Recessive Hereditary Spastic Paraplegia Research Articles

Clinical and genetic heterogeneity in hereditary spastic paraplegias: From SPG1 to SPG72 and still counting

Hereditary spastic paraplegias (HSPs) are genetically determined neurodegenerative disorders characterized by progressive weakness and spasticity of lower limbs, and are among the most clinically and genetically heterogeneous human diseases. All modes of inheritance have been described, and the recent technological revolution in molecular genetics has led to the identification of 76 different spastic gait disease-loci with 59 corresponding spastic paraplegia genes. Autosomal recessive HSP are usually associated with diverse additional features (referred to as complicated forms), contrary to autosomal dominant HSP, which are mostly pure. However, the identification of additional mutations and families has considerably enlarged the clinical spectra, and has revealed a huge clinical variability for almost all HSP; complicated forms have also been described for primary pure HSP subtypes, adding further complexity to the genotype-phenotype correlations. In addition, the introduction of next generation sequencing in clinical practice has revealed a genetic and phenotypic overlap with other neurodegenerative disorders (amyotrophic lateral sclerosis, neuropathies, cerebellar ataxias, etc.) and neurodevelopmental disorders, including intellectual disability. This review aims to describe the most recent advances in the field and to provide genotype-phenotype correlations that could help clinical diagnoses of this heterogeneous group of disorders.

Different expression levels of spartin cause broad spectrum of cellular consequences in human neuroblastoma cells.

Hereditary spastic paraplegia describes a diverse group of neurodegenerative conditions characterised by progressive spasticity and weakness of the lower limbs. Mutations in the SPG20 gene encoding spartin cause an autosomal recessive hereditary spastic paraplegia known as Troyer syndrome. To evaluate the cellular consequences of sustained spartin depletion in neuronal cells, we established several clonal SH-SY5Y cell lines with different level of spartin knockdown. Here, we report that cells with modest spartin downregulation show signs of neuronal differentiation such as increased neuritogenesis and cytoskeleton rearrangement. Interestingly, we also indicate that permanent high level spartin depletion results in impaired cell growth and multiple mitochondrial aberrations, which we speculate, arise as a result of chronic oxidative stress. Our studies demonstrate that the scale of spartin downregulation is the major factor that determines the severity of cellular consequences observed and suggest that there is a critical level of spartin expression which must be maintained for proper cellular functions.

Compound Heterozygous Autosomal Recessive Hereditary Spastic Paraplegia with Thin Corpus Callosum in Two Sisters of Non-Consanguineous Extraction in the North-West of Ireland (P2.242)

Compound Heterozygous Autosomal Recessive Hereditary Spastic Paraplegia with Thin Corpus Callosum in Two Sisters of Non-Consanguineous Extraction in the North-West of Ireland (P2.242)

Novel mutations in the PNPLA6 gene in Boucher-Neuhäuser syndrome.

On whole-exome sequencing, a novel compound heterozygous mutation (c.2923A>G/c.3523_3524insTGTCCG, p.T975A/p.1175_1176insVS) and a novel homozygous one (c.3534G>C, p.W1178C) in the PNPLA6 gene were identified in sporadic and familial Japanese patients with Boucher-Neuhäuser syndrome (BNS), respectively. However, we did not find any mutations in the PNPLA6 gene in 88 patients with autosomal recessive hereditary spastic paraplegia (ARHSP). Our study confirmed the earlier report that a PNPLA6 mutation causes BNS. This is the first report on PNPLA6 mutations in non-Caucasian patients. Meanwhile, PNPLA6 mutations might be extremely rare in Japanese ARHSP patients. Moreover, we first found hypersegmented neutrophils in two BNS patients with PNPLA6 mutations.

Alteration of ornithine metabolism leads to dominant and recessive hereditary spastic paraplegia

Alteration of ornithine metabolism leads to dominant and recessive hereditary spastic paraplegia

Identification of Novel Mutations inSpatacsinandApolipoprotein BGenes in a Patient with Spastic Paraplegia and Hypobetalipoproteinemia

Complicated hereditary spastic paraplegia (HSP) presents with complex neurological and nonneurological manifestations. We report a patient with autosomal recessive (AR) HSP in whom laboratory investigations revealed hypobetalipoproteinemia raising the possibility of a shared pathophysiology of these clinical features. A lipid profile of his parents disclosed a normal maternal lipid profile. However, the paternal lipid profile was similar to that of the patient suggesting autosomal dominant transmission of this trait. Whole exome sequence analysis was performed and novel mutations were detected in both the SPG11 and the APOB genes. Genetic testing of the parents showed that both APOB variants were inherited from the father while the SPG11 variants were inherited one from each parent. Our results indicate that, in this patient, the hypobetalipoproteinemia and spastic paraplegia are unrelated resulting from mutations in two independent genes. This clinical study provides support for the use of whole exome sequencing as a diagnostic tool for identification of mutations in conditions with complex presentations.

In-Frame Insertion Mutation in the SPG11 Gene Causes Autosomal Recessive Spastic Paraplegia with Thin Corpus Callosum “In A” Turkish Family with Late Age of Onset of the Phenotype

Autosomal recessive hereditary spastic paraplegia with thin corpus callosum (ARHSP-TCC) is one of the most prevalent forms of complex ARHSP. Mutations in the SPG11 gene are the most common cause for ARHSP-TCC and accounts for up to 70% of all cases. The mutational spectrum of SPG11 gene is broad as all types of DNA alterations are detected in the gene and most mutations lead to a premature truncation of the protein, suggesting “loss of function” as the likely pathogenic mechanism. In the current study, we report a consanguineous Turkish family with ARHSP inheritance manifesting white matter abnormalities including TCC with relatively late age of onset. Sequencing of SPG11 gene revealed a homozygous insertion of 15 nucleotides at position 6886 in exon 38 (c.6886_6900Dup15) leading to an in-frame insertion of five amino acids at codon 2296 (p.K2296_L2300Dup5), which resides within a predicted, highly conserved, intradiol ring-cleavage dioxygenase domain (2104 -2381 residues). In silico structural prediction of intradiol domain of the mutated Spatacsin protein revealed that the duplication of five amino acids leads to an extra turn in α-helix and a slightly longer loop region. Our structural analysis suggests that it is unlikely that insertion mutation (c.6886_6900Dup15) causes dysfunctional protein rather the minor conformation changes may elicit a “gain of function”, which may be detrimental to endogenous function of Spatacsin thus cause HSP.

Characterization of Alu and recombination-associated motifs mediating a large homozygous SPG7 gene rearrangement causing hereditary spastic paraplegia.

Spastic paraplegia type 7 (SPG7) is one of the most common forms of autosomal recessive hereditary spastic paraplegia (AR-HSP). Although over 77 different mutations have been identified in SPG7 patients, only 9 gross deletions have been reported with only a few of them being fully characterized. Here, we present a detailed description of a large homozygous intragenic SPG7 gene rearrangement involving a 5144-base pair (bp) genomic loss (c. 1450-446_1779 + 746 delinsAAAGTGCT) encompassing exons 11 to 13, identified in a Spanish AR-HSP family. Analysis of the deletion junction sequences revealed that the 5' breakpoint of this SPG7 gene deletion was located within highly homologous Alu sequences where the 3' breakpoint appears to be flanked by the core crossover hotspot instigator (chi)-like sequence (GCTGG). Furthermore, an 8-bp (AAAGTTGCT) conserved sequence at the breakpoint junction was identified, suggesting that the most likely mechanism for the occurrence of this rearrangement is by Alu microhomology and chi-like recombination-associated motif-mediated multiple exon deletion. Our results are consistent with non-allelic homologous recombination and non-homologous end joining in deletion mutagenesis for the generation of rearrangements. This study provides more evidence associating repeated elements as a genetic mechanism underlying neurodegenerative disorders, highlighting their importance in human diseases.

Spastic paraplegia proteins spastizin and spatacsin mediate autophagic lysosome reformation.

Autophagy allows cells to adapt to changes in their environment by coordinating the degradation and recycling of cellular components and organelles to maintain homeostasis. Lysosomes are organelles critical for terminating autophagy via their fusion with mature autophagosomes to generate autolysosomes that degrade autophagic materials; therefore, maintenance of the lysosomal population is essential for autophagy-dependent cellular clearance. Here, we have demonstrated that the two most common autosomal recessive hereditary spastic paraplegia gene products, the SPG15 protein spastizin and the SPG11 protein spatacsin, are pivotal for autophagic lysosome reformation (ALR), a pathway that generates new lysosomes. Lysosomal targeting of spastizin required an intact FYVE domain, which binds phosphatidylinositol 3-phosphate. Loss of spastizin or spatacsin resulted in depletion of free lysosomes, which are competent to fuse with autophagosomes, and an accumulation of autolysosomes, reflecting a failure in ALR. Moreover, spastizin and spatacsin were essential components for the initiation of lysosomal tubulation. Together, these results link dysfunction of the autophagy/lysosomal biogenesis machinery to neurodegeneration.

Japan Spastic Paraplegia Research Consortium (JASPAC)

Japan Spastic Paraplegia Research Consortium (JASPAC), a nationwide clinical and genetic survey of patients with hereditary spastic paraplegia (HSP), was started in 2006 as a project of the Research Committee for Ataxic Diseases of the Ministry of Health, Labor, and Welfare, Japan. To date (April 4, 2014), 448 indexed patients with HSP have been registered from 46 prefectures in Japan. We are now performing molecular testing of the HSP patients using Sanger sequencing (SPG4, SPG11, SPG31, and ARSACS), comparative genomic hybridization (CGH) array (SPG1, 2, 3A, 4, 5, 6, 7, 8, 10, 11, 13, 15, 17, 20, 21, 31, 33, 39, 42, ABCD1, alsin, and ARSACS), and resequencing microarray (SPG1, 2, 3A, 4, 5, 6, 7, 8, 10, 11, 13, 17, 20, 21, 31, 33, and ABCD1). In 206 Japanese families with autosomal dominant HSP, SPG4 was the most common form, accounting for 38%, followed by SPG3A (5%), SPG31 (5%), SPG10 (2%), and SPG8 (1%). In 88 patients with autosomal recessive HSP, although SPG11 was the most common form, accounting for 6%, most showed significant genetic heterogeneity. The results of molecular testing will be applicable to patients in terms of improved positive diagnosis, follow-up, and genetic counseling. JASPAC will contribute to elucidating the molecular mechanisms underlying HSP, and will facilitate the development of better treatments for HSP.

Novel EXOSC3 mutation causes complicated hereditary spastic paraplegia.

We describe two pairs of siblings from a consanguineous family manifesting autosomal recessive hereditary spastic paraplegia caused by a novel mutation in the EXOSC3 gene, previously reported in pontocerebellar hypoplasia type 1. Clinical findings included delayed motor milestones, early-onset spastic paraplegia, variable cognitive disability, and cerebellar signs. Cerebral imaging demonstrated enlarged cisterna magna and mild hypoplasia and atrophy of the lower vermis with a normal pons. Genetic analysis using homozygosity mapping followed by whole exome sequencing identified homozygous c.571G>T; p.G191C mutation in the EXOSC3 gene. We suggest that EXOSC3 mutations may present not only as pontocerebellar hypoplasia type 1, but also as a complicated form of hereditary spastic paraplegia without pontine hypoplasia or atrophy.

AP5Z1/SPG48 frequency in autosomal recessive and sporadic spastic paraplegia.

Hereditary spastic paraplegias (HSP) constitute a rare and highly heterogeneous group of neurodegenerative disorders, defined clinically by progressive lower limb spasticity and pyramidal weakness. Autosomal recessive HSP as well as sporadic cases present a significant diagnostic challenge. Mutations in AP5Z1, a gene playing a role in intracellular membrane trafficking, have been recently reported to be associated with spastic paraplegia type 48 (SPG48). Our objective was to determine the relative frequency and clinical relevance of AP5Z1 mutations in a large cohort of 127 HSP patients. We applied a targeted next-generation sequencing approach to analyze all coding exons of the AP5Z1 gene. With the output of high-quality reads and a mean coverage of 51-fold, we demonstrated a robust detection of variants. One 43-year-old female with sporadic complicated paraplegia showed two heterozygous nonsynonymous variants of unknown significance (VUS3; p.[R292W];[(T756I)]). Thus, AP5Z1 gene mutations are rare, at least in Europeans. Due to its low frequency, systematic genetic testing for AP5Z1 mutations is not recommended until larger studies are performed to add further evidence. Our findings demonstrate that amplicon-based deep sequencing is technically feasible and allows a compact molecular characterization of multiple HSP patients with high accuracy.

Motor protein mutations cause a new form of hereditary spastic paraplegia.

To identify a novel disease gene in 2 families with autosomal recessive hereditary spastic paraplegia (HSP). We used whole-exome sequencing to identify the underlying genetic disease cause in 2 families with apparently autosomal recessive spastic paraplegia. Endogenous expression as well as subcellular localization of wild-type and mutant protein were studied to support the pathogenicity of the identified mutations. In 2 families, we identified compound heterozygous or homozygous mutations in the kinesin gene KIF1C to cause hereditary spastic paraplegia type 58 (SPG58). SPG58 can be complicated by cervical dystonia and cerebellar ataxia. The same mutations in a heterozygous state result in a mild or subclinical phenotype. KIF1C mutations in SPG58 affect the domains involved in adenosine triphosphate hydrolysis and microtubule binding, key functions for this microtubule-based motor protein. KIF1C is the third kinesin gene involved in the pathogenesis of HSPs and is characterized by a mild dominant and a more severe recessive disease phenotype. The identification of KIF1C as an HSP disease gene further supports the key role of intracellular trafficking processes in the pathogenesis of hereditary axonopathies.

Clinical and genetic analysis of Taiwanese patients with hereditary spastic paraplegia type 5

Spastic paraplegia type 5 (SPG5) is an autosomal recessive (AR) hereditary spastic paraplegia (HSP) associated with pure or complicated phenotypes. This study aimed to screen SPG5 in Taiwanese HSP patients. Sequencing of the SPG5 gene, CYP7B1, was performed in a cohort of 25 ethnic Han Taiwanese patients with AR or sporadic HSP. Clinical information and magnetic resonance imaging (MRI) were analyzed in confirmed SPG5 patients. One (33%) AR kindred and four (18%) sporadic cases had CYP7B1 mutations. All of the SPG5 cases carried the mutation c.334 C>T (R112X). Haplotype analysis suggested a 'founder effect' in ethnic Hans for this mutation. The phenotype was either pure or complicated by cerebellar ataxia. For the primary HSP phenotype, there were profound dorsal column sensory deficits in all patients. Spine MRI showed thoraco-lumbar cord atrophy in some patients. Spastic paraplegia type 5 is a common cause of AR and sporadic HSPs that has a higher frequency in Taiwanese than in other ethnic groups. It is associated with a CYP7B1 founder mutation and its phenotype is characterized by pronounced dorsal column sensory loss, with cerebellar ataxia in some patients.

Further evidence that DDHD2 gene mutations cause autosomal recessive hereditary spastic paraplegia with thin corpus callosum

A. Magariello*, L. Citrigno*, S. Zuchner, M. Gonzalez, A. Patitucci, V. Sofia, F. L. Conforti, I. Pappalardo, R. Mazzei, C. Ungaro, M. Zappia and M. Muglia Institute of Neurological Sciences, National Research Council, Mangone (CS), Italy; Department of Human Genetics and Hussman Institute for Human Genomics, Miller School of Medicine, University of Miami, Miami, FL, USA; and Dipartimento G.F. Ingrassia Sezione di Neuroscienze Universit a di Catania, Catania, Italy

The role of spartin and its novel ubiquitin binding region in DALIS occurrence

Troyer syndrome is an autosomal recessive hereditary spastic paraplegia (HSP) caused by frameshift mutations in the SPG20 gene that results in a lack of expression of the truncated protein. Spartin is a multifunctional protein, yet only two conserved domains--a microtubule-interacting and trafficking domain and a plant-related senescence domain involved in cytokinesis and mitochondrial physiology, respectively--have been defined. We have shown that overexpressed spartin binds to the Ile44 hydrophobic pocket of ubiquitin, suggesting spartin might contain a ubiquitin-binding domain. In the present study, we demonstrate that spartin contributes to the formation of dendritic aggresome-like induced structures (DALIS) through a unique ubiquitin-binding region (UBR). Using short hairpin RNA, we knocked down spartin in RAW264.7 cells and found that DALIS frequency decreased; conversely, overexpression of spartin increased the percentage of cells containing DALIS. Using nuclear magnetic resonance spectroscopy, we characterized spartin's UBR and defined the UBR's amino acids that are key for ubiquitin binding. We also found that spartin, via the UBR, binds Lys-63-linked ubiquitin chains but does not bind Lys-48-linked ubiquitin chains. Finally, we demonstrate that spartin's role in DALIS formation depends on key residues within its UBR.

Sensory ataxia as a prominent clinical presentation in three families with mutations in CYP7B1

Pathogenic mutations in CYP7B1 account for SPG5, an autosomal recessive hereditary spastic paraplegia characterized by a complex phenotype including visual problems and cerebellar dysfunction. Sensory ataxia is not usually regarded as a typical clinical feature of SPG5. The purpose of this study was to describe six patients showing features of sensory ataxia as the prominent and/or initial symptoms of SPG5. Six patients from three distinct pedigrees (three women, three men; age 49.5 ± 18.2 years), all presenting gait unsteadiness and frequent falls since childhood, underwent clinical and molecular investigations. All showed marked sensory ataxic gait with positive Romberg's sign, as well as severely impaired position and vibration sense. Comparatively minor signs of pyramidal involvement were also detected. In four of the patients, brain MRI showed white matter hyperintensities on T2-weighted images. An already reported homozygous c.889A>G (p.T297A) mutation in SPG5/CYP7B1 was found in five patients from two families, whereas the remaining case harbored the novel c.250_251delC/p.L84Ffs*6 and c.266A>C/p.Y89S variants. Marked and enduring sensory ataxia can be a pivotal sign in SPG5, and expands the phenotypic spectrum associated with mutations in CYP7B1.

SPG35 contributes to the second common subtype of AR‐HSP in China: frequency analysis and functional characterization of FA2H gene mutations

Hereditary spastic paraplegias (HSPs) encompass a clinically and genetically heterogeneous group of neurodegenerative disorders. Recently, mutations in fatty acid 2-hydroxylase gene (FA2H) have been identified responsible for HSPs type 35 (SPG35). This study aims to define the contribution of FA2H to Chinese autosomal recessive HSP (AR-HSP) patients and provide insights into the enzymatic functions of the novel mutations. Direct sequencing of FA2H was conducted in 31 AR-HSP families and 55 sporadic cases without SPG11, SPG15, SPG5 and SPG7 gene mutations. Enzymatic activity of the mutated proteins was further examined. Three novel mutations were found in two Chinese families, including two compound heterozygous mutations (c.388C>T/p.L130F and c.506+6C>G) and one homozygous mutation (c.230T>G/p.L77R). The c.506+6C>G splice-site mutation led to the deletion of exon 3. Measurement of enzymatic functions revealed a significant reduction in the enzymatic activity of FA2H associated with p.L130F and p.L77R. Overall, our data widens the spectrum of the mutations on FA2H, and functional analyses indicate that these mutations severely impair the enzymatic activity of FA2H. Furthermore, frequency analysis shows that SPG35 is the second most common subtype of AR-HSP in China.

Loss of Association of REEP2 with Membranes Leads to Hereditary Spastic Paraplegia

Hereditary spastic paraplegias (HSPs) are clinically and genetically heterogeneous neurological conditions. Their main pathogenic mechanisms are thought to involve alterations in endomembrane trafficking, mitochondrial function, and lipid metabolism. With a combination of whole-genome mapping and exome sequencing, we identified three mutations in REEP2 in two families with HSP: a missense variant (c.107T>A [p.Val36Glu]) that segregated in the heterozygous state in a family with autosomal-dominant inheritance and a missense change (c.215T>A [p.Phe72Tyr]) that segregated in trans with a splice site mutation (c.105+3G>T) in a family with autosomal-recessive transmission. REEP2 belongs to a family of proteins that shape the endoplasmic reticulum, an organelle that was altered in fibroblasts from an affected subject. In vitro, the p.Val36Glu variant in the autosomal-dominant family had a dominant-negative effect; it inhibited the normal binding of wild-type REEP2 to membranes. The missense substitution p.Phe72Tyr, in the recessive family, decreased the affinity of the mutant protein for membranes that, together with the splice site mutation, is expected to cause complete loss of REEP2 function. Our findings illustrate how dominant and recessive inheritance can be explained by the effects and nature of mutations in the same gene. They have also important implications for genetic diagnosis and counseling in clinical practice because of the association of various modes of inheritance to this new clinico-genetic entity.

Hereditary spastic paraplegia: up to date

Hereditary spastic paraplegia (HSP) is a clinically and genetically heterogeneous group of neurodegenerative disorders that are clinically characterized by progressive spasticity and weakness of the lower limbs. HSP genetic loci are designated SPG1-72 in order of their discovery. In 206 Japanese families with autosomal dominant HSP, SPG4 was the most common form, accounting for 38%, followed by SPG3A (5%), SPG31 (5%), SPG10 (2%), and SPG8 (1%). We have identified novel mutations in the C12orf65 gene and the LYST gene in several Japanese families with autosomal recessive HSP. JASPAC will facilitate gene discovery and mechanistic understanding of HSP. The future challenge will be the establishment of treatment of HSP.