Selenoprotein biosynthesis defect causes progressive encephalopathy with elevated lactate

Expanding the Frontiers of Guardian Antioxidant Selenoproteins in Cardiovascular Pathophysiology

Significance: Physiological levels of reactive oxygen and nitrogen species (ROS/RNS) function as fundamental messengers for many cellular and developmental processes in the cardiovascular system. ROS/RNS involved in cardiac redox-signaling originate from diverse sources, and their levels are tightly controlled by key endogenous antioxidant systems that counteract their accumulation. However, dysregulated redox-stress resulting from inefficient removal of ROS/RNS leads to inflammation, mitochondrial dysfunction, and cell death, contributing to the development and progression of cardiovascular disease (CVD). Recent Advances: Basic and clinical studies demonstrate the critical role of selenium (Se) and selenoproteins (unique proteins that incorporate Se into their active site in the form of the 21st proteinogenic amino acid selenocysteine [Sec]), including glutathione peroxidase and thioredoxin reductase, in cardiovascular redox homeostasis, representing a first-line enzymatic antioxidant defense of the heart. Increasing attention has been paid to emerging selenoproteins in the endoplasmic reticulum (ER) (i.e., a multifunctional intracellular organelle whose disruption triggers cardiac inflammation and oxidative stress, leading to multiple CVD), which are crucially involved in redox balance, antioxidant activity, and calcium and ER homeostasis. Critical Issues: This review focuses on endogenous antioxidant strategies with therapeutic potential, particularly selenoproteins, which are very promising but deserve more detailed and clinical studies. Future Directions: The importance of selective selenoproteins in embryonic development and the consequences of their mutations and inborn errors highlight the need to improve knowledge of their biological function in myocardial redox signaling. This could facilitate the development of personalized approaches for the diagnosis, prevention, and treatment of CVD. Antioxid. Redox Signal. 40, 369-432.

Broadening the phenotype and genotype spectrum of novel mutations in pontocerebellar hypoplasia with a comprehensive molecular literature review

BACKGROUND

Pontocerebellar hypoplasia is an umbrella term describing a heterogeneous group of prenatal neurodegenerative disorders mostly affecting the pons and cerebellum, with 17 types associated with 25 genes. However, some types of PCH lack sufficient information, which highlights the importance of investigating and introducing more cases to further elucidate the clinical, radiological, and biochemical features of these disorders. The aim of this study is to provide an in-depth review of PCH and to identify disease genes and their inheritance patterns in 12 distinct Iranian families with clinically confirmed PCH.

METHODS

Cases included in this study were selected based on their phenotypic and genetic information available at the Center for Comprehensive Genetic Services. Whole-exome sequencing (WES) was used to discover the underlying genetic etiology of participants' problems, and Sanger sequencing was utilized to confirm any suspected alterations. We also conducted a comprehensive molecular literature review to outline the genetic features of the various subtypes of PCH.

RESULTS

This study classified and described the underlying etiology of PCH into three categories based on the genes involved. Twelve patients also were included, eleven of whom were from consanguineous parents. Ten different variations in 8 genes were found, all of which related to different types of PCH. Six novel variations were reported, including SEPSECS, TSEN2, TSEN54, AMPD2, TOE1, and CLP1. Almost all patients presented with developmental delay, hypotonia, seizure, and microcephaly being common features. Strabismus and elevation in lactate levels in MR spectroscopy were novel phenotypes for the first time in PCH types 7 and 9.

CONCLUSIONS

This study merges previously documented phenotypes and genotypes with unique novel ones. Due to the diversity in PCH, we provided guidance for detecting and diagnosing these heterogeneous groups of disorders. Moreover, since certain critical conditions, such as spinal muscular atrophy, can be a differential diagnosis, providing cases with novel variations and clinical findings could further expand the genetic and clinical spectrum of these diseases and help in better diagnosis. Therefore, six novel genetic variants and novel clinical and paraclinical findings have been reported for the first time. Further studies are needed to elucidate the underlying mechanisms and potential therapeutic targets for PCH.

Refining the phenotypic spectrum of CCDC88A-related PEHO-like syndrome

Progressive encephalopathy with edema, hypsarrhythmia, and optic atrophy (PEHO) and PEHO-like syndromes are very rare infantile disorders characterized by profound intellectual disability, hypotonia, convulsions, optic, and progressive brain atrophy. Many causative genes for PEHO and PEHO-like syndromes have been identified including CCDC88A. So far, only five patients from two unrelated families with biallelic CCDC88A variants have been reported in the literature. Herein, we describe a new family from Egypt with a lethal epileptic encephalopathy. Our patient was the youngest child born to a highly consanguineous couple and had a family history of five deceased sibs with the same condition. She presented with postnatal microcephaly, poor visual responsiveness, and epilepsy. Her brain MRI showed abnormal cortical gyration with failure of opercularization of the insula, hypogenesis of corpus callosum, colpocephaly, reduced white matter, hypoplastic vermis, and brain stem. Whole exome sequencing identified a new homozygous frameshift variant in CCDC88A gene (c.1795_1798delACAA, p.Thr599ValfsTer4). Our study presents the third reported family with this extremely rare disorder. We also reviewed all described cases to better refine the phenotypic spectrum associated with biallelic loss of function variants in the CCDC88A gene.

tRNA modification enzyme-dependent redox homeostasis regulates synapse formation and memory

Post-transcriptional modification of RNA regulates gene expression at multiple levels. ALKBH8 is a tRNA modifying enzyme that methylates wobble uridines in specific tRNAs to modulate translation. Through methylation of tRNA-selenocysteine, ALKBH8 promotes selenoprotein synthesis and regulates redox homeostasis. Pathogenic variants in ALKBH8 have been linked to intellectual disability disorders in the human population, but the role of ALKBH8 in the nervous system is unknown. Through in vivo studies in Drosophila , we show that ALKBH8 controls oxidative stress in the brain to restrain synaptic growth and support learning and memory. ALKBH8 null animals lack wobble uridine methylation and exhibit a global reduction in protein synthesis, including a specific decrease in selenoprotein levels. Loss of ALKBH8 or independent disruption of selenoprotein synthesis results in ectopic synapse formation. Genetic expression of antioxidant enzymes fully suppresses synaptic overgrowth in ALKBH8 null animals, confirming oxidative stress as the underlying cause of dysregulation. ALKBH8 animals also exhibit associative learning and memory impairments that are reversed by pharmacological antioxidant treatment. Together, these findings demonstrate the critical role of tRNA modification in redox homeostasis in the nervous system and reveal antioxidants as a potential therapy for ALKBH8-associated intellectual disability.

Significance Statement

tRNA modifying enzymes are emerging as important regulators of nervous system development and function due to their growing links to neurological disorders. Yet, their roles in the nervous system remain largely elusive. Through in vivo studies in Drosophila , we link tRNA methyltransferase-regulated selenoprotein synthesis to synapse development and associative memory. These findings demonstrate the key role of tRNA modifiers in redox homeostasis during nervous system development and highlight the potential therapeutic benefit of antioxidant-based therapies for cognitive disorders linked to dysregulation of tRNA modification.

"Alphabet" Selenoproteins: Implications in Pathology

Selenoproteins are a group of proteins containing selenium in the form of selenocysteine (Sec, U) as the 21st amino acid coded in the genetic code. Their synthesis depends on dietary selenium uptake and a common set of cofactors. Selenoproteins accomplish diverse roles in the body and cell processes by acting, for example, as antioxidants, modulators of the immune function, and detoxification agents for heavy metals, other xenobiotics, and key compounds in thyroid hormone metabolism. Although the functions of all this protein family are still unknown, several disorders in their structure, activity, or expression have been described by researchers. They concluded that selenium or cofactors deficiency, on the one hand, or the polymorphism in selenoproteins genes and synthesis, on the other hand, are involved in a large variety of pathological conditions, including type 2 diabetes, cardiovascular, muscular, oncological, hepatic, endocrine, immuno-inflammatory, and neurodegenerative diseases. This review focuses on the specific roles of selenoproteins named after letters of the alphabet in medicine, which are less known than the rest, regarding their implications in the pathological processes of several prevalent diseases and disease prevention.

Analysis of the Clinical Features and Imaging Findings of Pontocerebellar Hypoplasia Type 2D Caused by Mutations in SEPSECS Gene

Pontocerebellar hypoplasia type 2D (PCH2D) caused by SEPSECS gene mutations is very rare and only described in a few case reports. In this study, we analyzed the clinical features and imaging findings of these individuals, so as to provide references for the clinic. We reported a case of PCH2D caused by a new complex heterozygote mutation in SEPSECS gene, and reviewed the literatures to summarize the clinical features and imaging findings and compare the differences between early-onset patients (EOPs) and late-onset patients (LOPs). Of 23 PCH2D patients, 19 cases were early-onset and 4 cases were late-onset, with average ages of 4.1 ± 4.0 years and 21.8 ± 9.4 years, females were more prevalent (14/19). EOPs mainly distributed in Arab countries (10/14) and Finland (4/14), while LOPs in East Asia (3/3). EOPs develop severe initial symptoms at the average age of 4.1 ± 7.8 months or shortly after birth, while LOPs experienced mild developmental delay in infancy. Microcephaly (10/11), intellectual disability (10/11), decreased motor function (10/11), and spastic or dystonic quadriplegia (8/10) were the common clinical features of EOPs and LOPs. EOPs also presented with visual impairment (5/7), seizures (4/7), neonatal irritability/opisthotonus (3/7), tremors/myoclonus (3/7), dysmorphic features (3/7), and other symptoms. EOPs were characterized by cerebellar symptoms (4/4). Magnetic resonance imaging (MRI) revealed progressive cerebellar atrophy followed by less pronounced cerebral atrophy, and there was no pons atrophy in LOPs. Most patients of PCH2D were severe early-onset, and a few were late-onset with milder symptoms. EOPs and LOPs shared some common clinical features and MRI findings, but also had their own characteristics.

Integrative identification of hub genes in development of atrial fibrillation related stroke

BACKGROUND

As the most common arrhythmia, atrial fibrillation (AF) is associated with a significantly increased risk of stroke, which causes high disability and mortality. To date, the underlying mechanism of stroke occurring after AF remains unclear. Herein, we studied hub genes and regulatory pathways involved in AF and secondary stroke and aimed to reveal biomarkers and therapeutic targets of AF-related stroke.

METHODS

The GSE79768 and GSE58294 datasets were used to analyze AF- and stroke-related differentially expressed genes (DEGs) to obtain a DEG1 dataset. Weighted correlation network analysis (WGCNA) was used to identify modules associated with AF-related stroke in GSE66724 (DEG2). DEG1 and DEG2 were merged, and hub genes were identified based on protein-protein interaction networks. Gene Ontology terms were used to analyze the enriched pathways. The GSE129409 and GSE70887 were applied to construct a circRNA-miRNA-mRNA network in AF-related stroke. Hub genes were verified in patients using quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

We identified 3,132 DEGs in blood samples and 253 DEGs in left atrial specimens. Co-expressed hub genes of EIF4E3, ZNF595, ZNF700, MATR3, ACKR4, ANXA3, SEPSECS-AS1, and RNF166 were significantly associated with AF-related stroke. The hsa_circ_0018657/hsa-miR-198/EIF4E3 pathway was explored as the regulating axis in AF-related stroke. The qRT-PCR results were consistent with the bioinformatic analysis.

CONCLUSIONS

Hub genes EIF4E3, ZNF595, ZNF700, MATR3, ACKR4, ANXA3, SEPSECS-AS1, and RNF166 have potential as novel biomarkers and therapeutic targets in AF-related stroke. The hsa_circ_0018657/hsa-miR-198/EIF4E3 axis could play an important role regulating the development of AF-related stroke.

Selenoproteins in brain development and function

Expression of selenoproteins is widespread in neurons of the central nervous system. There is continuous evidence presented over decades that low levels of selenium or selenoproteins are linked to seizures and epilepsy indicating a failure of the inhibitory system. Many developmental processes in the brain depend on the thyroid hormone T3. T3 levels can be locally increased by the action of iodothyronine deiodinases on the prohormone T4. Since deiodinases are selenoproteins, it is expected that selenoprotein deficiency may affect development of the central nervous system. Studies in genetically modified mice or clinical observations of patients with rare diseases point to a role of selenoproteins in brain development and degeneration. In particular selenoprotein P is central to brain function by virtue of its selenium transport function into and within the brain. We summarize which selenoproteins are essential for the brain, which processes depend on selenoproteins, and what is known about genetic deficiencies of selenoproteins in humans. This review is not intended to cover the potential influence of selenium or selenoproteins on major neurodegenerative disorders in human.

Novel SEPSECS Pathogenic Variants Featuring Unusual Phenotype of Complex Movement Disorder With Thin Corpus Callosum: A Case Report

Objectives

To report a novel association between pathogenic variants in the SEPSECS gene and complex movement disorder with thin corpus callosum (TCC).

Methods

Clinical exome sequencing was performed in an adult patient with a genetically unsolved neurodegenerative disorder. The main clinical, neuroimaging, and genetic data were described.

Results

The c.865C > T (p.P289S) and c.1297T > C (p.Y433H) missense variants in SEPSECS (NM_016,955.3) were discovered.

Discussion

This case represents a novel form of early-onset pyramidal syndrome with optic nerve hypoplasia, which slowly evolved to extrapyramidal syndrome featuring dystonia-parkinsonism, associated with TCC, caused by SEPSECS pathogenic variants. This form enlarges the group of the so-called pyramidal-extrapyramidal syndromes, as well as complex hereditary spastic paraparesis with TCC.

Human Genetic Disorders Resulting in Systemic Selenoprotein Deficiency

Selenium, a trace element fundamental to human health, is incorporated as the amino acid selenocysteine (Sec) into more than 25 proteins, referred to as selenoproteins. Human mutations in SECISBP2, SEPSECS and TRU-TCA1-1, three genes essential in the selenocysteine incorporation pathway, affect the expression of most if not all selenoproteins. Systemic selenoprotein deficiency results in a complex, multifactorial disorder, reflecting loss of selenoprotein function in specific tissues and/or long-term impaired selenoenzyme-mediated defence against oxidative and endoplasmic reticulum stress. SEPSECS mutations are associated with a predominantly neurological phenotype with progressive cerebello-cerebral atrophy. Selenoprotein deficiency due to SECISBP2 and TRU-TCA1-1 defects are characterized by abnormal circulating thyroid hormones due to lack of Sec-containing deiodinases, low serum selenium levels (low SELENOP, GPX3), with additional features (myopathy due to low SELENON; photosensitivity, hearing loss, increased adipose mass and function due to reduced antioxidant and endoplasmic reticulum stress defence) in SECISBP2 cases. Antioxidant therapy ameliorates oxidative damage in cells and tissues of patients, but its longer term benefits remain undefined. Ongoing surveillance of patients enables ascertainment of additional phenotypes which may provide further insights into the role of selenoproteins in human biological processes.

Missense mutation in selenocysteine synthase causes cardio-respiratory failure and perinatal death in mice which can be compensated by selenium-independent GPX4

Selenoproteins are a small family of proteins containing the trace element selenium in form of the rare amino acid selenocysteine (Sec), which is decoded by the UGA codon. In humans, a number of pathogenic variants in genes encoding distinct selenoproteins or selenoprotein biosynthesis factors have been identified. Pathogenic variants in selenocysteine synthase (SEPSECS), which catalyzes the last step in Sec-tRNA[Ser]Sec biosynthesis, were reported in children suffering from progressive cerebello-cerebral atrophy. To understand the pathomechanism associated with SEPSECS deficiency, we generated a novel mouse model recapitulating the respective human pathogenic p.Y334C variant in the murine Sepsecs gene (SepsecsY334C). Unlike in patients, pups homozygous for the p.Y334C variant died perinatally with signs of cardio-respiratory failure. Perinatal death is reminiscent of the Sedaghatian spondylometaphyseal dysplasia disorder in humans, which is caused by pathogenic variants in the gene encoding the selenoprotein and key ferroptosis regulator glutathione peroxidase 4 (GPX4). Protein expression levels of distinct selenoproteins in SepsecsY334C/Y334C mice were found to be generally reduced in brain and isolated cortical neurons, while transcriptomics analysis uncovered an upregulation of NRF2-regulated genes. Crossbreeding of SepsecsY334C/Y334C mice with mice harboring a targeted mutation of the catalytically active Sec to Cys in GPX4 rescued perinatal death of SepsecsY334C/Y334C mice, showing that the cardio-respiratory defects of SepsecsY334C/Y334C mice were caused by the lack of GPX4. Like in SepsecsY334C/Y334C mice, selenoprotein expression levels remained low and NRF2-regulated genes remained highly expressed in these compound mutant mice, indicating that selenium-independent GPX4, along with a sustained antioxidant response are sufficient to compensate for dysfunctional Sec-tRNA[Ser]Sec biosynthesis. Our findings imply that children with pathogenic variants in SEPSECS or GPX4 may even benefit from treatments that incompletely compensate for impaired GPX4 activity.

Pathogenic Variants in Selenoproteins and Selenocysteine Biosynthesis Machinery

Selenium is incorporated into selenoproteins as the 21st amino acid selenocysteine (Sec). There are 25 selenoproteins encoded in the human genome, and their synthesis requires a dedicated machinery. Most selenoproteins are oxidoreductases with important functions in human health. A number of disorders have been associated with deficiency of selenoproteins, caused by mutations in selenoprotein genes or Sec machinery genes. We discuss mutations that are known to cause disease in humans and report their allele frequencies in the general population. The occurrence of protein-truncating variants in the same genes is also presented. We provide an overview of pathogenic variants in selenoproteins genes from a population genomics perspective.

Deletion of the SELENOP gene leads to CNS atrophy with cerebellar ataxia in dogs

We investigated a hereditary cerebellar ataxia in Belgian Shepherd dogs. Affected dogs developed uncoordinated movements and intention tremor at two weeks of age. The severity of clinical signs was highly variable. Histopathology demonstrated atrophy of the CNS, particularly in the cerebellum. Combined linkage and homozygosity mapping in a family with four affected puppies delineated a 52 Mb critical interval. The comparison of whole genome sequence data of one affected dog to 735 control genomes revealed a private homozygous structural variant in the critical interval, Chr4:66,946,539_66,963,863del17,325. This deletion includes the entire protein coding sequence of SELENOP and is predicted to result in complete absence of the encoded selenoprotein P required for selenium transport into the CNS. Genotypes at the deletion showed the expected co-segregation with the phenotype in the investigated family. Total selenium levels in the blood of homozygous mutant puppies of the investigated litter were reduced to about 30% of the value of a homozygous wildtype littermate. Genotyping >600 Belgian Shepherd dogs revealed an additional homozygous mutant dog. This dog also suffered from pronounced ataxia, but reached an age of 10 years. Selenop^-/- knock-out mice were reported to develop ataxia, but their histopathological changes were less severe than in the investigated dogs. Our results demonstrate that deletion of the SELENOP gene in dogs cause a defect in selenium transport associated with CNS atrophy and cerebellar ataxia (CACA). The affected dogs represent a valuable spontaneous animal model to gain further insights into the pathophysiological consequences of CNS selenium deficiency.

We studied a form of inherited ataxia in a family of Belgian Shepherd dogs that we termed CNS atrophy and cerebellar ataxia (CACA). Clinical signs were evident at 2 weeks of age and the affected puppies had to be euthanized at 4 weeks of age. The pedigree of the index family with 4 affected and 4 unaffected puppies suggested autosomal recessive inheritance. Using a purely positional cloning approach, we identified a complete deletion of the SELENOP gene as the most likely causative variant. SELENOP encodes selenoprotein P, a protein with multiple selenocysteine residues, which is required for the transport of selenium into the CNS. Selenium measurements in affected dogs demonstrated blood selenium levels of about 30% compared to normal control dogs. Genotyping a cohort of additional Belgian Shepherd dogs with unexplained ataxia identified another CACA case that had a relatively stable clinical condition and reached an age of 10 years. Selenop^-/- knock-out mice show a related but not identical ataxia phenotype. Our finding of a SELENOP gene deletion in CACA affected dogs identifies a spontaneous animal model to gain further insights into the pathophysiological consequences of CNS selenium deficiency.

Epigenome-Wide Association Study of Thyroid Function Traits Identifies Novel Associations of fT3 With KLF9 and DOT1L

CONTEXT

Circulating concentrations of free triiodothyronine (fT3), free thyroxine (fT4), and thyrotropin (TSH) are partly heritable traits. Recent studies have advanced knowledge of their genetic architecture. Epigenetic modifications, such as DNA methylation (DNAm), may be important in pituitary-thyroid axis regulation and action, but data are limited.

OBJECTIVE

To identify novel associations between fT3, fT4, and TSH and differentially methylated positions (DMPs) in the genome in subjects from 2 Australian cohorts.

METHOD

We performed an epigenome-wide association study (EWAS) of thyroid function parameters and DNAm using participants from: Brisbane Systems Genetics Study (median age 14.2 years, n = 563) and the Raine Study (median age 17.0 years, n = 863). Plasma fT3, fT4, and TSH were measured by immunoassay. DNAm levels in blood were assessed using Illumina HumanMethylation450 BeadChip arrays. Analyses employed generalized linear mixed models to test association between DNAm and thyroid function parameters. Data from the 2 cohorts were meta-analyzed.

RESULTS

We identified 2 DMPs with epigenome-wide significant (P < 2.4E-7) associations with TSH and 6 with fT3, including cg00049440 in KLF9 (P = 2.88E-10) and cg04173586 in DOT1L (P = 2.09E-16), both genes known to be induced by fT3. All DMPs had a positive association between DNAm and TSH and a negative association between DNAm and fT3. There were no DMPs significantly associated with fT4. We identified 23 differentially methylated regions associated with fT3, fT4, or TSH.

CONCLUSIONS

This study has demonstrated associations between blood-based DNAm and both fT3 and TSH. This may provide insight into mechanisms underlying thyroid hormone action and/or pituitary-thyroid axis function.

The Neurobiology of Selenium: Looking Back and to the Future

Eighteen years ago, unexpected epileptic seizures in Selenop-knockout mice pointed to a potentially novel, possibly underestimated, and previously difficult to study role of selenium (Se) in the mammalian brain. This mouse model was the key to open the field of molecular mechanisms, i.e., to delineate the roles of selenium and individual selenoproteins in the brain, and answer specific questions like: how does Se enter the brain; which processes and which cell types are dependent on selenoproteins; and, what are the individual roles of selenoproteins in the brain? Many of these questions have been answered and much progress is being made to fill remaining gaps. Mouse and human genetics have together boosted the field tremendously, in addition to traditional biochemistry and cell biology. As always, new questions have become apparent or more pressing with solving older questions. We will briefly summarize what we know about selenoproteins in the human brain, glance over to the mouse as a useful model, and then discuss new questions and directions the field might take in the next 18 years.

Case Report: A Relatively Mild Phenotype Produced by Novel Mutations in the SEPSECS Gene

Mutations in the human O-phosphoseryl-tRNA:selenocysteinyl-tRNA synthase gene (SEPSECS) are associated with progressive cerebello-cerebral atrophy (PCCA), also known as pontocerebellar hypoplasia type 2D (PCH2D). Early-onset profound developmental delay, progressive microcephaly, and hypotonia that develops toward severe spasticity have been previously reported with SEPSECS mutations. Herein we report a case with severe global developmental delay, myogenic changes in the lower limbs, and insomnia, but without progressive microcephaly and brain atrophy during infancy and toddlerhood in a child harboring the SEPSECS missense variant c.194A>G (p. Asn65Ser) and a novel splicing mutation c.701+1G>A. With these findings we communicate the first Chinese SEPSECS mutant case, and our report indicates that SEPSECS mutations can give rise to a milder phenotype.

Homozygous TAF1C variants are associated with a novel childhood-onset neurological phenotype

TATA-box binding protein associated factor, RNA polymerase I subunit C (TAF1C) is a component of selectivity factor 1 belonging to RNA polymerase I (Pol I) transcription machinery. We report two unrelated patients with homozygous TAF1C missense variants and an early onset neurological phenotype with severe global developmental delay. Clinical features included lack of speech and ambulation and epilepsy. MRI of the brain demonstrated widespread cerebral atrophy and frontal periventricular white matter hyperintensity. The phenotype resembled that of a previously described variant of UBTF, which encodes another transcription factor of Pol I. TAF1C variants were located in two conserved amino acid positions and were predicted to be deleterious. In patient-derived fibroblasts, TAF1C mRNA and protein expression levels were substantially reduced compared with healthy controls. We propose that the variants impairing TAF1C expression are likely pathogenic and relate to a novel neurological disease. This study expands the disease spectrum related to Pol I transcription machinery, associating the TAF1C missense variants with a severe neurological phenotype for the first time.

Clinical and genetic features of PEHO and PEHO-Like syndromes: A scoping review

Progressive encephalopathy with edema, hypsarrhythmia, and optic atrophy (PEHO) syndrome is a genetic neurological condition characterized by extreme cerebellar atrophy. PEHO-Like syndrome is comparable to PEHO syndrome, with the exception that there is no typical neuro-radiologic or neuro-ophthalmic findings. PEHO spectrum disorders are highly clinically and genetically heterogeneous, and this has challenged their diagnosis. This scoping review aims to summarize and discuss common clinical and genetic features of these syndromes to help future researches. This study was performed according to a six-stage methodology structure and PRISMA guideline. A systematic search of seven databases was performed to find eligible publications prior to June 2020. Articles screening and data extraction were independently performed by two reviewers and quantitative and qualitative analyses were conducted. Thirty-eight articles were identified that fulfill the inclusion criteria. Cerebellar atrophy was the main clinical difference between the two groups but data on optic atrophy and infantile spasms/hypsarrhythmia were not consistent with the previously essential diagnostic criteria. Genetic analysis was performed in several studies, leading to identification of pathogenic variants in different genes that caused these conditions due to different mechanisms. Genetic studies could revolutionize the diagnosis process and our understanding of the etiology of this challenging group of patients by providing targeted sequencing panels and exome- or genome-scale studies in the future.

Human Disorders Affecting the Selenocysteine Incorporation Pathway Cause Systemic Selenoprotein Deficiency

Significance: Generalized selenoprotein deficiency has been associated with mutations in SECISBP2, SEPSECS, and TRU-TCA1-1, 3 factors that are crucial for incorporation of the amino acid selenocysteine (Sec) into at least 25 human selenoproteins. SECISBP2 and TRU-TCA1-1 defects are characterized by a multisystem phenotype due to deficiencies of antioxidant and tissue-specific selenoproteins, together with abnormal thyroid hormone levels reflecting impaired hormone metabolism by deiodinase selenoenzymes. SEPSECS mutations are associated with a predominantly neurological phenotype with progressive cerebello-cerebral atrophy. Recent Advances: The recent identification of individuals with defects in genes encoding components of the selenocysteine insertion pathway has delineated complex and multisystem disorders, reflecting a lack of selenoproteins in specific tissues, oxidative damage due to lack of oxidoreductase-active selenoproteins and other pathways whose nature is unclear. Critical Issues: Abnormal thyroid hormone metabolism in patients can be corrected by triiodothyronine (T3) treatment. No specific therapies for other phenotypes (muscular dystrophy, male infertility, hearing loss, neurodegeneration) exist as yet, but their severity often requires supportive medical intervention. Future Directions: These disorders provide unique insights into the role of selenoproteins in humans. The long-term consequences of reduced cellular antioxidant capacity remain unknown, and future surveillance of patients may reveal time-dependent phenotypes (e.g., neoplasia, aging) or consequences of deficiency of selenoproteins whose function remains to be elucidated. The role of antioxidant therapies requires evaluation. Antioxid. Redox Signal. 33, 481-497.

Progress in the emerging role of selenoproteins in cardiovascular disease: focus on endoplasmic reticulum-resident selenoproteins

Cardiovascular diseases represent one of the most important health problems of developed countries. One of the main actors involved in the onset and development of cardiovascular diseases is the increased production of reactive oxygen species that, through lipid peroxidation, protein oxidation and DNA damage, induce oxidative stress and cell death. Basic and clinical research are ongoing to better understand the endogenous antioxidant mechanisms that counteract oxidative stress, which may allow to identify a possible therapeutic targeting/application in the field of stress-dependent cardiovascular pathologies. In this context, increasing attention is paid to the glutathione/glutathione-peroxidase and to the thioredoxin/thioredoxin-reductase systems, among the most potent endogenous antioxidative systems. These key enzymes, belonging to the selenoprotein family, have a well-established function in the regulation of the oxidative cell balance. The aim of the present review was to highlight the role of selenoproteins in cardiovascular diseases, introducing the emerging cardioprotective role of endoplasmic reticulum-resident members and in particular one of them, namely selenoprotein T or SELENOT. Accumulating evidence indicates that the dysfunction of different selenoproteins is involved in the susceptibility to oxidative stress and its associated cardiovascular alterations, such as congestive heart failure, coronary diseases, impaired cardiac structure and function. Some of them are under investigation as useful pathological biomarkers. In addition, SELENOT exhibited intriguing cardioprotective effects by reducing the cardiac ischemic damage, in terms of infarct size and performance. In conclusion, selenoproteins could represent valuable targets to treat and diagnose cardiovascular diseases secondary to oxidative stress, opening a new avenue in the field of related therapeutic strategies.

Prioritized brain selenium retention and selenoprotein expression: Nutritional insights into Parkinson's disease

Selenium (Se), an essential trace mineral, confers its physiological functions mainly through selenoproteins, most of which are oxidoreductases. Results from animal, epidemiological, and human genetic studies link Parkinson's disease to Se and certain selenoproteins. Parkinson's disease is characterized by multiple motor and non-motor symptoms that are difficult to diagnose at early stages of the pathogenesis. While irreversible, degenerative and age-related, the onset of Parkinson's disease may be delayed through proper dietary and environmental controls. One particular attribute of Se biology is that brain has the highest priority to receive and retain this nutrient even in Se deficiency. Thus, brain Se deficiency is rare; however, a strong body of recent evidence implicates selenoprotein dysfunction in Parkinson's disease. Direct and indirect evidence from mouse models implicate selenoprotein T, glutathione peroxidase 1, selenoprotein P and glutathione peroxidase 4 in counteracting Parkinson's disease through Se transportation to the brain and reduced oxidative stress. It is of future interest to further characterize the full selenoproteomes in various types of brain cells and elucidate the mechanism of their actions in Parkinson's disease.

Targeting ferroptosis: A novel therapeutic strategy for the treatment of mitochondrial disease-related epilepsy

BACKGROUND

Mitochondrial disease is a family of genetic disorders characterized by defects in the generation and regulation of energy. Epilepsy is a common symptom of mitochondrial disease, and in the vast majority of cases, refractory to commonly used antiepileptic drugs. Ferroptosis is a recently-described form of iron- and lipid-dependent regulated cell death associated with glutathione depletion and production of lipid peroxides by lipoxygenase enzymes. Activation of the ferroptosis pathway has been implicated in a growing number of disorders, including epilepsy. Given that ferroptosis is regulated by balancing the activities of glutathione peroxidase-4 (GPX4) and 15-lipoxygenase (15-LO), targeting these enzymes may provide a rational therapeutic strategy to modulate seizure. The clinical-stage therapeutic vatiquinone (EPI-743, α-tocotrienol quinone) was reported to reduce seizure frequency and associated morbidity in children with the mitochondrial disorder pontocerebellar hypoplasia type 6. We sought to elucidate the molecular mechanism of EPI-743 and explore the potential of targeting 15-LO to treat additional mitochondrial disease-associated epilepsies.

METHODS

Primary fibroblasts and B-lymphocytes derived from patients with mitochondrial disease-associated epilepsy were cultured under standardized conditions. Ferroptosis was induced by treatment with the irreversible GPX4 inhibitor RSL3 or a combination of pharmacological glutathione depletion and excess iron. EPI-743 was co-administered and endpoints, including cell viability and 15-LO-dependent lipid oxidation, were measured.

RESULTS

EPI-743 potently prevented ferroptosis in patient cells representing five distinct pediatric disease syndromes with associated epilepsy. Cytoprotection was preceded by a dose-dependent decrease in general lipid oxidation and the specific 15-LO product 15-hydroxyeicosatetraenoic acid (15-HETE).

CONCLUSIONS

These findings support the continued clinical evaluation of EPI-743 as a therapeutic agent for PCH6 and other mitochondrial diseases with associated epilepsy.

Selenium and GPX4, a vital symbiosis

Selenium has transitioned from an environmental poison and carcinogen to an essential micronutrient associated with a broad array of health promoting effects. These beneficial effects are now accepted to be linked to its incorporation into selenoproteins, a family of rare proteins utilizing a specialized translation machinery to integrate selenium in the form of selenocysteine. Despite this recognized role, much less is known regarding the actual role of selenium in these proteins. Here, we will provide the reader with an overview of the essential role of specific selenoproteins and their link to pathology based on mouse studies and relevant mutations discovered in humans. Additionally, we will cover recent insights linking a non-interchangeable role for selenium in glutathione peroxidase 4 and its function in suppressing ferroptosis. This critical dependency ultimately generates a strong reliance on metabolic pathways that regulate selenium metabolism and its incorporation into proteins, such as the mevalonate pathway.

Progressive cerebello-cerebral atrophy and progressive encephalopathy with edema, hypsarrhythmia and optic atrophy may be allelic syndromes

In 2003, a new syndrome was described in the Sephardi Jewish population, named progressive cerebello-cerebral atrophy (PCCA) based on the typical neuroradiological findings. Following the identification of the causal genes in 2010 and 2014, two types were defined: PCCA type 1 due to SEPSECS mutations and PCCA type 2 due to VPS53 mutations. Progressive encephalopathy with edema, hypsarrhythmia and optic atrophy (PEHO) was described in 1991 in Finland. The clinical and radiological phenotype resembles PCCA. The genetic background has been elusive for many years. Recently, mutations in multiple genes including SEPSECS have been described in patients with a PEHO-like syndrome. In 2007 two siblings of Moroccan-Jewish origin were diagnosed as having PEHO due to a severe developmental encephalopathy, limb and facial edema, intractable epilepsy, optic atrophy in one sibling and dysmorphic features. Six years ago an extensive workup, including whole exome sequencing, did not reveal the cause. Recently, a clinical reevaluation of the siblings suggested the possibility that they suffer from PCCA. A reanalysis of the exome data from 2014 revealed that the siblings indeed carried the two VPS53 mutations (exon 19 c.2084A>G p.(Gln695Arg) and c.1556 + 5G>A) and the parents were found to be carriers. The discovery that mutations in both VPS53 and SEPSECS can present with a PEHO-like phenotype, place PCCA and PEHO on the same clinical spectrum and suggest they may be allelic syndromes.

Consequences of mutations and inborn errors of selenoprotein biosynthesis and functions

In its 200 years of history, selenium has been defined first as a toxic element and finally as a micronutrient. Selenium is incorporated into selenoproteins as selenocysteine (Sec), the 21st proteinogenic amino acid codified by a stop codon. Specific biosynthetic factors recode UGA stop codon as Sec. The significance of selenoproteins in human health is manifested through the identification of patients with inborn errors in selenoproteins or their biosynthetic factors. Selenoprotein N-related myopathy was the first disease identified due to mutations in a selenoprotein gene. Mutations in GPX4 were linked to Sedaghatian disease, characterized by bone and brain anomalies and cardiorespiratory failure. Mutations in TXNRD2 produced familial glucocorticoid deficiency (FGD) and dilated cardiomyopathy (DCM). Genetic generalized epilepsy was associated with mutations in TXNRD1 gene. Mutations in biosynthetic factors as SEPSECS, SECISBP2 and even tRNA^[Ser]Sec, have been also related to diseases. Thus, SEPSECS mutations produce a neurodegenerative disease called now pontocerebellar hypoplasia type 2D (PCH2D). SECISBP2 syndrome, caused by SECISBP2 mutations, is a multifactorial disease affecting mainly thyroid metabolism, bone, inner ear and muscle. Similar symptoms were reproduced in a patient carrying a mutation in tRNA^[Ser]Sec gene, TRU-TCA1-1. This review describes human genetic disorders caused by selenoprotein deficiency. Human phenotypes will be compared with mouse models to explain the pathologic mechanisms of lack of selenoproteins.

A SEPSECS mutation in a 23-year-old woman with microcephaly and progressive cerebellar ataxia

Mutations in the SEPSECS gene are associated with pontocerebellar hypoplasia type 2D. Pontocerebellar hypoplasia (PCH) is a heterogeneous group of rare autosomal recessive neurodegenerative disorders, mainly affecting pons and cerebellum. Patients have severe motor and cognitive impairments and often die during infancy. Here, we report a 23-year-old woman with slowly progressive cerebellar ataxia and cognitive impairment, in whom a homozygous missense mutation in the SEPSECS gene (c.1321G>A; p.Gly441Arg) was identified with whole exome sequencing. Our findings underline that defects in selenoprotein synthesis can also result in milder cerebellar atrophy presenting at a later age.

Translation regulation of mammalian selenoproteins

BACKGROUND

Interest in selenium research has considerably grown over the last decades owing to the association of selenium deficiencies with an increased risk of several human diseases, including cancers, cardiovascular disorders and infectious diseases. The discovery of a genetically encoded 21^st amino acid, selenocysteine, is a fascinating breakthrough in molecular biology as it is the first addition to the genetic code deciphered in the 1960s. Selenocysteine is a structural and functional analog of cysteine, where selenium replaces sulfur, and its presence is critical for the catalytic activity of selenoproteins.

SCOPE OF REVIEW

The insertion of selenocysteine is a non-canonical translational event, based on the recoding of a UGA codon in selenoprotein mRNAs, normally used as a stop codon in other cellular mRNAs. Two RNA molecules and associated partners are crucial components of the selenocysteine insertion machinery, the Sec-tRNA^[Ser]Sec devoted to UGA codon recognition and the SECIS elements located in the 3'UTR of selenoprotein mRNAs.

MAJOR CONCLUSIONS

The translational UGA recoding event is a limiting stage of selenoprotein expression and its efficiency is regulated by several factors.

GENERAL SIGNIFICANCE

The control of selenoproteome expression is crucial for redox homeostasis and antioxidant defense of mammalian organisms. In this review, we summarize current knowledge on the co-translational insertion of selenocysteine into selenoproteins, and its layers of regulation.

On elongation factor eEFSec, its role and mechanism during selenium incorporation into nascent selenoproteins

BACKGROUND

Selenium, an essential dietary micronutrient, is incorporated into proteins as the amino acid selenocysteine (Sec) in response to in-frame UGA codons. Complex machinery ensures accurate recoding of Sec codons in higher organisms. A specialized elongation factor eEFSec is central to the process.

SCOPE OF REVIEW

Selenoprotein synthesis relies on selenocysteinyl-tRNA^Sec (Sec-tRNA^Sec), selenocysteine inserting sequence (SECIS) and other selenoprotein mRNA elements, an in-trans SECIS binding protein 2 (SBP2) protein factor, and eEFSec. The exact mechanisms of discrete steps of the Sec UGA recoding are not well understood. However, recent studies on mammalian model systems have revealed the first insights into these mechanisms. Herein, we summarize the current knowledge about the structure and role of mammalian eEFSec.

MAJOR CONCLUSIONS

eEFSec folds into a chalice-like structure resembling that of the archaeal and bacterial orthologues SelB and the initiation protein factor IF2/eIF5B. The three N-terminal domains harbor major functional sites and adopt an EF-Tu-like fold. The C-terminal domain 4 binds to Sec-tRNA^Sec and SBP2, senses distinct binding domains, and modulates the GTPase activity. Remarkably, GTP hydrolysis does not induce a canonical conformational change in eEFSec, but instead promotes a slight ratchet of domains 1 and 2 and a lever-like movement of domain 4, which may be critical for the release of Sec-tRNA^Sec on the ribosome.

GENERAL SIGNIFICANCE

Based on current findings, a non-canonical mechanism for elongation of selenoprotein synthesis at the Sec UGA codon is proposed. Although incomplete, our understanding of this fundamental biological process is significantly improved, and it is being harnessed for biomedical and synthetic biology initiatives. This article is part of a Special Issue entitled "Selenium research" in celebration of 200 years of selenium discovery, edited by Dr. Elias Arnér and Dr. Regina Brigelius-Flohe.

Disorders of metal metabolism

Trace elements are chemical elements needed in minute amounts for normal physiology. Some of the physiologically relevant trace elements include iodine, copper, iron, manganese, zinc, selenium, cobalt and molybdenum. Of these, some are metals, and in particular, transition metals. The different electron shells of an atom carry different energy levels, with those closest to the nucleus being lowest in energy. The number of electrons in the outermost shell determines the reactivity of such an atom. The electron shells are divided in sub-shells, and in particular the third shell has s, p and d sub-shells. Transition metals are strictly defined as elements whose atom has an incomplete d sub-shell. This incomplete d sub-shell makes them prone to chemical reactions, particularly redox reactions. Transition metals of biologic importance include copper, iron, manganese, cobalt and molybdenum. Zinc is not a transition metal, since it has a complete d sub-shell. Selenium, on the other hand, is strictly speaking a nonmetal, although given its chemical properties between those of metals and nonmetals, it is sometimes considered a metalloid. In this review, we summarize the current knowledge on the inborn errors of metal and metalloid metabolism.

ZNHIT3 is defective in PEHO syndrome, a severe encephalopathy with cerebellar granule neuron loss

Progressive encephalopathy with oedema, hypsarrhythmia, and optic atrophy (PEHO) syndrome is an early childhood onset, severe autosomal recessive encephalopathy characterized by extreme cerebellar atrophy due to almost total granule neuron loss. By combining homozygosity mapping in Finnish families with Sanger sequencing of positional candidate genes and with exome sequencing a homozygous missense substitution of leucine for serine at codon 31 in ZNHIT3 was identified as the primary cause of PEHO syndrome. ZNHIT3 encodes a nuclear zinc finger protein previously implicated in transcriptional regulation and in small nucleolar ribonucleoprotein particle assembly and thus possibly to pre-ribosomal RNA processing. The identified mutation affects a highly conserved amino acid residue in the zinc finger domain of ZNHIT3. Both knockdown and genome editing of znhit3 in zebrafish embryos recapitulate the patients' cerebellar defects, microcephaly and oedema. These phenotypes are rescued by wild-type, but not mutant human ZNHIT3 mRNA, suggesting that the patient missense substitution causes disease through a loss-of-function mechanism. Transfection of cell lines with ZNHIT3 expression vectors showed that the PEHO syndrome mutant protein is unstable. Immunohistochemical analysis of mouse cerebellar tissue demonstrated ZNHIT3 to be expressed in proliferating granule cell precursors, in proliferating and post-mitotic granule cells, and in Purkinje cells. Knockdown of Znhit3 in cultured mouse granule neurons and ex vivo cerebellar slices indicate that ZNHIT3 is indispensable for granule neuron survival and migration, consistent with the zebrafish findings and patient neuropathology. These results suggest that loss-of-function of a nuclear regulator protein underlies PEHO syndrome and imply that establishment of its spatiotemporal interaction targets will be the basis for developing therapeutic approaches and for improved understanding of cerebellar development.

Homozygous Truncating Variants in TBC1D23 Cause Pontocerebellar Hypoplasia and Alter Cortical Development

Pontocerebellar hypoplasia (PCH) is a heterogeneous group of rare recessive disorders with prenatal onset, characterized by hypoplasia of pons and cerebellum. Mutations in a small number of genes have been reported to cause PCH, and the vast majority of PCH cases are explained by mutations in TSEN54, which encodes a subunit of the tRNA splicing endonuclease complex. Here we report three families with homozygous truncating mutations in TBC1D23 who display moderate to severe intellectual disability and microcephaly. MRI data from available affected subjects revealed PCH, small normally proportioned cerebellum, and corpus callosum anomalies. Furthermore, through in utero electroporation, we show that downregulation of TBC1D23 affects cortical neuron positioning. TBC1D23 is a member of the Tre2-Bub2-Cdc16 (TBC) domain-containing RAB-specific GTPase-activating proteins (TBC/RABGAPs). Members of this protein family negatively regulate RAB proteins and modulate the signaling between RABs and other small GTPases, some of which have a crucial role in the trafficking of intracellular vesicles and are involved in neurological disorders. Here, we demonstrate that dense core vesicles and lysosomal trafficking dynamics are affected in fibroblasts harboring TBC1D23 mutation. We propose that mutations in TBC1D23 are responsible for a form of PCH with small, normally proportioned cerebellum and should be screened in individuals with syndromic pontocereballar hypoplasia.

Homozygous mutation in TXNRD1 is associated with genetic generalized epilepsy

Increased oxidative stress has been widely implicated in the pathogenesis in various forms of human epilepsy. Here, we report a homozygous mutation in TXNRD1 (thioredoxin reductase 1) in a family with genetic generalized epilepsy. TXNRD1 is an essential selenium-containing enzyme involved in detoxification of reactive oxygen species (ROS) and redox signaling. The TXNRD1 mutation p.Pro190Leu affecting a highly conserved amino acid residue was identified by whole-exome sequencing of blood DNA from the index patient. The detected mutation and its segregation within the family - all siblings of the index patient were homozygous and the parents heterozygous - were confirmed by Sanger sequencing. TXNRD1 activity was determined in subcellular fractions from a skeletal muscle biopsy and skin fibroblasts of the index patient and the expression levels of the mutated protein were assessed by ⁷⁵Se labeling and Western blot analysis. As result of the mutation, the activity of TXNRD1 was reduced in the patient's fibroblasts and skeletal muscle (to 34±3% and 16±8% of controls, respectively). In fibroblasts, we detected reduced ⁷⁵Se-labeling of the enzyme (41±3% of controls). An in-depth in vitro kinetic analysis of the recombinant mutated TXNRD1 indicated 30-40% lowered k_cat/Se values. Therefore, a reduced activity of the enzyme in the patient's tissue samples is explained by (i) lower enzyme turnover and (ii) reduced abundance of the mutated enzyme as confirmed by Western blotting and ⁷⁵Se labeling. The mutant fibroblasts were also found to be less resistant to a hydrogen peroxide challenge. Our data agree with a potential role of insufficient ROS detoxification for disease manifestation in genetic generalized epilepsy.

Genetics and genotype-phenotype correlations in early onset epileptic encephalopathy with burst suppression

OBJECTIVE

We sought to identify genetic causes of early onset epileptic encephalopathies with burst suppression (Ohtahara syndrome and early myoclonic encephalopathy) and evaluate genotype-phenotype correlations.

METHODS

We enrolled 33 patients with a referral diagnosis of Ohtahara syndrome or early myoclonic encephalopathy without malformations of cortical development. We performed detailed phenotypic assessment including seizure presentation, electroencephalography, and magnetic resonance imaging. We confirmed burst suppression in 28 of 33 patients. Research-based exome sequencing was performed for patients without a previously identified molecular diagnosis from clinical evaluation or a research-based epilepsy gene panel.

RESULTS

In 17 of 28 (61%) patients with confirmed early burst suppression, we identified variants predicted to be pathogenic in KCNQ2 (n = 10), STXBP1 (n = 2), SCN2A (n = 2), PNPO (n = 1), PIGA (n = 1), and SEPSECS (n = 1). In 3 of 5 (60%) patients without confirmed early burst suppression, we identified variants predicted to be pathogenic in STXBP1 (n = 2) and SCN2A (n = 1). The patient with the homozygous PNPO variant had a low cerebrospinal fluid pyridoxal-5-phosphate level. Otherwise, no early laboratory or clinical features distinguished the cases associated with pathogenic variants in specific genes from each other or from those with no prior genetic cause identified.

INTERPRETATION

We characterize the genetic landscape of epileptic encephalopathy with burst suppression, without brain malformations, and demonstrate feasibility of genetic diagnosis with clinically available testing in >60% of our cohort, with KCNQ2 implicated in one-third. This electroclinical syndrome is associated with pathogenic variation in SEPSECS. Ann Neurol 2017;81:419-429.

Crystal structures of the human elongation factor eEFSec suggest a non-canonical mechanism for selenocysteine incorporation

Selenocysteine is the only proteinogenic amino acid encoded by a recoded in-frame UGA codon that does not operate as the canonical opal stop codon. A specialized translation elongation factor, eEFSec in eukaryotes and SelB in prokaryotes, promotes selenocysteine incorporation into selenoproteins by a still poorly understood mechanism. Our structural and biochemical results reveal that four domains of human eEFSec fold into a chalice-like structure that has similar binding affinities for GDP, GTP and other guanine nucleotides. Surprisingly, unlike in eEF1A and EF-Tu, the guanine nucleotide exchange does not cause a major conformational change in domain 1 of eEFSec, but instead induces a swing of domain 4. We propose that eEFSec employs a non-canonical mechanism involving the distinct C-terminal domain 4 for the release of the selenocysteinyl-tRNA during decoding on the ribosome.

Structural basis for early-onset neurological disorders caused by mutations in human selenocysteine synthase

Selenocysteine synthase (SepSecS) catalyzes the terminal reaction of selenocysteine, and is vital for human selenoproteome integrity. Autosomal recessive inheritance of mutations in SepSecS-Ala239Thr, Thr325Ser, Tyr334Cys and Tyr429*-induced severe, early-onset, neurological disorders in distinct human populations. Although harboring different mutant alleles, patients presented remarkably similar phenotypes typified by cerebellar and cerebral atrophy, seizures, irritability, ataxia, and extreme spasticity. However, it has remained unclear how these genetic alterations affected the structure of SepSecS and subsequently elicited the development of a neurological pathology. Herein, our biophysical and structural characterization demonstrates that, with the exception of Tyr429*, pathogenic mutations decrease protein stability and trigger protein misfolding. We propose that the reduced stability and increased propensity towards misfolding are the main causes for the loss of SepSecS activity in afflicted patients, and that these factors contribute to disease progression. We also suggest that misfolding of enzymes regulating protein synthesis should be considered in the diagnosis and study of childhood neurological disorders.

Pontocerebellar hypoplasia type 2D and optic nerve atrophy further expand the spectrum associated with selenoprotein biosynthesis deficiency

BACKGROUND

The term Pontocerebellar hypoplasias collectively refers to a group of rare, heterogeneous and progressive disorders, which are frequently inherited in an autosomal recessive manner and usually have a prenatal onset. Mutations in the SEPSECS gene, leading to deficiency in selenoprotein biosynthesis, have been identified in recent times as the molecular etiology of different pre/perinatal onset neurological phenotypes, including cerebello-cerebral atrophy, Pontocerebellar hypoplasia type 2D and progressive encephalopathy with elevated lactate. These disorders share a similar spectrum of central (e.g., brain neurodegeneration with grey and white matter both involved) and peripheral (e.g., spasticity due to axonal neuropathy) nervous system impairment.

CASE PRESENTATION

We hereby describe a 9-year-old boy with (i) a typical Pontocerebellar hypoplasia type 2D phenotype (e.g. profound mental retardation, spastic quadriplegia, ponto-cerebellar hypoplasia and progressive cerebral atrophy); (ii) optic nerve atrophy and (iii) mild secondary mitochondrial myopathy detected by muscle biopsy and respiratory chain enzyme analysis. We performed whole exome sequencing which identified a homozygous mutation of the SEPSECS gene (c.1001T > C), confirming the clinical suspect of Pontocerebellar hypoplasia type 2D.

CONCLUSION

This report further corroborates the notion of a potential secondary mitochondrial dysfunction in the context of selenoprotein biosynthesis deficiency and also adds optic nerve atrophy as a new potential clinical feature within the SEPSECS-associated clinical spectrum. These findings suggest the presence of a possible shared genetic etiology among similar clinical entities characterized by the combination of progressive cerebello-cerebral and optic nerve atrophy and also stress the biological importance of selenoproteins in the regulation of neuronal and metabolic homeostasis.

Milder progressive cerebellar atrophy caused by biallelic SEPSECS mutations

Cerebellar atrophy is recognized in various types of childhood neurological disorders with clinical and genetic heterogeneity. Genetic analyses such as whole exome sequencing are useful for elucidating the genetic basis of these conditions. Pathological recessive mutations in Sep (O-phosphoserine) tRNA:Sec (selenocysteine) tRNA synthase (SEPSECS) have been reported in a total of 11 patients with pontocerebellar hypoplasia type 2, progressive cerebellocerebral atrophy or progressive encephalopathy, yet detailed clinical features are limited to only four patients. We identified two new families with progressive cerebellar atrophy, and by whole exome sequencing detected biallelic SEPSECS mutations: c.356A>G (p.Asn119Ser) and c.77delG (p.Arg26Profs*42) in family 1, and c.356A>G (p.Asn119Ser) and c.467G>A (p.Arg156Gln) in family 2. Their development was slightly delayed regardless of normal brain magnetic resonance imaging (MRI) in infancy. The progression of clinical symptoms in these families is evidently slower than in previously reported cases, and the cerebellar atrophy milder by brain MRI, indicating that SEPSECS mutations are also involved in milder late-onset cerebellar atrophy.

Mutation in human selenocysteine transfer RNA selectively disrupts selenoprotein synthesis

Selenium is a trace element that is essential for human health and is incorporated into more than 25 human selenocysteine-containing (Sec-containing) proteins via unique Sec-insertion machinery that includes a specific, nuclear genome-encoded, transfer RNA (tRNA[Ser]Sec). Here, we have identified a human tRNA[Ser]Sec mutation in a proband who presented with a variety of symptoms, including abdominal pain, fatigue, muscle weakness, and low plasma levels of selenium. This mutation resulted in a marked reduction in expression of stress-related, but not housekeeping, selenoproteins. Evaluation of primary cells from the homozygous proband and a heterozygous parent indicated that the observed deficit in stress-related selenoprotein production is likely mediated by reduced expression and diminished 2'-O-methylribosylation at uridine 34 in mutant tRNA[Ser]Sec. Moreover, this methylribosylation defect was restored by cellular complementation with normal tRNA[Ser]Sec. This study identifies a tRNA mutation that selectively impairs synthesis of stress-related selenoproteins and demonstrates the importance of tRNA modification for normal selenoprotein synthesis.