A New Case Series Suggests That SCA48 (ATX/STUB1) Is Primarily a Monogenic Disorder.

BACKGROUND: Monoallelic, pathogenic STUB1 variants cause autosomal dominant cerebellar ataxia (ATX-STUB1/SCA48). Recently, a genetic interaction between STUB1 variants and intermediate or high-normal CAG/CAA repeats in TBP was suggested, indicating digenic inheritance or a disease-modifying role for TBP expansions. OBJECTIVE: To determine the presence and impact of intermediate or high-normal TBP expansions in ataxic patients with heterozygous STUB1 variants. METHODS: We describe 21 patients with ataxia carrying a heterozygous STUB1 variant and determined TBP repeat length. RESULTS: A total of 15 of 21 patients (71%) carried a normal TBP<40 allele, 4 (19%) carried an intermediate TBP41-42 allele, and two carried a high-normal TBP40 allele (9.5%). Five of six carriers (83%) of both STUB1 variants and TBP40-42 alleles showed marked cognitive impairment. CONCLUSIONS: SCA48 is predominantly a monogenic disorder, because most patients carried an isolated, heterozygous STUB1 variant and presented with the typical combined phenotype of ataxia and cognitive dysfunction. Still, co-occurrence of TBP41-42 or high-normal TBP40 alleles was relatively frequent and associated with marked cognitive defects (28.5%), suggesting a modifying effect on clinical expression in some cases. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

The mitochondrial seryl-tRNA synthetase SARS2 modifies onset in spastic paraplegia type 4.

PURPOSE: Hereditary spastic paraplegia type 4 is extremely variable in age at onset; the same variant can cause onset at birth or in the eighth decade. We recently discovered that missense variants in SPAST, which influences microtubule dynamics, are associated with earlier onset and more severe disease than truncating variants, but even within the early and late-onset groups there remained significant differences in onset. Given the rarity of the condition, we adapted an extreme phenotype approach to identify genetic modifiers of onset. METHODS: We performed a genome-wide association study on 134 patients bearing truncating pathogenic variants in SPAST, divided into early- and late-onset groups (aged ≤15 and ≥45 years, respectively). A replication cohort of 419 included patients carrying either truncating or missense variants. Finally, age at onset was analyzed in the merged cohort (N = 553). RESULTS: We found 1 signal associated with earlier age at onset (rs10775533, P = 8.73E-6) in 2 independent cohorts and in the merged cohort (N = 553, Mantel-Cox test, P < .0001). Western blotting in lymphocytes of 20 patients showed that this locus tends to upregulate SARS2 expression in earlier-onset patients. CONCLUSION: SARS2 overexpression lowers the age of onset in hereditary spastic paraplegia type 4. Lowering SARS2 or improving mitochondrial function could thus present viable approaches to therapy.

Systematic analysis of short tandem repeats in 38,095 exomes provides an additional diagnostic yield.

PURPOSE: Expansions of a subset of short tandem repeats (STRs) have been implicated in approximately 30 different human genetic disorders. Despite extensive application of exome sequencing (ES) in routine diagnostic genetic testing, STRs are not routinely identified from these data. METHODS: We assessed diagnostic utility of STR analysis in exome sequencing by applying ExpansionHunter to 2,867 exomes from movement disorder patients and 35,228 other clinical exomes. RESULTS: We identified 38 movement disorder patients with a possible aberrant STR length. Validation by polymerase chain reaction (PCR) and/or repeat-primed PCR technologies confirmed the presence of aberrant expansion alleles for 13 (34%). For seven of these patients the genotype was compatible with the phenotypic description, resulting in a molecular diagnosis. We subsequently tested the remainder of our diagnostic ES cohort, including over 30 clinically and genetically heterogeneous disorders. Optimized manual curation yielded 167 samples with a likely aberrant STR length. Validations confirmed 93/167 (56%) aberrant expansion alleles, of which 48 were in the pathogenic range and 45 in the premutation range. CONCLUSION: Our work provides guidance for the implementation of STR analysis in clinical ES. Our results show that systematic STR evaluation may increase diagnostic ES yield by 0.2%, and recommend making STR evaluation a routine part of ES interpretation in genetic testing laboratories.

Clinical, genetic, and histological features of centronuclear myopathy in the Netherlands.

Centronuclear myopathy (CNM) is a genetically heterogeneous congenital myopathy characterized by muscle weakness, atrophy, and variable degrees of cardiorespiratory involvement. The clinical severity is largely explained by genotype (DNM2, MTM1, RYR1, BIN1, TTN, and other rarer genetic backgrounds), specific mutation(s), and age of the patient. The histopathological hallmark of CNM is the presence of internal centralized nuclei on muscle biopsy. Information on the phenotypical spectrum, subtype prevalence, and phenotype-genotype correlations is limited. To characterize CNM more comprehensively, we retrospectively assessed a national cohort of 48 CNM patients (mean age = 32 ± 24 years, range 0-80, 54% males) from the Netherlands clinically, histologically, and genetically. All information was extracted from entries in the patient's medical records, between 2000 and 2020. Frequent clinical features in addition to muscle weakness and hypotonia were fatigue and exercise intolerance in more mildly affected cases. Genetic analysis showed variants in four genes (18 DNM2, 14 MTM1, 9 RYR1, and 7 BIN1), including 16 novel variants. In addition to central nuclei, histologic examination revealed a large variability of myopathic features in the different genotypes. The identification and characterization of these patients contribute to trial readiness.

Clinical presentation and long-term follow-up of dopamine beta hydroxylase deficiency.

Dopamine beta hydroxylase (DBH) deficiency is an extremely rare autosomal recessive disorder with severe orthostatic hypotension, that can be treated with L-threo-3,4-dihydroxyphenylserine (L-DOPS). We aimed to summarize clinical, biochemical, and genetic data of all world-wide reported patients with DBH-deficiency, and to present detailed new data on long-term follow-up of a relatively large Dutch cohort. We retrospectively describe 10 patients from a Dutch cohort and 15 additional patients from the literature. We identified 25 patients (15 females) from 20 families. Ten patients were diagnosed in the Netherlands. Duration of follow-up of Dutch patients ranged from 1 to 21 years (median 13 years). All patients had severe orthostatic hypotension. Severely decreased or absent (nor)epinephrine, and increased dopamine plasma concentrations were found in 24/25 patients. Impaired kidney function and anemia were present in all Dutch patients, hypomagnesaemia in 5 out of 10. Clinically, all patients responded very well to L-DOPS, with marked reduction of orthostatic complaints. However, orthostatic hypotension remained present, and kidney function, anemia, and hypomagnesaemia only partially improved. Plasma norepinephrine increased and became detectable, while epinephrine remained undetectable in most patients. We confirm the core clinical characteristics of DBH-deficiency and the pathognomonic profile of catecholamines in body fluids. Impaired renal function, anemia, and hypomagnesaemia can be part of the clinical presentation. The subjective response to L-DOPS treatment is excellent and sustained, although the neurotransmitter profile in plasma does not normalize completely. Furthermore, orthostatic hypotension as well as renal function, anemia, and hypomagnesaemia improve only partially.

Cytidine Diphosphate-Ribitol Analysis for Diagnostics and Treatment Monitoring of Cytidine Diphosphate-l-Ribitol Pyrophosphorylase A Muscular Dystrophy.

BACKGROUND: Many muscular dystrophies currently remain untreatable. Recently, dietary ribitol has been suggested as a treatment for cytidine diphosphate (CDP)-l-ribitol pyrophosphorylase A (CRPPA, ISPD), fukutin (FKTN), and fukutin-related protein (FKRP) myopathy, by raising CDP-ribitol concentrations. Thus, to facilitate fast diagnosis, treatment development, and treatment monitoring, sensitive detection of CDP-ribitol is required. METHODS: An LC-MS method was optimized for CDP-ribitol in human and mice cells and tissues. RESULTS: CDP-ribitol, the product of CRPPA, was detected in all major human and mouse tissues. Moreover, CDP-ribitol concentrations were reduced in fibroblasts and skeletal muscle biopsies from patients with CRPPA myopathy, showing that CDP-ribitol could serve as a diagnostic marker to identify patients with CRPPA with severe Walker-Warburg syndrome and mild limb-girdle muscular dystrophy (LGMD) phenotypes. A screen for potentially therapeutic monosaccharides revealed that ribose, in addition to ribitol, restored CDP-ribitol concentrations and the associated O-glycosylation defect of α-dystroglycan. As the effect occurred in a mutation-dependent manner, we established a CDP-ribitol blood test to facilitate diagnosis and predict individualized treatment response. Ex vivo incubation of blood cells with ribose or ribitol restored CDP-ribitol concentrations in a patient with CRPPA LGMD. CONCLUSIONS: Sensitive detection of CDP-ribitol with LC-MS allows fast diagnosis of patients with severe and mild CRPPA myopathy. Ribose offers a readily testable dietary therapy for CRPPA myopathy, with possible applicability for patients with FKRP and FKTN myopathy. Evaluation of CDP-ribitol in blood is a promising tool for the evaluation and monitoring of dietary therapies for CRPPA myopathy in a patient-specific manner.

Mutations in BICD2, which encodes a golgin and important motor adaptor, cause congenital autosomal-dominant spinal muscular atrophy.

Spinal muscular atrophy (SMA) is a heterogeneous group of neuromuscular disorders caused by degeneration of lower motor neurons. Although functional loss of SMN1 is associated with autosomal-recessive childhood SMA, the genetic cause for most families affected by dominantly inherited SMA is unknown. Here, we identified pathogenic variants in bicaudal D homolog 2 (Drosophila) (BICD2) in three families afflicted with autosomal-dominant SMA. Affected individuals displayed congenital slowly progressive muscle weakness mainly of the lower limbs and congenital contractures. In a large Dutch family, linkage analysis identified a 9q22.3 locus in which exome sequencing uncovered c.320C>T (p.Ser107Leu) in BICD2. Sequencing of 23 additional families affected by dominant SMA led to the identification of pathogenic variants in one family from Canada (c.2108C>T [p.Thr703Met]) and one from the Netherlands (c.563A>C [p.Asn188Thr]). BICD2 is a golgin and motor-adaptor protein involved in Golgi dynamics and vesicular and mRNA transport. Transient transfection of HeLa cells with all three mutant BICD2 cDNAs caused massive Golgi fragmentation. This observation was even more prominent in primary fibroblasts from an individual harboring c.2108C>T (p.Thr703Met) (affecting the C-terminal coiled-coil domain) and slightly less evident in individuals with c.563A>C (p.Asn188Thr) (affecting the N-terminal coiled-coil domain). Furthermore, BICD2 levels were reduced in affected individuals and trapped within the fragmented Golgi. Previous studies have shown that Drosophila mutant BicD causes reduced larvae locomotion by impaired clathrin-mediated synaptic endocytosis in neuromuscular junctions. These data emphasize the relevance of BICD2 in synaptic-vesicle recycling and support the conclusion that BICD2 mutations cause congenital slowly progressive dominant SMA.

Copy number variants from 4800 exomes contribute to ~7% of genetic diagnoses in movement disorders, muscle disorders and neuropathies.

Various groups of neurological disorders, including movement disorders and neuromuscular diseases, are clinically and genetically heterogeneous. Diagnostic panel-based exome sequencing is a routine test for these disorders. Despite the success rates of exome sequencing, it results in the detection of causative sequence variants in 'only' 25-30% of cases. Copy number variants (CNVs), i.e. deletion or duplications, explain 10-20% of individuals with multisystemic phenotypes, such as co-existing intellectual disability, but may also have a role in disorders affecting a single system (organ), like neurological disorders with normal intelligence. In this study, CNVs were extracted from clinical exome sequencing reports of 4800 probands primarily with a movement disorder, myopathy or neuropathy. In 88 (~2%) probands, phenotype-matching CNVs were detected, representing ~7% of genetically confirmed cases. CNVs varied from involvement of over 100 genes to single exons and explained X-linked, autosomal dominant, or - recessive disorders, the latter due to either a homozygous CNV or a compound heterozygous CNV with a sequence variant on the other allele. CNVs were detected affecting genes where deletions or duplications are established as a common mechanism, like PRKN (in Parkinson's disease), DMD (in Duchenne muscular dystrophy) and PMP22 (in neuropathies), but also genes in which no intragenic CNVs have been reported to date. Analysis of CNVs as part of panel-based exome sequencing for genetically heterogeneous neurological diseases provides an additional diagnostic yield of ~2% without extra laboratory costs. Therefore it is recommended to perform CNV analysis for movement disorders, muscle disease, neuropathies, or any other single-system disorder.

Genetic characterization of primary lateral sclerosis.

BACKGROUND AND OBJECTIVES: Primary lateral sclerosis (PLS) is a motor neuron disease characterised by loss of the upper motor neurons. Most patients present with slowly progressive spasticity of the legs, which may also spread to the arms or bulbar regions. It is challenging to distinguish between PLS, early-stage amyotrophic lateral sclerosis (ALS) and hereditary spastic paraplegia (HSP). The current diagnostic criteria advise against extensive genetic testing. This recommendation is, however, based on limited data. METHODS: We aim to genetically characterize a PLS cohort using whole exome sequencing (WES) for genes associated with ALS, HSP, ataxia and movement disorders (364 genes) and C9orf72 repeat expansions. Patients fulfilling the definite PLS criteria by Turner et al. and with available DNA samples of sufficient quality were recruited from an on-going, population-based epidemiological study. Genetic variants were classified according to the ACMG criteria and assigned to groups based on disease association. RESULTS: WES was performed in 139 patients and the presence of repeat expansions in C9orf72 was analysed separately in 129 patients. This resulted in 31 variants of which 11 were (likely) pathogenic. (Likely) pathogenic variants resulted in 3 groups based on disease association: ALS-FTD (C9orf72, TBK1), pure HSP (SPAST, SPG7), "ALS-HSP-CMT overlap" (FIG4, NEFL, SPG11). DISCUSSION: In a cohort of 139 PLS patients, genetic analyses resulted in 31 variants (22%) of which 10 (7%) (likely) pathogenic associated with different diseases (predominantly ALS and HSP). Based on these results and the literature, we advise to consider genetic analyses in the diagnostic work-up for PLS.

The complexities of CACNA1A in clinical neurogenetics.

Variants in CACNA1A are classically related to episodic ataxia type 2, familial hemiplegic migraine type 1, and spinocerebellar ataxia type 6. Over the years, CACNA1A has been associated with a broader spectrum of phenotypes. Targeted analysis and unbiased sequencing of CACNA1A result not only in clear molecular diagnoses, but also in large numbers of variants of uncertain significance (VUS), or likely pathogenic variants with a phenotype that does not directly match the CACNA1A spectrum. Over the last years, targeted and clinical exome sequencing in our center has identified 41 CACNA1A variants. Ultimately, variants were considered pathogenic or likely pathogenic in 23 cases, with most phenotypes ranging from episodic or progressive ataxia to more complex ataxia syndromes, as well as intellectual disability and epilepsy. In two cases, the causality of the variant was discarded based on non-segregation or an alternative diagnosis. In the remaining 16 cases, the variant was classified as uncertain, due to lack of opportunities for segregation analysis or uncertain association with a non-classic phenotype. Phenotypic variability and the large number of VUS make CACNA1A a challenging gene for neurogenetic diagnostics. Accessible functional read-outs are clearly needed, especially in cases with a non-classic phenotype.

Neuromuscular Features in XL-MTM Carriers: A Cross-sectional Study in an Unselected Cohort.

BACKGROUND AND OBJECTIVE: X-linked myotubular myopathy (XL-MTM) is an early-onset congenital myopathy characterized by mild to severe muscle weakness in male individuals. The objective was to characterize the clinical spectrum of neuromuscular features in X-linked myotubular myopathy (XL-MTM) carriers. METHODS: We performed a nationwide cross-sectional study focusing on neuromuscular features in an unselected cohort of Dutch XL-MTM carriers. Participants were recruited from neuromuscular centers in the Netherlands and through the Dutch and European patient associations. Genetic results were collected. Carriers were classified based on ambulatory status and muscle weakness. We used a questionnaire focusing on medical and family history and neuromuscular symptoms. In addition, we performed a neurologic examination including manual muscle testing (MMT), timed up and go (TUG) test, and 6-minute walking test (6MWT). RESULTS: We included 21 carriers (20 genetically confirmed and 1 obligate), of whom 11 (52%) carriers were classified as manifesting, with severe (nonambulatory; n = 2), moderate (minimal independent ambulation/assisted ambulation; n = 2), mild (independent ambulation but with limb or axial muscle weakness; n = 3), and minimal (only facial muscle weakness, n = 4) phenotypes. Three of the manifesting carriers (2 severe and 1 moderate) were from families without genetically confirmed male XL-MTM patients. Furthermore, 7 manifesting carriers (1 moderate; 2 mild; and 4 minimal) were not classified as manifesting carriers before participation in our study. Three carriers reported a history of pneumothorax. The obstetric history revealed frequent polyhydramnios (50%) and reduced fetal movements (36%) in pregnancies of affected sons. Muscle weakness was most pronounced in proximal and limb girdle muscles. Other frequently reported signs included (asymmetric) facial weakness (73%), reduced or absent deep tendon reflexes (45%), scoliosis (40%), and ptosis (45%). Ten participants (48%) were classified as nonmanifesting. Manifesting carriers had lower functional testing scores on 6MWT and TUG compared with nonmanifesting carriers. DISCUSSION: This study showed that 52% of an unselected group of XL-MTM carriers has muscle weakness (3 of whom were previously unclassified as manifesting). This corresponds to findings of our recent questionnaire study on self-reported symptoms in XL-MTM carriers. These observations should raise awareness of the neuromuscular manifestations of the XL-MTM carrier state and provide important epidemiologic information required for future clinical trials.

Prevalence of intronic repeat expansions in RFC1 in Dutch patients with CANVAS and adult-onset ataxia.

Recently, an intronic biallelic (AAGGG)n repeat expansion in RFC1 was shown to be a cause of CANVAS and adult-onset ataxia in multiple populations. As the prevalence of the RFC1 repeat expansion in Dutch cases was unknown, we retrospectively tested 9 putative CANVAS cases and two independent cohorts (A and B) of 395 and 222 adult-onset ataxia cases, respectively, using the previously published protocol and, for the first time optical genome mapping to determine the size of the expanded RFC1 repeat. We identified the biallelic (AAGGG)n repeat expansion in 5/9 (55%) putative CANVAS patients and in 10/617 (1.6%; cohorts A + B) adult-onset ataxia patients. In addition to the AAGGG repeat motif, we observed a putative GAAGG repeat motif in the repeat expansion with unknown significance in two adult-onset ataxia patients. All the expanded (AAGGG)n repeats identified were in the range of 800-1299 repeat units. The intronic biallelic RFC1 repeat expansion thus explains a number of the Dutch adult-onset ataxia cases that display the main clinical features of CANVAS, and particularly when ataxia is combined with neuropathy. The yield of screening for RFC1 expansions in unselected cohorts is relatively low. To increase the current diagnostic yield in ataxia patients, we suggest adding RFC1 screening to the genetic diagnostic workflow by using advanced techniques that attain long fragments.

Reanalysis of exome negative patients with rare disease: a pragmatic workflow for diagnostic applications.

BACKGROUND: Approximately two third of patients with a rare genetic disease remain undiagnosed after exome sequencing (ES). As part of our post-test counseling procedures, patients without a conclusive diagnosis are advised to recontact their referring clinician to discuss new diagnostic opportunities in due time. We performed a systematic study of genetically undiagnosed patients 5 years after their initial negative ES report to determine the efficiency of diverse reanalysis strategies. METHODS: We revisited a cohort of 150 pediatric neurology patients originally enrolled at Radboud University Medical Center, of whom 103 initially remained genetically undiagnosed. We monitored uptake of physician-initiated routine clinical and/or genetic re-evaluation (ad hoc re-evaluation) and performed systematic reanalysis, including ES-based resequencing, of all genetically undiagnosed patients (systematic re-evaluation). RESULTS: Ad hoc re-evaluation was initiated for 45 of 103 patients and yielded 18 diagnoses (including 1 non-genetic). Subsequent systematic re-evaluation identified another 14 diagnoses, increasing the diagnostic yield in our cohort from 31% (47/150) to 53% (79/150). New genetic diagnoses were established by reclassification of previously identified variants (10%, 3/31), reanalysis with enhanced bioinformatic pipelines (19%, 6/31), improved coverage after resequencing (29%, 9/31), and new disease-gene associations (42%, 13/31). Crucially, our systematic study also showed that 11 of the 14 further conclusive genetic diagnoses were made in patients without a genetic diagnosis that did not recontact their referring clinician. CONCLUSIONS: We find that upon re-evaluation of undiagnosed patients, both reanalysis of existing ES data as well as resequencing strategies are needed to identify additional genetic diagnoses. Importantly, not all patients are routinely re-evaluated in clinical care, prolonging their diagnostic trajectory, unless systematic reanalysis is facilitated. We have translated our observations into considerations for systematic and ad hoc reanalysis in routine genetic care.

Biallelic Variants in the COLGALT1 Gene Causes Severe Congenital Porencephaly: A Case Report.

OBJECTIVE: We describe a third patient with brain small vessel disease 3 (BSVD3), being the first with a homozygous essential splice site variant in the COLGALT1 gene, with a more severe phenotype than the 2 children reported earlier. METHODS: Analysis of whole exome sequencing (WES) data of the child and parents was performed. We validated the missplicing of the homozygous variant using reverse transcription PCR and Sanger sequencing of the mRNA in a lymphocyte culture. RESULTS: The patient presented antenatally with porencephaly on ultrasound and MRI. Postnatally, he showed a severe developmental delay, refractory epilepsy, spastic quadriplegia, and a progressive hydrocephalus. WES revealed a homozygous canonical splice site variant NM_024656.3:c.625-2A>C. PCR and Sanger sequencing of the mRNA demonstrated that 2 cryptic splice sites are activated, causing a frameshift in the major transcript and in-frame deletion in a minor transcript. CONCLUSIONS: We report a third patient with biallelic pathogenic variants in COLGALT1, confirming the role of this gene in autosomal recessive BSVD3.

Spectrum of Clinical Features in X-Linked Myotubular Myopathy Carriers: An International Questionnaire Study.

OBJECTIVE: To characterize the spectrum of clinical features in a cohort of X-linked myotubular myopathy (XL-MTM) carriers, including prevalence, genetic features, clinical symptoms, and signs, as well as associated disease burden. METHODS: We performed a cross-sectional online questionnaire study among XL-MTM carriers. Participants were recruited from patient associations, medical centers, and registries in the United Kingdom, Germany, and the Netherlands. We used a custom-made questionnaire, the Checklist Individual Strength (CIS), the Frenchay Activities Index (FAI), the Short Form 12 (SF-12) health survey, and the McGill Pain Questionnaire. Carriers were classified as manifesting or nonmanifesting on the basis of self-reported ambulation and muscle weakness. RESULTS: The prevalence of manifesting carriers in this study population (n = 76) was 51%, subdivided into mild (independent ambulation, 39%), moderate (assisted ambulation, 9%), and severe (wheelchair dependent, 3%) phenotypes. In addition to muscle weakness, manifesting carriers frequently reported fatigue (70%) and exercise intolerance (49%). Manifesting carriers scored higher on the overall CIS (p = 0.001), the fatigue subscale (p < 0.001), and least severe pain subscale (p = 0.005) than nonmanifesting carriers. They scored lower on the FAI (p = 0.005) and the physical component of the SF-12 health survey (p < 0.001). CONCLUSIONS: The prevalence of manifesting XL-MTM carriers may be higher than currently assumed, most having a mild phenotype and a wide variety of symptoms. Manifesting carriers are particularly affected by fatigue, limitations of daily activities, pain, and reduced quality of life. Our findings should increase awareness and provide useful information for health care providers and future clinical trials.

De novo variants in CAMTA1 cause a syndrome variably associated with spasticity, ataxia, and intellectual disability.

Previously, intragenic CAMTA1 copy number variants (CNVs) have been shown to cause non-progressive, congenital ataxia with or without intellectual disability (OMIM#614756). However, ataxia, intellectual disability, and dysmorphic features were all incompletely penetrant, even within families. Here, we describe four patients with de novo nonsense, frameshift or missense CAMTA1 variants. All four patients predominantly manifested features of ataxia and/or spasticity. Borderline intellectual disability and dysmorphic features were both present in one patient only, and other neurological and behavioural symptoms were variably present. Neurodevelopmental delay was found to be mild. Our findings indicate that also nonsense, frameshift and missense variants in CAMTA1 can cause a spastic ataxia syndrome as the main phenotype.

KIF1A variants are a frequent cause of autosomal dominant hereditary spastic paraplegia.

Variants in the KIF1A gene can cause autosomal recessive spastic paraplegia 30, autosomal recessive hereditary sensory neuropathy, or autosomal (de novo) dominant mental retardation type 9. More recently, variants in KIF1A have also been described in a few cases with autosomal dominant spastic paraplegia. Here, we describe 20 KIF1A variants in 24 patients from a clinical exome sequencing cohort of 347 individuals with a mostly 'pure' spastic paraplegia. In these patients, spastic paraplegia was slowly progressive and mostly pure, but with a highly variable disease onset (0-57 years). Segregation analyses showed a de novo occurrence in seven cases, and a dominant inheritance pattern in 11 families. The motor domain of KIF1A is a hotspot for disease causing variants in autosomal dominant spastic paraplegia, similar to mental retardation type 9 and recessive spastic paraplegia type 30. However, unlike these allelic disorders, dominant spastic paraplegia was also caused by loss-of-function variants outside this domain in six families. Finally, three missense variants were outside the motor domain and need further characterization. In conclusion, KIF1A variants are a frequent cause of autosomal dominant spastic paraplegia in our cohort (6-7%). The identification of KIF1A loss-of-function variants suggests haploinsufficiency as a possible mechanism in autosomal dominant spastic paraplegia.

Panel-Based Exome Sequencing for Neuromuscular Disorders as a Diagnostic Service.

BACKGROUND: Neuromuscular disorders (NMDs) are clinically and genetically heterogeneous. Accurate molecular genetic diagnosis can improve clinical management, provides appropriate genetic counseling and testing of relatives, and allows potential therapeutic trials. OBJECTIVE: To establish the clinical utility of panel-based whole exome sequencing (WES) in NMDs in a population with children and adults with various neuromuscular symptoms. METHODS: Clinical exome sequencing, followed by diagnostic interpretation of variants in genes associated with NMDs, was performed in a cohort of 396 patients suspected of having a genetic cause with a variable age of onset, neuromuscular phenotype, and inheritance pattern. Many had previously undergone targeted gene testing without results. RESULTS: Disease-causing variants were identified in 75/396 patients (19%), with variants in the three COL6-genes (COL6A1, COL6A2 and COL6A3) as the most common cause of the identified muscle disorder, followed by variants in the RYR1 gene. Together, these four genes account for almost 25% of cases in whom a definite genetic cause was identified. Furthermore, likely pathogenic variants and/or variants of uncertain significance were identified in 95 of the patients (24%), in whom functional and/or segregation analysis should be used to confirm or reject the pathogenicity. In 18% of the cases with a disease-causing variant of which we received additional clinical information, we identified a genetic cause in genes of which the associated phenotypes did not match that of the patients. Hence, the advantage of panel-based WES is its unbiased approach. CONCLUSION: Whole exome sequencing, followed by filtering for NMD genes, offers an unbiased approach for the genetic diagnostics of NMD patients. This approach could be used as a first-tier test in neuromuscular disorders with a high suspicion of a genetic cause. With uncertain results, functional testing and segregation analysis are needed to complete the evidence.

Upstream SLC2A1 translation initiation causes GLUT1 deficiency syndrome.

Glucose transporter type 1 deficiency syndrome (GLUT1DS) is a neurometabolic disorder with a complex phenotypic spectrum but simple biomarkers in cerebrospinal fluid. The disorder is caused by impaired glucose transport into the brain resulting from variants in SCL2A1. In 10% of GLUT1DS patients, a genetic diagnosis can not be made. Using whole-genome sequencing, we identified a de novo 5'-UTR variant in SLC2A1, generating a novel translation initiation codon, severely compromising SLC2A1 function. This finding expands our understanding of the disease mechanisms underlying GLUT1DS and encourages further in-depth analysis of SLC2A1 non-coding regions in patients without variants in the coding region.