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.

Primary mitochondrial disorders and mimics: Insights from a large French cohort.

OBJECTIVE: The objective of this study was to evaluate the implementation of NGS within the French mitochondrial network, MitoDiag, from targeted gene panels to whole exome sequencing (WES) or whole genome sequencing (WGS) focusing on mitochondrial nuclear-encoded genes. METHODS: Over 2000 patients suspected of Primary Mitochondrial Diseases (PMD) were sequenced by either targeted gene panels, WES or WGS within MitoDiag. We described the clinical, biochemical, and molecular data of 397 genetically confirmed patients, comprising 294 children and 103 adults, carrying pathogenic or likely pathogenic variants in nuclear-encoded genes. RESULTS: The cohort exhibited a large genetic heterogeneity, with the identification of 172 distinct genes and 253 novel variants. Among children, a notable prevalence of pathogenic variants in genes associated with oxidative phosphorylation (OXPHOS) functions and mitochondrial translation was observed. In adults, pathogenic variants were primarily identified in genes linked to mtDNA maintenance. Additionally, a substantial proportion of patients (54% (42/78) and 48% (13/27) in children and adults, respectively), undergoing WES or WGS testing displayed PMD mimics, representing pathologies that clinically resemble mitochondrial diseases. INTERPRETATION: We reported the largest French cohort of patients suspected of PMD with pathogenic variants in nuclear genes. We have emphasized the clinical complexity of PMD and the challenges associated with recognizing and distinguishing them from other pathologies, particularly neuromuscular disorders. We confirmed that WES/WGS, instead of panel approach, was more valuable to identify the genetic basis in patients with "possible" PMD and we provided a genetic testing flowchart to guide physicians in their diagnostic strategy.

Loss of heterozygosity in CCM2 cDNA revealing a structural variant causing multiple cerebral cavernous malformations.

Loss-of-function variants in CCM1/KRIT1, CCM2/MGC4607, and CCM3/PDCD10 genes are identified in the vast majority of familial cases with multiple cerebral cavernous malformations. However, genomic DNA sequencing combined with large rearrangement screening fails to detect a pathogenic variant in 5% of the patients. We report a family with two affected members harboring multiple CCM lesions, one with severe hemorrhages and one asymptomatic. No causative variant was detected using DNA sequencing of the three CCM genes, CNV detection analysis, and RNA sequencing. However, a loss of heterozygosity in CCM2 was observed on cDNA sequences in one of the two affected members, which strongly suggested that this locus might be involved. Whole genome sequencing (WGS) identified a balanced structural variant on chromosome 7 with a breakpoint interrupting the CCM2 gene, preventing normal mRNA synthesis. These data underline the importance of WGS in undiagnosed patients with typical multiple CCM.

A Case Report of SYNE1 Deficiency-Mimicking Mitochondrial Disease and the Value of Pangenomic Investigations.

Mitochondrial disorders are characterized by a huge clinical, biochemical, and genetic heterogeneity, which poses significant diagnostic challenges. Several studies report that more than 50% of patients with suspected mitochondrial disease could have a non-mitochondrial disorder. Thus, only the identification of the causative pathogenic variant can confirm the diagnosis. Herein, we describe the diagnostic journey of a family suspected of having a mitochondrial disorder who were referred to our Genetics Department. The proband presented with the association of cerebellar ataxia, COX-negative fibers on muscle histology, and mtDNA deletions. Whole exome sequencing (WES), supplemented by a high-resolution array, comparative genomic hybridization (array-CGH), allowed us to identify two pathogenic variants in the non-mitochondrial SYNE1 gene. The proband and her affected sister were found to be compound heterozygous for a known nonsense variant (c.13258C>T, p.(Arg4420Ter)), and a large intragenic deletion that was predicted to result in a loss of function. To our knowledge, this is the first report of a large intragenic deletion of SYNE1 in patients with cerebellar ataxia (ARCA1). This report highlights the interest in a pangenomic approach to identify the genetic basis in heterogeneous neuromuscular patients with the possible cause of mitochondrial disease. Moreover, even rare copy number variations should be considered in patients with a phenotype suggestive of SYNE1 deficiency.

Splicing variants in NARS2 are associated with milder phenotypes and intra-familial variability.

Biallelic rare variants in NARS2 that encode the mitochondrial asparaginyl-tRNA synthetase are associated with a wide spectrum of clinical phenotypes ranging from severe neurodegenerative disorders to isolated mitochondrial myopathy or deafness. To date, only a small number of patients with NARS2 variants have been reported, and possible genotype-phenotype correlations are still lacking. Here, we present three siblings who had an early-onset hearing loss, while one developed severe symptoms in adulthood associated with early intellectual impairment, refractory seizures, moderate axonal sensorimotor neuropathy, and atypical psychiatric symptoms. Biochemical analysis revealed impairment of the activity and assembly of the respiratory chain complexes in this patient's muscle and fibroblasts. Whole Exome Sequencing allowed identification of a heterozygous variant NM_024678.5(NARS2):c.822G > C (p.Gln274His) that is known to be pathogenic and to affect splicing of the NARS2 gene, but was unable to detect a second variant in this gene. Coverage analysis and Sanger sequencing led to identification of a novel intronic deletion NM_024678.5(NARS2):c.922-21_922-19del in the three siblings in trans with the c.822G > C. Functional analysis by RT-PCR showed that this deletion was causing aberrant splicing and led to exon 9 skipping in NARS2 mRNA in patient fibroblasts. Our work expands the phenotype and genotype spectrum of NARS2-related disorders. We provide evidence of the pathogenic effect of a novel intronic deletion in the NARS2 gene and report on additional adult patients with a large intrafamilial variability associated with splice variants in this gene. More specifically, we detail the phenotype of the oldest living patient to date with NARS2 variants and, for the first time, we report the psychiatric symptoms associated with this gene. Our work confirms the complexity of genotype-phenotype correlation in patients with pathogenic NARS2 variants.

A recurrent RYR1 mutation associated with early-onset hypotonia and benign disease course.

The ryanodine receptor RyR1 is the main sarcoplasmic reticulum Ca2+ channel in skeletal muscle and acts as a connecting link between electrical stimulation and Ca2+-dependent muscle contraction. Abnormal RyR1 activity compromises normal muscle function and results in various human disorders including malignant hyperthermia, central core disease, and centronuclear myopathy. However, RYR1 is one of the largest genes of the human genome and accumulates numerous missense variants of uncertain significance (VUS), precluding an efficient molecular diagnosis for many patients and families. Here we describe a recurrent RYR1 mutation previously classified as VUS, and we provide clinical, histological, and genetic data supporting its pathogenicity. The heterozygous c.12083C>T (p.Ser4028Leu) mutation was found in thirteen patients from nine unrelated congenital myopathy families with consistent clinical presentation, and either segregated with the disease in the dominant families or occurred de novo. The affected individuals essentially manifested neonatal or infancy-onset hypotonia, delayed motor milestones, and a benign disease course differing from classical RYR1-related muscle disorders. Muscle biopsies showed unspecific histological and ultrastructural findings, while RYR1-typical cores and internal nuclei were seen only in single patients. In conclusion, our data evidence the causality of the RYR1 c.12083C>T (p.Ser4028Leu) mutation in the development of an atypical congenital myopathy with gradually improving motor function over the first decades of life, and may direct molecular diagnosis for patients with comparable clinical presentation and unspecific histopathological features on the muscle biopsy.

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.

DNAJC3 deficiency induces ß-cell mitochondrial apoptosis and causes syndromic young-onset diabetes.

OBJECTIVE: DNAJC3, also known as P58IPK, is an Hsp40 family member that interacts with and inhibits PKR-like ER-localized eIF2α kinase (PERK). Dnajc3 deficiency in mice causes pancreatic ß-cell loss and diabetes. Loss-of-function mutations in DNAJC3 cause early-onset diabetes and multisystemic neurodegeneration. The aim of our study was to investigate the genetic cause of early-onset syndromic diabetes in two unrelated patients, and elucidate the mechanisms of ß-cell failure in this syndrome. METHODS: Whole exome sequencing was performed and identified variants were confirmed by Sanger sequencing. DNAJC3 was silenced by RNAi in INS-1E cells, primary rat ß-cells, human islets, and induced pluripotent stem cell-derived ß-cells. ß-cell function and apoptosis were assessed, and potential mediators of apoptosis examined. RESULTS: The two patients presented with juvenile-onset diabetes, short stature, hypothyroidism, neurodegeneration, facial dysmorphism, hypoacusis, microcephaly and skeletal bone deformities. They were heterozygous compound and homozygous for novel loss-of-function mutations in DNAJC3. DNAJC3 silencing did not impair insulin content or secretion. Instead, the knockdown induced rat and human ß-cell apoptosis and further sensitized cells to endoplasmic reticulum stress, triggering mitochondrial apoptosis via the pro-apoptototic Bcl-2 proteins BIM and PUMA. CONCLUSIONS: This report confirms previously described features and expands the clinical spectrum of syndromic DNAJC3 diabetes, one of the five monogenic forms of diabetes pertaining to the PERK pathway of the endoplasmic reticulum stress response. DNAJC3 deficiency may lead to ß-cell loss through BIM- and PUMA-dependent activation of the mitochondrial pathway of apoptosis.

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.

Single Circulating Fetal Trophoblastic Cells Eligible for Non Invasive Prenatal Diagnosis: the Exception Rather than the Rule.

Non-Invasive Prenatal Diagnosis (NIPD), based on the analysis of circulating cell-free fetal DNA (cff-DNA), is successfully implemented for an increasing number of monogenic diseases. However, technical issues related to cff-DNA characteristics remain, and not all mutations can be screened with this method, particularly triplet expansion mutations that frequently concern prenatal diagnosis requests. The objective of this study was to develop an approach to isolate and analyze Circulating Trophoblastic Fetal Cells (CFTCs) for NIPD of monogenic diseases caused by triplet repeat expansion or point mutations. We developed a method for CFTC isolation based on DEPArray sorting and used Huntington's disease as the clinical model for CFTC-based NIPD. Then, we investigated whether CFTC isolation and Whole Genome Amplification (WGA) could be used for NIPD in couples at risk of transmitting different monogenic diseases. Our data show that the allele drop-out rate was 3-fold higher in CFTCs than in maternal cells processed in the same way. Moreover, we give new insights into CFTCs by compiling data obtained by extensive molecular testing by microsatellite multiplex PCR genotyping and by WGA followed by mini-exome sequencing. CFTCs appear to be often characterized by a random state of genomic degradation.

Single-fiber studies for assigning pathogenicity of eight mitochondrial DNA variants associated with mitochondrial diseases.

Whole mitochondrial DNA (mtDNA) sequencing is now systematically used in clinical laboratories to screen patients with a phenotype suggestive of mitochondrial disease. Next Generation Sequencing (NGS) has significantly increased the number of identified pathogenic mtDNA variants. Simultaneously, the number of variants of unknown significance (VUS) has increased even more, thus challenging their interpretation. Correct classification of the variants' pathogenicity is essential for optimal patient management, including treatment and genetic counseling. Here, we used single muscle fiber studies to characterize eight heteroplasmic mtDNA variants, among which were three novel variants. By applying the pathogenicity scoring system, we classified four variants as "definitely pathogenic" (m.590A>G, m.9166T>C, m.12293G>A, and m.15958A>T). Two variants remain "possibly pathogenic" (m.4327T>C and m.5672T>C) but should these be reported in a different family, they would be reclassified as "definitely pathogenic." We also illustrate the contribution of single-fiber studies to the diagnostic approach in patients harboring pathogenic variants with low level heteroplasmy.

Novel CCM2 missense variants abrogating the CCM1-CCM2 interaction cause cerebral cavernous malformations.

BACKGROUND: Cerebral cavernous malformations (CCMs) are vascular malformations mostly located within the central nervous system. Most deleterious variants are loss of function mutations in one of the three CCM genes. These genes code for proteins that form a ternary cytosolic complex with CCM2 as a hub. Very few CCM2 missense variants have been shown to be deleterious by modifying the ternary CCM complex stability. OBJECTIVES: To investigate the causality of novel missense CCM2 variants detected in patients with CCM. METHODS: The three CCM genes were screened in 984 patients referred for CCM molecular screening. Interaction between CCM1 and CCM2 proteins was tested using co-immunoprecipitation experiments for the CCM2 missense variants located in the phosphotyrosine binding (PTB) domain. RESULTS: 11 distinct CCM2 rare missense variants were found. Six variants predicted to be damaging were located in the PTB domain, four of them were novel. When co-transfected with CCM1 in HEK293T cells, a loss of interaction between CCM1 and CCM2 was observed for all six variants. CONCLUSION: We showed, using co-immunoprecipitation experiments, that CCM2 missense variants located in the PTB domain were actually damaging by preventing the normal interaction between CCM1 and CCM2. These data are important for diagnosis and genetic counselling, which are challenging in patients harbouring such variants.

A novel variant m.8561C>T in the overlapping region of MT-ATP6 and MT-ATP8 in a child with early-onset severe neurological signs.

Among mitochondrial diseases, isolated complex V (CV) deficiency represents a rare cause of respiratory chain (RC) dysfunction. In mammalian mitochondrial DNA (mtDNA), MT-ATP6 partly overlaps with MT-ATP8 making double mutations possible, yet extremely rarely reported principally in patients with cardiomyopathy. Here, we report a novel m.8561 C>T substitution in the overlapping region of MT-ATP6 and MT-ATP8 in a child with early-onset ataxia, psychomotor delay and microcephaly, enlarging the clinical manifestations spectrum associated with CV deficiency.

Cholic acid as a treatment for cerebrotendinous xanthomatosis in adults.

Cerebrotendineous xanthomatosis (CTX) is an autosomal recessive disorder of bile acids synthesis. Patients may present with a variety of clinical manifestations: bilateral cataract and chronic diarrhea during childhood, then occurrence of neurological debilitating symptoms in adulthood (cognitive decline, motor disorders). Plasma cholestanol is used as a diagnostic marker of CTX, and to monitor the response to the treatment. Current treatment for CTX is chenodeoxycholic acid (CDCA), which was reported to improve and/or stabilize clinical status and decrease levels of plasma cholestanol. Rare published reports have also suggested a potential efficacy of cholic acid (CA) in patients with CTX. In this retrospective Franco-Belgian multicentric study, we collected data from 12 patients treated with CA, evaluating their clinical status, cholestanol levels and adverse effects during the treatment period. The population was divided in two subgroups: treatment-naive (who never had CDCA prior to CA) and non-treatment-naive patients (who had CDCA prior to CA introduction). We found that treatment with CA significantly and strongly reduced cholestanol levels in all patients. Additionally, 10 out of 12 patients clinically improved or stabilized with CA treatment. Worsening was noted in one treatment-naïve patient and one non-treatment-naïve patient, but both patients experienced similar outcomes with CDCA treatment as well. No adverse effects were reported from patients with CA treatment, whereas elevated transaminases were observed in some patients while they were treated with CDCA. In conclusion, these findings suggest that CA may be a suitable alternative treatment for CTX, especially in patients with side effects related to CDCA.

NDUFS6 related Leigh syndrome: a case report and review of the literature.

The genetic causes of Leigh syndrome are heterogeneous, with a poor genotype-phenotype correlation. To date, more than 50 nuclear genes cause nuclear gene-encoded Leigh syndrome. NDUFS6 encodes a 13 kiloDaltons subunit, which is part of the peripheral arm of complex I and is localized in the iron-sulfur fraction. Only a few patients were reported with proven NDUFS6 pathogenic variants and all presented with severe neonatal lactic acidemia and complex I deficiency, leading to death in the first days of life. Here, we present a patient harboring two NDUFS6 variants with a phenotype compatible with Leigh syndrome. Although most of previous reports suggested that NDUFS6 pathogenic variants invariably lead to early neonatal death, this report shows that the clinical spectrum could be larger. We found a severe decrease of NDUFS6 protein level in patient's fibroblasts associated with a complex I assembly defect in patient's muscle and fibroblasts. These data confirm the importance of NDUFS6 and the Zn-finger domain for a correct assembly of complex I.

Whole genome paired-end sequencing elucidates functional and phenotypic consequences of balanced chromosomal rearrangement in patients with developmental disorders.

BACKGROUND: Balanced chromosomal rearrangements associated with abnormal phenotype are rare events, but may be challenging for genetic counselling, since molecular characterisation of breakpoints is not performed routinely. We used next-generation sequencing to characterise breakpoints of balanced chromosomal rearrangements at the molecular level in patients with intellectual disability and/or congenital anomalies. METHODS: Breakpoints were characterised by a paired-end low depth whole genome sequencing (WGS) strategy and validated by Sanger sequencing. Expression study of disrupted and neighbouring genes was performed by RT-qPCR from blood or lymphoblastoid cell line RNA. RESULTS: Among the 55 patients included (41 reciprocal translocations, 4 inversions, 2 insertions and 8 complex chromosomal rearrangements), we were able to detect 89% of chromosomal rearrangements (49/55). Molecular signatures at the breakpoints suggested that DNA breaks arose randomly and that there was no major influence of repeated elements. Non-homologous end-joining appeared as the main mechanism of repair (55% of rearrangements). A diagnosis could be established in 22/49 patients (44.8%), 15 by gene disruption (KANSL1, FOXP1, SPRED1, TLK2, MBD5, DMD, AUTS2, MEIS2, MEF2C, NRXN1, NFIX, SYNGAP1, GHR, ZMIZ1) and 7 by position effect (DLX5, MEF2C, BCL11B, SATB2, ZMIZ1). In addition, 16 new candidate genes were identified. Systematic gene expression studies further supported these results. We also showed the contribution of topologically associated domain maps to WGS data interpretation. CONCLUSION: Paired-end WGS is a valid strategy and may be used for structural variation characterisation in a clinical setting.

Molecular diagnosis of inherited peripheral neuropathies by targeted next-generation sequencing: molecular spectrum delineation.

PURPOSE: Inherited peripheral neuropathies (IPN) represent a large heterogenous group of hereditary diseases with more than 100 causative genes reported to date. In this context, targeted next-generation sequencing (NGS) offers the opportunity to screen all these genes with high efficiency in order to unravel the genetic basis of the disease. Here, we compare the diagnostic yield of targeted NGS with our previous gene by gene Sanger sequencing strategy. We also describe several novel likely pathogenic variants. DESIGN AND PARTICIPANTS: We have completed the targeted NGS of 81 IPN genes in a cohort of 123 unrelated patients affected with diverse forms of IPNs, mostly Charcot-Marie-Tooth disease (CMT): 23% CMT1, 52% CMT2, 9% distal hereditary motor neuropathy, 7% hereditary sensory and autonomic neuropathy and 6.5% intermediate CMT. RESULTS: We have solved the molecular diagnosis in 49 of 123 patients (~40%). Among the identified variants, 26 variants were already reported in the literature. In our cohort, the most frequently mutated genes are respectively: MFN2, SH3TC2, GDAP1, NEFL, GAN, KIF5A and AARS. Panel-based NGS was more efficient in familial cases than in sporadic cases (diagnostic yield 49%vs19%, respectively). NGS-based search for copy number variations, allowed the identification of three duplications in three patients and raised the diagnostic yield to 41%. This yield is two times higher than the one obtained previously by gene Sanger sequencing screening. The impact of panel-based NGS screening is particularly important for demyelinating CMT (CMT1) subtypes, for which the success rate reached 87% (36% only for axonal CMT2). CONCLUSION: NGS allowed to identify causal mutations in a shorter and cost-effective time. Actually, targeted NGS is a well-suited strategy for efficient molecular diagnosis of IPNs. However, NGS leads to the identification of numerous variants of unknown significance, which interpretation requires interdisciplinary collaborations between molecular geneticists, clinicians and (neuro)pathologists.

Targeted next generation sequencing with an extended gene panel does not impact variant detection in mitochondrial diseases.

BACKGROUND: Since the advent of next generation sequencing (NGS), several studies have tried to evaluate the relevance of targeted gene panel sequencing and whole exome sequencing for molecular diagnosis of mitochondrial diseases. The comparison between these different strategies is extremely difficult. A recent study analysed a cohort of patients affected by a mitochondrial disease using a NGS approach based on a targeted gene panel including 132 genes. This strategy led to identify the causative mutations in 15.2% of cases. The number of novel genes responsible for respiratory chain deficiency increases very rapidly. METHODS: In order to determine the impact of larger panels used as a first screening strategy on molecular diagnosis success, we analysed a cohort of 80 patients affected by a mitochondrial disease with a first mitochondrial DNA (mtDNA) NGS screening and secondarily a targeted mitochondrial panel of 281 nuclear genes. RESULTS: Pathogenic mtDNA abnormalities were identified in 4.1% (1/24) of children and 25% (14/56) of adult patients. The remaining 65 patients were analysed with our targeted mitochondrial panel and this approach enabled us to achieve an identification rate of 21.7% (5/23) in children versus 7.1% (3/42) in adults. CONCLUSIONS: Our results confirm that larger gene panels do not improve diagnostic yield of mitochondrial diseases due to (i) their very high genetic heterogeneity, (ii) the ongoing discovery of novel genes and (iii) mutations in genes apparently not related to mitochondrial function that lead to secondary respiratory chain deficiency.