De novo and inherited monoallelic variants in TUBA4A cause ataxia and spasticity.

Alpha-tubulin 4A encoding gene (TUBA4A) has been associated with familial amyotrophic lateral sclerosis (fALS) and fronto-temporal dementia (FTD), based on identification of likely pathogenic variants in patients from distinct ALS and FTD cohorts. By screening a multicentric French cohort of 448 unrelated probands presenting with cerebellar ataxia, we identified ultra-rare TUBA4A missense variants, all being absent from public databases and predicted pathogenic by multiple in-silico tools. In addition, gene burden analyses in the 100,000 genomes project (100KGP) showed enrichment of TUBA4A rare variants in the inherited ataxia group compared to controls (OR: 57.0847 [10.2- 576.7]; p = 4.02 x10-07). Altogether, we report 12 patients presenting with spasticity and/or cerebellar ataxia and harboring a predicted pathogenic TUBA4A missense mutation, including 5 confirmed de novo cases and a mutation previously reported in a large family presenting with spastic ataxia. Cultured fibroblasts from 3 patients harboring distinct TUBA4A missense showed significant alterations in microtubule organisation and dynamics, providing insight of TUBA4A variants pathogenicity. Our data confirm the identification of a hereditary spastic ataxia disease gene with variable age of onset, expanding the clinical spectrum of TUBA4A associated phenotypes.

RFC1 nonsense and frameshift variants cause CANVAS: clues for an unsolved pathophysiology.

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) is an inherited late-onset neurological disease caused by bi-allelic AAGGG pentanucleotide expansions within intron 2 of RFC1. Despite extensive studies, the pathophysiological mechanism of these intronic expansions remains elusive. We screened by clinical exome sequencing two unrelated patients presenting with late-onset ataxia. A repeat-primer polymerase chain reaction was used for RFC1 AAGGG intronic expansion identification. RFC1 mRNA expression was assessed by quantitative reverse transcription-polymerase chain reaction. We identified the first two CANVAS affected patients who are compound heterozygous for RFC1 truncating variants (p.Arg388* and c.575delA, respectively) and a pathological AAGGG expansion. RFC1 expression studies in whole blood showed a significant reduction of RFC1 mRNA for both patients compared to three patients with bi-allelic RFC1 expansions. In conclusion, this observation provides clues that suggest bi-allelic RFC1 conditional loss-of-function as the cause of the disease.

High rate of hypomorphic variants as the cause of inherited ataxia and related diseases: study of a cohort of 366 families.

PURPOSE: Diagnosis of inherited ataxia and related diseases represents a real challenge given the tremendous heterogeneity and clinical overlap of the various causes. We evaluated the efficacy of molecular diagnosis of these diseases by sequencing a large cohort of undiagnosed families. METHODS: We analyzed 366 unrelated consecutive patients with undiagnosed ataxia or related disorders by clinical exome-capture sequencing. In silico analysis was performed with an in-house pipeline that combines variant ranking and copy-number variant (CNV) searches. Variants were interpreted according to American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) guidelines. RESULTS: We established the molecular diagnosis in 46% of the cases. We identified 35 mildly affected patients with causative variants in genes that are classically associated with severe presentations. These cases were explained by the occurrence of hypomorphic variants, but also rarely suspected mechanisms such as C-terminal truncations and translation reinitiation. CONCLUSION: A significant fraction of the clinical heterogeneity and phenotypic overlap is explained by hypomorphic variants that are difficult to identify and not readily predicted. The hypomorphic C-terminal truncation and translation reinitiation mechanisms that we identified may only apply to few genes, as it relies on specific domain organization and alterations. We identified PEX10 and FASTKD2 as candidates for translation reinitiation accounting for mild disease presentation.

Developmental Consequences of Defective ATG7-Mediated Autophagy in Humans.

BACKGROUND: Autophagy is the major intracellular degradation route in mammalian cells. Systemic ablation of core autophagy-related (ATG) genes in mice leads to embryonic or perinatal lethality, and conditional models show neurodegeneration. Impaired autophagy has been associated with a range of complex human diseases, yet congenital autophagy disorders are rare. METHODS: We performed a genetic, clinical, and neuroimaging analysis involving five families. Mechanistic investigations were conducted with the use of patient-derived fibroblasts, skeletal muscle-biopsy specimens, mouse embryonic fibroblasts, and yeast. RESULTS: We found deleterious, recessive variants in human ATG7, a core autophagy-related gene encoding a protein that is indispensable to classical degradative autophagy. Twelve patients from five families with distinct ATG7 variants had complex neurodevelopmental disorders with brain, muscle, and endocrine involvement. Patients had abnormalities of the cerebellum and corpus callosum and various degrees of facial dysmorphism. These patients have survived with impaired autophagic flux arising from a diminishment or absence of ATG7 protein. Although autophagic sequestration was markedly reduced, evidence of basal autophagy was readily identified in fibroblasts and skeletal muscle with loss of ATG7. Complementation of different model systems by deleterious ATG7 variants resulted in poor or absent autophagic function as compared with the reintroduction of wild-type ATG7. CONCLUSIONS: We identified several patients with a neurodevelopmental disorder who have survived with a severe loss or complete absence of ATG7, an essential effector enzyme for autophagy without a known functional paralogue. (Funded by the Wellcome Centre for Mitochondrial Research and others.).

Biallelic RFC1-expansion in a French multicentric sporadic ataxia cohort.

OBJECTIVE: Cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS) is a recessively inherited multisystem ataxia compromising cerebellar, vestibular, and sensory nerves, which has been associated to a pathogenic AAGGG(n) biallelic expansion repeat in the RFC1 gene. Our objective was to assess its prevalence in a French cohort of patients with idiopathic sporadic late-onset ataxia (ILOA), idiopathic early-onset ataxia (IEOA), or Multiple System Atrophy of Cerebellar type (MSA-C). METHODS: 163 patients were recruited in 3 French tertiary centers: 100 ILOA, 21 IEOA, and 42 patients with possible or probable MSA-C. RESULTS: A pathogenic biallelic RFC1 AAGGG(n) repeat expansion was found in 15 patients: 15/100 in the ILOA group, but none in the IEOA and MSA-C subgroups. 14/15 patients had a CANVAS phenotype. Only 1/15 had isolated cerebellar ataxia, but also shorter biallelic expansions. Two RFC1 AAGGG(n) alleles were found in 78% of patients with a CANVAS phenotype. In one post-mortem case, the pathophysiological involvement of cerebellum and medullar posterior columns was found. CONCLUSION: Our study confirms the genetic heterogeneity of the CANVAS and that RFC1 repeat expansions should be searched for preferentially in case of unexplained ILOA associated with a sensory neuronopathy, but not particularly in patients classified as MSA-C.

Expanding the clinical spectrum of STIP1 homology and U-box containing protein 1-associated ataxia.

BACKGROUND: STUB1 has been first associated with autosomal recessive (SCAR16, MIM# 615768) and later with dominant forms of ataxia (SCA48, MIM# 618093). Pathogenic variations in STUB1 are now considered a frequent cause of cerebellar ataxia. OBJECTIVE: We aimed to improve the clinical, radiological, and molecular delineation of SCAR16 and SCA48. METHODS: Retrospective collection of patients with SCAR16 or SCA48 diagnosed in three French genetic centers (Montpellier, Strasbourg and Nancy). RESULTS: Here, we report four SCAR16 and nine SCA48 patients from two SCAR16 and five SCA48 unrelated French families. All presented with slowly progressive cerebellar ataxia. Additional findings included cognitive decline, dystonia, parkinsonism and swallowing difficulties. The age at onset was highly variable, ranging from 14 to 76 years. Brain MRI showed marked cerebellar atrophy in all patients. Phenotypic findings associated with STUB1 pathogenic variations cover a broad spectrum, ranging from isolated slowly progressive ataxia to severe encephalopathy, and include extrapyramidal features. We described five new pathogenic variations, two previously reported pathogenic variations, and two rare variants of unknown significance in association with STUB1-related disorders. We also report the first pathogenic variation associated with both dominant and recessive forms of inheritance (SCAR16 and SCA48). CONCLUSION: Even though differences are observed between the recessive and dominant forms, it appears that a continuum exists between these two entities. While adding new symptoms associated with STUB1 pathogenic variations, we insist on the difficulty of genetic counselling in STUB1-related pathologies. Finally, we underscore the usefulness of DAT-scan as an additional clue for diagnosis.

ATP8A2-related disorders as recessive cerebellar ataxia.

ATP8A2-related disorders are autosomal recessive conditions that associate encephalopathy with or without hypotonia, psychomotor delay, abnormal movements, chorea, tremor, optic atrophy and cerebellar atrophy (CARMQ4). Through a multi-centric collaboration, we identified six point mutations (one splice site and five missense mutations) involving ATP8A2 in six individuals from five families. Two patients from one family with the homozygous p.Gly585Val mutation had a milder presentation without encephalopathy. Expression and functional studies of the missense mutations demonstrated that protein levels of four of the five missense variants were very low and lacked phosphatidylserine-activated ATPase activity. One variant p.Ile215Leu, however, expressed at normal levels and displayed phospholipid-activated ATPase activity similar to the non-mutated protein. We therefore expand for the first time the phenotype related to ATP8A2 mutations to less severe forms characterized by cerebellar ataxia without encephalopathy and suggest that ATP8A2 should be analyzed for all cases of syndromic or non-syndromic recessive or sporadic ataxia.

Mini-Exome Coupled to Read-Depth Based Copy Number Variation Analysis in Patients with Inherited Ataxias.

Next-generation sequencing (NGS) has an established diagnostic value for inherited ataxia. However, the need of a rigorous process of analysis and validation remains challenging. Moreover, copy number variations (CNV) or dynamic expansions of repeated sequence are classically considered not adequately detected by exome sequencing technique. We applied a strategy of mini-exome coupled to read-depth based CNV analysis to a series of 33 patients with probable inherited ataxia and onset <50 years. The mini-exome consisted of the capture of 4,813 genes having associated clinical phenotypes. Pathogenic variants were found in 42% and variants of uncertain significance in 24% of the patients. These results are comparable to those from whole exome sequencing and better than previous targeted NGS studies. CNV and dynamic expansions of repeated CAG sequence were identified in three patients. We identified both atypical presentation of known ataxia genes (ATM, NPC1) and mutations in genes very rarely associated with ataxia (ERCC4, HSD17B4). We show that mini-exome bioinformatics data analysis allows the identification of CNV and dynamic expansions of repeated sequence. Our study confirms the diagnostic value of the proposed genetic analysis strategy. We also provide an algorithm for the multidisciplinary process of analysis, interpretation, and validation of NGS data.

A false positive newborn screening result due to a complex allele carrying two frequent CF-causing variants.

The detection of two frequent CFTR disease-causing variations in the context of a newborn screening program (NBS) usually leads to the diagnosis of cystic fibrosis (CF) and a relevant genetic counseling in the family. In the present study, CF-causing variants p.Phe508del (F508del) and c.3140-26A>G (3272-26A>G) were identified on a neonate with positive ImmunoReactive Trypsinogen test by the Elucigene™ CF30 kit. The CF diagnosis initially suggested, despite three inconclusive Sweat Chloride Tests (SCT), was finally ruled out after the familial segregation study combined with a negative SCT. Haplotype studies, based on the comparison of 80 p.Phe508del haplotypes, suggested a probable de novo occurrence of c.3140-26A>G on the p.Phe508del ancestral allele in this family. This false positive case emphasizes the importance of SCT in the NBS strategy. Moreover, it raises the need for familial segregation studies in CF and in overall molecular diagnosis strategy of autosomal recessive diseases.

Genes for spinocerebellar ataxia with blindness and deafness (SCABD/SCAR3, MIM# 271250 and SCABD2).

Ataxia is a symptom that is often associated with syndromic inherited diseases. We previously reported the linkage of a novel syndrome, ataxia with blindness and deafness (SCAR3/SCABD, OMIM# 271250), to chromosome 6p21-p23 by linkage mapping of an Arab Israeli consanguineous family. We have now identified by whole-exome sequencing a homozygous missense mutation in the Arab Israeli family in the SLC52A2 gene located in 8qter, therefore excluding linkage of this family to 6p. We confirmed the involvement of SLC52A2 by the identification of a second mutation in an independent family with an identical syndromic presentation, which we suggest to name SCABD2. SCABD2 is therefore allelic to Brown-Vialleto-Van Laere syndrome type 2 defined by prominent motoneuronopathy and deafness, and also caused by SLC52A2 mutations. In the course of this project, we identified a clinically similar family with a homozygous missense mutation in PEX6, which is located in 6p21. Therefore, despite false linkage in the initial family, SCABD1/SCAR3 is located in 6p21 and is caused by PEX6 mutations. Both SLC52A2 and PEX6 should be included in screening panels for the diagnosis of syndromic inherited ataxias, particularly as patients with mutations in SLC52A2 can be ameliorated by riboflavin supplementation.

Novel deletions involving the USH2A gene in patients with Usher syndrome and retinitis pigmentosa.

PURPOSE: The aim of the present work was to identify and characterize large rearrangements involving the USH2A gene in patients with Usher syndrome and nonsyndromic retinitis pigmentosa. METHODS: The multiplex ligation-dependent probe amplification (MLPA) technique combined with a customized array-based comparative genomic hybridization (aCGH) analysis was applied to 40 unrelated patients previously screened for point mutations in the USH2A gene in which none or only one pathologic mutation was identified. RESULTS: We detected six large deletions involving USH2A in six out of the 40 cases studied. Three of the patients were homozygous for the deletion, and the remaining three were compound heterozygous with a previously identified USH2A point mutation. In five of these cases, the patients displayed Usher type 2, and the remaining case displayed nonsyndromic retinitis pigmentosa. The exact breakpoint junctions of the deletions found in USH2A in four of these cases were characterized. CONCLUSIONS: Our study highlights the need to develop improved efficient strategies of mutation screening based upon next generation sequencing (NGS) that reduce cost, time, and complexity and allow simultaneous identification of all types of disease-causing mutations in diagnostic procedures.

Enrichment of LOVD-USHbases with 152 USH2A genotypes defines an extensive mutational spectrum and highlights missense hotspots.

Alterations of USH2A, encoding usherin, are responsible for more than 70% of cases of Usher syndrome type II (USH2), a recessive disorder that combines moderate to severe hearing loss and retinal degeneration. The longest USH2A transcript encodes usherin isoform b, a 5,202-amino-acid transmembrane protein with an exceptionally large extracellular domain consisting notably of a Laminin N-terminal domain and numerous Laminin EGF-like (LE) and Fibronectin type III (FN3) repeats. Mutations of USH2A are scattered throughout the gene and mostly private. Annotating these variants is therefore of major importance to correctly assign pathogenicity. We have extensively genotyped a novel cohort of 152 Usher patients and identified 158 different mutations, of which 93 are newly described. Pooling this new data with the existing pathogenic variants already incorporated in USHbases reveals several previously unappreciated features of the mutational spectrum. We show that parts of the protein are more likely to tolerate single amino acid variations, whereas others constitute pathogenic missense hotspots. We have found, in repeated LE and FN3 domains, a nonequal distribution of the missense mutations that highlights some crucial positions in usherin with possible consequences for the assessment of the pathogenicity of the numerous missense variants identified in USH2A.

Experience of targeted Usher exome sequencing as a clinical test.

We show that massively parallel targeted sequencing of 19 genes provides a new and reliable strategy for molecular diagnosis of Usher syndrome (USH) and nonsyndromic deafness, particularly appropriate for these disorders characterized by a high clinical and genetic heterogeneity and a complex structure of several of the genes involved. A series of 71 patients including Usher patients previously screened by Sanger sequencing plus newly referred patients was studied. Ninety-eight percent of the variants previously identified by Sanger sequencing were found by next-generation sequencing (NGS). NGS proved to be efficient as it offers analysis of all relevant genes which is laborious to reach with Sanger sequencing. Among the 13 newly referred Usher patients, both mutations in the same gene were identified in 77% of cases (10 patients) and one candidate pathogenic variant in two additional patients. This work can be considered as pilot for implementing NGS for genetically heterogeneous diseases in clinical service.

Usher syndrome type 2 caused by activation of an USH2A pseudoexon: implications for diagnosis and therapy.

USH2A sequencing in three affected members of a large family, referred for the recessive USH2 syndrome, identified a single pathogenic alteration in one of them and a different mutation in the two affected nieces. As the patients carried a common USH2A haplotype, they likely shared a mutation not found by standard sequencing techniques. Analysis of RNA from nasal cells in one affected individual identified an additional pseudoexon (PE) resulting from a deep intronic mutation. This was confirmed by minigene assay. This is the first example in Usher syndrome (USH) with a mutation causing activation of a PE. The finding of this alteration in eight other individuals of mixed European origin emphasizes the importance of including RNA analysis in a comprehensive diagnostic service. Finally, this mutation, which would not have been found by whole-exome sequencing, could offer, for the first time in USH, the possibility of therapeutic correction by antisense oligonucleotides (AONs).

Non-USH2A mutations in USH2 patients.

We have systematically analyzed the two known minor genes involved in Usher syndrome type 2, DFNB31 and GPR98, for mutations in a cohort of 31 patients not linked to USH2A. PDZD7, an Usher syndrome type 2 (USH2) related gene, was analyzed when indicated. We found that mutations in GPR98 contribute significantly to USH2. We report 17 mutations in 10 individuals, doubling the number of GPR98 mutations reported to date. In contrast to mutations in usherin, the mutational spectrum of GPR98 predominantly results in a truncated protein product. This is true even when the mutation affects splicing, and we have incorporated a splicing reporter minigene assay to show this, where appropriate. Only two mutations were found which we believe to be genuine missense changes. Discrepancy in the mutational spectrum between GPR98 and USH2A is discussed. Only two patients were found with mutations in DFNB31, showing that mutations of this gene contribute to only a very small extent to USH2. Close examination of the clinical details, where available, for patients in whom no mutation was found in USH2A, GPR98, or DFNB31, showed that most of them had atypical features. In effect, these three genes account for the vast majority of USH2 patients and their analysis provide a robust pathway for routine molecular diagnosis.

Nasal epithelial cells are a reliable source to study splicing variants in Usher syndrome.

We have shown that nasal ciliated epithelium, which can be easily biopsied under local anesthetic, provides a good source of RNA transcripts from eight of the nine known genes that cause Usher syndrome, namely, MYO7A, USH1C, CDH23, PCDH15, USH1G for Usher type 1, and USH2A, GPR98, WHRN for Usher type 2. Furthermore, the known or predicted effect on mRNA splicing of eight variants was faithfully reproduced in the biopsied sample as measured by nested RT-PCR. These included changes at the canonical acceptor site, changes within the noncanonical acceptor site and both synonymous and nonsynonymous amino acid changes. This shows that mRNA analysis by this method will help in assessing the pathogenic effect of variants, which is a major problem in the molecular diagnosis of Usher syndrome.

The USH2A c.2299delG mutation: dating its common origin in a Southern European population.

Usher syndrome type II is the most common form of Usher syndrome. USH2A is the main responsible gene of the three known to be disease causing. It encodes two isoforms of the protein usherin. This protein is part of an interactome that has an essential role in the development and function of inner ear hair cells and photoreceptors. The gene contains 72 exons spanning over a region of 800 kb. Although numerous mutations have been described, the c.2299delG mutation is the most prevalent in several populations. Its ancestral origin was previously suggested after the identification of a common core haplotype restricted to 250 kb in the 5' region that encodes the short usherin isoform. By extending the haplotype analysis over the 800 kb region of the USH2A gene with a total of 14 intragenic single nucleotide polymorphisms, we have been able to define 10 different c.2299delG haplotypes, showing high variability but preserving the previously described core haplotype. An exhaustive c.2299delG/control haplotype study suggests that the major source of variability in the USH2A gene is recombination. Furthermore, we have evidenced twice the amount of recombination hotspots located in the 500 kb region that covers the 3' end of the gene, explaining the higher variability observed in this region when compared with the 250 kb of the 5' region. Our data confirm the common ancestral origin of the c.2299delG mutation.

Ex vivo splicing assays of mutations at noncanonical positions of splice sites in USHER genes.

Molecular diagnosis in Usher syndrome type 1 and 2 patients led to the identification of 21 sequence variations located in noncanonical positions of splice sites in MYO7A, CDH23, USH1C, and USH2A genes. To establish experimentally the splicing pattern of these substitutions, whose impact on splicing is not always predictable by available softwares, ex vivo splicing assays were performed. The branch-point mapping strategy was also used to investigate further a putative branch-point mutation in USH2A intron 43. Aberrant splicing was demonstrated for 16 of the 21 (76.2%) tested sequence variations. The mutations resulted more frequently in activation of a nearby cryptic splice site or use of a de novo splice site than exon skipping (37.5%). This study allowed the reclassification as splicing mutations of one silent (c.7872G>A (p.Glu2624Glu) in CDH23) and four missense mutations (c.2993G>A (p.Arg998Lys) in USH2A, c.592G>A (p.Ala198Thr), c.3503G>C [p.Arg1168Pro], c.5944G>A (p.Gly1982Arg) in MYO7A), whereas it provided clues about a role in structure/function in four other cases: c.802G>A (p.Gly268Arg), c.653T>A (p.Val218Glu) (USH2A), and c.397C>T (p.His133Tyr), c.3502C>T (p.Arg1168Trp) (MYO7A). Our data provide insights into the contribution of splicing mutations in Usher genes and illustrate the need to define accurately their splicing outcome for diagnostic purposes.