Abnormal Paraplegin Expression in Swollen Neurites, τ- and α-Synuclein Pathology in a Case of Hereditary Spastic Paraplegia SPG7 with an Ala510Val Mutation

Abstract

Mutations in the SPG7 gene are the most frequent cause of autosomal recessive hereditary spastic paraplegias and spastic ataxias. Ala510Val is the most common SPG7 mutation, with a frequency of up to 1% in the general population. Here we report the clinical, genetic, and neuropathological findings in a homozygous Ala510Val SPG7 case with spastic ataxia. Neuron loss with associated gliosis was found in the inferior olivary nucleus, the dentate nucleus of the cerebellum, the substantia nigra and the basal nucleus of Meynert. Neurofilament and/or paraplegin accumulation was observed in swollen neurites in the cerebellar and cerebral cortex. This case also showed subcortical τ-pathology in an unique distribution pattern largely restricted to the brainstem. α-synuclein containing Lewy bodies (LBs) were observed in the brainstem and the cortex, compatible with a limbic pattern of Braak LB-Disease stage 4. Taken together, this case shows that the spectrum of pathologies in SPG7 can include neuron loss of the dentate nucleus and the inferior olivary nucleus as well as neuritic pathology. The progressive supranuclear palsy-like brainstem predominant pattern of τ pathology and α-synuclein containing Lewy bodies in our SPG7 cases may be either coincidental or related to SPG7 in addition to neuron loss and neuritic pathology.

Keywords: Lewy bodies; SPG7; ataxia; coiled bodies; neurofibrillary tangles; paraplegin; spastic ataxia; spastic paraplegia; tau.

Figure 1

Midsagittal T1 weighted MRI of the SGP7 index patient at age 70 years. The MRI shows mild vermian cerebellar atrophy (arrows).

Figure 2

Pedigree (a) and electropherograms (b) of the p.Ala510Val SPG7 family. The p.Ala510Val SPG7 variant was observed in a homozygous state in both affected subjects for whom DNA was available, namely in the index patient (arrow) and his sister. She was similarly affected with progressive spastic-ataxic gait disorder and cPEO, starting at age 50 years. The unaffected children of the index patient carried this mutation in a heterozygous state. Pedigree symbols for a: squares: male; circle: female; diamond: gender not specified; black filled symbols: affected by disease; white symbols: healthy; symbols cross by a line: deceased. Color codes for b: G (guanin): black, T (thymine): red, C (cytosine): blue, A (adenine): green.

Figure 3

Moderate neuron loss and gliosis in the inferior olivary nucleus (ION) (arrows in a,b) and in the dentate nucleus (dentate nucl.) of the SPG7 case (arrows in c,d); a,c show overviews of the ION (a) and the dentate nucleus (c), whereas at increased magnification (b,d) a moderately reduced neuron frequency could be observed when comparing with control cases as depicted for control case number three (e,f). Arrows in e,f indicate normal neuron densities in ION (e) and the dentate nucleus (f). Calibration bar in f valid for a: 320 µm; c: 710 µm; b,d–f: 130 µm.

Figure 4

Paraplegin expression in the frontal neocortex of SPG7 case (a) and of a non-diseased control (case No. 2) (b); (c) depicts the negative control for the SPG7 case by omitting the primary antibody to document the specificity of the immunostainings. Despite an overall increased staining intensity in the SPG7 case that can be explained by varying staining intensities, neurons were slightly stained in both the SPG7 case and in the control (arrowheads in a,b), whereas neurites exhibited paraplegin only in the SPG7 case (arrows in a) and not in the healthy control brain. Similarly, neurites exhibit paraplegin in the Purkinje cell layer of the cerebellum in the SPG7 case (arrows in d) but not in the control (case number four) (e); Here only the perikarya of the Purkinje cells were labeled mildly (e); The presence of paraplegin positive neurites in the cerebellum was associated with high numbers of neurites accumulating 68 kDa neurofilaments. Some of these neurites appear swollen (arrows in f); In the control (case number four) there was no neuritic accumulation of the 68 kDa neurofilament protein (g); Calibration bar in c valid for a–c: 35 µm; d,e: 40 µm; f,g: 50 µm.

Figure 5

(a) Lewy-bodies (arrows) and Lewy neurites (arrowhead) are detected in the locus coeruleus with the anti-α-synunclein (α-syn) antibody; (b,c) NFTs are detected in the neurons of the substantia nigra with an antibody directed against abnormal phosphorylated τ-protein (arrow in b) as well as with the Gallyas silver method (arrow in c). Neuropil threads were also detected (arrowhead in c); (d,e) the abnormal τ-protein aggregates in the substantia nigra nerve cells (arrowhead in d) and in neuropil threads contained 4rp τ (arrows in d) but no 3rp τ (e); Lewy bodies (red arrows in d) occurring inside τ-positive NFTs (arrowhead in d) did not exhibit immunoreactivity with anti-τ antibodies such as anti-rp4 τ; (f) coiled bodies were detected with an antibody raised against abnormal phosphorylated τ-protein in the globus pallidum (arrows); (g) neuronal silver-stained inclusions (arrow) as well as neuropil threads (arrowheads) also occurred in the cerebellar dentate nucleus; (h) single glial inclusions with a tufted astrocyte-like pattern were detected in the putamen by Gallyas silver staining; (i) NFTs (arrowhead) and LBs (large and small arrows) were also detectable with an antibodies directed against p62 as documented in the substantia nigra; (j) Although paraplegin was detected in neurites (arrowheads) and neurons of the substantia nigra (arrow), Lewy bodies were not specifically labeled (red arrow); (k) neuritic accumulation of paraplegin was also observed in the dentate nucleus; and (l) antibodies against the 68 kDa subunit of neurofilaments also showed some neurites with neurofilament accumulation. Calibration bar in f valid for a–f, i–l: 30 µm; g,h: 15 µm.