Long-term course and mutational spectrum of spatacsin-linked spastic paraplegia

Delving into the complexity of hereditary spastic paraplegias: how unexpected phenotypes and inheritance modes are revolutionizing their nosology

Hereditary spastic paraplegias (HSP) are rare neurodegenerative diseases sharing the degeneration of the corticospinal tracts as the main pathological characteristic. They are considered one of the most heterogeneous neurological disorders. All modes of inheritance have been described for the 84 different loci and 67 known causative genes implicated up to now. Recent advances in molecular genetics have revealed clinico-genetic heterogeneity of these disorders including their clinical and genetic overlap with other diseases of the nervous system. The systematic analysis of a large set of genes, including exome sequencing, is unmasking unusual phenotypes or inheritance modes associated with mutations in HSP genes and related genes involved in various neurological diseases. A new nosology may emerge after integration and understanding of these new data to replace the current classification. Collectively, functions of the known genes implicate the disturbance of intracellular membrane dynamics and trafficking as the consequence of alterations of cytoskeletal dynamics, lipid metabolism and organelle structures, which represent in fact a relatively small number of cellular processes that could help to find common curative approaches, which are still lacking.

Hereditary spastic paraplegia: clinical-genetic characteristics and evolving molecular mechanisms

Hereditary spastic paraplegia (HSP) is a group of clinically and genetically heterogeneous neurological disorders characterized by pathophysiologic hallmark of length-dependent distal axonal degeneration of the corticospinal tracts. The prominent features of this pathological condition are progressive spasticity and weakness of the lower limbs. To date, 72 spastic gait disease-loci and 55 spastic paraplegia genes (SPGs) have been identified. All modes of inheritance (autosomal dominant, autosomal recessive, and X-linked) have been described. Recently, a late onset spastic gait disorder with maternal trait of inheritance has been reported, as well as mutations in genes not yet classified as spastic gait disease. Several cellular processes are involved in its pathogenesis, such as membrane and axonal transport, endoplasmic reticulum membrane modeling and shaping, mitochondrial function, DNA repair, autophagy, and abnormalities in lipid metabolism and myelination processes. Moreover, recent evidences have been found about the impairment of endosome membrane trafficking in vesicle formation and about the involvement of oxidative stress and mtDNA polymorphisms in the onset of the disease. Interactome networks have been postulated by bioinformatics and biological analyses of spastic paraplegia genes, which would contribute to the development of new therapeutic approaches.

Overlapping phenotypes in complex spastic paraplegias SPG11, SPG15, SPG35 and SPG48

Hereditary spastic paraplegias are a heterogeneous group of neurodegenerative disorders, clinically classified in pure and complex forms. Genetically, more than 70 different forms of spastic paraplegias have been characterized. A subgroup of complicate recessive forms has been distinguished for the presence of thin corpus callosum and white matter lesions at brain imaging. This group includes several genetic entities, but most of the cases are caused by mutations in the KIAA1840 (SPG11) and ZFYVE26 genes (SPG15). We studied a cohort of 61 consecutive patients with complicated spastic paraplegias, presenting at least one of the following features: mental retardation, thin corpus callosum and/or white matter lesions. DNA samples were screened for mutations in the SPG11/KIAA1840, SPG15/ZFYVE26, SPG21/ACP33, SPG35/FA2H, SPG48/AP5Z1 and SPG54/DDHD2 genes by direct sequencing. Sequence variants were found in 30 of 61 cases: 16 patients carried SPG11/KIAA1840 gene variants (26.2%), nine patients carried SPG15/ZFYVE26 variants (14.8%), three patients SPG35/FA2H (5%), and two patients carried SPG48/AP5Z1 gene variants (3%). Mean age at onset was similar in patients with SPG11 and with SPG15 (range 11-36), and the phenotype was mostly indistinguishable. Extrapyramidal signs were observed only in patients with SPG15, and epilepsy in three subjects with SPG11. Motor axonal neuropathy was found in 60% of cases with SPG11 and 70% of cases with SPG15. Subjects with SPG35 had intellectual impairment, spastic paraplegia, thin corpus callosum, white matter hyperintensities, and cerebellar atrophy. Two families had a late-onset presentation, and none had signs of brain iron accumulation. The patients with SPG48 were a 5-year-old child, homozygous for a missense SPG48/AP5Z1 variant, and a 51-year-old female, carrying two different nonsense variants. Both patients had intellectual deficits, thin corpus callosum and white matter lesions. None of the cases in our cohort carried mutations in the SPG21/ACP33 and SPG54/DDH2H genes. Our study confirms that the phenotype of patients with SPG11 and with SPG15 is homogeneous, whereas cases with SPG35 and with SPG48 cases present overlapping features, and a broader clinical spectrum. The large group of non-diagnosed subjects (51%) suggests further genetic heterogeneity. The observation of common clinical features in association with defects in different causative genes, suggest a general vulnerability of the corticospinal tract axons to a wide spectrum of cellular alterations.

Dysfunction of spatacsin leads to axonal pathology in SPG11-linked hereditary spastic paraplegia

Hereditary spastic paraplegias are a group of inherited motor neuron diseases characterized by progressive paraparesis and spasticity. Mutations in the spastic paraplegia gene SPG11, encoding spatacsin, cause an autosomal-recessive disease trait; however, the precise knowledge about the role of spatacsin in neurons is very limited. We for the first time analyzed the expression and function of spatacsin in human forebrain neurons derived from human pluripotent stem cells including lines from two SPG11 patients and two controls. SPG11 patients'-derived neurons exhibited downregulation of specific axonal-related genes, decreased neurite complexity and accumulation of membranous bodies within axonal processes. Altogether, these data point towards axonal pathologies in human neurons with SPG11 mutations. To further corroborate spatacsin function, we investigated human pluripotent stem cell-derived neurons and mouse cortical neurons. In these cells, spatacsin was located in axons and dendrites. It colocalized with cytoskeletal and synaptic vesicle (SV) markers and was present in synaptosomes. Knockdown of spatacsin in mouse cortical neurons evidenced that the loss of function of spatacsin leads to axonal instability by downregulation of acetylated tubulin. Finally, time-lapse assays performed in SPG11 patients'-derived neurons and spatacsin-silenced mouse neurons highlighted a reduction in the anterograde vesicle trafficking indicative of impaired axonal transport. By employing SPG11 patient-derived forebrain neurons and mouse cortical neurons, this study provides the first evidence that SPG11 is implicated in axonal maintenance and cargo trafficking. Understanding the cellular functions of spatacsin will allow deciphering mechanisms of motor cortex dysfunction in autosomal-recessive hereditary spastic paraplegia.

Novel mutations c.[5121_5122insAG]+[6859C>T] of the SPG11 gene associated with cerebellum hypometabolism in a Chinese case of hereditary spastic paraplegia with thin corpus callosum

Hereditary spastic paraplegia (HSP) is a very heterogeneous disease, both genetically and clinically. To date, approximately 52 loci and 31 genes have been reported to be involved in the causality of HSP. The pattern of inheritance of the disease can be autosomal dominant, autosomal recessive, or X-linked recessive. Autosomal recessive HSP with thin corpus callosum (ARHSP-TCC) is one form of this disease, and a recessive gene, SPG11, is responsible for 41-77% of all ARHSP-TCC cases. SPG11 encodes the protein SPATACSIN, which is most prominently expressed in the cerebellum. However, little is known about its function. Despite diverse clinical presentations, diffuse hypometabolism in the cerebellum has not been reported previously. We have identified an HSP-TCC patient that presented with prominent intellectual disability rather than spasticity. (18)Fluorodeoxyglucose positron emission tomography/computed tomography ((18)FDG-PET/CT) examination showed diffuse hypometabolism in both cerebella. Mutation screening of the SPG11 gene using Sanger sequencing identified the novel compound heterozygous mutation c.[5121_5122insAG]+[6859C>T] (p.[I1708RfsX2]+[Q2287X]) in the patient. The mother bears the c.5121_5122insAG mutation, which results in a frameshift and is predicted to truncate the 735 amino acids from the C-terminus, and the father carries the c.6859C>T mutation, which terminates the 157 amino acids from the C-terminus. Therefore, these mutations may result in the loss of function of wild-type SPATACSIN. Our results suggest that SPATACSIN may be involved in cerebella metabolism, and the novel mutations provide more data for the mutational spectrum of this gene, which will aid in the development of quick and accurate genetic diagnostic tools for this disease.

Immunohistochemical localization of spatacsin in α-synucleinopathies

Spatacsin (SPG11) is a major mutated gene in autosomal recessive spastic paraplegia with thin corpus callosum (ARHSP-TCC) and is responsible for juvenile Parkinsonism. To elucidate the role of spatacsin in the pathogenesis of α-synucleinopathies, an immunohistochemical investigation was performed on the brain of patients with Parkinson's disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy (MSA) using anti-spatacsin antibody. In PD, Lewy bodies (LBs) in the brain stem were positive for spatacsin. These LBs showed intense staining in their peripheral portions and occasionally in the central cores. Lewy neurites were also spatacsin-positive. In DLB, cortical LBs were immunolabeled by spatacsin. In MSA, glial cytoplasmic inclusions (GCI) and a small fraction of neuronal cytoplasmic inclusions (NCI) were positive for spatacsin. The widespread accumulation of spatacsin observed in pathologic α-synuclein-containing inclusions suggests that spatacsin may be involved in the pathogenesis of α-synucleinopathies.

Microstructural integrity of cerebral fiber tracts in hereditary spastic paraparesis with SPG11 mutation

BACKGROUND AND PURPOSE

ARHSP-TCC is characterized by progressive leg spasticity, ataxia, and cognitive dysfunction. Although mutations in the human SPG11 gene were identified as responsible for ARHSP-TCC, the cerebral fiber integrity has not been assessed systemically. The objective of this study was to assess cerebral fiber integrity and its clinical significance in patients with ARHSP-TCC.

MATERIALS AND METHODS

Five patients from 2 families who were clinically and genetically confirmed to have ARHSP-TCC were examined by neuropsychological evaluation and DSI of the brain. We performed voxel-based GFA analysis for global white matter evaluation, tractography-based analysis for tract-to-tract comparisons, and tract-specific analysis of the CST to evaluate microstructural integrity along the axonal direction.

RESULTS

The neuropsychological evaluation revealed widespread cognitive decline across all domains. Voxel-based analysis showed global reduction of GFA in the cerebral white matter. Tractography-based analysis revealed a significant reduction of the microstructural integrity in all neural fiber types, while commissure and association fibers had more GFA reduction than projection fibers (P < .00001). Prefrontal and motor portions of the CC were most severely affected among all fiber tracts (P < .00001, P = .018). Tract-specific analysis of the CST validated a "dying-back" phenomenon (R(2) = 0.68, P < .00001).

CONCLUSIONS

There was a characteristic gradation in the reduction of microstructural integrity among fiber types and within the CC in patients with the SPG11 mutation. The dying-back process in CST might explain the pathogenic mechanisms for ARHSP-TCC.

Rapidly deteriorating course in Dutch hereditary spastic paraplegia type 11 patients

Although SPG11 is the most common complicated hereditary spastic paraplegia, our knowledge of the long-term prognosis and life expectancy is limited. We therefore studied the disease course of all patients with a proven SPG11 mutation as tested in our laboratory, the single Dutch laboratory providing SPG11 mutation analysis, between 1 January 2009 and 1 January 2011. We identified nine different SPG11 mutations, four of which are novel, in nine index patients. Eighteen SPG11 patients from these nine families were studied by means of a retrospective chart analysis and additional interview/examination. Ages at onset were between 4 months and 14 years; 39% started with learning difficulties rather than gait impairment. Brain magnetic resonance imaging showed a thin corpus callosum and typical periventricular white matter changes in the frontal horn region (known as the 'ears-of the lynx'-sign) in all. Most patients became wheelchair bound after a disease duration of 1 to 2 decades. End-stage disease consisted of loss of spontaneous speech, severe dysphagia, spastic tetraplegia with peripheral nerve involvement and contractures. Several patients died of complications between ages 30 and 48 years, 3-4 decades after onset of gait impairment. Other relevant features during the disease were urinary and fecal incontinence, obesity and psychosis. Our study of 18 Dutch SPG11-patients shows the potential serious long-term consequences of SPG11 including a possibly restricted life span.

Hereditary spastic paraplegias: one disease for many genes, and still counting

Hereditary spastic paraplegias (HSPs) are genetically heterogeneous Mendelian disorders characterized by spastic gait with stiffness and weakness in the legs and an associated plethora of neurological or extraneurological signs in "complicated" forms. Major advances have been made during the past two decades in our understanding of their molecular bases with the identification of a large number of gene loci and the cloning of a set of them. The combined genetic and clinical information obtained has permitted a new, molecularly-driven classification and an improved diagnosis of these conditions. This represents a prerequisite for better counseling in families and more appropriate therapeutic options. However, further heterogeneity is expected and new insight into the possible mechanisms anticipated.

Amyotrophic lateral sclerosis: new insights into underlying molecular mechanisms and opportunities for therapeutic intervention

Recent years have witnessed a renewed interest in the pathogenic mechanisms of amyotrophic lateral sclerosis (ALS), a late-onset progressive degeneration of motor neurons. The discovery of new genes associated with the familial form of the disease, along with a deeper insight into pathways already described for this disease, has led scientists to reconsider previous postulates. While protein misfolding, mitochondrial dysfunction, oxidative damage, defective axonal transport, and excitotoxicity have not been dismissed, they need to be re-examined as contributors to the onset or progression of ALS in the light of the current knowledge that the mutations of proteins involved in RNA processing, apparently unrelated to the previous "old partners," are causative of the same phenotype. Thus, newly envisaged models and tools may offer unforeseen clues on the etiology of this disease and hopefully provide the key to treatment.

Novel SPG11 mutations in Chinese families with hereditary spastic paraplegia with thin corpus callosum

BACKGROUND

Hereditary spastic paraplegia is a clinically and genetically heterogeneous neurodegenerative disorder characterized by progressive spasticity of the lower limbs. Mutations in SPG11 gene have been recently identified as a major cause of hereditary spastic paraplegia with thin corpus callosum.

METHODS

Two unrelated Chinese families were examined by clinical evaluation, mutation analysis of SPG11, detailed neuropsychological assessment and diffusion tensor imaging.

RESULTS

Both patients presented with spastic paraparesis and learning disability. Two novel and one known mutations in SPG11 were detected through genetic analysis. Cognitive impairment was found with severe deficits in domains such as executive functions and memory. Magnetic resonance imaging showed thin corpus callosum while diffusion tensor imaging revealed increased mean diffusion and decreased fractional anisotropy in the corpus callosum and subcortical white matter in frontal, temporal lobe compared with the healthy controls.

CONCLUSIONS

This study widens the spectrum of mutations in SPG11. The application of detailed neuropsychological tests and diffusion tensor imaging could detect cerebral subtle involvement even in early stage of the disease.

Autosomal recessive hereditary spastic paraplegia with thin corpus callosum: a novel mutation in the SPG11 gene and further evidence for genetic heterogeneity

BACKGROUND AND PURPOSE

Autosomal Recessive Hereditary Spastic Paraplegia with Thin Corpus Callosum (AR-HSPTCC) is a clinically and genetically heterogeneous complicated form of spastic paraplegia. Two AR-HSPTCC loci have been assigned to chromosome 15q13-15 (SPG11) and chromosome 8p12-p11.21 respectively. Mutations in the SPG11 gene, encoding the spatacsin protein, have been found in the majority of SPG11 families. In this study, involvement of the SPG11 or 8p12-p11.21 loci was investigated in five Italian families, of which four consanguineous.

METHODS

Families were tested for linkage to the SPG11 or 8p12-p11.21 loci and the SPG11 gene was screened in all the affected individuals.

RESULTS

Linkage was excluded in the four consanguineous families. In the only SPG11-linked family the same homozygous haplotype 4.2 cM across the SPG11 locus was shared by all the three affected siblings. A novel c.2608A>G mutation predicted to affect the splicing was found in exon 14 of the SPG11 gene.

DISCUSSION

This collection of families contributes to highlight the intra and inter locus heterogeneity in AR-HSPTCC, already remarked in previous reports. In particular, it confirms heterogeneity amongst Italian families and reports a new mutation predicted to affect splicing in the spatacsin gene.

Neurotransmitter abnormalities and response to supplementation in SPG11

OBJECTIVE

To report the detection of secondary neurotransmitter abnormalities in a group of SPG11 patients and describe treatment with l-dopa/carbidopa and sapropterin.

DESIGN

Case reports.

SETTING

National Institutes of Health in the Undiagnosed Disease Program; Children's National Medical Center in the Myelin Disorders Bioregistry Program.

PATIENTS

Four SPG11 patients with a clinical picture of progressive spastic paraparesis complicated by extrapyramidal symptoms and maculopathy.

INTERVENTIONS

L-Dopa/carbidopa and sapropterin.

RESULTS

3/4 patients presented secondary neurotransmitter abnormalities; 4/4 partially responded to L-dopa as well as sapropterin.

CONCLUSIONS

In the SPG11 patient with extrapyramidal symptoms, a trial of L-dopa/carbidopa and sapropterin and/or evaluation of cerebrospinal fluid neurotransmitters should be considered.

Spatacsin and spastizin act in the same pathway required for proper spinal motor neuron axon outgrowth in zebrafish

Hereditary spastic paraplegias (HSPs) are rare neurological conditions caused by degeneration of the long axons of the cerebrospinal tracts, leading to locomotor impairment and additional neurological symptoms. There are more than 40 different causative genes, 24 of which have been identified, including SPG11 and SPG15 mutated in complex clinical forms. Since the vast majority of the causative mutations lead to loss of function of the corresponding proteins, we made use of morpholino-oligonucleotide (MO)-mediated gene knock-down to generate zebrafish models of both SPG11 and SPG15 and determine how invalidation of the causative genes (zspg11 and zspg15) during development might contribute to the disease. Micro-injection of MOs targeting each gene caused locomotor impairment and abnormal branching of spinal cord motor neurons at the neuromuscular junction. More severe phenotypes with abnormal tail developments were also seen. Moreover, partial depletion of both proteins at sub-phenotypic levels resulted in the same phenotypes, suggesting for the first time, in vivo, a genetic interaction between these genes. In conclusion, the zebrafish orthologues of the SPG11 and SPG15 genes are important for proper development of the axons of spinal motor neurons and likely act in a common pathway to promote their proper path finding towards the neuromuscular junction.

Alu elements mediate large SPG11 gene rearrangements: further spatacsin mutations

PURPOSE

Hereditary spastic paraplegias compose a group of neurodegenerative disorders with a large clinical and genetic heterogeneity. Among the autosomal recessive forms, spastic paraplegia type 11 is the most common.

METHODS

To better understand the spastic paraplegia type 11 mutation spectrum, we studied a group of 54 patients with hereditary spastic paraplegia. Mutation screening was performed by PCR amplification of SPG11 coding regions and intron boundaries, followed by sequencing. For the detection of large gene rearrangements, we performed multiplex ligation-dependent probe amplification.

RESULTS

We report 13 families with spastic paraplegia type 11 carrying either novel or previously identified mutations. We describe a complex entire SPG11 rearrangement and show that large gene rearrangements are frequent among patients with spastic paraplegia type 11. Moreover, we mapped the deletion breakpoints of three different large SPG11 deletions and provide evidence for Alu microhomology-mediated exon deletion.

CONCLUSION

Our analysis shows that the high number of repeated elements in SPG11 together with the presence of recombination hotspots and the high intrinsic instability of the 15q locus all contribute toward making this genomic region more prone to large gene rearrangements. These findings enlarge the amount of data relating repeated elements with neurodegenerative disorders and highlight their importance in human disease and genome evolution.

The use of next-generation sequencing in movement disorders

New advances in genomic technology are being introduced at a greater speed and are revolutionizing the field of genetics for both complex and Mendelian diseases. For instance, during the past few years, genome-wide association studies (GWAS) have identified a large number of significant associations between genomic loci and movement disorders such as Parkinson's disease and progressive supranuclear palsy. GWAS are carried out through the use of high-throughput SNP genotyping arrays, which are also used to perform linkage analyses in families previously considered statistically underpowered for genetic analyses. In inherited movement disorders, using this latter technology, it has repeatedly been shown that mutations in a single gene can lead to different phenotypes, while the same clinical entity can be caused by mutations in different genes. This is being highlighted with the use of next-generation sequencing technologies and leads to the search for genes or genetic modifiers that contribute to the phenotypic expression of movement disorders. Establishing an accurate genome-epigenome-phenotype relationship is becoming a major challenge in the post-genomic research that should be facilitated through the implementation of both functional and cellular analyses.

An analysis of exome sequencing for diagnostic testing of the genes associated with muscle disease and spastic paraplegia

In this study, we assess exome sequencing (ES) as a diagnostic alternative for genetically heterogeneous disorders. Because ES readily identified a previously reported homozygous mutation in the CAPN3 gene for an individual with an undiagnosed limb girdle muscular dystrophy, we evaluated ES as a generalizable clinical diagnostic tool by assessing the targeting efficiency and sequencing coverage of 88 genes associated with muscle disease (MD) and spastic paraplegia (SPG). We used three exome-capture kits on 125 individuals. Exons constituting each gene were defined using the UCSC and CCDS databases. The three exome-capture kits targeted 47-92% of bases within the UCSC-defined exons and 97-99% of bases within the CCDS-defined exons. An average of 61.2-99.5% and 19.1-99.5% of targeted bases per gene were sequenced to 20X coverage within the CCDS-defined MD and SPG coding exons, respectively. Greater than 95-99% of targeted known mutation positions were sequenced to ≥1X coverage and 55-87% to ≥20X coverage in every exome. We conclude, therefore, that ES is a rapid and efficient first-tier method to screen for mutations, particularly within the CCDS annotated exons, although its application requires disclosure of the extent of coverage for each targeted gene and supplementation with second-tier Sanger sequencing for full coverage.

Structural and metabolic damage in brains of patients with SPG11-related spastic paraplegia as detected by quantitative MRI

The goal of this work was to assess brain structural and metabolic abnormalities of subjects with SPG11 and their relevance to clinical disability by using quantitative magnetic resonance (MR) metrics. Autosomal recessive hereditary spastic paraplegia (AR-HSP) with thin corpus callosum and cognitive decline is a complex neurological disorder caused by mutations in the SPG11 gene in most cases. Little is known about the process leading to corticospinal and white matter degeneration. We performed conventional MRI/MR spectroscopic imaging ((1)H-MRSI) examinations in 10 HSP patients carrying an SPG11 mutation and in 10 demographically matched healthy controls (HC). We measured in each subject cerebral white matter hyperintensities (WMHs), normalized global and cortical brain volumes, and (1)H-MRSI-derived central brain levels of N-acetylaspartate (NAA) and choline (Cho) normalized to creatine (Cr). Clinical disability was assessed according to patients' autonomy in walking. Conventional MRI showed WMHs in all patients. Global brain volumes were lower in patients than in HC (p < 0.001). Decreased values were diffusely found also in cortical regions (p < 0.01). On (1)H-MRSI, NAA/Cr values were lower in SPG11 patients than in HC (p = 0.002). Cho/Cr values did not differ between patients and HC. Cerebral volume decreases and NAA/Cr in the corona radiata correlated closely with increasing disability scores (p < 0.05). Quantitative MR measures propose that widespread structural and metabolic brain damage occur in SPG11 patients. The correlation of these MR metrics with measures of patients' disease severity suggests that they might represent adequate surrogate markers of disease outcome.

Novel mutations in SPG11 cause hereditary spastic paraplegia associated with early-onset levodopa-responsive Parkinsonism

BACKGROUND

Autosomal recessive hereditary spastic paraplegia with thin corpus callosum is a neurodegenerative disorder characterized by spastic paraparesis, cognitive impairment, and peripheral neuropathy. The neuroradiologic hallmarks are thin corpus callosum and periventricular white matter changes. Mutations in the SPG11 gene have been identified to be a major cause of autosomal recessive hereditary spastic paraplegia with thin corpus callosum and recently also proven to be responsible for juvenile parkinsonism associated with spastic paraplegia.

METHODS

We describe one Italian autosomal recessive hereditary spastic paraplegia with thin corpus callosum patient who unusually presented at onset, 16 years, with parkinsonism-like features, responsive to dopaminergic therapy. Then the clinical picture evolved and became more complex. A brain magnetic resonance imaging scan showed thin corpus callosum and hyperintense T(2)-weighted lesions in periventricular regions, and the (123)I-ioflupane single-photon emission coupled tomography was abnormal.

RESULTS

Genetic analysis detected two novel mutations, a c.3664insT variant in compound heterozygosity with a c.6331insG mutation, in SPG11.

DISCUSSION

This case confirms the high genetic and clinical heterogeneity associated with SPG11 mutations. It also offers further evidence that parkinsonism may initiate autosomal recessive hereditary spastic paraplegia with thin corpus callosum and that parkinsonian symptoms can have variable dopaminergic response in these patients.

Novel SPG11 mutations in Asian kindreds and disruption of spatacsin function in the zebrafish

Autosomal recessive hereditary spastic paraplegia with thin corpus callosum (HSP-TCC) maps to the SPG11 locus in the majority of cases. Mutations in the KIAA1840 gene, encoding spatacsin, have been shown to underlie SPG11-linked HSP-TCC. The aim of this study was to perform candidate gene analysis in HSP-TCC subjects from Asian families and to characterize disruption of spatacsin function during zebrafish development. Homozygosity mapping and direct sequencing were used to assess the ACCPN, SPG11, and SPG21 loci in four inbred kindreds originating from the Indian subcontinent. Four novel homozygous SPG11 mutations (c.442+1G>A, c.2146C>T, c.3602_3603delAT, and c.4846C>T) were identified, predicting a loss of spatacsin function in each case. To investigate the role of spatacsin during development, we additionally ascertained the complete zebrafish spg11 ortholog by reverse transcriptase PCR and 5′ RACE. Analysis of transcript expression through whole-mount in situ hybridization demonstrated ubiquitous distribution, with highest levels detected in the brain. Morpholino antisense oligonucleotide injection was used to knock down spatacsin function in zebrafish embryos. Examination of spg11 morphant embryos revealed a range of developmental defects and CNS abnormalities, and analysis of axon pathway formation demonstrated an overall perturbation of neuronal differentiation. These data confirm loss of spatacsin as the cause of SPG11-linked HSP-TCC in Asian kindreds, expanding the mutation spectrum recognized in this disorder. This study represents the first investigation in zebrafish addressing the function of a causative gene in autosomal recessive HSP and identifies a critical role for spatacsin during early neural development in vivo.

Early-onset L-dopa-responsive parkinsonism with pyramidal signs due to ATP13A2, PLA2G6, FBXO7 and spatacsin mutations

Seven autosomal recessive genes associated with juvenile and young-onset Levodopa-responsive parkinsonism have been identified. Mutations in PRKN, DJ-1, and PINK1 are associated with a rather pure parkinsonian phenotype, and have a more benign course with sustained treatment response and absence of dementia. On the other hand, Kufor-Rakeb syndrome has additional signs, which distinguish it clearly from Parkinson's disease including supranuclear vertical gaze palsy, myoclonic jerks, pyramidal signs, and cognitive impairment. Neurodegeneration with brain iron accumulation type I (Hallervorden-Spatz syndrome) due to mutations in PANK2 gene may share similar features with Kufor-Rakeb syndrome. Mutations in three other genes, PLA2G6 (PARK14), FBXO7 (PARK15), and Spatacsin (SPG11) also produce clinical similar phenotypes in that they presented with rapidly progressive parkinsonism, initially responsive to Levodopa treatment but later, developed additional features including cognitive decline and loss of Levodopa responsiveness. Here, using homozygosity mapping and sequence analysis in families with complex parkinsonisms, we identified genetic defects in the ATP13A2 (1 family), PLA2G6 (1 family) FBXO7 (2 families), and SPG11 (1 family). The genetic heterogeneity was surprising given their initially common clinical features. On careful review, we found the FBXO7 cases to have a phenotype more similar to PRKN gene associated parkinsonism. The ATP13A2 and PLA2G6 cases were more seriously disabled with additional swallowing problems, dystonic features, severe in some, and usually pyramidal involvement including pyramidal weakness. These data suggest that these four genes account for many cases of Levodopa responsive parkinsonism with pyramidal signs cases formerly categorized clinically as pallido-pyramidal syndrome.

A new locus (SPG46) maps to 9p21.2-q21.12 in a Tunisian family with a complicated autosomal recessive hereditary spastic paraplegia with mental impairment and thin corpus callosum

Hereditary spastic paraplegia (HSP) with thin corpus callosum (TCC) and mental impairment is a frequent subtype of complicated HSP, often inherited as an autosomal recessive (AR) trait. It is clear from molecular genetic analyses that there are several underlying causes of this syndrome, with at least six genetic loci identified to date. However, SPG11 and SPG15 are the two major genes for this entity. To map the responsible gene in a large AR-HSP-TCC family of Tunisian origin, we investigated a consanguineous family with a diagnosis of AR-HSP-TCC excluded for linkage to the SPG7, SPG11, SPG15, SPG18, SPG21, and SPG32 loci. A genome-wide scan was undertaken using 6,090 SNP markers covering all chromosomes. The phenotypic presentation in five patients was suggestive of a complex HSP that associated an early-onset spastic paraplegia with mild handicap, mental deterioration, congenital cataract, cerebellar signs, and TCC. The genome-wide search identified a single candidate region on chromosome 9, exceeding the LOD score threshold of +3. Fine mapping using additional markers narrowed the candidate region to a 45.1-Mb interval (15.4 cM). Mutations in three candidate genes were excluded. The mapping of a novel AR-HSP-TCC locus further demonstrates the extensive genetic heterogeneity of this condition. We propose that testing for this locus should be performed, after exclusion of mutations in SPG11 and SPG15 genes, in AR-HSP-TCC families, especially when cerebellar ataxia and cataract are present.

SPATACSIN mutations cause autosomal recessive juvenile amyotrophic lateral sclerosis

The mutation of the spatacsin gene is the single most common cause of autosomal recessive hereditary spastic paraplegia with thin corpus callosum. Common clinical, pathological and genetic features between amyotrophic lateral sclerosis and hereditary spastic paraplegia motivated us to investigate 25 families with autosomal recessive juvenile amyotrophic lateral sclerosis and long-term survival for mutations in the spatascin gene. The inclusion criterion was a diagnosis of clinically definite amyotrophic lateral sclerosis according to the revised El Escorial criteria. The exclusion criterion was a diagnosis of hereditary spastic paraplegia with thin corpus callosum in line with an established protocol. Additional pathological and genetic evaluations were also performed. Surprisingly, 12 sequence alterations in the spatacsin gene (one of which is novel, IVS30 + 1 G > A) were identified in 10 unrelated pedigrees with autosomal recessive juvenile amyotrophic lateral sclerosis and long-term survival. The countries of origin of these families were Italy, Brazil, Canada, Japan and Turkey. The variants seemed to be pathogenic since they co-segregated with the disease in all pedigrees, were absent in controls and were associated with amyotrophic lateral sclerosis neuropathology in one member of one of these families for whom central nervous system tissue was available. Our study indicates that mutations in the spatascin gene could cause a much wider spectrum of clinical features than previously recognized, including autosomal recessive juvenile amyotrophic lateral sclerosis.

SPG11 mutations cause Kjellin syndrome, a hereditary spastic paraplegia with thin corpus callosum and central retinal degeneration

Autosomal recessive hereditary spastic paraplegia (ARHSP) with thin corpus callosum (TCC) is genetically heterogenous and approximately 35% of patients carry mutations in either of the SPG11 or SPG15 genes. Disease onset is during the first three decades of life with spastic paraplegia and mental impairment. Peripheral neuropathy and amyotrophy may occur. Kjellin syndrome is characterized by central retinal degeneration in addition to ARHSP-TCC and the disease is associated with mutations in the SPG15 gene. We identified five patients in four unrelated kindreds with spastic paraplegia and mental impairment. Magnetic resonance imaging revealed TCC, atrophy elsewhere in the brain and increased T2 signal intensity in the periventricular white matter. Probands from the four kindreds were screened for mutations in the SPG11 gene. All patients were found homozygous or compound heterozygous for truncating SPG11 mutations of which four are reported for the first time. Ophthalmological investigations revealed that the four index cases have central retinal degeneration consistent with Kjellin syndrome. PET examinations with N-[11C-methyl]-L-deuterodeprenyl (DED) and fluor-18 2-fluorodeoxyglucose (FDG) were performed in two patients with Kjellin syndrome. We observed a reduced glucose uptake in the thalami, anterior cingulum, and sensorimotor cortex indicating neuronal loss, and an increased DED binding in the thalami and pons which suggests astrogliosis. From our results we extend the SPG11 associated phenotype to comprise also Kjellin syndrome, previously found to be associated with mutations in the SPG15 gene. We anticipate that degeneration of the central retina is a common and previously unrecognized feature in SPG11 related disease.

SPG11 spastic paraplegia. A new cause of juvenile parkinsonism

Autosomal recessive hereditary spastic paraplegia (AR HSP) with thin corpus callosum (TCC) is a rare neurodegenerative disorder often caused by mutations in the gene encoding for spatacsin at the SPG11 locus on chromosome 15q. The disease is characterized by progressive spastic paraparesis and mental retardation which occur during the first two decades of life and frequently with peripheral neuropathy. Brain magnetic resonance imaging (MRI) reveals typical TCC with periventricular white matter changes. We describe two patients, of Turkish descent, from the same consanguineous family and affected with SPG11 in association with unusual early-onset parkinsonism. Parkinsonism occurred during the very early stages of SPG11 in both patients, being in one the inaugural symptom of the disease presented as a resting tremor with akinesia, rigidity and expressing an initial moderate levodopa-response that progressively weakened. The second patient presented a resting tremor with mild akinesia and no levodopa-response. Both patients were affected with progressive spastic paraparesis which had initially occurred at 15 and 12 years of age, respectively, in association with mild mental retardation and an axonal polyneuropathy. TCC with periventricular white matter changes (PWMC) was evident by MRI and (123)I-ioflupane SPECT was abnormal. Genetic analysis detected for both patients a new c.704_705delAT, p.H235RfsX12 homozygous mutation in SPG11. This report provides evidence that parkinsonism may initiate SPG11-linked HSP TCC and that SPG11 may cause juvenile parkinsonism.

Widespread thalamic and cerebellar degeneration in a patient with a complicated hereditary spastic paraplegia (HSP)

The hereditary spastic paraplegias (HSP) are a heterogeneous group of familial movement disorders sharing progressive spastic paraplegia as a common disease sign. In the present study, we performed the first pathoanatomical investigation of the central nervous degeneration of a female patient with a complicated HSP form who suffered from progressive spastic paraplegia, dysarthria, emotional symptoms, cognitive decline and a variety of additional neuropsychological deficits. This pathoanatomical investigation revealed in addition to loss of layer V Betz pyramidal cells in the primary motor cortex, widespread cerebellar neurodegeneration (i.e., loss of Purkinje cells and neuronal loss in the deep cerebellar nuclei), extensive and severe neuronal loss in a large number of thalamic nuclei, involvement of some brainstem nuclei, as well as damage to descending (i.e., lateral and ventral corticospinal tracts) and ascending (i.e., dorsal and ventral spinocerebellar tracts, gracile fascicle) fiber tracts. In view of their known functional role, damage to these central nervous gray and white matter components offers explanations for the patient's pyramidal signs, her cerebellar, psychiatric and neuropsychological disease symptoms.

Novel mutations of the SPG11 gene in hereditary spastic paraplegia with thin corpus callosum

BACKGROUND

Hereditary spastic paraplegia with thin corpus callosum (HSP-TCC) is a clinically and genetically heterogeneous neurodegenerative disorder with genetic linkage to multi-loci. Recently pathogenic mutations in the KIAA1840 (now named SPG11) for SPG11, the major HSP-TCC locus, were identified; at least 42 different mutations have been detected.

OBJECTIVE

To study the clinical features and identify the SPG11 gene mutations in Chinese patients with HSP-TCC.

METHODS

Three kindreds with an autosomal recessive HSP-TCC and 5 cases with sporadic HSP-TCC in Chinese Hans were recruited. Detailed clinical history, neurological examination, MRI, electromyography, Mini Mental State Examination (MMSE), Spastic Paraplegia Rating Scale (SPRS) were presented. DNA samples of the 8 families were collected and mutation analysis of SPG11 gene was carried out by direct DNA sequencing.

RESULTS

Except for one patient whose age at onset was 3 years old, 10 patients manifested a relatively similar combination of adolescence-onset cognitive decline and spastic paraparesis with TCC on brain MRI. We identified 10 novel and one known mutations in our 8 HSP-TCC families, which were two nonsense mutations (c.5977C>T/p.Q1993X, c.4668T>A/p.Y1556X), three small deletions (c.6898_6899delCT/p.L2300AfsX2338, c.3719_3720delTA/p.I1240VfsX263, c.733_734delAT/p.M245VfsX246), four small insertions (c.7088_7089insATTA/p.Y2363X, c.2163_2164insT/p.I722YfsX731, c.7101_7102insT/p.K2368X, c.6790_6791insC/p.L2264PfsX2339), one deletion/insertion (c.654_655delinsG/p.S218RfsX219), and one splice mutation (c.7151+4_7151+7delAGTA/p.K2384fsX2386). Each family has a different mutation and all the mutations are predicted to cause early protein truncation.

CONCLUSION

This study widens the mutation spectrum of the SPG11 gene and the mutations in the SPG11 gene are also the major causative gene for HSP-TCC in the Chinese Hans. Screening of the whole gene is recommended in clinical practice.

Identification of a heterozygous genomic deletion in the spatacsin gene in SPG11 patients using high-resolution comparative genomic hybridization

Mutations in the spatacsin gene have recently been identified as the genetic cause of autosomal-recessive spastic paraplegia (SPG) with thin corpus callosum, mapping to chromosome 15p13-21. While several nonsense and frameshift mutations as well as splice mutations have been identified, large genomic deletions have not yet been found, potentially due to the absence of an efficient analysis tool. After complete sequencing of 12 autosomal recessive hereditary spastic paraplegia patients with suggestive clinical signs, we were able to define nine SPG11 cases but were left with three patients in which only one SPG11 mutation could be identified by direct sequencing. In these patients, we performed high-resolution comparative genomic hybridization using a predesigned human chromosome 15 tiling array with an average spacing of 100 bp. Data analysis suggested heterozygous genomic deletion within the spatacsin gene in all three patients. In one patient, a relatively small genomic deletion (8.2 kb) could be validated by quantitative polymerase chain reaction (PCR) and long-range PCR, allowing the diagnosis of the deletion of exons 31 through 34. For two patients, quantitative PCR validation could not confirm a genomic deletion. As high density tiling arrays are available for the entire human genome, we suggest this approach for the screening of heterozygous genomic deletions in candidate genes down to a few kilobases.

Clinical heterogeneity and genotype-phenotype correlations in hereditary spastic paraplegia because of Spatacsin mutations (SPG11)

BACKGROUND

Autosomal recessive hereditary spastic paraplegia (ARHSP) with thin corpus callosum is a distinct and usually severe form of complex hereditary spastic paraplegia classified as SPG11. Recently mutations on SPG11 gene (KIAA1840), which is localized to chromosome 15q13-q15, were shown to cause the majority of SPG11 cases.

METHODS

We analysed the 40 coding exons of this gene in the probands from eight families with complex ARHSP, four of these families had a thin corpus callosum and two has mild thinning.

RESULTS

Three families were identified with novel mutations in the SPG11 gene. One family was of Asian origin with a homozygous nonsense mutation and had a very severe phenotype but only very mild thinning of the corpus callosum. In the other two English families the parents were unrelated and the mutations were compound heterozygotes. In these two families the phenotype was mild and both probands had a thin corpus callosum.

CONCLUSION

Given the probable mechanism of action of the mutations in the Spatacsin gene, we discuss the probable genotype phenotype correlations in these families. This study confirms the frequent occurrence of Spatacsin mutations in complex ARHSP with genotype phenotype effects and exposes the spectrum of clinical heterogeneity in SPG11.