Abstracts

Rationale: Malformations of cortical development (MCD) are important causes of intractable epilepsies, often accompanied with developmental delays and neurological deficits. Recent genetic studies have identified that somatic (mosaic), gain-of-function mutations in PIK3CA are commonly associated with diverse MCD syndromes including megalencephaly-capillary malformation (MCAP), hemimegalencephaly and focal cortical dysplasia. PIK3CA is a gene that encodes the catalytic subunit of phosphoinositide 3-kinases. These enzymes are critically involved in many cellular processes including cell growth, proliferation, differentiation, motility, survival and intracellular trafficking; and have been implicated in the pathogenesis of many types of cancer. We hypothesize that the epilepsy and brain developmental phenotypes caused by the PIK3CA activating mutations correlate with the severity of each mutation and the extent of distribution of its mosaicism. To test this hypothesis, we generated two lines of mutant mice carrying a conditional PIK3CA gain-of-function mutation. We are studying the role of these mutations in the etiology of the structural abnormalities and epilepsy related to MCD. Methods: Two lines of mice carrying a Pik3ca gain-of-function mutation were generated by crossing a floxed-stop-Pik3ca E545K mouse (1) with a constitutive Nestin-Cre mouse for embryonic activation of the mutated gene and (2) with a tamoxifen inducible Nestin-Cre mouse induced on post-natal day zero and day one (P0/P1). The neuropathology of the mice was characterized during embryogenesis and post-natal development using specific markers to assess general brain tissue morphology, progenitor proliferation, cortical layering and cell morphology. Seizure susceptibility was assessed using subcutaneous injection of Pentylenetetrazole and simultaneous video-electroencephalography records were collected in a subset of mice on a PowerLab 8/35 data acquisition unit (AD Instruments, Colorado Spring, Co) to characterize seizure semiology. Results: Mutant mice from the two lines successfully recapitulated the main clinical features of PIK3CA-related the human neurodevelopmental disorders. Embryonic activation of Pik3ca E545K caused dramatic megalencephaly (MEG, brain overgrowth), abnormal histology, and increased seizure susceptibility in the mice. In contrast, P0/P1 activation of the same mutated gene allele lead to focal cortical dysplasia characterized by unaltered brain size, dispersed abnormal cytoarchitecture, and greatly increased seizure susceptibility. These findings reveal critical periods for pik3ca involvement in specific process of neurodevelopment and epileptogenesis. Conclusions: These findings demonstrate that these mouse models are invaluable tools for investigations of the roles of PIK3CA mutations in the etiology of related MCDs, epilepsy, developmental delays, and neurological deficits. Furthermore these mice will permit studies of novel treatments of these conditions.