Hereditary spastic paraplegia type 35 in a Turkish girl with fatty acid hydroxylase-associated neurodegeneration

OBJECTIVES

The fatty acid 2-hydroxylase gene (FA2H) compound heterozygous or homozygous variants that cause spastic paraplegia type 35 (SPG35) (OMIM # 612319) are autosomal recessive HSPs. FA2H gene variants in humans have been shown to be associated with not only SPG35 but also leukodystrophy and neurodegeneration with brain iron accumulation.

CASE PRESENTATION

A patient with a spastic gait since age seven was admitted to the paediatric metabolism department. She was born to consanguineous, healthy Turkish parents and had no family history of neurological disease. She had normal developmental milestones and was able to walk at 11 months. At age seven, she developed a progressive gait disorder with increased muscle tone in her lower limbs, bilateral ankle clonus and dysdiadochokinesis. She had frequent falls and deteriorating school performance. Despite physiotherapy, her spastic paraplegia was progressive. Whole exome sequencing (WES) identified a homozygous NM_024306.5:c.460C>T missense variant in the FA2H gene, of which her parents were heterozygous carriers. A brain MRI showed a slight reduction in the cerebellar volume with no iron deposits.

CONCLUSIONS

Pathogenic variants of the FA2H gene have been linked to neurodegeneration with iron accumulation in the brain, leukodystrophy and SPG35. When patients developed progressive gait deterioration since early childhood even if not exhibited hypointensity in the basal ganglia detected by neuroimaging, FA2H-related neurodegeneration with brain iron accumulation should be ruled out. FA2H/SPG35 disease is characterised by notable clinical and imaging variability, as well as phenotypic diversity.

Novel TFG mutation causes autosomal-dominant spastic paraplegia and defects in autophagy

BACKGROUND

Mutations in the tropomyosin receptor kinase fused (TFG) gene are associated with various neurological disorders, including autosomal recessive hereditary spastic paraplegia (HSP), autosomal dominant hereditary motor and sensory neuropathy with proximal dominant involvement (HMSN-P) and autosomal dominant type of Charcot-Marie-Tooth disease type 2.

METHODS

Whole genome sequencing and whole-exome sequencing were used, followed by Sanger sequencing for validation. Haplotype analysis was performed to confirm the inheritance mode of the novel TFG mutation in a large Chinese family with HSP. Additionally, another family diagnosed with HMSN-P and carrying the reported TFG mutation was studied. Clinical data and muscle pathology comparisons were drawn between patients with HSP and patients with HMSN-P. Furthermore, functional studies using skin fibroblasts derived from patients with HSP and patients with HMSN-P were conducted to investigate the pathomechanisms of TFG mutations.

RESULTS

A novel heterozygous TFG variant (NM_006070.6: c.125G>A (p.R42Q)) was identified and caused pure HSP. We further confirmed that the well-documented recessively inherited spastic paraplegia, caused by homozygous TFG mutations, exists in a dominantly inherited form. Although the clinical features and muscle pathology between patients with HSP and patients with HMSN-P were distinct, skin fibroblasts derived from both patient groups exhibited reduced levels of autophagy-related proteins and the presence of TFG-positive puncta.

CONCLUSIONS

Our findings suggest that autophagy impairment may serve as a common pathomechanism among different clinical phenotypes caused by TFG mutations. Consequently, targeting autophagy may facilitate the development of a uniform treatment for TFG-related neurological disorders.

DDHD2, whose mutations cause spastic paraplegia type 54, enhances lipophagy via engaging ATG8 family proteins

Hereditary spastic paraplegia (HSP) is a group of inherited neurodegenerative disorders characterized by progressive lower limb spasticity and weakness. One subtype of HSP, known as SPG54, is caused by biallelic mutations in the DDHD2 gene. The primary pathological feature observed in patients with SPG54 is the massive accumulation of lipid droplets (LDs) in the brain. However, the precise mechanisms and roles of DDHD2 in regulating lipid homeostasis are not yet fully understood. Through Affinity Purification-Mass Spectroscopy (AP-MS) analysis, we identify that DDHD2 interacts with multiple members of the ATG8 family proteins (LC3, GABARAPs), which play crucial roles in lipophagy. Mutational analysis reveals the presence of two authentic LIR motifs in DDHD2 protein that are essential for its binding to LC3/GABARAPs. We show that DDHD2 deficiency leads to LD accumulation, while enhanced DDHD2 expression reduces LD formation. The LC3/GABARAP-binding capacity of DDHD2 and the canonical autophagy pathway both contribute to its LD-eliminating activity. Moreover, DDHD2 enhances the colocalization between LC3B and LDs to promote lipophagy. LD·ATTEC, a small molecule that tethers LC3 to LDs to enhance their autophagic clearance, effectively counteracts DDHD2 deficiency-induced LD accumulation. These findings provide valuable insights into the regulatory roles of DDHD2 in LD catabolism and offer a potential therapeutic approach for treating SPG54 patients.

Neuroinflammatory disease signatures in SPG11-related hereditary spastic paraplegia patients

Biallelic loss of SPG11 function constitutes the most frequent cause of complicated autosomal recessive hereditary spastic paraplegia (HSP) with thin corpus callosum, resulting in progressive multisystem neurodegeneration. While the impact of neuroinflammation is an emerging and potentially treatable aspect in neurodegenerative diseases and leukodystrophies, the role of immune cells in SPG11-HSP patients is unknown. Here, we performed a comprehensive immunological characterization of SPG11-HSP, including examination of three human postmortem brain donations, immunophenotyping of patients' peripheral blood cells and patient-specific induced pluripotent stem cell-derived microglia-like cells (iMGL). We delineate a previously unknown role of innate immunity in SPG11-HSP. Neuropathological analysis of SPG11-HSP patient brain tissue revealed profound microgliosis in areas of neurodegeneration, downregulation of homeostatic microglial markers and cell-intrinsic accumulation of lipids and lipofuscin in IBA1+ cells. In a larger cohort of SPG11-HSP patients, the ratio of peripheral classical and intermediate monocytes was increased, along with increased serum levels of IL-6 that correlated with disease severity. Stimulation of patient-specific iMGLs with IFNγ led to increased phagocytic activity compared to control iMGL as well as increased upregulation and release of proinflammatory cytokines and chemokines, such as CXCL10. On a molecular basis, we identified increased STAT1 phosphorylation as mechanism connecting IFNγ-mediated immune hyperactivation and SPG11 loss of function. STAT1 expression was increased both in human postmortem brain tissue and in an Spg11-/- mouse model. Application of an STAT1 inhibitor decreased CXCL10 production in SPG11 iMGL and rescued their toxic effect on SPG11 neurons. Our data establish neuroinflammation as a novel disease mechanism in SPG11-HSP patients and constitute the first description of myeloid cell/ microglia activation in human SPG11-HSP. IFNγ/ STAT1-mediated neurotoxic effects of hyperreactive microglia upon SPG11 loss of function indicate that immunomodulation strategies may slow down disease progression.

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.

Advanced Structural Magnetic Resonance Imaging of the Spinal Cord: Technical Aspects and Clinical Use

For a long time, technical obstacles have hampered the acquisition of high-resolution images and the development of reliable processing protocols for spinal cord (SC) MRI. Fortunately, this scenario has changed in the past 5-10 years, due to hardware and software improvements. Nowadays, with advanced protocols, SC MRI is considered a useful tool for several inherited and acquired neurologic diseases, not only for diagnosis approach but also for pathophysiological unraveling and as a biomarker for disease monitoring and clinical trials. In this review, we address advanced SC MRI sequences for macrostructural and microstructural evaluation, useful semiautomatic and automatic processing tools and clinical applications on several neurologic conditions such as hereditary cerebellar ataxia, hereditary spastic paraplegia, motor neuron diseases and multiple sclerosis.

Biallelic DDHD2 mutations in patients with adult-onset complex hereditary spastic paraplegia

OBJECTIVE

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative disorders characterized by slowly progressive lower limb spasticity and weakness. HSP type 54 (SPG54) is autosomal recessively inherited and caused by mutations in the DDHD2 gene. This study investigated the clinical characteristics and molecular features of DDHD2 mutations in a cohort of Taiwanese patients with HSP.

METHODS

Mutational analysis of DDHD2 was performed for 242 unrelated Taiwanese patients with HSP. The clinical, neuroimaging, and genetic features of the patients with biallelic DDHD2 mutations were characterized. A cell-based study was performed to assess the effects of the DDHD2 mutations on protein expression.

RESULTS

SPG54 was diagnosed in three patients. Among them, two patients carried compound heterozygous DDHD2 mutations, p.[R112Q];[Y606*] and p.[R112Q];[p.D660H], and the other one was homozygous for the DDHD2 p.R112Q mutation. DDHD2 p.Y606* is a novel mutation, whereas DDHD2 p.D660H and p.R112Q have been reported in the literature. All three patients manifested adult onset complex HSP with additional cerebellar ataxia, polyneuropathy, or cognitive impairment. Brain proton magnetic resonance spectroscopy revealed an abnormal lipid peak in thalamus of all three patients. In vitro studies demonstrated that all the three DDHD2 mutations were associated with a considerably lower DDHD2 protein level.

INTERPRETATION

SPG54 was detected in approximately 1.2% (3 of 242) of the Taiwanese HSP cohort. This study expands the known mutational spectrum of DDHD2, provides molecular evidence of the pathogenicity of the DDHD2 mutations, and underlines the importance of considering SPG54 as a potential diagnosis of adult-onset HSP.

Hereditary spastic paraplegias proteome: common pathways and pathogenetic mechanisms

INTRODUCTION

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative disorders characterized by progressive spasticity and weakness of the lower limbs. These conditions are caused by lesions in the neuronal pyramidal tract and exhibit clinical and genetic variability. Ongoing research focuses on understanding the underlying mechanisms of HSP onset, which ultimately lead to neuronal degeneration. Key molecular mechanisms involved include axonal transport, cytoskeleton dynamics, myelination abnormalities, membrane trafficking, organelle morphogenesis, ER homeostasis, mitochondrial dysfunction, and autophagy deregulation.

AREAS COVERED

This review aims to provide an overview of the shared pathogenetic mechanisms in various forms of HSPs. By examining disease-causing gene products and their associated functional pathways, this understanding could lead to the discovery of new therapeutic targets and the development of treatments to modify the progression of the disease.

EXPERT OPINION

Investigating gene functionality is crucial for identifying shared pathogenetic pathways underlying different HSP subtypes. Categorizing protein function and identifying pathways aids in finding biomarkers, predicting early onset, and guiding treatment for a better quality of life. Targeting shared mechanisms enables efficient and cost-effective therapies. Prospects involve identifying new disease-causing genes, refining molecular processes, and implementing findings in diagnosis, key for advancing HSP understanding and developing effective treatments.

Drosophila SPG12 ortholog, reticulon-like 1, governs presynaptic ER organization and Ca2+ dynamics

Neuronal endoplasmic reticulum (ER) appears continuous throughout the cell. Its shape and continuity are influenced by ER-shaping proteins, mutations in which can cause distal axon degeneration in Hereditary Spastic Paraplegia (HSP). We therefore asked how loss of Rtnl1, a Drosophila ortholog of the human HSP gene RTN2 (SPG12), which encodes an ER-shaping protein, affects ER organization and the function of presynaptic terminals. Loss of Rtnl1 depleted ER membrane markers at Drosophila presynaptic motor terminals and appeared to deplete narrow tubular ER while leaving cisternae largely unaffected, thus suggesting little change in resting Ca2+ storage capacity. Nevertheless, these changes were accompanied by major reductions in activity-evoked Ca2+ fluxes in the cytosol, ER lumen, and mitochondria, as well as reduced evoked and spontaneous neurotransmission. We found that reduced STIM-mediated ER-plasma membrane contacts underlie presynaptic Ca2+ defects in Rtnl1 mutants. Our results show the importance of ER architecture in presynaptic physiology and function, which are therefore potential factors in the pathology of HSP.

Research on clinical and molecular genetics of hereditary spastic paraplegia 11 patients in China

The hereditary spastic paraplegia (HSP) is a rare hereditary disease in nervous system due to the damage of corticospinal tract. HSP has various inheritance modes, including autosomal dominant inheritance, autosomal recessive inheritance, X-linked inheritance, and mitochondrial inheritance in some cases. At present, there are at least 80 subtypes of HSP. Hereditary spastic paraplegia type 11 (SPG11) is the most common subtype in autosomal recessive inheritance, and its pathogenic factor is KIAA1840 gene, which encodes spatacsin protein. A total of 52 SPG11 patients aged from 4-24 years old have been reported. Their initial symptoms were gait disturbance and/or mental retardation. As the disease develops, they may present with mental retardation, sphincter disturbance, decreased vision, ataxia, amyotrophy, pes arcuatus, ophthalmoplegia, peripheral neuropathy, and others. Except agenesis of the corpus callosum and periventricular white matter changes, patients might show cortical atrophy, ventricular dilation, and cerebellar atrophy, and so on. Chinese SPG11 patients manifested significant clinical and genetical heterogeneity and no obvious gender difference. Of them, 37 pathogenic mutations of KIAA1840 gene were detected, which all introduced truncated mutation of spatacsin protein. KIAA1840 gene frameshift mutation is the most common type of mutation.

Autologous iPSC-Derived Human Neuromuscular Junction to Model the Pathophysiology of Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) is a heterogeneous group of genetic neurodegenerative disorders, characterized by progressive lower limb spasticity and weakness resulting from retrograde axonal degeneration of motor neurons (MNs). Here, we generated in vitro human neuromuscular junctions (NMJs) from five HSP patient-specific induced pluripotent stem cell (hiPSC) lines, by means of microfluidic strategy, to model disease-relevant neuropathologic processes. The strength of our NMJ model lies in the generation of lower MNs and myotubes from autologous hiPSC origin, maintaining the genetic background of the HSP patient donors in both cell types and in the cellular organization due to the microfluidic devices. Three patients characterized by a mutation in the SPG3a gene, encoding the ATLASTIN GTPase 1 protein, and two patients with a mutation in the SPG4 gene, encoding the SPASTIN protein, were included in this study. Differentiation of the HSP-derived lines gave rise to lower MNs that could recapitulate pathological hallmarks, such as axonal swellings with accumulation of Acetyl-α-TUBULIN and reduction of SPASTIN levels. Furthermore, NMJs from HSP-derived lines were lower in number and in contact point complexity, denoting an impaired NMJ profile, also confirmed by some alterations in genes encoding for proteins associated with microtubules and responsible for axonal transport. Considering the complexity of HSP, these patient-derived neuronal and skeletal muscle cell co-cultures offer unique tools to study the pathologic mechanisms and explore novel treatment options for rescuing axonal defects and diverse cellular processes, including membrane trafficking, intracellular motility and protein degradation in HSP.

Age-Dependent Increase in Schmidt-Lanterman Incisures and a Cadm4-Associated Membrane Skeletal Complex in Fatty Acid 2-hydroxylase Deficient Mice: a Mouse Model of Spastic Paraplegia SPG35

PNS and CNS myelin contain large amounts of galactocerebroside and sulfatide with 2-hydroxylated fatty acids. The underlying hydroxylation reaction is catalyzed by fatty acid 2-hydroxylase (FA2H). Deficiency in this enzyme causes a complicated hereditary spastic paraplegia, SPG35, which is associated with leukodystrophy. Mass spectrometry-based proteomics of purified myelin isolated from sciatic nerves of Fa2h-deficient (Fa2h^−/−) mice revealed an increase in the concentration of the three proteins Cadm4, Mpp6 (Pals2), and protein band 4.1G (Epb41l2) in 17-month-old, but not in young (4 to 6-month-old), Fa2h^−/− mice. These proteins are known to form a complex, together with the protein Lin7, in Schmidt-Lanterman incisures (SLIs). Accordingly, the number of SLIs was significantly increased in 17-month-old but not 4-month-old Fa2h^−/− mice compared to age-matched wild-type mice. On the other hand, the relative increase in the SLI frequency was less pronounced than expected from Cadm4, Lin7, Mpp6 (Pals2), and band 4.1G (Epb41l2) protein levels. This suggests that the latter not only reflect the higher SLI frequency but that the concentration of the Cadm4 containing complex itself is increased in the SLIs or compact myelin of Fa2h^−/− mice and may potentially play a role in the pathogenesis of the disease. The proteome data are available via ProteomeXchange with identifier PXD030244.

Chinese patients with hereditary spastic paraplegias (HSPs): a protocol for a hospital-based cohort study

INTRODUCTION

Hereditary spastic paraplegias (HSPs) are uncommon but not rare neurodegenerative diseases. More than 100 pathogenic genes and loci related to spastic paraplegia symptoms have been reported. HSPs have the same core clinical features, including progressive spasticity in the lower limbs, though HSPs are heterogeneous (eg, clinical signs, MRI features, gene mutation). The age of onset varies greatly, from infant to adulthood. In addition, the slow and variable rates of disease progression in patients with HSP represent a substantial challenge for informative assessment of therapeutic efficacy. To address this, we are undertaking a prospective cohort study to investigate genetic-clinical characteristics, find surrogates for monitoring disease progress and identify clinical readouts for treatment.

METHODS AND ANALYSIS

In this case-control cohort study, we will enrol 200 patients with HSP and 200 healthy individuals in parallel. Participants will be continuously assessed for 3 years at 12-month intervals. Six aspects, including clinical signs, genetic spectrum, cognitive competence, MRI features, potential biochemical indicators and nerve electrophysiological factors, will be assessed in detail. This study will observe clinical manifestations and disease severity based on different molecular mechanisms, including oxidative stress, cholesterol metabolism and microtubule dynamics, all of which have been proposed as potential treatment targets or modalities. The analysis will also assess disease progression in different types of HSPs and cellular pathways with a longitudinal study using t tests and χ² tests.

ETHICS AND DISSEMINATION

The study was granted ethics committee approval by the first affiliated hospital of Fujian Medical University (MRCTA, ECFAH of FMU (2019)194) in 2019. Findings will be disseminated via presentations and peer-reviewed publications. Dissemination will target different audiences, including national stakeholders, researchers from different disciplines and the general public.

TRIAL REGISTRATION NUMBER

NCT04006418.

Current Knowledge of Endolysosomal and Autophagy Defects in Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) refers to a group of neurological disorders involving the degeneration of motor neurons. Due to their clinical and genetic heterogeneity, finding common effective therapeutics is difficult. Therefore, a better understanding of the common pathological mechanisms is necessary. The role of several HSP genes/proteins is linked to the endolysosomal and autophagic pathways, suggesting a functional convergence. Furthermore, impairment of these pathways is particularly interesting since it has been linked to other neurodegenerative diseases, which would suggest that the nervous system is particularly sensitive to the disruption of the endolysosomal and autophagic systems. In this review, we will summarize the involvement of HSP proteins in the endolysosomal and autophagic pathways in order to clarify their functioning and decipher some of the pathological mechanisms leading to HSP.

A novel homozygous KY variant causing a complex neurological disorder

Mutations in the gene kyphoscoliosis peptidase (KY) are known to cause myofibrillar myopathy-7 and hereditary spastic paraplegia. We investigated the genetic cause of a complex neurological phenotype in a consanguineous Pakistani family with four affected members, manifesting lower limb spasticity and weakness, toe walking, pes equinovarus, and a speech disorder. Genome-wide linkage analysis with microsatellite markers delineated chromosome 3q22.2-q24 harboring the disease gene. Whole exome sequencing was performed for two subjects, identifying a homozygous 14-bp frameshift deletion NM_178554.6:c.842_855del; p(Val281GlyfsTer18) in KY. The variant segregated with the phenotype and was absent from public databases and 100 ethnically matched controls. We confirm a novel homozygous KY variant causing a complex neurological phenotype in this family. A review of previously reported KY variants suggests that variants in this gene can cause a spectrum of neurological phenotypes.

Human ankyrins and their contribution to disease biology: An update

Ankyrins (Ank)are ubiquitously expressed proteins that play a critical role in the integrity of cytoskeleton and cellular signalling. Their presence in metazoans and evolutionary conserved protein primary sequence indicates their functional significance. Tissue-specific isoforms and an array of transcript variants make this protein one of the indispensable cellular components. Membrane-binding domains consist of ankyrin repeats that bind with several functional membrane proteins that enable maintaining cellular integrity. Cytosolic ankyrins help in cellular signal transduction. Linkage studies and recent genome-wide association studies uncovered the pathogenic roles of ankyrins (ankyrin-R, ankyrin-B and ankyrin-G) in several diseases, such as hereditary spherocytosis, long QT syndrome, intellectual disability, and CRASH syndrome, among several others. Identification of Ank3 in celiac disease may potentially explain the link between neuronal health and immunity. It is thus warranted to investigate the role of neuronal factors in immune diseases and vice versa. In this review, we briefly discussed the contribution of ankyrin genes to human diseases.

A novel missense mutation (c.1006C>T) of SPG20 gene associated with Troyer syndrome

The number of gene mutations involved in the hereditary spastic paraplegias is rapidly growing due to the expansion of the frontiers of genomic research by next-generation DNA sequencing platforms. Nevertheless, a comprehensive genetic diagnosis method remains yet unavailable for these diseases. In the current research, an 8-year-old boy with short stature and developmental delay impairment, from a nonconsanguineous family, was referred to our genetic lab. Firstly, based on the physician recommendation, the patient was evaluated by tandem mass spectrometry (MS/MS) for the quantitative examination of amino acids, and then the patient was genetically investigated by karyotype analysis and whole-exome sequencing (WES) technique. Subsequently, targeted Sanger sequencing was applied to confirm the presence of the candidate variant in all the members of the family and screening the other patients for Troyer syndrome. Analysis of inherited metabolic disorders by tandem MS/MS showed the state of all the family members as normal and also karyotyping indicated no chromosomal aberration in the patient. Further investigation by WES technique indicated a homozygous missense variant in the SPG20 gene, c.1006C[T. Targeted sequencing result of the mutation confirmed homozygote state for the affected case and a heterozygote genotype for his parents. The mutation was classified as pathogenic. Detection of novel variants especially pathogenic variantin the SPG20 gene was associated with Troyer syndrome, which encodes a multifunctional protein termed Spartin, assist in improving genotype-phenotype correlation of genetic variants and may facilitate initial diagnosis of Troyer syndrome.

Hereditary Myelopathies

PURPOSE OF REVIEW

Hereditary myelopathies are very diverse genetic disorders, and many of them represent a widespread neurodegenerative process rather than isolated spinal cord dysfunction. This article reviews various types of inherited myelopathies, with emphasis on hereditary spastic paraplegias and spastic ataxias.

RECENT FINDINGS

The ever-growing number of myelopathy-causing genes and broadening of phenotype-genotype correlations makes the molecular diagnosis of inherited myelopathies a daunting task. This article emphasizes the main phenotypic clusters among inherited myelopathies that can facilitate the diagnostic process. This article focuses on newly identified genetic causes and the most important identifying clinical features that can aid the diagnosis, including the presence of a characteristic age of onset and additional neurologic signs such as leukodystrophy, thin corpus callosum, or amyotrophy.

SUMMARY

The exclusion of potentially treatable causes of myelopathy remains the most important diagnostic step. Syndromic diagnosis can be supported by molecular diagnosis, but the genetic diagnosis at present does not change the management. Moreover, a negative genetic test does not exclude the diagnosis of a hereditary myelopathy because comprehensive molecular testing is not yet available, and many disease-causing genes remain unknown.

GJA1 Variants Cause Spastic Paraplegia Associated with Cerebral Hypomyelination

Oculodentodigital dysplasia is an autosomal dominant disorder due to GJA1 variants characterized by dysmorphic features. Neurologic symptoms have been described in some patients but without a clear neuroimaging pattern. To understand the pathophysiology underlying neurologic deficits in oculodentodigital dysplasia, we studied 8 consecutive patients presenting with hereditary spastic paraplegia due to GJA1 variants. Clinical disease severity was highly variable. Cerebral MR imaging revealed variable white matter abnormalities, consistent with a hypomyelination pattern, and bilateral hypointense signal of the basal ganglia on T2-weighted images and/or magnetic susceptibility sequences, as seen in neurodegeneration with brain iron accumulation diseases. Patients with the more prominent basal ganglia abnormalities were the most disabled ones. This study suggests that GJA1-related hereditary spastic paraplegia is a complex neurodegenerative disease affecting both the myelin and the basal ganglia. GJA1 variants should be considered in patients with hereditary spastic paraplegia presenting with brain hypomyelination, especially if associated with neurodegeneration and a brain iron accumulation pattern.

Novel SPG11 Mutations in a Patient with Symptoms Mimicking Multiple Sclerosis

We describe the cases of two sisters with spastic paraplegia 11 (SPG11). The younger sister developed relapsing lesions in the brain white matter with enhancement during the acute phase that mimicked multiple sclerosis (MS). The elevation of myelin basic protein in the cerebrospinal fluid (CSF) suggested demyelination, but a normal IgG index, the absence of oligoclonal bands, and the ineffectiveness of steroid treatment indicate that an autoimmune mechanism may not have been involved. In these affected sisters, we identified novel compound heterozygous mutations in the SPG11 gene. Our cases indicate the possible existence of a broader phenotypic spectrum of SPG11 mutations.

Diseases of ganglioside biosynthesis: An expanding group of congenital disorders of glycosylation

Among the numerous congenital disorders of glycosylation concerning glycoproteins, only a single mutation in ganglioside biosynthesis had been reported until a few years ago: one in the ST3GAL5 gene, encoding GM3 synthase. More recently, additional mutations in the same gene were reported, together with several distinct mutations in the B4GALNT1 gene, encoding GM2/GD2/GA2 synthase. Patients suffering from ST3GAL5 deficiency present a devastating syndrome characterized by early onset and dramatic neurological and cognitive impairment, sometimes associated with dyspigmentation and an increased blood lactate concentration. On the other hand, B4GALNT1 mutations give rise to a form of complicated hereditary spastic paraplegia (HSP), previously referred to as HSP26. It is characterized by the late onset of lower limb weakness and mild to moderate intellectual impairment, which is usually not progressive. In addition to the most typical signs, some patients present ocular and endocrine signs, pes cavus, and psychiatric illness. Since the nineties, mice lacking genes for single glycosyltransferases involved in ganglioside biosynthesis, including ST3GAL5 and B4GALNT1, were created and studied. The resulting phenotypes were frequently mild or very mild, so double knock-out animals were created to effectively study the function of gangliosides. The main clinical and biochemical features of patients suffering from GM3 synthase or GM2/GD2/GA2 synthase deficiency, compared with the phenotypes described in mice that are null for single or multiple glycosyltransferase genes, provide suggestions to improve the recognition of novel mutations and potentially related disorders.

Bile acid analysis in human disorders of bile acid biosynthesis

Bile acids facilitate the absorption of lipids in the gut, but are also needed to maintain cholesterol homeostasis, induce bile flow, excrete toxic substances and regulate energy metabolism by acting as signaling molecules. Bile acid biosynthesis is a complex process distributed across many cellular organelles and requires at least 17 enzymes in addition to different metabolite transport proteins to synthesize the two primary bile acids, cholic acid and chenodeoxycholic acid. Disorders of bile acid synthesis can present from the neonatal period to adulthood and have very diverse clinical symptoms ranging from cholestatic liver disease to neuropsychiatric symptoms and spastic paraplegias. This review describes the different bile acid synthesis pathways followed by a summary of the current knowledge on hereditary disorders of human bile acid biosynthesis with a special focus on diagnostic bile acid profiling using mass spectrometry.

Establishment of SPAST mutant induced pluripotent stem cells (iPSCs) from a hereditary spastic paraplegia (HSP) patient

Human skin fibroblasts were isolated from a 40-year-old hereditary spastic paraplegia patient carrying an intronic splice site mutation (c.1687+2T>A) in SPAST, leading to hereditary spastic paraplegia type 4 (SPG4). Fibroblasts were reprogrammed using episomal plasmids carrying hOCT4, hSOX2, hKLF4, hL-MYC and hLIN28. The generated transgene-free line iPS-SPG4-splice retained the specific mutation with no additional genomic aberrations, expressed pluripotency markers and was able to differentiate into cells of all germ layers in vitro. The generated iPS-SPG4-splice line might be a useful platform to study the pathomechanism of SPG4.

Modeling Axonal Phenotypes with Human Pluripotent Stem Cells

Impaired axonal development and degeneration are implicated in many debilitating disorders, such as hereditary spastic paraplegia (HSP), amyotrophic lateral sclerosis (ALS), and periphery neuropathy. Human pluripotent stem cells (hPSCs) have provided researchers with an excellent resource for modeling human neuropathologic processes including axonal defects in vitro. There are a number of steps that are crucial when developing an hPSC-based model of a human disease, including generating induced pluripotent stem cells (iPSCs), differentiating those cells to affected cell types, and identifying disease-relevant phenotypes. Here, we describe these steps in detail, focusing on the neurodegenerative disorder HSP.

In Vivo Evidence for Lysosome Depletion and Impaired Autophagic Clearance in Hereditary Spastic Paraplegia Type SPG11

Hereditary spastic paraplegia (HSP) is characterized by a dying back degeneration of corticospinal axons which leads to progressive weakness and spasticity of the legs. SPG11 is the most common autosomal-recessive form of HSPs and is caused by mutations in SPG11. A recent in vitro study suggested that Spatacsin, the respective gene product, is needed for the recycling of lysosomes from autolysosomes, a process known as autophagic lysosome reformation. The relevance of this observation for hereditary spastic paraplegia, however, has remained unclear. Here, we report that disruption of Spatacsin in mice indeed causes hereditary spastic paraplegia-like phenotypes with loss of cortical neurons and Purkinje cells. Degenerating neurons accumulate autofluorescent material, which stains for the lysosomal protein Lamp1 and for p62, a marker of substrate destined to be degraded by autophagy, and hence appears to be related to autolysosomes. Supporting a more generalized defect of autophagy, levels of lipidated LC3 are increased in Spatacsin knockout mouse embryonic fibrobasts (MEFs). Though distinct parameters of lysosomal function like processing of cathepsin D and lysosomal pH are preserved, lysosome numbers are reduced in knockout MEFs and the recovery of lysosomes during sustained starvation impaired consistent with a defect of autophagic lysosome reformation. Because lysosomes are reduced in cortical neurons and Purkinje cells in vivo, we propose that the decreased number of lysosomes available for fusion with autophagosomes impairs autolysosomal clearance, results in the accumulation of undegraded material and finally causes death of particularly sensitive neurons like cortical motoneurons and Purkinje cells in knockout mice.

Autophagy is a degradative pathway for the removal and subsequent recycling of dysfunctional intracellular components. The material destined for degradation is initially enclosed by a double membrane, the autophagosome. In autolysosomes, which result from fusion of autophagosomes with lysosomes, the material is finally broken down. Recent in vitro data suggested that the protein Spatacsin plays a pivotal role in the regeneration of lysosomes from autolysosomes. Spatacsin is encoded by SPG11, the most common gene mutated in autosomal recessive hereditary spastic paraplegia. Here we show that mice devoid of Spatacsin develop symptoms consistent with spastic paraplegia and progressively loose cortical motoneurons and Purkinje cells. In these mice degenerating neurons have a reduced number of lysosomes available for fusion with autophagosomes and consequently accumulate autolysosome-derived material over time. In the long term this causes death of particularly sensitive neurons like cortical motoneurons and Purkinje cells.

Multimodal MRI-based study in patients with SPG4 mutations

Mutations in the SPG4 gene (SPG4-HSP) are the most frequent cause of hereditary spastic paraplegia, but the extent of the neurodegeneration related to the disease is not yet known. Therefore, our objective is to identify regions of the central nervous system damaged in patients with SPG4-HSP using a multi-modal neuroimaging approach. In addition, we aimed to identify possible clinical correlates of such damage. Eleven patients (mean age 46.0 ± 15.0 years, 8 men) with molecular confirmation of hereditary spastic paraplegia, and 23 matched healthy controls (mean age 51.4 ± 14.1years, 17 men) underwent MRI scans in a 3T scanner. We used 3D T1 images to perform volumetric measurements of the brain and spinal cord. We then performed tract-based spatial statistics and tractography analyses of diffusion tensor images to assess microstructural integrity of white matter tracts. Disease severity was quantified with the Spastic Paraplegia Rating Scale. Correlations were then carried out between MRI metrics and clinical data. Volumetric analyses did not identify macroscopic abnormalities in the brain of hereditary spastic paraplegia patients. In contrast, we found extensive fractional anisotropy reduction in the corticospinal tracts, cingulate gyri and splenium of the corpus callosum. Spinal cord morphometry identified atrophy without flattening in the group of patients with hereditary spastic paraplegia. Fractional anisotropy of the corpus callosum and pyramidal tracts did correlate with disease severity. Hereditary spastic paraplegia is characterized by relative sparing of the cortical mantle and remarkable damage to the distal portions of the corticospinal tracts, extending into the spinal cord.

Souffle/Spastizin controls secretory vesicle maturation during zebrafish oogenesis

During oogenesis, the egg prepares for fertilization and early embryogenesis. As a consequence, vesicle transport is very active during vitellogenesis, and oocytes are an outstanding system to study regulators of membrane trafficking. Here, we combine zebrafish genetics and the oocyte model to identify the molecular lesion underlying the zebrafish souffle (suf) mutation. We demonstrate that suf encodes the homolog of the Hereditary Spastic Paraplegia (HSP) gene SPASTIZIN (SPG15). We show that in zebrafish oocytes suf mutants accumulate Rab11b-positive vesicles, but trafficking of recycling endosomes is not affected. Instead, we detect Suf/Spastizin on cortical granules, which undergo regulated secretion. We demonstrate genetically that Suf is essential for granule maturation into secretion competent dense-core vesicles describing a novel role for Suf in vesicle maturation. Interestingly, in suf mutants immature, secretory precursors accumulate, because they fail to pinch-off Clathrin-coated buds. Moreover, pharmacological inhibition of the abscission regulator Dynamin leads to an accumulation of immature secretory granules and mimics the suf phenotype. Our results identify a novel regulator of secretory vesicle formation in the zebrafish oocyte. In addition, we describe an uncharacterized cellular mechanism for Suf/Spastizin activity during secretion, which raises the possibility of novel therapeutic avenues for HSP research.

Oocytes of egg laying animals frequently represent the biggest cell type of a species. The size of the egg is a consequence of active transport processes, e.g. the import of yolk proteins, which results in the massive storage of vesicles. In addition, secretory vesicles termed cortical granules are stored in the oocyte to be discharged right after fertilization during cortical reaction, which also occurs in mammals. Their secretion leads to chorion expansion, which prevents the lethal entry of additional sperm and protects the developing embryo against physical damage. Mutants with a defect in membrane transport are successful tools to discover genes regulating vesicle formation. We molecularly identify the disrupted gene in the recessive maternal-effect mutation souffle, which encodes a homolog of human SPASTIZIN. SPASTIZIN was previously implicated in endocytosis, but our cellular analysis of mutant oocytes connects this gene also with the regulation of cortical granule exocytosis. More precisely, we show that Suf/Spastizin is crucial for the maturation of cortical granules into secretion competent vesicles describing a novel role for this protein. Since SPASITIZN causes the disease Hereditary Spastic Paraplegia in humans, our results will help to decipher the pathogenesis of this neurodegenerative disorder.

Molecular aspects of hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by progressive lower limbs spasticity and weakness. What was first thought to be a small group of rare Mendelian disorder has now become a large group that includes many complex syndromes. While large families with defined modes of inheritance were used for the initial HSP gene discovery, new sequencing technologies have recently allowed the study of small families, with the identification of many new disease causative genes. These discoveries are slowly leading to a better understanding of the molecular mechanisms underlying HSP with the identification of precise disease pathways. These insights may lead to new therapeutic strategies for what is a group of largely untreatable diseases. This review looks at the key players involved in HSP and where they act in their specific pathways.

Infantile-onset ascending hereditary spastic paralysis: a case report and brief literature review

BACKGROUND

Infantile-onset ascending hereditary spastic paralysis (IAHSP) is a rare, early-onset autosomal recessive motor neuron disease associated with mutations in ALS2.

AIM

We studied a 17-year-old boy who had features of IAHSP. We also reviewed the current literature on ALS2-related syndromes.

METHODS

Clinical and neuroimaging studies were performed. Blood DNA analyses were combined with mRNA studies in cultured skin fibroblasts.

RESULTS

Like previously described cases, the patient presented with severe spastic paraparesis and showed rapid progression of paresis to the upper limbs. He also developed bulbar involvement and severe scoliosis during childhood. In blood DNA we identified a novel splice-site homozygous mutation in ALS2 (c.3836+1G > T), producing exon skipping in fibroblast mRNA and predicting premature protein truncation.

CONCLUSIONS

This case adds to the allelic heterogeneity of IAHSP. Review of the pertinent literature indicates a fairly homogeneous clinical picture in IAHSP that should facilitate molecular confirmation and prevention of long-term complications.

Syndromes presenting adducted thumb with/without clubfoot and Dundar syndrome

Congenital adducted thumb has been called variously as congenital clasped thumb, thumb in palm deformity or flexion adduction deformity of the thumb. This condition can be an isolated anomaly or associated with several genetic disorders. The syndromes that include adducted thumb as a cardinal feature such as Dundar Syndrome are few in the literature. This syndrome is an autosomal-recessive very rare disorder characterized by typical facial appearance with dysmorphic features that includes wasted build, hyperextensible, thin and translucent skin with atrophic scarring, severe congenital contractures of fingers and thumbs, club feet, severe kyphoscoliosis, joint instability, muscular hypotonia, and ocular involvement. Heart, kidney, and/or intestinal defects can also be observed. Up to date the syndrome is described in few families in the literature. Here we discuss the syndromes that include adducted thumb as a cardinal feature and also the differential diagnosis of the Dundar Syndrome according to the literature.

A hereditary spastic paraplegia mouse model supports a role of ZFYVE26/SPASTIZIN for the endolysosomal system

Hereditary spastic paraplegias (HSPs) are characterized by progressive weakness and spasticity of the legs because of the degeneration of cortical motoneuron axons. SPG15 is a recessively inherited HSP variant caused by mutations in the ZFYVE26 gene and is additionally characterized by cerebellar ataxia, mental decline, and progressive thinning of the corpus callosum. ZFYVE26 encodes the FYVE domain-containing protein ZFYVE26/SPASTIZIN, which has been suggested to be associated with the newly discovered adaptor protein 5 (AP5) complex. We show that Zfyve26 is broadly expressed in neurons, associates with intracellular vesicles immunopositive for the early endosomal marker EEA1, and co-fractionates with a component of the AP5 complex. As the function of ZFYVE26 in neurons was largely unknown, we disrupted Zfyve26 in mice. Zfyve26 knockout mice do not show developmental defects but develop late-onset spastic paraplegia with cerebellar ataxia confirming that SPG15 is caused by ZFYVE26 deficiency. The morphological analysis reveals axon degeneration and progressive loss of both cortical motoneurons and Purkinje cells in the cerebellum. Importantly, neuron loss is preceded by accumulation of large intraneuronal deposits of membrane-surrounded material, which co-stains with the lysosomal marker Lamp1. A density gradient analysis of brain lysates shows an increase of Lamp1-positive membrane compartments with higher densities in Zfyve26 knockout mice. Increased levels of lysosomal enzymes in brains of aged knockout mice further support an alteration of the lysosomal compartment upon disruption of Zfyve26. We propose that SPG15 is caused by an endolysosomal membrane trafficking defect, which results in endolysosomal dysfunction. This appears to be particularly relevant in neurons with highly specialized neurites such as cortical motoneurons and Purkinje cells.

Hereditary spastic paraplegias (HSPs) are inherited disorders characterized by progressive weakness and spasticity of the legs. In HSP patients, nerve fibers connecting cortical motoneurons with spinal cord neurons are progressively lost. HSP subtype 15 (SPG15) is caused by mutations in ZFYVE26, and is characterized by additional cerebellar symptoms. We show that the Zfyve26 protein is broadly expressed in the brain. At the subcellular level Zfyve26 localizes to an intracellular compartment in the endocytic pathway from the plasma membrane to lysosomes, which is part of the degradative system of the cell. Closely resembling the human disease, mice deficient for Zfyve26 develop a progressive spastic gait disorder with cerebellar symptoms and degeneration of both neurons of the motor cortex and Purkinje cells in the cerebellum. Importantly, this degeneration is characterized by the intracellular accumulation of abnormal deposits, which stain positive for the lysosomal marker Lamp1. As Zfyve26 has been shown to interact with the newly identified adaptor complex AP5, which is supposed to be involved in cargo trafficking in the endolysosomal compartment, endolysosomal dysfunction may be caused by a targeting defect upon disruption of Zfyve26. As highly specialized neurons like cortical motoneurons and cerebellar Purkinje cells degenerate, these neurons appear to be particularly dependent on proper endolysosomal function.

Mutations in CYP2U1, DDHD2 and GBA2 genes are rare causes of complicated forms of hereditary spastic paraparesis

Complicated hereditary spastic paraplegias (HSP) are a heterogeneous group of HSP characterized by spasticity associated with a variable combination of neurologic and extra-neurologic signs and symptoms. Among them, HSP with thin corpus callosum and intellectual disability is a frequent subtype, often inherited as a recessive trait (ARHSP-TCC). Within this heterogeneous subgroup, SPG11 and SPG15 represent the most frequent subtypes. We analyzed the mutation frequency of three genes associated with early-onset forms of ARHSP with and without TCC, CYP2U1/SPG56, DDHD2/SPG54 and GBA2/SPG46, in a large population of selected complicated HSP patients by using a combined approach of traditional-based and amplicon-based high-throughput pooled-sequencing. Three families with mutations were identified, one for each of the genes analyzed. Novel homozygous mutations were identified in CYP2U1 (c.1A>C/p.Met1?) and in GBA2 (c.2048G>C/p.Gly683Arg), while the homozygous mutation found in DDHD2 (c.1978G>C/p.Asp660His) had been previously reported in a compound heterozygous state. The phenotypes associated with the CYP2U1 and DDHD2 mutations overlap the SPG56 and the SPG54 subtypes, respectively, with few differences. By contrast, the GBA2 mutated patients show phenotypes combining typical features of both the SPG46 subtype and the recessive ataxia form, with marked intrafamilial variability thereby expanding the spectrum of clinical entities associated with GBA2 mutations. Overall, each of three genes analyzed shows a low mutation frequency in a general population of complicated HSP (<1 % for either CYP2U1 or DDHD2 and approximately 2 % for GBA2). These findings underline once again the genetic heterogeneity of ARHSP-TCC and the clinical overlap between complicated HSP and the recessive ataxia syndromes.

A spastic paraplegia mouse model reveals REEP1-dependent ER shaping

Axonopathies are a group of clinically diverse disorders characterized by the progressive degeneration of the axons of specific neurons. In hereditary spastic paraplegia (HSP), the axons of cortical motor neurons degenerate and cause a spastic movement disorder. HSP is linked to mutations in several loci known collectively as the spastic paraplegia genes (SPGs). We identified a heterozygous receptor accessory protein 1 (REEP1) exon 2 deletion in a patient suffering from the autosomal dominantly inherited HSP variant SPG31. We generated the corresponding mouse model to study the underlying cellular pathology. Mice with heterozygous deletion of exon 2 in Reep1 displayed a gait disorder closely resembling SPG31 in humans. Homozygous exon 2 deletion resulted in the complete loss of REEP1 and a more severe phenotype with earlier onset. At the molecular level, we demonstrated that REEP1 is a neuron-specific, membrane-binding, and membrane curvature-inducing protein that resides in the ER. We further show that Reep1 expression was prominent in cortical motor neurons. In REEP1-deficient mice, these neurons showed reduced complexity of the peripheral ER upon ultrastructural analysis. Our study connects proper neuronal ER architecture to long-term axon survival.

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.

Phenotypic heterogeneity in hereditary motor neuropathy type V: a new case report series

Previous studies have revealed a wide phenotypic heterogeneity in hereditary motor neuropathy type V in which upper and lower motor neurons and peripheral motor axons are variously affected, even within the same family. In this case series, we describe the genetic, clinical and electrophysiological features of patients belonging to a four-generation Italian family. Because of a possible anticipation phenomenon, the disorder became apparent at an earlier age as it passed to the next generation, with a median age of onset of 65 years for the first 2 generations, 32 for the third, and 13.5 for the fourth. The symptoms at onset varied considerably among the sufferers, with a predominant impairment of the hands in seven cases, the impairment of the four limbs in one patient and only of the lower limbs in another. Also muscle atrophy was variable, from very mild to severe (wasting of the distal muscles of the limbs). Moreover, electrophysiological results were heterogeneous, including cases with isolated and with diffuse axonal motor neuropathy, and one case of motor sensory polyneuropathy. A novel polymorphism G→T was also found in the Berardinelli-Seip congenital lipodystrophy 2 gene on intron 4. This broad phenotypic and genotypic spectrum calls the clinician attention to this rare and still insufficiently known disease.

Whole-exome sequencing identifies homozygous AFG3L2 mutations in a spastic ataxia-neuropathy syndrome linked to mitochondrial m-AAA proteases

We report an early onset spastic ataxia-neuropathy syndrome in two brothers of a consanguineous family characterized clinically by lower extremity spasticity, peripheral neuropathy, ptosis, oculomotor apraxia, dystonia, cerebellar atrophy, and progressive myoclonic epilepsy. Whole-exome sequencing identified a homozygous missense mutation (c.1847G>A; p.Y616C) in AFG3L2, encoding a subunit of an m-AAA protease. m-AAA proteases reside in the mitochondrial inner membrane and are responsible for removal of damaged or misfolded proteins and proteolytic activation of essential mitochondrial proteins. AFG3L2 forms either a homo-oligomeric isoenzyme or a hetero-oligomeric complex with paraplegin, a homologous protein mutated in hereditary spastic paraplegia type 7 (SPG7). Heterozygous loss-of-function mutations in AFG3L2 cause autosomal-dominant spinocerebellar ataxia type 28 (SCA28), a disorder whose phenotype is strikingly different from that of our patients. As defined in yeast complementation assays, the AFG3L2(Y616C) gene product is a hypomorphic variant that exhibited oligomerization defects in yeast as well as in patient fibroblasts. Specifically, the formation of AFG3L2(Y616C) complexes was impaired, both with itself and to a greater extent with paraplegin. This produced an early-onset clinical syndrome that combines the severe phenotypes of SPG7 and SCA28, in additional to other "mitochondrial" features such as oculomotor apraxia, extrapyramidal dysfunction, and myoclonic epilepsy. These findings expand the phenotype associated with AFG3L2 mutations and suggest that AFG3L2-related disease should be considered in the differential diagnosis of spastic ataxias.

Fusing a lasting relationship between ER tubules

Atlastin is an integral membrane GTPase localized to the endoplasmic reticulum (ER). In vitro and in vivo analyses indicate that atlastin is a membrane fusogen capable of driving membrane fusion, suggesting a role in ER structure and maintenance. Interestingly, mutations in the human atlastin-1 gene, SPG3A, cause a form of autosomal dominant hereditary spastic paraplegia (HSP). The etiology of HSP is unclear, but two predominant forms of the disorder are caused by mutant proteins that affect ER structure, formation and maintenance in motor neurons. In this review, we describe the current knowledge about the molecular mechanism of atlastin function and its potential role in HSP. Greater understanding of the function of atlastin and associated proteins should provide important insight into normal ER biogenesis and maintenance, as well as the pathology of disease.

Axonal integrity of corticospinal projections to the upper limbs in patients with pure hereditary spastic paraplegia

OBJECTIVE

Patients with hereditary spastic paraplegia (HSP) show pathological findings when transcranial magnetic stimulation (TMS) is used to test corticospinal projections to the lower limbs. However, TMS studies on the pathways to the upper limbs revealed inconsistent results. Standard clinical TMS procedures are not well suited for testing axonal integrity, which is thought to be affected in HSP. More appropriate measures can be achieved by testing corticospinal projections with the triple stimulation technique (TST).

METHODS

TST was used to test axonal integrity of corticospinal projections to the upper limbs in 15 patients with pure HSP (13 of whom were tested positive for SPG 4) and 15 healthy control subjects.

RESULTS

TST measurements revealed normal values for corticospinal transmission in all 15 patients with pure HSP, as well as in all healthy control subjects. No differences between groups could be found.

CONCLUSIONS

Axonal integrity of projections to the upper limbs is unimpaired in patients with pure HSP. The pathological mechanisms leading to spasticity and motor disability seem to be restricted to those fibres of the corticospinal pathways projecting to the lower limbs.

SIGNIFICANCE

Abnormal corticospinal function to the upper limbs seems to be incompatible with pure HSP.

Expanding the clinical spectrum of SPG11 gene mutations in recessive hereditary spastic paraplegia with thin corpus callosum

Hereditary spastic paraplegia (HSP) represents a large group of neurological disorders characterized by progressive spasticity of the lower limbs. One subtype of HSP shows an autosomal recessive form of inheritance with thin corpus callosum (ARHSP-TCC), and displays genetic heterogeneity with four known loci. We identified a consanguineous Egyptian family with five affected individuals with ARHSP-TCC. We found linkage to the SPG11 locus and identified a novel homozygous p.Q498X stop codon mutation in exon 7 in the SPG11 gene encoding Spatacsin. Cognitive impairment and polyneuropathy, reported as frequent in SPG11, were not evident. This family supports the importance of SPG11 as a frequent cause for ARHSP-TCC, and expands the clinical SPG11 spectrum.

Clinical and genetic analysis of a Korean family with hereditary spastic paraplegia type 3

Hereditary spastic paraplegia (HSP) is a neurodegenerative disease characterized by progressive spasticity in the lower extremities. Mutations in the atlastin GTPase 1 (ATL1) gene cause approximately 10% of autosomal dominantly inherited HSP. For many subjects with an ATL1 mutation, spastic gait begins in early childhood and does not significantly worsen, even over many years; such cases resemble spastic diplegic cerebral palsy. Herein we report a heterozygous R239C mutation in the ATL1 gene in a Korean family. The family members exhibited early onset pure spastic paraplegia and had been previously diagnosed with the diplegic form of cerebral palsy. We suggest that spastic paraplegia type 3 (SPG3A) be included in the differential diagnosis of early onset spastic paraplegia. To the best of our knowledge, this is the first report of a genetically confirmed family affected with SPG3A in Korea.

Sequence variants in SPAST, SPG3A and HSPD1 in hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is a group of clinically and genetically heterogeneous neurodegenerative disorders characterized by progressive spasticity and weakness in the lower limbs. The most common forms of autosomal dominant HSP, SPG4 and SPG3, are caused by sequence variants in the SPAST and SPG3A genes, respectively. The pathogenic variants are scattered all over these genes and many variants are unique to a specific family. The phenotype in SPG4 patients can be modified by a variant in SPAST (p.Ser44Leu) and recently, a variant in HSPD1, the gene underlying SPG13, was reported as a second genetic modifier in SPG4 patients. In this study HSP patients were screened for variants in SPG3A, SPAST and HSPD1 in order to identify disease causing variations. SPAST was sequenced in all patients whereas subsets were sequenced in HSPD1 and in selected exons of SPG3A. SPG4 patients and their HSP relatives were genotyped for the modifying variant in HSPD1. We report six new sequence variants in SPAST including a fourth non synonymous sequence variant in exon 1 and two synonymous changes of which one has been found in a HSP patient previously, but never in controls. Of the novel variants in SPAST four were interpreted as disease causing. In addition one new disease causing sequence variant and one non pathogenic non synonymous variant were found in SPG3A. In HSPD1 we identified a sporadic patient homozygote for the potential modifying variation. The effect of the modifying HSPD1 variation was not supported by identification in one SPG4 family.

Point mutations and a large intragenic deletion in SPG11 in complicated spastic paraplegia without thin corpus callosum

BACKGROUND

Hereditary spastic paraplegia (HSP) with thin corpus callosum (HSP-TCC) is a frequent subtype of complicated HSP clinically characterised by slowly progressive spastic paraparesis with cognitive impairment and thin corpus callosum (TCC). SPG11, the gene associated with the major locus involved, encodes spatacsin, a protein of unknown function.

METHODS

Different types of mutations were identified in patients with the complex form of HSP (cHSP) including TCC. We screened a series of 45 index patients with different types of cHSP with (n = 10) and without (n = 35) TCC.

RESULTS

Ten mutations, of which five are novel, were detected in seven patients. Of importance, three out of seven mutated patients present with cHSP without TCC. Among the novel mutations identified, we characterised a large intragenic rearrangement deleting 2.6 kb of the SPG11 gene. The rearrangement is due to non-allelic homologous recombination between Alu sequences flanking the breakpoints.

CONCLUSIONS

These findings expand the mutation spectrum of SPG11 and suggest that SPG11 mutations may occur more frequently in familial than sporadic forms of cHSP without TCC. This helps to define further clinical and molecular criteria for a correct diagnosis of the SPG11 related form of cHSP. In addition, the intragenic deletion detected here, and the mechanism involved, both provide clues to address the issue of SPG11 missing mutant alleles previously reported.

SPG11--the most common type of recessive spastic paraplegia in Norway?

BACKGROUND

Hereditary spastic paraplegias (HSP) are neurodegenerative diseases mainly characterized by lower limb spasticity with additional neurological symptoms and signs in complicated forms. Among the many autosomal recessive forms, SPG11 appears to be one of the most frequent.

OBJECTIVE

Our objective was to select potential SPG11 patients based on phenotypes in our material, identify eventual disease-causing variants with the collaboration of laboratories abroad, estimate the frequency and spectrum of SPG11-mutations and describe their associated phenotypes.

MATERIAL AND METHODS

Two isolated cases and two affected members of one family with cognitive impairment and confirmed thin corpus callosum on magnetic resonance imaging were selected from our database for inclusion into a multicenter study. Results - Mutations were found in the two isolated cases but not in the proband of the family.

CONCLUSION

We present the first SPG11-HSP in the Norwegian population. SPG11 should be suspected in patients with isolated or recessive HSP, thin corpus callosum and mental retardation.

[The clinical characteristics of a pedigree with incompletely penetrated autosomal dominant hereditary spastic paraplegia and its exclusion analysis of genetic loci]

OBJECTIVE

To describe the clinical features of a big family with incompletely penetrated autosomal dominant hereditary spastic paraplegia (SPG) and perform the exclusion analysis of genetic loci.

METHODS

The clinical information of this SPG family was analyzed retrospectively. Exclusion analysis of the known autosomal dominant SPG loci was performed by using multiplex fluorescence PCR, capillary electrophoresis and Linkage package.

RESULTS

There were eleven affected members available in this SPG family and the age at onset ranged from 2 to 10 years. The first symptoms were a bilateral, symmetrical, progressive lower limb weakness and spasticity. Patients presented with spasticity and hyperreflexia, positive Babinski sign and scissors gait, and the upper limbs were involved more severely than the lower limbs. No urinary inconsistence, sensory impairment, nystagmus and dementia were found. Genetic analysis showed that this family was consistent with autosomal dominant inheritance. The linkage analysis and mutation analysis revealed this family was not linked to the known autosomal dominant loci.

CONCLUSION

This SPG family had typical "pure" clinical symptoms. The age at onset was early and the signs in the upper limbs were more obvious than those in the lower limbs. The result of linkage analysis shows that this family represents a new SPG subtype.

SPG11: a consistent clinical phenotype in a family with homozygous spatacsin truncating mutation

Hereditary spastic paraplegias (HSP) are a heterogeneous group of neurodegenerative disorders leading to progressive spasticity of the lower limbs. Here, we describe clinical and genetic features in an Italian family affected by autosomal recessive HSP (ARHSP) with mental impairment and thin corpus callosum (TCC). In both affected subjects, genetic analysis revealed the presence of a homozygous small deletion (733_734delAT) leading to a frameshift (M245VfsX) within the coding region of SPG11 gene, encoding spatacsin. This finding is the first independent confirmation that spatacsin loss of function mutations cause ARHPS-TCC.

Hereditary spastic paraplegia 3A associated with axonal neuropathy

OBJECTIVE

To study the frequency and distribution of mutations in SPG3A in a large cohort of patients with hereditary spastic paraplegia.

DESIGN

We screened a large cohort of 182 families and isolated cases with pure or complex hereditary spastic paraplegia phenotypes, which were negative for mutations in SPG4.

RESULTS

In 12 probands (6.6%), we identified 12 different SPG3A mutations (11 missense and 1 insertion/frameshift) of which 7 were novel and 3 were de novo. We found incomplete penetrance in 1 family (G482V). In most cases, SPG3A mutations were associated with an early age at onset (mean, 3 y); however, in 1 family (R495W mutation), symptoms started later (mean, 14 y) with clear intrafamilial variability (8-28 y). Six patients with an SPG3A mutation (F151S, Q191R, M408T, G469A, R495W) originating from 5 unrelated families presented with a complex form of hereditary spastic paraplegia associated with a neuropathy (17%). Our electrophysiological and pathological findings confirmed an axonal sensory-motor neuropathy. There was no correlation between the genotype and the presence of a neuropathy.

CONCLUSIONS

We conclude that mutations in SPG3A represent an important cause of patients in the overall hereditary spastic paraplegia population. SPG3A is more often associated with a neuropathy than previously assumed. Therefore, patients with a bipyramidal syndrome and a neuropathy should be screened for mutations in SPG3A.