Structural basis of thiamine transport and drug recognition by SLC19A3

Thiamine (vitamin B1) functions as an essential coenzyme in cells. Humans and other mammals cannot synthesise this vitamin de novo and thus have to take it up from their diet. Eventually, every cell needs to import thiamine across its plasma membrane, which is mainly mediated by the two specific thiamine transporters SLC19A2 and SLC19A3. Loss of function mutations in either of these transporters lead to detrimental, life-threatening metabolic disorders. SLC19A3 is furthermore a major site of drug interactions. Many medications, including antidepressants, antibiotics and chemotherapeutics are known to inhibit this transporter, with potentially fatal consequences for patients. Despite a thorough functional characterisation over the past two decades, the structural basis of its transport mechanism and drug interactions has remained elusive. Here, we report seven cryo-electron microscopy (cryo-EM) structures of the human thiamine transporter SLC19A3 in complex with various ligands. Conformation-specific nanobodies enable us to capture different states of SLC19A3's transport cycle, revealing the molecular details of thiamine recognition and transport. We identify seven previously unknown drug interactions of SLC19A3 and present structures of the transporter in complex with the inhibitors fedratinib, amprolium and hydroxychloroquine. These data allow us to develop an understanding of the transport mechanism and ligand recognition of SLC19A3.

The Impacts of Slc19a3 Deletion and Intestinal SLC19A3 Insertion on Thiamine Distribution and Brain Metabolism in the Mouse

The Thiamine Transporter 2 (THTR2) encoded by SLC19A3 plays an ill-defined role in the maintenance of tissue thiamine, thiamine monophosphate, and thiamine diphosphate (TDP) levels. To evaluate the impact of THTR2 on tissue thiamine status and metabolism, we expressed the human SLC19A3 transgene in the intestine of total body Slc19a3 knockout (KO) mice. Male and female wildtype (WT) and transgenic (TG) mice were fed either 17 mg/kg (1×) or 85 mg/kg (5×) thiamine hydrochloride diet, while KOs were only fed the 5× diet. Thiamine vitamers in plasma, red blood cells, duodenum, brain, liver, kidney, heart, and adipose tissue were measured. Untargeted metabolomics were performed on the brain tissues of groups with equivalent plasma thiamine. KO mice had ~two- and ~three-fold lower plasma and brain thiamine levels than WT on the 5× diet. Circulating vitamers were sensitive to diet and equivalent in TG and WT mice. However, TG had 60% lower thiamine but normal brain TDP levels regardless of diet, with subtle differences in the heart and liver. The loss of THTR2 reduced levels of nucleic acid and amino acid derivatives in the brain. Therefore, mutation or inhibition of THTR2 may alter the brain metabolome and reduce the thiamine reservoir for TDP biosynthesis.

Synthetic Thioesters of Thiamine: Promising Tools for Slowing Progression of Neurodegenerative Diseases

Thiamine (vitamin B1) is essential for the brain. This is attributed to the coenzyme role of thiamine diphosphate (ThDP) in glucose and energy metabolism. The synthetic thiamine prodrug, the thioester benfotiamine (BFT), has been extensively studied and has beneficial effects both in rodent models of neurodegeneration and in human clinical studies. BFT has no known adverse effects and improves cognitive outcomes in patients with mild Alzheimer's disease. In cell culture and animal models, BFT has antioxidant and anti-inflammatory properties that seem to be mediated by a mechanism independent of the coenzyme function of ThDP. Recent in vitro studies show that another thiamine thioester, O,S-dibenzoylthiamine (DBT), is even more efficient than BFT, especially with respect to its anti-inflammatory potency, and is effective at lower concentrations. Thiamine thioesters have pleiotropic properties linked to an increase in circulating thiamine concentrations and possibly in hitherto unidentified open thiazole ring derivatives. The identification of the active neuroprotective metabolites and the clarification of their mechanism of action open extremely promising perspectives in the field of neurodegenerative, neurodevelopmental, and psychiatric conditions. The present review aims to summarize existing data on the neuroprotective effects of thiamine thioesters and give a comprehensive account.

Focal lesions following intracerebral gene therapy for mucopolysaccharidosis IIIA

OBJECTIVE

Mucopolysaccharidosis type IIIA (MPSIIIA) caused by recessive SGSH variants results in sulfamidase deficiency, leading to neurocognitive decline and death. No disease-modifying therapy is available. The AAVance gene therapy trial investigates AAVrh.10 overexpressing human sulfamidase (LYS-SAF302) delivered by intracerebral injection in children with MPSIIIA. Post-treatment MRI monitoring revealed lesions around injection sites. Investigations were initiated in one patient to determine the cause.

METHODS

Clinical and MRI details were reviewed. Stereotactic needle biopsies of a lesion were performed; blood and CSF were sampled. All samples were used for viral studies. Immunohistochemistry, electron microscopy, and transcriptome analysis were performed on brain tissue of the patient and various controls.

RESULTS

MRI revealed focal lesions around injection sites with onset from 3 months after therapy, progression until 7 months post therapy with subsequent stabilization and some regression. The patient had transient slight neurological signs and is following near-normal development. No evidence of viral or immunological/inflammatory cause was found. Immunohistochemistry showed immature oligodendrocytes and astrocytes, oligodendrocyte apoptosis, strong intracellular and extracellular sulfamidase expression and hardly detectable intracellular or extracellular heparan sulfate. No activation of the unfolded protein response was found.

INTERPRETATION

Results suggest that intracerebral gene therapy with local sulfamidase overexpression leads to dysfunction of transduced cells close to injection sites, with extracellular spilling of lysosomal enzymes. This alters extracellular matrix composition, depletes heparan sulfate, impairs astrocyte and oligodendrocyte function, and causes cystic white matter degeneration at the site of highest gene expression. The AAVance trial results will reveal the potential benefit-risk ratio of this therapy.

Child Neurology: Infantile Biotin Thiamine Responsive Basal Ganglia Disease: Case Report and Brief Review

Biotin thiamine responsive basal ganglia disease (BTRBGD) is an inherited autosomal recessive disorder that results from the inability of thiamine to cross the blood-brain barrier. It is considered a treatable condition if vitamin supplementation, most commonly with thiamine and biotin, is initiated early. BTRBGD can present as an infantile form, classical childhood form, or adult Wernicke-like encephalopathy. The infantile form is often the most severe and portends a worse prognosis with high mortality despite vitamin supplementation. We present a two-month-old who presented with irritability, opisthotonos, and abnormal eye movements who was found to have compound heterozygous variants in the SLC19A3 gene inherited in trans, including one known pathogenic intronic variant and a novel variant presumed to be pathogenic. She was therefore diagnosed with infantile BTRBGD. In this report, we discuss the differential for infantile BTRBGD, the clinical and radiologic features of BTRBGD, and describe a rapid, positive response to early vitamin supplementation in an infant with a likely pathogenic novel variant in SLC19A3.

Leigh syndrome

Leigh syndrome, or subacute necrotizing encephalomyelopathy, was initially recognized as a neuropathological entity in 1951. Bilateral symmetrical lesions, typically extending from the basal ganglia and thalamus through brainstem structures to the posterior columns of the spinal cord, are characterized microscopically by capillary proliferation, gliosis, severe neuronal loss, and relative preservation of astrocytes. Leigh syndrome is a pan-ethnic disorder usually with onset in infancy or early childhood, but late-onset forms occur, including in adult life. Over the last six decades it has emerged that this complex neurodegenerative disorder encompasses more than 100 separate monogenic disorders associated with enormous clinical and biochemical heterogeneity. This chapter discusses clinical, biochemical and neuropathological aspects of the disorder, and postulated pathomechanisms. Known genetic causes, including defects of 16 mitochondrial DNA (mtDNA) genes and approaching 100 nuclear genes, are categorized into disorders of subunits and assembly factors of the five oxidative phosphorylation enzymes, disorders of pyruvate metabolism and vitamin and cofactor transport and metabolism, disorders of mtDNA maintenance, and defects of mitochondrial gene expression, protein quality control, lipid remodeling, dynamics, and toxicity. An approach to diagnosis is presented, together with known treatable causes and an overview of current supportive management options and emerging therapies on the horizon.

Exome-wide Analysis of De Novo and Rare Genetic Variants in Patients With Brain Arteriovenous Malformation

BACKGROUND AND OBJECTIVES

Brain arteriovenous malformation (bAVM) is a congenital disorder and a leading cause of hemorrhagic stroke. Germline genetic variants play an essential role in the pathogenesis of brain arteriovenous malformation. However, the biological relevance of the disease-associated genes identified in previous studies is elusive. In this study, we aim to systematically investigate the contribution of germline variants to bAVM and explore the critical molecular pathways underlying the pathogenesis of bAVM.

METHODS

Probands with sporadic bAVM were consecutively recruited into this study from November 2015 to November 2018 and underwent exome sequencing. The controls were aggregated from individuals who were not known to have vascular malformation and underwent exome sequencing for clinical or research purposes. The retained control dataset included 4609 individuals, including 251 individuals with parental samples sequenced. We firstly compared de novo variants in cases and controls and performed a pathway enrichment analysis. A gene-based rare variant association analysis was then performed to identify genes whose variants were significantly enriched in cases.

RESULTS

We collected an exome-sequenced bAVM cohort consisting of 152 trios and 40 singletons. By firstly focusing on de novo variants, we observed a significant mutational burden of de novo likely gene-disrupting variants in cases versus controls. By performing a pathway enrichment analysis of all nonsynonymous de novo variants identified in cases, we found the angiopoietin-like protein 8 (ANGPTL8) regulatory pathway to be significantly enriched in patients with bAVM. Through an exome-wide rare variant association analysis utilizing 4394 in-house exome data as controls, we identified SLC19A3 as a disease-associated gene for bAVM. In addition, we found that the SLC19A3 variants in cases are preferably located at the N' side of the SLC19A3 protein. These findings implicate a phenotypic extension of SLC19A3-related disorders with a domain-specific effect.

DISCUSSION

This study provides insights into the biological basis of bAVM by identifying novel molecular pathways and candidate genes.

DTYMK is essential for genome integrity and neuronal survival

Nucleotide metabolism is a complex pathway regulating crucial cellular processes such as nucleic acid synthesis, DNA repair and proliferation. This study shows that impairment of the biosynthesis of one of the building blocks of DNA, dTTP, causes a severe, early-onset neurodegenerative disease. Here, we describe two unrelated children with bi-allelic variants in DTYMK, encoding dTMPK, which catalyzes the penultimate step in dTTP biosynthesis. The affected children show severe microcephaly and growth retardation with minimal neurodevelopment. Brain imaging revealed severe cerebral atrophy and disappearance of the basal ganglia. In cells of affected individuals, dTMPK enzyme activity was minimal, along with impaired DNA replication. In addition, we generated dtymk mutant zebrafish that replicate this phenotype of microcephaly, neuronal cell death and early lethality. An increase of ribonucleotide incorporation in the genome as well as impaired responses to DNA damage were observed in dtymk mutant zebrafish, providing novel pathophysiological insights. It is highly remarkable that this deficiency is viable as an essential component for DNA cannot be generated, since the metabolic pathway for dTTP synthesis is completely blocked. In summary, by combining genetic and biochemical approaches in multiple models we identified loss-of-function of DTYMK as the cause of a severe postnatal neurodegenerative disease and highlight the essential nature of dTTP synthesis in the maintenance of genome stability and neuronal survival.

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.

Report of the Largest Chinese Cohort With SLC19A3 Gene Defect and Literature Review

Thiamine metabolism dysfunction syndrome 2 (THMD2) is a rare metabolic disorder caused by SLC19A3 mutations, inherited in autosomal recessive pattern. As a treatable disease, early diagnosis and therapy with vitamin supplementation is important to improve the prognosis. So far, the reported cases were mainly from Saudi Arab regions, and presented with relatively simple clinical course because of the hot spot mutation (T422A). Rare Chinese cases were described until now. In this study, we investigated 18 Chinese THMD2 patients with variable phenotypes, and identified 23 novel SLC19A3 mutations, which expanded the genetic and clinical spectrum of the disorder. Meanwhile, we reviewed all 146 reported patients from different countries. Approximately 2/3 of patients presented with classical BTBGD, while 1/3 of patients manifested as much earlier onset and poor prognosis, including infantile Leigh-like syndrome, infantile spasms, neonatal lactic acidosis and infantile BTBGD. Literature review showed that elevated lactate in blood and CSF, as well as abnormal OXPHOS activities of muscle or skin usually correlated with infantile phenotypes, which indicated poor outcome. Brainstem involvement on MRI was more common in deceased cases. Thiamine supplementation is indispensable in the treatment of THMD2, whereas combination of biotin and thiamine is not superior to thiamine alone. But biotin supplementation does work in some patients. Genotypic-phenotypic correlation remains unclear which needs further investigation, and biallelic truncated mutations usually led to more severe phenotype.

Early Infantile Thiamine Transporter-2 Deficiency with Epileptic Spasms—A Phenotypic Spectrum with a Novel Mutation

Epileptic seizures are a frequent feature of thiamine transporter deficiency that may present as a clinical continuum between severe epileptic encephalopathy and mixed focal or generalized seizures. Thiamine metabolism dysfunction syndrome 2 (MIM: 607483) or biotin-thiamine-responsive basal ganglia disease (BTBGD) due to biallelic pathogenic mutation in the SLC19A3 gene is a well-recognized cause of early infantile encephalopathy with a Leigh syndrome-like presentation and a lesser-known phenotype of atypical infantile spasms. We reported a 4-month-old infant who presented with progressive epileptic spasms since 1 month of age, psychomotor retardation, and lactic acidosis. Magnetic resonance imaging (MRI) revealed altered signal intensities in bilateral thalamic and basal ganglia, cerebellum, brainstem, cortical and subcortical white matter. Whole exome sequencing identified a homozygous ENST00000258403.3: c.871G > C (p.Gly291Arg) variant in the SLC19A3 gene. We elucidate the features in the proband, which were an amalgamation of both the above subtypes of the SLC19A3 associated with early infantile encephalopathy. We also highlight the features which were atypical for either “Leigh syndrome-like” or “atypical infantile spasm” phenotypes and suggest that the two separate entities can be merged as a clinical continuum. Treatment outcome with high-dose biotin and thiamine is promising. In addition, we report a novel pathogenic variant in the SLC19A3 gene.

Biotin-Thiamine-Responsive Basal Ganglia Disease in Children: A Treatable Neurometabolic Disorder

Biotin-thiamine-responsive basal ganglia disease is a rare, autosomal recessive, treatable, neurometabolic disorder associated with biallelic pathogenic variations in the SLC19A3 gene. The condition may present as an early-childhood encephalopathy, an early-infantile lethal encephalopathy with lactic acidosis, with or without infantile spasms, or a late-onset Wernicke-like encephalopathy. The key radiological features are bilateral, symmetrical lesions in the caudate, putamen, and medial thalamus, with variable extension into the brain stem, cerebral cortex, and cerebellum. Treatment is life long and includes initiation of high dose biotin and thiamine. Genetic testing confirms the diagnosis. The prognosis depends on the time from diagnosis to the time of vitamin supplementation. The genotype-phenotype correlations are not clear yet, but the early infantile phenotype portends a poorer prognosis. We provide a brief overview of the disorder and emphasize the initiation of high-dose biotin and thiamine in infants and children with unexplained encephalopathy and basal ganglia involvement.

Cerebral Microangiopathy in Leukoencephalopathy With Cerebral Calcifications and Cysts: A Pathological Description

Leukoencephalopathy with calcifications and cysts (LCC) is a neurological syndrome recently associated with pathogenic variants in SNORD118. We report autopsy neuropathological findings from an individual with genetically confirmed LCC. Histologic studies included staining of formalin-fixed paraffin-embedded tissue sections by hematoxylin and eosin, elastic van Gieson, and luxol fast blue. Immunohistochemistry stains against glial fibrillary acidic protein, proteolipid protein, phosphorylated neurofilament, CD31, alpha-interferon, LN3, and inflammatory markers were performed. Gross examination revealed dark tan/gray appearing white matter with widespread calcifications. Microscopy revealed a diffuse destructive process due to a vasculopathy with secondary ischemic lesions and mineralization. The vasculopathy involved clustered small vessels, resembling vascular malformations, and sporadic lymphocytic infiltration of vessel walls. The white matter was also diffusely abnormal, with concurrent loss of myelin and axons, tissue rarefaction with multifocal cystic degeneration, and the presence of foamy macrophages, secondary calcifications, and astrogliosis. The midbrain, pons, and cerebellum were diffusely involved. It is not understood why variants in SNORD118 result in a disorder that predominantly causes neurological disease and significantly disrupts the cerebral vasculature. Clinical and radiological benefit was recently reported in an LCC patient treated with Bevacizumab; it is important that these patients are rapidly diagnosed and trial of this treatment modality is considered in appropriate circumstances.

Targeted Therapies for Leigh Syndrome: Systematic Review and Steps Towards a 'Treatabolome'

BACKGROUND

Leigh syndrome (LS) is the most frequent paediatric clinical presentation of mitochondrial disease. The clinical phenotype of LS is highly heterogeneous. Though historically the treatment for LS is largely supportive, new treatments are on the horizon. Due to the rarity of LS, large-scale interventional studies are scarce, limiting dissemination of information of therapeutic options to the wider scientific and clinical community.

OBJECTIVE

We conducted a systematic review of pharmacological therapies of LS following the guidelines for FAIR-compliant datasets.

METHODS

We searched for interventional studies within Clincialtrials.gov and European Clinical trials databases. Randomised controlled trials, observational studies, case reports and case series formed part of a wider MEDLINE search.

RESULTS

Of the 1,193 studies initially identified, 157 met our inclusion criteria, of which 104 were carried over into our final analysis. Treatments for LS included very few interventional trials using EPI-743 and cysteamine bitartrate. Wider literature searches identified case series and reports of treatments repleting glutathione stores, reduction of oxidative stress and restoration of oxidative phosphorylation.

CONCLUSIONS

Though interventional randomised controlled trials have begun for LS, the majority of evidence remains in case reports and case series for a number of treatable genes, encoding cofactors or transporter proteins of the mitochondria. Our findings will form part of the international expert-led Solve-RD efforts to assist clinicians initiating treatments in patients with treatable variants of LS.

Single gene, two diseases, and multiple clinical presentations: Biotin-thiamine-responsive basal ganglia disease

AIM

To present seven new genetically confirmed cases of biotin-thiamin-responsive basal ganglia disease (BTBGD) with different clinical and brain magnetic resonance imaging (MRI) characteristics.

MATERIAL AND METHODS

Genetic variants, clinical presentations, brain MRI findings, treatment response, and prognosis of seven selected patients with BTBGD, diagnosed with SLC19A3 mutations were described.

RESULTS

Among seven patients diagnosed with BTBGD, two had early infantile form, four had classic childhood form, and one was asymptomatic. Four different homozygous variants were found in the SLC19A3. Two patients with early infantile form presented with encephalopathy, dystonia, and refractory seizure in the neonatal period and have different variants. Their MRI findings were similar and pathognomonic for the early infantile form. Three siblings had same variants: one presented seizure and encephalopathy at the age of 4 months, one presented seizure at 14 years, and another was asymptomatic at 20 years. Only one of them had normal MRI findings, and the others MRI findings were similar and suggestive of the classic form. Other two siblings; one of them presented with developmental delay, seizure, and dystonia at 18 months and the other presented with subacute encephalopathy and ataxia at 20 months. Their MRI findings were also similar and suggestive of the classic form.

CONCLUSION

BTBGD may present with dissimilar clinical characteristics or remain asymptomatic for a long time period even in a family or patients with same variants. Brain MRI patterns may be important for the early diagnosis of BTBGD that would save children's lives.

Eleven novel mutations and clinical characteristics in seven Chinese patients with thiamine metabolism dysfunction syndrome

Thiamine metabolism dysfunction syndrome (THMD) comprises a group of clinically and genetically heterogeneous encephalopathies with autosomal recessive inheritance. Four genes, SLC19A3, SLC25A19, SLC19A2, and TPK1, are associated with this disorder. This study aimed to explore the clinical, biochemical and molecular characteristics of seven Chinese patients with THMD. Targeted next-generation sequencing of mitochondrial DNA and nuclear DNA was used to identify the causative mutations. The patients presented with subacute encephalopathy between the ages of 1-27 months. Brain magnetic resonance imaging (MRI) revealed abnormalities in the basal ganglia, indicating Leigh syndrome. Urine α-ketoglutarate in five patients was elevated. In four patients, five novel mutations (c.1276_1278delTAC, c.265A > C, c.197T > C, c.850T > C, whole gene deletion) were found in SLC19A3, which is associated with THMD2. In two patients, four novel mutations (c.194C > T, c.454C > A, c.481G > A, and c.550G > C) were identified in SLC25A19, supporting a diagnosis of THMD4. In one patient, two novel mutations (c.395T > C and c.614-1G > A) were detected in TPK1, which is indicative of THMD5. The patients received thiamine, biotin, and symptomatic therapy, upon which six patients demonstrated clinical improvement. Our findings expanded the phenotypic and genotypic spectrum of THMD, with eleven novel mutations identified in seven Chinese patients. Early diagnosis and treatment have a significant impact on prognosis.

Biotin–Thiamine Responsive Basal Ganglia Disease: A Treatable Metabolic Encephalopathy—Not to Be Missed!

Biotin–thiamine responsive basal ganglia disease (BTBGD) is an autosomal recessive neurometabolic disorder, characterized by encephalopathy, extrapyramidal signs following mild infection, trauma or surgery and is potentially reversible with treatment. We describe a 15-month-old female child of Indian-Muslim origin with characteristic clinical and radiological features of BTBGD that showed complete resolution with treatment. A comparison with previously reported cases reveals a different mutation (exon 2 vs. exon 5 in middle east cases) in the SLC19A3 gene that could be specific for the Indian subcontinent. We also emphasize the importance of a trial of vitamins in patients with acute metabolic encephalopathy.

Genetic heterogeneity in Leigh syndrome: Highlighting treatable and novel genetic causes

Leigh syndrome (LS), the most common childhood mitochondrial disorder, has characteristic clinical and neuroradiologic features. Mutations in more than 75 genes have been identified in both the mitochondrial and nuclear genome, implicating a high degree of genetic heterogeneity in LS. To profile these genetic signatures and understand the pathophysiology of LS, we recruited 64 patients from 62 families who were clinically diagnosed with LS at Seoul National University Children's Hospital. Mitochondrial genetic analysis followed by whole-exome sequencing was performed on 61 patients. Pathogenic variants in mitochondrial DNA were identified in 18 families and nuclear DNA mutations in 22. The following 17 genes analyzed in 40 families were found to have genetic complexity: MTATP6, MTND1, MTND3, MTND5, MTND6, MTTK, NDUFS1, NDUFV1, NDUFAF6, SURF1, SLC19A3, ECHS1, PNPT1, IARS2, NARS2, VPS13D, and NAXE. Two treatable cases had biotin-thiamine responsive basal ganglia disease, and another three were identified as having defects in the newly recognized genes (VPS13D or NAXE). Variants in the nuclear genes that encoded mitochondrial aminoacyl tRNA synthetases were present in 27.3% of cases. Our findings expand the genetic and clinical spectrum of LS, showing genetic heterogeneity and highlighting treatable cases and those with novel genetic causes.

Metachromatic leukodystrophy and transplantation: remyelination, no cross-correction

OBJECTIVE

In metachromatic leukodystrophy, a lysosomal storage disorder due to decreased arylsulfatase A activity, hematopoietic stem cell transplantation may stop brain demyelination and allow remyelination, thereby halting white matter degeneration. This is the first study to define the effects and therapeutic mechanisms of hematopoietic stem cell transplantation on brain tissue of transplanted metachromatic leukodystrophy patients.

METHODS

Autopsy brain tissue was obtained from eight (two transplanted and six nontransplanted) metachromatic leukodystrophy patients, and two age-matched controls. We examined the presence of donor cells by immunohistochemistry and microscopy. In addition, we assessed myelin content, oligodendrocyte numbers, and macrophage phenotypes. An unpaired t-test, linear regression or the nonparametric Mann-Whitney U-test was performed to evaluate differences between the transplanted, nontransplanted, and control group.

RESULTS

In brain tissue of transplanted patients, we found metabolically competent donor macrophages expressing arylsulfatase A distributed throughout the entire white matter. Compared to nontransplanted patients, these macrophages preferentially expressed markers of alternatively activated, anti-inflammatory cells that may support oligodendrocyte survival and differentiation. Additionally, transplanted patients showed higher numbers of oligodendrocytes and evidence for remyelination. Contrary to the current hypothesis on therapeutic mechanism of hematopoietic cell transplantation in metachromatic leukodystrophy, we detected no enzymatic cross-correction to resident astrocytes and oligodendrocytes.

INTERPRETATION

In conclusion, donor macrophages are able to digest accumulated sulfatides and may play a neuroprotective role for resident oligodendrocytes, thereby enabling remyelination, albeit without evidence of cross-correction of oligo- and astroglia. These results emphasize the importance of immunomodulation in addition to the metabolic correction, which might be exploited for improved outcomes.

Complete loss of KCNA1 activity causes neonatal epileptic encephalopathy and dyskinesia

Background

Since 1994, over 50 families affected by the episodic ataxia type 1 disease spectrum have been described with mutations in KCNA1, encoding the voltage-gated K⁺ channel subunit Kv1.1. All of these mutations are either transmitted in an autosomal-dominant mode or found as de novo events.

Methods

A patient presenting with a severe combination of dyskinesia and neonatal epileptic encephalopathy was sequenced by whole-exome sequencing (WES). A candidate variant was tested using cellular assays and patch-clamp recordings.

Results

WES revealed a homozygous variant (p.Val368Leu) in KCNA1, involving a conserved residue in the pore domain, close to the selectivity signature sequence for K⁺ ions (TVGYG). Functional analysis showed that mutant protein alone failed to produce functional channels in homozygous state, while coexpression with wild-type produced no effects on K⁺ currents, similar to wild-type protein alone. Treatment with oxcarbazepine, a sodium channel blocker, proved effective in controlling seizures.

Conclusion

This newly identified variant is the first to be reported to act in a recessive mode of inheritance in KCNA1. These findings serve as a cautionary tale for the diagnosis of channelopathies, in which an unreported phenotypic presentation or mode of inheritance for the variant of interest can hinder the identification of causative variants and adequate treatment choice.

Genetic defects of thiamine transport and metabolism: A review of clinical phenotypes, genetics, and functional studies

Thiamine is a crucial cofactor involved in the maintenance of carbohydrate metabolism and participates in multiple cellular metabolic processes within the cytosol, mitochondria, and peroxisomes. Currently, four genetic defects have been described causing impairment of thiamine transport and metabolism: SLC19A2 dysfunction leads to diabetes mellitus, megaloblastic anemia and sensory-neural hearing loss, whereas SLC19A3, SLC25A19, and TPK1-related disorders result in recurrent encephalopathy, basal ganglia necrosis, generalized dystonia, severe disability, and early death. In order to achieve early diagnosis and treatment, biomarkers play an important role. SLC19A3 patients present a profound decrease of free-thiamine in cerebrospinal fluid (CSF) and fibroblasts. TPK1 patients show decreased concentrations of thiamine pyrophosphate in blood and muscle. Thiamine supplementation has been shown to improve diabetes and anemia control in Rogers' syndrome patients due to SLC19A2 deficiency. In a significant number of patients with SLC19A3, thiamine improves clinical outcome and survival, and prevents further metabolic crisis. In SLC25A19 and TPK1 defects, thiamine has also led to clinical stabilization in single cases. Moreover, thiamine supplementation leads to normal concentrations of free-thiamine in the CSF of SLC19A3 patients. Herein, we present a literature review of the current knowledge of the disease including related clinical phenotypes, treatment approaches, update of pathogenic variants, as well as in vitro and in vivo functional models that provide pathogenic evidence and propose mechanisms for thiamine deficiency in humans.

A homozygous ATAD1 mutation impairs postsynaptic AMPA receptor trafficking and causes a lethal encephalopathy

Members of the AAA+ superfamily of ATPases are involved in the unfolding of proteins and disassembly of protein complexes and aggregates. ATAD1 encoding the ATPase family, AAA+ domain containing 1-protein Thorase plays an important role in the function and integrity of mitochondria and peroxisomes. Postsynaptically, Thorase controls the internalization of excitatory, glutamatergic AMPA receptors by disassembling complexes between the AMPA receptor-binding protein, GRIP1, and the AMPA receptor subunit GluA2. Using whole-exome sequencing, we identified a homozygous frameshift mutation in the last exon of ATAD1 [c.1070_1071delAT; p.(His357Argfs*15)] in three siblings who presented with a severe, lethal encephalopathy associated with stiffness and arthrogryposis. Biochemical and cellular analyses show that the C-terminal end of Thorase mutant gained a novel function that strongly impacts its oligomeric state, reduces stability or expression of a set of Golgi, peroxisomal and mitochondrial proteins and affects disassembly of GluA2 and Thorase oligomer complexes. Atad1-/- neurons expressing Thorase mutantHis357Argfs*15 display reduced amount of GluA2 at the cell surface suggesting that the Thorase mutant may inhibit the recycling back and/or reinsertion of AMPA receptors to the plasma membrane. Taken together, our molecular and functional analyses identify an activating ATAD1 mutation as a new cause of severe encephalopathy and congenital stiffness.

Biotin-Thiamine Responsive Encephalopathy: Report of an Egyptian Family with a Novel SLC19A3 Mutation and Review of the Literature

Biotin-thiamine responsive basal ganglia disease (BTRBGD) is an autosomal recessive neurometabolic disorder with poor genotype-phenotype correlation, caused by mutations in the SLC19A3 gene on chromosome 2q36.6. The disease is characterized by three stages: stage 1 is a sub-acute encephalopathy often triggered by febrile illness; stage 2 is an acute encephalopathy with seizures, loss of motor function, developmental regression, dystonia, external ophthalmoplegia, dysphagia, and dysarthria; stage 3 is represented by chronic or slowly progressive encephalopathy. Clinical and biochemical findings, as well as the magnetic resonance imaging (MRI) pattern, resemble those of Leigh's syndrome, so that BTRBGD can be misdiagnosed as a mitochondrial encephalopathy.Here we report the clinical and radiological phenotypes of two siblings diagnosed with BTRBGD in which a novel SLC19A3 mutation (NM_025243.3: c.548C > T; p.Ala183Val) was found by whole exome sequencing (WES) of the family members.

Novel Homozygous Variant in TTC19 Causing Mitochondrial Complex III Deficiency with Recurrent Stroke-Like Episodes: Expanding the Phenotype

A 7-year-old boy with family history of consanguinity presented with developmental delay and recurrent hemiplegia involving both sides of the body, with variable facial and ocular involvement. Brain MRI showed bilateral striatal necrosis with cystic degeneration and lactate peaks on spectroscopy. Biochemical testing demonstrated mildly elevated lactate and pyruvate. Whole-exome sequencing revealed a novel homozygous pathogenic frameshift mutation in gene TTC19, diagnostic of mitochondrial complex III deficiency.

SLC19A3 related disorder: Treatment implication and clinical outcome of 2 new patients

Encephalopathies with neostriatal involvement constitute a heterogeneous group of acquired and genetically inherited conditions that include Bilateral Striatal Necrosis (BSN) and other Striatal Lesions (SL) (Tonduti et al). We describe two new patients suffering from BSN due to biallelic SLC19A3 mutations. In the first patient vitamin supplementation was started early on, resulting in the remission of the clinical picture, and an almost complete normalization of the neuroradiological findings. In the second one treatment was started late, compliance was irregular and the resulting clinical outcome was poor. The clinical outcome of our two patients confirms and further stresses the importance of the early administration of vitamin supplementation in all patients presenting with neostriatal lesions, or clear bilateral striatal necrosis. Patient 2 didn't present any additional episode of acute decompensation after the age of 20 years despite having completely stopped treatment. This suggests the existence of an age dependency of thiamin requirement in humans.

High predictive value of brain MRI imaging in primary mitochondrial respiratory chain deficiency

Background

Because the mitochondrial respiratory chain (RC) is ubiquitous, its deficiency can theoretically give rise to any symptom in any organ or tissue at any age with any mode of inheritance, owing to the twofold genetic origin of respiratory enzyme machinery, that is, nuclear and mitochondrial. Not all respiratory enzyme deficiencies are primary and secondary or artefactual deficiency is frequently observed, leading to a number of misleading conclusions and inappropriate investigations in clinical practice. This study is aimed at investigating the potential role of brain MRI in distinguishing primary RC deficiency from phenocopies and other aetiologies.

Methods

Starting from a large series of 189 patients (median age: 3.5 years (8 days–56 years), 58% males) showing signs of RC enzyme deficiency, for whom both brain MRIs and disease-causing mutations were available, we retrospectively studied the positive predictive value (PPV) and the positive likelihood ratio (LR+) of brain MRI imaging and its ability to discriminate between two groups: primary deficiency of the mitochondrial RC machinery and phenocopies.

Results

Detection of (1) brainstem hyperintensity with basal ganglia involvement (P≤0.001) and (2) lactate peak with either brainstem or basal ganglia hyperintensity was highly suggestive of primary RC deficiency (P≤0.01). Fourteen items had a PPV>95% and LR+ was greater than 9 for seven signs. Biallelic SLC19A3 mutations represented the main differential diagnosis. Non-significant differences between the two groups were found for cortical/subcortical atrophy, leucoencephalopathy and involvement of caudate nuclei, spinothalamic tract and corpus callosum.

Conclusion

Based on these results and owing to invasiveness of skeletal muscle biopsies and cost of high-throughput DNA sequencing, we suggest giving consideration to brain MRI imaging as a diagnostic marker and an informative investigation to be performed in patients showing signs of RC enzyme deficiency.

Compound heterozygous SLC19A3 mutations further refine the critical promoter region for biotin-thiamine-responsive basal ganglia disease

Mutations in the gene SLC19A3 result in thiamine metabolism dysfunction syndrome 2, also known as biotin-thiamine-responsive basal ganglia disease (BTBGD). This neurometabolic disease typically presents in early childhood with progressive neurodegeneration, including confusion, seizures, and dysphagia, advancing to coma and death. Treatment is possible via supplement of biotin and/or thiamine, with early treatment resulting in significant lifelong improvements. Here we report two siblings who received a refined diagnosis of BTBGD following whole-genome sequencing. Both children inherited compound heterozygous mutations from unaffected parents; a missense single-nucleotide variant (p.G23V) in the first transmembrane domain of the protein, and a 4808-bp deletion in exon 1 encompassing the 5′ UTR and minimal promoter region. This deletion is the smallest promoter deletion reported to date, further defining the minimal promoter region of SLC19A3. Unfortunately, one of the siblings died prior to diagnosis, but the other is showing significant improvement after commencement of therapy. This case demonstrates the power of whole-genome sequencing for the identification of structural variants and subsequent diagnosis of rare neurodevelopmental disorders.

Early Infantile Leigh-like SLC19A3 Gene Defects Have a Poor Prognosis: Report and Review

Solute carrier family 19 (thiamine transporter), member 3 (SCL19A3) gene defect produces an autosomal recessive neurodegenerative disorder associated with different phenotypes and acronyms. One of the common presentations is early infantile lethal Leigh-like syndrome. We report a case of early infantile Leigh-like SLC19A3 gene defects of patients who died at 4 months of age with no response to a high dose of biotin and thiamine. In addition, we report a novel mutation that was not reported previously. Finally, we review the literature regarding early infantile Leigh-like SLC19A3 gene defects and compare the literature with our patient.

Psychological Assessment of Patients With Biotin-Thiamine-Responsive Basal Ganglia Disease

Biotin-thiamine-responsive basal ganglia disease is a devastating autosomal recessive inherited neurological disorder. We conducted a retrospective chart review of all patients with biotin-thiamine-responsive basal ganglia disease who underwent a formal psychological assessment. Six females and 3 males were included. Five patients (56%) had an average IQ, two patients (22%) had mild delay, and two (22%) had severe delay. A normal outcome was directly related to the time of diagnosis and initiation of treatment. Early diagnosis and immediate commencement of treatment were associated with a favorable outcome and vice versa. The most affected domain was visual motor integration, while understanding and mathematical problem-solving were the least affected. In summary, this is the first study discussing the psychological assessment of patients with biotin-thiamine-responsive basal ganglia disease. The results of this study alert clinicians to consider prompt initiation of biotin and thiamine in any patient presenting with neuroregression and a basal ganglia lesion on a brain magnetic resonance imaging.

Thiamine deficiency in childhood with attention to genetic causes: Survival and outcome predictors

Primary and secondary conditions leading to thiamine deficiency have overlapping features in children, presenting with acute episodes of encephalopathy, bilateral symmetric brain lesions, and high excretion of organic acids that are specific of thiamine-dependent mitochondrial enzymes, mainly lactate, alpha-ketoglutarate, and branched chain keto-acids. Undiagnosed and untreated thiamine deficiencies are often fatal or lead to severe sequelae. Herein, we describe the clinical and genetic characterization of 79 patients with inherited thiamine defects causing encephalopathy in childhood, identifying outcome predictors in patients with pathogenic SLC19A3 variants, the most common genetic etiology. We propose diagnostic criteria that will aid clinicians to establish a faster and accurate diagnosis so that early vitamin supplementation is considered. Ann Neurol 2017;82:317-330.

Multiregional Tumor Drug-Uptake Imaging by PET and Microvascular Morphology in End-Stage Diffuse Intrinsic Pontine Glioma

Inadequate tumor uptake of the vascular endothelial growth factor antibody bevacizumab could explain lack of effect in diffuse intrinsic pontine glioma. Methods: By combining data from a PET imaging study using ⁸⁹Zr-labeled bevacizumab and an autopsy study, a 1-on-1 analysis of multiregional in vivo and ex vivo ⁸⁹Zr-bevacizumab uptake, tumor histology, and vascular morphology in a diffuse intrinsic pontine glioma patient was performed. Results: In vivo ⁸⁹Zr-bevacizumab measurements showed heterogeneity between lesions. Additional ex vivo measurements and immunohistochemistry of cervicomedullary metastasis samples showed uptake to be highest in the area with marked microvascular proliferation. In the primary pontine tumor, all samples showed similar vascular morphology. Other histologic features were similar between the samples studied. Conclusion: In vivo ⁸⁹Zr-bevacizumab PET serves to identify heterogeneous uptake between tumor lesions, whereas subcentimeter intralesional heterogeneity could be identified only by ex vivo measurements. ⁸⁹Zr-bevacizumab uptake is enhanced by vascular proliferation, although our results suggest it is not the only determinant of intralesional uptake heterogeneity.

Early and lethal neurodegeneration with myasthenic and myopathic features: A new ALG14-CDG

OBJECTIVE

To describe the presentation and identify the cause of a new clinical phenotype, characterized by early severe neurodegeneration with myopathic and myasthenic features.

METHODS

This case study of 5 patients from 3 families includes clinical phenotype, serial MRI, electrophysiologic testing, muscle biopsy, and full autopsy. Genetic workup included whole exome sequencing and segregation analysis of the likely causal mutation.

RESULTS

All 5 patients showed severe muscular hypotonia, progressive cerebral atrophy, and therapy-refractory epilepsy. Three patients had congenital contractures. All patients died during their first year of life. In 2 of our patients, electrophysiologic testing showed abnormal decrement, but treatment with pyridostigmine led only to temporary improvement. Causative mutations in ALG14 were identified in all patients. The mutation c.220 G>A (p.Asp74Asn) was homozygous in 2 patients and heterozygous in the other 3 patients. Additional heterozygous mutations were c.422T>G (p.Val141Gly) and c.326G>A (p.Arg109Gln). In all cases, parents were found to be heterozygous carriers. None of the identified variants has been described previously.

CONCLUSIONS

We report a genetic syndrome combining myasthenic features and severe neurodegeneration with therapy-refractory epilepsy. The underlying cause is a glycosylation defect due to mutations in ALG14. These cases broaden the phenotypic spectrum associated with ALG14 congenital disorders of glycosylation as previously only isolated myasthenia has been described.

Thiamine metabolism is critical for regulating correlated growth of dendrite arbors and neuronal somata

Thiamine is critical for cellular function, as its phosphorylated and active form, thiamine diphosphate (TDP), acts as coenzyme for three key enzymes in glucose metabolism. Mutations in thiamine transporter, TDP synthesizing enzyme or carrier, including solute carrier family 19 member 3 (SLC19A3), thiamine pyrophosphokinase (TPK1) and solute carrier family 25 member 19 (SLC25A19), have been associated with developmental neurological disorders, including microcephaly and Leigh syndrome. However, little is known about how thiamine metabolism regulates neuronal morphology at the cellular level. Here, using primary rat hippocampal neuronal cultures, we showed that reducing the expression of Tpk1, Slc25a19 or Slc19a3 in individual neurons significantly reduced dendrite complexity, as measured by total dendritic branch tip number (TDBTN) and total dendritic branch length (TDBL). The specificity of the RNAi effects were verified by overexpression of RNAi resistant human constructs. Importantly, changes in both TDBTN and TDBL tightly correlated with reduction in soma size, demonstrating coordinated regulation of soma and dendrite growth by thiamine. The requirement of thiamine metabolism for coordinated somata and dendrite growth is highly consistent with the microcephaly and neurodegenerative phenotypes observed in thiamine loss-of-function diseases.

Inborn Errors of Metabolism and Epilepsy: Current Understanding, Diagnosis, and Treatment Approaches

Inborn errors of metabolism (IEM) are a rare cause of epilepsy, but seizures and epilepsy are frequently encountered in patients with IEM. Since these disorders are related to inherited enzyme deficiencies with resulting effects on metabolic/biochemical pathways, the term “metabolic epilepsy” can be used to include these conditions. These epilepsies can present across the life span, and share features of refractoriness to anti-epileptic drugs, and are often associated with co-morbid developmental delay/regression, intellectual, and behavioral impairments. Some of these disorders are amenable to specific treatment interventions; hence timely and appropriate diagnosis is critical to improve outcomes. In this review, we discuss those disorders in which epilepsy is a dominant feature and present an approach to the clinical recognition, diagnosis, and management of these disorders, with a greater focus on primarily treatable conditions. Finally, we propose a tiered approach that will permit a clinician to systematically investigate, identify, and treat these rare disorders.

High-dose thiamine prevents brain lesions and prolongs survival of Slc19a3-deficient mice

SLC19A3 deficiency, also called thiamine metabolism dysfunction syndrome-2 (THMD2; OMIM 607483), is an autosomal recessive neurodegenerative disorder caused by mutations in SLC19A3, the gene encoding thiamine transporter 2. To investigate the molecular mechanisms of neurodegeneration in SLC19A3 deficiency and whether administration of high-dose thiamine prevents neurodegeneration, we generated homozygous Slc19a3 E314Q knock-in (KI) mice harboring the mutation corresponding to the human SLC19A3 E320Q, which is associated with the severe form of THMD2. Homozygous KI mice and previously reported homozygous Slc19a3 knock-out (KO) mice fed a thiamine-restricted diet (thiamine: 0.60 mg/100 g food) died within 30 and 12 days, respectively, with dramatically decreased thiamine concentration in the blood and brain, acute neurodegeneration, and astrogliosis in the submedial nucleus of the thalamus and ventral anterior-lateral complex of the thalamus. These findings may bear some features of thiamine-deficient mice generated by pyrithiamine injection and a thiamine-deficient diet, suggesting that the primary cause of THMD2 could be thiamine pyrophosphate (TPP) deficiency. Next, we analyzed the therapeutic effects of high-dose thiamine treatment. When the diet was reverted to a conventional diet (thiamine: 1.71 mg/100 g food) after thiamine restriction, all homozygous KO mice died. In contrast, when the diet was changed to a high-thiamine diet (thiamine: 8.50 mg/100 g food) after thiamine restriction, more than half of homozygous KO mice survived, without progression of brain lesions. Unexpectedly, when the high-thiamine diet of recovered mice was reverted to a conventional diet, some homozygous KO mice died. These results showed that acute neurodegeneration caused by thiamine deficiency is preventable in most parts, and prompt high-dose thiamine administration is critical for the treatment of THMD2. However, reduction of thiamine should be performed carefully to prevent recurrence after recovery of the disease.

Biotin-thiamine responsive basal ganglia disease: Identification of a pyruvate peak on brain spectroscopy, novel mutation in SLC19A3, and calculation of prevalence based on allele frequencies from aggregated next-generation sequencing data

Biotin-thiamine responsive basal ganglia disease is an inborn error of metabolism caused by mutations in SLC19A3, encoding a transporter of thiamine across the plasma membrane. We report a novel mutation identified in the homozygous state in a patient with typical brain MRI changes. In addition, this patient had markedly elevated CSF pyruvate, a low lactate-to-pyruvate molar ratio, and an abnormal pyruvate peak at 2.4 ppm on brain magnetic resonance spectroscopy. Using aggregated exome sequencing data, we calculate the carrier frequency of mutations in SLC19A3 as 1 in 232 individuals in the general population, for an estimated prevalence of the disease of approximately 1 in 215,000 individuals. The disease is thus more frequent than previously recognized, and the presence of a pyruvate peak on spectroscopy could serve as an important diagnostic clue.

The Pediatric Cerebellum in Inherited Neurodegenerative Disorders: A Pattern-recognition Approach

Evaluation of imaging studies of the cerebellum in inherited neurodegenerative disorders is aided by attention to neuroimaging patterns based on anatomic determinants, including biometric analysis, hyperintense signal of structures, including the cerebellar cortex, white matter, dentate nuclei, brainstem tracts, and nuclei, the presence of cysts, brain iron, or calcifications, change over time, the use of diffusion-weighted/diffusion tensor imaging and T2*-weighted sequences, magnetic resonance spectroscopy; and, in rare occurrences, the administration of contrast material.

Hypocapnic hypothesis of Leigh disease

Leigh syndrome (LS) is a neurogenetic disorder of children caused by mutations in at least 75 genes which impair mitochondrial bioenergetics. The changes have typical localization in basal ganglia and brainstem, and typical histological picture of spongiform appearance, vascular proliferation and gliosis. ATP deprivation, free radicals and lactate accumulation are suspected to be the causes. Hypocapnic hypothesis proposed in the paper questions the energy deprivation as the mechanism of LS. We assume that the primary harmful factor is hypocapnia (decrease in pCO₂) and respiratory alkalosis (increase in pH) due to hyperventilation, permanent or in response to stress. Inside mitochondria, the pH signal of high pH/low bicarbonate ion (HCO^-₃) is transmitted by soluble adenyl cyclase (sAC) through cAMP dependent manner. The process can initiate brain lesions (necrosis, apoptosis, hypervascularity) in OXPHOS deficient cells residing at the LS area of the brain. The major message of the article is that it is not the ATP depletion but intracellular alkalization (and/or hyperoxia?) which seem to be the cause of LS. The paper includes suggestions concerning the methodology for further research on the LS mechanism and for therapeutic strategy.

Neurological Disorders Associated with Striatal Lesions: Classification and Diagnostic Approach

Neostriatal abnormalities can be observed in a very large number of neurological conditions clinically dominated by the presence of movement disorders. The neuroradiological picture in some cases has been described as "bilateral striatal necrosis" (BSN). BSN represents a condition histo-pathologically defined by the involvement of the neostriata and characterized by initial swelling of putamina and caudates followed by degeneration and cellular necrosis. After the first description in 1975, numerous acquired and hereditary conditions have been associated with the presence of BSN. At the same time, a large number of disorders involving neostriata have been described as BSN, in some cases irrespective of the presence of signs of cavitation on MRI. As a consequence, the etiological spectrum and the nosographic boundaries of the syndrome have progressively become less clear. In this study, we review the clinical and radiological features of the conditions associated with MRI evidence of bilateral striatal lesions. Based on MRI findings, we have distinguished two groups of disorders: BSN and other neostriatal lesions (SL). This distinction is extremely helpful in narrowing the differential diagnosis to a small group of known conditions. The clinical picture and complementary exams will finally lead to the diagnosis. We provide an update on the etiological spectrum of BSN and propose a diagnostic flowchart for clinicians.

Treatable Inborn Errors of Metabolism Due to Membrane Vitamin Transporters Deficiency

B vitamins act as cofactors for strategic metabolic processes. The SLC19 gene family of solute carriers has a significant structural similarity, transporting substrates with different structure and ionic charge. Three proteins of this family are expressed ubiquitously and mediate the transport of 2 important water-soluble vitamins, folate, and thiamine. SLC19A1 transports folate and SLC19A2 and SLC19A3 transport thiamine. PCFT and FOLR1 ensure intestinal absorption and transport of folate through the blood-brain barrier and SLC19A25 transports thiamine into the mitochondria. Several damaging genetic defects in vitamin B transport and metabolism have been reported. The most relevant feature of thiamine and folate transport defects is that both of them are treatable disorders. In this article, we discuss the biology and transport of thiamine and folate, as well as the clinical phenotype of the genetic defects.

A case report of biotin-thiamine-responsive basal ganglia disease in a Saudi child: Is extended genetic family study recommended?

BACKGROUND

Biotin-thiamine-responsive basal ganglia disease (BTRBGD) is a neurometabolic autosomal recessive (AR) disorder characterized by subacute encephalopathy with confusion, convulsions, dysarthria, and dystonia. The disease is completely reversible if treated early with biotin and thiamine, and can be fatal if left untreated.We herein present our experience with in an extended family study of an index case of BTRBGD aiming to support its AR mode of inheritance, diagnose asymptomatic and missed symptomatic cases, and provide family screening with proper genetic counseling.

METHODS

An index case of BTRBGD and his family underwent thorough clinical and radiological assessment along with genetic molecular testing.

RESULTS

Two-and-half years old Saudi male child whose parents are consanguineous fulfilled the clinical and magnetic resonance imaging (MRI) criteria of BTRBGD. He was proved by molecular genetic testing to have homozygous mutation of c.1264A>G (p.Thr422Ala) in the SLC19A3 gene of BTRBGD. Extended clinical, radiological, and genetic family study revealed 2 affected members: a neglected symptomatic cousin with subtle neurological affection and an asymptomatic brother carrying the disease mutation in homozygous status. Heterozygous pattern was detected in his parents, his grandma and grandpa, his aunt and her husband, 2 siblings, and 1 cousin while 1 sibling and 2 cousins were negative to this mutation.Treatment of the patient and his diseased cousin with biotin and thiamine was initiated with gradual improvement of symptoms within few days. Treatment of his asymptomatic brother was also initiated.

CONCLUSION

BTRBGD requires high index of suspicion in any child presenting with unexplained subacute encephalopathy, abnormal movement, and characteristic MRI findings. Extended family study is crucial to diagnose asymptomatic diseased cases and those with subtle neurological symptoms.

Whole Exome Sequencing Reveals a BSCL2 Mutation Causing Progressive Encephalopathy with Lipodystrophy (PELD) in an Iranian Pediatric Patient

Background:

Progressive encephalopathy with or without lipodystrophy is a rare autosomal recessive childhood-onset seipin-associated neurodegenerative syndrome, leading to developmental regression of motor and cognitive skills. In this study, we introduce a patient with developmental regression and autism. The causative mutation was found by exome sequencing.

Methods:

The proband showed a generalized hypertonia and regression of all developmental milestones. Based on the advantages of next-generation sequencing (NGS), whole exome sequencing (WES) was requested. The functional significance of variants was evaluated by NGS-specific prediction servers. Sanger sequencing was used for segregation analysis in the family.

Results:

There was no specific sign in the clinical and paraclinical investigations of the patient to establish a conclusive clinical diagnosis. WES detected a known homozygous nonsense mutation in BSCL2 (NM_001122955.3:c. 985C>T; p.Arg329*). The variant is segregating in the pedigree with an autosomal recessive pattern.

Conclusion:

Exome sequencing is a robust method for identifying the candidate gene variants in Mendelian traits.

New perspective in diagnostics of mitochondrial disorders: two years' experience with whole-exome sequencing at a national paediatric centre

Background

Whole-exome sequencing (WES) has led to an exponential increase in identification of causative variants in mitochondrial disorders (MD).

Methods

We performed WES in 113 MD suspected patients from Polish paediatric reference centre, in whom routine testing failed to identify a molecular defect. WES was performed using TruSeqExome enrichment, followed by variant prioritization, validation by Sanger sequencing, and segregation with the disease phenotype in the family.

Results

Likely causative mutations were identified in 67 (59.3 %) patients; these included variants in mtDNA (6 patients) and nDNA: X-linked (9 patients), autosomal dominant (5 patients), and autosomal recessive (47 patients, 11 homozygotes). Novel variants accounted for 50.5 % (50/99) of all detected changes. In 47 patients, changes in 31 MD-related genes (ACAD9, ADCK3, AIFM1, CLPB, COX10, DLD, EARS2, FBXL4, MTATP6, MTFMT, MTND1, MTND3, MTND5, NAXE, NDUFS6, NDUFS7, NDUFV1, OPA1, PARS2, PC, PDHA1, POLG, RARS2, RRM2B, SCO2, SERAC1, SLC19A3, SLC25A12, TAZ, TMEM126B, VARS2) were identified. The ACAD9, CLPB, FBXL4, PDHA1 genes recurred more than twice suggesting higher general/ethnic prevalence. In 19 cases, variants in 18 non-MD related genes (ADAR, CACNA1A, CDKL5, CLN3, CPS1, DMD, DYSF, GBE1, GFAP, HSD17B4, MECP2, MYBPC3, PEX5, PGAP2, PIGN, PRF1, SBDS, SCN2A) were found. The percentage of positive WES results rose gradually with increasing probability of MD according to the Mitochondrial Disease Criteria (MDC) scale (from 36 to 90 % for low and high probability, respectively). The percentage of detected MD-related genes compared with non MD-related genes also grew with the increasing MD likelihood (from 20 to 97 %). Molecular diagnosis was established in 30/47 (63.8 %) neonates and in 17/28 (60.7 %) patients with basal ganglia involvement. Mutations in CLPB, SERAC1, TAZ genes were identified in neonates with 3-methylglutaconic aciduria (3-MGA) as a discriminative feature. New MD-related candidate gene (NDUFB8) is under verification.

Conclusions

We suggest WES rather than targeted NGS as the method of choice in diagnostics of MD in children, including neonates with 3-MGA aciduria, who died without determination of disease cause and with limited availability of laboratory data. There is a strong correlation between the degree of MD diagnosis by WES and MD likelihood expressed by the MDC scale.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-016-0930-9) contains supplementary material, which is available to authorized users.

Biotin and Thiamine Responsive Basal Ganglia Disease--A vital differential diagnosis in infants with severe encephalopathy

We report two siblings of Swedish origin with infantile Biotin and Thiamine Responsive Basal Ganglia Disease (BTRBG).

CASE REPORT

Initial symptoms were in both cases lethargia, with reduced contact and poor feeding from the age of 5 weeks. Magnetic resonance imaging showed altered signal in the basal ganglia, along with grey and white matter abnormalities. The diagnosis BTRBG was not recognized in the first sibling who died at the age of 8 weeks. The second sibling was started on biotin and thiamine immediately upon development of symptoms, leading to clinical improvement and partial reversion of the magnetic resonance imaging findings. Genetic analysis of the SLC19A3 gene identified two mutations, c.74dupT and c.1403delA, carried in compound heterozygous form in both boys, each inherited from one parent.

COMMENTS

The first mutation has previously been described in children with BTRBG, and the second mutation is novel. Although the clinical picture in BTRGB is very severe it is also rather unspecific and the diagnosis may be missed.

CONCLUSION

This report highlights the importance of considering biotin and thiamine treatment also in a European infant born to non-consanguineous parents, who presents with symptoms of acute/subacute encephalopathy.