Abstracts

Rationale: The neurodevelopmental disorder Angelman syndrome is most often caused by deletion of the maternally-derived chromosome 15q11-q13 region, which includes not only the UBE3A gene but also genes for the GABA-A receptor subunits beta3, alpha5, and gamma3. GABAergic dysfunction has been hypothesized to contribute to the development of epilepsy in this condition. In the present study, analysis of GABA-A receptor subunit expression and function was performed in cerebral cortex from three subjects with Angelman syndrome and compared to that from control tissue. Methods: Postmortem frozen neocortical specimens were obtained from the NICHD Brain and Tissue Bank for Neurodevelopmental Disorders at the University of Maryland. The membrane fraction of the frozen tissue was isolated and subjected to Western blot analysis. An additional membrane fraction was injected into Xenopus oocytes, resulting in incorporation of the brain membrane vesicles with their associated receptors into the oocyte cellular membrane. Two-electrode voltage clamp analysis of GABA-A receptor currents was then performed. Results: As expected, protein expression levels of the beta3 and alpha5 GABA-A receptor subunits were significantly decreased in Angelman syndrome cortex when compared with controls. Expression of the gamma2 subunit was also depressed. In contrast, beta2 subunit expression was substantially elevated, while alpha1 expression was unchanged. Functional studies of GABA-A receptor pharmacology revealed unchanged receptor affinity and modulation by benzodiazepines or neurosteroids. Current enhancement by barbiturates was actually increased in Angelman syndrome cortex, while sensitivity to current inhibition by zinc was decreased. Conclusions: Alterations in GABA-A receptor subunit expression and pharmacology in Angelman syndrome may contribute to its neurodevelopmental phenotype and predicts differential responses to GABAergic drugs.