Abstracts

RATIONALE: Human hemimegalencephaly (HMEG) is a cortical dysplasia syndrome characterized by unilateral enlargement of one cerebral hemisphere and cytoarchitectural abnormalities such as laminar disorganization, neuronal cytomegaly, and heterotopia. HMEG is highly associated with severe and intractable seizures. The molecular and developmental pathogenesis of HMEG is unknown. In view of the pervasive disruption of organized lamination in HMEG, we hypothesized that altered expression of select candidate genes may contribute to the formation of HMEG during brain development.
The objective of this study is to define changes in gene expression that may account for disorganized laminar and cellular architecture in HMEG and that may aid in understanding why HMEG is associated with epilepsy.
METHODS: We implemented cDNA array analysis as a strategy to evaluate the expression of select candidate genes in human HMEG specimens. We obtained 8 HMEG surgical specimens from patients with intractable epilepsy. Poly (A) mRNA was amplified from whole HMEG sections (10 sections per case) or from tissue homogenates and after radiolabeling, was used to probe cDNA arrays containing over 150 full-length cDNAs. Candidate cDNAs included neurotransmitter receptor subunits, uptake sites, growth factors, transcription factors, and cell signaling molecules relevant to brain development. The relative expression of each mRNA was determined and compared to expression in post-mortem control cortex (ANOVA; p[lt]0.05). The expression of several of these mRNAs was corroborated at the protein level by Western analysis, ligand binding assays, or immunohistochemistry.
RESULTS: When compared with post-mortem control brain samples (n=4), we identified differential expression of more than 50 genes in all candidate gene families. For example, the expression of c-fos, OTX-1, and IGF-1 was significantly reduced in HMEG whereas angiopoietin-1, angiogenin, VEGF, CREB, aFGF, FGF receptor subunit, and HGF expression was significantly enhanced (P[lt]0.05). The expression of GluR1, 2, 4, and 5, as well as several GABAA receptor subunits was reduced in HMEG. Significant reductions in NMDA receptor subunit mRNAs were corroborated at the functional protein level as determined by 125I-MK-801 binding assays.
CONCLUSIONS: These results demonstrate new insights into candidate genes and proteins whose altered expression in HMEG may lead to abnormal cytoarchitecture and epileptogenesis. Select alterations in transcription factor gene expression may lead to aberrant expression of additional downstream mRNAs. Changes in growth and angiogenesis factor mRNA expression may lead to neural and vascular proliferation during brain development. Altered expression of neurotransmitter subunits may lead to changes in neural excitability leading to enhanced propensity for seizures in HMEG patients.
[Supported by: MH01658, NS39938, the Esther A. and Joseph Klingenstein Fund, and Parents Against Childhood Epilepsy (P.A.C.E.).]