Bourneville and Taylor: a developing story?

Molecular neuropathology of epilepsy-associated glioneuronal malformations

Glioneuronal malformations (malformations of cortical development [MCD]) include focal cortical dysplasias (FCD) as well as highly differentiated glioneuronal tumors (i.e. gangliogliomas) and constitute frequent findings in patients with pharmacoresistent focal epilepsies. Tailored resection strategies evolved as promising treatment options and allow a systematic neuropathologic and molecular biologic examination of the epileptogenic area in these patients. The histopathologic appearance and immunophenotype of glioneuronal lesions are, however, characterized by numerous similarities and suggest impaired proliferation, migration, and differentiation of neural precursor cells to play a pathogenetic role. Recent studies point toward molecular alterations within a variety of genes and pathways involved in development of the central nervous system, neuronal growth, and maturation. Compromised signaling within insulin- or reelin-transduction cascades are common findings and were associated with specific MCD entities. Unraveling pathogenic mechanisms may advance refined classification systems for epilepsy-associated malformations and open new avenues for the development of targeted treatment strategies in pharmacoresistent focal epilepsies associated with cortical malformations.

Distinct Allelic Variants of TSC1 and TSC2 in Epilepsy-Associated Cortical Malformations Without Balloon Cells

Epilepsy-associated malformations of cortical development (MCDs) comprise a variety of dysplastic and neoplastic lesions of yet undetermined molecular pathology. Histopathologic similarities between MCDs and dysplastic brain lesions in the autosomal inherited neurocutaneous phacomatosis tuberous sclerosis (TSC), which affects the TSC1 and/or TSC2 genes, suggest common pathogenetic mechanisms. Previous studies revealed different alterations of TSC1 and TSC2 in epilepsy-associated malformations and glio-neuronal tumors despite histopathologic similarities. In order to examine current clinico-pathologic classification systems of cortical malformations on the molecular level, we carried out a mutational analysis of TSC1 and TSC2 in a series of surgical specimens obtained from patients with FCD without Taylor type balloon cells (FCDIIa; n = 20), architectural dysplasias (FCDI; n = 15), nodular cortical heterotopias (NCH; n = 4), and heterotopic white matter neurons (WMNH; n = 19). In FCDIIa, abundant genomic polymorphisms were detected in TSC2 (intron 4) but no allelic variants observed in exon 17 of TSC1. This allelic distribution pattern is in contrast to findings in FCDI and WMNH but also to those previously reported in FCDIIb (Taylor's balloon cell type). The latter revealed increased frequencies of specific alleles only in TSC1. The determination of characteristic molecular genetic alterations in specific epilepsy-associated malformations will support a comprehensive clinico-pathologic classification system and help to identify molecular pathways with potential pathogenetic relevance. Our work is supported by DFG (SFB TR3 [AJB], DFG Bl 421/1-1 [IB]), BONFOR, and Deutsche Krebshilfe.