Abstracts

Rationale: In patients with Tuberous Sclerosis Complex (TSC), there are phenotypic differences between those with TSC1 and TSC2 mutations. Although TSC gene products hamartin and tuberin function together as a dimer regulating the PI3K pathway, there is a divergence in their regulation. Cdk1 and IKKβ phosphorylate hamartin, while Erk, Akt, MK2, AMPK and RSK1 phosphorylate tuberin. Thus, in vivo factors such as inflammation may have differential effects in patients with TSC1 and TSC2 mutations. Cortical tubers express large numbers of activated microglia, macrophages and T lymphocytes, thus suggesting activation of inflammatory pathways. Under conditions which cause activation of inflammatory pathways, indoleamine 2,3-dioxygenase expression (IDO, the rate limiting enzyme of tryptophan metabolism by the kynurenine pathway in brain) is induced by IFN-γ. Major vault protein (MVP), a protein associated with multidrug resistance, has been found in tubers, and is also induced by IFN-γ. Thus, the effect of inflammation in epileptogenic tubers on induction of IDO and MVP might differ between patients with TSC1 and TSC2 mutations. In this study, we test this hypothesis by measuring tryptophan metabolism in vivo with α[11C]-methyl-L-tryptophan (AMT) positron emission tomography (PET) and expression of IDO and MVP by western blot in tubers resected for control of epilepsy in patients with TSC1 and TSC2 mutations. Methods: Twelve children (9 m-16 y) with TSC underwent epilepsy surgery to remove epileptogenic tubers. Subgroup analysis was performed based on the data available (see Table 1). Ten patients had genetic testing (TSC1 N = 4, TSC2 N = 5, and 1 had neither mutation). Nine patients had both epileptogenic tuber and a relatively normal cortical area (control region) resected. Dynamic AMT PET data were fit to a kinetic model to estimate the unidirectional uptake rate constant (K complex) for AMT in tuber and control regions. Western blotting was performed on surgical specimens with antibodies against MVP, IDO, and actin. Densitometry of IDO and MVP levels was normalized to actin levels. Results: Tryptophan metabolism was elevated in tubers as evidenced by elevated K-complex in tubers compared to control regions in 7/12 patients (0.0077 vs. 0.0062 mL/g/min, p = 0.004, paired t-test). Tissue from two patients analyzed by western blot showed that expression of IDO in tubers was higher than in control regions (Fig 1A). Analysis of tissue from nine patients showed that tubers had higher MVP compared to control regions (0.87 vs. 0.61 a.u., p = 0.007, Fig 1B). Furthermore, MVP expression was higher in tubers of the TSC2 group compared to tubers from TSC1 patients (0.93 vs. 0.71 a.u., p = 0.04). Conclusions: Increased tryptophan metabolism measured in vivo is related to increased expression of IDO in resected epileptogenic tubers. Higher expression of MVP in patients with TSC2 may be related to a more severe course of the disease compared to patients with TSC1. IDO and MVP might represent novel therapeutic targets for patients with TSC and may be individualized for type of mutation.