Abstracts

Rationale: Type IIB Focal Cortical Dysplasia (FCD) possess distinctive neuro-pathologic and clinical features, and affected patients show severe epilepsy course. In the attempt of elucidating the molecular and cellular mechanisms underscoring the intrinsic epileptogenicity in human FCDs, we have undertaken a combined morphologic and molecular approach on specimens from type IIB FCD epileptic patients subjected to epilepsy surgery. Methods: We analyzed specimens from four pediatric and five adult type IIB FCD patients surgically treated for drug-resistant focal epilepsy. As controls, we also examined seven non-epileptic patients of comparable age with non-invasive brain tumors whose specimens were taken from cortical areas away from the tumor. Patients were treated either at the Epilepsy Surgery Center C. Munari of the Niguarda General Hospital or at the Neurosurgery Department of the Neurological Institute. In epileptic patients, high-resolution MRI and electroclinical analysis including SEEG recordings were used to carefully define the epileptogenic areas. In these epileptogenic areas as well as in adjacent cortical areas we have performed a careful morphologic/morphometric analysis. We have also performed a molecular analysis of NMDA receptor complex composition, using protein fractions specifically enriched in post-synaptic densities. Results: Our morphologic data revealed that dysmorphic neurons with particularly large perykarya (soma size greater than 1100 m2) were present only in type IIB FCD patients with a longer (>10 years) epilepsy history. We indeed found a strong and statistically significant positive correlation between the percentage of these larger dysmorphic neurons and duration of epilepsy (r=0,8; p<0,05). These dysmorphic neurons were characterized by huge nucleus and abnormal perinuclear intracytoplasmic Nissl aggregates, possibly reflecting the abnormal over-expression of neurofilaments. We also found, in all patients, a reduction of neuronal cell density in both supra- and infra-granular layers of the epileptogenic area in comparison to the adjacent less affected cortex. In addition, this decreased cell density was more pronounced in patients with longer epilepsy history. Finally, in all considered type IIB FCD patients, in agreement with previous data, we found increased expression of NMDA regulatory subunits in the post-synaptic membranes of dysmorphic pyramidal neurons. A concomitant increase of NMDA scaffolding protein levels was present in the post-synaptic fraction of pediatric patients, possibly reflecting different synapse maturation and neuronal architecture. Conclusions: Our morphometric data support the hypothesis that cellular abnormalities observed in type IIB FCD epileptic patients could be worsened by a long history of severe, repeated seizures. These findings suggest that FCD lesions are the origin of but can be also affected by epilepsy itself. In addition, our data confirm previous data indicating that abnormalities of NMDA receptor complex and associated proteins are consistently associated with, and may sustain epileptogenesis of FCD IIB patients.