Establishing a Drosophila model for Angelman syndrome

Drosophila models for human diseases have helped in advancing our knowledge on human diseases and the discovery of potential treatments. Angelman syndrome is a rare neurological disorder that results in severe mental retardation and loss of motor coordination. The disease is caused by loss-of-function mutations in the UBE3A gene encoding a HECT domain ubiquitin protein ligase. Drosophila dube3a is the fly homolog of human UBE3A and their protein products share ~55% similarity in amino acid sequence along the entire length of the proteins. My goal was to develop a Drosophila AS model that will allow us to identify the AS-associated substrate(s) of the Drosophila UBE3A homolog and ultimately, to determine why the lack of UBE3A protein causes Angelman syndrome in humans. Dube3a is present in the embryonic, larval and adult central nervous system, including the adult mushroom bodies, which is the center for learning and memory. I have generated dube3a knock-out flies and they appear normal externally, but display abnormal locomotor behaviors. Flies that overexpress wild-type dube3a in the nervous system also display locomotion defects, and these overexpression phenotypes are dependent on the presence of a conserved cysteine residue essential for HECT domain E3 enzymatic activity. Targeted overexpression of dube3a in the eye, the wing, or ubiquitously causes rough eyes, curly wings and lethality, respectively. These morphological abnormalities in the eye or wing depend on the critical catalytic cysteine of Dube3a. Overexpression of mutant dube3a carrying AS-associated point mutations does not elicit such defects, suggesting they act as loss-of-function mutants. Taken together, dube3a mutants are a candidate fly model for Angelman syndrome, and the flies that overexpress dube3a in the eye or wing are useful for genetic screens to identify the elusive UBE3A substrates relevant to Angelman syndrome.