Heterozygous PNPT1 Variants Cause Spinocerebellar Ataxia Type 25.

OBJECTIVE: Dominant spinocerebellar ataxias (SCA) are characterized by genetic heterogeneity. Some mapped and named loci remain without a causal gene identified. Here we applied next generation sequencing (NGS) to uncover the genetic etiology of the SCA25 locus. METHODS: Whole-exome and whole-genome sequencing were performed in families linked to SCA25, including the French family in which the SCA25 locus was originally mapped. Whole exome sequence data were interrogated in a cohort of 796 ataxia patients of unknown etiology. RESULTS: The SCA25 phenotype spans a slowly evolving sensory and cerebellar ataxia, in most cases attributed to ganglionopathy. A pathogenic variant causing exon skipping was identified in the gene encoding Polyribonucleotide Nucleotidyltransferase PNPase 1 (PNPT1) located in the SCA25 linkage interval. A second splice variant in PNPT1 was detected in a large Australian family with a dominant ataxia also mapping to SCA25. An additional nonsense variant was detected in an unrelated individual with ataxia. Both nonsense and splice heterozygous variants result in premature stop codons, all located in the S1-domain of PNPase. In addition, an elevated type I interferon response was observed in blood from all affected heterozygous carriers tested. PNPase notably prevents the abnormal accumulation of double-stranded mtRNAs in the mitochondria and leakage into the cytoplasm, associated with triggering a type I interferon response. INTERPRETATION: This study identifies PNPT1 as a new SCA gene, responsible for SCA25, and highlights biological links between alterations of mtRNA trafficking, interferonopathies and ataxia. ANN NEUROL 2022;92:122-137.

A Recurrent Mutation in CACNA1G Alters Cav3.1 T-Type Calcium-Channel Conduction and Causes Autosomal-Dominant Cerebellar Ataxia.

Hereditary cerebellar ataxias (CAs) are neurodegenerative disorders clinically characterized by a cerebellar syndrome, often accompanied by other neurological or non-neurological signs. All transmission modes have been described. In autosomal-dominant CA (ADCA), mutations in more than 30 genes are implicated, but the molecular diagnosis remains unknown in about 40% of cases. Implication of ion channels has long been an ongoing topic in the genetics of CA, and mutations in several channel genes have been recently connected to ADCA. In a large family affected by ADCA and mild pyramidal signs, we searched for the causative variant by combining linkage analysis and whole-exome sequencing. In CACNA1G, we identified a c.5144G>A mutation, causing an arginine-to-histidine (p.Arg1715His) change in the voltage sensor S4 segment of the T-type channel protein Cav3.1. Two out of 479 index subjects screened subsequently harbored the same mutation. We performed electrophysiological experiments in HEK293T cells to compare the properties of the p.Arg1715His and wild-type Cav3.1 channels. The current-voltage and the steady-state activation curves of the p.Arg1715His channel were shifted positively, whereas the inactivation curve had a higher slope factor. Computer modeling in deep cerebellar nuclei (DCN) neurons suggested that the mutation results in decreased neuronal excitability. Taken together, these data establish CACNA1G, which is highly expressed in the cerebellum, as a gene whose mutations can cause ADCA. This is consistent with the neuropathological examination, which showed severe Purkinje cell loss. Our study further extends our knowledge of the link between calcium channelopathies and CAs.

Loss of association of REEP2 with membranes leads to hereditary spastic paraplegia.

Hereditary spastic paraplegias (HSPs) are clinically and genetically heterogeneous neurological conditions. Their main pathogenic mechanisms are thought to involve alterations in endomembrane trafficking, mitochondrial function, and lipid metabolism. With a combination of whole-genome mapping and exome sequencing, we identified three mutations in REEP2 in two families with HSP: a missense variant (c.107T>A [p.Val36Glu]) that segregated in the heterozygous state in a family with autosomal-dominant inheritance and a missense change (c.215T>A [p.Phe72Tyr]) that segregated in trans with a splice site mutation (c.105+3G>T) in a family with autosomal-recessive transmission. REEP2 belongs to a family of proteins that shape the endoplasmic reticulum, an organelle that was altered in fibroblasts from an affected subject. In vitro, the p.Val36Glu variant in the autosomal-dominant family had a dominant-negative effect; it inhibited the normal binding of wild-type REEP2 to membranes. The missense substitution p.Phe72Tyr, in the recessive family, decreased the affinity of the mutant protein for membranes that, together with the splice site mutation, is expected to cause complete loss of REEP2 function. Our findings illustrate how dominant and recessive inheritance can be explained by the effects and nature of mutations in the same gene. They have also important implications for genetic diagnosis and counseling in clinical practice because of the association of various modes of inheritance to this new clinico-genetic entity.

Loss of function of glucocerebrosidase GBA2 is responsible for motor neuron defects in hereditary spastic paraplegia.

Spastic paraplegia 46 refers to a locus mapped to chromosome 9 that accounts for a complicated autosomal-recessive form of hereditary spastic paraplegia (HSP). With next-generation sequencing in three independent families, we identified four different mutations in GBA2 (three truncating variants and one missense variant), which were found to cosegregate with the disease and were absent in controls. GBA2 encodes a microsomal nonlysosomal glucosylceramidase that catalyzes the conversion of glucosylceramide to free glucose and ceramide and the hydrolysis of bile acid 3-O-glucosides. The missense variant was also found at the homozygous state in a simplex subject in whom no residual glucocerebrosidase activity of GBA2 could be evidenced in blood cells, opening the way to a possible measurement of this enzyme activity in clinical practice. The overall phenotype was a complex HSP with mental impairment, cataract, and hypogonadism in males associated with various degrees of corpus callosum and cerebellar atrophy on brain imaging. Antisense morpholino oligonucleotides targeting the zebrafish GBA2 orthologous gene led to abnormal motor behavior and axonal shortening/branching of motoneurons that were rescued by the human wild-type mRNA but not by applying the same mRNA containing the missense mutation. This study highlights the role of ceramide metabolism in HSP pathology.

Alteration of ganglioside biosynthesis responsible for complex hereditary spastic paraplegia.

Hereditary spastic paraplegias (HSPs) form a heterogeneous group of neurological disorders. A whole-genome linkage mapping effort was made with three HSP-affected families from Spain, Portugal, and Tunisia and it allowed us to reduce the SPG26 locus interval from 34 to 9 Mb. Subsequently, a targeted capture was made to sequence the entire exome of affected individuals from these three families, as well as from two additional autosomal-recessive HSP-affected families of German and Brazilian origins. Five homozygous truncating (n = 3) and missense (n = 2) mutations were identified in B4GALNT1. After this finding, we analyzed the entire coding region of this gene in 65 additional cases, and three mutations were identified in two subjects. All mutated cases presented an early-onset spastic paraplegia, with frequent intellectual disability, cerebellar ataxia, and peripheral neuropathy as well as cortical atrophy and white matter hyperintensities on brain imaging. B4GALNT1 encodes ß-1,4-N-acetyl-galactosaminyl transferase 1 (B4GALNT1), involved in ganglioside biosynthesis. These findings confirm the increasing interest of lipid metabolism in HSPs. Interestingly, although the catabolism of gangliosides is implicated in a variety of neurological diseases, SPG26 is only the second human disease involving defects of their biosynthesis.

CIC inactivating mutations identify aggressive subset of 1p19q codeleted gliomas.

OBJECTIVE: CIC gene is frequently mutated in oligodendroglial tumors with 1p19q codeletion. However, clinical and biological impact remain poorly understood. METHODS: We sequenced the CIC gene on 127 oligodendroglial tumors (109 with the 1p19q codeletion) and analyzed patients' outcome. We compared magnetic resonance imaging, transcriptomic profile, and CIC protein expression of CIC wild-type (WT) and mutant gliomas. We compared the level of expression of CIC target genes on Hs683-IDH1(R132H) cells transfected with lentivirus encoding mutant and WT CIC. RESULTS: We found 63 mutations affecting 60 of 127 patients, virtually all 1p19q codeleted and IDH mutated (59 of 60). In the 1p19q codeleted gliomas, CIC mutations were associated with a poorer outcome by uni- (p = 0.001) and multivariate analysis (p < 0.016). CIC mutation prognostic impact was validated on the TCGA cohort. CIC mutant grade II codeleted gliomas spontaneously grew faster than WTs. Transcriptomic analysis revealed an enrichment of proliferative pathways and oligodendrocyte precursor cell gene expression profile in CIC mutant gliomas, with upregulation of normally CIC repressed genes ETV1, ETV4, ETV5, and CCND1. Various missense mutations resulted in CIC protein expression loss. Moreover, a truncating CIC mutation resulted in a defect of nuclear targeting of CIC protein to the nucleus in a human glioma cell line expressing IDH1(R132H) and overexpression of CCND1 and other new target genes of CIC, such as DUSP4 and SPRED1. INTERPRETATION: CIC mutations result in protein inactivation with upregulation of CIC target genes, activation of proliferative pathways, inhibition of differentiation, and poorer outcome in patients with a 1p19q codeleted glioma.

The impact of rare variants in FUS in essential tremor.

OBJECTIVE: We analyzed the coding region of the Fused in Sarcoma (FUS) gene in familial essential tremor (ET) and reviewed previous studies assessing FUS variants in ET. BACKGROUND: ET is often a familial disorder with an autosomal dominant inheritance pattern. A potentially causative variant in FUS has been identified in one ET family. Subsequent studies described further putatively causal variants. METHODS: We performed DNA sequencing of FUS in 85 unrelated, familial German and French definite ET patients. RESULTS: We did not find novel variants affecting the protein sequence. Seven previously published studies and data from the exome variant server (EVS) showed that rare exonic variants in FUS are not more frequent in ET than in the general population. CONCLUSIONS: Our findings provide no evidence for a role of rare genetic variants in the pathogenesis of ET, apart from the initially published FUS mutation segregating in a large ET family.

TP53 and p53 statuses and their clinical impact in diffuse low grade gliomas.

TP53 is a pivotal gene frequently mutated in diffuse gliomas and particularly in astrocytic tumors. The majority of studies dedicated to TP53 in gliomas were focused on mutational hotspots located in exons 5-8. Recent studies have suggested that TP53 is also mutated outside the classic mutational hotspots reported in gliomas. Therefore, we have sequenced all TP53 coding exons in a retrospective series of 61 low grade gliomas (LGG) using high throughput sequencing technology. In addition, TP53 mutational status was correlated with: (i) p53 expression, (ii) tumor type, (iii) chromosome arms 1p/19q status and (iv) clinical features of patients. The cohort included 32 oligodendrogliomas (O), 21 oligoastrocytomas (M) and 8 astrocytomas (A). TP53 mutation was detected in 52.4% (32/61) of tumors (34% of O, 71.4% of M and 75% of A). All mutations (38 mutations in 32 samples) were detected in exons 4, 5, 6, 7, 8 and 10. Missense and non-missense mutations, including seven novel mutations, were detected in 42.6 and 9.8% of tumors respectively. TP53 mutations were almost mutually exclusive with 1p/19q co-deletion and were associated with: (i) astrocytic phenotype, (ii) younger age, (iii) p53 expression. Using a threshold of 10% p53-positive tumor cells, p53 expression is an interesting surrogate marker for missense TP53 mutations (Se = 92%; Sp = 79.4%) but not for non-missense mutation (18.4% of mutations). TP53 and p53 statuses were not prognostic in LGG. In conclusion, we have identified novel TP53 mutations in LGG. TP53 mutations outside exons 4-8 are rare. Although it remains imperfect, p53 expression with a threshold of 10% is a good surrogate marker for missense TP53 mutations and appears helpful in the setting of LGG phenotype diagnosis.

Mutations in KCND3 cause spinocerebellar ataxia type 22.

OBJECTIVE: To identify the causative gene in spinocerebellar ataxia (SCA) 22, an autosomal dominant cerebellar ataxia mapped to chromosome 1p21-q23. METHODS: We previously characterized a large Chinese family with progressive ataxia designated SCA22, which overlaps with the locus of SCA19. The disease locus in a French family and an Ashkenazi Jewish American family was also mapped to this region. Members from all 3 families were enrolled. Whole exome sequencing was performed to identify candidate mutations, which were narrowed by linkage analysis and confirmed by Sanger sequencing and cosegregation analyses. Mutational analyses were also performed in 105 Chinese and 55 Japanese families with cerebellar ataxia. Mutant gene products were examined in a heterologous expression system to address the changes in protein localization and electrophysiological functions. RESULTS: We identified heterozygous mutations in the voltage-gated potassium channel Kv4.3-encoding gene KCND3: an in-frame 3-nucleotide deletion c.679_681delTTC p.F227del in both the Chinese and French pedigrees, and a missense mutation c.1034G>T p.G345V in the Ashkenazi Jewish family. Direct sequencing of KCND3 further identified 3 mutations, c.1034G>T p.G345V, c.1013T>C p.V338E, and c.1130C>T p.T377M, in 3 Japanese kindreds. Immunofluorescence analyses revealed that the mutant p.F227del Kv4.3 subunits were retained in the cytoplasm, consistent with the lack of A-type K(+) channel conductance in whole cell patch-clamp recordings. INTERPRETATION: Our data identify the cause of SCA19/22 in patients of diverse ethnic origins as mutations in KCND3. These findings further emphasize the important role of ion channels as key regulators of neuronal excitability in the pathogenesis of cerebellar degeneration.

A genome-scale DNA repair RNAi screen identifies SPG48 as a novel gene associated with hereditary spastic paraplegia.

DNA repair is essential to maintain genome integrity, and genes with roles in DNA repair are frequently mutated in a variety of human diseases. Repair via homologous recombination typically restores the original DNA sequence without introducing mutations, and a number of genes that are required for homologous recombination DNA double-strand break repair (HR-DSBR) have been identified. However, a systematic analysis of this important DNA repair pathway in mammalian cells has not been reported. Here, we describe a genome-scale endoribonuclease-prepared short interfering RNA (esiRNA) screen for genes involved in DNA double strand break repair. We report 61 genes that influenced the frequency of HR-DSBR and characterize in detail one of the genes that decreased the frequency of HR-DSBR. We show that the gene KIAA0415 encodes a putative helicase that interacts with SPG11 and SPG15, two proteins mutated in hereditary spastic paraplegia (HSP). We identify mutations in HSP patients, discovering KIAA0415/SPG48 as a novel HSP-associated gene, and show that a KIAA0415/SPG48 mutant cell line is more sensitive to DNA damaging drugs. We present the first genome-scale survey of HR-DSBR in mammalian cells providing a dataset that should accelerate the discovery of novel genes with roles in DNA repair and associated medical conditions. The discovery that proteins forming a novel protein complex are required for efficient HR-DSBR and are mutated in patients suffering from HSP suggests a link between HSP and DNA repair.

Gene expression of circadian genes and CIART in bipolar disorder: A preliminary case-control study.

Based on the observed circadian rhythms disruptions and sleep abnormalities in bipolar disorders (BD), a chronobiological model has been proposed suggesting that core clock genes play a central role in the vulnerability to the disorder. In this context, the analysis of circadian genes expression levels is particularly relevant, however studies focused on the whole set of core clock genes are scarce. We compared the levels of expression of 19 circadian genes (including the recently described circadian repressor (CIART)) in 37 euthymic individuals with BD and 20 healthy controls (HC), using data obtained by RNA sequencing of lymphoblastoid cell lines and validated the results using RT-qPCR. RNA sequencing data showed that CIART gene expression was correlated with those of ARNTL, ARNTL2, DBP, PER2 and TIMELESS. Data from RNA sequencing showed that the level of expression of four circadian genes (ARNTL, ARNTL2, BHLHE41 and CIART) discriminated individuals with BD from HC. We replicated this result using RT-qPCR for ARNTL and CIART. This study suggests that an imbalance between activation/repression of the transcription within the circadian system in individuals with BD as compared to HC and as such opens avenues for further research in larger independent samples combining both expression and epigenetic analyses.

REEP1 mutations in SPG31: frequency, mutational spectrum, and potential association with mitochondrial morpho-functional dysfunction.

Hereditary spastic paraplegias (HSP) constitute a heterogeneous group of neurodegenerative disorders characterized at least by slowly progressive spasticity of the lower limbs. Mutations in REEP1 were recently associated with a pure dominant HSP, SPG31. We sequenced all exons of REEP1 and searched for rearrangements by multiplex ligation-dependent probe amplification (MLPA) in a large panel of 175 unrelated HSP index patients from kindreds with dominant inheritance (AD-HSP), with either pure (n = 102) or complicated (n = 73) forms of the disease, after exclusion of other known HSP genes. We identified 12 different heterozygous mutations, including two exon deletions, associated with either a pure or a complex phenotype. The overall mutation rate in our clinically heterogeneous sample was 4.5% in French families with AD-HSP. The phenotype was restricted to pyramidal signs in the lower limbs in most patients but nine had a complex phenotype associating axonal peripheral neuropathy (= 5/11 patients) including a Silver-like syndrome in one patient, and less frequently cerebellar ataxia, tremor, dementia. Interestingly, we evidenced abnormal mitochondrial network organization in fibroblasts of one patient in addition to defective mitochondrial energy production in both fibroblasts and muscle, but whether these anomalies are directly or indirectly related to the mutations remains uncertain.

Kjellin syndrome: long-term neuro-ophthalmologic follow-up and novel mutations in the SPG11 gene.

OBJECTIVE: Kjellin's syndrome is a hereditary neuro-ophthalmologic syndrome. We describe the clinical phenotypes of 7 patients, identifying the responsible mutations for 4 of them. A 10-year ophthalmologic and neurologic follow-up of 5 patients allowed us to describe the disease's characteristics, early symptoms and progression, associated ocular signs, and retinal changes in carriers. DESIGN: Retrospective clinical study and molecular genetics investigation. PARTICIPANTS: The records of 7 patients with Kjellin's syndrome were analyzed retrospectively. METHODS: All patients underwent full neurologic and ophthalmologic examinations. The neurologic examinations included assessments of initial symptoms, intelligence quotient tests, psychologic tests, and either magnetic resonance imaging or computed tomography. The ophthalmologic examinations included visual acuity on an Early Treatment Diabetic Retinopathy Study chart, intraocular pressure color vision assessment, slit-lamp and fundus examination, Goldmann perimetry, fundus autofluorescence, optical coherence tomography and fluorescein angiography, electro-oculography, electroretinography, and flash visual evoked potentials. Direct sequencing of the SPG11 and SPG15 genes and gene-dosage analysis for the former were performed for 4 of these index patients. MAIN OUTCOME MEASURES: Identification of new mutations in the SPG11 gene, validating its implication in Kjellin's syndrome. RESULTS: The first signs appear before the age of 10 years, with late verbal development and difficulty running and walking. Life expectancy is between 30 and 40 years. The secondary ophthalmologic symptoms only moderately affect visual acuity. In addition to the classic symptoms, 3 of the 7 patients displayed small whitish lens opacities, and 3 neurologically unaffected parents (father or mother), all heterozygous carriers, exhibited whitish retinal dots. All the patients who were tested carried SPG11, not SPG15, mutations. CONCLUSIONS: Neurologic signs of SPG11 mutations emerge in early infancy, with walking and language difficulties. Onset of paraplegia occurs at the end of the first decade or during the second decade. Retinal changes, an integral part of SPG11 mutations in this series of patients, are only observed once the paraplegia has become apparent.

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

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

Spinocerebellar ataxia type 11 (SCA11) is an uncommon cause of dominant ataxia among French and German kindreds.

BACKGROUND: At least 28 loci have been linked to autosomal dominant spinocerebellar ataxia (ADCA). Causative genes have been cloned for 10 nucleotide repeat expansions (SCA1, 2, 3, 6, 7, 8, 10, 12, 17 and 31) and six genes with classical mutations (SCA5, 13, 14, 15/16, 27 and 28). Recently, a large British pedigree linked to SCA11 has been reported to carry a mutation in the TTBK2 gene. In order to assess the prevalence and phenotypic spectrum of SCA11, the authors screened 148 index patients of predominantly German (n=69) and French (n=79) descent with ADCA tested negative for a panel of SCA mutations (SCA1, 2, 3, 6, 7 and 17), for mutations in TTBK2. METHODS: In the German ADCA cohort, the complete coding sequence of the TTBK2 gene was PCR-amplified and screened for mutations by high-resolution-melting (HRM) analysis. In the French cohort, exons known to carry mutations were directly sequenced. For both cohorts, the gene-dosage alterations were assessed using a customised multiplex ligation probe amplification (MLPA) assay. RESULTS: In two of 148 ADCA families--one German and one French--the authors identified a potentially disease-causing SCA11 mutation. Interestingly, both carried an identical two-basepair deletion (c.1306_1307delGA, p.D435fs448X in exon 12) leading to a premature stop codon. Gene-dosage alterations were not detected in the TTBK2 gene. Clinically, the SCA11 patients had phenotypic characteristics as described before presenting with slowly progressive almost pure cerebellar ataxia with normal life expectancy. CONCLUSION: SCA11 presented as ADCA III according to Harding's classification and is a rare cause of spinocerebellar ataxia in Caucasians accounting for less than 1% of dominant ataxias in central Europe.

Complicated forms of autosomal dominant hereditary spastic paraplegia are frequent in SPG10.

Hereditary spastic paraplegias (HSP) constitute a heterogeneous group of neurodegenerative disorders characterized by slowly progressive spasticity of the lower extremities. Only a few different mutations in the SPG10 gene, KIF5A, have been described in pure dominant forms of the disease. We sequenced the motor domain of KIF5A in a large panel of 205 European HSP patients with either pure or complicated forms of the disease. We identified eight different heterozygous missense mutations, seven novels, in eight different families of French origin. Residue R280 was a mutational hot spot. Interestingly, the patients in 7/8 families had a complex phenotype, with peripheral neuropathy, severe upper limb amyotrophy (Silver syndrome-like), mental impairment, parkinsonism, deafness and/or retinitis pigmentosa as variably associated features. We report the largest series of SPG10 families described so far, which extends both the mutational spectrum of the disease and its phenotype, which now includes complicated forms of HSP. SPG10 mutations were found in 10% of our complicated forms of HSP, suggesting that mutations in KIF5A represent the major cause of complicated AD-HSP in France.

Fe/S protein assembly gene IBA57 mutation causes hereditary spastic paraplegia.

OBJECTIVE: To present the clinical, molecular, and cell biological findings in a family with an autosomal recessive form of hereditary spastic paraplegia characterized by a combination of spastic paraplegia, optic atrophy, and peripheral neuropathy (SPOAN). METHODS: We used a combination of whole-genome linkage analysis and exome sequencing to map the disease locus and to identify the responsible gene. To analyze the physiologic consequences of the disease, we used biochemical and cell biological methods. RESULTS: Ten members of a highly consanguineous family manifested a childhood-onset SPOAN-like phenotype with slow progression into late adulthood. We mapped this disorder to a locus on chromosome 1q and identified a homozygous donor splice-site mutation in the IBA57 gene, previously implicated in 2 infants with lethal perinatal encephalomyopathy. This gene encodes the mitochondrial iron-sulfur (Fe/S) protein assembly factor IBA57. In addition to a severely decreased amount of normal IBA57 messenger RNA, a patient's cells expressed an aberrantly spliced messenger RNA with a premature stop codon. Lymphoblasts contained 10-fold-lower levels of wild-type, but no signs of truncated IBA57 protein. The decrease in functional IBA57 resulted in reduced levels and activities of several mitochondrial [4Fe-4S] proteins, including complexes I and II, while mitochondrial [2Fe-2S] proteins remained normal. CONCLUSIONS: Our findings reinforce the suggested specific function of IBA57 in mitochondrial [4Fe-4S] protein maturation and provide additional evidence for its role in human disease. The less decreased IBA57 protein level in this family explains phenotypic differences compared with the previously described lethal encephalomyopathy with no functional IBA57.

GRID2 mutations span from congenital to mild adult-onset cerebellar ataxia.

OBJECTIVES: In a large family of Algerian origin, we aimed to identify the genetic mutation segregating with simultaneous presence of adult-onset, paucisymptomatic, slowly progressive, cerebellar ataxia in 7 adults and congenital ataxia in 1 child, and then to assess the involvement of GRID2 mutations in 144 patients with congenital cerebellar ataxia. METHODS: We used a combined approach of linkage analysis and whole-exome sequencing in one family, and a targeted gene panel sequencing approach in 144 congenital ataxias. RESULTS: In the large family with spinocerebellar ataxia, we identified a missense mutation (c.1966C>G/p.Leu656Val) in the GRID2 gene, in a heterozygous state in adults, and in a homozygous state in one child with congenital ataxia, compatible with a semidominant transmission pattern. In 144 patients affected with congenital ataxia, we identified 2 missense de novo GRID2 mutations in 2 children (c.1960G>A/p.Ala654Thr, c.1961C>A/p.Ala654Asp). They affect the same amino acid as the previously described Lurcher mutation in mice; the variant in the large family concerns a nearby amino acid. CONCLUSIONS: In humans, GRID2 had only been involved in ataxia through complete loss-of-function mutations due to exon deletions. We report the first point mutations in this gene, with putative gain-of-function mechanisms, and a semidominant transmission as was observed in the Lurcher mice model. Of note, cerebellar ataxia is the core phenotype, but with variable severity ranging from very mild adult-onset to congenital-onset ataxias linked to both the heterozygous and homozygous state of the variant, and the position of the mutation.

DEPDC5 mutations in families presenting as autosomal dominant nocturnal frontal lobe epilepsy.

OBJECTIVE: To study the prevalence of DEPDC5 mutations in a series of 30 small European families with a phenotype compatible with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). METHODS: Thirty unrelated families referred with ADNFLE were recruited in France, Italy, Germany, Belgium, and Norway. Whole-exome sequencing was performed in 10 probands and direct sequencing of the DEPDC5 coding sequence in 20 probands. Testing for nonsense-mediated messenger RNA decay (NMD) was performed in lymphoblastic cells. RESULTS: Exome sequencing revealed a splice acceptor mutation (c.2355-2A>G) in DEPDC5 in the proband of a German family. In addition, 3 nonsense DEPDC5 mutations (p.Arg487*, p.Arg1087*, and p.Trp1369*) were detected in the probands of 2 French and one Belgian family. The nonsense mutations p.Arg487* and p.Arg1087* were targeted by NMD, leading to the degradation of the mutated transcripts. At the clinical level, 78% of the patients with DEPDC5 mutations were drug resistant. CONCLUSIONS: DEPDC5 loss-of-function mutations were found in 13% of the families with a presentation of ADNFLE. The rate of drug resistance was high in patients with DEPDC5 mutations. Small ADNFLE pedigrees with DEPDC5 mutations might actually represent a part of the broader familial focal epilepsy with variable foci phenotype.