New insights into a spectrum of developmental malformations related to mTOR dysregulations: challenges and perspectives

Abstract

In recent years the role of the mammalian target of rapamycin (mTOR) pathway has emerged as crucial for normal cortical development. Therefore, it is not surprising that aberrant activation of mTOR is associated with developmental malformations and epileptogenesis. A broad spectrum of malformations of cortical development, such as focal cortical dysplasia (FCD) and tuberous sclerosis complex (TSC), have been linked to either germline or somatic mutations in mTOR pathway-related genes, commonly summarised under the umbrella term 'mTORopathies'. However, there are still a number of unanswered questions regarding the involvement of mTOR in the pathophysiology of these abnormalities. Therefore, a monogenetic disease, such as TSC, can be more easily applied as a model to study the mechanisms of epileptogenesis and identify potential new targets of therapy. Developmental neuropathology and genetics demonstrate that FCD IIb and hemimegalencephaly are the same diseases. Constitutive activation of mTOR signalling represents a shared pathogenic mechanism in a group of developmental malformations that have histopathological and clinical features in common, such as epilepsy, autism and other comorbidities. We seek to understand the effect of mTOR dysregulation in a developing cortex with the propensity to generate seizures as well as the aftermath of the surrounding environment, including the white matter.

Keywords: epilepsy; epileptogenesis; focal cortical dysplasia; mTORopathies; malformations; rapamycin; tuberous sclerosis complex.

Figure 1

A schematic overview of the mTOR pathway. A schematic overview of the mTORC1 signalling pathway showing the proteins that are affected by mutations of different mTORpathies (FCD, TSC, megalencephaly and hemimegencephaly) as summarised in Table 1. Mutations related to FCD are indicated with a red star, mutations related to TSC with a blue star, mutations related to megalencephaly with a light green star, and mutations related to hemimegencephaly with a dark green star. IRS1, insulin receptor substrate 1; PI3K, PI3kinase; PDK1, phosphoinositide‐dependent kinase‐1; PTEN, phosphatase and tensin homologue; AKT, protein kinase B; BRAF, v‐raf murine sarcoma viral oncogene homolog B1; MEK, mitogen activated protein kinase; ERK, extracellular signal‐regulated kinase; LKB1, tumor suppressor liver kinase B1; STRADα, STE20‐related kinase adaptor alpha; AMPK, AMP‐activated protein kinase; TBC1D5, TBC1 Domain Family Member 5; RHEB, ras homolog enriched in brain; mTORC1, mammalian target of rapamycin complex 1; DEPDC5, DEP Domain Containing 5; NPRL2, NPR2 Like, GATOR1 Complex Subunit; NPRL3, NPR3 Like, GATOR1 Complex Subunit; GATOR1, Gap Activity TOward Rags 1; S6K1, p70S6kinase; S6, ribosomal S6 protein; 4EBP1, eIF4E‐binding protein 1; eIF4E, binding of eukaryotic translation.

Figure 2

Lesion differences in mTORopathies. (A) A schematic overview showing the concept of mosaicism in the brain where somatic mutations can occur during development with or without an underlying germline mutation. Somatic mutations that appear during early development (in blue) can lead to hemimegalencephaly (in blue). Somatic mutations that occur later in development (in purple) or second‐hit mutations (in green) that occur when a germline mutation is present (in orange) can lead to focal lesions (indicated in purple and green). Both time and location of the somatic mutations can influence the affected region and size of the brain. (B) A schematic overview showing both the location and the histological characteristics of TSC tubers, FCD and SEGAs.

Figure 3

Macroscopy and histology in mTORopathies. (A–K) Tuberous sclerosis complex (TSC). (A) MRI showing a TSC lesion (histologically proven; arrow). (B–D) Coronal sections of the brain (32‐year‐old patient with a germline mutation in the TSC2 gene; Boer et al. 2008a; Aronica et al. 2012a) showing several regions with blurring of the cortex/white matter junction (arrows in B and C) and subependymal nodules (arrowheads in B and arrows in D). (B,C) A high magnification of abnormal brain regions with loss of a distinct cortex/white matter junction and mushroom‐like appearance of the gyri, indicating the presence of tubers. (D) A high magnification of the SENs which appear as firm oval‐shaped structures projecting into the ventricles. (E,F) Histological stains: Luxol fast blue‐PAS‐staining (E) and haematoxylin and eosin (H&E; panel F), showing cortical tubers (arrows) and subependymal nodules (arrowheads; Boer et al. 2008a; Aronica et al. 2012a). (G) phospho‐S6 ribosomal protein (pS6) staining positive in giant cells of a tuber (Prabowo et al. 2013). (H,I) NeuN staining showing difference in the architecture between the perituberal cortex (H) and the tuberal cortex (I) with dyslamination within the cortical tuber. (J,K) H&E staining of a tuber (J) and SEGA (K). Tuber showing large dysmorphic neurones (arrows), calcification (arrowheads) and a giant cell in insert (Boer et al. 2008a; Aronica et al. 2012a). SEGA showing giant cells (arrowheads) with a mixed glial background and blood vessels (Bongaarts et al. 2017). (L–S) Focal cortical dysplasia (FCD). (L) Coronal MRI image revealing a focal lesion at the left frontal lobe (arrow). (M) Coronal section of the brain showing a region with blurring of the cortex/white matter junction (indicated with arrow). (N) SMI32 staining showing accumulation of non‐phosphorylated neurofilaments (indicated with arrow). (O) H&E staining of FCD showing dysmorphic neurones (arrow) and balloon cells (asterisk). (P) Vimentin staining showing balloon cells (arrow and insert). (Q) Higher magnification of (N) showing accumulation of non‐phosphorylated neurofilaments (SMI32 antibody; arrows indicate dysmorphic neurones). (R,S) NeuN staining showing difference in the architecture between the control cortex (R) and FCDIIb cortex (S) with cortical dyslamination. (T–W) Hemimegalencephaly (HME; Boer et al. 2008a; Aronica et al. 2012a). (T) Coronal section showing diffuse and severe hypertrophy of the left hemisphere with lateral ventricular dilation, periventricular cysts, pachygyria and thickened cortex. (U) Loss of lamination with irregular distribution of neuronal cells and clusters of NeuN‐positive elements in Layer I. (V,W) Nissl staining showing the difference in the architecture of the control cortex (‘normal’ side; V) and the affected left side (HME; W). HME (W and insert) show severe cortical disorganisation with loss of lamination and presence of cytomegalic neurones (insert in W). Scale bars: (G) 60 μm, (H/I) 250 μm, (J/K) 40 μm, (O–Q) 50 μm, (R,S) 200 μm, (U) 320 μm, (W) 200 μm. (M,L) Courtesy of T. Veersema and K. Braun, University Medical Centre Utrecht, Utrecht, The Netherlands.