Epilepsy in the mTORopathies: opportunities for precision medicine

Abstract

The mechanistic target of rapamycin signalling pathway serves as a ubiquitous regulator of cell metabolism, growth, proliferation and survival. The main cellular activity of the mechanistic target of rapamycin cascade funnels through mechanistic target of rapamycin complex 1, which is inhibited by rapamycin, a macrolide compound produced by the bacterium Streptomyces hygroscopicus. Pathogenic variants in genes encoding upstream regulators of mechanistic target of rapamycin complex 1 cause epilepsies and neurodevelopmental disorders. Tuberous sclerosis complex is a multisystem disorder caused by mutations in mechanistic target of rapamycin regulators TSC1 or TSC2, with prominent neurological manifestations including epilepsy, focal cortical dysplasia and neuropsychiatric disorders. Focal cortical dysplasia type II results from somatic brain mutations in mechanistic target of rapamycin pathway activators MTOR, AKT3, PIK3CA and RHEB and is a major cause of drug-resistant epilepsy. DEPDC5, NPRL2 and NPRL3 code for subunits of the GTPase-activating protein (GAP) activity towards Rags 1 complex (GATOR1), the principal amino acid-sensing regulator of mechanistic target of rapamycin complex 1. Germline pathogenic variants in GATOR1 genes cause non-lesional focal epilepsies and epilepsies associated with malformations of cortical development. Collectively, the mTORopathies are characterized by excessive mechanistic target of rapamycin pathway activation and drug-resistant epilepsy. In the first large-scale precision medicine trial in a genetically mediated epilepsy, everolimus (a synthetic analogue of rapamycin) was effective at reducing seizure frequency in people with tuberous sclerosis complex. Rapamycin reduced seizures in rodent models of DEPDC5-related epilepsy and focal cortical dysplasia type II. This review outlines a personalized medicine approach to the management of epilepsies in the mTORopathies. We advocate for early diagnostic sequencing of mechanistic target of rapamycin pathway genes in drug-resistant epilepsy, as identification of a pathogenic variant may point to an occult dysplasia in apparently non-lesional epilepsy or may uncover important prognostic information including, an increased risk of sudden unexpected death in epilepsy in the GATORopathies or favourable epilepsy surgery outcomes in focal cortical dysplasia type II due to somatic brain mutations. Lastly, we discuss the potential therapeutic application of mechanistic target of rapamycin inhibitors for drug-resistant seizures in GATOR1-related epilepsies and focal cortical dysplasia type II.

Keywords: GATOR1-related epilepsies; everolimus; focal cortical dysplasia type II; the mTORopathies; tuberous sclerosis complex.

Graphical Abstract

Figure 1

The mTOR cascade and its regulators. Mechanistic target of rapamycin (mTOR) combines with protein binding partners to form mTOR complex 1 (mTORC1). The downstream substrates of mTORC1 signalling include ribosomal S6 kinase (S6K) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1). Rapamycin inhibits mTORC1 signalling by binding with FK506 binding protein 1 A 12 kDa (FKBP12). Three pathways converge to regulate mTORC1 signalling. Hamartin (TSC1), tuberin (TSC2) and TBC1 domain family member 7 (TBC1D7) form a protein complex that indirectly inhibits mTORC1 signalling via Ras homologue enriched in brain (Rheb). (A) Growth factors stimulate phosphoinositide 3-kinase (PI3K) to trigger phosphoinositide-dependent kinase 1 (PDK1) to phosphorylate and activate Akt. TSC2 is repressed by Akt activation, which has a disinhibitory effect on mTORC1 signalling. Phosphatase and tensin homologue (PTEN) is a negative regulator of the PI3K-Akt pathway. (B) The energy-sensing arm is regulated by the STE20-related kinase adaptor alpha (STRADα) and liver kinase B (LKB) complex. In response to depleted ATP, the STRADα/LKB complex inhibits mTORC1 signalling by activating TSC2 via phosphorylation of adenosine monophosphate-activated kinase (AMPK). (C) The amino acid-sensing pathway is regulated by GTPase-activating protein (GAP) activity towards Rags 1 complex (GATOR1). GATOR1 is composed of three subunits: Dishevelled, Egl-10, and Pleckstrin domain-containing protein 5 (DEPDC5); nitrogen permease regulator-like 2 and 3 (NPRL2, NPRL3). The GATOR2 complex inhibits GATOR1 in response to increasing amino acid levels, resulting in mTORC1 disinhibition, facilitating pathways for cell growth. When amino acid levels are low GATOR1 directly inhibits mTORC1 activity. The KICSTOR (KPTN, ITFG2, C12orf66 and SZT2-containing regulator of mTORC1) complex scaffolds GATOR1 to the lysosomal surface (adapted from Peter B. Crino’s review with permission from Springer Nature).

Figure 2

A personalised medicine approach to the management of tuberous sclerosis complex-related epilepsy. This figure outlines a therapeutic and prognostic framework, utilizing genetic and molecular data for the management of TSC-related epilepsy. Early genetic testing for TSC1 or TSC2 mutations is recommended for infants with phenotypic features of TSC. TSC2 mutations are associated with a more severe neurological phenotype. In pre-symptomatic TSC, serial EEG monitoring is recommended, as pre-emptive vigabatrin at the onset of epileptiform abnormalities is associated with better long-term epilepsy outcomes. Evidence-based treatment options for TSC-related DRE include everolimus, CBD and tuberectomy with resection of surrounding perituberal tissue. Early treatment with everolimus in seizure-naïve TSC patients may improve long-term epilepsy and cognitive outcomes. Evidence-based recommendations are highlighted in blue and potential future applications are highlighted in gold. ASM, anti-seizure medication; CBD, cannabidiol; DRE, drug-resistant epilepsy; mTORi, mechanistic target of rapamycin inhibitor; TSC, tuberous sclerosis complex.

Figure 3

A personalised medicine approach to the management of GATOR1-related epilepsies. Early diagnostic sequencing of mTOR pathway genes is recommended in non-lesional epilepsies and in epilepsies due to FCD or HME. LoF DEPDC5 and NPRL3 variants are associated with severe epilepsy phenotypes. Treatment options for GATOR1-related DRE include epilepsy surgery and potentially, mTOR inhibitors. Evidence-based recommendations are highlighted in blue and potential future applications are highlighted in gold. FCD, focal cortical dysplasia; GATOR1, GAP activity towards Rags 1 complex; HME, hemimegalencephaly; LoF, loss-of-function; mTORi, mechanistic target of rapamycin inhibitor; SUDEP, sudden unexpected death in epilepsy.

Figure 4

A personalised medicine approach to the management of epilepsy due to FCD type II and hemimegalencephaly. Epilepsy surgery for FCD type II and HME due to pathogenic variants in mTOR pathway genes is associated with good surgical outcomes. CSF liquid biopsy represents a less invasive option for identification of somatic mutations in cases without detectable mTOR pathway germline mutations. mTOR inhibitors may offer an alternative therapeutic strategy for individuals with inaccessible lesions or persistent seizures after surgery. PIK3CA and AKT inhibitors are promising targeted therapies for individuals with PIKC3A- and AKT3-associated MCD. Evidence-based recommendations are highlighted in blue and potential future applications are highlighted in gold. FCD, focal cortical dysplasia; HME, hemimegalencephaly; MCD, malformation of cortical development; mTORi, mechanistic target of rapamycin inhibitor.