An Investigation of the Expression of G₁-Phase Cell Cycle Proteins in Focal Cortical Dysplasia Type IIB

Abstract

Balloon cells (BCs) are the pathologic hallmark of focal cortical dysplasia type IIB, a common cause of pharmacoresistent epilepsy. Expression of markers of cell immaturity and of the proliferation marker minichromosome maintenance protein 2 (mcm2) have been previously shown in BCs, suggesting that these cells might represent a pool of less-differentiated cells licensed for replication. An alternative explanation is that these cells are the remnants of early cortical plate cells that have failed to differentiate or to be eliminated during development and are arrested in the cell cycle, a hypothesis that this study aims to explore. Using immunohistochemical methods and semiquantitative analysis in 19 cases of focal cortical dysplasia (ages 1Y81 years), we studied the expression of cell cycle proteins important either in regulating progression through the G 1 phase or inducing cell arrest and promoting premature senescence. Only a small fraction of BCs expressed geminin, suggesting that few BCs enter the S phase or complete the cell cycle. Variable expression of nonphosphorylated retinoblastoma protein (Rb), cdk4, and p53 was noted in BCs. Cyclin E, D1, cdk2, phosphorylated Rb (795 and 807/ 811), and checkpoint 2 expression levels were low in BCs. These findings suggest early rather than late G 1 arrest. Cell senescence could be induced by an undefined cerebral insult during development or alternatively represent a physiologic replicative senescence. These findings also suggest that dysregulation of cell cycle pathways may occur in focal cortical dysplasia, which opens further areas for exploration as potential new treatment avenues.

The application of cortical layer markers in the evaluation of cortical dysplasias in epilepsy

“…The striking Pax6/GFAP co-expression we observed in BC supports their origin from radial glial cells as previously proposed [10, 32]. A proliferative capacity of BC has also been demonstrated [55], although many appear to be arrested in G1 phase of the cell cycle [54]. Pax6 regulates exit from the cell cycle, controlling cortical cell number [45].…”

Basic Mechanisms of Epileptogenesis in Pediatric Cortical Dysplasia

“…Intriguingly, they also express the vesicular glutamate transporter VGLUT2 [116], as well as the primarily astrocytic markers EAAT2/GLT1 [118]. Expression of undifferentiated cell markers suggests that balloon cells suffered an arrest in development and differentiation [119,120]. Further, the chloride transporters NKCC1 and KCC2 also are differentially expressed.…”

Characteristic expression of p57/Kip2 in balloon cells in focal cortical dysplasia

“…It has been hypothesized that balloon cells may be arrested in cell cycle G1, based on the findings that they frequently express G1 markers in their nuclei. 13 Concerning the cell cycle, a biological study demonstrated that p57/Kip2 was strongly expressed in association with the late G1 phase of developing cells of the CNS. 20 Therefore, the p57/Kip2 selected expression in the balloon cells in the present study appears compatible with the notion that they are arrested in G1.…”

“…It has been hypothesized that balloon cells may be arrested in cell cycle G1, based on the findings that they frequently express G1 markers in their nuclei . Concerning the cell cycle, a biological study demonstrated that p57/Kip2 was strongly expressed in association with the late G1 phase of developing cells of the CNS .…”

“…Another morphological and electrophysiological study of FCD IIb has also proposed that balloon cells represent radial glial cells that have failed to undergo postnatal degeneration . Moreover, an immunohistochemical study examining the expression of cell cycle proteins, including non‐phosphorylated retinoblastoma protein, cdk4 and p53, in balloon cells in FCD IIb has hypothesized that these cells are the remnants of early cortical cells, which have failed to undergo differentiation or elimination during development and have undergone cell cycle arrest . Thus, dysregulation of cell cycle proteins may be a pivotal mechanism underlying the dysplastic features of balloon cells, including abnormal cell fate determination, cell immaturity, and subsequent abnormal morphology.…”