The pathophysiology of tuberous sclerosis complex

Abstract

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder that results from mutations in the TSC1 or TSC2 genes (Crino et al., 2006). Although skin, kidney, heart, eye, and lung can be affected, brain involvement is associated with the most significant patient morbidity. TSC is highly associated with epilepsy that is often medically intractable, as well as cognitive disability and autism. Focal developmental malformations of the cerebral cortex known as tubers are identified in more than 80% of individuals with TSC (Fig. 1). These occur as either single or multiple lesions and are believed to form between weeks 8 and 20 of human gestation; fetal magnetic resonance imaging (MRI) has demonstrated tubers by 19 weeks of gestation. The cellular components of tubers include dysmorphic neurons, giant cells, and enhanced numbers of astrocytes. Tubers are widely believed to represent the neuropathologic substrates for neurologic disease in TSC. However, there is a growing body of evidence to suggest that more subtle neuropathologic changes present throughout the brain may also contribute to the neurologic features of TSC. Top left, fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) showing cortical tubers (arrows). Middle, postmortem specimen showing surface anatomy of a tuber (arrow); right, immunohistochemical labeling of tuber section with antibodies recognizing phosphorylated S6 protein in giant cells (arrows). Note loss of cortical lamination within tubers. Recent translational investigations (for reviews see, Huang & Manning, 2008; Dunlop & Tee, 2009) have demonstrated that the TSC1 and TSC2 encoded proteins bind as cytoplasmic heterodimers and act to inhibit the activity of the serine kinase mammalian target of rapamycin (mTOR; Fig. 1). Select posttranslational modifications of TSC1 and TSC2, for example, phosphorylation, can lead to protein activation or inhibition. In the setting of nutrient and growth factor, for example, insulin-like growth factor (IGF1), stimulation, TSC2 is phosphorylated and releases mTOR inhibition, thereby permitting mTOR-mediated phosphorylation of several downstream proteins including S6Kinase, S6, and 4E-BP-1, as well as facilitating cellular growth via effects on protein translation and to a lesser extent on gene transcription through signal transducers and activators of transcription (STAT3) and myc. In TSC, loss of function mutations leads to constitutive mTOR kinase activity and unregulated cell growth. Indeed, it is widely believed that hyperactive mTOR signaling is associated with enhanced cell size and increased cell proliferation characteristic of lesions in TSC. Several labs have shown that cells in tubers and subependymal giant cell astrocytomas exhibit robust phosphorylation of S6 protein in keeping with hyperactive mTOR signaling (Baybis et al., 2004; Chan et al., 2004). There are approximately 700 allelic mutant TSC1 and TSC2 gene variants that exhibit variable penetrance and pleiotropy. Furthermore, a clear genotype–phenotype correlation has not been established, although in general patients with TSC2-associated disease may be more severely affected. Understanding epilepsy in TSC remains a challenge. Studies examining neurotransmitter receptor subunit expression in tuber specimens have demonstrated cell specific alterations in α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) and N-methyl-d-aspartate (NMDA) subunit expression (White et al., 2001; Talos et al., 2008). Slice recordings of resected tubers reveal enhanced excitability and epileptiform discharges. However, the early events that lead to seizure onset and in particular infantile spasms in TSC remain to be defined. The development of at least four conditional TSC mouse mutant strains over the last decade has provided investigators with several models to study abnormal brain development and epilepsy in TSC. These knockout strains variably exhibit abnormal neurogenesis, enhanced astrocyte proliferation, and spontaneous seizures. The Tsc1GFAPcre mouse exhibits spontaneous seizures by 3–4 weeks of age, and preclinical studies have shown that daily administration of rapamycin, a highly selective mTOR antagonist, shortly after birth prevents the onset of spontaneous seizures (Zeng et al., 2008). These findings provide proof-of-principle evidence that rapamycin may be effective in patients with TSC, and clinical trials are in progress. Recent work in our lab has demonstrated that focal knockdown of Tsc2 in the developing mouse brain leads to significant alterations in cortical lamination within a restricted cortical region, which can be studied as a model of human tubers. Other major developments in TSC research have been the identification that TSC1 and TSC2 contribute to dendritogenesis and dendritic spine formation in hippocampal neurons in an mTOR-dependent fashion (Tavazoie et al., 2005) and that the TSC proteins facilitate establishment of axonal polarization (Choi et al., 2008). Another recent development has been the demonstration that other signaling cascades including mitogen activated protein kinase (MAPK), vascular endothelial growth factor (VEGF), and epidermal growth factor receptor (EGFR) may be activated in TSC, suggesting potentially new pathways for therapy development. Indeed, the identification of activated proinflammatory cytokines in resected tubers by several labs also suggests that other pathways aside from mTOR may be activated in TSC. A growing body of evidence now suggests that there may be structural abnormalities in the TSC brain that are subtle and distinct from tubers. Both MRI-based and histopathologic analyses suggest that minor changes in the subcortical white matter and subcortical structures, such as the thalamus and cerebellum, may contribute to neuropsychological manifestations of TSC including autism (Ridler et al., 2001; Boer et al., 2008). In support of this finding, behavioral studies in the TSC2 heterozygous mouse reveal selective deficits despite a paucity of anatomic abnormalities. It is hoped that future research will define the roles of the TSC1 and TSC2 proteins in neural progenitor cell development and cortical lamination. There is a clear need to understand the cellular and molecular mechanisms leading to seizures so that new treatment approaches can be formulated. Department of Defense CDMRP TSC Initiative and NINDS NS045021. The author has no conflict of interest to disclose.