Clinical and pathologic findings in hereditary spastic paraparesis with spastin mutation

Neuroimaging in Hereditary Spastic Paraplegias: Current Use and Future Perspectives

Hereditary spastic paraplegias (HSP) are a large group of genetic diseases characterized by progressive degeneration of the long tracts of the spinal cord, namely the corticospinal tracts and dorsal columns. Genotypic and phenotypic heterogeneity is a hallmark of this group of diseases, which makes proper diagnosis and management often challenging. In this scenario, magnetic resonance imaging (MRI) emerges as a valuable tool to assist in the exclusion of mimicking disorders and in the detailed phenotypic characterization. Some neuroradiological signs have been reported in specific subtypes of HSP and are therefore helpful to guide genetic testing/interpretation. In addition, advanced MRI techniques enable detection of subtle structural abnormalities not visible on routine scans in the spinal cord and brain of subjects with HSP. In particular, quantitative spinal cord morphometry and diffusion tensor imaging look promising tools to uncover the pathophysiology and to track progression of these diseases. In the current review article, we discuss the current use and future perspectives of MRI in the context of HSP.

Resting state fMRI studies in SPG4-linked hereditary spastic paraplegia

OBJECTIVE

The study aimed to investigate the functional alterations of spontaneous brain activity in patients with spastic paraplegia type 4 (SPG4), and the relationship with the severity of spasticity.

METHODS

Twelve patients with SPG4 and ten healthy controls underwent resting-state functional magnetic resonance imaging (rs-fMRI). Amplitude of low-frequency fluctuation (ALFF) and regional homogeneity (ReHo) were used to characterize regional neural function, and functional connectivity (FC) was used to evaluate the functional integration of the brain network.

RESULTS

Compared to healthy controls, patients with SPG4 exhibited significantly decreased ReHo values in the medial superior frontal gyrus. ALFF values were lower in left insula and higher in right precentral and superior frontal gyrus of the patient group. Increased ALFF values in the right precentral gyrus negatively correlated with Spastic Paraplegia Rating Scale (SPRS) scores in the patients. The connectivity study showed that the SPG4 patients had one increased FC between the left middle frontal gyrus to the left middle orbitofrontal gyrus, and pairs of decreased FC.

CONCLUSIONS

Our findings confirm that the baseline regional neural activity and interregional connectivity are altered in many brain regions in patients with SPG4, and certain changes are correlated with disease severity, providing potential diagnostic markers for SPG4.

Tau missorting and spastin-induced microtubule disruption in neurodegeneration: Alzheimer Disease and Hereditary Spastic Paraplegia

In Alzheimer Disease (AD), the mechanistic connection of the two major pathological hallmarks, namely deposition of Amyloid-beta (Aβ) in the form of extracellular plaques, and the pathological changes of the intracellular protein Tau (such as phosphorylation, missorting, aggregation), is not well understood. Genetic evidence from AD and Down Syndrome (Trisomy 21), and animal models thereof, suggests that aberrant production of Aβ is upstream of Tau aggregation, but also points to Tau as a critical effector in the pathological process. Yet, the cascade of events leading from increased levels of Aβ to Tau-dependent toxicity remains a matter of debate.

Using primary neurons exposed to oligomeric forms of Aβ, we have found that Tau becomes mislocalized (missorted) into the somatodendritic compartment. Missorting of Tau correlates with loss of microtubules and downstream consequences such as loss of mature spines, loss of synaptic activity, and mislocalization of mitochondria.

In this cascade, missorting of Tau induces mislocalization of TTLL6 (Tubulin-Tyrosine-Ligase-Like 6) into the dendrites. TTLL6 induces polyglutamylation of microtubules, which acts as a trigger for spastin mediated severing of dendritic microtubules. Loss of microtubules makes cells unable to maintain transport of mitochondria, which in turn results in synaptic dysfunction and loss of mature spines. These pathological changes are absent in TauKO derived primary neurons. Thus, Tau mediated mislocalization of TTLL6 and spastin activation reveals a pathological gain of function for Tau and spastin in this cellular model system of AD.

In contrast, in hereditary spastic paraplegia (HSP) caused by mutations of the gene encoding spastin (spg4 alias SPAST), spastin function in terms of microtubule severing is decreased at least for the gene product of the mutated allele, resulting in overstable microtubules in disease model systems. Whether total spastin severing activity or microtubule stability in human disease is also affected is not yet clear. No human disease has been associated so far with the long-chain polyglutamylation enzyme TTLL6, or the other TTLLs (1,5,11) possibly involved.

Here we review the findings supporting a role for Tau, spastin and TTLL6 in AD and other tauopathies, HSP and neurodegeneration, and summarize possible therapeutic approaches for AD and HSP.

Spastin related hereditary spastic paraplegia with dysplastic corpus callosum

Thin corpus callosum has been recently observed in two patients with an autosomal dominant trait of hereditary spastic paraplegia (HSP) linked to a novel mutation in the spastin gene (SPG4). In the same two patients cerebellar atrophy has been found. Reportedly, in other members of the same family, there has been a variable presence of mental retardation. We report on the clinical and genetic investigation of an Austrian family with a novel mutation in the spastin gene. Genetic analysis of the SPG4 locus revealed a mutation (C1120A) and a known intronic polymorphism (996-47G>A) of the spastin gene. In one affected family member, previously undescribed dysplasia of the corpus callosum (CC) was found in conjunction with otherwise uncomplicated HSP. Dysplastic CC was not paralleled with cortical atrophy, cognitive impairment or other phenotypic variations. Two further affected family members showed the same mutation and polymorphism, but no evidence of CC abnormalities. We conclude that apparently pure HSP may present with MRI features of dysplastic CC. This finding extended the spastin-related phenotype which is distinct from previous reports of thin CC in HSP.