Truncating mutations of SPAST associated with hereditary spastic paraplegia indicate greater accumulation and toxicity of the M1 isoform of spastin

Role of Residues Undergoing Hereditary Spastic Paraplegias Mutations: Insights from Simulating the Spiral to Ring Transition in Katanin

Several dozen mutations in the M87 isoform of the spastin enzyme have been associated with mobility impairment in hereditary spastic paraplegias. Some of them impact the structural determinants of two functional conformations of the protein: spiral and ring. Here we investigate the possible patterns between these disease-related residues in spastin and aligned regions in the closely related protein katanin toward their role in the transition of the two conformations, which is essential for both enzymes' function. By performing a variety of molecular simulations (including metadynamics) on katanin, we suggest that about one-fourth of the known M87 spastin disease-associated mutations also affect the interconversion and/or the stability of a previously unrecognized intermediate of the katanin transition. The protocol used here can be applied to the study of conformational changes in other large biomolecular complexes.

UCHL1-Mediated Spastin Degradation Regulates Microtubule Severing and Hippocampal Neurite Outgrowth

As a key component of the cytoskeleton, microtubule dynamic provides structural support for neurite outgrowth. Spastin, a microtubule severing enzyme associated with hereditary spastic paraplegia (HSP), is crucial for the growth and branching of neuronal processes. Thus, the activity and function of spastin need to be strictly regulated. However, the mechanism by which spastin protein levels are regulated is still poorly understood. In the current study, we showed that UCHL1 interacted with spastin via mass spectrometry, GST-pulldown and immunoprecipitation assays. Overexpression of UCHL1 decreased the protein level of spastin, while the genetic knockdown of UCHL1 increased that of spastin. CHX chase assay showed that UCHL1 regulated the protein degradation of spastin. Application of proteasome inhibitor MG-132 suppressed UCHL1-mediated spastin degradation. Furthermore, overexpression or knockout of UCHL1 can inhibit or restore spastin-mediated microtubule severing, thereby regulating neuronal length and branch formation. These findings reveal the important regulatory mechanism of UCHL1 on spastin-mediated neurite outgrowth.

Unraveling Isoform Complexity: The Roles of M1- and M87-Spastin in Spastic Paraplegia 4 (SPG4)

Spastic Paraplegia 4 (SPG4) is a debilitating neurodegenerative disorder characterized by progressive muscle weakness and spasticity in the lower limbs, often leading to gait impairment. Central to SPG4 pathology is the die-back degeneration of corticospinal tracts, primarily driven by mutations in the spastin protein encoded by the SPAST gene. SPAST gives rise to two major spastin isoforms, M1- and M87-spastin, which are generated from distinct translation initiation sites. Although spastin is implicated in various cellular functions, the specific roles of each isoform in the pathogenesis of SPG4 remain poorly understood. This review offers an overview of the genetic and structural organization of the M1- and M87-spastin isoforms, highlighting their distinct and overlapping functions, and exploring their potential roles in the haploinsufficiency and gain-of-toxicity mechanisms underlying SPG4. We also present a novel perspective on the evolutionary emergence of M1-spastin and its potential unique involvement in the pathogenesis of SPG4. Drawing upon the latest research, we propose an intriguing hypothesis regarding the hetero-oligomerization of M1- and M87-spastin, exploring how their interaction may drive disease progression and open new avenues for therapeutic intervention. By integrating the current research with these fresh insights, we seek to illuminate the complex molecular mechanisms driving SPG4 and foster the development of innovative therapeutic strategies. This review not only incorporates existing knowledge but also lays the groundwork for future studies aimed at uncovering the isoform-specific roles of spastin in SPG4, with the ultimate goal of advancing targeted treatments for this challenging neurodegenerative disorder. © 2024 International Parkinson and Movement Disorder Society.

From spastic paraplegia to infantile neurodegenerative disorder: Expanding the phenotypic spectrum associated with biallelic SPAST variants

PURPOSE

Heterozygous pathogenic variants in SPAST are known to cause Hereditary Spastic Paraplegia 4 (SPG4), the most common form of HSP, characterized by progressive bilateral lower limbs spasticity with frequent sphincter disorders. However, there are very few descriptions in the literature of patients carrying biallelic variants in SPAST.

METHODS

Targeted Sanger sequencing, panel sequencing and exome sequencing were used to identify the genetic causes in 9 patients from 6 unrelated families with symptoms of HSP or infantile neurodegenerative disorder.

RESULTS

We describe 5 patients with pure HSP with a variable age of onset, mostly in infancy, and 4 patients with profound intellectual disability and progressively worsening tetrapyramidal syndrome. The patients' parents, heterozygous carriers of pathogenic SPAST variants, included both asymptomatic carriers and patients with classic forms of SPG4.

CONCLUSION

Biallelic variants of SPAST may explain cases of hereditary spastic paraplegia with autosomal recessive inheritance. Furthermore, some biallelic variants may also cause psychomotor regression with an infantile neurodegenerative disorder, associated with a tetrapyramidal syndrome, a new phenotype associated with the SPAST gene.

Novel SPAST Deletion Mutation in an American Family With Hereditary Spastic Paraplegia: A Case Report

The diverse group of neurodegenerative disorders known as hereditary spastic paraplegia (HSP) is characterized by spasticity and weakness of the bilateral lower extremity due to degeneration of the corticospinal tract. The pathogenesis of HSP is broad, with autosomal dominant, autosomal recessive, X-linked recessive, mitochondrial inheritance, and de novo mutations reported, along with remarkable heterogeneity of mutations and clinical presentation. Of these, the most common subtype of HSP is HSP type 4 (HSP-SPG4), a result of mutations in the SPAST gene (chromosome 2p22.3) that leads to impaired activity of the microtubule-severing protein spastin. Typically presenting as an uncomplicated, autosomal dominant form of the disease, HSP-SPG4 has been documented worldwide with vast genomic variance across the SPAST gene. Despite common features in clinical phenotypes, a clear link between SPAST gene variants and disease presentation remains vague. Here, we report a novel 26.1 kb deletion in the SPAST gene (del exons 4-7) in a US family with previously undiagnosed HSP-SPG4.

Cul-4 inhibition rescues spastin levels and reduces defects in hereditary spastic paraplegia models

Hereditary spastic paraplegias (HSPs) are degenerative motor neuron diseases characterized by progressive spasticity and weakness in the lower limbs. The most common form of HSP is due to SPG4 gene haploinsufficiency. SPG4 encodes the microtubule severing enzyme spastin. Although, there is no cure for SPG4-HSP, strategies to induce a spastin recovery are emerging as promising therapeutic approaches. Spastin protein levels are regulated by poly-ubiquitination and proteasomal-mediated degradation, in a neddylation-dependent manner. However, the molecular players involved in this regulation are unknown. Here, we show that the Cullin-4-RING E3 ubiquitin ligase complex (CRL4) regulates spastin stability. Inhibition of CRL4 increases spastin levels by preventing its poly-ubiquitination and subsequent degradation in spastin-proficient and in patient derived SPG4 haploinsufficient cells. To evaluate the role of CRL4 complex in spastin regulation in vivo, we developed a Drosophila melanogaster model of SPG4 haploinsufficiency which show alterations of synapse morphology and locomotor activity, recapitulating phenotypical defects observed in patients. Downregulation of the CRL4 complex, highly conserved in Drosophila, rescues spastin levels and the phenotypical defects observed in flies. As a proof of concept of possible pharmacological treatments, we demonstrate a recovery of spastin levels and amelioration of the SPG4-HSP-associated defects both in the fly model and in patient-derived cells by chemical inactivation of the CRL4 complex with NSC1892. Taken together, these findings show that CRL4 contributes to spastin stability regulation and that it is possible to induce spastin recovery and rescue of SPG4-HSP defects by blocking the CRL4-mediated spastin degradation.

Pluripotent Stem Cells as a Preclinical Cellular Model for Studying Hereditary Spastic Paraplegias

Hereditary spastic paraplegias (HSPs) comprise a family of degenerative diseases mostly hitting descending axons of corticospinal neurons. Depending on the gene and mutation involved, the disease could present as a pure form with limb spasticity, or a complex form associated with cerebellar and/or cortical signs such as ataxia, dysarthria, epilepsy, and intellectual disability. The progressive nature of HSPs invariably leads patients to require walking canes or wheelchairs over time. Despite several attempts to ameliorate the life quality of patients that have been tested, current therapeutical approaches are just symptomatic, as no cure is available. Progress in research in the last two decades has identified a vast number of genes involved in HSP etiology, using cellular and animal models generated on purpose. Although unanimously considered invaluable tools for basic research, those systems are rarely predictive for the establishment of a therapeutic approach. The advent of induced pluripotent stem (iPS) cells allowed instead the direct study of morphological and molecular properties of the patient's affected neurons generated upon in vitro differentiation. In this review, we revisited all the present literature recently published regarding the use of iPS cells to differentiate HSP patient-specific neurons. Most studies have defined patient-derived neurons as a reliable model to faithfully mimic HSP in vitro, discovering original findings through immunological and -omics approaches, and providing a platform to screen novel or repurposed drugs. Thereby, one of the biggest hopes of current HSP research regards the use of patient-derived iPS cells to expand basic knowledge on the disease, while simultaneously establishing new therapeutic treatments for both generalized and personalized approaches in daily medical practice.

A novel truncated variant in SPAST results in spastin accumulation and defects in microtubule dynamics

OBJECTIVE

Haploinsufficiency is widely accepted as the pathogenic mechanism of hereditary spastic paraplegias type 4 (SPG4). However, there are some cases that cannot be explained by reduced function of the spastin protein encoded by SPAST. The aim of this study was to identify the causative variant of SPG4 in a large Chinese family and explore its pathological mechanism.

MATERIALS AND METHODS

A five-generation family with 49 members including nine affected (4 males and 5 females) and 40 unaffected individuals in Mongolian nationality was recruited. Whole exome sequencing was employed to investigate the genetic etiology. Western blotting and immunofluorescence were used to analyze the effects of the mutant proteins in vitro.

RESULTS

A novel frameshift variant NM_014946.4: c.483_484delinsC (p.Val162Leufs*2) was identified in SPAST from a pedigree with SPG4. The variant segregated with the disease in the family and thus determined as the disease-causing variant. The c.483_484delinsC variant produced two truncated mutants (mutant M1 and M87 isoforms). They accumulated to a higher level and presented increased stability than their wild-type counterparts and may lost the microtubule severing activity.

CONCLUSION

SPAST mutations leading to premature stop codons do not always act through haploinsufficiency. The potential toxicity to the corticospinal tract caused by the intracellular accumulation of truncated spastin should be considered as the pathological mechanism of SPG4.

Long-read sequencing revealing intragenic deletions in exome-negative spastic paraplegias

Hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurodegenerative disorders characterized by progressive spasticity and weakness in the lower extremities. To date, a total of 88 types of SPG are known. To diagnose HSP, multiple technologies, including microarray, direct sequencing, multiplex ligation-dependent probe amplification, and short-read next-generation sequencing, are often chosen based on the frequency of HSP subtypes. Exome sequencing (ES) is commonly used. We used ES to analyze ten cases of HSP from eight families. We identified pathogenic variants in three cases (from three different families); however, we were unable to determine the cause of the other seven cases using ES. We therefore applied long-read sequencing to the seven undetermined HSP cases (from five families). We detected intragenic deletions within the SPAST gene in four families, and a deletion within PSEN1 in the remaining family. The size of the deletion ranged from 4.7 to 12.5 kb and involved 1-7 exons. All deletions were entirely included in one long read. We retrospectively performed an ES-based copy number variation analysis focusing on pathogenic deletions, but were not able to accurately detect these deletions. This study demonstrated the efficiency of long-read sequencing in detecting intragenic pathogenic deletions in ES-negative HSP patients.

Alu Retrotransposition Event in SPAST Gene as a Novel Cause of Hereditary Spastic Paraplegia

OBJECTIVES

To diagnose the molecular cause of hereditary spastic paraplegia (HSP) observed in a four-generation family with autosomal dominant inheritance.

METHODS

Multiplex ligation-dependent probe amplification (MLPA), whole-exome sequencing (WES), and RNA sequencing (RNA-seq) of peripheral blood leukocytes were performed. Reverse transcription polymerase chain reaction (RT-PCR) and Sanger sequencing were used to characterize target regions of SPAST.

RESULTS

A 121-bp AluYb9 insertion with a 30-bp poly-A tail flanked by 15-bp direct repeats on both sides was identified in the edge of intron 16 in SPAST that segregated with the disease phenotype.

CONCLUSIONS

We identified an intronic AluYb9 insertion inducing splicing alteration in SPAST causing pure HSP phenotype that was not detected by routine WES analysis. Our findings suggest RNA-seq is a recommended implementation for undiagnosed cases by first-line diagnostic approaches. © 2023 International Parkinson and Movement Disorder Society.

Spastin is required for human immunodeficiency virus-1 efficient replication through cooperation with the endosomal sorting complex required for transport (ESCRT) protein

Human immunodeficiency virus-1 (HIV-1) encodes simply 15 proteins and thus depends on multiple host cellular factors for virus reproduction. Spastin, a microtubule severing protein, is an identified HIV-1 dependency factor, but the mechanism regulating HIV-1 is unclear. Here, the study showed that knockdown of spastin inhibited the production of the intracellular HIV-1 Gag protein and new virions through enhancing Gag lysosomal degradation. Further investigation showed that increased sodium tolerance 1 (IST1), the subunit of endosomal sorting complex required for transport (ESCRT), could interact with the MIT domain of spastin to regulate the intracellular Gag production. In summary, spastin is required for HIV-1 replication, while spastin-IST1 interaction facilitates virus production by regulating HIV-1 Gag intracellular trafficking and degradation. Spastin may serve as new target for HIV-1 prophylactic and therapy.

A novel missense mutation in SPAST causes hereditary spastic paraplegia in male members of a family: A case report

Hereditary spastic paraplegia (HSP) comprises a group of hereditary and neurodegenerative diseases that are characterized by axonal degeneration or demyelination of bilateral corticospinal tracts in the spinal cord; affected patients exhibit progressive spasticity and weakness in the lower limbs. The most common manifestation of HSP is spastic paraplegia type 4 (SPG4), which is caused by mutations in the spastin (SPAST) gene. The present study reports the clinical characteristics of affected individuals and sequencing analysis of a mutation that caused SPG4 in a family. All affected family members exhibited spasticity and weakness of the lower limbs and, notably, only male members of the family were affected. Whole‑exome sequencing revealed that all affected individuals had a novel c.1785C>A (p. Ser595Arg) missense mutation in SPAST. Bioinformatics analysis revealed changes in both secondary and tertiary structures of the mutated protein. The novel missense mutation in SPAST supported the diagnosis of SPG4 in this family and expands the spectrum of pathogenic mutations that cause SPG4. Analysis of SPAST sequences revealed that most pathogenic mutations occurred in the AAA domain of the protein, which may have a close relationship with SPG4 pathogenesis.

A novel truncating variant of SPAST associated with hereditary spastic paraplegia indicates a haploinsufficiency pathogenic mechanism

Introduction

Hereditary spastic paraplegias (HSPs) are genetic neurodegenerative diseases. The most common form of pure HSP that is inherited in an autosomal dominant manner is spastic paraplegia type 4 (SPG4), which is caused by mutations in the SPAST gene. Different theories have been proposed as the mechanism underlying SPAST-HSP for different types of genetic mutations, including gain- and loss-of-function mechanisms. To better understand the mutation mechanisms, we performed genetic analysis and investigated a truncating SPAST variant that segregated with disease in one family.

Objectives and methods

We described a pure HSP pedigree with family members across four generations. We performed genetic analysis and investigated a novel frameshift pathogenic variant (c.862_863dupAC, p. H289Lfs*27) in this family. We performed reverse transcription-polymerase chain reaction (RT-PCR), Sanger sequencing, and quantitative RT-PCR using total RNA from an Epstein-Barr virus-induced lymphoblastoid cell line produced from the proband. We also performed Western blotting on cell lysates to investigate if the protein expression of spastin is affected by this variant.

Results

This variant (c.862_863dupAC, p. H289Lfs*27) co-segregated with pure HSP in this family and is not registered in any public database. Measurement of SPAST transcripts in lymphoblasts from the proband demonstrated a reduction of SPAST transcript levels through likely nonsense-mediated mRNA decay. Immunoblot analyses demonstrated a reduction of spastin protein expression levels in lymphoblasts.

Conclusion

We report an SPG4 family with a novel heterozygous frameshift variant p.H289Lfs*27 in SPAST. Our study implies haploinsufficiency as the pathogenic mechanism for this variant and expands the known mutation spectrum of SPAST.

The Role of Spastin in Axon Biology

Neurons are highly polarized cells with elaborate shapes that allow them to perform their function. In neurons, microtubule organization—length, density, and dynamics—are essential for the establishment of polarity, growth, and transport. A mounting body of evidence shows that modulation of the microtubule cytoskeleton by microtubule-associated proteins fine tunes key aspects of neuronal cell biology. In this respect, microtubule severing enzymes—spastin, katanin and fidgetin—a group of microtubule-associated proteins that bind to and generate internal breaks in the microtubule lattice, are emerging as key modulators of the microtubule cytoskeleton in different model systems. In this review, we provide an integrative view on the latest research demonstrating the key role of spastin in neurons, specifically in the context of axonal cell biology. We focus on the function of spastin in the regulation of microtubule organization, and axonal transport, that underlie its importance in the intricate control of axon growth, branching and regeneration.

A novel variant of SPAST in a pedigree with pure hereditary spastic paraplegia in Yunnan Province

BACKGROUND

Hereditary spastic paraplegia (HSP) is a rare group of genetically heterogeneous, neurodegenerative disorders. The aim of this study was to identify pathological candidate genes and variants in a large pedigree cohort of 11 purely HSP patients in Yunnan Province.

METHODS

Whole-exome sequencing (WES) was applied to 2 HSP patients and 1 control patient to screen out the candidate gene variants. Then, filtration and verification of these pathological variants were performed by Sanger sequencing.

RESULTS

After the raw data were filtered, two genes with novel variations (SPAST: c.1510 C>T, p.Gln504X, RefSeq.NM_199436; DNAJC16: c.718 C>T, p.Q240X, Ref Seq NM_015291) were identified. The accession numbers of the genes in the ClinVar database were SCV001573094 and SCV001573804, respectively. One gene with a reported single nucleotide polymorphism (CPT1C: rs150853576) was filtered as a candidate variant. Using Sanger sequencing, the novel SPAST gene (protein: Spastin) variant leading to a predicted premature termination and an 18% deletion of the SPAST/spastic paraplegia type 4 (SPG4) protein was confirmed to exist only in affected individuals. The candidate CPT1C and DNAJC16 variants were verified in almost all HSP patients, with one exception.

CONCLUSIONS

Considering that the clinical symptoms and time of onset of HSP are highly heterogeneous, the SPAST as a genotype-phenotype cosegregated variant might be the causative gene of this pedigree, and the other two variants might present cumulative risks to the occurrence and progression of HSP. These three candidate genes with or without novel variants may be potential contributors to disease onset, and therefore useful diagnostic and therapeutic biomarkers. Further research is required to confirm the functions of these genes.

Modeling gain-of-function and loss-of-function components of SPAST-based hereditary spastic paraplegia using transgenic mice

Hereditary spastic paraplegia (HSP) is a disease in which dieback degeneration of corticospinal tracts, accompanied by axonal swellings, leads to gait deficiencies. SPG4-HSP, the most common form of the disease, results from mutations of human spastin gene (SPAST), which is the gene that encodes spastin, a microtubule-severing protein. The lack of a vertebrate model that recapitulates both the etiology and symptoms of SPG4-HSP has stymied the development of effective therapies for the disease. hSPAST-C448Y mice, which express human mutant spastin at the ROSA26 locus, display corticospinal dieback and gait deficiencies but not axonal swellings. On the other hand, mouse spastin gene (Spast)-knockout (KO) mice display axonal swellings but not corticospinal dieback or gait deficiencies. One possibility is that reduced spastin function, resulting in axonal swellings, is not the cause of the disease but exacerbates the toxic effects of the mutant protein. To explore this idea, Spast-KO and hSPAST-C448Y mice were crossbred, and the offspring were compared with the parental lines via histological and behavioral analyses. The crossbred animals displayed axonal swellings as well as earlier onset, worsened gait deficiencies and corticospinal dieback compared with the hSPAST-C448Y mouse. These results, together with observations on changes in histone deacetylases 6 and tubulin modifications in the axon, indicate that each of these three transgenic mouse lines is valuable for investigating a different component of the disease pathology. Moreover, the crossbred mice are the best vertebrate model to date for testing potential therapies for SPG4-HSP.

A Novel SPAST Mutation Results in Spastin Accumulation and Defects in Microtubule Dynamics

Background

Haploinsufficiency is widely accepted as the pathogenic mechanism of spastic paraplegia type 4 (SPG4). However, there are some cases that cannot be explained by reduced function of the spastin protein encoded by SPAST.

Objectives

To identify the causative gene of autosomal dominant hereditary spastic paraplegia in three large Chinese families and explore the pathological mechanism of a spastin variant.

Methods

Three large Chinese hereditary spastic paraplegia families with a total of 247 individuals (67 patients) were investigated, of whom 59 members were recruited to the study. Genetic testing was performed to identify the causative gene. Western blotting and immunofluorescence were used to analyze the effects of the mutant proteins in vitro.

Results

In the three hereditary spastic paraplegia families, of whom three index cases were misdiagnosed as other types of neurological diseases, a novel c.985dupA (p.Met329Asnfs*3) variant in SPAST was identified and was shown to cosegregate with the phenotype in the three families. The c.985dupA mutation produced two truncated mutants (mutant M1 and M87 isoforms) that accumulated to a higher level than their wild‐type counterparts. Furthermore, the mutant M1 isoform heavily decorated the microtubules and rendered them resistant to depolymerization. In contrast, the mutant M87 isoform was diffusely localized in both the nucleus and the cytoplasm, could not decorate microtubules, and was not able to promote microtubule disassembly.

Conclusions

SPAST mutations leading to premature stop codons do not always act through haploinsufficiency. The truncated spastin may damage the corticospinal tracts through an isoform‐specific toxic effect.

Therapeutic Strategies for Mutant SPAST-Based Hereditary Spastic Paraplegia

Mutations of the SPAST gene that encodes the microtubule-severing enzyme called spastin are the chief cause of Hereditary Spastic Paraplegia. Growing evidence indicates that pathogenic mutations functionally compromise the spastin protein and endow it with toxic gain-of-function properties. With each of these two factors potentially relevant to disease etiology, the present article discusses possible therapeutic strategies that may ameliorate symptoms in patients suffering from SPAST-based Hereditary Spastic Paraplegia, which is usually termed SPG4-HSP.

Anticipation Can Be More Common in Hereditary Spastic Paraplegia with SPAST Mutations Than It Appears

BACKGROUND AND OBJECTIVE

Hereditary spastic paraplegia (HSP) is a heterogeneous neurodegenerative disorder with lower-limb spasticity and weakness. Different patterns of inheritance have been identified in HSP. Most autosomal-dominant HSPs (AD-HSPs) are associated with mutations of the SPAST gene (SPG4), leading to a pure form of HSP with variable age-at-onset (AAO). Anticipation, an earlier onset of disease, as well as aggravation of symptoms in successive generations, may be correlated to SPG4. Herein, we suggested that anticipation might be a relatively common finding in SPG4 families.

METHODS

Whole-exome sequencing was done on DNA of 14 unrelated Iranian AD-HSP probands. Data were analyzed, and candidate variants were PCR-amplified and sequenced by the Sanger method, subsequently checked in family members to co-segregation analysis. Multiplex ligation-dependent probe amplification (MLPA) was done for seven probands. Clinical features of the probands were recorded, and the probable anticipation was checked in these families. Other previous reported SPG4 families were investigated to anticipation.

RESULTS

Our findings showed that SPG4 was the common subtype of HSP; three families carried variants in the KIF5A, ATL1, and MFN2 genes, while five families harbored mutations in the SPAST gene. Clinical features of only SPG4 families indicated decreasing AAO in affected individuals of the successive generations, and this difference was significant (p-value <0.05).

CONCLUSION

It seems SPAST will be the first candidate gene in families that manifests a pure form of AD-HSP and anticipation. Therefore, it may be a powerful situation of genotype-phenotype correlation. However, the underlying mechanism of anticipation in these families is not clear yet.

Genotype-phenotype associations in hereditary spastic paraplegia: a systematic review and meta-analysis on 13,570 patients

AIMS

The hereditary spastic paraplegias (HSPs) are a heterogeneous group of inherited neurodegenerative disorders. Although, several genotype-phenotype studies have carried out on HSPs, the association between genotypes and clinical phenotypes remain incomplete since most studies are small in size or restricted to a few genes. Accordingly, this study provides the systematic meta-analysis of genotype-phenotype associations in HSP.

METHODS AND RESULTS

We retrieved literature on genotype-phenotype associations in patients with HSP and mutated SPAST, REEP1, ATL1, SPG11, SPG15, SPG7, SPG35, SPG54, SPG5. In total, 147 studies with 13,570 HSP patients were included in our meta-analysis. The frequency of mutations in SPAST (25%) was higher than REEP1 (3%), as well as ATL1 (5%) in AD-HSP patients. As for AR-HSP patients, the rates of mutations in SPG11 (18%), SPG15 (7%) and SPG7 (13%) were higher than SPG5 (5%), as well as SPG35 (8%) and SPG54 (7%). The mean age of AD-HSP onset for ATL1 mutation-positive patients was earlier than patients with SPAST, REEP1 mutations. Also, the tendency toward younger age at AR-HSP onset for SPG35 was higher than other mutated genes. It is noteworthy that the mean age at HSP onset ranged from infancy to adulthood. As for the gender distribution, the male proportion in SPG7-HSP (90%) and REEP1-HSP (78%) was markedly high. The frequency of symptoms was varied among patients with different mutated genes. The rates of LL weakness, superficial sensory abnormalities, neuropathy, and deep sensory impairment were noticeably high in REEP1 mutations carriers. Also, in AR-HSP patients with SPG11 mutations, the presentation of symptoms including pes cavus, Neuropathy, and UL spasticity was higher.

CONCLUSION

Our comprehensive genotype-phenotype assessment of available data displays that the mean age at disease onset and particular sub-phenotypes are associated with specific mutated genes which might be beneficial for a diagnostic procedure and differentiation of the specific mutated genes phenotype among diverse forms of HSP.

Mutations disrupting neuritogenesis genes confer risk for cerebral palsy

In addition to commonly associated environmental factors, genomic factors may cause cerebral palsy. We performed whole-exome sequencing of 250 parent-offspring trios, and observed enrichment of damaging de novo mutations in cerebral palsy cases. Eight genes had multiple damaging de novo mutations; of these, two (TUBA1A and CTNNB1) met genome-wide significance. We identified two novel monogenic etiologies, FBXO31 and RHOB, and showed that the RHOB mutation enhances active-state Rho effector binding while the FBXO31 mutation diminishes cyclin D levels. Candidate cerebral palsy risk genes overlapped with neurodevelopmental disorder genes. Network analyses identified enrichment of Rho GTPase, extracellular matrix, focal adhesion and cytoskeleton pathways. Cerebral palsy risk genes in enriched pathways were shown to regulate neuromotor function in a Drosophila reverse genetics screen. We estimate that 14% of cases could be attributed to an excess of damaging de novo or recessive variants. These findings provide evidence for genetically mediated dysregulation of early neuronal connectivity in cerebral palsy.

A newly identified NES sequence present in spastin regulates its subcellular localization and microtubule severing activity

Spastin, a microtubule-severing AAA ATPase, regulates microtubule dynamics and plays important roles in cell division and neurogenesis. Mutations in the spastin-coding gene SPAST lead to neurodegenerative disorders and cause spastic paraplegia type 4. Spastin has two main isoforms, M1 and M87, that differ only in the presence or absence of 86 N-terminal amino acids and have alternative splicing variants that lack exon4. The N-terminal region of M1 contains a hydrophobic domain, nuclear localization signal (NLS), and nuclear export signal (NES), which partly explains the differences in the two isoforms' localization. However, the mechanisms involved in regulating isoform localization, and the effects of localization on spastin functions are not fully understood. We found endogenous M1 and M87 shuttled between the nucleus and cytoplasm during the cell cycle. We identified a NES (amino acids 195-204) that spans the microtubule-interacting and endosomal-trafficking domain and exon4 region. Furthermore, the NES sequence contains both the coiled-coil and exon4 region of spastin isoforms. Highly conserved leucine 195 in exon3 and the two residues in exon4 are crucial for predicted coiled-coil formation. Mutations in NES or leptomycin B treatment reduced cytoplasmic localization and microtubule fragmentation in M87 rather than in M1. Phosphomimetic mutation of threonine 306 adjacent to the NLS (amino acids 309-312) inhibited nuclear transport of M87. Our results indicate that the newly identified NES in the spastin isoforms containing exon4 regulates the subcellular localization of spastin in coordination with NLS controlled by the phosphorylation state of spastin, and is involved in microtubule severing.

Spastin MIT Domain Disease-Associated Mutations Disrupt Lysosomal Function

The hereditary spastic paraplegias (HSPs) are genetic motor neuron diseases characterized by progressive degeneration of corticospinal tract axons. Mutations in SPAST, encoding the microtubule-severing ATPase spastin, are the most common causes of HSP. The broad SPAST mutational spectrum indicates a haploinsufficiency pathogenic mechanism in most cases. Most missense mutations cluster in the ATPase domain, where they disrupt the protein's ability to sever microtubules. However, several putative missense mutations in the protein's microtubule interacting and trafficking (MIT) domain have also been described, but the pathogenicity of these mutations has not been verified with functional studies. Spastin promotes endosomal tubule fission, and defects in this lead to lysosomal enzyme mistrafficking and downstream lysosomal abnormalities. We investigated the function of three disease-associated spastin MIT mutants and found that none was able to promote normal endosomal tubule fission, lysosomal enzyme receptor trafficking, or lysosomal morphology. One of the mutations affected recruitment of spastin to endosomes, a property that requires the canonical function of the MIT domain in binding endosomal sorting complex required for transport (ESCRT)-III proteins. However, the other mutants did not affect spastin's endosomal recruitment, raising the possibility of pathologically important non-canonical roles for the MIT domain. In conclusion, we demonstrate that spastin MIT mutants cause functional abnormalities related to the pathogenesis of HSP. These mutations do not directly affect spastin's microtubule-severing capacity, and so we identify a new molecular pathological mechanism by which spastin mutations may cause disease.

Novel mutations in the SPAST gene cause hereditary spastic paraplegia

BACKGROUND

Mutations in the SPAST gene are the most frequent cause of hereditary spastic paraplegia (HSP). We aim to extend the mutation spectrum of spastic paraplegia 4 (SPG4) and carried out experiment in vitro to explore the influence of the SPAST gene mutation on the function of corresponding protein.

METHODS

Whole-exome sequencing (WES) combined with multiplex ligation-dependent probe amplification (MLPA) were performed in a cohort of 150 patients clinically diagnosed with HSP. We focus on screening for mutations in SPAST gene and carrying out functional experiments to assess the effects of the novel variants.

RESULTS

A total of 34 different mutations in the SPAST gene were identified, of which 10 were novel, including 1 missense (c.1479T > A), 1 nonsense (c.766G > T), 3 splicing (c.1413 + 1_1413+4delGTAA, c.1729-1G > A and c.1536+2T > G) and 5 frameshift mutations (c.1094delC, c.885dupA, c.517_518delAG, c.280delG and c.908dupC). For 7 novel non-splicing mutations, functional study showed that accumulated M1 spastin colcocalized with microtubules which was different from a uniformly diffused M87 spastin. While an impairment in severing activity was observed in both mutant M1 and mutant M87, except for c.280delG. All 3 novel splicing variants w ere predicted to affect splicing by using bioinformatic programs. However, only c.1536+2T > G had no influence on splice site in vitro, which conflicts with the in-silico analysis.

CONCLUSION

We genetically diagnosed 40 SPG4 patients. All the novel non-splicing mutations except for c.280delG were certified to exert an effect on the microtubule-severing and all the novel splicing mutations other than c.1536+2T > G would cause abnormal splicing of the spastin.

New hypothesis for the etiology of SPAST-based hereditary spastic paraplegia

Mutations of the SPAST gene are the chief cause of hereditary spastic paraplegia. Controversy exists in the medical community as to whether the etiology of the disease is haploinsufficiency or toxic gain-of-function properties of the mutant spastin proteins. In recognition of strong reasons that support each possible mechanism, here we present a novel perspective, based in part on new studies with mouse models and in part on the largest study to date on patients with the disease. We posit that haploinsufficiency does not cause the disease but makes the corticospinal tracts vulnerable to a second hit, which is usually the mutant spastin proteins but could also be proteins generated by mutations of other genes that may or may not cause the disease on their own.

Hereditary spastic paraplegia: gain-of-function mechanisms revealed by new transgenic mouse

Mutations of the SPAST gene, which encodes the microtubule-severing protein spastin, are the most common cause of hereditary spastic paraplegia (HSP). Haploinsufficiency is the prevalent opinion as to the mechanism of the disease, but gain-of-function toxicity of the mutant proteins is another possibility. Here, we report a new transgenic mouse (termed SPASTC448Y mouse) that is not haploinsufficient but expresses human spastin bearing the HSP pathogenic C448Y mutation. Expression of the mutant spastin was documented from fetus to adult, but gait defects reminiscent of HSP (not observed in spastin knockout mice) were adult onset, as is typical of human patients. Results of histological and tracer studies on the mouse are consistent with progressive dying back of corticospinal axons, which is characteristic of the disease. The C448Y-mutated spastin alters microtubule stability in a manner that is opposite to the expectations of haploinsufficiency. Neurons cultured from the mouse display deficits in organelle transport typical of axonal degenerative diseases, and these deficits were worsened by depletion of endogenous mouse spastin. These results on the SPASTC448Y mouse are consistent with a gain-of-function mechanism underlying HSP, with spastin haploinsufficiency exacerbating the toxicity of the mutant spastin proteins. These findings reveal the need for a different therapeutic approach than indicated by haploinsufficiency alone.

Functional differences of short and long isoforms of spastin harboring missense mutation

Mutations of the SPG4 (SPAST) gene encoding for spastin protein are the main causes of hereditary spastic paraplegia. Spastin binds to microtubules and severs them through the enzymatic activity of its AAA domain. Several missense mutations located in this domain lead to stable, nonsevering spastins that decorate a subset of microtubules, suggesting a possible negative gain-of-function mechanism for these mutants. Of the two main isoforms of spastin, only mutations of the long isoform, M1, are supposed to be involved in the onset of the pathology, leaving the role of the ubiquitously expressed shorter one, M87, not fully investigated and understood. Here, we show that two isoforms of spastin harboring the same missense mutation bind and bundle different subsets of microtubules in HeLa cells, and likely stabilize them by increasing the level of acetylated tubulin. However, only mutated M1 has the ability to interact with wild-type M1, and decorates a subset of perinuclear microtubules associated with the endoplasmic reticulum that display higher resistance to microtubule depolymerization and increased intracellular ionic strength, compared with those decorated by mutated M87. We further show that only mutated M1 decorates microtubules of proximal axons and dendrites, and strongly impairs axonal transport in cortical neurons through a mechanism likely independent of the microtubule-severing activity of this protein.

Summary: Long and short isoforms of spastin (SPG4) harboring the same missense mutation show different intracellular localization, resistance to pharmacological treatments and effects on axonal cargo transport.

Missense mutation of SPAST protein (I344K) results in loss of ATPase activity and prolonged the half-life, implicated in autosomal dominant hereditary spastic paraplegia

The spastin protein (SPAST) contains an ATPase with diverse cellular activities (AAA) domain and regulates microtubule dynamics. Missense mutations of the SPAST gene are frequently detected in patients with hereditary spastic paraplegias (HSPs) and represent the main reason of loss of SPAST function; however, the pathogenicity of mutant SPAST is heterogeneous. Here, SPAST variant with an I344K mutation (I344K-SPAST) was identified in a Korean family with autosomal dominant-type HSP. We investigated the role of the I344K-SPAST in HSP to provide a therapeutic mechanism. The I344K-SPAST mutation prolonged the half-life of the protein compared to wild-type SPAST (WT-SPAST) in cells by modulating post-translational modifications for proteasomal degradation. I344K-SPAST was localized in microtubule but defective in microtubule severing and ATPase activity compared to WT-SPAST in vitro and in cells. Mutant M87 isoform harboring the same mutation with I344K-M1 SPAST also increased protein stability and loss of MT severing activity, but the pathogenicity was not stronger than I344K-M1 SPAST in neurite outgrowth. Overexpression of I344K-SPAST resulted in microtubule accumulation following inhibited neurite growth in neuroblastoma, neural progenitor cells and mouse primary cortical neurons. Conversely, these pathogenic effects of I344K-SPAST were reduced by overexpression of WT-M1 SPAST in a dose dependent manner since WT-SPAST could interact with I344K-SPAST. Our data therefore provide proof-of-concept that gene transfer of WT-M1 SPAST may serve as a valid therapeutic option for HSPs.