A multi-institutional study reveals the link between loss of WDR37 gene to a new neurological syndrome that is characterized by brain malformations and a range of symptoms including visual impairment, epilepsy, developmental delays and intellectual disability. The study was published in the American Journal of Human Genetics.
This collaborative study was led by Dr. Hugo Bellen, professor at Baylor College of Medicine, investigator in the Jan and Dan Duncan Neurological Research Institute at Texas Children’s Hospital and Howard Hughes Medical Institute; and Dr. May Christine Malicdan, staff scientist at the National Institutes of Health. The study also involved other researchers affiliated with the Undiagnosed Diseases Network (UDN), a national initiative whose mission is to find the genetic cause of rare and new diseases.
The researchers received information about the first patient through the UDN. Subsequent searches on GeneMatcher and MyGene2 databases uncovered four other patients who presented with similar clinical features – tissue loss in essential structures of the eyes, facial distortion, seizures, feeding difficulties, developmental delays, intellectual disability, and poor verbal development. Most of these patients also had congenital cardiac defects and low muscle tone. Furthermore, imaging analyses revealed several remarkably identical abnormalities in critical brain regions in these patients.
To identify the gene(s) responsible for these defects, the team sequenced and analyzed entire genomes (i.e. the entire complement of all genes in a person) or exomes (i.e. only the protein-coding part of the genome) of these patients and their parents.
“Interestingly, the five patients carried spontaneous mutation in the same gene, WDR37. This gene belongs to the WD40 family of proteins, whose domains fold to serve as ‘platforms’ to facilitate crucial protein-protein interactions. The mutations we identified results in a ‘misspelling’ or errors in the WDR37 protein, which we hypothesized might be the reason for the symptoms in these patients,” Dr. Oguz Kanca, lead author of the study and postdoctoral researcher in the Bellen lab, said.
The WDR37 gene contains a known coiled region and seven WD40 repeats (WDRs), stretches of ~40 amino acids (i.e. building blocks of proteins). Although based on previous analysis it was known that human genomes have a low tolerance for loss-of-function of WDR37, very little is known about its biological function.
Since the WDR domain is structurally similar across species, researchers in the Bellen lab used Drosophila melanogaster (fruit flies) to tease apart the biological function of this gene. They used CRISPR-Cas9 technology to precisely delete both copies of this gene in fruit flies. Initial experiments showed the flies lacking this gene survived/reproduced normally and did not display obvious morphological defects. They first showed the gene was expressed both in neurons and muscles. They then performed a battery of behavioral assays to test neuromuscular and motor function.
“In flies, bang sensitivity is commonly used as a readout of paralysis and seizures. It measures how fast flies can recover balance/coordination after experiencing a mechanical stress. Wild-type flies recovered quickly after being shaken, whereas flies lacking wdr37 remained uncoordinated and seized for a protracted period of time. A second behavioral defect that was observed in mutant flies possibly relates to a lack of muscular strength, reminiscent of low muscle tone in our patient cohort. The flies were unable to hold onto the walls of the vials in a climbing assay and repeatedly fell down. Finally, we observed disruptions in copulation, an interaction that involves stereotypical sequence of physical movements. Normally, male flies cling to the backside of the females when mating, and maintain this position by gripping the female’s abdomen with their legs. However, flies lacking wdr37 are unable to maintain their grip during copulation and the majority (71%) fell down,” Kanca said.
Interestingly, when Kanca expressed the normal version of human WDR37 gene in flies that lack wdr37, he observed a complete reversal of these behavioral defects showing the human gene functions properly in the fly. On the other hand, when variant/mutant forms of human WDR37 present in the patients were expressed, they did not result in an improvement of the above behaviors. Together, these results provide compelling evidence that the variants in WDR37 are responsible for the symptoms seen in the five patients.
“While follow-up studies in different animal and cell culture models are needed to better understand the precise mechanism of this novel disorder, this study reveals WDR37 is associated with a novel syndrome and offers many avenues for future exploration,” Bellen concluded.