Genetic and functional analyses demonstrate a role for abnormal glycinergic signaling in autism.

Autism spectrum disorder (ASD) is a common neurodevelopmental condition characterized by marked genetic heterogeneity. Recent studies of rare structural and sequence variants have identified hundreds of loci involved in ASD, but our knowledge of the overall genetic architecture and the underlying pathophysiological mechanisms remains incomplete. Glycine receptors (GlyRs) are ligand-gated chloride channels that mediate inhibitory neurotransmission in the adult nervous system but exert an excitatory action in immature neurons. GlyRs containing the α2 subunit are highly expressed in the embryonic brain, where they promote cortical interneuron migration and the generation of excitatory projection neurons. We previously identified a rare microdeletion of the X-linked gene GLRA2, encoding the GlyR α2 subunit, in a boy with autism. The microdeletion removes the terminal exons of the gene (GLRA2(Δex8-9)). Here, we sequenced 400 males with ASD and identified one de novo missense mutation, p.R153Q, absent from controls. In vitro functional analysis demonstrated that the GLRA2(Δex8)(-)(9) protein failed to localize to the cell membrane, while the R153Q mutation impaired surface expression and markedly reduced sensitivity to glycine. Very recently, an additional de novo missense mutation (p.N136S) was reported in a boy with ASD, and we show that this mutation also reduced cell-surface expression and glycine sensitivity. Targeted glra2 knockdown in zebrafish induced severe axon-branching defects, rescued by injection of wild type but not GLRA2(Δex8-9) or R153Q transcripts, providing further evidence for their loss-of-function effect. Glra2 knockout mice exhibited deficits in object recognition memory and impaired long-term potentiation in the prefrontal cortex. Taken together, these results implicate GLRA2 in non-syndromic ASD, unveil a novel role for GLRA2 in synaptic plasticity and learning and memory, and link altered glycinergic signaling to social and cognitive impairments.

A new locus for autosomal recessive spastic paraplegia (SPG32) on chromosome 14q12-q21.

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative disorders characterized by progressive spasticity of the lower limbs. Here, we performed a genome-wide linkage analysis on a consanguineous family presenting an autosomal recessive form of HSP associated with mild mental retardation, brainstem dysraphia, and clinically asymptomatic cerebellar atrophy. We have mapped the disease locus SPG32 to chromosome 14q12-q21 within a 30-cM interval, which excludes the atlastin gene.

Motor system abnormalities in hereditary spastic paraparesis type 4 (SPG4) depend on the type of mutation in the spastin gene.

BACKGROUND: Hereditary spastic paraparesis (HSP) denotes a group of inherited neurological disorders with progressive lower limb spasticity as their clinical hallmark; a large proportion of autosomal dominant HSP belongs to HSP type 4, which has been linked to the SPG4 locus on chromosome 2. A variety of mutations have been identified within the SPG4 gene product, spastin. OBJECTIVE: Correlation of genotype and electrophysiological phenotype. MATERIAL: Two large families with HSP linked to the SPG4 locus with a very similar disease with respect to age of onset, progression, and severity of symptoms. METHODS: Mutation analysis was performed by PCR from genomic DNA and cDNA, and direct sequencing. The motor system was evaluated using transcranial magnetic stimulation. RESULTS: Patients differ in several categories depending on the type of mutation present. CONCLUSIONS: For the first time in hereditary spastic paraparesis, a phenotypic correlate of a given genetic change in the spastin gene has been shown.

Mutation analysis of the spastin gene (SPG4) in patients with hereditary spastic paraparesis.

BACKGROUND: Hereditary spastic paraparesis is a genetically heterogeneous condition. Recently, mutations in the spastin gene were reported in families linked to the common SPG4 locus on chromosome 2p21-22. OBJECTIVES: To study a population of patients with hereditary spastic paraparesis for mutations in the spastin gene (SPG4) on chromosome 2p21-22. METHODS: DNA from 32 patients (12 from families known to be linked to SPG4) was analysed for mutations in the spastin gene by single strand conformational polymorphism analysis and sequencing. All patients were also examined clinically. RESULTS: Thirteen SPG4 mutations were identified, 11 of which are novel. These mutations include missense, nonsense, frameshift, and splice site mutations, the majority of which affect the AAA cassette. We also describe a nucleotide substitution outside this conserved region which appears to behave as a recessive mutation. CONCLUSIONS: Recurrent mutations in the spastin gene are uncommon. This reduces the ease of mutation detection as a part of the diagnostic work up of patients with hereditary spastic paraparesis. Our findings have important implications for the presumed function of spastin and schemes for mutation detection in HSP patients.

Hereditary spastic paraplegia caused by mutations in the SPG4 gene.

Autosomal dominant hereditary spastic paraplegia (AD-HSP) is a genetically heterogeneous neurodegenerative disorder characterised by progressive spasticity of the lower limbs. The SPG4 locus at 2p21-p22 accounts for 40-50% of all AD-HSP families. The SPG4 gene was recently identified. It is ubiquitously expressed in adult and foetal tissues and encodes spastin, an ATPase of the AAA family. We have now identified four novel SPG4 mutations in German AD-HSP families, including one large family for which anticipation had been proposed. Mutations include one frame-shift and one missense mutation, both affecting the Walker motif B. Two further mutations affect two donor splice sites in introns 12 and 16, respectively. RT-PCR analysis of both donor splice site mutations revealed exon skipping and reduced stability of aberrantly spliced SPG4 mRNA. All mutations are predicted to cause loss of functional protein. In conclusion, we confirm in German families that SPG4 mutations cause AD-HSP. Our data suggest that SPG4 mutations exert their dominant effect not by gain of function but by haploinsufficiency. If a threshold level of spastin were critical for axonal preservation, such threshold dosage effects might explain the variable expressivity and incomplete penetrance of SPG4-linked AD-HSP.

A new locus for autosomal dominant pure spastic paraplegia, on chromosome 2q24-q34.

Hereditary spastic paraplegia (HSP) comprises a group of clinically and genetically heterogeneous disorders causing progressive spasticity and weakness of the lower limbs. We report a large family of French descent with autosomal dominant pure HSP. We excluded genetic linkage to the known loci causing HSP and performed a genomewide search. We found evidence for linkage of the disorder to polymorphic markers on chromosome 2q24-q34: a maximum LOD score of 3. 03 was obtained for marker D2S2318. By comparison with families having linkage to the major locus of pure autosomal dominant HSP (SPG4 on chromosome 2p), there were significantly more patients without Babinski signs, with increased reflexes in the upper limbs, and with severe functional handicaps.

Spectrum of SPG4 mutations in autosomal dominant spastic paraplegia.

Autosomal dominant hereditary spastic paraplegia (AD-HSP) is a group of genetically heterogeneous neurodegenerative disorders characterized by pro- gressive spasticity of the lower limbs. Five AD-HSP loci have been mapped to chromosomes 14q, 2p, 15q, 8q and 12q. The SPG4 locus at 2p21-p22 has been shown to account for approximately 40% of all AD-HSP families. SPG4 encoding spastin, a putative nuclear AAA protein, has recently been identified. Here, sequence analysis of the 17 exons of SPG4 in 87 unrelated AD-HSP patients has resulted in the detection of 34 novel mutations. These SPG4 mutations are scattered along the coding region of the gene and include all types of DNA modification including missense (28%), nonsense (15%) and splice site point (26.5%) mutations as well as deletions (23%) and insertions (7.5%). The clinical analysis of the 238 mutation carriers revealed a high proportion of both asymptomatic carriers (14/238) and patients unaware of symptoms (45/238), and permitted the redefinition of this frequent form of AD-HSP.

No evidence for long CAG/CTG repeats in families with spastic paraplegia linked to chromosome 2p21-24.

Autosomal dominant familial spastic paraplegia (AD-FSP) is a genetically heterogeneous, neurodegenerative disorder characterized by spasticity and progressive weakness in the lower limbs. Anticipation has been suggested to occur and an association between expanded CAG/CTG repeats and AD-FSP linked to the SPG4 locus (2p21-p24) has been described. In this study, 42 affected individuals from six SPG4 families were screened for expanded CAG/CTG repeats using the repeat expansion detection (RED) method. Large RED products (range 180-240 nucleotides) corresponding in size to repeats at the ERDA1 locus were detected in eight patients and at the CTG 18.1 locus in one patient. The large ERDA1 repeats did not segregate with the disorder within families. Mean age at onset and index of severity were not significantly different between patients with or without expanded RED products. Furthermore, no abnormal proteins were found by Western blot in 15 selected patient samples as compared with controls, using the 1C2 antibody, which detects long polyglutamine stretches. Thus, in contrast to previous reports, our study provides evidence against the hypothesis that a large translated CAG repeat expansion is the basis of SPG4. We propose that mechanisms other than large pathogenic CAG/CTG repeats may account for the disease in the SPG4 families tested here.

Spastin, a new AAA protein, is altered in the most frequent form of autosomal dominant spastic paraplegia.

Autosomal dominant hereditary spastic paraplegia (AD-HSP) is a genetically heterogeneous neurodegenerative disorder characterized by progressive spasticity of the lower limbs. Among the four loci causing AD-HSP identified so far, the SPG4 locus at chromosome 2p2-1p22 has been shown to account for 40-50% of all AD-HSP families. Using a positional cloning strategy based on obtaining sequence of the entire SPG4 interval, we identified a candidate gene encoding a new member of the AAA protein family, which we named spastin. Sequence analysis of this gene in seven SPG4-linked pedigrees revealed several DNA modifications, including missense, nonsense and splice-site mutations. Both SPG4 and its mouse orthologue were shown to be expressed early and ubiquitously in fetal and adult tissues. The sequence homologies and putative subcellular localization of spastin suggest that this ATPase is involved in the assembly or function of nuclear protein complexes.

A fine integrated map of the SPG4 locus excludes an expanded CAG repeat in chromosome 2p-linked autosomal dominant spastic paraplegia.

Autosomal dominant hereditary spastic paraplegia (AD-HSP) is a genetically heterogeneous disorder characterized by progressive spasticity of the lower limbs. A major locus (SPG4) causing AD-HSP in about 40% of the families was mapped to chromosome 2p. The analysis of six SPG4-linked AD-HSP families using the RED procedure previously showed the expansion of a CAG repeat in affected individuals. To identify the gene responsible for this form of HSP, we have constructed a 3.5-Mb YAC contig flanked by loci D2S400 and D2S367, have subcloned five of these YACs spanning the candidate region into cosmids, and screened these cosmid libraries for the presence of CAG repeat sequences. Four CAG repeats have been identified but none of them is expanded in 26 patients from 13 SPG4-linked AD-HSP families. A gene map comprising 21 transcripts was established using expressed sequence tags (ESTs) assigned previously to this region of 2p21-p22 with radiation hybrid panels GeneBridge 4 and G3. Full-length cDNAs corresponding to the 14 ESTs mapping to the SPG4 interval flanked by loci D2S352 and D2S2347 were isolated and sequenced. None contains a CAG repeat in its coding sequence. Finally, we have assembled a BAC contig composed of 37 clones that were also screened for the presence of CAG repeats; this failed to detect additional repeats to those identified on YACs.

Clinical heterogeneity of autosomal recessive spastic paraplegias: analysis of 106 patients in 46 families.

BACKGROUND: Hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurodegenerative disorders characterized by progressive and predominant spasticity of the lower limbs, in which dominant, recessive, and X-linked forms have been described. While autosomal dominant HSP has been extensively studied, autosomal recessive HSP is less well known and is considered a rare condition. OBJECTIVE: To analyze the clinical presentation in a large group of patients with autosomal recessive HSP from Portugal and Algeria to define homogeneous groups that could serve as a guide for future molecular studies. RESULTS: Clinical features in 106 patients belonging to 46 Portuguese and Algerian families with autosomal recessive HSP are presented, as well as the results of molecular studies in 23 of these families. Five phenotypes are defined: (1) pure early-onset families, (2) pure lateonset families, (3) complex families with mental retardation, (4) complex families with mental retardation and peripheral neuropathy, and (5) complex families with cerebellar ataxia. Six additional families have specific complex presentations, each of which is unique in the present series. Pyramidal signs in the upper limbs and pes cavus are frequent findings, while pseudobulbar signs, including dysarthria, dysphagia, and brisk jaw jerks, are more frequent in the complex forms. The complex forms have a poorer prognosis, while pure forms, particularly those with early onset, are more benign. One Algerian pure early-onset kindred was linked to the locus on chromosome 8, previously reported in 4 Tunisian families. Two of the Portuguese kindreds with complex forms (one with mental retardation and the other associated with hypoplasia of the corpus callosum) showed linkage to the locus recently identified on chromosome 16. CONCLUSIONS: Although autosomal recessive HSP represents a heterogeneous group of diseases, some phenotypes can be defined by analyzing a large group of patients. The fact that only one Algerian family was linked to chromosome 8 suggests that this is a rare localization even in kindreds with the same ethnic background. Linkage to chromosome 16 was found in 2 clinically diverse Portuguese kindreds, illustrating that this locus is also rare and may correspond to different phenotypes.

Quality assessment of whole genome mapping data in the refined familial spastic paraplegia interval on chromosome 14q.

Autosomal dominant familial spastic paraplegia (AD-FSP) is a genetically heterogeneous neurodegenerative disorder characterized by progressive spasticity of the lower limbs. Three loci on chromosome 14q (SPG3), 2p (SPG4), and 15q (SPG6) were shown to be responsible for AD-FSP. Analysis of recombination events in three SPG3-linked families allowed us to narrow the critical interval from 9 to 5 cM. An approximately 5-Mb YAC contig comprising 32 clones and 90 STSs was built from D14S301 to D14S991, encompassing this region of 14q21. Fifty-six ESTs assigned previously to this region with radiation hybrid (RH) panels Genebridge 4 and G3 were precisely localized on the YAC contig. The 90 STSs positioned on the contig were tested on the TNG RH panel to compare our YAC-based map with an RH map at a high level of resolution. Comparison between our map and the whole genome mapping data on this interval of chromosome 14q is discussed.

[Clinical and molecular genetic analysis of 4 Swiss families with the pure form of hereditary spastic spinal paralysis]. / Klinische und molekulargenetische Analyse bei vier schweizerischen Familien mit der reinen Form der hereditären spastischen Spinalparalyse.

Hereditary spastic paraplegia (HSP) is a rare neurodegenerative disease of the spinal cord with a progressive gait disorder, associated with other neurological abnormalities in the complicated form. A cluster of families with this disorder in the central part of the country has long been known to Swiss neurologists. In the present report, we describe our clinical and molecular findings in four large families originating from this region and suffering from a pure HSP form. Clinical presentation was similar in the four families. The age of onset varied widely from 2 to 70 years with the appearance of a gait disorder, which slowly progressed to wheelchair confinement after 30-70 years. No other neurological abnormality was found except for impairment of the vibration sense and sphincter abnormalities. In three families an association with markers of the SPG4 locus on chromosome 2 was found. In the fourth, the largest one, no linkage could be found with either SPG4, or with the other two known loci, SPG3 on chromosome 14 and SPG6 on chromosome 15. These data demonstrate the genetic heterogeneity in HSP, even in families from the same region. They also suggest the presence of at least one additional locus for the pure form.

Mapping of a complicated familial spastic paraplegia to locus SPG4 on chromosome 2p.

Autosomal dominant familial spastic paraplegia (AD-FSP) is a degenerative disorder of the central motor system characterised by progressive spasticity of the lower limbs. AD-FSP has been divided into pure and complicated forms. Pure AD-FSP is genetically heterogeneous; three loci have been mapped to chromosomes 14q (SPG3), 2p (SPG4), and 15q (SPG6), whereas no loci responsible for complicated forms have been identified to date. Here we report linkage to the SPG4 locus in a three generation family with AD-FSP complicated by dementia and epilepsy. Assuming that both forms of AD-FSP are caused by mutations involving the same FSP gene, analysis of recombination events in this family positions the SPG4 gene within a 0 cM interval flanked by loci D2S2255 and D2S2347.

Human desmin gene: cDNA sequence, regional localization and exclusion of the locus in a familial desmin-related myopathy.

Desmin is a muscle-specific intermediate filament that is encoded by a gene assigned to human chromosome 2q35. Desmin-related myopathies are inherited disorders characterized by an intrasarcoplasmic accumulation of desmin. Recently, the knockout of the desmin gene was shown to generate a myopathic syndrome in transgenic mice, suggesting that functional abnormality of desmin may generate similar clinical symptoms in mouse and human. To determine the potential role of the desmin gene in a well-defined desmin-related myopathy (autosomal dominant form of Fardeau), human desmin cDNAs obtained from affected and unaffected individuals were cloned, sequenced and compared. No obvious mutation was detected. A BssHII restriction fragment length polymorphism (RFLP) was identified in exon 6 of the desmin gene. This RFLP was associated with a previously identified EcoRV RFLP in exon 4 to generate a tetra-allelic system, which was tested for linkage to the desmin-related myopathy in three families. The human desmin gene was localized within an 11-cM interval on chromosome 2q using a panel of radiation hybrids. This 11-cM region was clearly excluded by linkage analysis in the three desmin-related myopathy families using a set of highly polymorphic microsatellite markers. These results suggest that the desmin gene is not primarily involved in this disease.

Phenotype of autosomal dominant spastic paraplegia linked to chromosome 2.

We report the clinical features of 12 families with autosomal dominant spastic paraplegia (ADSP) linked to the SPG4 locus on chromosome 2p, the major locus for this disorder that accounts for approximately 40% of the families. Among 93 gene carriers, 32 (34%) were unaware of symptoms but were clinically affected. Haplotype reconstruction showed that 90% of the asymptomatic gene carriers presented increased reflexes and/or extensor plantar responses independent of age at examination. The mean age at onset was 29 years, ranging from 1 to 63 years. Intra- as well as inter-familial variability of age at onset was important, but did not result from anticipation. Phenotype-genotype correlations and comparison with SPG3 and SPG5 families indicated that despite the variability of age at onset, SPG4 is a single genetic entity but no clinical features distinguish individual SPG4 patients from those with SPG3 or SPG5 mutations.

Autosomal dominant spastic paraplegia with anticipation maps to a 4-cM interval on chromosome 2p21-p24 in a large German family.

Autosomal dominant familial spastic paraplegias (AD-FSP) are a group of genetically heterogeneous diseases characterised by a progressive spasticity of the lower limbs. Three loci have already been identified by genetic linkage studies on chromosomes 2p, 14q and 15q. Here we present linkage data from a large German family displaying AD-FSP with anticipation which confirms the existence of the FSP2 locus on chromosome 2p. The recombination events observed in our family define the critical region for the FSP2 gene to be within a 4-cM interval, flanked by markers D2S400 and D2S367. Moreover, clinical data from our family show evidence of anticipation, a phenomenon caused by trinucleotide expansion in several other neurodegenerative diseases.

A comprehensive genetic map of the human genome based on 5,264 microsatellites.

The great increase in successful linkage studies in a number of higher eukaryotes during recent years has essentially resulted from major improvements in reference genetic linkage maps, which at present consist of short tandem repeat polymorphisms of simple sequences or microsatellites. We report here the last version of the Généthon human linkage map. This map consists of 5,264 short tandem (AC/TG)n repeat polymorphisms with a mean heterozygosity of 70%. The map spans a sex-averaged genetic distance of 3,699 cM and comprises 2,335 positions, of which 2,032 could be ordered with an odds ratio of at least 1,000:1 against alternative orders. The average interval size is 1.6 cM; 59% of the map is covered by intervals of 2 cM at most and 1% remains in intervals above 10 cM.

Pure familial spastic paraplegia: clinical and genetic analysis of nine Belgian pedigrees.

We ascertained 9 multigeneration Belgian families with pure dominant spastic paraplegia (SPG) for clinical and genetic studies. Linkage was examined using simple tandem repeat (STR) markers located near the 5 loci for familial SPG on chromosomes Xq28 (SPG1), Xq21.3-q22 (SPG2), 2p21-p24 (SPG4), 14q12-q23 (SPG3) and 15q11.1 (SPG6). Positive linkage results were obtained only for markers at the SPG4 locus mapping the SPG4 gene between D2S400 and D2S367, a region of 4 cM. In order to facilitate the positional cloning of the SPG4 gene, we constructed a contiguous YAC map covering the SPG4 candidate region. Our physical mapping data indicate that the SPG4 gene resides within maximal 5 Mb.