Molecular neuropathology of epilepsy-associated glioneuronal malformations

Neuropathology of focal epilepsies: a critical review

A broad spectrum of structural lesions can be histopathologically identified in surgical brain specimens obtained from patients with focal, therapy-refractory epilepsies. In our experience with 4512 tissue samples collected at the German Neuropathological Reference Center for Epilepsy Surgery, three clinicopathological entities are most common: mesial temporal sclerosis (40%), long-term epilepsy-associated tumors (27%), and malformations of cortical development (13%). Notwithstanding, a systematic histopathological and molecular-genetic analysis is mandatory to unravel the underlying pathogenic mechanism of epilepsy-associated lesions and may contribute to our current understanding of pharmacoresistance and epileptogenesis. However, an interdisciplinary approach is necessary to further explore predictive parameters with respect to postsurgical seizure relief and memory impairment, and also to identify new pharmacological targets.

Pi3K-mTOR signaling and AMOG expression in epilepsy-associated glioneuronal tumors

Gangliogliomas (GGs) and dysembryoplastic neuroepithelial tumors (DNTs) represent the most frequent type of neoplasms in pediatric medically intractable epilepsy. Several data suggest a pathogenetic relationship between GGs and other glioneuronal malformations of cortical development (MCDs), including activation of the Pi3K-mTOR signaling pathway. To further reveal these pathogenetic similarities, we investigated immunocytochemically the expression of phosphorylated (p)-PDK1, p-AKT, p-mTOR, p-4E-BP1, p-eIF4G, p-p70S6K and p-S6, the effector proteins ERM (ezrin/radixin/moesin) and the pathway regulator AMOG (adhesion molecule on glia) in both GGs and DNTs. Components of the Pi3K-mTOR signaling pathway were observed in a higher percentage of neuronal cells in GGs compared with control cortex. In DNTs, the expression of these components was low and comparable with the expression in control samples. Strong immunoreactivity for ERM was observed in GGs, but not in DNTs. Additionally, AMOG was strongly expressed within GGs (but not in DNTs) in CD34-positive precursor cells. These findings support the previously suggested pathogenic relationship between GG and MCDs concerning activation of the Pi3K-mTOR signaling pathway and suggest a different pathogenetic origin for DNTs. The strong expression of AMOG within the precursor cells of GG may represent an additional marker for the diagnostic evaluation of these glioneuronal lesions.

Assessment and surgical outcomes for mild type I and severe type II cortical dysplasia: a critical review and the UCLA experience

Recent findings on the clinical, electroencephalography (EEG), neuroimaging, and surgical outcomes are reviewed comparing patients with Palmini type I (mild) and type II (severe) cortical dysplasia. Resources include peer-reviewed studies on surgically treated patients and a subanalysis of the 2004 International League Against Epilepsy (ILAE) Survey of Pediatric Epilepsy Surgery. These sources were supplemented with data from University of California, Los Angeles (UCLA). Cortical dysplasia is the most frequent histopathologic substrate in children, and the second most common etiology in adult epilepsy surgery patients. Cortical dysplasia patients present with seizures at an earlier age than other surgically treated etiologies, and 33-50% have nonlocalized scalp EEG and normal magnetic resonance imaging (MRI) scans. 2-((18)F)Fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) is positive in 75-90% of cases. After complete resection, 80% of patients are seizure free compared with 20% with incomplete resections. Compared with type I, patients with type II cortical dysplasia present at younger ages, have higher seizure frequencies, and are extratemporal. Type I dysplasia is found more often in adult patients in the temporal lobe and is often MRI negative. These findings identify characteristics of patients with mild and severe cortical dysplasia that define surgically treated epilepsy syndromes. The authors discuss future challenges to identifying and treating medically refractory epilepsy patients with cortical dysplasia.

FDG-PET/MRI coregistration improves detection of cortical dysplasia in patients with epilepsy

OBJECTIVE

Patients with cortical dysplasia (CD) are difficult to treat because the MRI abnormality may be undetectable. This study determined whether fluorodeoxyglucose (FDG)-PET/MRI coregistration enhanced the recognition of CD in epilepsy surgery patients.

METHODS

Patients from 2004-2007 in whom FDG-PET/MRI coregistration was a component of the presurgical evaluation were compared with patients from 2000-2003 without this technique. For the 2004-2007 cohort, neuroimaging and clinical variables were compared between patients with mild Palmini type I and severe Palmini type II CD.

RESULTS

Compared with the 2000-2003 cohort, from 2004-2007 more CD patients were detected, most had type I CD, and fewer cases required intracranial electrodes. From 2004-2007, 85% of type I CD cases had normal non-University of California, Los Angeles (UCLA) MRI scans. UCLA MRI identified CD in 78% of patients, and 37% of type I CD cases had normal UCLA scans. EEG and neuroimaging findings were concordant in 52% of type I CD patients, compared with 89% of type II CD patients. FDG-PET scans were positive in 71% of CD cases, and type I CD patients had less hypometabolism compared with type II CD patients. Postoperative seizure freedom occurred in 82% of patients, without differences between type I and type II CD cases.

CONCLUSIONS

Incorporating fluorodeoxyglucose-PET/MRI coregistration into the multimodality presurgical evaluation enhanced the noninvasive identification and successful surgical treatment of patients with cortical dysplasia (CD), especially for the 33% of patients with nonconcordant findings and those with normal MRI scans from mild type I CD.

Array analysis of epilepsy-associated gangliogliomas reveals expression patterns related to aberrant development of neuronal precursors

Gangliogliomas, the most frequent neoplasms in patients with pharmacoresistant focal epilepsies, are characterized by histological combinations of glial and dysplastic neuronal elements, a highly differentiated phenotype and rare gene mutations. Their molecular basis and relationship to other low-grade brain tumours are not completely understood. Systematic investigations of altered gene expression in gangliogliomas have been hampered by their cellular complexity, the lack of suitable control tissue and of sensitive expression profiling approaches. Here, we have used discrete microdissected ganglioglioma and adjacent control brain tissue obtained from the neurosurgical access to the tumour of identical patients (n = 6) carefully matched for equivalent glial and neuronal elements in an amount sufficient for oligonucleotide microarray hybridization without repetitive amplification. Multivariate statistical analysis identified a rich profile of genes with altered expression in gangliogliomas. Many differentially expressed transcripts related to intra- and intercellular signalling including protein kinase C and its target NELL2 in identical ganglioglioma cell components as determined by real-time quantitative RT-PCR (qRT-PCR) and in situ hybridization. We observed the LIM-domain-binding 2 (LDB2) transcript, critical for brain development during embryogenesis, as one of the strongest reduced mRNAs in gangliogliomas. Subsequent qRT-PCR in dysembryoplastic neuroepithelial tumours (n = 7) revealed partial expression similarities as well as marked differences from gangliogliomas. The demonstrated gene expression profile differentiates gangliogliomas from other low-grade primary brain tumours. shRNA-mediated silencing of LDB2 resulted in substantially aberrant dendritic arborization in cultured developing primary hippocampal neurons. The present data characterize novel molecular mechanisms operating in gangliogliomas that contribute to the development of dysplastic neurons and an aberrant neuronal network.

Comprehensive characterization of genomic aberrations in gangliogliomas by CGH, array-based CGH and interphase FISH

Gangliogliomas are generally benign neuroepithelial tumors composed of dysplastic neuronal and neoplastic glial elements. We screened 61 gangliogliomas [World Health Organization (WHO) grade I] for genomic alterations by chromosomal and array-based comparative genomic hybridization (CGH). Aberrations were detected in 66% of gangliogliomas (mean +/- SEM = 2.5 +/- 0.5 alterations/tumor). Frequent gains were on chromosomes 7 (21%), 5 (16%), 8 (13%), 12 (12%); frequent losses on 22q (16%), 9 (10%), 10 (8%). Recurrent partial imbalances comprised the minimal overlapping regions dim(10)(q25) and enh(12)(q13.3-q14.1). Unsupervised cluster analysis of genomic profiles detected two major subgroups (group I: complete gain of 7 and additional gains of 5, 8 or 12; group II: no major recurring imbalances, mainly losses). A comparison with low-grade gliomas (astrocytomas WHO grade II) showed chromosome 5 gain to be significantly more frequent in gangliogliomas. Interphase fluorescence in situ hybridization (FISH) identified the aberrations to be contained in a subpopulation of glial but not in neuronal cells. Two gangliogliomas and their anaplastic recurrences (WHO grade III) were analyzed. Losses of CDKN2A/B and DMBT1 or a gain/amplification of CDK4 found in the anaplastic tumors were already present in the respective gangliogliomas by array CGH and interphase FISH. In summary, genomic profiling in a large series of gangliogliomas could distinguish genetic subgroups even in this low-grade tumor.

Defining the spectrum of international practice in pediatric epilepsy surgery patients

PURPOSE

The Pediatric Epilepsy Surgery Sub-commission of the International League Against Epilepsy conducted a survey to determine the frequency of epilepsy procedures and etiologies.

METHODS

Data were gathered from 20 programs in the United States, Europe, and Australia on 543 patients (<18 years) for calendar year 2004.

RESULTS

Age at seizure onset was 1 year or less in 46% of patients. Intracranial electrodes were used in 27% of patients. The most common final operation was lobar and focal resections of the frontal and temporal lobes (41%), followed by cerebral hemispherectomy (16%), vagus nerve stimulator (16%), and multilobar resections (13%). Multiple-subpial transections were uncommon procedures (0.6%). The most frequent etiologies were cortical dysplasia (42%), tumors (19%), and atrophic lesions and strokes (10%). Less common were vascular lesions (1.5%), Rasmussen encephalitis (3%), Sturge-Weber (3%), and operations for infantile spasms (4%), and Landau-Kleffner/ESES (<2%). Children <4 years generally presented with daily seizures from cortical dysplasia and underwent multilobar or hemispherectomy. Previous surgeries before 2004 were reported in 22%, and occurred more commonly in patients with tumors (40%), hypothalamic hamartomas (47%), and vascular lesions (67%). U.S. centers reported more vagus nerve stimulator procedures, operated more often on children with no or subtle MRI findings, and used intracranial electrodes and functional neuroimaging more frequently than programs in Europe and Australia.

CONCLUSIONS

The results of this international survey show the feasibility of international collaborations in determining epilepsy etiologies and procedures in children, and suggest differences in clinical practice between regions of the world. These results can guide the design of future studies in producing guidelines for therapy-resistant pediatric epilepsy surgery patients.

Gene expression profile analysis of epilepsy-associated gangliogliomas

Gangliogliomas (GG) constitute the most frequent tumor entity in young patients undergoing surgery for intractable epilepsy. The histological composition of GG, with the presence of dysplastic neurons, corroborates their maldevelopmental origin. However, their histogenesis, the pathogenetic relationship with other developmental lesions, and the molecular alterations underlying the epileptogenicity of these tumors remain largely unknown. We performed gene expression analysis using the Affymetrix Gene Chip System (U133 plus 2.0 array). We used GENMAPP and the Gene Ontology database to identify global trends in gene expression data. Our analysis has identified various interesting genes and processes that are differentially expressed in GG when compared with normal tissue. The immune and inflammatory responses were the most prominent processes expressed in GG. Several genes involved in the complement pathway displayed a high level of expression compared with control expression levels. Higher expression was also observed for genes involved in cell adhesion, extracellular matrix and proliferation processes. We observed differential expression of genes as cyclin D1 and cyclin-dependent kinases, essential for neuronal cell cycle regulation and differentiation. Synaptic transmission, including GABA receptor signaling was an under-expressed process compared with control tissue. These data provide some suggestions for the molecular pathogenesis of GG. Furthermore, they indicate possible targets that may be investigated in order to dissect the mechanisms of epileptogenesis and possibly counteract the epileptogenic process in these developmental lesions.

Correlation of MRI and histopathology in epileptogenic parietal and occipital lobe lesions

INTRODUCTION

To analyze the diagnostic accuracy of MRI in patients undergoing parietal and occipital lobe epilepsy surgery.

METHODS

In a retrospective study, we analyzed MRI scans and neuropathology reports of 42 patients who had undergone resective epilepsy surgery in the parietal and occipital lobe between 1998 and 2003. We evaluated, whether lesions were precisely characterized by MRI and whether lesion characterization allowed to estimate postsurgical seizure outcome.

RESULTS

Within the categories epilepsy associated tumors, focal cortical dysplasias, vascular malformations, scarring, and others, MRI was concordant with histopathology in 36 of 42 (86%) lesions. Among the discordant lesions, one lesion was re-classified following MRI-histopathology synopsis, another two lesions represented new tumor entities (angiocentric neuroepithelial tumor, isomorphic astrocytoma) which have been described recently. Seizure freedom (Engel class I) one year following surgery was achieved in 25 patients (60%). Seizure outcome was different for lesion categories (Engel class I: epilepsy associated tumors, 62%; focal cortical dysplasias, 71%; vascular malformations, 75%; scarring, 40%), and was unchanged if no lesion was found on preoperative MRI.

CONCLUSION

If MRI and histopathology are discordant, not only the MRI findings may be debatable. MRI lesion detection is important, since chance of seizure freedom is low if no lesion is detected.

Mechanisms of epileptogenesis in tuberous sclerosis complex and related malformations of cortical development with abnormal glioneuronal proliferation

Malformations of cortical development (MCDs) are increasingly recognized as causes of medically intractable epilepsy. In order to develop more effective, rational therapies for refractory epilepsy related to MCDs, it is important to achieve a better understanding of the underlying mechanisms of epileptogenesis, but this is complicated by the wide variety of different radiographic, histopathological, and molecular features of these disorders. A subset of MCDs share a number of characteristic cellular and molecular abnormalities due to early defects in neuronal and glial proliferation and differentiation and have a particularly high incidence of epilepsy, suggesting that this category of MCDs with abnormal glioneuronal proliferation may also share a common set of primary mechanisms of epileptogenesis. This review critically analyzes both clinical and basic science evidence for overlapping mechanisms of epileptogenesis in this group of disorders, focusing on tuberous sclerosis complex, focal cortical dysplasia with balloon cells, and gangliogliomas. Specifically, the role of lesional versus perilesional regions, circuit versus cellular/molecular defects, and nonneuronal factors, such as astrocytes, in contributing to epileptogenesis in these MCDs is examined. An improved understanding of these various factors involved in epileptogenesis has direct clinical implications for optimizing current treatments or developing novel therapeutic approaches for epilepsy in these disorders.

Mutational and expression analysis of CDK1, cyclinA2 and cyclinB1 in epilepsy-associated glioneuronal lesions

Gangliogliomas and focal cortical dysplasias (FCDs) constitute glioneuronal lesions, which are frequently encountered in biopsy specimens of patients with pharmacoresistant focal epilepsy and relate to impaired differentiation and migration of neural precursors. However, their molecular pathogenesis and relationship are still largely enigmatic. Recent data suggest several components of the insulin-pathway, including TSC1 and TSC2 mutated in tuberous sclerosis complex (TSC), to be altered in gangliogliomas and FCD with Taylor type balloon cells (FCD(IIb)). The proteins tuberin (TSC2) and hamartin (TSC1) constitute a tumour suppressor mechanism involved in cell-cycle control. Hamartin and/or tuberin were reported to colocalize and/or interact with CDK1, cyclinB1 and cyclinA2 that are critically involved in cell-size and cell-growth control. Here, we have carried out mutational and expression analyses of CDK1, cyclinB1 and cyclinA2 in gangliogliomas and FCD(IIb). Mutational screening was performed by single-strand conformation polymorphism analysis in gangliogliomas (n = 20), FCD(IIb) (n = 35) and controls. CyclinB1 revealed a polymorphism (G to A, cDNA Position 966, GenBank: NM_031966) in exon 7 with similar frequencies in FCD(IIb), gangliogliomas and control specimens (FCD n = 9/35; gangliogliomas n = 5/20; control n = 20/100). We used real-time reverse transcription polymerase chain reaction to determine expression levels of CDK1, cyclinB1 and cyclinA2 in 10 FCD(IIb) and nine gangliogliomas compared with unaffected adjacent control tissue of the same patients. We observed significantly lower expression of CDK1 and cyclinA2 in FCD(IIb) vs. controls whereas no significant expression differences were present for CDK1, cyclinB1 and cyclinA2 in gangliogliomas. Our data strongly argue against mutational events of CDK1, cyclinB1 and cyclinA2 to play a role in gangliogliomas or FCD(IIb). However, a potential functional significance of lower expression for the cell-size and cell-cycle regulators CDK1 and cyclinA2 in FCD(IIb) composed of large dysplastic neurones and balloon cells needs to be further resolved.

Differential Pi3K-pathway activation in cortical tubers and focal cortical dysplasias with balloon cells

Balloon cells of distinct focal cortical dysplasias type IIb (FCD(IIb)) and giant cells of cortical tubers in tuberous sclerosis (TSC) constitute neuropathological hallmarks and cytological similarities. In TSC, frequent mutations in the TSC1 or TSC2 genes result in mTOR-signaling activity. Here, we addressed whether Pi3K-pathway activation differentiates balloon cells from giant cells. We used immunohistochemistry with antibodies against p-PDK1 (S241), p-Akt (S473), p-tuberin (T1462), p-p70(S6K) (T389), p-p70(S6K) (T229) and phalloidin-staining to analyze stress fiber formation in balloon cells of FCD(IIb) (n = 23) compared with cortical tuber giant cells (n = 5) and adjacent normal CNS tissue as control. We have further established an in vitro assay to assess potential phosphorylation between Akt and S6. We observed phosphorylated (p-)PDK1, p-Akt, p-tuberin, and p-p70-kDa S6-kinase (p-p70(S6K); residue T229) in balloon cells, whereas giant cells showed only equivalent levels of p-tuberin, p-p70(S6K) and stress fibers. Furthermore, Pi3K-cascade activity in balloon cells may reflect pathway "cross-talk". An in vitro assay revealed S6, a major target of p70(S6K), to increase phosphorylation of Akt. Our data suggest recruitment of different Pi3K-cascade factors in the molecular pathogenesis of giant cells in cortical tubers vs. balloon cells in FCD(IIb) and provides new implications for the development of treatment strategies for these cortical malformations.