Pathogenic Potential of a PCK1 Gene Variant in Cytosolic PEPCK Deficiency: A Compelling Case Study.

BACKGROUND Cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) deficiency is an extremely rare autosomal recessive inherited error of metabolism in which gluconeogenesis is impaired, resulting in life-threatening episodes of hypoglycemia and metabolic acidosis. The diagnosis of gluconeogenesis disorders is challenging. In the diagnostic pathway, the molecular test plays a paramount role. CASE REPORT The aim of the paper is to present the case report of a girl with recurrent episodes of severe hypoglycemia, in whom molecular diagnosis enabled the confirmation of PEPCK - C deficiency. The patient experienced 4 episodes of severe hypoglycemia. Most of them were accompanied by hyperlacticaemia, metabolic acidosis, and elevated liver enzymes. All of the metabolic decompensations were triggered by infectious agents. The episodes resolved after continuous infusion of high-dose glucose. Due to the recurrent character of the disease, a genetic condition was suspected. The differential diagnosis included metabolic and endocrinological causes of hypoglycemia. Two variants in the PCK1 gene were detected: c.265G>A p.(Glu89Lys) in exon 3 and c.925G>A p.(Gly309Arg) in exon 6. As c.925G>A p.(Gly309Arg) is a known pathogenic variant, the second variant was first described in June 2023 in the ClinVar database and described as "with unknown clinical significance". CONCLUSIONS According to the clinical symptoms observed in the presented case, the variant c.265G>A p.(Glu89Lys) in PCK1 gene should be considered likely pathogenic. We suggest considering molecular diagnostics in every patient presented with recurrent, severe hypoglycemia with accompanying liver damage as most accurate, feasible, and reliable method.

Efficacy and Safety of Ketogenic Diet Treatment in Pediatric Patients with Mitochondrial Disease.

Mitochondrial diseases (MDs) are a heterogeneous group of disorders resulting from abnormal mitochondrial function. Currently, there is no causal treatment for MDs. The aim of the study was to assess the effectiveness and safety of the ketogenic diet (KD) in patients with MD and to analyse selected biochemical and clinical parameters evaluating the effectiveness of KD treatment in patients with MDs. A total of 42 paediatric patients were assigned to four groups: group 1-patients with MD in whom KD treatment was started (n = 11); group 2-patients with MD remaining on an ordinary diet (n = 10); group 3-patients without MD in whom KD treatment was initiated (n = 10), group 4-patients without MD on a regular diet (n = 11). Clinical improvement was observed in 9/11 patients with MD treated with KD. Among patients with MD without KD, the clinical condition deteriorated in 7/10 patients, improved in 2/10 patients, and remained unchanged in one patient. Adverse events of KD occurred with a comparable frequency in groups 1 and 3. There was no significant difference in changes in biomarker concentrations over the course of the study among patients treated and untreated with KD.

The NBN founder mutation-Evidence for a country specific difference in age at cancer manifestation.

BACKGROUND: Nijmegen breakage syndrome (NBS) is an autosomal-recessive chromosome instability disorder characterized by, among others, hypersensitivity to X-irradiation and an exceptionally high risk for lymphoid malignancy. The vast majority of NBS patients is homozygous for a common Slavic founder mutation, c.657del5, of the NBN gene, which is involved in the repair of DNA double-strand breaks (DSBs). The founder mutation also predisposes heterozygous carriers to cancer, apparently however, with a higher risk in the Czech Republic/Slovakia (CS) than in Poland. AIM: To examine whether the age of cancer manifestation and cancer death of NBN homozygotes is different between probands from CS and Poland. METHODS: The study is restricted to probands born until 1989, before replacement of the communist regime by a democratic system in CS and Poland, and a substantial transition of the health care systems. Moreover, all patients were recruited without knowledge of their genetic status since the NBN gene was not identified until 1998. RESULTS: Here, we show that cancer manifestation of NBN homozygotes is at a significantly earlier age in probands from CS than from Poland. This is explained by the difference in natural and medical radiation exposure, though within the permissible dosage. CONCLUSION: It is reasonable to assume that this finding also sheds light on the higher cancer risk of NBN heterozygotes in CS than in Poland. This has implications for genetic counseling and individualized medicine also of probands with other DNA repair defects.

Genotypic and phenotypic spectrum of infantile liver failure due to pathogenic TRMU variants.

PURPOSE: This study aimed to define the genotypic and phenotypic spectrum of reversible acute liver failure (ALF) of infancy resulting from biallelic pathogenic TRMU variants and determine the role of cysteine supplementation in its treatment. METHODS: Individuals with biallelic (likely) pathogenic variants in TRMU were studied within an international retrospective collection of de-identified patient data. RESULTS: In 62 individuals, including 30 previously unreported cases, we described 47 (likely) pathogenic TRMU variants, of which 17 were novel, and 1 intragenic deletion. Of these 62 individuals, 42 were alive at a median age of 6.8 (0.6-22) years after a median follow-up of 3.6 (0.1-22) years. The most frequent finding, occurring in all but 2 individuals, was liver involvement. ALF occurred only in the first year of life and was reported in 43 of 62 individuals; 11 of whom received liver transplantation. Loss-of-function TRMU variants were associated with poor survival. Supplementation with at least 1 cysteine source, typically N-acetylcysteine, improved survival significantly. Neurodevelopmental delay was observed in 11 individuals and persisted in 4 of the survivors, but we were unable to determine whether this was a primary or a secondary consequence of TRMU deficiency. CONCLUSION: In most patients, TRMU-associated ALF was a transient, reversible disease and cysteine supplementation improved survival.

Genetic Profile of Left Ventricular Noncompaction Cardiomyopathy in Children-A Single Reference Center Experience.

BACKGROUND: Left ventricular noncompaction cardiomyopathy (LVNC) is a rare cardiac disorder characterised by the presence of a two-layer myocardium with prominent ventricular trabeculation, intertrabecular deep depressions and an increased risk of heart failure, atrial and ventricular arrhythmias and systemic thromboembolic events in affected patients. The heterogeneous molecular aetiology solved in 10%-50% of patients more frequently involves sarcomeric, cytoskeletal or ion channel protein dysfunction-mainly related to causative MYH7, TTN or MYBPC3 variants. The aim of the study was to determine the molecular spectrum of isolated LVNC in a group of children examined in a single paediatric reference centre. METHODS: Thirty-one paediatric patients prospectively diagnosed with LVNC by echocardiography and cardiovascular magnetic resonance examination were recruited into the study group. The molecular analysis included next-generation sequencing (gene panel or whole exome) and classic Sanger sequencing. All selected variants with high priority were co-segregated in the available parents. RESULTS: We identified 16 distinct variants in 11 genes in 16 patients (52%), including 10 novel alterations. The most frequent defects in our cohort were found in the genes HCN4 (n = 4), MYH7 (n = 2) and PRDM16 (n = 2). Other likely disease-causing variants were detected in ACTC1, ACTN2, HCCS, LAMA4, MYH6, RBM20, TAFFAZIN and TTN. Patients with established molecular defects more often presented with arrhythmia, thromboembolic events and death, whereas the predominant symptoms in patients with no identified molecular defects were heart failure and the presence of late gadolinium enhancement. CONCLUSION: This study expands the genetic and clinical spectrum of childhood LVNC. Although the molecular aetiology of LVNC varies widely, the comprehensive testing of a wide panel of cardiomyopathy-related genes helped to identify underlying molecular defects in more than half of the children in the study group. The molecular spectrum in our cohort correlated with the occurrence of arrhythmia, death and a family history of cardiomyopathy. We confirmed that genetic testing is an integral part of the work-up and management LVNC in children.

The fibroblast growth factor 21 concentration in children with mitochondrial disease does not depend on the disease stage, but rather on the disease genotype.

ABSTRACT: The fibroblast growth factor 21 (FGF21) is a new biomarker of mitochondrial diseases (MD). FGF21 concentration may be used to define the severity of mitochondrial disease. AIM OF THE STUDY: The study objective was to verify if the FGF21 concentration in paediatric patients with MD was correlated with the disease severity and stage and to assess the correlation between FGF21 levels and the genetic background of MD. MATERIAL AND METHODS: The disease stage in MD subjects was determined on the basis of the International Paediatric Mitochondrial Disease Scale (IPMDS) and the concentrations of FGF21, lactic and pyruvic acids, alanine and creatine kinase in serum were assessed in those patients. RESULTS: The median age of children with MD (n = 32) was 33 months (range: 2-213), in the control group (n = 21) the median age was 42 months (range: 8-202). The concentrations of FGF21, lactic acid and pyruvic acid were higher in MD patients than in the control group. No correlation between the disease severity (IPMDS) and serum FGF21 concentration was found. The FGF21 concentration was higher in patients whose MD resulted from nuclear gene damage (nDNA), median FGF21 = 1022 (84-8873) pg/ml, than in patients with MD resulting from mitochondrial damage (mtDNA), median FGF21 = 736 (188-2906) pg/ml, or with an abnormal variant in the PDHA1 gene, median FGF21 = 58 (25-637) pg/ml. CONCLUSIONS: There is no correlation between the stage of MD and FGF21 level. Higher FGF21 values are seen in patients whose MD results from an abnormal nDNA variant rather than mtDNA damage.

Clinical implementation of RNA sequencing for Mendelian disease diagnostics.

BACKGROUND: Lack of functional evidence hampers variant interpretation, leaving a large proportion of individuals with a suspected Mendelian disorder without genetic diagnosis after whole genome or whole exome sequencing (WES). Research studies advocate to further sequence transcriptomes to directly and systematically probe gene expression defects. However, collection of additional biopsies and establishment of lab workflows, analytical pipelines, and defined concepts in clinical interpretation of aberrant gene expression are still needed for adopting RNA sequencing (RNA-seq) in routine diagnostics. METHODS: We implemented an automated RNA-seq protocol and a computational workflow with which we analyzed skin fibroblasts of 303 individuals with a suspected mitochondrial disease that previously underwent WES. We also assessed through simulations how aberrant expression and mono-allelic expression tests depend on RNA-seq coverage. RESULTS: We detected on average 12,500 genes per sample including around 60% of all disease genes-a coverage substantially higher than with whole blood, supporting the use of skin biopsies. We prioritized genes demonstrating aberrant expression, aberrant splicing, or mono-allelic expression. The pipeline required less than 1 week from sample preparation to result reporting and provided a median of eight disease-associated genes per patient for inspection. A genetic diagnosis was established for 16% of the 205 WES-inconclusive cases. Detection of aberrant expression was a major contributor to diagnosis including instances of 50% reduction, which, together with mono-allelic expression, allowed for the diagnosis of dominant disorders caused by haploinsufficiency. Moreover, calling aberrant splicing and variants from RNA-seq data enabled detecting and validating splice-disrupting variants, of which the majority fell outside WES-covered regions. CONCLUSION: Together, these results show that streamlined experimental and computational processes can accelerate the implementation of RNA-seq in routine diagnostics.

Clinical Presentation of Left Ventricular Noncompaction Cardiomyopathy and Bradycardia in Three Families Carrying HCN4 Pathogenic Variants.

BACKGROUND: Left ventricular noncompaction (LVNC) is a genetically and phenotypically heterogeneous cardiomyopathy in which myocardium consists of two, distinct compacted and noncompacted layers, and prominent ventricular trabeculations and deep intertrabecular recesses are present. LVNC is associated with an increased risk of heart failure, atrial and ventricular arrhythmias and thromboembolic events. Familial forms of primary sinus bradycardia have been attributed to alterations in HCN4. There are very few reports about the association between HCN4 and LVNC. The aim of our study was to characterize the clinical phenotype of families with LVNC and sinus bradycardia caused by pathogenic variants of the HCN4 gene. METHODS: From March 2008 to July 2021, we enrolled six patients from four families with diagnosed isolated LVNC based on the clinical presentation, family history and echocardiographic and cardiovascular magnetic resonance (CMR) evidence of LVNC. Next generation sequencing (NGS) analysis was undertaken for the evaluation of the molecular basis of the disease in each family. RESULTS: A total of six children (median age 11 years) were recruited and followed prospectively for the median of 12 years. All six patients were diagnosed with LVNC by echocardiography, and five participants additionally by CMR. The presence of late gadolinium enhancement (LGE) was found in three children. Sinus bradycardia and dilation of the ascending aorta occurred in five studied patients. In four patients from three families, the molecular studies demonstrated the presence of rare heterozygous HCN4 variants. CONCLUSION: (1) The HCN4 molecular variants influence the presence of a complex LVNC phenotype, sinus bradycardia and dilation of the ascending aorta. (2) The HCN4 alteration may be associated with the early presentation of clinical symptoms and the severe course of the disease. (3) It is particularly important to assess myocardial fibrosis not only within the ventricles, but also in the atria in patients with LVNC and sinus bradycardia.

DNAJC30 defect: a frequent cause of recessive Leber hereditary optic neuropathy and Leigh syndrome.

The recent description of biallelic DNAJC30 variants in Leber hereditary optic neuropathy (LHON) and Leigh syndrome challenged the longstanding assumption for LHON to be exclusively maternally inherited and broadened the genetic spectrum of Leigh syndrome, the most frequent paediatric mitochondrial disease. Herein, we characterize 28 so far unreported individuals from 26 families carrying a homozygous DNAJC30 p.Tyr51Cys founder variant, 24 manifesting with LHON, two manifesting with Leigh syndrome, and two remaining asymptomatic. This collection of unreported variant carriers confirms sex-dependent incomplete penetrance of the homozygous variant given a significant male predominance of disease and the report of asymptomatic homozygous variant carriers. The autosomal recessive LHON patients demonstrate an earlier age of disease onset and a higher rate of idebenone-treated and spontaneous recovery of vision in comparison to reported figures for maternally inherited disease. Moreover, the report of two additional patients with childhood- or adult-onset Leigh syndrome further evidences the association of DNAJC30 with Leigh syndrome, previously only reported in a single childhood-onset case.

Clinico-radiological features, molecular spectrum, and identification of prognostic factors in developmental and epileptic encephalopathy due to inosine triphosphate pyrophosphatase (ITPase) deficiency.

Developmental and epileptic encephalopathy 35 (DEE 35) is a severe neurological condition caused by biallelic variants in ITPA, encoding inosine triphosphate pyrophosphatase, an essential enzyme in purine metabolism. We delineate the genotypic and phenotypic spectrum of DEE 35, analyzing possible predictors for adverse clinical outcomes. We investigated a cohort of 28 new patients and reviewed previously described cases, providing a comprehensive characterization of 40 subjects. Exome sequencing was performed to identify underlying ITPA pathogenic variants. Brain MRI (magnetic resonance imaging) scans were systematically analyzed to delineate the neuroradiological spectrum. Survival curves according to the Kaplan-Meier method and log-rank test were used to investigate outcome predictors in different subgroups of patients. We identified 18 distinct ITPA pathogenic variants, including 14 novel variants, and two deletions. All subjects showed profound developmental delay, microcephaly, and refractory epilepsy followed by neurodevelopmental regression. Brain MRI revision revealed a recurrent pattern of delayed myelination and restricted diffusion of early myelinating structures. Congenital microcephaly and cardiac involvement were statistically significant novel clinical predictors of adverse outcomes. We refined the molecular, clinical, and neuroradiological characterization of ITPase deficiency, and identified new clinical predictors which may have a potentially important impact on diagnosis, counseling, and follow-up of affected individuals.

Early treatment of biotin-thiamine-responsive basal ganglia disease improves the prognosis.

BACKGROUND: Biotin-thiamine-responsive basal ganglia disease (BTBGD) is an autosomal recessive neurometabolic disorder associated with pathogenic variants in SLC19A3 gene. The clinical picture includes symptoms of subacute encephalopathy (e.g. confusion, dysphagia, dysarthria, and seizures), which respond very well to early treatment with thiamine and biotin. METHOD: A retrospective review of clinical characteristics, magnetic resonance imaging and molecular findings in 3 patients with BTBGD. RESULTS: The first symptoms in all patients occurred at 12-24 months of age and they had subacute encephalopathy, ataxia and dystonia. The baseline magnetic resonance imaging demonstrated abnormal signal intensity in the basal ganglia with atrophy and necrosis of the basal ganglia during follow-up in two patients. One patient was diagnosed and the treatment was initiated after a long period from symptoms onset and he is currently severely affected, with dystonia, quadriparesis and seizures. The other two patients were diagnosed early in life and are currently stable on treatment, without the clinical symptoms. Genetic testing demonstrated pathogenic variants in SLC19A3 gene. CONCLUSION: To avoid diagnostic errors and delayed or incorrect treatment, BTBGD must be recognized early. Adequate prompt treatment gives the chance of significant clinical improvement. Unexplained encephalopathy and MRI abnormalities including bilateral abnormal signal in the basal ganglia should alert the clinician to consider BTBGD in the differential, and the treatment with biotin and thiamine should be introduced immediately.

NBAS deficiency due to biallelic c.2809C > G variant presenting with recurrent acute liver failure with severe hyperammonemia, acquired microcephaly and progressive brain atrophy.

Biallelic pathogenic variants in the neuroblastoma amplified sequence (NBAS) gene were firstly (2015) identified as a cause of fever-triggered recurrent acute liver failure (RALF). Since then, some patients with NBAS deficiency presenting with neurologic features, including a motor delay, intellectual disability, muscular hypotonia and a mild brain atrophy, have been reported. Here, we describe a case of pediatric patient diagnosed with NBAS deficiency due to a homozygous c.2809C > G, p.(Pro937Ala) variant presenting with RALF with severe hyperammonemia, acquired microcephaly and progressive brain atrophy. Not reported in the literature findings include severe hyperammonemia during ALF episode, and neurologic features in the form of acquired progressive microcephaly with brain atrophy. The latter raises the hypothesis about a primary neurologic phenotype in NBAS deficiency.

The Indices of Cardiovascular Magnetic Resonance Derived Atrial Dynamics May Improve the Contemporary Risk Stratification Algorithms in Children with Hypertrophic Cardiomyopathy.

INTRODUCTION: The most efficient risk stratification algorithms are expected to deliver robust and indefectible identification of high-risk children with hypertrophic cardiomyopathy (HCM). Here we compare algorithms for risk stratification in primary prevention in HCM children and investigate whether novel indices of biatrial performance improve these algorithms. METHODS AND RESULTS: The endpoints were defined as sudden cardiac death, resuscitated cardiac arrest, or appropriate implantable cardioverter-defibrillator discharge. We examined the prognostic utility of classic American College of Cardiology/American Heart Association (ACC/AHA) risk factors, the novel HCM Risk-Kids score and the combination of these with indices of biatrial dynamics. The study consisted of 55 HCM children (mean age 12.5 ± 4.6 years, 69.1% males); seven had endpoints (four deaths, three appropriate ICD discharges). A strong trend (DeLong p = 0.08) was observed towards better endpoint identification performance of the HCM Risk-Kids Model compared to the ACC/AHA strategy. Adding the atrial conduit function component significantly improved the prediction capabilities of the AHA/ACC Model (DeLong p = 0.01) and HCM Risk-Kids algorithm (DeLong p = 0.04). CONCLUSIONS: The new HCM Risk-Kids individualised algorithm and score was capable of identifying high-risk children with very good accuracy. The inclusion of one of the atrial dynamic indices improved both risk stratification strategies.

Expanding the clinical and genetic spectrum of FDXR deficiency by functional validation of variants of uncertain significance.

Ferrodoxin reductase (FDXR) deficiency is a mitochondrial disease described in recent years primarily in association with optic atrophy, acoustic neuropathy, and developmental delays. Here, we identified seven unpublished patients with FDXR deficiency belonging to six independent families. These patients show a broad clinical spectrum ranging from Leigh syndrome with early demise and severe infantile-onset encephalopathy, to milder movement disorders. In total nine individual pathogenic variants, of which seven were novel, were identified in FDXR using whole exome sequencing in suspected mitochondrial disease patients. Over 80% of these variants are missense, a challenging variant class in which to determine pathogenic consequence, especially in the setting of nonspecific phenotypes and in the absence of a reliable biomarker, necessitating functional validation. Here we implement an Arh1-null yeast model to confirm the pathogenicity of variants of uncertain significance in FDXR, bypassing the requirement for patient-derived material.

Acute liver failure due to DGUOK deficiency-is liver transplantation justified?

BACKGROUND: Deoxyguanosine kinase (DGUOK) deficiency is one of the causes of the hepatocerebral form of mitochondrial depletion syndrome (MDS). It is characterized by an early onset of liver failure with concomitant neurological deterioration. In the current literature, there are only few reports regarding long-term observation of children with DGUOK deficiency. Liver transplantation (LTx) is controversial due to extrahepatic involvement and unpredictable outcome. METHODS: Five patients (2 boys) from 4 different families with hepatocerebral MDS associated with DGUOK mutations diagnosed with liver failure were treated in our hospital between 2010-2019. RESULTS: In all children clinical symptoms developed within the first days of live and hypoglycemia (hypoketotic), conjugated hyperbilirubinemia (cholestasis), severe lactic acidosis, and coagulopathy were observed. Two neonates had low birth-weight for gestational age and failed to thrive. Mild neurological involvement as hypotonia was observed in all children. Three children died at the age of 2, 6 months and 6,5 months of age, respectively, due to end-stage liver failure. In one case, LTx was not considered, in two patients (sisters) parents did not agree to this procedure. LTx was subsequently performed in two patients at the age of 6 and 7 months, respectively, one from deceased, and one from living related donor, in both before the final confirmation of DGUOK mutations. One boy died 2 months after LTx due to post-LTx procedure-related complications; one is still alive with 3years of follow-up, with good liver function and mild neurological disturbances. The diagnosis of DGUOK deficiency was confirmed by biallelic DGUOK mutations detection. Equally, patients were compound heterozygotes (three cases) and homozygotes (two cases). Three known molecular variants, including regulatory substitutions (c.1A>G, c.3G>A) and in-frame insertion (c.813_814insTTT) were identified. CONCLUSIONS: Prognosis in patients with DGUOK deficiency is generally poor. Based on a review of the literature and our experience liver transplantation in selected patients with DGUOK mutation does not appear to be contraindicated, especially in those without or with minimal neurologic abnormalities.

Clinical picture and treatment effects in 5 patients with Methylmalonic aciduria related to MMAA mutations.

INTRODUCTION: Methylmalonic Aciduria (MMA) is a heterogeneous group of rare diseases leading to accumulation of methylmalonic acid in body fluids. One of the causes of the disease is the methylmalonic aciduria, cblA type (cblA - type MMA), conditioned by a mutation in the MMAA gene, which is essential for the proper functioning of a cofactor of the methylmalonyl-CoA mutase. The symptoms of the disease, depending on the cause, may manifest themselves at different ages. Most patients are sensitive to high doses of hydroxycobalamin, which is associated with better prognosis. MATERIAL AND METHOD: The purpose of the study was to retrospectively analyze the clinical picture and effects of treatment of patients with methylmalonic aciduria related to mutation in the MMAA gene. RESULTS: Five patients with diagnosed cblA - type MMA were presented. At the time of diagnosis the median of age was 18.8 months, but the symptoms had already appeared since infancy, as recurrent vomiting and delayed psychomotor development. Significant excretion of methylmalonic acid in urine and metabolic acidosis traits with significantly increased anionic gap were observed in all patients. All of them were sensitive to the treatment with vitamin B12. The median of therapy duration and observation is 12.2 years. During the treatment, good metabolic control was achieved in all patients, but their cognitive development is delayed. Three patients have renal failure and pharmacologically treated arterial hypertension. CONCLUSIONS: Patients with a mutation in the MMAA gene are sensitive to treatment with hydroxocobalamine, but the inclusion of appropriate treatment does not protect against neurodevelopmental disorders and chronic kidney disease.

Leigh syndrome caused by mutations in MTFMT is associated with a better prognosis.

Objectives: Mitochondrial methionyl-tRNA formyltransferase (MTFMT) is required for the initiation of translation and elongation of mitochondrial protein synthesis. Pathogenic variants in MTFMT have been associated with Leigh syndrome (LS) and mitochondrial multiple respiratory chain deficiencies. We sought to elucidate the spectrum of clinical, neuroradiological and molecular genetic findings of patients with bi-allelic pathogenic variants in MTFMT. Methods: Retrospective cohort study combining new cases and previously published cases. Results: Thirty-eight patients with pathogenic variants in MTFMT were identified, including eight new cases. The median age of presentation was 14 months (range: birth to 17 years, interquartile range [IQR] 4.5 years), with developmental delay and motor symptoms being the most frequent initial manifestation. Twenty-nine percent of the patients survived into adulthood. MRI headings in MTFMT pathogenic variants included symmetrical basal ganglia changes (62%), periventricular and subcortical white matter abnormalities (55%), and brainstem lesions (48%). Isolated complex I and combined respiratory chain deficiencies were identified in 31% and 59% of the cases, respectively. Reduction of the mitochondrial complex I and complex IV subunits was identified in the fibroblasts (13/13). Sixteen pathogenic variants were identified, of which c.626C>T was the most common. Seventy-four percent of the patients were alive at their last clinical review (median 6.8 years, range: 14 months to 31 years, IQR 14.5 years). Interpretation: Patients that harbour pathogenic variants in MTFMT have a milder clinical phenotype and disease progression compared to LS caused by other nuclear defects. Fibroblasts may preclude the need for muscle biopsy, to prove causality of any novel variant.