Novel UBE3B mutations: report of eight patients with Kaufman oculocerebrofacial syndrome with additional clinical findings from a highly consanguineous population.

Biallelic mutations in UBE3B cause Kaufman oculocerebrofacial syndrome (KOS; OMIM 244450) with a wide range of clinical manifestations. In this study, we employed genetic analyses including homozygosity mapping, candidate gene sequencing, whole exome sequencing, and confirmatory Sanger sequencing on eight patients from three unrelated consanguineous families. Our analysis yielded three different novel variants in UBE3B : a missense substitution [NM_130466.4: c.2975C>T; (p.Pro992Leu)] in the HECT domain in family 1, a 3-bp deletion within exon 14 [c.1692_1694delCTC; (p.Ser565del)] leading to removal of a serine residue in family 2, and a splice donor site variant in intron eight of UBE3B (c.630 + 1G>T) in family 3. Blepharophimosis, telecanthus, ptosis, intellectual disability and abnormal lipid profile were similar to those found in previously reported KOS patients. Longitudinal follow-up revealed rather marfanoid body habitus of the patients in family 1. This study reports eight patients from Saudi Arabia with novel deleterious variants in UBE3B and adds to the phenotypic spectrum of KOS.

SHQ1-associated neurodevelopmental disorder: Report of the first homozygous variant in unrelated patients and review of the literature.

Compound heterozygous mutations in SHQ1 have been associated with a rare and severe neurological disorder characterized by global developmental delay (GDD), cerebellar degeneration coupled with seizures, and early-onset dystonia. Currently, only five affected individuals have been documented in the literature. Here, we report three children from two unrelated families harboring a homozygous variant in the gene but with a milder phenotype than previously described. The patients had GDD and seizures. Magnetic resonance imaging analyses revealed diffuse white matter hypomyelination. Sanger sequencing confirmed the whole-exome sequencing results and revealed full segregation of the missense variant (SHQ1:c.833 T > C; p.I278T) in both families. We performed a comprehensive in silico analysis using different prediction classifiers and structural modeling of the variant. Our findings demonstrate that this novel homozygous variant in SHQ1 is likely to be pathogenic and leads to the clinical features observed in our patients.

A Novel Homozygous Founder Variant of RTN4IP1 in Two Consanguineous Saudi Families.

The genetic architecture of mitochondrial disease continues to expand and currently exceeds more than 350 disease-causing genes. Bi-allelic variants in RTN4IP1, also known as Optic Atrophy-10 (OPA10), lead to early-onset recessive optic neuropathy, atrophy, and encephalopathy in the afflicted patients. The gene is known to encode a mitochondrial ubiquinol oxidoreductase that interacts with reticulon 4 and is thought to be a mitochondrial antioxidant NADPH oxidoreductase. Here, we describe two unrelated consanguineous families from the northern region of Saudi Arabia harboring a missense variant (RTN4IP1:NM_032730.5; c.475G

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.

Genetics of ataxia telangiectasia in a highly consanguineous population.

Ataxia telangiectasia (AT) is a rare autosomal recessive multisystemic disorder. It usually presents in toddler years with progressive ataxia and oculomotor apraxia, or less commonly, in the late-first or early-second decade of life with mixed movement disorders. Biallelic mutations in ataxia telangiectasia mutated gene (ATM) cause AT phenotype, a disease not well documented in Saudi Arabia, a highly consanguineous society. We studied several Saudi AT patients, identified ATM variants, and investigated associated clinical features. We included 17 patients from 12 consanguineous families. All patients had comprehensive clinical and radiological assessment, and most were examined through whole-exome sequencing (WES). Selected individuals were analyzed using various genetic approaches. We identified five different ATM variants in our patients: three previously reported mutations, and two novel variants. Nearly all patients had classical AT presentation except for two patients with a milder phenotype. Among the three known variants, a deletion causing truncation (c.381delA resulting in p.(Val128Ter)) was identified in 13 patients. Two patients harboured the other two truncating variants, (c.9001_9002delAG resulting in p.Ser3001Phefs*6) and (c.9066delA resulting in p.Glu3023Alafs*10) and two patients had novel compound heterozygous variants (NM_000051.3:Paternal Allele:c.8762C > G;p.Thr2921Arg and Maternal Allele:c.1057T > C;p.Cys353Arg). We speculate that c.381delA is a founder mutation in our population. This study provides a genotype-phenotype relationship in a previously unstudied consanguineous population. Our findings contribute to improve local clinical care, therapy, and genetic counseling.

SLC25A42-associated mitochondrial encephalomyopathy: Report of additional founder cases and functional characterization of a novel deletion.

SLC25A42 is the main transporter of coenzyme A (CoA) into mitochondria. To date, 15 individuals have been reported to have one of two bi-allelic homozygous missense variants in the SLC25A42 as the cause of mitochondrial encephalomyopathy, of which 14 of them were of Saudi origin and share the same founder variant, c.871A > G:p.Asn291Asp. The other subject was of German origin with a variant at canonical splice site, c.380 + 2 T > A. Here, we describe the clinical manifestations and the disease course in additional six Saudi patients from four unrelated consanguineous families. While five patients have the Saudi founder p.Asn291Asp variant, one subject has a novel deletion. Functional analyses on fibroblasts obtained from this patient revealed that the deletion causes significant decrease in mitochondrial oxygen consumption and ATP production compared to healthy individuals. Moreover, extracellular acidification rate revealed significantly reduced glycolysis, glycolytic capacity, and glycolytic reserve as compared to control individuals. There were no changes in the mitochondrial DNA (mtDNA) content of patient fibroblasts. Immunoblotting experiments revealed significantly diminished protein expression due to the deletion. In conclusion, we report additional patients with SLC25A42-associated mitochondrial encephalomyopathy. Our study expands the molecular spectrum of this condition and provides further evidence of mitochondrial dysfunction as a central cause of pathology. We therefore propose that this disorder should be included in the differential diagnosis of any patient with an unexplained motor and speech delay, recurrent encephalopathy with metabolic acidosis, intermittent or persistent dystonia, lactic acidosis, basal ganglia lesions and, especially, of Arab ethnicity. Finally, deep brain stimulation should be considered in the management of patients with life altering dystonia.

Bi-allelic variants in HOPS complex subunit VPS41 cause cerebellar ataxia and abnormal membrane trafficking.

Membrane trafficking is a complex, essential process in eukaryotic cells responsible for protein transport and processing. Deficiencies in vacuolar protein sorting (VPS) proteins, key regulators of trafficking, cause abnormal intracellular segregation of macromolecules and organelles and are linked to human disease. VPS proteins function as part of complexes such as the homotypic fusion and vacuole protein sorting (HOPS) tethering complex, composed of VPS11, VPS16, VPS18, VPS33A, VPS39 and VPS41. The HOPS-specific subunit VPS41 has been reported to promote viability of dopaminergic neurons in Parkinson's disease but to date has not been linked to human disease. Here, we describe five unrelated families with nine affected individuals, all carrying homozygous variants in VPS41 that we show impact protein function. All affected individuals presented with a progressive neurodevelopmental disorder consisting of cognitive impairment, cerebellar atrophy/hypoplasia, motor dysfunction with ataxia and dystonia, and nystagmus. Zebrafish disease modelling supports the involvement of VPS41 dysfunction in the disorder, indicating lysosomal dysregulation throughout the brain and providing support for cerebellar and microglial abnormalities when vps41 was mutated. This provides the first example of human disease linked to the HOPS-specific subunit VPS41 and suggests the importance of HOPS complex activity for cerebellar function.

Hematological findings associated with tubulin-folding cofactors D-related encephalopathy: Expanding the phenotype.

The dysfunction of microtubules (α/ß-tubulin polymers) underlies a wide range of nervous system genetic abnormalities. Defects in TBCD, a tubulin-folding cofactor, cause diseases highlighted with early-onset encephalopathy with or without neurodegeneration, intellectual disability, seizures, microcephaly and tetraparaperesis. Utilizing various molecular methods, we describe nine patients from four unrelated families with two novel exon 18 variants in TBCD exhibiting the typical neurological phenotype of the disease. Interestingly, all the investigated patients had previously unreported hematological findings in the form of neutropenia and mild degree of anemia and thrombocytopenia. In addition to delineating the neurological phenotype in several patients with TBCD variants, our study stresses on the new association of neutropenia, in particular, with the disease.

Pyrostigmine therapy in a patient with VAMP1-related congenital myasthenic syndrome.

Congenital myasthenic syndrome comprises several genetic disorders that impair neuromuscular junction transmission. Causative mutations occur in at least 30 genes, approximately 6-8% of which are presynaptic. One such gene, VAMP1, encodes vesicle-associated membrane protein-1, which is crucial in the formation and fusion of synaptic vesicles with the presynaptic membrane at the neuromuscular junction. VAMP1 mutations are associated with two main phenotypes: a) autosomal recessive congenital myasthenic syndrome and b) autosomal dominant spastic ataxia 1. We report a girl from a consanguineous Saudi family presenting with hypotonia, developmental delay, feeding difficulties and floppiness since birth. Comprehensive genetic testing revealed a homozygous splicing mutation in VAMP1. RT-PCR confirmed the presence of an aberrant transcript causing skipping of exon 2 in the gene.

Rett Syndrome, a Neurodevelopmental Disorder, Whole-Transcriptome, and Mitochondrial Genome Multiomics Analyses Identify Novel Variations and Disease Pathways.

Rett syndrome (RTT) is a severe neurodevelopmental disorder reported worldwide in diverse populations. RTT is diagnosed primarily in females, with clinical findings manifesting early in life. Despite the variable rates across populations, RTT has an estimated prevalence of ∼1 in 10,000 live female births. Among 215 Saudi Arabian patients with neurodevelopmental and autism spectrum disorders, we identified 33 patients with RTT who were subsequently examined by genome-wide transcriptome and mitochondrial genome variations. To the best of our knowledge, this is the first in-depth molecular and multiomics analyses of a large cohort of Saudi RTT cases with a view to informing the underlying mechanisms of this disease that impact many patients and families worldwide. The patients were unrelated, except for 2 affected sisters, and comprised of 25 classic and eight atypical RTT cases. The cases were screened for methyl-CpG binding protein 2 (MECP2), CDKL5, FOXG1, NTNG1, and mitochondrial DNA (mtDNA) variants, as well as copy number variations (CNVs) using a genome-wide experimental strategy. We found that 15 patients (13 classic and two atypical RTT) have MECP2 mutations, 2 of which were novel variants. Two patients had novel FOXG1 and CDKL5 variants (both atypical RTT). Whole mtDNA sequencing of the patients who were MECP2 negative revealed two novel mtDNA variants in two classic RTT patients. Importantly, the whole-transcriptome analysis of our RTT patients' blood and further comparison with previous expression profiling of brain tissue from patients with RTT revealed 77 significantly dysregulated genes. The gene ontology and interaction network analysis indicated potentially critical roles of MAPK9, NDUFA5, ATR, SMARCA5, RPL23, SRSF3, and mitochondrial dysfunction, oxidative stress response and MAPK signaling pathways in the pathogenesis of RTT genes. This study expands our knowledge on RTT disease networks and pathways as well as presents novel mutations and mtDNA alterations in RTT in a population sample that was not previously studied.

Loss of UGP2 in brain leads to a severe epileptic encephalopathy, emphasizing that bi-allelic isoform-specific start-loss mutations of essential genes can cause genetic diseases.

Developmental and/or epileptic encephalopathies (DEEs) are a group of devastating genetic disorders, resulting in early-onset, therapy-resistant seizures and developmental delay. Here we report on 22 individuals from 15 families presenting with a severe form of intractable epilepsy, severe developmental delay, progressive microcephaly, visual disturbance and similar minor dysmorphisms. Whole exome sequencing identified a recurrent, homozygous variant (chr2:64083454A > G) in the essential UDP-glucose pyrophosphorylase (UGP2) gene in all probands. This rare variant results in a tolerable Met12Val missense change of the longer UGP2 protein isoform but causes a disruption of the start codon of the shorter isoform, which is predominant in brain. We show that the absence of the shorter isoform leads to a reduction of functional UGP2 enzyme in neural stem cells, leading to altered glycogen metabolism, upregulated unfolded protein response and premature neuronal differentiation, as modeled during pluripotent stem cell differentiation in vitro. In contrast, the complete lack of all UGP2 isoforms leads to differentiation defects in multiple lineages in human cells. Reduced expression of Ugp2a/Ugp2b in vivo in zebrafish mimics visual disturbance and mutant animals show a behavioral phenotype. Our study identifies a recurrent start codon mutation in UGP2 as a cause of a novel autosomal recessive DEE syndrome. Importantly, it also shows that isoform-specific start-loss mutations causing expression loss of a tissue-relevant isoform of an essential protein can cause a genetic disease, even when an organism-wide protein absence is incompatible with life. We provide additional examples where a similar disease mechanism applies.

Identification of novel genomic imbalances in Saudi patients with congenital heart disease.

BACKGROUND: Quick genetic diagnosis of a patient with congenital heart disease (CHD) is quite important for proper health care and management. Copy number variations (CNV), chromosomal imbalances and rearrangements have been frequently associated with CHD. Previously, due to limitations of microscope based standard karyotyping techniques copious CNVs and submicroscopic imbalances could not be detected in numerous CHD patients. The aim of our study is to identify cytogenetic abnormalities among the selected CHD cases (n = 17) of the cohort using high density oligo arrays. RESULTS: Our screening study indicated that six patients (~35%) have various cytogenetic abnormalities. Among the patients, only patient 2 had a duplication whereas the rest carried various deletions. The patients 1, 4 and 6 have only single large deletions throughout their genome; a 3.2 Mb deletion on chromosome 7, a 3.35 Mb deletion on chromosome 3, and a 2.78 Mb a deletion on chromosome 2, respectively. Patients 3 and 5 have two deletions on different chromosomes. Patient 3 has deletions on chromosome 2 (2q24.1; 249 kb) and 16 (16q22.2; 1.8 Mb). Patient 4 has a 3.35 Mb an interstitial deletion on chromosome 3 (3q13.2q13.31).Based on our search on the latest available literature, our study is the first inclusive array CGH evaluation on Saudi cohort of CHD patients. CONCLUSIONS: This study emphasizes the importance of the arrays in genetic diagnosis of CHD. Based on our results the high resolution arrays should be utilized as first-tier diagnostic tool in clinical care as suggested before by others. Moreover, previously evaluated negative CHD cases (based on standard karyotyping methods) should be re-examined by microarray based cytogenetic methods.

A Novel Homozygous Mutation in SPTBN2 Leads to Spinocerebellar Ataxia in a Consanguineous Family: Report of a New Infantile-Onset Case and Brief Review of the Literature.

The objective of this study was the identification of likely genes and mutations associated with an autosomal recessive (AR) rare spinocerebellar ataxia (SCA) phenotype in two patients with infantile onset, from a consanguineous family. Using genome-wide SNP screening, autozygosity mapping, targeted Sanger sequencing and nextgen sequencing, family segregation analysis, and comprehensive neuropanel, we discovered a novel mutation in SPTBN2. Next, we utilized multiple sequence alignment of amino acids from various species as well as crystal structures provided by protein data bank (PDB# 1WYQ and 1WJM) to model the mutation site and its effect on ß-III-spectrin. Finally, we used various bioinformatic classifiers to determine pathogenicity of the missense variant. A comprehensive clinical and diagnostic workup including radiological exams were performed on the patients as part of routine patient care. The homozygous missense variant (c.1572C>T; p.R414C) detected in exon 2 was fully segregated in the family and absent in a large ethnic cohort as well as publicly available data sets. Our comprehensive targeted sequencing approaches did not reveal any other likely candidate variants or mutations in both patients. The two male siblings presented with delayed motor milestones and cognitive and learning disability. Brain MRI revealed isolated cerebellar atrophy more marked in midline inferior vermis at ages of 3 and 6.5 years. Sequence alignments of the amino acids for ß-III-spectrin indicated that the arginine at 414 is highly conserved among various species and located towards the end of first spectrin repeat domain. Inclusive bioinformatic analysis predicted that the variant is to be damaging and disease causing. In addition to the novel mutation, a brief literature review of the previously reported mutations as well as clinical comparison of the cases were also presented. Our study reviews the previously reported SPTBN2 mutations and cases. Moreover, the novel mutation, p.R414C, adds up to the literature for the infantile-onset form of autosomal recessive ataxia associated with SPTBN2. Previously, few SPTBN2 recessive mutations have been reported in humans. Animal models especially the ß-III-/- mouse model provided insights into early coordination and gait deficit suggestive of loss-of-function. It is expected to see more recessive SPTBN2 mutations appearing in the literature during the upcoming years.

Further delineation of the phenotypic spectrum of ISCA2 defect: A report of ten new cases.

Iron-Sulfur Cluster (ISC) biogenesis is a vital cellular process required to produce various ISC-containing proteins. These ISC proteins are responsible for essential functions such as glycine cleavage and the formation of lipoic acid, an essential cofactor of respiratory chain complexes. Defects in ISC biogenesis lead to multiple mitochondrial dysfunction syndromes including: ISCA2 with infantile onset leukodystrophy. Recently, a founder mutation, c.229G > A, p.Gly77Ser in ISCA2 was reported to cause Multiple Mitochondrial Dysfunction Syndrome type 4. In a retrospective review of children diagnosed with the ISCA2 defect, we were able to identify ten new patients who were not reported previously with the identical founder mutation. High CSF glycine levels and elevated glycine peaks on MR spectroscopy were demonstrated in all tested probands. All patients were between 3 and 7 months of age with a triad of neurodevelopmental regression, nystagmus and optic atrophy and leukodystrophy. MRI findings were typical in the patients with diffuse, abnormal white matter signal in the cerebrum, cerebellum, brain stem and spinal cord. The patients ended up in a vegetative state, and often premature death due to respiratory infections. We alert clinicians to consider the ISCA2 defect as a differential diagnosis of infantile onset leukodystrophies affecting the brain as well as the spinal cord, especially in the presence of elevated CSF glycine or elevated glycine peaks in MR spectroscopy.

A new association between CDK5RAP2 microcephaly and congenital cataracts.

INTRODUCTION: Primary microcephaly type 3 is a genetically heterogeneous condition caused by a homozygous or compound heterozygous mutation in CDK5 regulatory subunit associated protein 2 (CDK5RAP2) and characterized by reduced head circumference (<5th percentile) with additional phenotypes varying from pigmentary abnormalities to sensorineural hearing loss. Until now, congenital cataracts have not been reported in patients with primary microcephaly type 3. CLINICAL REPORT: We report multiple affected family members from a consanguineous Saudi family with microcephaly and congenital cataracts. We utilized a next-generation sequencing-based microcephaly gene panel that revealed a CDK5RAP2 variant (c.4055A>G; p.Glu1352Gly) as the most plausible candidate for the likely etiology in this family. Then we performed family segregation analysis using Sanger sequencing, autozygosity mapping, and whole exome sequencing, all of which revealed no other possible disease-causing variants. CONCLUSION: Here we report on a new clinical manifestation of CDK5RAP2 and expand the phenotype of primary microcephaly type 3.

Molecular and clinical spectra of FBXL4 deficiency.

F-box and leucine-rich repeat protein 4 (FBXL4) is a mitochondrial protein whose exact function is not yet known. However, cellular studies have suggested that it plays significant roles in mitochondrial bioenergetics, mitochondrial DNA (mtDNA) maintenance, and mitochondrial dynamics. Biallelic pathogenic variants in FBXL4 are associated with an encephalopathic mtDNA maintenance defect syndrome that is a multisystem disease characterized by lactic acidemia, developmental delay, and hypotonia. Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined deficiencies of multiple electron transport complexes, and mtDNA depletion. Since its initial description in 2013, 36 different pathogenic variants in FBXL4 were reported in 50 affected individuals. In this report, we present 37 additional affected individuals and 11 previously unreported pathogenic variants. We summarize the clinical features of all 87 individuals with FBXL4-related mtDNA maintenance defect, review FBXL4 structure and function, map the 47 pathogenic variants onto the gene structure to assess the variants distribution, and investigate the genotype-phenotype correlation. Finally, we provide future directions to understand the disease mechanism and identify treatment strategies.

KCNA4 deficiency leads to a syndrome of abnormal striatum, congenital cataract and intellectual disability.

BACKGROUND: Voltage-gated potassium channels are highly diverse proteins representing the most complex class of voltage-gated ion channels from structural and functional perspectives. Deficiency of these channels usually results in various human disorders. OBJECTIVES: To describe a novel autosomal recessive syndrome associated with KCNA4 deficiency leading to congenital cataract, abnormal striatum, intellectual disability and attention deficit hyperactivity disorder. METHODS: We used SNP arrays, linkage analyses, autozygosity mapping, whole-exome sequencing, RT-PCR and two-electrode voltage-clamp recording. RESULTS: We identified a missense variant (p.Arg89Gln) in KCNA4 in four patients from a consanguineous family manifesting a novel syndrome of congenital cataract, abnormal striatum, intellectual disability and attention deficit hyperactivity disorder. The variant was fully segregated with the disease and absent in 747 ethnically matched exomes. Xenopus oocytes were injected with human Kv1.4 wild-type mRNA, R89Q and WT/R89Q channels. The wild type had mean current amplitude that was significantly greater than those recorded from the cells expressing the same amount of mutant mRNA. Co-expression of the wild type and mutant mRNAs resulted in mean current amplitude that was significantly different from that of the wild type. RT-PCR indicated that KCNA4 is present in mouse brain, lens and retina. KCNA4 interacts with several molecules including synaptotagmin I, DLG1 and DLG2. The channel co-localises with cholinergic amacrine and rod bipolar cells in rats and is widely distributed in the central nervous system. Based on previous studies, the channel is highly expressed in outer retina, rod inner segments, hippocampus and concentrated in axonal membranes. CONCLUSION: KCNA4 (Kv1.4) is implicated in a novel syndrome characterised by striatal thinning, congenital cataract and attention deficit hyperactivity disorder. Our study highlights potassium channels' role in ocular and neuronal genetics.

ISCA2 mutation causes infantile neurodegenerative mitochondrial disorder.

BACKGROUND: There are numerous nuclear genes that cause mitochondrial disorders and clinically and genetically heterogeneous disorders whose aetiology often remains unsolved. In this study, we aim to investigate an autosomal recessive syndrome causing leukodystrophy and neuroregression. We studied six patients from five unrelated consanguineous families. METHODS: Patients underwent full neurological, radiological, genetic, metabolic and dysmorphological examinations. Exome sequencing coupled with autozygosity mapping, Sanger sequencing, microsatellite haplotyping, standard and molecular karyotyping and whole mitochondrial DNA sequencing were used to identify the genetic cause of the syndrome. Immunohistochemistry, transmission electron microscopy, confocal microscopy, dipstick assays, quantitative PCR, reverse transcription PCR and quantitative reverse transcription PCR were performed on different tissue samples from the patients. RESULTS: We identified a homoallelic missense founder mutation in ISCA2 leading to mitochondrial depletion and reduced complex I activity as well as decreased ISCA2, ISCA1 and IBA57 expression in fibroblasts. MRI indicated similar white matter abnormalities in the patients. Histological examination of the skeletal muscle showed mild to moderate variation in myofibre size and the presence of many randomly distributed atrophic fibres. CONCLUSIONS: Our data demonstrate that ISCA2 deficiency leads to a hereditary mitochondrial neurodegenerative white matter disease in infancy.