Co-expression of cyclin D1 and phosphorylated ribosomal S6 proteins in hemimegalencephaly

Abstract

Hemimegalencephaly (HMEG) is a developmental brain malformation highly associated with epilepsy. Balloon cells (BCs) and cytomegalic neurons (CNs) are frequently observed in HMEG specimens. Cytomegaly in developmental brain malformations may reXect in aberrant activation of the mTOR and -catenin signaling cascades, known regulators of cell size. We hypothesized that there is aberrant co-expression of phospho-ribosomal S6 (P-S6) protein, a downstream eVector of the mTOR cascade, as well as cyclin D1, a downstream eVector of the -catenin pathway, in BCs and cytomegalic neurons in HMEG. We hypothesized that mutations in PTEN (a cause of HMEG associated with Proteus syndrome), TSC1 or TSC2 (tuberous sclerosis complex) genes, which are known to modulate -catenin and mTOR signaling could cause sporadic HMEG. Expression of cyclin D1, phospho-p70 S6 kinase (P-p70S6K, another mTOR cascade kinase), P-S6, MAP2, NeuN, or GFAP was determined by immunohistochemistry in HMEG brain tissue (n = 7 specimens). Cyclin D1, Pp70S6K, and P-S6 proteins were co-localized in BCs and CNs in the enlarged hemisphere but not in the unaVected hemisphere or in morphologically normal tissue. Cyclin D1 and P-S6 proteins were not detected in GFAP-labeled astrocytes. Sequencing of PTEN, TSC1, and TSC2 genes in cytomegalic cells co-expressing cyclin D1 and P-S6 proteins did not reveal mutations. Selective expression of cyclin D1and P-S6 in cytomegalic cells in HMEG suggests co-activation of the -catenin and mTOR cascades. PTEN, TSC1, or TSC2 gene mutations were not detected suggesting that sporadic HMEG is distinct from HMEG associated with Proteus syndrome or tuberous sclerosis complex.

mTOR Signaling in Epilepsy: Insights from Malformations of Cortical Development

“…Enhanced mTOR signaling in HME was first shown in BCs and DNs (Ljungberg et al 2006;Aronica et al 2007). In both studies, the specimens did not show enhanced mTOR activation in every cell (mTOR activation was observed in DNs and BCs), and, thus, it was postulated that HME forms as a consequence of somatic mutations in mTOR regulatory genes during brain development (Fig.…”

“…Enhanced mTOR signaling was first identified in FCDIIB (Baybis et al 2004;Miyata et al 2004), and sets the stage for subsequent studies (see below) showing mTOR activation in HME (Ljungberg et al 2006;Aronica et al 2007) and GG (Samadani et al 2007). Phospho-p70S6K and phospho-S6 isoforms were detected by immunohistochemistry in resected FCDIIB specimens.…”

Epilepsy Related to Developmental Tumors and Malformations of Cortical Development

“…Recent studies point to the role of 2 converging cell signaling pathways (Wnt/β-catenin and mTOR) in the pathogenesis of HME [245,[249][250][251]. The identification of mTOR signaling overactivation in HME (as in TSC and FCD IIb) suggests a pathogenic link between these malformations, possibly representing a spectrum of disorders of mTOR signaling (so-called "TORopathies") [118,252,253].…”

Somatic Mutations in TSC1 and TSC2 Cause Focal Cortical Dysplasia

“…In addition, TSC (MIM: 191100) and hemimegalencephaly (HME) are known to share several pathological characteristics with FCDII, such as cytomegalic neurons and epilepsy, and to be linked to aberrantly hyperactivated mTOR signaling. [13][14][15][16] Interestingly, TSC and HME are caused by germline or somatic mutations in upstream regulators of mTOR kinase, including PIK3CA (MIM: 171834), PIK3R2 (MIM: 603157), AKT3 (MIM: 611223), TSC1…”