New spastic paraplegia phenotype associated to mutation of NFU1

Patient-specific variants of NFU1/NFU-1 disrupt cholinergic signaling in a model of multiple mitochondrial dysfunctions syndrome 1

Neuromuscular dysfunction is a common feature of mitochondrial diseases and frequently presents as ataxia, spasticity and/or dystonia, all of which can severely impact individuals with mitochondrial diseases. Dystonia is one of the most common symptoms of multiple mitochondrial dysfunctions syndrome 1 (MMDS1), a disease associated with mutations in the causative gene (NFU1) that impair iron-sulfur cluster biogenesis. We have generated Caenorhabditis elegans strains that recreated patient-specific point variants in the C. elegans ortholog (nfu-1) that result in allele-specific dysfunction. Each of these mutants, Gly147Arg and Gly166Cys, have altered acetylcholine signaling at neuromuscular junctions, but opposite effects on activity and motility. We found that the Gly147Arg variant was hypersensitive to acetylcholine and that knockdown of acetylcholine release rescued nearly all neuromuscular phenotypes of this variant. In contrast, we found that the Gly166Cys variant caused predominantly postsynaptic acetylcholine hypersensitivity due to an unclear mechanism. These results are important for understanding the neuromuscular conditions of MMDS1 patients and potential avenues for therapeutic intervention.

Phenotypic continuum of NFU1-related disorders

Bi-allelic variants in Iron-Sulfur Cluster Scaffold (NFU1) have previously been associated with multiple mitochondrial dysfunctions syndrome 1 (MMDS1) characterized by early-onset rapidly fatal leukoencephalopathy. We report 19 affected individuals from 10 independent families with ultra-rare bi-allelic NFU1 missense variants associated with a spectrum of early-onset pure to complex hereditary spastic paraplegia (HSP) phenotype with a longer survival (16/19) on one end and neurodevelopmental delay with severe hypotonia (3/19) on the other. Reversible or irreversible neurological decompensation after a febrile illness was common in the cohort, and there were invariable white matter abnormalities on neuroimaging. The study suggests that MMDS1 and HSP could be the two ends of the NFU1-related phenotypic continuum.

Genetic and genomic signatures in ethanol withdrawal seizure-prone and seizure-resistant mice implicate genes involved in epilepsy and neuronal excitability

Alcohol withdrawal is a clinically important consequence and potential driver of Alcohol Use Disorder. However, susceptibility to withdrawal symptoms, ranging from craving and anxiety to seizures and delirium, varies greatly. Selectively bred Withdrawal Seizure-Prone (WSP) and Seizure-Resistant (WSR) mice are an animal model of differential susceptibility to withdrawal and phenotypes with which withdrawal severity correlates. To identify innate drivers of alcohol withdrawal severity, we performed a multi-omic study of the WSP and WSR lines and F2 mice derived from them, using genomic, genetic, and transcriptomic analyses. Genes implicated in seizures and epilepsy were over-represented among those that segregated between WSP and WSR mice and that displayed differential expression in F2 mice high and low in withdrawal. Quantitative trait locus (QTL) analysis of ethanol withdrawal convulsions identified several genome-wide significant loci and pointed to genes that modulate potassium channel function and neural excitability. Perturbations of expression of genes involved in synaptic transmission, including GABAergic and glutamatergic genes, were prominent in prefrontal cortex transcriptome. Expression QTL (eQTL) analysis fine mapped genes within the peak ethanol withdrawal QTL regions. Genetic association analysis in human subjects provided converging evidence for the involvement of those genes in severity of alcohol withdrawal and dependence. Our results reveal a polygenic network and neural signaling pathways contributing to ethanol withdrawal seizures and related phenotypes that overlap with genes modulating epilepsy and neuronal excitability.

Molecular Basis of Rare Diseases Associated to the Maturation of Mitochondrial [4Fe-4S]-Containing Proteins

The importance of mitochondria in mammalian cells is widely known. Several biochemical reactions and pathways take place within mitochondria: among them, there are those involving the biogenesis of the iron–sulfur (Fe-S) clusters. The latter are evolutionarily conserved, ubiquitous inorganic cofactors, performing a variety of functions, such as electron transport, enzymatic catalysis, DNA maintenance, and gene expression regulation. The synthesis and distribution of Fe-S clusters are strictly controlled cellular processes that involve several mitochondrial proteins that specifically interact each other to form a complex machinery (Iron Sulfur Cluster assembly machinery, ISC machinery hereafter). This machinery ensures the correct assembly of both [2Fe-2S] and [4Fe-4S] clusters and their insertion in the mitochondrial target proteins. The present review provides a structural and molecular overview of the rare diseases associated with the genes encoding for the accessory proteins of the ISC machinery (i.e., GLRX5, ISCA1, ISCA2, IBA57, FDX2, BOLA3, IND1 and NFU1) involved in the assembly and insertion of [4Fe-4S] clusters in mitochondrial proteins. The disease-related missense mutations were mapped on the 3D structures of these accessory proteins or of their protein complexes, and the possible impact that these mutations have on their specific activity/function in the frame of the mitochondrial [4Fe-4S] protein biogenesis is described.

GLRX5-associated [Fe-S] cluster biogenesis disorder: further characterisation of the neurological phenotype and long-term outcome

BACKGROUND

Identification and characterisation of monogenic causes of complex neurological phenotypes are important for genetic counselling and prognostication. Bi-allelic pathogenic variants in the gene encoding GLRX5, a protein involved in the early steps of Fe-S cluster biogenesis, are rare and cause two distinct phenotypes: isolated sideroblastic anemia and a neurological phenotype with variant non-ketotic hyperglycinemia. In this study, we analysed the evolution of clinical and MRI findings and long-term outcome of patients with GLRX5 mutations.

METHODS

Four patients from three Australian families of Lebanese descent were identified. All patients presented in childhood and were followed up into adult life through multiple clinical assessments. All were prescribed sodium benzoate.

RESULTS

All patients (all females, age range 18-56 years) showed a complex neurological phenotype characterised by varying combinations of spastic paraparesis, length-dependent motor/sensory-motor axonal polyneuropathy, and psychiatric disturbances with variable intellectual disability. All had non-ketotic hyperglycinemia and a homozygous pathogenic c.151_153delAAG (p.K51del) change in GLRX5. Motor disability gradually progressed reaching moderate disability during adolescence and moderately severe disability during adult life. The major MRI finding was the upper cervical spinal cord signal changes with contrast enhancement noted in all and additional leukoencephalopathy in one. On follow up MRI, the white matter lesions diminished on a subsequent scan and then remained static over time. The spinal cord showed gliotic changes. Two patients have previously demonstrated low pyruvate dehydrogenase complex deficiency but none had plasma lactate elevation, nor biochemical evidence of branch-chain keto-dehydrogenase deficiency. Glycine levels reduced in patients that tolerated sodium benzoate, possibly stabilising clinical manifestations.

CONCLUSIONS

This report demonstrates that the p.K51del GLRX5 variant causes a distinct and predictable neurological phenotype. The clinical assessments spanning from childhood to adult life enable physicians to infer the natural history of GLRX5 related neurological disorder. There may be widespread metabolic consequences, and optimal management is unknown.

Mitochondrial iron-sulfur cluster biogenesis and neurological disorders

Iron-sulfur clusters (ISCs) are highly conserved moieties embedded into numerous crucial proteins in almost all bacteria, plants and mammals. As such, ISC biosynthesis is critical to cellular function. The pathway was first characterized in bacteria by the late 1990s, and over the subsequent 20 years there has been increasing understanding of its components in humans. Defects in the ISC pathway are now associated with many different human disease states, such as Friedreich ataxia and ISCU myopathy. Whilst the disorders have variable clinical features, most involve neurological phenotypes. There are common biochemical signatures in most of these conditions, as a lack of ISCs causes deficiencies of target proteins including Complex I, II and III, aconitase and lipoic acid. This review focuses on the disorders of ISC biogenesis that have been described in the literature to-date. Key clinical, biochemical and neuroradiological features will be discussed, providing a reference point for clinicians diagnosing and managing these patients. Therapies are mostly supportive at this stage. However, the improved understanding of the pathophysiology of these conditions could pave the way for disease-modifying therapies in the near future.

Allele-specific mitochondrial stress induced by Multiple Mitochondrial Dysfunctions Syndrome 1 pathogenic mutations modeled in Caenorhabditis elegans

Multiple Mitochondrial Dysfunctions Syndrome 1 (MMDS1) is a rare, autosomal recessive disorder caused by mutations in the NFU1 gene. NFU1 is responsible for delivery of iron-sulfur clusters (ISCs) to recipient proteins which require these metallic cofactors for their function. Pathogenic variants of NFU1 lead to dysfunction of its target proteins within mitochondria. To date, 20 NFU1 variants have been reported and the unique contributions of each variant to MMDS1 pathogenesis is unknown. Given that over half of MMDS1 individuals are compound heterozygous for different NFU1 variants, it is valuable to investigate individual variants in an isogenic background. In order to understand the shared and unique phenotypes of NFU1 variants, we used CRISPR/Cas9 gene editing to recreate exact patient variants of NFU1 in the orthologous gene, nfu-1 (formerly lpd-8), in C. elegans. Five mutant C. elegans alleles focused on the presumptive iron-sulfur cluster interaction domain were generated and analyzed for mitochondrial phenotypes including respiratory dysfunction and oxidative stress. Phenotypes were variable between the mutant nfu-1 alleles and generally presented as an allelic series indicating that not all variants have lost complete function. Furthermore, reactive iron within mitochondria was evident in some, but not all, nfu-1 mutants indicating that iron dyshomeostasis may contribute to disease pathogenesis in some MMDS1 individuals.

Functional mitochondria are essential to life in eukaryotes, but they can be perterbured by inherent dysfunction of important proteins or stressors. Mitochondrial dysfunction is the root cause of dozens of diseases many of which involve complex phenotypes. One such disease is Multiple Mitochondrial Dysfunctions Syndrome 1, a pediatric-fatal disease that is poorly understood in part due to the lack of clarity about how mutations in the causative gene, NFU1, affect protein function and phenotype development and severity. Here we employ the power of CRISPR/Cas9 gene editing in the small nematode Caenorhabditis elegans to recreate five patient-specific mutations known to cause Multiple Mitochondrial Dysfunctions Syndrome 1. We are able to analyze each of these mutations individually, evaluate how mitochondrial dysfunction differs between them, and whether or not the phenotypes can be improved. We find that there are meaningful differences between each mutation which not only effects the types of stress that develop, but also the ability to rescue deleterious phenotypes. This work thus provides insight into disease pathogenesis and establishes a foundation for potential future therapeutic intervention.

A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation

Mitochondrial proteins carrying iron-sulfur (Fe-S) clusters are involved in essential cellular pathways such as oxidative phosphorylation, lipoic acid synthesis, and iron metabolism. NFU1, BOLA3, IBA57, ISCA2, and ISCA1 are involved in the last steps of the maturation of mitochondrial [4Fe-4S]-containing proteins. Since 2011, mutations in their genes leading to five multiple mitochondrial dysfunction syndromes (MMDS types 1 to 5) were reported. The aim of this systematic review is to describe all reported MMDS-patients. Their clinical, biological, and radiological data and associated genotype will be compared to each other. Despite certain specific clinical elements such as pulmonary hypertension or dilated cardiomyopathy in MMDS type 1 or 2, respectively, nearly all of the patients with MMDS presented with severe and early onset leukoencephalopathy. Diagnosis could be suggested by high lactate, pyruvate, and glycine levels in body fluids. Genetic analysis including large gene panels (Next Generation Sequencing) or whole exome sequencing is needed to confirm diagnosis.

Molecular Basis of Multiple Mitochondrial Dysfunctions Syndrome 2 Caused by CYS59TYR BOLA3 Mutation

Multiple mitochondrial dysfunctions syndrome (MMDS) is a rare neurodegenerative disorder associated with mutations in genes with a vital role in the biogenesis of mitochondrial [4Fe-4S] proteins. Mutations in one of these genes encoding for BOLA3 protein lead to MMDS type 2 (MMDS2). Recently, a novel phenotype for MMDS2 with complete clinical recovery was observed in a patient containing a novel variant (c.176G > A, p.Cys59Tyr) in compound heterozygosity. In this work, we aimed to rationalize this unique phenotype observed in MMDS2. To do so, we first investigated the structural impact of the Cys59Tyr mutation on BOLA3 by NMR, and then we analyzed how the mutation affects both the formation of a hetero-complex between BOLA3 and its protein partner GLRX5 and the iron-sulfur cluster-binding properties of the hetero-complex by various spectroscopic techniques and by experimentally driven molecular docking. We show that (1) the mutation structurally perturbed the iron-sulfur cluster-binding region of BOLA3, but without abolishing [2Fe-2S]2+ cluster-binding on the hetero-complex; (2) tyrosine 59 did not replace cysteine 59 as iron-sulfur cluster ligand; and (3) the mutation promoted the formation of an aberrant apo C59Y BOLA3-GLRX5 complex. All these aspects allowed us to rationalize the unique phenotype observed in MMDS2 caused by Cys59Tyr mutation.

A genetic mimic of cerebral palsy: Homozygous NFU1 mutation with marked intrafamilial phenotypic variation

BACKGROUND

Genetic defects in the NFU1, an iron-sulfur cluster scaffold protein coding gene, which is vital in the final stage of assembly for iron sulfur proteins, have been defined as multiple mitochondrial dysfunctions syndrome I. This disorder is a severe autosomal recessive disease with onset in early infancy. It is characterized by disruption of the energy metabolism, resulting in weakness, neurological regression, hyperglycinemia, lactic acidosis, and early death.

PATIENT DESCRIPTION

This report documents the case of a 27-month-old girl, who showed clinical signs and symptoms of spastic paraparesis with a relapsing-remitting course. The patient had a sister with a severe phenotype who died at the age of 16 months.

RESULTS

Magnetic resonance imaging revealed hyperintensity of the cerebral white matter that was more prominent in the frontal regions, with milder involvement in the posterior periventricular regions. There was also evidence of partial cystic degeneration and cavitation in the frontal regions. In addition, she had hyperglycinemia. Homozygous NM_001002755.4:c.565G>A (p.Gly189Arg) mutation was identified in the NFU1 gene; this had not previously been reported as homozygous.

CONCLUSION

Hyperglycinemia and cavitating leukodystrophy are suggestive of an NFU1 mutation diagnosis. An intrafamilial phenotypic variation has not been published in NFU1-associated disorders before. Presenting with spasticity as a rare phenotype, NFU1 mutations could be considered a genetic mimic of cerebral palsy.

Multiple mitochondrial dysfunctions syndrome 1: An unusual cause of developmental pulmonary hypertension

Pulmonary hypertension (pHTN) is a severe, life-threatening disease, which can be idiopathic or associated with an underlying syndrome or genetic diagnosis. Here we discuss a patient who presented with severe pHTN and was later found to be compound heterozygous for pathogenic variants in the NFU1 gene causing multiple mitochondrial dysfunctions syndrome 1 (MMDS1). Review of autopsy slides from an older sibling revealed the same diagnosis along with pulmonary findings consistent with a developmental lung disorder. In particular, these postmortem, autopsy findings have not been described previously in humans with this mitochondrial syndrome and suggest a possible developmental basis for the severe pHTN seen in this disease. Given the rarity of patients reported with MMDS1, we review the current state of knowledge of this disease and our novel management strategies for pHTN and MMDS1-associated complications in this population.

"Idiopathic" pulmonary arterial hypertension in early infancy: Excluding NFU1 deficiency

NFU1 deficiency is a rare metabolic disorder affecting iron-sulfur cluster synthesis, an essential pathway for lipoic acid-dependent enzymatic activities and mitochondrial respiratory chain complexes. It is a little-known cause of pulmonary arterial hypertension (PAH), while PAH is a prominent feature of the disease. We herein report on a female infant diagnosed as having idiopathic PAH since 1 month of age, who did not respond to bosentan plus sildenafil. NFU1 deficiency was only suggested and confirmed at 10 months of age when she demonstrated neurological deterioration along with high glycine levels in body fluids. Unexplained PAH in early infancy should prompt clinicians to perform amino acid chromatography searching for high glycine levels. Early recognition will avoid further invasive procedures and enable appropriate genetic counseling to be offered. No effective treatment is currently able to prevent the fatal course of this metabolic condition.

Patisiran, an RNAi therapeutic for the treatment of hereditary transthyretin-mediated amyloidosis

Hereditary transthyretin-mediated amyloidosis is a rapidly progressive, heterogeneous disease caused by the accumulation of misfolded transthyretin protein as amyloid fibrils at multiple sites, and is characterized by peripheral sensorimotor neuropathy, autonomic neuropathy and/or cardiomyopathy. Current treatment options have limited efficacy and often do not prevent disease progression. Patisiran is a novel RNA interference therapeutic that specifically reduces production of both wild-type and mutant transthyretin protein. In Phase II, III and long-term extension studies in patients with hereditary transthyretin-mediated amyloidosis, patisiran has consistently slowed or improved progression of neuropathy. In addition, the Phase III trial demonstrated significant improvements in quality of life measures and indicators of cardiomyopathy. Here, we highlight efficacy and safety data from the patisiran clinical trial programme.

Expanding the phenotype of IBA57 mutations: related leukodystrophy can remain asymptomatic

Biallelic mutations in IBA57 cause a mitochondrial disorder with a broad phenotypic spectrum that ranges from severe intellectual disability to adolescent-onset spastic paraplegia. Only 21 IBA57 mutations have been reported, therefore the phenotypic spectrum of IBA57-related mitochondrial disease has not yet been fully elucidated. In this study, we performed whole-exome sequencing on a Sepharadi Jewish and Japanese family with leukodystrophy. We identified four novel biallelic variants in IBA57 in the two families: one frameshift insertion and three missense variants. The three missense variants were predicted to be disease-causing by multiple in silico tools. The 29-year-old Sepharadi Jewish male had infantile-onset optic atrophy with clinically asymptomatic leukodystrophy involving periventricular white matter. The 19-year-old younger brother, with the same compound heterozygous IBA57 variants, had a similar clinical course until 7 years of age. However, he then developed a rapidly progressive spastic paraparesis following a febrile illness. A 7-year-old Japanese girl had developmental regression, spastic quadriplegia, and abnormal periventricular white matter signal on brain magnetic resonance imaging performed at 8 months of age. She had febrile convulsions at the age of 18 months and later developed epilepsy. In summary, we have identified four novel IBA57 mutations in two unrelated families. Consequently, we describe a patient with infantile-onset optic atrophy and asymptomatic white matter involvement, thus broadening the phenotypic spectrum of biallelic IBA57 mutations.

Clinical and genetic aspects of defects in the mitochondrial iron-sulfur cluster synthesis pathway

Iron-sulfur clusters are evolutionarily conserved biological structures which play an important role as cofactor for multiple enzymes in eukaryotic cells. The biosynthesis pathways of the iron-sulfur clusters are located in the mitochondria and in the cytosol. The mitochondrial iron-sulfur cluster biosynthesis pathway (ISC) can be divided into at least twenty enzymatic steps. Since the description of frataxin deficiency as the cause of Friedreich's ataxia, multiple other deficiencies in ISC biosynthesis pathway have been reported. In this paper, an overview is given of the clinical, biochemical and genetic aspects reported in humans affected by a defect in iron-sulfur cluster biosynthesis.

Fe-S cluster assembly in the supergroup Excavata

The majority of established model organisms belong to the supergroup Opisthokonta, which includes yeasts and animals. While enlightening, this focus has neglected protists,  organisms that represent the bulk of eukaryotic diversity and are often regarded as primitive eukaryotes. One of these is the “supergroup” Excavata, which comprises unicellular flagellates of diverse lifestyles and contains species of medical importance, such as Trichomonas, Giardia, Naegleria, Trypanosoma and Leishmania. Excavata exhibits a continuum in mitochondrial forms, ranging from classical aerobic, cristae-bearing mitochondria to mitochondria-related organelles, such as hydrogenosomes and mitosomes, to the extreme case of a complete absence of the organelle. All forms of mitochondria house a machinery for the assembly of Fe–S clusters, ancient cofactors required in various biochemical activities needed to sustain every extant cell. In this review, we survey what is known about the Fe–S cluster assembly in the supergroup Excavata. We aim to bring attention to the diversity found in this group, reflected in gene losses and gains that have shaped the Fe–S cluster biogenesis pathways.

Iron-sulfur cluster biosynthesis and trafficking - impact on human disease conditions

Iron-sulfur clusters (Fe-S) are one of the most ancient, ubiquitous and versatile classes of metal cofactors found in nature. Proteins that contain Fe-S clusters constitute one of the largest families of proteins, with varied functions that include electron transport, regulation of gene expression, substrate binding and activation, radical generation, and, more recently discovered, DNA repair. Research during the past two decades has shown that mitochondria are central to the biogenesis of Fe-S clusters in eukaryotic cells via a conserved cluster assembly machinery (ISC assembly machinery) that also controls the synthesis of Fe-S clusters of cytosolic and nuclear proteins. Several key steps for synthesis and trafficking have been determined for mitochondrial Fe-S clusters, as well as the cytosol (CIA - cytosolic iron-sulfur protein assembly), but detailed mechanisms of cluster biosynthesis, transport, and exchange are not well established. Genetic mutations and the instability of certain steps in the biosynthesis and maturation of mitochondrial, cytosolic and nuclear Fe-S cluster proteins affects overall cellular iron homeostasis and can lead to severe metabolic, systemic, neurological and hematological diseases, often resulting in fatality. In this review we briefly summarize the current molecular understanding of both mitochondrial ISC and CIA assembly machineries, and present a comprehensive overview of various associated inborn human disease states.

Steps Toward Understanding Mitochondrial Fe/S Cluster Biogenesis

Iron-sulfur clusters (Fe/S clusters) are essential cofactors required throughout the clades of biology for performing a myriad of unique functions including nitrogen fixation, ribosome assembly, DNA repair, mitochondrial respiration, and metabolite catabolism. Although Fe/S clusters can be synthesized in vitro and transferred to a client protein without enzymatic assistance, biology has evolved intricate mechanisms to assemble and transfer Fe/S clusters within the cellular environment. In eukaryotes, the foundation of all cellular clusters starts within the mitochondria. The focus of this review is to detail the mitochondrial Fe/S biogenesis (ISC) pathway along with the Fe/S cluster transfer steps necessary to mature Fe/S proteins. New advances in our understanding of the mitochondrial Fe/S biogenesis machinery will be highlighted. Additionally, we will address various experimental approaches that have been successful in the identification and characterization of components of the ISC pathway.

Mammalian Fe-S proteins: definition of a consensus motif recognized by the co-chaperone HSC20

Iron-sulfur (Fe-S) clusters are inorganic cofactors that are fundamental to several biological processes in all three kingdoms of life. In most organisms, Fe-S clusters are initially assembled on a scaffold protein, ISCU, and subsequently transferred to target proteins or to intermediate carriers by a dedicated chaperone/co-chaperone system. The delivery of assembled Fe-S clusters to recipient proteins is a crucial step in the biogenesis of Fe-S proteins, and, in mammals, it relies on the activity of a multiprotein transfer complex that contains the chaperone HSPA9, the co-chaperone HSC20 and the scaffold ISCU. How the transfer complex efficiently engages recipient Fe-S target proteins involves specific protein interactions that are not fully understood. This mini review focuses on recent insights into the molecular mechanism of amino acid motif recognition and discrimination by the co-chaperone HSC20, which guides Fe-S cluster delivery.

Understanding the molecular basis for multiple mitochondrial dysfunctions syndrome 1 (MMDS1): impact of a disease-causing Gly189Arg substitution on NFU1

Iron-sulfur (Fe/S) cluster-containing proteins constitute one of the largest protein classes, with highly varied function. Consequently, the biosynthesis of Fe/S clusters is evolutionarily conserved and mutations in intermediate Fe/S cluster scaffold proteins can cause disease, including multiple mitochondrial dysfunctions syndrome (MMDS). Herein, we have characterized the impact of defects occurring in the MMDS1 disease state that result from a point mutation (p.Gly189Arg) near the active site of NFU1, an Fe/S scaffold protein. In vitro investigation into the structure-function relationship of the Gly189Arg derivative, along with two other variants, reveals that substitution at position 189 triggers structural changes that increase flexibility, decrease stability, and alter the monomer-dimer equilibrium toward monomer, thereby impairing the ability of the Gly189X derivatives to receive an Fe/S cluster from physiologically relevant sources.

Biallelic Mutations in LIPT2 Cause a Mitochondrial Lipoylation Defect Associated with Severe Neonatal Encephalopathy

Lipoate serves as a cofactor for the glycine cleavage system (GCS) and four 2-oxoacid dehydrogenases functioning in energy metabolism (α-oxoglutarate dehydrogenase [α-KGDHc] and pyruvate dehydrogenase [PDHc]), or amino acid metabolism (branched-chain oxoacid dehydrogenase, 2-oxoadipate dehydrogenase). Mitochondrial lipoate synthesis involves three enzymatic steps catalyzed sequentially by lipoyl(octanoyl) transferase 2 (LIPT2), lipoic acid synthetase (LIAS), and lipoyltransferase 1 (LIPT1). Mutations in LIAS have been associated with nonketotic hyperglycinemia-like early-onset convulsions and encephalopathy combined with a defect in mitochondrial energy metabolism. LIPT1 deficiency spares GCS deficiency and has been associated with a biochemical signature of combined 2-oxoacid dehydrogenase deficiency leading to early death or Leigh-like encephalopathy. We report on the identification of biallelic LIPT2 mutations in three affected individuals from two families with severe neonatal encephalopathy. Brain MRI showed major cortical atrophy with white matter abnormalities and cysts. Plasma glycine was mildly increased. Affected individuals' fibroblasts showed reduced oxygen consumption rates, PDHc, α-KGDHc activities, leucine catabolic flux, and decreased protein lipoylation. A normalization of lipoylation was observed after expression of wild-type LIPT2, arguing for LIPT2 requirement in intramitochondrial lipoate synthesis. Lipoic acid supplementation did not improve clinical condition nor activities of PDHc, α-KGDHc, or leucine metabolism in fibroblasts and was ineffective in yeast deleted for the orthologous LIP2.

A leaky splicing mutation in NFU1 is associated with a particular biochemical phenotype. Consequences for the diagnosis

Mutations in NFU1 were recently identified in patients with fatal encephalopathy. NFU1 is an iron-sulfur cluster protein necessary for the activity of the mitochondrial respiratory chain complexes I-II and the synthesis of lipoic acid. We report two NFU1 compound heterozygous individuals with normal complex I and lipoic acid-dependent enzymatic activities and low, but detectable, levels of lipoylated proteins. We demonstrated a leaky splicing regulation due to a splice site mutation (c.545+5G>A) that produces small amounts of wild type NFU1 mRNA that might result in enough protein to partially lipoylate and restore the activity of lipoic acid-dependent enzymes and the assembly and activity of complex I. These results allowed us to gain insights into the molecular basis underlying this disease and should be considered for the diagnosis of NFU1 patients.