Neuropathology of FMR1-premutation carriers presenting with dementia and neuropsychiatric symptoms

Abstract

CGG repeat expansions within the premutation range (55-200) of the FMR1 gene can lead to Fragile X-associated tremor/ataxia syndrome and Fragile X-associated neuropsychiatric disorders. These CGG repeats are translated into a toxic polyglycine-containing protein, FMRpolyG. Pathology of Fragile X-associated tremor/ataxia syndrome and Fragile X-associated neuropsychiatric disorders comprises FMRpolyG- and p62-positive intranuclear inclusions. Diagnosing a FMR1-premutation carrier remains challenging, as the clinical features overlap with other neurodegenerative diseases. Here, we describe two male cases with Fragile X-associated neuropsychiatric disorders-related symptoms and mild movement disturbances and novel pathological features that can attribute to the variable phenotype. Macroscopically, both donors did not show characteristic white matter lesions on MRI; however, vascular infarcts in cortical- and sub-cortical regions were identified. Immunohistochemistry analyses revealed a high number of FMRpolyG intranuclear inclusions throughout the brain, which were also positive for p62. Importantly, we identified a novel pathological vascular phenotype with inclusions present in pericytes and endothelial cells. Although these results need to be confirmed in more cases, we propose that these vascular lesions in the brain could contribute to the complex symptomology of FMR1-premutation carriers. Overall, our report suggests that Fragile X-associated tremor/ataxia syndrome and Fragile X-associated neuropsychiatric disorders may present diverse clinical involvements resembling other types of dementia, and in the absence of genetic testing, FMRpolyG can be used post-mortem to identify premutation carriers.

Keywords: FMR1-premutation; FXAND; FXTAS; neuropathology; nuclear inclusions.

Graphical Abstract

Figure 1

Clinical course and imaging results of Donors 1 and 2. Clinical course (A) shows moment in time of MRI. General mild atrophy is observed (B, F). Ventricles are slightly dilated and Donor 1 shows periventricular white matter lesions (C), Donor 2 shows an increased signal due to an infarct (G). Slight cerebellar atrophy can be seen in both donors (D, E, H, I). Enlargement of the fourth ventricle, but no FXTAS-characteristic middle cerebral peduncle sign can be seen. T2-fluid-attenuated inversion recovery of Donor 2 does not show increased insular and middle cerebral peduncle signal intensities (J, K), no abnormalities in vasculature (K) and no thinning of the corpus callosum (M). CVA, cerebrovascular accident; PNP, polyneuropathy; VD, vascular dementia; PD, Parkinson’s disease; AD, Alzheimer’s disease; CBD, corticobasal degeneration; FTD, frontotemporal dementia; PSP, progressive supranuclear palsy.

Figure 2

p62-Pathology in Donor 2. Abundant p62-positive nuclear inclusions in the frontal cortex grey matter (A, B) and white matter (C, D). The hippocampus shows nuclear inclusions in the CA4 neuron (white arrowhead) and the dentate gyrus (E, F). In the cerebellum, inclusions are seen in the white matter and granular layer (G, H). No inclusions were seen in the Purkinje cells, whereas most Bergmann glia showed inclusions (H). Pathology was also seen in the dopaminergic neurons (white arrowhead) and astrocytes (black arrowhead) in the substantia nigra (I), ependymal (J), extra-axial cells of the third nerve (K) and choroid plexus cells (L). Scale bar A is 50 μm; Scale bar B, D, F, H, I–L is 10 μm; scale bar C, E, G is 100 μm.

Figure 3

Vascular pathology of Donors 1 and 2. Brown iron-like deposits are seen in both donors (A, E), which stain blue in a Perls staining (B, F). Both donors showed abnormal increased iron deposits in the ventral part of the substantia nigra (C, G). Throughout the brain, inclusions were seen in the endothelial cells (black arrowhead) and the pericytes (white arrowhead) (D, H). Scale bar A, B, D, E, F, H is 10 µm; scale bar C, G is 0.5 cm.

Figure 4

Figure 4 FMRpolyG-positivity in Donor 2. FMRpolyG (1C5)-positive inclusions are seen in the frontal cortex (A), hippocampus (B), substantia nigra (C) and cerebellum (D). Inclusions are indicated with white arrowheads. Co-localization of p62 and FMRpolyG (NTF1) immunopositivity in the Bergmann glia of the cerebellum in Donor 2 and spinocerebellar ataxia 3 (SCA3) donor. Bergmann glia shown with DAPI (E, I) showed inclusions with p62 (F, J) and FMRpolyG (G, K). Co-localization of the immunopositivity shows overlap in FXAND (H) and not in SCA3 (L). Scale bar is 10 μm.