The Troyer syndrome. A recessive form of spastic paraplegia with distal muscle wasting

Review of the Genetic Spectrum of Hereditary Spastic Paraplegias in the Middle East and North Africa Regions

Background and Objectives

Hereditary spastic paraplegias (HSPs) are inherited neurodegenerative disorders, and their classification is based on inheritance mode, allelic variants, and clinical presentation. Despite global occurrence, research, especially in the Middle East and North Africa (MENA) regions, is lacking, underscoring the need for further investigation. The objective of this study was to improve the regions' clinical practice and public health, and this study aims to gather data on HSP prevalence, pathogenic variants, and patient characteristics in MENA countries.

Methods

A systematic literature review encompassing PubMed, MEDLINE, and Google Scholar was conducted. Quality assessment was performed on the included studies. Data extraction and analysis provided insights into HSP's current status in the region.

Results

Iran had the highest number of patients with HSP, followed by Tunisia. SPG11 (19.8%), FA2H (8.5%), and ZFYVE26 (7.7%) were the most frequently found genes in the cases. Autosomal recessive HSP with thin corpus callosum was common among the affected patients, with SPG11 identified as the primary cause.

Discussion

Our analysis highlights genetic diversity and regional prevalence variations. Despite limited research in MENA countries, we stress the importance of further investigation to address gaps in understanding and improve patient care and public health initiatives.

Comparison of long- and short-rest periods during high-intensity interval exercise on transcriptomic responses in equine skeletal muscle

The purpose of this study was to elucidate the skeletal muscle transcriptomic response unique to rest duration during high-intensity interval exercise. Thoroughbred horses performed three 1-min bouts of exercise at their maximal oxygen uptake (10.7-12.5 m/s), separated by 15 min (long) or 2 min (short) walking at 1.7 m/s. Gluteus medius muscle was collected before and at 4 h after the exercise and used for RNA sequencing. We identified 1,756 and 1,421 differentially expressed genes in response to the long and short protocols, respectively, using DEseq2 analysis [false discovery rate (FDR) cutoff = 0.05, minimal fold change = 1.5]. The overall transcriptional response was partially aligned, with 43% (n = 949) of genes altered in both protocols, whereas no discordant directional changes were observed. K-means clustering and gene set enrichment analyses based on Gene Ontology biological process terms showed that genes associated with muscle adaptation and development were upregulated regardless of exercise conditions; genes related to immune and cytokine responses were more upregulated following the long protocol, and protein folding and temperature response were highly expressed after the short protocol. We found that 11 genes were upregulated to a greater extent by the short protocol and one was by the long protocol, with GNA13, SPART, PHAF1, and PTX3 identified as potential candidates for skeletal muscle remodeling. Our results suggest that altered metabolic fluctuations dependent on the intermittent pattern of interval exercise modulate skeletal muscle gene expression, and therefore, rest interval length could be an important consideration in optimizing skeletal muscle adaptation.NEW & NOTEWORTHY This is the first study to address the comparison of transcriptional responses to high-intensity interval exercise with two different rest periods in skeletal muscle. The expression of genes related to metabolic adaptations altered in both conditions, while genes associated with immune and cytokine responses and protein folding and temperature response were varied with the length of the rest period. These results provide evidence for rest duration-specific transcriptional response to high-intensity interval training.

[Developmental and epileptic encephalopathy produced by the ATP1A2 mutation]

A case of DEE98, a rare developmental and epileptic encephalopathy related to previously reported the de novo missense mutation p.Arg908Gln in the ATP1A2 gene, is described. A girl examined first time in 11 months had microcephaly, severe mental and motor delay, strabismus, spastic paraparesis and pachypolymicrogyria on brain MRI that is atypical for DEE98. Epilepsy with polymorphic seizures started at the age of 15 months. There was a remission lasting 9 months, after which seizures renewed. DEE98 was diagnosed at the age of 2 years 9 months by exome sequencing verified by trio Sanger sequencing. Another finding from high-throughput exome sequencing were two previously undescribed heterozygous variants of uncertain pathogenicity in the SPART gene, which causes autosomal recessive spastic paraplegia type 20 (SPG20); Sanger sequencing confirmed the trans position of the variants. The common clinical sign with typical SPG20 was early spastic paraparesis with contractures; other symptoms did not coincide. Considering the phenotypic diversity of SPG20 and the possibility of a combination of two independent diseases, we performed an additional study of the pathogenicity of SPART variants at the mRNA level: pathogenicity was not confirmed, and there were no grounds to diagnose SPG20.

Mutant SPART causes defects in mitochondrial protein import and bioenergetics reversed by Coenzyme Q

Pathogenic variants in SPART cause Troyer syndrome, characterized by lower extremity spasticity and weakness, short stature and cognitive impairment, and a severe mitochondrial impairment. Herein, we report the identification of a role of Spartin in nuclear-encoded mitochondrial proteins. SPART biallelic missense variants were detected in a 5-year-old boy with short stature, developmental delay and muscle weakness with impaired walking distance. Patient-derived fibroblasts showed an altered mitochondrial network, decreased mitochondrial respiration, increased mitochondrial reactive oxygen species and altered Ca2+ versus control cells. We investigated the mitochondrial import of nuclear-encoded proteins in these fibroblasts and in another cell model carrying a SPART loss-of-function mutation. In both cell models the mitochondrial import was impaired, leading to a significant decrease in different proteins, including two key enzymes involved in CoQ10 (CoQ) synthesis, COQ7 and COQ9, with a severe reduction in CoQ content, versus control cells. CoQ supplementation restored cellular ATP levels to the same extent shown by the re-expression of wild-type SPART, suggesting CoQ treatment as a promising therapeutic approach for patients carrying mutations in SPART.

The hereditary spastic paraplegias

The hereditary spastic paraplegias (HSPs) are a group of more than 90 genetic disorders in which lower extremity spasticity and weakness are either the primary neurologic impairments ("uncomplicated HSP") or when accompanied by other neurologic deficits ("complicated HSP"), important features of the clinical syndrome. Various genetic types of HSP are inherited such as autosomal dominant, autosomal recessive, X-linked, and maternal (mitochondrial) traits. Symptoms that begin in early childhood may be nonprogressive and resemble spastic diplegic cerebral palsy. Symptoms that begin later, typically progress insidiously over a number of years. Genetic testing is able to confirm the diagnosis for many subjects. Insights from gene discovery indicate that abnormalities in diverse molecular processes underlie various forms of HSP, including disturbance in axon transport, endoplasmic reticulum morphogenesis, vesicle transport, lipid metabolism, and mitochondrial function. Pathologic studies in "uncomplicated" HSP have shown axon degeneration particularly involving the distal ends of corticospinal tracts and dorsal column fibers. Treatment is limited to symptom reduction including amelioration of spasticity, reducing urinary urgency, proactive physical therapy including strengthening, stretching, balance, and agility exercise.

Novel phenotype with prominent cerebellar oculomotor dysfunction in spastic paraplegia type 39

Objectives

The term hereditary spastic paraplegia comprises an ever-expanding array of neurological disorders with distinct aetiologies. Spastic paraplegia gene 39 is one of the many genetically defined types with features of other organs and neurological systems in addition to paraspasticity. We describe a large kindred with a novel clinical phenotype as, in addition to spastic paraplegia, affected subjects suffered from a prominent cerebellar oculomotor dysfunction with two hitherto undescribed mutations of PNPLA6.

Methods

Three of five genetically tested family members of a large kindred were affected by spastic gait and a unique and prominent cerebellar oculomotor dysfunction. Further clinical, imaging, laboratory and videonystagmographic data were analyzed. Genetic analysis was done using next-generation sequencing.

Results

The most salient clinical feature, in addition to paraspasticity, in three of five subjects was cerebellar oculomotor dysfunction with an upbeating nystagmus provoked by downward gaze. Genetic analysis revealed two hitherto unknown sequence variants in the PNPLA6 gene, a splice-site variant c.1635 + 3G > T and a missense variant c.3401A > T, p.(Asp1134Val). In addition to cerebellar oculomotor dysfunction, compound-heterozygous siblings presented with paraspasticity and a moderate hypogonadotropic hypogonadism in the female. A paternal uncle being homozygous for the splice-site variant of PNPLA6 presented with increased lower limb reflexes and an unstable gait. Treatment with 4-aminopyridine, a potassium channel blocker, lead to meaningful improvement of clinical symptoms.

Conclusions

The unique and prominent cerebellar ocular motor disorder in our family broadens the spectrum of clinical phenotypes associated with variations in the PNLA6 gene. The finding of paraspasticity with cerebellar oculomotor dysfunction alongside inconspicuous brainstem imaging may raise suspicion of complex HSP with PNPLA6 mutations.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00415-022-11313-6.

13q Deletion Syndrome Involving RB1: Characterization of a New Minimal Critical Region for Psychomotor Delay

Retinoblastoma (RB) is an ocular tumor of the pediatric age caused by biallelic inactivation of the RB1 gene (13q14). About 10% of cases are due to gross-sized molecular deletions. The deletions can involve the surrounding genes delineating a contiguous gene syndrome characterized by RB, developmental anomalies, and peculiar facial dysmorphisms. Overlapping deletions previously found by traditional and/or molecular cytogenetic analysis allowed to define some critical regions for intellectual disability (ID) and multiple congenital anomalies, with key candidate genes. In the present study, using array-CGH, we characterized seven new patients with interstitial 13q deletion involving RB1. Among these cases, three patients with medium or large 13q deletions did not present psychomotor delay. This allowed defining a minimal critical region for ID that excludes the previously suggested candidate genes (HTR2A, NUFIP1, PCDH8, and PCDH17). The region contains 36 genes including NBEA, which emerged as the candidate gene associated with developmental delay. In addition, MAB21L1, DCLK1, EXOSC8, and SPART haploinsufficiency might contribute to the observed impaired neurodevelopmental phenotype. In conclusion, this study adds important novelties to the 13q deletion syndrome, although further studies are needed to better characterize the contribution of different genes and to understand how the haploinsufficiency of this region can determine ID.

Dwarfism in Troyer syndrome: a family with SPG20 compound heterozygous mutations and a literature review

Troyer syndrome is an autosomal recessive disease characterized by spastic paralysis, dysarthria, distal amyotrophy, and short stature. Recently, two siblings (an older brother and a younger sister) were admitted to our hospital for the chief complaints of "short stature and intellectual disability." Through whole exome sequencing of the sister, who is the proband, it was found that her SPG20 gene had compound heterozygous mutations: c.364_365delAT (p.Met122Valfs^* 2) and c.892delA (p.Thr298Glnfs^* 30). Target testing revealed that the brother had the same genotype as the sister, and the former mutation originated from the father, while the latter mutation originated from the mother. In summary, this is the first report of Troyer syndrome in a family caused by SPG20 compound heterozygous mutations. A novel SPG20 mutation was found, namely c.892delA (p.Thr298Glnfs^* 30). In addition, we also summarize these Troyer syndrome patients' heights and their clinical characteristics, and provide a brief review of all known pathogenic mutations of SPG20.

Complexity of Generating Mouse Models to Study the Upper Motor Neurons: Let Us Shift Focus from Mice to Neurons

Motor neuron circuitry is one of the most elaborate circuitries in our body, which ensures voluntary and skilled movement that requires cognitive input. Therefore, both the cortex and the spinal cord are involved. The cortex has special importance for motor neuron diseases, in which initiation and modulation of voluntary movement is affected. Amyotrophic lateral sclerosis (ALS) is defined by the progressive degeneration of both the upper and lower motor neurons, whereas hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS) are characterized mainly by the loss of upper motor neurons. In an effort to reveal the cellular and molecular basis of neuronal degeneration, numerous model systems are generated, and mouse models are no exception. However, there are many different levels of complexities that need to be considered when developing mouse models. Here, we focus our attention to the upper motor neurons, which are one of the most challenging neuron populations to study. Since mice and human differ greatly at a species level, but the cells/neurons in mice and human share many common aspects of cell biology, we offer a solution by focusing our attention to the affected neurons to reveal the complexities of diseases at a cellular level and to improve translational efforts.

The role of TGF-β superfamily signaling in neurological disorders

The TGF-β superfamily signaling is involved in a variety of biological processes during embryogenesis and in adult tissue homeostasis. Faulty regulation of the signaling pathway that transduces the TGF-β superfamily signals accordingly leads to a number of ailments, such as cancer and cardiovascular, metabolic, urinary, intestinal, skeletal, and immune diseases. In recent years, a number of studies have elucidated the essential roles of TGF-βs and BMPs during neuronal development in the maintenance of appropriate innervation and neuronal activity. The new advancement implicates significant roles of the aberrant TGF-β superfamily signaling in the pathogenesis of neurological disorders. In this review, we compile a number of reports implicating the deregulation of TGF-β/BMP signaling pathways in the pathogenesis of cognitive and neurodegenerative disorders in animal models and patients. We apologize in advance that the review falls short of providing details of the role of TGF-β/BMP signaling or mechanisms underlying the pathogenesis of neurological disorders. The goal of this article is to reveal a gap in our knowledge regarding the association between TGF-β/BMP signaling pathways and neuronal tissue homeostasis and development and facilitate the research with a potential to develop new therapies for neurological ailments by modulating the pathways.

Novel SPG20 mutation in an extended family with Troyer syndrome

Troyer Syndrome (TRS) is a rare autosomal recessive complicated spastic paraplegia disorder characterized by various neurological and musculoskeletal manifestations. Pathogenicity stems from mutations in SPG20 which encodes Spartin, a multifunctional protein that is thought to be essential for neuron viability. Here we report on the clinical and molecular characterization of TRS in five patients from an extended consanguineous family in the United Arab Emirates. Molecular analysis involved Whole Exome Sequencing and Sanger sequencing for identification and confirmation of the causative variant respectively. In silico tools including CADD and Polyphen-2 were used to assess pathogenicity of the variant. The clinical description of these patients included spastic paraparesis, motor and cognitive delay, gait abnormalities, musculoskeletal features, as well as white matter abnormalities and emotional liability. Molecular analysis revealed a novel homozygous missense mutation in SPG20 (c.1324G > C; p.Ala442Pro) occurring at an evolutionarily conserved residue in the Plant-Related Senescence domain of Spartin. The mutation segregated with the clinical phenotype in all patients. In silico algorithms predict the mutation to be disease causing, and the variant had not been previously reported in public or ethnic specific variant repositories.

SPG20 mutation in three siblings with familial hereditary spastic paraplegia

Troyer syndrome (MIM#275900) is an autosomal recessive form of complicated hereditary spastic paraplegia. It is characterized by progressive lower extremity spasticity and weakness, dysarthria, distal amyotrophy, developmental delay, short stature, and subtle skeletal abnormalities. It is caused by deleterious mutations in the SPG20 gene, encoding spartin, on Chromosome 13q13. Until now, six unrelated families with a genetically confirmed diagnosis have been reported. Here we report the clinical findings in three brothers of a consanguineous Moroccan family, aged 24, 17, and 7 yr old, with spastic paraplegia, short stature, motor and cognitive delay, and severe intellectual disability. Targeted exon capture and sequencing showed a homozygous nonsense mutation in the SPG20 gene, c.1369C>T (p.Arg457*), in the three affected boys.

Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay

A new syndrome of autosomal recessive spastic ataxia has been isolated in the Charlevoix-Saguenay region of Quebec. This syndrome is remarkably homogeneous and includes: spasticity, dysarthria, distal muscle wasting, foot deformities, truncal ataxia, absence of sensory evoked potentials in the lower limbs, retinal striation reminiscent of early Leber's atrophy and the frequent presence (57%) of a prolapse of the mitral valve. Biochemically, many cases show impaired pyruvate oxidation, others have hyperbilirubinaemia and some have low serum β-lipoproteins and HDL apoproteins. These features are similar to those found in trypical Friedreich's ataxia.

Oxidative Stress in Caenorhabditis elegans: Protective Effects of Spartin

Troyer syndrome is caused by a mutation in the SPG20 gene, which results in complete loss of expression of the protein spartin. We generated a genetic model of Troyer syndrome in worms to explore the locomotor consequences of a null mutation of the Caenorhabditis elegans SPG20 orthologue, F57B10.9, also known as spg-20. Spg-20 mutants showed decreased length, crawling speed, and thrashing frequency, and had a shorter lifespan than wild-type animals. These results suggest an age-dependent decline in motor function in mutant animals. The drug paraquat was used to induce oxidative stress for 4 days in the animals. We measured survival rate and examined locomotion by measuring crawling speed and thrashing frequency. After 4 days of paraquat exposure, 77% of wild-type animals survived, but only 38% of spg-20 mutant animals survived. Conversely, animals overexpressing spg-20 had a survival rate of 95%. We also tested lifespan after a 1 hour exposure to sodium azide. After a 24 hour recovery period, 87% of wild type animals survived, 57% of spg-20 mutant animals survived, and 82% of animals overexpressing spg-20 survived. In the behavioral assays, spg-20 mutant animals showed a significant decrease in both crawling speed and thrashing frequency compared with wild-type animals. Importantly, the locomotor phenotype for both crawling and thrashing was rescued in animals overexpressing spg-20. The animals overexpressing spg-20 had crawling speeds and thrashing frequencies similar to those of wild-type animals. These data suggest that the protein F57B10.9/SPG-20 might have a protective role against oxidative stress.

Recurrent null mutation in SPG20 leads to Troyer syndrome

Troyer syndrome is an autosomal recessive form of complex hereditary spastic paraplegia. To date, the disorder has only been described in the Amish and in kindred from Oman. In Amish, all affected individuals have a homozygous one nucleotide deletion; c.1110delA. In the Omani kindred, all affected have a homozygous two nucleotides deletion; c.364_365delTA (p.Met122ValfsTer2). Here we report the results of homozygosity mapping and whole exome sequencing in two siblings of a consanguineous Turkish family with mild intellectual disability, spastic paraplegia, and muscular dystrophy. We identified the same deletion that has been identified in the Omani kindred, but haplotype analysis suggests a recurrent event, and not a founder mutation. We summarize current knowledge of Troyer syndrome, and propose wider use of whole exome sequencing in routine diagnostics. This applies in particular to nonspecific phenotypes with high heterogeneity, such as spastic paraplegia, intellectual disability, and muscular dystrophy, since in such cases the assignment of a definite diagnosis is frequently delayed.

De novo partial deletion in GRID2 presenting with complicated spastic paraplegia

INTRODUCTION

Complex forms of spastic paraplegia (SPG) are rare and genetically heterogeneous. In apparently sporadic cases, analysis of known SPG genes often fails to reveal a mutation.

METHODS

We report a 24-year-old patient with a syndrome of spastic paraplegia, ataxia, frontotemporal dementia, and lower motor neuron involvement.

RESULTS

Screening of the patient's genome for copy number variation identified a novel 276 kb deletion spanning the first exon of the GRID2 gene. MRI scan showed atrophy of the cerebellum, and electromyography revealed a chronic disorder of motor neurons or their axons. A deletion in GRID2, coding for the glutamate receptor delta-2 subunit precursor protein, was excluded in either parent, suggesting that the deletion in the index patient occurred de novo.

CONCLUSIONS

We hypothesize that the deletion identified here is the cause of our patient's clinical presentation, due to the resemblance to the GRID2 mutation phenotype in mouse models.

Hereditary spastic paraplegia: clinico-pathologic features and emerging molecular mechanisms

Hereditary spastic paraplegia (HSP) is a syndrome designation describing inherited disorders in which lower extremity weakness and spasticity are the predominant symptoms. There are more than 50 genetic types of HSP. HSP affects individuals of diverse ethnic groups with prevalence estimates ranging from 1.2 to 9.6 per 100,000. Symptoms may begin at any age. Gait impairment that begins after childhood usually worsens very slowly over many years. Gait impairment that begins in infancy and early childhood may not worsen significantly. Postmortem studies consistently identify degeneration of corticospinal tract axons (maximal in the thoracic spinal cord) and degeneration of fasciculus gracilis fibers (maximal in the cervico-medullary region). HSP syndromes thus appear to involve motor-sensory axon degeneration affecting predominantly (but not exclusively) the distal ends of long central nervous system (CNS) axons. In general, proteins encoded by HSP genes have diverse functions including (1) axon transport (e.g. SPG30/KIF1A, SPG10/KIF5A and possibly SPG4/Spastin); (2) endoplasmic reticulum morphology (e.g. SPG3A/Atlastin, SPG4/Spastin, SPG12/reticulon 2, and SPG31/REEP1, all of which interact); (3) mitochondrial function (e.g. SPG13/chaperonin 60/heat-shock protein 60, SPG7/paraplegin; and mitochondrial ATP6); (4) myelin formation (e.g. SPG2/Proteolipid protein and SPG42/Connexin 47); (5) protein folding and ER-stress response (SPG6/NIPA1, SPG8/K1AA0196 (Strumpellin), SGP17/BSCL2 (Seipin), "mutilating sensory neuropathy with spastic paraplegia" owing to CcT5 mutation and presumably SPG18/ERLIN2); (6) corticospinal tract and other neurodevelopment (e.g. SPG1/L1 cell adhesion molecule and SPG22/thyroid transporter MCT8); (7) fatty acid and phospholipid metabolism (e.g. SPG28/DDHD1, SPG35/FA2H, SPG39/NTE, SPG54/DDHD2, and SPG56/CYP2U1); and (8) endosome membrane trafficking and vesicle formation (e.g. SPG47/AP4B1, SPG48/KIAA0415, SPG50/AP4M1, SPG51/AP4E, SPG52/AP4S1, and VSPG53/VPS37A). The availability of animal models (including bovine, murine, zebrafish, Drosophila, and C. elegans) for many types of HSP permits exploration of disease mechanisms and potential treatments. This review highlights emerging concepts of this large group of clinically similar disorders.

Spartin regulates synaptic growth and neuronal survival by inhibiting BMP-mediated microtubule stabilization

Troyer syndrome is a hereditary spastic paraplegia caused by human spartin (SPG20) gene mutations. We have generated a Drosophila disease model showing that Spartin functions presynaptically with endocytic adaptor Eps15 to regulate synaptic growth and function. Spartin inhibits bone morphogenetic protein (BMP) signaling by promoting endocytic degradation of BMP receptor wishful thinking (Wit). Drosophila fragile X mental retardation protein (dFMRP) and Futsch/MAP1B are downstream effectors of Spartin and BMP signaling in regulating microtubule stability and synaptic growth. Loss of Spartin or elevation of BMP signaling induces age-dependent progressive defects resembling hereditary spastic paraplegias, including motor dysfunction and brain neurodegeneration. Null spartin phenotypes are prevented by administration of the microtubule-destabilizing drug vinblastine. Together, these results demonstrate that Spartin regulates both synaptic development and neuronal survival by controlling microtubule stability via the BMP-dFMRP-Futsch pathway, suggesting that impaired regulation of microtubule stability is a core pathogenic component in Troyer syndrome.

Spg20-/- mice reveal multimodal functions for Troyer syndrome protein spartin in lipid droplet maintenance, cytokinesis and BMP signaling

Hereditary spastic paraplegias (HSPs; SPG1-48) are inherited neurological disorders characterized by lower extremity spasticity and weakness. Loss-of-function mutations in the SPG20 gene encoding spartin cause autosomal recessive Troyer syndrome (SPG20), which has additional features of short stature, cognitive deficits and distal amyotrophy. To identify cellular impairments underlying Troyer syndrome, we generated Spg20-/- mice, which exhibit progressive gait defects. Although gross central nervous system pathology appeared largely normal, cerebral cortical neurons cultured from neonatal Spg20-/- mice exhibited increased axon branching, a phenotype suppressed by reintroducing spartin and which required its interaction with the endosomal sorting complex required for transport (ESCRT)-III protein IST1. Analysis of the bone morphogenetic protein (BMP) signaling pathway in Spg20-/- embryonic fibroblasts indicated that Smad1/5 phosphorylation is modestly elevated, possibly due to alterations in BMP receptor trafficking. Cytokinesis was impaired in embryonic fibroblasts cultured from Spg20-/- mice, and binucleated chondrocytes were prominent in epiphyseal growth plates of bones in Spg20-/- mice, perhaps explaining the short stature of patients. Finally, adipose tissue from Spg20-/- female mice exhibited increased lipid droplet (LD) numbers and alterations in perilipin levels, supporting a role for spartin in LD maintenance. Taken together, our results support multimodal functions for spartin that provide important insights into HSP pathogenesis.

SPG20 protein spartin associates with cardiolipin via its plant-related senescence domain and regulates mitochondrial Ca2+ homeostasis

Hereditary spastic paraplegias (HSPs) are a group of neurological disorders characterized clinically by spasticity of lower limbs and pathologically by degeneration of the corticospinal tract. Troyer syndrome is an autosomal recessive HSP caused by a frameshift mutation in the spartin (SPG20) gene. Previously, we established that this mutation results in a lack of expression of the truncated mutant spartin protein. Spartin is involved in many cellular processes and associates with several intracellular organelles, including mitochondria. Spartin contains a conserved plant-related senescence domain at its C-terminus. However, neither the function of this domain nor the roles of spartin in mitochondrial physiology are currently known. In this study, we determined that the plant-related senescence domain of spartin interacts with cardiolipin but not with two other major mitochondrial phospholipids, phosphatidylcholine and phosphatidylethanolamine. We also found that knockdown of spartin by small interfering RNA in a human neuroblastoma cell line resulted in depolarization of the mitochondrial membrane. In addition, depletion of spartin resulted in a significant decrease in both mitochondrial calcium uptake and mitochondrial membrane potential in cells treated with thapsigargin. Our results suggest that impairment of mitochondrial calcium uptake might contribute to the neurodegeneration of long corticospinal axons and the pathophysiology of Troyer syndrome.

Motor neuron disease due to neuropathy target esterase gene mutation: clinical features of the index families

Recently, we reported that mutations in the neuropathy target esterase (NTE) gene cause autosomal recessive motor neuron disease (NTE-MND). We describe clinical, neurophysiologic, and neuroimaging features of affected subjects in the index families. NTE-MND subjects exhibited progressive lower extremity spastic weakness that began in childhood and was later associated with atrophy of distal leg and intrinsic hand muscles. NTE-MND resembles Troyer syndrome, except that short stature, cognitive impairment, and dysmorphic features, which often accompany Troyer syndrome, are not features of NTE-MND. Early onset, symmetry, and slow progression distinguish NTE-MND from typical amyotrophic lateral sclerosis. NTE is implicated in organophosphorus compound-induced delayed neurotoxicity (OPIDN). NTE-MND patients have upper and lower motor neuron deficits that are similar to OPIDN. Motor neuron degeneration in subjects with NTE mutations supports the role of NTE and its biochemical cascade in the molecular pathogenesis of OPIDN and possibly other degenerative neurologic disorders.

SPG20 protein spartin is recruited to midbodies by ESCRT-III protein Ist1 and participates in cytokinesis

Hereditary spastic paraplegias (HSPs, SPG1-46) are inherited neurological disorders characterized by lower extremity spastic weakness. Loss-of-function SPG20 gene mutations cause an autosomal recessive HSP known as Troyer syndrome. The SPG20 protein spartin localizes to lipid droplets and endosomes, and it interacts with tail interacting protein 47 (TIP47) as well as the ubiquitin E3 ligases atrophin-1-interacting protein (AIP)4 and AIP5. Spartin harbors a domain contained within microtubule-interacting and trafficking molecules (MIT) at its N-terminus, and most proteins with MIT domains interact with specific ESCRT-III proteins. Using yeast two-hybrid and in vitro surface plasmon resonance assays, we demonstrate that the spartin MIT domain binds with micromolar affinity to the endosomal sorting complex required for transport (ESCRT)-III protein increased sodium tolerance (Ist)1 but not to ESCRT-III proteins charged multivesicular body proteins 1-7. Spartin colocalizes with Ist1 at the midbody, and depletion of Ist1 in cells by small interfering RNA significantly decreases the number of cells where spartin is present at midbodies. Depletion of spartin does not affect Ist1 localization to midbodies but markedly impairs cytokinesis. A structure-based amino acid substitution in the spartin MIT domain (F24D) blocks the spartin-Ist1 interaction. Spartin F24D does not localize to the midbody and acts in a dominant-negative manner to impair cytokinesis. These data suggest that Ist1 interaction is important for spartin recruitment to the midbody and that spartin participates in cytokinesis.

Developmental and degenerative features in a complicated spastic paraplegia

OBJECTIVE

We sought to explore the genetic and molecular causes of Troyer syndrome, one of several complicated hereditary spastic paraplegias (HSPs). Troyer syndrome had been thought to be restricted to the Amish; however, we identified 2 Omani families with HSP, short stature, dysarthria and developmental delay-core features of Troyer syndrome-and a novel mutation in the SPG20 gene, which is also mutated in the Amish. In addition, we analyzed SPG20 expression throughout development to infer how disruption of this gene might generate the constellation of developmental and degenerative Troyer syndrome phenotypes.

METHODS

Clinical characterization of 2 non-Amish families with Troyer syndrome was followed by linkage and sequencing analysis. Quantitative polymerase chain reaction and in situ hybridization analysis of SPG20 expression were carried out in embryonic and adult human and mouse tissue.

RESULTS

Two Omani families carrying a novel SPG20 mutation displayed clinical features remarkably similar to the Amish patients with Troyer syndrome. SPG20 mRNA is expressed broadly but at low relative levels in the adult brain; however, it is robustly and specifically expressed in the limbs, face, and brain during early morphogenesis.

INTERPRETATION

Null mutations in SPG20 cause Troyer syndrome, a specific clinical entity with developmental and degenerative features. Maximal expression of SPG20 in the limb buds and forebrain during embryogenesis may explain the developmental origin of the skeletal and cognitive defects observed in this disorder.

Identification of novel spartin-interactors shows spartin is a multifunctional protein

Hereditary spastic paraplegia describes a group of neurodegenerative diseases characterized by lower limb progressive weakness and spasticity. Troyer syndrome is an autosomal recessive form of hereditary spastic paraplegia caused by a frameshift mutation (1110delA) in the SPG20 gene encoding spartin protein, the cellular function of which remains unknown. Knowledge about spartin-interactors is also very limited. In this study, we apply a broad spectrum of proteomics techniques to identify novel spartin-binding proteins. We used a Tandem Affinity Purification technique followed by HPLC-mass spectrometry to characterize potential spartin-binding partners. Selected putative interactions were confirmed by co-immunoprecipitation experiments. We identified 94 potential spartin-binding proteins which were grouped into functional categories. We performed co-immunoprecipitation experiments to confirm that spartin interacts with GRP78, GRP75 and nucleolin proteins. Additionally, our mass spectrometry results confirmed previously published information about spartin interaction with ubiquitin and the E3 ubiquitin-protein ligases, AIP4/Itch and AIP5/WWP1. Our studies suggest that spartin is a multifunctional protein and for the first time we suggest a role for spartin in protein folding and turnover both in mitochondria and endoplasmic reticulum. We also show for the first time interaction between spartin and a nucleolar protein, nucleolin.

Low cancer incidence rates in Ohio Amish

BACKGROUND

The Amish have not been previously studied for cancer incidence, yet they have the potential to help in the understanding of its environmental and genetic contributions. The purpose of this study was to estimate the incidence of cancer among the largest Amish population.

METHODS

Adults from randomly selected households were interviewed and a detailed cancer family history was taken. Using both the household interview data and a search of the Ohio cancer registry data, a total of 191 cancer cases were identified between the years 1996 and 2003.

RESULTS

The age-adjusted cancer incidence rate for all cancers among the Amish adults was 60% of the age-adjusted adult rate in Ohio (389.5/10(5) vs. 646.9/10(5); p < 0.0001). The incidence rate for tobacco-related cancers in the Amish was 37% of the rate for Ohio adults (p < 0.0001). The incidence rate for non-tobacco-related cancers in the Amish was 72% of the age-adjusted adult rate in Ohio (p = 0.0001).

CONCLUSION

Cancer incidence is low in the Ohio Amish. These data strongly support reduction of cancer incidence by tobacco abstinence but cannot be explained solely on this basis. Understanding these contributions may help to identify additional important factors to target to reduce cancer among the non-Amish.

Genetic studies in the Amish community

The Amish community was established in Pennsylvania, Ohio and Indiana. They form a distinct and biologically isolated community by virtue of their strong cultural and religious beliefs. This paper outlines aspects of the Amish culture and reviews some of the recent genetic studies that have been undertaken in this community.

A novel form of juvenile recessive ALS maps to loci on 6p25 and 21q22

We describe a novel form of juvenile recessive ALS (JRALS) affecting four of six offspring from a consanguineous first cousin marriage. The syndrome is characterized by early and prominent upper motor neuron signs, along with striking amyotrophy of the upper and lower limbs and bulbar involvement. After excluding linkage to loci with known association to ALS and other motor neuron diseases, we used a homozygosity mapping approach to identify loci on chromosomes 6p25 and 21q22, each with an equal probability of linkage to the trait (with a LOD score=3.1, the maximum possible given the family structure). Mutation analysis of seven candidate genes that are expressed in the CNS or have roles in neuronal function did not reveal any pathogenic mutations. Identification of additional families will help to distinguish between which of the two autosomal loci contains the disease-causing gene, or whether this is a digenic trait.

A 60-year tale of spots, maps, and genes

This is an account of almost 60 years' experience in the clinical delineation of genetic disorders, mapping genes on chromosomes, and cataloging human disease-related genes and genetic disorders. The origins of medical genetics as a clinical specialty, of the Human Genome Project, of genomics (including the term), and of HUGO are recounted.

Lack of spartin protein in Troyer syndrome: a loss-of-function disease mechanism?

BACKGROUND

Hereditary spastic paraplegias (SPG1-SPG33) are characterized by progressive spastic weakness of the lower limbs. A nucleotide deletion (1110delA) in the (SPG20; OMIM 275900) spartin gene is the origin of autosomal recessive Troyer syndrome. This mutation is predicted to cause premature termination of the spartin protein. However, it remains unknown whether this truncated spartin protein is absent or is present and partially functional in patients.

OBJECTIVE

To determine whether the truncated spartin protein is present or absent in cells derived from patients with Troyer syndrome.

DESIGN

Case report.

SETTING

Academic research.

PATIENTS

We describe a new family with Troyer syndrome due to the 1110delA mutation.

MAIN OUTCOME MEASURES

We cultured primary fibroblasts and generated lymphoblasts from affected individuals, carriers, and control subjects and subjected these cells to immunoblot analyses.

RESULTS

Spartin protein is undetectable in several cell lines derived from patients with Troyer syndrome.

CONCLUSIONS

Our data suggest that Troyer syndrome results from complete loss of spartin protein rather than from the predicted partly functional fragment. This may reflect increased protein degradation or impaired translation.

Neuropathy target esterase gene mutations cause motor neuron disease

The possibility that organophosphorus (OP) compounds contribute to motor neuron disease (MND) is supported by association of paraoxonase 1 polymorphisms with amyotrophic lateral sclerosis (ALS) and the occurrence of MND in OP compound-induced delayed neuropathy (OPIDN), in which neuropathy target esterase (NTE) is inhibited by organophosphorylation. We evaluated a consanguineous kindred and a genetically unrelated nonconsanguineous kindred in which affected subjects exhibited progressive spastic paraplegia and distal muscle wasting. Affected subjects resembled those with OPIDN and those with Troyer Syndrome due to SPG20/spartin gene mutation (excluded by genetic linkage and SPG20/spartin sequence analysis). Genome-wide analysis suggested linkage to a 22 cM homozygous locus (D19S565 to D19S884, maximum multipoint LOD score 3.28) on chromosome 19p13 to which NTE had been mapped (GenBank AJ004832). NTE was a candidate because of its role in OPIDN and the similarity of our patients to those with OPIDN. Affected subjects in the consanguineous kindred were homozygous for disease-specific NTE mutation c.3034A-->G that disrupted an interspecies conserved residue (M1012V) in NTE's catalytic domain. Affected subjects in the nonconsanguineous family were compound heterozygotes: one allele had c.2669G-->A mutation, which disrupts an interspecies conserved residue in NTE's catalytic domain (R890H), and the other allele had an insertion (c.2946_2947insCAGC) causing frameshift and protein truncation (p.S982fs1019). Disease-specific, nonconserved NTE mutations in unrelated MND patients indicates NTE's importance in maintaining axonal integrity, raises the possibility that NTE pathway disturbances contribute to other MNDs including ALS, and supports the role of NTE abnormalities in axonopathy produced by neuropathic OP compounds.

Spastic paraplegia 5: Locus refinement, candidate gene analysis and clinical description

Thirty-three different loci for hereditary spastic paraplegias (HSP) have been mapped, and 15 responsible genes have been identified. Autosomal recessive spastic paraplegias (ARHSPs) usually have clinically complex phenotypes but the SPG5, SPG24, and SPG28 loci are considered to be associated with pure forms of the disease. We performed a genome-wide scan in a large French family. Fine mapping of the refined SPG5 region on chromosome 8q12 was performed in another 17 ARHSP families with additional microsatellite markers. After exclusion of known ARHSP loci, the genome-wide screen provided evidence of linkage with a maximal multipoint lod score of 2.6 in the D8S1113-D8S1699 interval. This interval partially overlapped SPG5 and reduced it to a 5.9 megabase (Mb)-region between D8S1113 and D8S544. In a family of Algerian origin from a series of 17 other ARHSP kindreds, linkage to the SPG5 locus was supported by a multipoint lod score of 2.3. The direct sequencing of the coding exons of seven candidate genes did not detect mutations/polymorphisms in the index cases of both linked families. The phenotype of the two SPG5-linked families consisted of spastic paraparesis associated with deep sensory loss. In several patients with long disease durations, there were also mild cerebellar signs. The frequency of SPG5 was approximately 10% (2/18) in our series of ARHSP families with pure or complex forms. We have refined the SPG5 locus to a 3.8 cM interval and extended the phenotype of this form of ARHSP to include slight cerebellar signs.

The genetics of hereditary spastic paraplegia and implications for drug therapy

Hereditary spastic paraplegia (HSP) comprises a group of clinically and genetically heterogeneous diseases that affect the upper motor neurons and their axonal projections. A total of 30 chromosomal loci have been identified for autosomal dominant, recessive and X-linked HSP. The underlying genes for 15 of these loci have been described. The molecular dissection of the cellular functions of the related gene products has already greatly advanced our understanding of the most critical pathways involved in HSP. It is hoped that in the foreseeable future this knowledge will begin to translate into novel pharmacological approaches for this devastating disease.

Troyer syndrome protein spartin is mono-ubiquitinated and functions in EGF receptor trafficking

Troyer syndrome is an autosomal recessive hereditary spastic paraplegia caused by mutation in the spartin (SPG20) gene, which encodes a widely expressed protein of unknown function. This mutation results in premature protein truncation and thus might signify a loss-of-function disease mechanism. In this study, we have found that spartin is mono-ubiquitinated and functions in degradation of the epidermal growth factor receptor (EGFR). Upon EGF stimulation, spartin translocates from the cytoplasm to the plasma membrane and colocalizes with internalized EGF-Alexa. Knockdown of spartin by small interfering RNA decreases the rate of EGFR degradation and also affects EGFR internalization, recycling, or both. Furthermore, overexpression of spartin results in a prominent decrease in EGFR degradation. Taken together, our data suggest that spartin is involved in the intracellular trafficking of EGFR and that impaired endocytosis may underlie the pathogenesis of Troyer syndrome.

Thin corpus callosum and amyotrophy in spastic paraplegia—Case report and review of literature

We report the clinical, structural, functional and genetic characterization of a 37-year-old Caucasian female, presenting as a sporadic case of complicated spastic paraplegia with thin corpus callosum (CC), cognitive impairment, amyotrophy of the hand muscles and a sensorimotor neuropathy and review the literature for spastic paraplegia with thin CC. Magnetic resonance imaging (MRI) examination revealed a thin CC with fronto-parietal cortical atrophy. 18Fluordesoxyglucose positron emission tomography (FDG-PET) showed reduced cortical and thalamic metabolism. By transcranial magnetic stimulation, we delineated a severe impairment of transcallosal inhibition. Sequence analysis did not reveal disease causing mutations in the genes SLC12A6 (Andermann), Spastin (SPG 4), BSCL2 (SPG 17) and Spartin (SPG 20). We reviewed the literature for HSP with thin CC and found 113 HSP patients with thin CC previously described (35 with linkage to chromosome 15q13-15). Thin CC and peripheral neuropathy often appear together in spastic paraplegia and might be indicative for combined degeneration mechanism of central and peripheral axons.

Hereditary spastic paraplegia

The hereditary spastic paraplegias (HSPs) comprise a large group of inherited neurologic disorders. HSP is classified according to the mode of inheritance, the HSP locus when known, and whether the spastic paraplegia syndrome occurs alone or is accompanied by additional neurologic or systemic abnormalities. Analysis of 11 recently discovered HSP genes provides insight into HSP pathogenesis. Hereditary spastic paraplegia is a clinical diagnosis for which laboratory confirmation is sometimes possible, and careful exclusion of alternate and co-existing disorders is an important element in HSP diagnosis. Treatment for HSP is presently limited to symptomatic reduction of muscle spasticity, reduction in urinary urgency, and strength and gait improvement through physical therapy. Prenatal genetic testing in HSP is possible for some individuals with the increasing availability of HSP gene analysis.

The Troyer syndrome (SPG20) protein spartin interacts with Eps15

The hereditary spastic paraplegias comprise a group of inherited neurological disorders in which the primary manifestation is spastic weakness of the lower extremities. Troyer syndrome is an autosomal recessive form of spastic paraplegia caused by a frameshift mutation in the spartin (SPG20) gene. Currently, neither the localization nor the functions of the spartin protein are known. In this study, we generated anti-spartin antibodies and found that spartin is both cytosolic and membrane-associated. Using a yeast two-hybrid approach, we screened an adult human brain library for binding partners of spartin. We identified Eps15, a protein known to be involved in endocytosis and the control of cell proliferation. This interaction was confirmed by fusion protein "pull-down" experiments as well as a cellular redistribution assay. Our results suggest that spartin might be involved in endocytosis, vesicle trafficking, or mitogenic activity, and that impairment in one of these processes may underlie the long axonopathy in patients with Troyer syndrome.

Troyer syndrome revisited. A clinical and radiological study of a complicated hereditary spastic paraplegia

Troyer syndrome, originally described in 1967 in an Old Order Amish population, is a complicated form of hereditary spastic paraplegia (HSP) inherited in an autosomal recessive fashion and slowly progressive. The cardinal features are spastic paraparesis, pseudobulbar palsy and distal amyotrophy, together with mild developmental delay and subtle skeletal abnormalities. We report a detailed evaluation of 21 cases of Troyer syndrome in the same Amish population, including three from the original study. Imaging of the brain revealed white matter abnormalities, particularly in the temporoparietal periventricular area. This study, coupled with the recent identification of the gene responsible (SPG20, encoding spartin), increases our understanding of this form of HSP.

X-linked motor and sensory neuropathy with pyramidal signs and cerebral white matter lesions

We report two brothers with hereditary motor and sensory neuropathies and pyramidal signs. Electrophysiological evaluation revealed polyneuropathy and involvement of the central motor, somatosensory, and auditory pathways. Brain magnetic resonance imaging studies showed diffuse white matter lesions, and sural nerve biopsy identified a reduction in the large myelinated nerve fibers. The patients' mother and sister exhibited similar, but milder neurologic findings suggesting that the genetic defect may be X-linked; however, a point mutation in the connexin 32 gene was negative.

Is the transportation highway the right road for hereditary spastic paraplegia?

The term "hereditary spastic paraplegia" (HSP) refers to a genetically and clinically diverse group of disorders whose primary feature is progressive spasticity of the lower extremities. The condition arises because of degeneration of the longest motor and sensory axons on the spinal cord, which appear to be most sensitive to the underlying mutations. The marked genetic heterogeneity in HSP, with 20 loci chromosomally mapped and eight genes now identified, suggests that a number of defective cellular processes may be shown to result in the disease. Although previous studies have suggested a mitochondrial basis for at least one form of the disease, a mechanism common to a number of the other genes mutated in HSP has remained elusive until now. The identification of the most recent genes for the condition suggests that aberrant cellular-trafficking dynamics may be a common process responsible for the specific pattern of neurodegeneration seen in HSP.

SPG20 is mutated in Troyer syndrome, an hereditary spastic paraplegia

Troyer syndrome (TRS) is an autosomal recessive complicated hereditary spastic paraplegia (HSP) that occurs with high frequency in the Old Order Amish. We report mapping of the TRS locus to chromosome 13q12.3 and identify a frameshift mutation in SPG20, encoding spartin. Comparative sequence analysis indicates that spartin shares similarity with molecules involved in endosomal trafficking and with spastin, a molecule implicated in microtubule interaction that is commonly mutated in HSP.

Hereditary spastic paraplegia

The hereditary spastic paraplegias are a large group of clinically similar disorders. Seventeen different HSP loci have been discovered thus far. Different genetic forms of uncomplicated HSP are clinically very similar. Except for the average age at which symptoms appear, different genetic types of uncomplicated HSP cannot be distinguished reliably by clinical parameters alone. For most subjects, HSP is a diagnosis of exclusion. The differential diagnosis includes treatable disorders as well as those for which the prognosis is quite different from HSP. Even with the emerging availability of laboratory testing for HSP gene mutations, it is still essential that alternative disorders be excluded by careful history, examination, laboratory studies, neuroimaging, and neurophysiologic evaluation. Uncomplicated HSP is due to axonal degeneration at the ends of the longest motor (corticospinal tract) and sensory (dorsal column fibers) in the spinal cord. The observation that some forms begin in childhood and are essentially nonprogressive while other forms begin in adulthood and are slowly progressive raises the possibility that some forms of HSP (e.g.; those associated with LICAM gene mutations and possibly those due to SPG3A mutations) are neurodevelopmental disorders; and other forms are truly neurodegenerative disorders. The mechanisms by which spastin, atlastin, and paraplegin mutations cause axonal degeneration that results in clinically similar forms of HSP are not known. Nonetheless, the identification of these genes and the ability to generate animal models of these forms of HSP will permit direct exploration of the molecular basis of HSP.

Autosomal dominant juvenile amyotrophic lateral sclerosis

Juvenile amyotrophic lateral sclerosis (ALS) is a form of chronic motor neuron disease characterized by combined upper and lower motor neuron symptoms and signs with onset prior to age 25 years. We report the clinical and electrodiagnostic findings in 49 affected family members and neuropathological findings from two autopsies of a Maryland kindred with autosomal dominant juvenile ALS linked to the chromosome 9q34 region (ALS4). Patients ranged in age from 12 to 85 years (mean 45 years) and the mean age of onset was 17 years. Distal weakness and atrophy was associated with pyramidal signs (43/49) and normal sensation (44/49). Motor conduction studies (n = 8) showed reduced evoked amplitudes and normal conduction parameters. Sensory conduction studies (n = 8), quantitative sensory testing (n = 4) and intracutaneous sensory fibres in skin biopsies (n = 6) were normal in all patients tested. Electromyography showed distal more than proximal chronic partial denervation and reinnervation (n = 8). Post-mortem spinal cord tissue demonstrated atrophic spinal cords with marked loss of anterior horn cells and degeneration of corticospinal tracts, as well as loss of neurons in the dorsal root ganglia and degeneration of the posterior columns. Axonal spheroids were present in the grey matter of the spinal cord, the dorsal root entry zones and the peripheral nerves. Motor and sensory roots, as well as peripheral nerves, showed significant axonal loss. Swellings were prominent around motor neurons, probably representing changes in presynaptic terminals. These studies define autosomal dominant juvenile ALS linked to the chromosome 9q34 region (ALS4) and extend the clinical, pathological and genetic heterogeneity of familial ALS and juvenile ALS.

Autosomal recessive spastic ataxia of Charlevoix-Saguenay

A form of autosomal recessive spastic ataxia unique to the Charlevoix-Saguenay area was clinically identified 20 years ago in patients from that region. This region of Québec, Canada, was once considered a genetic isolate. First noted at gait initiation, signs of ataxia slowly progress along with spasticity of the four limbs, slurred speech, and followed by distal amyotrophy. Early diagnosis relies on the presence of prominent myelinated fibers embedding retinal blood vessels at funduscopy and marked saccadic alteration of ocular smooth pursuit. Imaging of the posterior fossa shows cerebellar vermis atrophy and nerve conduction studies reveal loss of sensory and reduced motor conduction velocities. The clinical features are consistent with a developmental defect in myelination of both retinal and peripheral nerve fibers. The cause of this defect and the progressive axonal degeneration in the corticospinal and spinocerebellar tracts, as well as in the peripheral nerves is still unknown. Results of recent molecular genetic linkage analysis have located the gene locus to chromosome 13q12. Further research is needed to define where this hereditary spastic ataxia stands in the classification of the early onset spinocerebellar degenerations.