Novel mutations in the Atlastin gene (SPG3A) in families with autosomal dominant hereditary spastic paraplegia and evidence for late onset forms of HSP linked to the SPG3A locus

Identification of atlastin genetic modifiers in a model of hereditary spastic paraplegia in Drosophila

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative disorders characterized by progressive dysfunction of corticospinal motor neurons. Mutations in Atlastin1/Spg3, a small GTPase required for membrane fusion in the endoplasmic reticulum, are responsible for 10% of HSPs. Patients with the same Atlastin1/Spg3 mutation present high variability in age at onset and severity, suggesting a fundamental role of the environment and genetic background. Here, we used a Drosophila model of HSPs to identify genetic modifiers of decreased locomotion associated with atlastin knockdown in motor neurons. First, we screened for genomic regions that modify the climbing performance or viability of flies expressing atl RNAi in motor neurons. We tested 364 deficiencies spanning chromosomes two and three and found 35 enhancer and four suppressor regions of the climbing phenotype. We found that candidate genomic regions can also rescue atlastin effects at synapse morphology, suggesting a role in developing or maintaining the neuromuscular junction. Motor neuron-specific knockdown of 84 genes spanning candidate regions of the second chromosome identified 48 genes required for climbing behavior in motor neurons and 7 for viability, mapping to 11 modifier regions. We found that atl interacts genetically with Su(z)2, a component of the Polycomb repressive complex 1, suggesting that epigenetic regulation plays a role in the variability of HSP-like phenotypes caused by atl alleles. Our results identify new candidate genes and epigenetic regulation as a mechanism modifying neuronal atl pathogenic phenotypes, providing new targets for clinical studies.

Lipid Droplets in the Pathogenesis of Hereditary Spastic Paraplegia

Hereditary spastic paraplegias (HSPs) are genetically heterogeneous conditions caused by the progressive dying back of the longest axons in the central nervous system, the corticospinal axons. A wealth of data in the last decade has unraveled disturbances of lipid droplet (LD) biogenesis, maturation, turnover and contact sites in cellular and animal models with perturbed expression and function of HSP proteins. As ubiquitous organelles that segregate neutral lipid into a phospholipid monolayer, LDs are at the cross-road of several processes including lipid metabolism and trafficking, energy homeostasis, and stress signaling cascades. However, their role in brain cells, especially in neurons remains enigmatic. Here, we review experimental findings linking LD abnormalities to defective function of proteins encoded by HSP genes, and discuss arising questions in the context of the pathogenesis of HSP.

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

AIMS

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

METHODS AND RESULTS

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

CONCLUSION

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

[Common forms of hereditary spastic paraplegias]

A group of hereditary spastic paraplegias includes about 80 spastic paraplegia genes (SPG): forms with identified (almost 70) or only mapped (about 10) genes. Methods of next generation sequencing (NGS), along with new SPG discovering, modify knowledge about earlier delineated SPG. Clinical and genetic characteristics of common autosomal dominant (SPG4, SPG3, SPG31) and autosomal recessive (SPG11, SPG7, SPG15) forms are presented.

Three novel mutations in 20 patients with hereditary spastic paraparesis

Hereditary spastic paraparesis (HSP) constitutes both genetic and clinically heterogeneous group of upper motor neuron diseases. Half of the individuals with autosomal dominant (AD) HSP have mutations in SPAST, ATL1, and REEP1 genes. This study was conducted to elucidate the genetic etiology of patients with the pure type AD-HSP diagnosis. The patient group consisted of 23 individuals from 6 families in Turkey. In the first step of work, Sanger sequencing (SS) was performed in ATL1, SPAST, and REEP1 genes and the second phase whole-exome sequencing (WES) was performed following SS analysis for the patients with no detected mutations in these genes. The results of this study revealed that in ATL1, 6 patients have previously reported c.776C > A mutation and 6 patients have novel c.470 T > C mutation. In SPAST, 3 patients have novel c.1072G > C mutation and 2 patients have novel c.1099-1G > C mutation. WES was performed in three patients, who had no detected mutation in these genes with SS analysis. In this approach, as previously reported c.1859 T > C mutation in KIAA0196 was detected, and it was confirmed with the patient's relatives by SS. In three of patients, no HSP-associated variant could be identified in SS and WES. With this study, the molecular genetic etiology in 20 of 23 (87%) individuals that were included in this study with the utilization of SS and WES was elucidated. Utilization of SS and WES methods have enabled the identification of genetic etiology of HSP further with appropriate genetic counseling that was provided to the patients.

Clinical features and genotype-phenotype correlation analysis in patients with ATL1 mutations: A literature reanalysis

Background

The hereditary spastic paraplegias (HSPs) are a group of clinically and genetically heterogeneous disorders. Approximately 10% of the autosomal dominant (AD) HSPs (ADHSPs) have the spastic paraplegia 3A (SPG3A) genotype which is caused by ATL1 gene mutations. Currently there are more than 60 reported ATL1 gene mutations and the genotype-phenotype correlation remains unclear. The study aims to investigate the genotype-phenotype correlation in SPG3A patients.

Methods

We performed a reanalysis of the clinical features and genotype-phenotype correlations in 51 reported studies exhibiting an ATL1 gene mutation.

Results

Most HSPs-SPG3A patients exhibited an early age at onset (AAO) of <10 years old, and showed an autosomal dominant pure spastic paraplegia. We found that 14% of the HSPs-SPG3A patients presented complicated phenotypes, with distal atrophy being the most common complicated symptom. The AAO of each mutation group was not statistically significant (P > 0.05). The mutational spectrum associated with ATL1 gene mutation is wide, and most mutations are missense mutations, but do not involve the functional motif of ATL1 gene encoded atlastin-1 protein.

Conclusions

Our findings indicate that there is no clear genotype-phenotype correlation in HSPs-SPG3A patients. We also find that exons 4, 7, 8 and 12 are mutation hotspots in ATL1 gene.

Electronic supplementary material

The online version of this article (doi:10.1186/s40035-017-0079-3) contains supplementary material, which is available to authorized users.

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

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

ER network formation and membrane fusion by atlastin1/SPG3A disease variants

Atlastin catalyzes GTP-dependent membrane fusion to form the ER network. Mutations in atlastin1 cause the disease hereditary spastic paraplegia (HSP), implying that defects in ER membrane fusion cause HSP. Surprisingly, several disease variants are functional in assays for ER network formation and membrane fusion, warranting rethinking of HSP causation by atlastin1 mutations.

At least 38 distinct missense mutations in the neuronal atlastin1/SPG3A GTPase are implicated in an autosomal dominant form of hereditary spastic paraplegia (HSP), a motor-neurological disorder manifested by lower limb weakness and spasticity and length-dependent axonopathy of corticospinal motor neurons. Because the atlastin GTPase is sufficient to catalyze membrane fusion and required to form the ER network, at least in nonneuronal cells, it is logically assumed that defects in ER membrane morphogenesis due to impaired fusion activity are the primary drivers of SPG3A-associated HSP. Here we analyzed a subset of established atlastin1/SPG3A disease variants using cell-based assays for atlastin-mediated ER network formation and biochemical assays for atlastin-catalyzed GTP hydrolysis, dimer formation, and membrane fusion. As anticipated, some variants exhibited clear deficits. Surprisingly however, at least two disease variants, one of which represents that most frequently identified in SPG3A HSP patients, displayed wild-type levels of activity in all assays. The same variants were also capable of co-redistributing ER-localized REEP1, a recently identified function of atlastins that requires its catalytic activity. Taken together, these findings indicate that a deficit in the membrane fusion activity of atlastin1 may be a key contributor, but is not required, for HSP causation.

High frequency of SPG4 in Taiwanese families with autosomal dominant hereditary spastic paraplegia

BACKGROUND

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative diseases characterized by progressive spasticity and weakness of the lower limbs. SPG4, SPG3A and SPG31 are the three leading causes of autosomal dominant (AD) HSPs.

METHODS

A total of 20 unrelated AD-HSP families were recruited for clinical and genetic assessment. Detection of mutations in SPG4, SPG3A and SPG31 genes was conducted according to a standard protocol. Genotype-phenotype correlations and determinants for disease severity and progression were analyzed.

RESULTS

Mutations in the SPG4 gene (SPAST) were detected in 18 (90%) of the AD-HSP families. Mutations in SPG4, SPG3A and SPG31 genes were not detected in the remaining two families. Considerable variations in clinical features were noted, even for mutation carriers from the same family. Mutations causing complete loss of the spastin AAA cassette were associated with earlier onset of disease (20 ± 18 years) compared with those with preservation of partial or total AAA cassette (32 ± 19 years, p = 0.041). For those with SPG4 mutations, disease severity was related to the patients' current age, and the progression rate of disease was positively correlated with age at onset.

CONCLUSIONS

SPG4 accounts for most of the AD-HSP cases in Taiwanese, with a frequency significantly higher than in other populations. SPAST mutations which predict complete loss of the spastin AAA cassette were associated with an earlier onset of disease.

Mutation and clinical characteristics of autosomal-dominant hereditary spastic paraplegias in China

BACKGROUND

Hereditary spastic paraplegias constitute a heterogeneous group of inherited neurodegenerative disorders. To date, there has been no systematic mutation and clinical analysis for a large group of autosomal-dominant hereditary spastic paraplegias in China.

OBJECTIVE

The purpose of this study was to investigate the mutation frequencies and the clinical phenotypes of Chinese spastic paraplegia patients.

METHODS

Direct sequencing and a multiplex ligation-dependent probe amplification assay were applied to detect the mutations of SPAST and ATL1 in 54 autosomal-dominant hereditary spastic paraplegia probands and 66 isolated cases. Next, mutations in NIPA1, KIF5A, REEP1 and SLC33A1 were detected in the negative patients. Subsets of spastic paraplegia patients were genotyped for the modifying variants. Further, detailed clinical data regarding the genetically diagnosed families were analysed.

RESULTS

Altogether, 27 families were diagnosed as SPG4, 3 as SPG3A and 1 as SPG6. No mutations in KIF5A, REEP1 or SLC33A1 were found; 9 SPAST mutations were novel. There was no p.S44L or p.P45Q variant in SPAST and no p.G563A variant in HSPD1 in either the 120 spastic paraplegia patients or the 500 controls. There was a remarkable clinical difference between the SPG4 and non-SPG4 patients and even between genders among the SPG4 patients. Non-penetrance and remarkable gender difference were observed in some SPG4 and SPG3A families.

CONCLUSIONS

Our data confirm that hereditary spastic paraplegias in China represent a heterogeneous group of genetic neurodegenerative disorders in autosomal-dominant and apparently sporadic forms. Novel genotype-phenotype correlations were established. © 2014 S. Karger AG, Basel.

Evidence for autosomal recessive inheritance in SPG3A caused by homozygosity for a novel ATL1 missense mutation

Hereditary spastic paraplegias (HSPs) comprise a heterogeneous group of disorders characterized by progressive spasticity and weakness of the lower limbs. Autosomal dominant and 'pure' forms of HSP account for ∼80% of cases in Western societies of whom 10% carry atlastin-1 (ATL1) gene mutations. We report on a large consanguineous family segregating six members with early onset HSP. The pedigree was compatible with both autosomal dominant and autosomal recessive inheritance. Whole-exome sequencing and segregation analysis revealed a homozygous novel missense variant c.353G>A, p.(Arg118Gln) in ATL1 in all six affected family members. Seven heterozygous carriers, five females and two males, showed no clinical signs of HSP with the exception of sub-clinically reduced vibration sensation in one adult female. Our combined findings show that homozygosity for the ATL1 missense variant remains the only plausible cause of HSP, whereas heterozygous carriers are asymptomatic. This apparent autosomal recessive inheritance adds to the clinical complexity of spastic paraplegia 3A and calls for caution using directed genetic screening in HSP.

A conserved role for atlastin GTPases in regulating lipid droplet size

Lipid droplets (LDs) are the major fat storage organelles in eukaryotic cells, but how their size is regulated is unknown. Using genetic screens in C. elegans for LD morphology defects in intestinal cells, we found that mutations in atlastin, a GTPase required for homotypic fusion of endoplasmic reticulum (ER) membranes, cause not only ER morphology defects, but also a reduction in LD size. Similar results were obtained after depletion of atlastin or expression of a dominant-negative mutant, whereas overexpression of atlastin had the opposite effect. Atlastin depletion in Drosophila fat bodies also reduced LD size and decreased triglycerides in whole animals, sensitizing them to starvation. In mammalian cells, co-overexpression of atlastin-1 and REEP1, a paralog of the ER tubule-shaping protein DP1/REEP5, generates large LDs. The effect of atlastin-1 on LD size correlates with its activity to promote membrane fusion in vitro. Our results indicate that atlastin-mediated fusion of ER membranes is important for LD size regulation.

ATL1 and REEP1 mutations in hereditary and sporadic upper motor neuron syndromes

SPAST mutations are the most common cause of autosomal dominant hereditary spastic paraplegias (AD-HSPs), but many spastic paraplegia patients are found to carry no mutations in this gene. In order to assess the contribution of ATL1 and REEP1 in AD-HSP, we performed mutational analysis in 27 SPAST-negative AD-HSP families. We found three novel ATL1 mutations and one REEP1 mutation in five index-patients. In 110 patients with sporadic adult-onset upper motor neuron syndromes, a novel REEP1 mutation was identified in one patient. Apart from a significantly younger age at onset in ATL1 patients and restless legs in some, the clinical phenotype of ATL1 and REEP1 was similar to other pure AD-HSPs.

Mutation screening of spastin, atlastin, and REEP1 in hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) comprises a group of clinically and genetically heterogeneous diseases that affect the upper motor neurons and their axonal projections. Over 40 chromosomal loci have been identified for autosomal dominant, recessive, and X-linked HSP. Mutations in the genes atlastin, spastin and REEP1 are estimated to account for up to 50% of autosomal-dominant HSP and currently guide the molecular diagnosis of HSP. Here, we report the mutation screening results of 120 HSP patients from North America for spastin, atlastin, and REEP1, with the latter one partially reported previously. We identified mutations in 36.7% of all tested HSP patients and describe 20 novel changes in spastin and atlastin. Our results add to a growing number of HSP disease-associated variants and confirm the high prevalence of atlastin, spastin, and REEP1 mutations in the HSP patient population.

Clinical and genetic findings in a series of Italian children with pure hereditary spastic paraplegia

BACKGROUND

hereditary spastic paraplegias (HSP) are a group of neurodegenerative disorders characterized by progressive lower extremity spastic weakness. SPG7, SPG4 and SPG3A are some of the autosomal genes recently found as mutated in recessive or dominant forms of HSP in childhood. SPG31 is more often associated with a pure spastic paraplegia phenotype, but genotype-phenotype correlation is still unclear. The aims of the current study was: (i) to verify the mutational frequency of SPG4, SPG3A, SPG31 and SPG7 genes in our very-well-selected childhood sample, and (ii) to improve our knowledge about the clinical and electrophysiological HSP phenotypes and their possible correlation with a specific mutation.

METHODS

a sample of 14 Italian children affected by pure HSP (mean age at diagnosis 5.9 years) was extensively investigated with electrophysiological, neuroradiological and genetic tests.

RESULTS

three SPG4 mutations were identified in three patients: two novel missense mutations, both sporadic, and one multiexonic deletion already reported. A novel large deletion in SPG31 gene involving exons 2-5 was also detected in one young patient. No mutations in the SPG7 and in the SPG3A genes were found.

CONCLUSIONS

our data confirm that HSP represent a heterogeneous group of genetic neurodegenerative disorders, also in sporadic or autosomal recessive early onset forms. Multiplex Ligation-dependent Probe Amplification-based mutation screening for SPG4 and SPG31 genes would be added to sequencing-based screening of SPG4, SPG31 and SPG3A genes in the routine diagnosis of HSP children.

Mutational spectrum of the SPG4 (SPAST) and SPG3A (ATL1) genes in Spanish patients with hereditary spastic paraplegia

Background

Hereditary Spastic Paraplegias (HSP) are characterized by progressive spasticity and weakness of the lower limbs. At least 45 loci have been identified in families with autosomal dominant (AD), autosomal recessive (AR), or X-linked hereditary patterns. Mutations in the SPAST (SPG4) and ATL1 (SPG3A) genes would account for about 50% of the ADHSP cases.

Methods

We defined the SPAST and ATL1 mutational spectrum in a total of 370 unrelated HSP index cases from Spain (83% with a pure phenotype).

Results

We found 50 SPAST mutations (including two large deletions) in 54 patients and 7 ATL1 mutations in 11 patients. A total of 33 of the SPAST and 3 of the ATL1 were new mutations. A total of 141 (31%) were familial cases, and we found a higher frequency of mutation carriers among these compared to apparently sporadic cases (38% vs. 5%). Five of the SPAST mutations were predicted to affect the pre-mRNA splicing, and in 4 of them we demonstrated this effect at the cDNA level. In addition to large deletions, splicing, frameshifting, and missense mutations, we also found a nucleotide change in the stop codon that would result in a larger ORF.

Conclusions

In a large cohort of Spanish patients with spastic paraplegia, SPAST and ATL1 mutations were found in 15% of the cases. These mutations were more frequent in familial cases (compared to sporadic), and were associated with heterogeneous clinical manifestations.

The effect of HSP-causing mutations in SPG3A and NIPA1 on the assembly, trafficking, and interaction between atlastin-1 and NIPA1

Despite its genetic heterogeneity, hereditary spastic paraplegia (HSP) is characterized by similar clinical phenotypes, suggesting that a common biochemical pathway underlies its pathogenesis. In support of this hypothesis, we used a combination of immunoprecipitation, confocal microscopy, and flow cytometry to demonstrate that two HSP-associated proteins, atlastin-1 and NIPA1, are direct binding partners, and interestingly, that the endogenous expression and trafficking of these proteins is highly dependent upon their coexpression. In addition, we demonstrated that the cellular distribution of atlastin-1:NIPA1 complexes was dramatically altered by HSP-causing mutations, as missense mutations in atlastin-1 (R239C and R495W) and NIPA1 (T45R and G106R) caused protein sequestration in the Golgi complex (GC) and endoplasmic reticulum (ER), respectively. Moreover, we demonstrated that HSP-causing mutations in both atlastin-1 and NIPA1 reduced axonal and dendritic sprouting in cultured rat cortical neurons. Together, these findings support the hypothesis that NIPA1 and atlastin-1 are members of a common biochemical pathway that supports axonal maintenance, which may explain in part the characteristic degeneration of long spinal pathways observed in patients with HSP.

A total of 220 patients with autosomal dominant spastic paraplegia do not display mutations in the SLC33A1 gene (SPG42)

The most frequent causes of autosomal dominant (AD) hereditary spastic paraplegias (HSP) (ADHSP) are mutations in the SPAST gene (SPG4 locus). However, roughly 60% of patients are negative for SPAST mutations, despite their family history being compatible with AD inheritance. A mutation in the gene for an acetyl-CoA transporter (SLC33A1) has recently been reported in one Chinese family to cause ADHSP-type SPG42. In this study, we screened 220 independent SPAST mutation-negative ADHSP samples for mutations in the SLC33A1 gene by high-resolution melting curve analysis. Conspicuous samples were validated by direct sequencing. Moreover, copy number variations affecting SLC33A1 were screened by multiplex ligation-dependent probe amplification assay. We could not identify potentially disease-causing mutations in our patients either by mutation scanning or by gene dosage analysis, as for the latter specific positive controls are not available to date. As our sample represents ADHSP patients for whom SPAST mutations and almost in all cases ATL1 and REEP1 mutations had been excluded, we consider SLC33A1 gene mutations as being very rare in a European ADHSP cohort, if present at all. To date, as SPG42 has still not been identified in a second, unrelated family, systematic genetic testing for SLC33A1 mutations is not recommended.

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.

Novel SPG3A and SPG4 mutations in dominant spastic paraplegia families

OBJECTIVES

The hereditary spastic paraplegias (HSP) are a genetically and clinically heterogeneous group of neurodegenerative disorders, mainly characterized by a progressive spasticity and weakness of the lower limbs. Mutations in the SPG4 and SPG3A genes are responsible for approximately 50% of autosomal dominant HSP. To genetically diagnose the Portuguese families with HSP, mutation analysis was performed for the SPG4 and SPG3A genes.

PATIENTS AND METHODS

Analysis was performed by polymerase chain reaction, followed by denaturing high performance liquid chromatography (DHPLC), in 61 autosomal dominant (AD)-HSP families and 19 unrelated patients without family history.

RESULTS

Ten novel mutations were identified: one in the SPG3A and nine in the SPG4 genes; three known mutations in the SPG4 were also found. Most of the novel mutations were frameshift or nonsense (80%), resulting in a dysfunctional protein.

CONCLUSIONS

The SPG4 and SPG3A analysis allowed the identification of 10 novel mutations and the genetic diagnosis of approximately a quarter of our AD-HSP families.

A missense mutation in SLC33A1, which encodes the acetyl-CoA transporter, causes autosomal-dominant spastic paraplegia (SPG42)

Hereditary spastic paraplegias (HSPs), characterized by progressive and bilateral spasticity of the legs, are usually caused by developmental failure or degeneration of motor axons in the corticospinal tract. There are considerable interfamilial and intrafamilial variations in age at onset and severity of spasticity. Genetic studies also showed that there are dozens of genetic loci, on multiple chromosomes, that are responsible for HSPs. Through linkage study of a pedigree of HSP with autosomal-dominant inheritance, we mapped the causative gene to 3q24-q26. Screening of candidate genes revealed that the HSP is caused by a missense mutation in the gene for acetyl-CoA transporter (SLC33A1). It is predicted that the missense mutation, causing the change of the highly conserved serine to arginine at the codon 113 (p. S113R), disrupts the second transmembrane domain in the transporter and reverses the orientation of all of the descending domains. Knockdown of Slc33a1 in zebrafish caused a curve-shaped tail and defective axon outgrowth from the spinal cord. Although the wild-type human SLC33A1 was able to rescue the phenotype caused by Slc33a1 knockdown in zebrafish, the mutant SLC33A1 (p.S113R) was not, suggesting that S113R mutation renders SLC33A1 nonfunctional and one that wild-type allele is not sufficient for sustaining the outgrowth and maintenance of long motor axons in human heterozygotes. Thus, our study illustrated a critical role of acetyl-CoA transporter in motor-neuron development and function.

Analysis of CYP7B1 in non-consanguineous cases of hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is a neurodegenerative condition defined clinically by lower limb spasticity and weakness. Homozygous mutations in CYP7B1 have been identified in several consanguineous families that represented HSP type 5 (SPG5), one of the many genetic forms of the disease. We used direct sequencing and multiplex ligation-dependent probe amplification to screen for CYP7B1 alterations in apparently sporadic HSP patients (n = 12) as well as index patients from non-consanguineous families with recessive (n = 8) and dominant (n = 8) transmission of HSP. One sporadic patient showing HSP as well as optic atrophy carried a homozygous nonsense mutation. Compound heterozygosity was observed in a recessive family with a clinically pure phenotype. A heterozygous missense change segregated in a small dominant family. We also found a significant association of a known coding polymorphism with cerebellar signs complicating a primary HSP phenotype. Our findings suggest CYP7B1 alterations to represent a rather frequent cause of HSP that should be considered in patients with various clinical presentations.

Autosomal dominant hereditary spastic paraplegia: novel mutations in the REEP1 gene (SPG31)

Background

Mutations in the SPG4 gene (spastin) and in the SPG3A gene (atlastin) account for the majority of 'pure' autosomal dominant form of hereditary spastic paraplegia (HSP). Recently, mutations in the REEP1 gene were identified to cause autosomal dominant HSP type SPG31. The purpose of this study was to determine the prevalence of REEP1 mutations in a cohort of 162 unrelated Caucasian index patients with 'pure' HSP and a positive family history (at least two persons per family presented symptoms).

Methods

162 patients were screened for mutations by, both, DHPLC and direct sequencing.

Results

Ten mutations were identified in the REEP1 gene, these included eight novel mutations comprising small insertions/deletions causing frame shifts and subsequently premature stop codons, one nonsense mutation and one splice site mutation as well as two missense mutations. Both missense mutations and the splice site mutation were not identified in 170 control subjects.

Conclusion

In our HSP cohort we found pathogenic mutations in 4.3% of cases with autosomal dominant inheritance. Our results confirm the previously observed mutation range of 3% to 6.5%, respectively, and they widen the spectrum of REEP1 mutations.

REEP1 mutation spectrum and genotype/phenotype correlation in hereditary spastic paraplegia type 31

Mutations in the receptor expression enhancing protein 1 (REEP1) have recently been reported to cause autosomal dominant hereditary spastic paraplegia (HSP) type SPG31. In a large collaborative effort, we screened a sample of 535 unrelated HSP patients for REEP1 mutations and copy number variations. We identified 13 novel and 2 known REEP1 mutations in 16 familial and sporadic patients by direct sequencing analysis. Twelve out of 16 mutations were small insertions, deletions or splice site mutations. These changes would result in shifts of the open-reading-frame followed by premature termination of translation and haploinsufficiency. Interestingly, we identified two disease associated variations in the 3'-UTR of REEP1 that fell into highly conserved micro RNA binding sites. Copy number variation analysis in a subset of 133 HSP index patients revealed a large duplication of REEP1 that involved exons 2-7 in an Irish family. Clinically most SPG31 patients present with a pure spastic paraplegia; rare complicating features were restricted to symptoms or signs of peripheral nerve involvement. Interestingly, the distribution of age at onset suggested a bimodal pattern with the appearance of initial symptoms of disease either before the age of 20 years or after the age of 30 years. The overall mutation rate in our clinically heterogeneous sample was 3.0%; however, in the sub-sample of pure HSP REEP1 mutations accounted for 8.2% of all patients. These results firmly establish REEP1 as a relatively frequent autosomal dominant HSP gene for which genetic testing is warranted. We also establish haploinsufficiency as the main molecular genetic mechanism in SPG31, which should initiate and guide functional studies on REEP1 with a focus on loss-of-function mechanisms. Our results should be valid as a reference for mutation frequency, spectrum of REEP1 mutations, and clinical phenotypes associated with SPG31.

Characterization of a novel SPG3A deletion in a French-Canadian family

Hereditary spastic paraplegias (HSPs) are characterized by progressive lower limb spasticity and weakness. Mutations in the SPG3A gene, which encodes the large guanosine triphosphatase atlastin, are the second most common cause of autosomal dominant hereditary spastic paraplegia. In a large SPG3A screen of 70 hereditary spastic paraplegia subjects, a novel in-frame deletion, p.del436N, was identified. Characterization of this deletion showed that it affects neither the guanosine triphosphatase activity of atlastin nor interactions between atlastin and spastin. Interestingly, immunoblot analysis of lymphoblasts from affected patients demonstrated a significant reduction in atlastin protein levels, supporting a loss-of-function disease mechanism.

Loss of spastic paraplegia gene atlastin induces age-dependent death of dopaminergic neurons in Drosophila

Hereditary spastic paraplegias (HSPs) are human genetic disorders causing increased stiffness and overactive muscle reflexes in the lower extremities. atlastin (atl) is one of the major genes in which mutations result in HSP. We generated a Drosophila model of HSP that has a null mutation in atl. As they aged, atl null flies were paralyzed by mechanical shock such as bumping or vortexing. Furthermore, the flies showed age-dependent degeneration of dopaminergic neurons. These phenotypes were rescued by targeted expression of atl in dopaminergic neurons or feeding L-DOPA or SK&F 38393, an agonist of dopamine receptor. Our data raised the possibility that one of the causes of HSP disease symptoms in human patients with alt mutations is malfunction or degeneration of dopaminergic neurons.

Transient kinetic investigation of GTP hydrolysis catalyzed by interferon-gamma-induced hGBP1 (human guanylate binding protein 1)

Within the family of large GTP-binding proteins, human guanylate binding protein 1 (hGBP1) belongs to a subgroup of interferon-inducible proteins. GTP hydrolysis activity of these proteins is much higher compared with members of other GTPase families and underlies mechanisms that are not understood. The large GTP-binding proteins form self-assemblies that lead to stimulation of the catalytic activity. The unique result of GTP hydrolysis catalyzed by hGBP1 is GDP and GMP. We investigated this reaction mechanism by transient kinetic methods using radioactively labeled GTP as well as fluorescent probes. Substrate binding and formation of the hGBP1 homodimer are fast as no lag phase is observed in the time courses of GTP hydrolysis. Instead, multiple turnover experiments show a rapid burst of P(i) formation prior to the steady state phase, indicating a rate-limiting step after GTP cleavage. Both molecules are catalytically active and cleave off a phosphate ion in the first step. Then bifurcation into catalytic inactivation, probably by irreversible dissociation of the dimer, and into GDP hydrolysis is observed. The second cleavage step is even faster than the first step, implying a rapid rearrangement of the nucleotide within the catalytic center of hGBP1. We could also show that the release of the products, including the phosphate ions, is fast and not limiting the steady state activity. We suggest that slow dissociation of the GMP-bound homodimer gives rise to the burst behavior and controls the steady state. The assembled forms of the GDP- and GMP-bound states of hGBP1 are accessible only through GTP binding and hydrolysis and achieve a lifetime of a few seconds.

The molecular genetics of non-ALS motor neuron diseases

Hereditary disorders of voluntary motor neurons are individually relatively uncommon, but have the potential to provide significant insights into motor neuron function in general and into the mechanisms underlying the more common form of sporadic Amyotrophic Lateral Sclerosis. Recently, mutations in a number of novel genes have been associated with Lower Motor Neuron (HSPB1, HSPB8, GARS, Dynactin), Upper Motor Neuron (Spastin, Atlastin, Paraplegin, HSP60, KIF5A, NIPA1) or mixed ALS-like phenotypes (Alsin, Senataxin, VAPB, BSCL2). In comparison to sporadic ALS these conditions are usually associated with slow progression, but as experience increases, a wide variation in clinical phenotype has become apparent. At the molecular level common themes are emerging that point to areas of specific vulnerability for motor neurons such as axonal transport, endosomal trafficking and RNA processing. We review the clinical and molecular features of this diverse group of genetically determined conditions and consider the implications for the broad group of motor neuron diseases in general.

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.

Mutation in KIF5A can also cause adult-onset hereditary spastic paraplegia

Autosomal dominant hereditary spastic paraplegia (AD HSP) linked to chromosome 12q (SPG10) is caused by mutations in the neuronal kinesin heavy-chain KIF5A gene. This is a rare cause of AD HSP, and only two disease-causing mutations have been reported thus far. In both instances, affected individuals harboring mutations in the KIF5A gene displayed symptom onset at a very early age. Here we present the results of clinical and genetic analyses of a large kindred with uncomplicated AD HSP. We were able to establish a definitive linkage to the SPG10 locus, and sequencing of the KIF5A gene revealed a heterozygous missense mutation 1,035 A>G in exon 10, resulting in tyrosine-to-cysteine substitution. This mutation is located in a highly conserved kinesin motor domain of the neuronal kinesin heavy-chain protein, but in contrast to two previously reported missense mutations, the age of symptom onset in our family was much later, with an average age of 36.1+/-4 years. Our results demonstrate that mutations in the KIF5A gene can also be associated with an adult age of onset of AD HSP.

Traffic accidents: Molecular genetic insights into the pathogenesis of the hereditary spastic paraplegias

The hereditary spastic paraplegias (HSPs) comprise a clinically and genetically diverse group of inherited neurological disorders in which the primary manifestation is progressive spasticity and weakness of the lower limbs. The identification of over 25 genetic loci and 11 gene products for these disorders has yielded new insights into the molecular pathways involved in the pathogenesis of HSPs. In particular, causative mutations in proteins implicated in mitochondrial function, intracellular transport and trafficking, axonal development, and myelination have been identified. In many cases, the proper intracellular trafficking and distribution of molecules and organelles are ultimately thought to be involved in HSP pathogenesis. In fact, deficits in intracellular cargo trafficking and transport are concordant with the length dependence of the distal axonopathy of upper motor neurons observed in HSP patients. Through a better understanding of the functions of the HSP gene products, novel therapeutic targets for treatment and prevention are being identified.

Spastin and atlastin, two proteins mutated in autosomal-dominant hereditary spastic paraplegia, are binding partners

The pure hereditary spastic paraplegias (HSPs) are a group of conditions in which there is a progressive length-dependent degeneration of the distal ends of the corticospinal tract axons, resulting in spastic paralysis of the legs. Pure HSPs are most frequently inherited in an autosomal-dominant pattern and are commonly caused by mutations either in the SPG4 gene spastin or in the SPG3A gene atlastin. To identify binding partners for spastin, we carried out a yeast two-hybrid screen on a brain cDNA library, using spastin as bait. Remarkably, nearly all of the positive interacting prey clones coded for atlastin. We have verified the physiological relevance of this interaction using co-immunoprecipitation, glutathione S-transferase pull-down and intracellular co-localization experiments. We show that the spastin domain required for binding to atlastin lies within the N-terminal 80 residues of the protein, a region that is only present in the predominantly cytoplasmic, full-length spastin isoform. These data suggest that spastin and atlastin function in the same biochemical pathway and that it is the cytoplasmic function of spastin which is important for the pathogenesis of HSP. They also provide further evidence for a physiological and pathological role of spastin in membrane dynamics.

The R495W mutation in SPG3A causes spastic paraplegia associated with axonal neuropathy

Mutations in the SPG3A gene cause a form of pure, early-onset autosomal dominant hereditary spastic paraplegia linked to chromosome 14q. The encoded protein, atlastin, is a putative member of the dynamin superfamily of large GTPases involved in cellular trafficking patterns. We report a new atlastin mutation causing spastic paraplegia in association with axonal neuropathy in an Italian family.

Early onset autosomal dominant spastic paraplegia caused by novel mutations in SPG3A

Hereditary spastic paraplegia (HSP) is a group of neurodegenerative disorders mainly characterized by progressive spasticity of the lower limbs. The major features of HSP are a marked phenotypic variability both among and within families and an extended genetic heterogeneity. More than 20 HSP loci and 10 spastic paraplegia genes (SPG) have been identified to date, including the genes responsible for the two most frequent forms of autosomal dominant spastic paraplegia (AD-HSP), encoding spastin (SPG4) and atlastin (SPG3A), respectively. To date, only eight mutations have been described in the atlastin gene, which was reported to account for about 10% of all AD-HSP families. We investigated 15 German and French AD-HSP families, including the 3 large pedigrees that allowed the mapping and subsequent refinement of the SPG3A locus. Three novel mutations were found in exons 4, 9, and 12 of the atlastin gene and the common R239C mutation located in exon 7 was confirmed in a 7th family of European origin. Overall, the comparison of the clinical data for all SPG3A-HSP families reported to date failed to reveal any genotype/phenotype correlation as demonstrated for other forms of AD-HSP. However, it confirmed the early onset of this form of HSP, which was observed in almost all affected individuals with a mutation in the atlastin gene.