Tuberous sclerosis complex is associated with a novel human tauopathy

Abstract

Tuberous sclerosis complex (TSC) is a neurogenetic disorder leading to epilepsy, developmental delay, and neurobehavioral dysfunction. The syndrome is caused by pathogenic variants in TSC1 (coding for hamartin) or TSC2 (coding for tuberin). Recently, we reported a progressive frontotemporal dementia-like clinical syndrome in a patient with a mutation in TSC1, but the neuropathological changes seen in adults with TSC with or without dementia have yet to be systematically explored. Here, we examined neuropathological findings in adults with TSC (n = 11) aged 30-58 years and compared them to age-matched patients with epilepsy unrelated to TSC (n = 9) and non-neurological controls (n = 10). In 3 of 11 subjects with TSC, we observed a neurofibrillary tangle-predominant "TSC tauopathy" not seen in epilepsy or non-neurological controls. This tauopathy was observed in the absence of pathological amyloid beta, TDP-43, or alpha-synuclein deposition. The neurofibrillary tangles in TSC tauopathy showed a unique pattern of post-translational modifications, with apparent differences between TSC1 and TSC2 mutation carriers. Tau acetylation (K274, K343) was prominent in both TSC1 and TSC2, whereas tau phosphorylation at a common phospho-epitope (S202) was observed only in TSC2. TSC tauopathy was observed in selected neocortical, limbic, subcortical, and brainstem sites and showed a 3-repeat greater than 4-repeat tau isoform pattern in both TSC1 and TSC2 mutation carriers, but no tangles were immunolabeled with MC1 or p62 antibodies. The findings suggest that individuals with TSC are at risk for a unique tauopathy in mid-life and that tauopathy pathogenesis may involve TSC1, TSC2, and related molecular pathways.

Keywords: Acetylation; Neurofibrillary tangle; TSC1; TSC2; Tau; Tauopathy; Tuberous sclerosis complex (TSC).

Fig. 1. TSC tauopathy is a neurofibrillary tauopathy with prominent tau acetylation

Immunohistochemical staining with CP13 and MAB359 antibodies in TSC subjects carrying pathogenic variants in TSC1 (TS 6, a-b, e-f, i-j, m-n, q-r, u-v) and TSC2 (TS 10, c-d, g-h, k-l, o-p, s-t, w-x). Representative images of immunostaining for CP13 and MAB359 in inferior frontal gyrus (IFG, a-d), superior frontal gyrus (SFG, e-h), middle insula (mINS, i-l), amygdala (Amy, m-p), hippocampal CA2 (Hip-CA2, q-t) and temporal pole (TP, u-x) are shown, highlighting neurofibrillary tangles (arrowheads) in the insets. All sections were counterstained with hematoxylin. Scale bars in panel x are 100 µm and 25 µm (inset) and apply to all images.

Fig. 2. TSC tauopathy most prominently involves neocortical and paralimbic fronto-temporal-insular regions, ventral striatum, and amygdala

Heat map represents MAB359 (ac-tau, K274) immunoreactive NFT density averaged, region-wise, across the three subjects (TS 6, 7, and 10) with TSC tauopathy. The pattern emphasizes cortical, subcortical, and limbic regions and does not conform to the Braak NFT staging system. Figures are adapted mainly from sections 9, 18, 32, 54, and 82 of the Allen Human Brain Reference Atlas (atlas.brain-map.org), and from plates 10, 22, 30, and 34 of Olszewski and Baxter’s Cytoarchitecture of the Human Brainstem.

Fig. 3. TSC tauopathy shows a unique pattern of immunohistochemical staining characteristics

Immunohistochemical staining performed on inferior frontal gyrus with various tau antibodies and p62 is shown in subjects with TSC tauopathy (TS 6 and TS 10). Representative images highlight immunostaining for 3R tau (RD3, a-e), 4R tau (RD4, f-i), p-tau S396–404 (PHF1, j-m), ac-tau K343 (acK343, n-r), conformational tau (MC1, s-u) and p62 (v-x). Neither TSC1 nor TSC2 subjects showed MC1 (s and t) or p62 (v and w) immunoreactivity. Positive controls are shown for MC1 (u, inferior temporal gyrus in a subject with AD) and p62 (x, cerebellum in a subject with C9orf72-FTD, to capture p62-positive dipeptide repeat protein inclusions). Boxes in panels u and x are magnified in insets. All sections were counterstained with hematoxylin. Scale bar: 100 µm in panel x and applies to all other images, 25 µm in panel r and all other insets (u, x).

Fig. 4. TSC1 and TSC2 variants show differing tau phosphorylation and acetylation in TSC

Immunoblots performed on frozen tissue homogenates from inferior frontal gyrus tissue of control, non-TSC epilepsy (EP 6, EP 9), and TSC subjects with TSC1 (TS 6) or TSC2 mutations (TS 7, TS 10). Antibodies used include CP13, PHF1, and MAB359 and were normalized to total protein stain. Ac-tau and, to a lesser extent, p-tau, was more abundant in subjects with TSC tauopathy compared with other groups.