REPORT FASTKD2 Nonsense Mutation in an Infantile Mitochondrial Encephalomyopathy Associated with Cytochrome C Oxidase Deficiency
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CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector REPORT FASTKD2 Nonsense Mutation in an Infantile Mitochondrial Encephalomyopathy Associated with Cytochrome C Oxidase Deficiency Daniele Ghezzi,1 Ann Saada,2 Pio D’Adamo,3 Erika Fernandez-Vizarra,1 Paolo Gasparini,3 Valeria Tiranti,1 Orly Elpeleg,2 and Massimo Zeviani1,* In two siblings we found a mitochondrial encephalomyopathy, characterized by developmental delay, hemiplegia, convulsions, asym- metrical brain atrophy, and low cytochrome c oxidase (COX) activity in skeletal muscle. The disease locus was identified on chromosome 2 by homozygosity mapping; candidate genes were prioritized for their known or predicted mitochondrial localization and then se- quenced in probands and controls. A homozygous nonsense mutation in the KIAA0971 gene segregated with the disease in the proband family. The corresponding protein is known as fas activated serine-threonine kinase domain 2, FASTKD2. Confocal immunofluorescence colocalized a tagged recombinant FASTKD2 protein with mitochondrial markers, and membrane-potential-dependent in vitro mito- chondrial import was demonstrated in isolated mitochondria. In staurosporine-induced-apoptosis experiments, decreased nuclear fragmentation was detected in treated mutant versus control fibroblasts. In conclusion, we found a loss-of-function mutation in a gene segregating with a peculiar mitochondrial encephalomyopathy associated with COX deficiency in skeletal muscle. The corresponding protein is localized in the mitochondrial inner compartment. Preliminary data indicate that FASTKD2 plays a role in mitochondrial apoptosis. Mitochondrial disorders are clinical phenotypes associated and proteins may be considered as a candidate for disease. with abnormalities of the terminal component of aerobic Therefore, it is not surprising that in spite of the tremen- energy metabolism, i.e., oxidative phosphorylation (OX- dous expansion of scientific and medical knowledge in PHOS), a complex pathway linking cellular respiration to the field, the genetic etiology of many mitochondrial clin- adenosine triphosphate (ATP) synthesis. OXPHOS is car- ical syndromes still remains elusive, the mechanisms lead- ried out in the inner mitochondrial membrane by the ing to disease are poorly understood, and the very function mitochondrial respiratory chain (MRC). The MRC is com- of several factors involved in mitochondrial disorders is posed of a series of multiheteromeric enzymes resulting not, or is at best partially, known. Approximately 60% of from a dual genetic contribution, i.e., the nuclear and biochemically defined OXPHOS disorders still fail to get the mitochondrial genomes. Whereas the 13 MRC proteins diagnosed at the molecular level. To identify additional encoded by mtDNA are all essential for OXPHOS, they nuclear genes that cause mitochondrial disease pheno- make up only a tiny fraction of the total number of pro- types, we have embarked on a systematic, genome-wide teins involved in a functional MRC, most of which are in linkage-analysis project based on homozygosity mapping fact nucleus encoded and imported into mitochondria. of consanguineous multiplex families characterized by As a consequence, mitochondrial disease can be due to mu- mitochondrial recessive traits. tations in either mtDNA or nuclear DNA genes function- The family reported in this paper (Figure 1) is composed ally related to MRC and OXPHOS.1 In fact, a huge number of first-cousin parents of Bedouin origin, with an offspring of pathogenic mutations of mtDNA have been building up of seven siblings, two of whom are affected by a peculiar, in the last two decades, in association with a wide spec- early-onset encephalomyopathy. trum of clinical presentations. Albeit at a lesser rate, sub- Patient II-1, a female, was born at term after an unevent- stantial progress has also been made in the characteriza- ful pregnancy by caesarean section due to nonprogression tion of nuclear OXPHOS-related genes responsible for of labor. Birth weight was 3800 grams. Bilateral congenital mitochondrial syndromes.2 hip dislocation was managed by abduction splinting. In addition to approximately 80 MRC structural sub- Marked irritability and inconsolable cry were noted since units, OXPHOS nuclear genes encode a remarkable num- birth. Nonetheless, social and motor development was ber of factors that control the formation, turnover, and reported as normal until 7 months of age. At that time, activity of MRC, including mtDNA biogenesis and expres- she suffered from febrile illness and developed refractory sion, and their adaptation to tissue- and development-spe- generalized tonic-clonic convulsions. Because atypical cific needs.3–5 The size of the mitochondrial proteome is pneumonia was suspected, she was treated with oral eryth- not precisely known, but the most recent studies have set romycin. Brain MRI examination disclosed generalized it up to more than 1400 entries.6,7 Each of these genes symmetric atrophy (Figure 2A). In the following years, 1Division of Molecular Neurogenetics, Foundation IRCCS Neurological Institute ‘‘C. Besta,’’ 20126 Milan, Italy; 2Metabolic Disease Unit, Hadassah-Hebrew University Medical Center, 91120 Jerusalem, Israel; 3Division of Medical Genetics, IRCCS Burlo Garofolo - University of Trieste, 34137 Trieste, Italy *Correspondence: [email protected] DOI 10.1016/j.ajhg.2008.08.009. ª2008 by The American Society of Human Genetics. All rights reserved. The American Journal of Human Genetics 83, 415–423, September 12, 2008 415 Figure 1. Pedigree and Haplotypes Filled symbols indicate affected individuals. Each haplotype is indicated by a distinct color. her epileptic disease persisted, the psychomotor develop- the other MRC complexes, as well as that of the pyruvate ment was markedly delayed, and left side hemiplegia en- dehydrogenase complex (PDHc), were within the control sued, with facial-nerve involvement. Brain CT scan per- range. The respiratory-chain activities were measured 2 yr formed at 5 yr of age revealed severe atrophic changes on later in the muscle homogenate by a second laboratory, the right hemisphere (Figure 2B). Because of fixed contrac- at the Istituto Neurologico ‘‘Carlo Besta’’ in Milan, Italy. tures, the hip and ankle tendons were surgically released at Again, the activity of COX, normalized to that of citrate 7 yr of age. Echocardiography, abdominal ultrasound, synthase (CS), was severely reduced, to 14% of the control blood count, and liver and renal functions were repeatedly mean, while the activities of complex II and III were in the normal and plasma lactate was mildly increased (2.4–3.2 lower limit of the normal or just below it (Table S1 available mM in different samples; normal values [n.v.] < 1.8 mM). online). The activity of complex I could not be measured At 14 yr she obeyed simple commands in two languages, because of the scarcity of the material. In spite of the iso- could recognize colors, and had a 20 word vocabulary. She lated COX defect detected in muscle homogenate, no gross could sit herself up and move herself relatively smoothly abnormality of COX and succinate dehydrogenase (SDH) along the floor while sitting, but was never able to stand histochemical reactions were reported in muscle, and the or walk. Her hearing was intact but the eyesight was im- trichrome Gomori stain was normal. The activities of all paired because of bilateral optic atrophy. She had left spas- MRC complexes were normal in cultured skin fibroblasts tic hemiparesis; on the right side the muscle tone was (Table S1). decreased with reduced strength (3/5). Myoclonic and ge- Patient II-6, the 6th child of the couple, was born at term, lastic seizures were partly controlled by carbamazepine, with a birth weight of 4150 gr. His early development was valproate, and clonazepam. The EEG revealed bilateral ep- uneventful, and at 10 months he could sit himself up and ileptic activity, which was more prominent over the left crawl freely. After a febrile gastroenteritis at 1 yr of age, the hemisphere. Biochemical assays were first performed in patient experienced subacute neurological deterioration isolated mitochondria at Hadassah-Hebrew University with muscle hypotonia and extrapyramidal movements, Medical Center in Jerusalem, Israel. As shown in Table 1, mainly on the left limbs. Brain MRI disclosed increased sig- the specific activity of cytochrome c oxidase (COX) was re- nal intensities on the left nucleus caudatus, globus pal- duced to 21% of the control mean (3.1 nmoles/min/mg lidus, and crus cerebri. The lateral ventricles and the basal versus a control mean of 14.8 5 3.6). The activities of cysternae were of normal size (Figure 2C). A convulsive 416 The American Journal of Human Genetics 83, 415–423, September 12, 2008 Figure 2. Brain Imaging (A) Magnetic resonance imaging (MRI) T1- weighted sequence on a transverse section of supratentorial brain of patient II-1 at disease onset (7 months of age). (B) Computerized tomography (CT) scan of the brain in patient II-1 at 5 yr. (C) MRI T2-weighted sequence on a trans- verse section of supratentorial brain in patient II-6 at disease onset (1 yr of age). (D) CT scan of the same brain section in patient II-1 at 30 months. (Figure 2D). At four years, he was bed-ridden with neither communica- tion nor any voluntary activity. There was bilateral optic atrophy and stra- bismus. His general muscle tone was decreased with hyperreflexia and dys- tonic posturing. The gag reflex was weak and he was fed via gastrostome. He had two episodes of deep-vein thrombosis at sites of central-line catheters, but blood coagulation screen was negative. Echocardiogra- phy, abdominal ultrasound, blood count, blood gases, liver and renal function tests, urinary organic acids, plasma very-long-chain fatty acids, plasma lactate, and amino acid levels were normal. CSF lactate was in- creased to 3.8 mM (n.v. < 1.8 mM), disorder, first noted at around one year, became refractory and the activity of COX in his lymphocytes was reported to treatment at the third year of life. The patient went into as markedly decreased.