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Background Leigh syndrome is a progressive neurodegenerative disorder, associated with primary or secondary dysfunction of the mitochondrial oxidative phosphorylation. Despite the fact that Leigh syndrome is the most common phenotype of mitochondrial disorders in children, longitudinal natural history data is missing. This study was undertaken to assess the phenotypic and genotypic spectrum of patients with Leigh syndrome, characterise the clinical course and identify predictors of survival in a large cohort of patients. Methods This is a retrospective study of patients with Leigh syndrome that have been followed at eight centers specialising in mitochondrial diseases in Europe; Gothenburg, Rotterdam, Helsinki, Copenhagen, Stockholm, Brussels, Bergen and Oulu. Results A total of 130 patients were included (78 males; 52 females), of whom 77 patients had identified pathogenic mutations. The median age of disease onset was 7 months, with 80.8% of patients presenting by the age of 2 years. The most common clinical features were abnormal motor findings, followed by abnormal ocular findings. Epileptic seizures were reported in 40% of patients. Approximately 44% of patients experienced acute exacerbations requiring hospitalisation during the previous year, mainly due to infections. The presence of pathological signs at birth and a history of epileptic seizures were associated with higher occurrence of acute exacerbations and/or relapses. Increased lactate in the cerebrospinal fluid was significantly correlated to a more severe disease course, characterised by early onset before 6 months of age, acute exacerbations and/or relapses, as well as brainstem involvement. 39% of patients had died by the age of 21 years, at a median age of 2.4 years. Disease onset before 6 months of age, failure to thrive, brainstem lesions on neuroimaging and intensive care treatment were significantly associated with poorer survival. Conclusions This is a multicenter study performed in a large cohort of patients with Leigh syndrome. Our data help define the natural history of Leigh syndrome and identify novel predictors of disease severity and long-term prognosis.

Genetic leukoencephalopathies are a heterogeneous group of central nervous system disorders with white matter involvement. In a Finnish patient, we identified a novel homozygous disease-causing variant in HIKESHI, c.11G>C, p.(Cys4Ser), leading to hypomyelinating leukoencephalopathy with periventricular cysts and vermian atrophy. A founder Ashkenazi-Jewish disease-causing variant recently linked Hikeshi and its heat-shock protective function to leukoencephalopathy. In our patient, clinical features of lower limb spasticity, optic atrophy, nystagmus, and severe developmental delay were similar to reported patients. Additional features included vermian atrophy, epileptic seizures, and an ovarian tumor. Structural modeling and protein analyses revealed that modified interactions inside Hikeshi's hydrophobic pockets induce protein instability. The patient's cells showed impaired nuclear translocation of HSP70 during heat shock, and decreased ERO1-Lα, an endoplasmic reticulum (ER) oxidoreductase. Overall, we show that: (1) the clinical spectrum associated with Hikeshi deficiency extends to leukoencephalopathy with vermian atrophy and epilepsy; (2) the cellular disease process involves both nuclear chaperone and ER functions.

Mutations in SUCLA2, encoding the ß-subunit of succinyl-CoA synthetase of Krebs cycle, are one cause of mitochondrial DNA depletion syndrome. Patients have been reported to have severe progressive childhood-onset encephalomyopathy, and methylmalonic aciduria, often leading to death in childhood. We studied two families, with children manifesting with slowly progressive mitochondrial encephalomyopathy, hearing impairment and transient methylmalonic aciduria, without mtDNA depletion. The other family also showed dominant inheritance of bilateral retinoblastoma, which coexisted with mitochondrial encephalomyopathy in one patient. We found a variant in SUCLA2 leading to Asp333Gly change, homozygous in one patient and compound heterozygous in one. The latter patient also carried a deletion of 13q14 of the other allele, discovered with molecular karyotyping. The deletion spanned both SUCLA2 and RB1 gene regions, leading to manifestation of both mitochondrial disease and retinoblastoma. We made a homology model for human succinyl-CoA synthetase and used it for structure-function analysis of all reported pathogenic mutations in SUCLA2. On the basis of our model, all previously described mutations were predicted to result in decreased amounts of incorrectly assembled protein or disruption of ADP phosphorylation, explaining the severe early lethal manifestations. However, the Asp333Gly change was predicted to reduce the activity of the otherwise functional enzyme. On the basis of our findings, SUCLA2 mutations should be analyzed in patients with slowly progressive encephalomyopathy, even in the absence of methylmalonic aciduria or mitochondrial DNA depletion. In addition, an encephalomyopathy in a patient with retinoblastoma suggests mutations affecting SUCLA2.

Muscle biopsy is the gold standard for diagnosis of mitochondrial disorders because of the lack of sensitive biomarkers in serum. Fibroblast growth factor 21 (FGF-21) is a growth factor with regulatory roles in lipid metabolism and the starvation response, and concentrations are raised in skeletal muscle and serum in mice with mitochondrial respiratory chain deficiencies. We investigated in a retrospective diagnostic study whether FGF-21 could be a biomarker for human mitochondrial disorders. We assessed samples from adults and children with mitochondrial disorders or non-mitochondrial neurological disorders (disease controls) from seven study centres in Europe and the USA, and recruited healthy volunteers (healthy controls), matched for age where possible, from the same centres. We used ELISA to measure FGF-21 concentrations in serum or plasma samples (abnormal values were defined as >200 pg/mL). We compared these concentrations with values for lactate, pyruvate, lactate-to-pyruvate ratio, and creatine kinase in serum or plasma and calculated sensitivity, specificity, and positive and negative predictive values for all biomarkers. We analysed serum or plasma from 67 patients (41 adults and 26 children) with mitochondrial disorders, 34 disease controls (22 adults and 12 children), and 74 healthy controls. Mean FGF-21 concentrations in serum were 820 (SD 1151) pg/mL in adult and 1983 (1550) pg/mL in child patients with respiratory chain deficiencies and 76 (58) pg/mL in healthy controls. FGF-21 concentrations were high in patients with mitochondrial disorders affecting skeletal muscle but not in disease controls, including those with dystrophies. In patients with abnormal FGF-21 concentrations in serum, the odds ratio of having a muscle-manifesting mitochondrial disease was 132·0 (95% CI 38·7–450·3). For the identification of muscle-manifesting mitochondrial disease, the sensitivity was 92·3% (95% CI 81·5–97·9%) and specificity was 91·7% (84·8–96·1%). The positive and negative predictive values for FGF-21 were 84·2% (95% CI 72·1–92·5%) and 96·1 (90·4–98·9%). The accuracy of FGF-21 to correctly identify muscle-manifesting respiratory chain disorders was better than that for all conventional biomarkers. The area under the receiver-operating-characteristic curve for FGF-21 was 0·95; by comparison, the values for other biomarkers were 0·83 lactate (p=0·037, 0·83 for pyruvate (p=0·015), 0·72 for the lactate-to-pyruvate ratio (p=0·0002), and 0·77 for creatine kinase (p=0·013). Measurement of FGF-21 concentrations in serum identified primary muscle-manifesting respiratory chain deficiencies in adults and children and might be feasible as a first-line diagnostic test for these disorders to reduce the need for muscle biopsy. Sigrid Jusélius Foundation, Jane and Aatos Erkko Foundation, Molecular Medicine Institute of Finland, University of Helsinki, Helsinki University Central Hospital, Academy of Finland, Novo Nordisk, Arvo and Lea Ylppö Foundation.

The BCS1L gene encodes a chaperone responsible for assembly of respiratory chain complex III (CIII). A homozygous point mutation (232A→G) has been found as the genetic etiology for fetal growth retardation, amino aciduria, cholestasis, iron overload, lactic acidosis, and early death (GRACILE) syndrome (MIM 603358). Variable phenotypes have been found with other mutations. Our aim was to assess whether 232A→G or other BCS1L mutations were present in infants ( n  = 21) of Finnish origin with severe, lethal disease compatible with mitochondrial disorder. A further aim was to confirm the GRACILE genotype–phenotype constancy ( n  = 8). Three new cases with homozygous 232A→G mutation were identified; all had the primary GRACILE characteristics. No other mutations were found in the gene in other cases. All infants with GRACILE syndrome had the typical mutation. In conclusion, the rather homogenous population of Finns seems to have a specific BCS1L mutation that, as homozygous state, causes GRACILE syndrome, whereas other mutations are rare or not occurring. Thus, the novel clinical implication of this study is to screen for BCS1L mutations only if CIII is dysfunctioning or lacking Rieske protein, and to assess 232A→G mutation in cases with GRACILE syndrome.

Background The genetic complexity of infantile cardiomyopathies is remarkable, and the importance of mitochondrial translation defects as a causative factor is only starting to be recognised. We investigated the genetic basis for infantile onset recessive hypertrophic cardiomyopathy in two siblings. Methods and results Analysis of respiratory chain enzymes revealed a combined deficiency of complexes I and IV in the heart and skeletal muscle. Exome sequencing uncovered a homozygous mutation (L156R) in MRPL44 of both siblings. MRPL44 encodes a protein in the large subunit of the mitochondrial ribosome and is suggested to locate in close proximity to the tunnel exit of the yeast mitochondrial ribosome. We found severely reduced MRPL44 levels in the patient's heart, skeletal muscle and fibroblasts suggesting that the missense mutation affected the protein stability. In patient fibroblasts, decreased MRPL44 affected assembly of the large ribosomal subunit and stability of 16S rRNA leading to complex IV deficiency. Despite this assembly defect, de novo mitochondrial translation was only mildly affected in fibroblasts suggesting that MRPL44 may have a function in the assembly/stability of nascent mitochondrial polypeptides exiting the ribosome. Retroviral expression of wild-type MRPL44 in patient fibroblasts rescued the large ribosome assembly defect and COX deficiency. Conclusions These findings indicate that mitochondrial ribosomal subunit defects can generate tissue-specific manifestations, such as cardiomyopathy.

Aim  Children with motor disabilities are at increased risk of compromised bone health. This study evaluated prevalence and risk factors of low bone mass and fractures in these children. Method  This cross‐sectional cohort study evaluated bone health in 59 children (38 males, 21 females; median age 10y 11mo) with motor disability (Gross Motor Function Classification System levels II–V). Bone mineral density (BMD) in the lumbar spine was measured with dual‐energy X‐ray absorptiometry; BMD values were corrected for bone size (bone mineral apparent density [BMAD]) and skeletal maturity, and compared with normative data. Spinal radiographs were obtained to assess vertebral morphology. Blood biochemistry included vitamin D concentration and other parameters of calcium homeostasis. Results  Ten children (17%) had sustained in total 14 peripheral fractures; lower‐limb fractures predominated. Compression fractures were present in 25%. The median spinal BMAD z‐score was −1.0 (range −5.0 to 2.0); it was −0.6 in those without fractures and −1.7 in those with fractures (p=0.004). Vitamin D insufficiency was present in 59% of participants (serum 25‐hydroxyvitamin D <50nmol/l) and hypercalciuria in 27%. Low BMAD z‐score and hypercalciuria were independent predictors for fractures. Interpretation  Children with motor disability are at high risk of peripheral and vertebral fractures and low BMD. Evaluation of bone health and prevention of osteoporosis should be included in the follow‐up.