An Undiagnosed Diseases Network (UDN) study led by Dr. Hugo Bellen, investigator at the Jan and Dan Duncan Neurological Research Institute (NRI) at Texas Children’s Hospital and professor at the Baylor College of Medicine, has found mutations in transportin-2 (TNPO2) gene to be the underlying cause of a novel neurodevelopmental disorder.
The study provides the first experimental evidence of the association between variations in TNPO2 and a new neurological disorder in humans. It was published in the American Journal of Human Genetics.
“To uncover the underlying cause for the patient’s symptoms, we utilized a multi-pronged approach. Using a combination of in-depth clinical tests, genome sequencing and functional studies in fruit flies, our team showed that disruption in the function of transportin-2 protein was the likely common cause of the symptoms experienced by the patients in this cohort,” Bellen said.
The study started with an individual with global developmental delay and no definitive diagnosis who presented at the UDN, a unique national collaborative which brings together clinical and research experts from across the country to solve the most challenging cases using advanced technologies. The network is often the last resort for families of severely affected individuals (‘medical mystery cases’), who despite years of testing are unable to get a definitive diagnosis. The lack of a definitive diagnosis severely limits a family’s options to receive appropriate clinical treatments and support.
To find a definitive diagnosis, researchers sequenced and compared the genomes of the affected individual and parents to find a change in the patient’s DNA that is unique and not normally present in unaffected individuals. They found that this individual carried mutations in the transportin-2 (TNPO2) gene. This was the first time that this gene was shown to be involved in a neurological disorder. To conclude that mutations in TNPO2 gene mutation were the cause of neurological symptoms in this patient, the team looked for other patients with TNPO2 mutations to see if they had similar neurological defects.
The lead clinical investigator for this study, Dr. Queenie Tan at Duke University, and her team scoured GeneMatcher, a virtual matchmaking website that connects researchers with clinicians around the globe who oversee similar medical mystery cases. With this helpful tool, they identified fourteen patients with mutations in this gene. Upon closer examination of the clinical features, they found that majority of the identified individuals shared several overlapping developmental and neurological symptoms such as intellectual disability, autism, seizures, distorted facial features, feeding difficulties and poor muscle tone.
Fruit flies as a diagnostic tool
To understand how functional alteration of TNPO2 protein was responsible for the symptoms in these patients, the Bellen lab turned to fruit flies to performed an in-depth functional analysis.
The human and fly versions of TNPO2 protein share significant structural and functional similarities. They shuttle approximately 60 known cargo proteins between the nucleus and cytoplasm of cells, including neurons.
The team found that flies with TNPO2 mutations could not survive beyond the larval stages, demonstrating that this is a critical developmental gene. Since little was known about the role of this protein in the context of the nervous system, the investigators further studied its role in the fly neurons. They found that the fly version of this gene is critical for the maintenance and function of neurons. Further, disruption of fly TNPO2’s activity, either by deletion or upregulation, can lead to different developmental deformities in the eye and wing. Overall, these phenotypes are consistent with the neurological and morphological symptoms seen in the patients.
“Interestingly, when we overexpressed six of the fifteen disease-causing human variants of TNPO2, we observed phenotypes similar to the loss of the fly version of this gene”, said Dr. Lindsey Goodman, a postdoctoral associate in the Bellen lab and the first author of the study. “However, the severity of the phenotype varied for each variant and the effects were localized to specific tissues in some of the variants. Finally, we noticed a close correlation between how severe the symptoms were to a specific domain within the TNPO2 protein that was impacted in that variant. This led us to conclude that mutations in certain domains of TNPO2 are more deleterious than the mutations in other domains”.
This study describes the first foray into the role of TNPO2 in a neurological disorder and provides a solid foundational base for researchers to further dissect the underlying mechanisms and targets, which could eventually lead to a therapy for this novel disorder.
Other authors involved in the study and their institutional affiliations can be found here. The study was supported by the National Institutes of Health, Taiwan Merit Scholarships Program, NIHR Professorship, The Sir Jules Thorn Award for Biomedical Research, Rosetrees Trust and the Cerebral Palsy Alliance.