Results of a new study from the University of North Carolina at Chapel Hill may help pave the way to a treatment for a neurogenetic disorder often misdiagnosed as cerebral palsy or autism.
Known as Angelman syndrome, or AS, its most characteristic feature is the absence or near absence of speech throughout the person's life. Occurring in one in 15,000 live births, other AS characteristics include intellectual and developmental delay, severe intellectual disability, seizures, sleep disturbance, motor and balance disorders. Individuals with the syndrome typically have a happy, excitable demeanor with frequent smiling, laughter, and hand flapping.
No effective therapies exist for AS, which arises from mutations or deletions of the gene Ube3a on chromosome 15. The Ube3a protein produced by the gene is a key component of a molecular pathway that is very important to all cells, especially brain neurons by helping them pass electrical or chemical signals to other neurons via the synapse.
Angelman syndrome is linked to mutations or deletions in the Ube3a gene inherited from the mother; thus, the maternal allele. In most tissues of the body, both the maternal and paternal alleles are expressed. But in rodents and humans, the paternal Ube3a allele is intact but silent, or dormant.
What apparently accounts for the dormancy of that allele is a strand of ribonucleic acid known as antisense RNA, which in terms of gene expression keeps paternal Ube3a silenced, or off. Once referred to as the genome's "dark matter," antisense RNA makes no functioning gene product, but works to repress expression of another gene by binding to its RNA.
"We wanted to determine if there could be a way to "awaken" the dormant allele and restore Ube3a expression in neurons," said neuroscientist Benjamin D. Philpot, PhD, associate professor of cell and molecular physiology, one of three senior investigators in the study and a member of the UNC Neuroscience Center.
In a report of the research published online Dec. 21,2011 in the journal Nature, the interdisciplinary team of UNC scientists say they have found a way to "awaken" the paternal allele of Ube3a, which could lead to a potential treatment strategy for AS.
"We have taken advantage of a tool that allows us to distinguish between active and inactive alleles," Philpot said. "That tool is a modified mouse that's engineered so that the Ube3a gene has a fluorescent 'reporter' gene attached to it, which tells you when the gene is on or when it's off. When the gene is on, neurons will fluoresce in yellow, but won't when the gene is off."
Other 'tools' available on the UNC campus come from study senior author Bryan L. Roth, MD, PhD, Michael Hooker Distinguished Professor of Pharmacology and Translational Proteomics and director of the National Institute of Mental Health Psychoactive Drug Screening Program. These include highly automated robotics of the sort normally found in major pharmaceutical companies: fluid handling robotics and automated high-content imaging that combine the molecular tools of modern cell biology with automated high resolution microscopy and robotic handling (see http://pdspdb.unc.edu/download/robotLab2011.php).
Using a library of FDA-approved drugs obtained from the National Institute of Health (the NIH Clinical Collection) the UNC team discovered that irinotecan, a topoisomerase (TOPO-EYE-SOM-ERASE) inhibitor known to be active in the central nervous system -- robustly 'awakened' Ube3a. Subsequently, the team identified the FDA approved medication topotecan and several other topoisomerase inhibitors as drugs which can 'awaken' Ube3a.