Identification of Disease-Causing Mutations by Functional Complementation of Patient-Derived Fibroblast Cell Lines

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

Identification of Autophagy as a Functional Target Suitable for the Pharmacological Treatment of Mitochondrial Membrane Protein-Associated Neurodegeneration (MPAN) In Vitro

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is a relentlessly progressive neurodegenerative disorder caused by mutations in the C19orf12 gene. C19orf12 has been implicated in playing a role in lipid metabolism, mitochondrial function, and autophagy, however, the precise functions remain unknown. To identify new robust cellular targets for small compound treatments, we evaluated reported mitochondrial function alterations, cellular signaling, and autophagy in a large cohort of MPAN patients and control fibroblasts. We found no consistent alteration of mitochondrial functions or cellular signaling messengers in MPAN fibroblasts. In contrast, we found that autophagy initiation is consistently impaired in MPAN fibroblasts and show that C19orf12 expression correlates with the amount of LC3 puncta, an autophagy marker. Finally, we screened 14 different autophagy modulators to test which can restore this autophagy defect. Amongst these compounds, carbamazepine, ABT-737, LY294002, oridonin, and paroxetine could restore LC3 puncta in the MPAN fibroblasts, identifying them as novel potential therapeutic compounds to treat MPAN. In summary, our study confirms a role for C19orf12 in autophagy, proposes LC3 puncta as a functionally robust and consistent readout for testing compounds, and pinpoints potential therapeutic compounds for MPAN.

Identification and characterization of novel MPC1 gene variants causing mitochondrial pyruvate carrier deficiency

Pyruvate, the end product of glycolysis, is a key metabolic molecule enabling mitochondrial adenosine triphosphate synthesis and takes part in multiple biosynthetic pathways within mitochondria. The mitochondrial pyruvate carrier (MPC) plays a vital role in transporting pyruvate from the cytosol into the organelle. In humans, MPC is a hetero-oligomeric complex formed by the MPC1 and MPC2 paralogs that are both necessary to stabilize each other and form a functional MPC. MPC deficiency (OMIM#614741) due to pathogenic MPC1 variants is a rare autosomal recessive disease involving developmental delay, microcephaly, growth failure, and increased serum lactate and pyruvate. To date, two MPC1 variants in four cases have been reported, though only one with a detailed clinical description. Herein, we report three novel pathogenic MPC1 variants in six patients from three unrelated families, identified within European, Kuwaiti, and Chinese mitochondrial disease patient cohorts, one of whom presented as a Leigh-like syndrome. Functional analysis in primary fibroblasts from the patients revealed decreased expression of MPC1 and MPC2. We rescued pyruvate-driven oxygen consumption rate in patient's fibroblasts by reconstituting with wild-type MPC1. Complementing homozygous MPC1 mutant cDNA with CRISPR-deleted MPC1 C2C12 cells verified the mechanism of variants: unstable MPC complex or ablated pyruvate uptake activity. Furthermore, we showed that glutamine and beta-hydroxybutyrate were alternative substrates to maintain mitochondrial respiration when cells lack pyruvate. In conclusion, we expand the clinical phenotypes and genotypes associated with MPC deficiency, with our studies revealing glutamine as a potential therapy for MPC deficiency.

Characterising a homozygous two-exon deletion in UQCRH: comparing human and mouse phenotypes

Mitochondrial disorders are clinically and genetically diverse, with isolated complex III (CIII) deficiency being relatively rare. Here, we describe two affected cousins, presenting with recurrent episodes of severe lactic acidosis, hyperammonaemia, hypoglycaemia and encephalopathy. Genetic investigations in both cases identified a homozygous deletion of exons 2 and 3 of UQCRH, which encodes a structural complex III (CIII) subunit. We generated a mouse model with the equivalent homozygous Uqcrh deletion (Uqcrh-/- ), which also presented with lactic acidosis and hyperammonaemia, but had a more severe, non-episodic phenotype, resulting in failure to thrive and early death. The biochemical phenotypes observed in patient and Uqcrh-/- mouse tissues were remarkably similar, displaying impaired CIII activity, decreased molecular weight of fully assembled holoenzyme and an increase of an unexpected large supercomplex (SXL ), comprising mostly of one complex I (CI) dimer and one CIII dimer. This phenotypic similarity along with lentiviral rescue experiments in patient fibroblasts verifies the pathogenicity of the shared genetic defect, demonstrating that the Uqcrh-/- mouse is a valuable model for future studies of human CIII deficiency.

Phenotypes and genotypes of mitochondrial diseases with mtDNA variations in Chinese children: A multi-center study

Mitochondrial DNA (mtDNA) associated mitochondrial diseases hold a crucial position but comprehensive and systematic studies are relatively rare. Among the 262 patients of four children's hospitals in China, 96%-point mutations (30 alleles in 11 genes encoding tRNA, rRNA, Complex I and V) and 4%-deletions (seven of ten had not been reported before) were identified as the cause of 14 phenotypes. MILS presented the highest genetic heterogeneity, while the m.3243A > G mutation was the only "hotspot" mutation with a wide range of phenotypes. The degrees of heteroplasmy in the leukocytes of MM were higher than MELAS. The heteroplasmy level of patients was higher than that in mild and carrier group, while we found low-level heteroplasmy pathogenic mutations as well. Some homoplasmic variations (e.g., m.9176 T > C mutation) are having high incomplete penetrance. For a suspected MELAS, m.3243A > G mutation was recommended to detect first; while for a suspected LS, trios-WES and mtDNA genome sequencing by NGS were recommended first in both blood and urine.

Identification of a novel m.3955G > A variant in MT-ND1 associated with Leigh syndrome

Leigh syndrome (LS) is one of the most common mitochondrial diseases in children, for which at least 90 causative genes have been identified. However, many LS patients have no genetic diagnosis, indicating that more disease-related genes remain to be identified. In this study, we identified a novel variant, m.3955G > A, in mitochondrially encoded NADH:ubiquinone oxidoreductase core subunit 1 (MT-ND1) in two unrelated LS patients, manifesting as infancy-onset frequent seizures, neurodegeneration, elevated lactate levels, and bilateral symmetrical lesions in the brainstem, basal ganglia, and thalamus. Transfer of the mutant mtDNA with m.3955G > A into cybrids disturbed the MT-ND1 expression and CI assembly, followed by remarkable mitochondrial dysfunction, reactive oxygen species production, and mitochondrial membrane potential reduction. Our findings demonstrated the pathogenicity of the novel m.3955G > A variant, and extend the spectrum of pathogenic mtDNA variants.

Biallelic COA7-Variants Leading to Developmental Regression With Progressive Spasticity and Brain Atrophy in a Chinese Patient

Objective

The cytochrome c oxidase assembly factor 7 (COA7) gene encodes a protein localized to mitochondria that is involved in the assembly of mitochondrial respiratory chain complex IV. Here, we report the clinical, genetic and biochemical analysis of a female patient with suspected mitochondrial disorder and novel variants in COA7, that presented with a considerably different phenotype and age of onset than the five COA7 patients reported to date.

Methods

We performed trio-exome sequencing in the affected patient and both parents. To verify the pathogenicity of the detected variants in COA7, mitochondrial enzyme activities and oxygen consumption rate were investigated in fibroblasts of the patient and her parents.

Results

A Chinese girl was referred at 9 months of age with a history of developmental delay and regression since 3 months of age. In the following months, she lost previously acquired skills and developed progressive spasticity of the lower extremities. Trio-exome sequencing revealed compound heterzygous variants in COA7 (c.511G > A/p.Ala171Thr and c.566A > G/p.Asn189Ser). Functional validation experiments revealed isolated complex IV deficiency and a significantly reduced mitochondrial respiration rate in patient-derived fibroblasts.

Interpretation

Hitherto, characteristic features of COA7 patients were described as slowly progressing neuropathy and spinocerebellar ataxia, starting at the toddler age and progressing into adulthood. In contrast, our patient was reported to show developmental delay from 3 months of age, which was found to be due to a rapidly progressive encephalopathy and brain atrophy seen at 9 months of age. Unexpectedly, the genetic investigation revealed a COA7-associated mitochondrial disease, which was confirmed functionally. Thus, this report broadens the genetic and clinical spectrum of this heterogeneous mitochondriopathy and highlights the value of the presented unbiased approach.

Case Report: A Variant Non-ketotic Hyperglycinemia With GLRX5 Mutations: Manifestation of Deficiency of Activities of the Respiratory Chain Enzymes

Objective: Variant non-ketotic hyperglycinaemia (NKH) is a rare disorder characterized by variable clinical, biochemical, and imaging features. The variant form of NKH is rare and characterized by variable clinical, biochemical and imaging features. Subjects: Herein, we report a girl with variant NKH with two mutations in glutaredoxin 5 (GLRX5), which has been described in only three patients. Results: The clinical and biochemical phenotypes of the patient are also described. She suffered from developmental regression associated with spasticity, developmental delay, anemia and optic atrophy. The mitochondrial leukoencephalopathy was used to designate these disorders. An increased T2 signal from the medulla oblongata to the C6 spinal region was also observed on spinal cord MRI. Tandem mass analysis of a dried blood sample revealed elevated levels of glycine. The patient has two compound heterozygous mutations (c.151_153 del AAG and c.196C>T) in the GLRX5 gene. The c.196C>T mutation led to a stop codon (p.Q66Ter). Activities of mitochondrial respiratory chain (MRC) complexes II+III in the patient's fibroblasts were abnormal. Conclusions: We present the case of a girl with variant NKH who manifested spasticity and bilateral cavitating leukoencephalopathy. The patient had a deficiency of a respiratory chain enzyme, and this is the first report. Genetic testing is important for physicians to evaluate suspected variant NKH patients and to provide proper genetic counseling.

Identification of a Novel Variant in MT-CO3 Causing MELAS

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) is a maternally inherited mitochondrial disease. Most cases of MELAS are caused by the m.3243A > G variant in the MT-TL1 gene encoding tRNALeu^(UUR). However, the genetic cause in 10% of patients with MELAS is unknown. We investigated the pathogenicity of the novel mtDNA variant m.9396G > A/MT-CO3 (p.E64K), which affects an extremely conserved amino acid in the CO3 subunit of mitochondrial respiratory chain (MRC) complex IV (CIV) in a patient with MELAS. Biochemical assays of a muscle biopsy confirmed remarkable CIV deficiency, and pathological examination showed ragged red fibers and generalized COX non-reactive muscle fibers. Transfer of the mutant mtDNA into cybrids impaired CIV assembly, followed by remarkable mitochondrial dysfunction and ROS production. Our findings highlight the pathogenicity of a novel m.9396G > A variant and extend the spectrum of pathogenic mtDNA variants.

Whole genome and exome sequencing identify NDUFV2 mutations as a new cause of progressive cavitating leukoencephalopathy

Background

Progressive cavitating leukoencephalopathy (PCL) is thought to result from mutations in nuclear genes affecting mitochondrial function and energy metabolism. To date, mutations in two subunits of complex I, NDUFS1 and NDUFV1, have been reported to be related to PCL.

Methods

Patients underwent clinical examinations, brain MRI, skin biopsy and muscle biopsy. Whole-genome or whole-exome sequencing was performed on the index patients from two unrelated families with PCL. The effects of the mutations were examined through complementation of the NDUFV2 mutation by cDNA expression.

Results

The common clinical features of the patients in this study were recurring episodes of acute or subacute developmental regression that appeared in the first years of life, followed by gradual remissions and prolonged periods of stability. MRI showed leukoencephalopathy with multiple cavities. Three novel NDUFV2 missense mutations were identified in these families. Complex I deficiency was confirmed in affected individuals’ fibroblasts and a muscle biopsy. Functional and structural analyses revealed that these mutations affect the structural stability and function of the NDUFV2 protein, indicating that defective NDUFV2 function is responsible for the phenotypes in these individuals.

Conclusions

Here, we report the clinical presentations, neuroimaging and molecular and functional analyses of novel mutations in NDUFV2 in two sibling pairs of two Chinese families presenting with PCL. We hereby expand the knowledge on the clinical phenotypes associated with mutations in NDUFV2 and the genotypes causative for PCL.

Impaired complex I repair causes recessive Leber's hereditary optic neuropathy

Leber’s hereditary optic neuropathy (LHON) is the most frequent mitochondrial disease and was the first to be genetically defined by a point mutation in mitochondrial DNA (mtDNA). A molecular diagnosis is achieved in up to 95% of cases, the vast majority of which are accounted for by 3 mutations within mitochondrial complex I subunit–encoding genes in the mtDNA (mtLHON). Here, we resolve the enigma of LHON in the absence of pathogenic mtDNA mutations. We describe biallelic mutations in a nuclear encoded gene, DNAJC30, in 33 unsolved patients from 29 families and establish an autosomal recessive mode of inheritance for LHON (arLHON), which to date has been a prime example of a maternally inherited disorder. Remarkably, all hallmarks of mtLHON were recapitulated, including incomplete penetrance, male predominance, and significant idebenone responsivity. Moreover, by tracking protein turnover in patient-derived cell lines and a DNAJC30-knockout cellular model, we measured reduced turnover of specific complex I N-module subunits and a resultant impairment of complex I function. These results demonstrate that DNAJC30 is a chaperone protein needed for the efficient exchange of complex I subunits exposed to reactive oxygen species and integral to a mitochondrial complex I repair mechanism, thereby providing the first example to our knowledge of a disease resulting from impaired exchange of assembled respiratory chain subunits.

Whole exome sequencing identifies a novel homozygous MECR mutation in a Chinese patient with childhood-onset dystonia and basal ganglia abnormalities, without optic atrophy

Childhood-onset dystonia with optic atrophy and basal ganglia abnormalities is an extremely rare autosomal recessive mitochondrial disease caused by biallelic mutations in MECR. Using whole-exome sequencing, we identified a novel homozygous MECR mutation (c.910G > T, p.Asp304Tyr) in a Chinese patient with childhood-onset dystonia and basal ganglia abnormalities, without optic atrophy. With lipoic acid treatment, the disease progression was under control, and neither visual impairment nor optic atrophy was observed. To our knowledge, this is the first study about MECR-related mitochondrial disease in a Chinese patient and the first to report that supplementation with lipoic acid is a possible effective therapeutic strategy for this disease.

Novel ECHS1 mutations in Leigh syndrome identified by whole-exome sequencing in five Chinese families: case report

Background

Short-chain enoyl-CoA hydratase deficiency (ECHS1D), also known as ECHS1 deficiency, is a rare inborn metabolic disorder with clinical presentations characterized by Leigh syndrome (LS). Thirty-four different pathogenic mutations have been identified from over 40 patients to date.

Case presentation

Here, we report five Chinese patients with clinical syndromes typified as LS. Despite different initial symptoms, all patients presented developmental regression, dystonia, common radiological features such as symmetrical bilateral brain abnormalities, and similar metabolic results such as elevated plasma lactate and 2,3-dihydroxy-2-methylbutyrate. Utilizing whole-exome sequencing (WES), we identified eight distinct variants in ECHS1, with six novel variants, and the remaining two variants have been previously reported. Interestingly, one of the six novel variants, c.463G > A (p.Gly155Ser), was detected in three patients from unrelated families, suggesting a potential founder effect already described for a few mutations in LS. Incorporating both genetic analysis and medical results, including magnetic resonance imaging (MRI), electroencephalography (EEG), and biochemical testing, our study enriched the mutation spectrum of the ECHS1 gene and confirmed the phenotypic presentations of LS.

Conclusions

The severity of ECHS1 deficiency seems to vary. It was affected by both genetics and external environmental factors that lead to increased metabolism. Our study enriched the mutation spectrum of the ECHS1 gene, confirmed the phenotypic presentations, and highlighted the importance of the valine catabolic pathway in Leigh syndrome. Further studies are required to examine the potential founder mutation c.463G > A (p.Gly155Ser) and the role of ECHS1 in relevant pathways.

Genetics of mitochondrial diseases: Identifying mutations to help diagnosis

Mitochondrial diseases are amongst the most genetically and phenotypically diverse groups of inherited diseases. The vast phenotypic overlap with other disease entities together with the absence of reliable biomarkers act as driving forces for the integration of unbiased methodologies early in the diagnostic algorithm, such as whole exome sequencing (WES) and whole genome sequencing (WGS). Such approaches are used in variant discovery and in combination with high-throughput functional assays such as transcriptomics in simultaneous variant discovery and validation. By capturing all genes, they not only increase the diagnostic rate in heterogenous mitochondrial disease patients, but accelerate novel disease gene discovery, and are valuable in side-stepping the risk of overlooking unexpected or even treatable genetic disease diagnoses.

Bi-Allelic UQCRFS1 Variants Are Associated with Mitochondrial Complex III Deficiency, Cardiomyopathy, and Alopecia Totalis

Isolated complex III (CIII) deficiencies are among the least frequently diagnosed mitochondrial disorders. Clinical symptoms range from isolated myopathy to severe multi-systemic disorders with early death and disability. To date, we know of pathogenic variants in genes encoding five out of 10 subunits and five out of 13 assembly factors of CIII. Here we describe rare bi-allelic variants in the gene of a catalytic subunit of CIII, UQCRFS1, which encodes the Rieske iron-sulfur protein, in two unrelated individuals. Affected children presented with low CIII activity in fibroblasts, lactic acidosis, fetal bradycardia, hypertrophic cardiomyopathy, and alopecia totalis. Studies in proband-derived fibroblasts showed a deleterious effect of the variants on UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular respiration. Complementation studies via lentiviral transduction and overexpression of wild-type UQCRFS1 restored mitochondrial function and rescued the cellular phenotype, confirming UQCRFS1 variants as causative for CIII deficiency. We demonstrate that mutations in UQCRFS1 can cause mitochondrial disease, and our results thereby expand the clinical and mutational spectrum of CIII deficiencies.

The diagnosis of inborn errors of metabolism by an integrative "multi-omics" approach: A perspective encompassing genomics, transcriptomics, and proteomics

Given the rapidly decreasing cost and increasing speed and accessibility of massively parallel technologies, the integration of comprehensive genomic, transcriptomic, and proteomic data into a "multi-omics" diagnostic pipeline is within reach. Even though genomic analysis has the capability to reveal all possible perturbations in our genetic code, analysis typically reaches a diagnosis in just 35% of cases, with a diagnostic gap arising due to limitations in prioritization and interpretation of detected variants. Here we review the utility of complementing genetic data with transcriptomic data and give a perspective for the introduction of proteomics into the diagnostic pipeline. Together these methodologies enable comprehensive capture of the functional consequence of variants, unobtainable by the analysis of each methodology in isolation. This facilitates functional annotation and reprioritization of candidate genes and variants-a promising approach to shed light on the underlying molecular cause of a patient's disease, increasing diagnostic rate, and allowing actionability in clinical practice.

Arabidopsis thaliana alternative dehydrogenases: a potential therapy for mitochondrial complex I deficiency? Perspectives and pitfalls

BACKGROUND

Complex I (CI or NADH:ubiquinone oxidoreductase) deficiency is the most frequent cause of mitochondrial respiratory chain defect. Successful attempts to rescue CI function by introducing an exogenous NADH dehydrogenase, such as the NDI1 from Saccharomyces cerevisiae (ScNDI1), have been reported although with drawbacks related to competition with CI. In contrast to ScNDI1, which is permanently active in yeast naturally devoid of CI, plant alternative NADH dehydrogenases (NDH-2) support the oxidation of NADH only when the CI is metabolically inactive and conceivably when the concentration of matrix NADH exceeds a certain threshold. We therefore explored the feasibility of CI rescue by NDH-2 from Arabidopsis thaliana (At) in human CI defective fibroblasts.

RESULTS

We showed that, other than ScNDI1, two different NDH-2 (AtNDA2 and AtNDB4) targeted to the mitochondria were able to rescue CI deficiency and decrease oxidative stress as indicated by a normalization of SOD activity in human CI-defective fibroblasts. We further demonstrated that when expressed in human control fibroblasts, AtNDA2 shows an affinity for NADH oxidation similar to that of CI, thus competing with CI for the oxidation of NADH as opposed to our initial hypothesis. This competition reduced the amount of ATP produced per oxygen atom reduced to water by half in control cells.

CONCLUSIONS

In conclusion, despite their promising potential to rescue CI defects, due to a possible competition with remaining CI activity, plant NDH-2 should be regarded with caution as potential therapeutic tools for human mitochondrial diseases.

Clinical, biochemical and genetic spectrum of 70 patients with ACAD9 deficiency: is riboflavin supplementation effective?

BACKGROUND

Mitochondrial acyl-CoA dehydrogenase family member 9 (ACAD9) is essential for the assembly of mitochondrial respiratory chain complex I. Disease causing biallelic variants in ACAD9 have been reported in individuals presenting with lactic acidosis and cardiomyopathy.

RESULTS

We describe the genetic, clinical and biochemical findings in a cohort of 70 patients, of whom 29 previously unpublished. We found 34 known and 18 previously unreported variants in ACAD9. No patients harbored biallelic loss of function mutations, indicating that this combination is unlikely to be compatible with life. Causal pathogenic variants were distributed throughout the entire gene, and there was no obvious genotype-phenotype correlation. Most of the patients presented in the first year of life. For this subgroup the survival was poor (50% not surviving the first 2 years) comparing to patients with a later presentation (more than 90% surviving 10 years). The most common clinical findings were cardiomyopathy (85%), muscular weakness (75%) and exercise intolerance (72%). Interestingly, severe intellectual deficits were only reported in one patient and severe developmental delays in four patients. More than 70% of the patients were able to perform the same activities of daily living when compared to peers.

CONCLUSIONS

Our data show that riboflavin treatment improves complex I activity in the majority of patient-derived fibroblasts tested. This effect was also reported for most of the treated patients and is mirrored in the survival data. In the patient group with disease-onset below 1 year of age, we observed a statistically-significant better survival for patients treated with riboflavin.

Mutations in PPCS, Encoding Phosphopantothenoylcysteine Synthetase, Cause Autosomal-Recessive Dilated Cardiomyopathy

Coenzyme A (CoA) is an essential metabolic cofactor used by around 4% of cellular enzymes. Its role is to carry and transfer acetyl and acyl groups to other molecules. Cells can synthesize CoA de novo from vitamin B5 (pantothenate) through five consecutive enzymatic steps. Phosphopantothenoylcysteine synthetase (PPCS) catalyzes the second step of the pathway during which phosphopantothenate reacts with ATP and cysteine to form phosphopantothenoylcysteine. Inborn errors of CoA biosynthesis have been implicated in neurodegeneration with brain iron accumulation (NBIA), a group of rare neurological disorders characterized by accumulation of iron in the basal ganglia and progressive neurodegeneration. Exome sequencing in five individuals from two unrelated families presenting with dilated cardiomyopathy revealed biallelic mutations in PPCS, linking CoA synthesis with a cardiac phenotype. Studies in yeast and fruit flies confirmed the pathogenicity of identified mutations. Biochemical analysis revealed a decrease in CoA levels in fibroblasts of all affected individuals. CoA biosynthesis can occur with pantethine as a source independent from PPCS, suggesting pantethine as targeted treatment for the affected individuals still alive.

The functional genomics laboratory: functional validation of genetic variants

Currently, one of the main challenges in human molecular genetics is the interpretation of rare genetic variants of unknown clinical significance. A conclusive diagnosis is of importance for the patient to obtain certainty about the cause of the disease, for the clinician to be able to provide optimal care to the patient and to predict the disease course, and for the clinical geneticist for genetic counseling of the patient and family members. Conclusive evidence for pathogenicity of genetic variants is therefore crucial. This review gives an introduction to the problem of the interpretation of genetic variants of unknown clinical significance in view of the recent advances in genetic screening, and gives an overview of the possibilities for functional tests that can be performed to answer questions about the function of genes and the functional consequences of genetic variants ("functional genomics") in the field of inborn errors of metabolism (IEM), including several examples of functional genomics studies of mitochondrial disorders and several other IEM.

NDUFB8 Mutations Cause Mitochondrial Complex I Deficiency in Individuals with Leigh-like Encephalomyopathy

Respiratory chain complex I deficiency is the most frequently identified biochemical defect in childhood mitochondrial diseases. Clinical symptoms range from fatal infantile lactic acidosis to Leigh syndrome and other encephalomyopathies or cardiomyopathies. To date, disease-causing variants in genes coding for 27 complex I subunits, including 7 mitochondrial DNA genes, and in 11 genes encoding complex I assembly factors have been reported. Here, we describe rare biallelic variants in NDUFB8 encoding a complex I accessory subunit revealed by whole-exome sequencing in two individuals from two families. Both presented with a progressive course of disease with encephalo(cardio)myopathic features including muscular hypotonia, cardiac hypertrophy, respiratory failure, failure to thrive, and developmental delay. Blood lactate was elevated. Neuroimaging disclosed progressive changes in the basal ganglia and either brain stem or internal capsule. Biochemical analyses showed an isolated decrease in complex I enzymatic activity in muscle and fibroblasts. Complementation studies by expression of wild-type NDUFB8 in cells from affected individuals restored mitochondrial function, confirming NDUFB8 variants as the cause of complex I deficiency. Hereby we establish NDUFB8 as a relevant gene in childhood-onset mitochondrial disease.