and functional analysis, and zebrafish animal models. Interested faculty, students, or postdocs can contact the program directly through its website.
When working together pays off
Already the MBGD program has produced impressive results. A collaboration with Professor of Molecular Physiology and Biophysics Richard O’Brien, for instance, is investigating G6PC1, a membrane-embedded enzyme that is linked to glycogen storage disorders. These disorders are characterized by abnormal storage or use of sugar that can cause a wide spectrum of health issues, most commonly affecting the liver and muscles. The Mchaourab and O’Brien labs, in an effort spearheaded by Derek Claxton, research assistant professor of molecular physiology and biophysics, were the first to purify G6PC1 in its functional form and are now performing experiments to figure out exactly what its function is and the impact of various mutations. They are also using cryo-electron microscopy, which uses an electron stream instead of light to visualize a frozen sample, to determine its precise structure.
A second collaboration with Galli, who is now a professor of surgery and director of the Center for Inter-systemic Networks and Enteric Medical Advances at the University of Alabama at Birmingham, is enabling study into a mutation in the dopamine transporter associated with infantile parkinsonism, which is a rare neurological condition that appears during infancy that causes tremors, movement difficulties, and global developmental delays.
Normally the dopamine transporter tightly controls the flow of ions (electrically charged atoms) in and out of the cell. Structural analyses by the researchers showed that the mutation makes the protein unable to regulate that flow, so that ions move in and out at random. When expressed in a fruit fly model, the mutation produces flies with abnormal wing function, resembling the movement difficulties caused by the disease. The researchers were even able to use that model to identify a potential treatment: chloroquine, an antimalarial drug. These findings were recently published in eLIFE.
According to Mchaourab, participating in these interdisciplinary projects and establishing the MBGD program has benefited his own lab by encouraging them to “think outside the box.” The MBGD program is an example of a collaborative effort that brings together experts from many different points along the continuum of bench-to-bedside—and as the early results show, this can be a very powerful approach.