Abstracts

Rationale: Malformations of cortical development such as cortical dysplasia (CD) and tuberous sclerosis (TSC) are commonly associated with drug resistant epilepsy. There are two major types of isolated cortical dysplasia - cortical dysplasia type I (CD type I) and cortical dysplasia II (CD type II). Both are characterized by cortical dyslamination with ectopic and dysplastic neurons, while only CD type II is associated with cytomegalic neurons. Electrophysiological studies in resected tissue have shown that cytomegalic neurons found in dysplastic tissue are hyperexcitable. Molecular studies performed to identify the cause of dysplasia and hyperexcitability have shown activation of mammalian target of rapamycin (mTOR) pathway. The cause of mTOR pathway hyperactivity in CD is unknown. One potential explanation for hyperactivation of the mTOR pathway is phosphorylation of the upstream mTOR regulator, TSC2 at the S664 site by extracellular regulated kinase (ERK). In the studies presented we sought to characterize ERK and mTOR pathway activation and interaction in malformations of cortical development such as CD and TSC. Methods: Human brain tissue sections obtained after resection from epilepsy surgery were stained with antibodies against phosphorylated ERK (pERK), TSC2 (ERK phosphorylation site S664; pTSC2), S6K1 (phosphorylation site T389; pS6K1), and S6 (phosphorylation site S240/244; pS6). Tissue staining with these antibodies was correlated with the neuropathological and clinical history. Results: IHC analysis of brain sections obtained from individuals with CD and TSC undergoing epilepsy surgery showed increased pS6K1 and pS6 labeling of dysplastic neurons and cytomegalic neurons. However, increased staining for pERK and pTSC2 was evident only in the cytomegalic neurons. Correlation studies confirmed previous reports of positive correlation between MRI lesion representative of CD type II and better surgical outcome with pERK staining. Conclusions: Our findings suggest that the mTOR pathway is aberrantly activated in CD type I and II and TSC, while the ERK pathway is activated only in CD type II and TSC. These findings provide a molecular as well as the previously described anatomical distinction for CD. Our findings reveal ERK and mTOR pathway cross-talk at the level of TSC2 in cytomegalic neurons in epilepsy, thereby identifying providing a potential mechanism for mTOR upregulation in CD type II.