Expansion of the neurodevelopmental phenotype of individuals with EEF1A2 variants and genotype-phenotype study.

Translation elongation factor eEF1A2 constitutes the alpha subunit of the elongation factor-1 complex, responsible for the enzymatic binding of aminoacyl-tRNA to the ribosome. Since 2012, 21 pathogenic missense variants affecting EEF1A2 have been described in 42 individuals with a severe neurodevelopmental phenotype including epileptic encephalopathy and moderate to profound intellectual disability (ID), with neurological regression in some patients. Through international collaborative call, we collected 26 patients with EEF1A2 variants and compared them to the literature. Our cohort shows a significantly milder phenotype. 83% of the patients are walking (vs. 29% in the literature), and 84% of the patients have language skills (vs. 15%). Three of our patients do not have ID. Epilepsy is present in 63% (vs. 93%). Neurological examination shows a less severe phenotype with significantly less hypotonia (58% vs. 96%), and pyramidal signs (24% vs. 68%). Cognitive regression was noted in 4% (vs. 56% in the literature). Among individuals over 10 years, 56% disclosed neurocognitive regression, with a mean age of onset at 2 years. We describe 8 novel missense variants of EEF1A2. Modeling of the different amino-acid sites shows that the variants associated with a severe phenotype, and the majority of those associated with a moderate phenotype, cluster within the switch II region of the protein and thus may affect GTP exchange. In contrast, variants associated with milder phenotypes may impact secondary functions such as actin binding. We report the largest cohort of individuals with EEF1A2 variants thus far, allowing us to expand the phenotype spectrum and reveal genotype-phenotype correlations.

AGAP1-associated endolysosomal trafficking abnormalities link gene-environment interactions in neurodevelopmental disorders.

AGAP1 is an Arf1 GTPase-activating protein that regulates endolysosomal trafficking. Damaging variants have been linked to cerebral palsy and autism. We report three new cases in which individuals had microdeletion variants in AGAP1. The affected individuals had intellectual disability (3/3), autism (3/3), dystonia with axial hypotonia (1/3), abnormalities of brain maturation (1/3), growth impairment (2/3) and facial dysmorphism (2/3). We investigated mechanisms potentially underlying AGAP1 variant-mediated neurodevelopmental impairments using the Drosophila ortholog CenG1a. We discovered reduced axon terminal size, increased neuronal endosome abundance and elevated autophagy compared to those in controls. Given potential incomplete penetrance, we assessed gene-environment interactions. We found basal elevation in the phosphorylation of the integrated stress-response protein eIF2α (or eIF2A) and inability to further increase eIF2α phosphorylation with subsequent cytotoxic stressors. CenG1a-mutant flies had increased lethality from exposure to environmental insults. We propose a model wherein disruption of AGAP1 function impairs endolysosomal trafficking, chronically activating the integrated stress response and leaving AGAP1-deficient cells susceptible to a variety of second-hit cytotoxic stressors. This model may have broader applicability beyond AGAP1 in instances where both genetic and environmental insults co-occur in individuals with neurodevelopmental disorders.

AGAP1-associated endolysosomal trafficking abnormalities link gene-environment interactions in a neurodevelopmental disorder.

AGAP1 is an Arf1 GAP that regulates endolysosomal trafficking. Damaging variants have been linked to cerebral palsy and autism. We report 3 new individuals with microdeletion variants in AGAP1 . Affected individuals have intellectual disability (3/3), autism (3/3), dystonia with axial hypotonia (1/3), abnormalities of brain maturation (1/3), growth impairment (2/3) and facial dysmorphism (2/3). We investigated mechanisms potentially underlying AGAP1 neurodevelopmental impairments using the Drosophila ortholog, CenG1a . We discovered reduced axon terminal size, increased neuronal endosome abundance, and elevated autophagy at baseline. Given potential incomplete penetrance, we assessed gene-environment interactions. We found basal elevation in phosphorylation of the integrated stress-response protein eIF2α and inability to further increase eIF2α-P with subsequent cytotoxic stressors. CenG1a -mutant flies have increased lethality from exposure to environmental insults. We propose a model wherein disruption of AGAP1 function impairs endolysosomal trafficking, chronically activating the integrated stress response, and leaving AGAP1-deficient cells susceptible to a variety of second hit cytotoxic stressors. This model may have broader applicability beyond AGAP1 in instances where both genetic and environmental insults co-occur in individuals with neurodevelopmental disorders. Summary statement: We describe 3 additional patients with heterozygous AGAP1 deletion variants and use a loss of function Drosophila model to identify defects in synaptic morphology with increased endosomal sequestration, chronic autophagy induction, basal activation of eIF2α-P, and sensitivity to environmental stressors.

Mutation in ZDHHC15 Leads to Hypotonic Cerebral Palsy, Autism, Epilepsy, and Intellectual Disability.

OBJECTIVE: To determine whether mutations reported for ZDHHC15 can cause mixed neurodevelopmental disorders, we performed both functional studies on variant pathogenicity and ZDHHC15 function in animal models. METHODS: We examined protein function of 4 identified variants in ZDHHC15 in a yeast complementation assay and locomotor defects of loss-of-function genotypes in a Drosophila model. RESULTS: Although we assessed multiple patient variants, only 1 (p.H158R) affected protein function. We report a patient with a diagnosis of hypotonic cerebral palsy, autism, epilepsy, and intellectual disability associated with this bona fide damaging X-linked variant. Features include tall forehead with mild brachycephaly, down-slanting palpebral fissures, large ears, long face, facial muscle hypotonia, high-arched palate with dental crowding, and arachnodactyly. The patient had mild diminished cerebral volume, with left-sided T2/FLAIR hyperintense periatrial ovoid lesion. We found that loss-of-function mutations in orthologs of this gene cause flight and coordinated movement defects in Drosophila. CONCLUSIONS: Our findings support a functional expansion of this gene to a role in motor dysfunction. Although ZDHHC15 mutations represent a rare cause of neurodevelopmental disability, candidate variants need to be carefully assessed before pathogenicity can be determined.

Damaging de novo missense variants in EEF1A2 lead to a developmental and degenerative epileptic-dyskinetic encephalopathy.

Heterozygous de novo variants in the eukaryotic elongation factor EEF1A2 have previously been described in association with intellectual disability and epilepsy but never functionally validated. Here we report 14 new individuals with heterozygous EEF1A2 variants. We functionally validate multiple variants as protein-damaging using heterologous expression and complementation analysis. Our findings allow us to confirm multiple variants as pathogenic and broaden the phenotypic spectrum to include dystonia/choreoathetosis, and in some cases a degenerative course with cerebral and cerebellar atrophy. Pathogenic variants appear to act via a haploinsufficiency mechanism, disrupting both the protein synthesis and integrated stress response functions of EEF1A2. Our studies provide evidence that EEF1A2 is highly intolerant to variation and that de novo pathogenic variants lead to an epileptic-dyskinetic encephalopathy with both neurodevelopmental and neurodegenerative features. Developmental features may be driven by impaired synaptic protein synthesis during early brain development while progressive symptoms may be linked to an impaired ability to handle cytotoxic stressors.

Bi-allelic variants in RNF170 are associated with hereditary spastic paraplegia.

Alterations of Ca2+ homeostasis have been implicated in a wide range of neurodegenerative diseases. Ca2+ efflux from the endoplasmic reticulum into the cytoplasm is controlled by binding of inositol 1,4,5-trisphosphate to its receptor. Activated inositol 1,4,5-trisphosphate receptors are then rapidly degraded by the endoplasmic reticulum-associated degradation pathway. Mutations in genes encoding the neuronal isoform of the inositol 1,4,5-trisphosphate receptor (ITPR1) and genes involved in inositol 1,4,5-trisphosphate receptor degradation (ERLIN1, ERLIN2) are known to cause hereditary spastic paraplegia (HSP) and cerebellar ataxia. We provide evidence that mutations in the ubiquitin E3 ligase gene RNF170, which targets inositol 1,4,5-trisphosphate receptors for degradation, are the likely cause of autosomal recessive HSP in four unrelated families and functionally evaluate the consequences of mutations in patient fibroblasts, mutant SH-SY5Y cells and by gene knockdown in zebrafish. Our findings highlight inositol 1,4,5-trisphosphate signaling as a candidate key pathway for hereditary spastic paraplegias and cerebellar ataxias and thus prioritize this pathway for therapeutic interventions.

De Novo Pathogenic Variants in CACNA1E Cause Developmental and Epileptic Encephalopathy with Contractures, Macrocephaly, and Dyskinesias.

Developmental and epileptic encephalopathies (DEEs) are severe neurodevelopmental disorders often beginning in infancy or early childhood that are characterized by intractable seizures, abundant epileptiform activity on EEG, and developmental impairment or regression. CACNA1E is highly expressed in the central nervous system and encodes the α1-subunit of the voltage-gated CaV2.3 channel, which conducts high voltage-activated R-type calcium currents that initiate synaptic transmission. Using next-generation sequencing techniques, we identified de novo CACNA1E variants in 30 individuals with DEE, characterized by refractory infantile-onset seizures, severe hypotonia, and profound developmental impairment, often with congenital contractures, macrocephaly, hyperkinetic movement disorders, and early death. Most of the 14, partially recurring, variants cluster within the cytoplasmic ends of all four S6 segments, which form the presumed CaV2.3 channel activation gate. Functional analysis of several S6 variants revealed consistent gain-of-function effects comprising facilitated voltage-dependent activation and slowed inactivation. Another variant located in the domain II S4-S5 linker results in facilitated activation and increased current density. Five participants achieved seizure freedom on the anti-epileptic drug topiramate, which blocks R-type calcium channels. We establish pathogenic variants in CACNA1E as a cause of DEEs and suggest facilitated R-type calcium currents as a disease mechanism for human epilepsy and developmental disorders.

Homozygous EEF1A2 mutation causes dilated cardiomyopathy, failure to thrive, global developmental delay, epilepsy and early death.

Eukaryotic elongation factor 1A (EEF1A), is encoded by two distinct isoforms, EEF1A1 and EEF1A2; whereas EEF1A1 is expressed almost ubiquitously, EEF1A2 expression is limited such that it is only detectable in skeletal muscle, heart, brain and spinal cord. Currently, the role of EEF1A2 in normal cardiac development and function is unclear. There have been several reports linking de novo dominant EEF1A2 mutations to neurological issues in humans. We report a pair of siblings carrying a homozygous missense mutation p.P333L in EEF1A2 who exhibited global developmental delay, failure to thrive, dilated cardiomyopathy and epilepsy, ultimately leading to death in early childhood. A third sibling also died of a similar presentation, but DNA was unavailable to confirm the mutation. Functional genomic analysis was performed in S. cerevisiae and zebrafish. In S. cerevisiae, there was no evidence for a dominant-negative effect. Previously identified putative de novo mutations failed to complement yeast strains lacking the EEF1A ortholog showing a major growth defect. In contrast, the introduction of the mutation seen in our family led to a milder growth defect. To evaluate its function in zebrafish, we knocked down eef1a2 expression using translation blocking and splice-site interfering morpholinos. EEF1A2-deficient zebrafish had skeletal muscle weakness, cardiac failure and small heads. Human EEF1A2 wild-type mRNA successfully rescued the morphant phenotype, but mutant RNA did not. Overall, EEF1A2 appears to be critical for normal heart function in humans, and its deficiency results in clinical abnormalities in neurologic function as well as in skeletal and cardiac muscle defects.

Loss-of-Function Mutations in FRRS1L Lead to an Epileptic-Dyskinetic Encephalopathy.

Glutamatergic neurotransmission governs excitatory signaling in the mammalian brain, and abnormalities of glutamate signaling have been shown to contribute to both epilepsy and hyperkinetic movement disorders. The etiology of many severe childhood movement disorders and epilepsies remains uncharacterized. We describe a neurological disorder with epilepsy and prominent choreoathetosis caused by biallelic pathogenic variants in FRRS1L, which encodes an AMPA receptor outer-core protein. Loss of FRRS1L function attenuates AMPA-mediated currents, implicating chronic abnormalities of glutamatergic neurotransmission in this monogenic neurological disease of childhood.

The phosphatase Ptc7 induces coenzyme Q biosynthesis by activating the hydroxylase Coq7 in yeast.

The study of the components of mitochondrial metabolism has potential benefits for health span and lifespan because the maintenance of efficient mitochondrial function and antioxidant capacity is associated with improved health and survival. In yeast, mitochondrial function requires the tight control of several metabolic processes such as coenzyme Q biosynthesis, assuring an appropriate energy supply and antioxidant functions. Many mitochondrial processes are regulated by phosphorylation cycles mediated by protein kinases and phosphatases. In this study, we determined that the mitochondrial phosphatase Ptc7p, a Ser/Thr phosphatase, was required to regulate coenzyme Q6 biosynthesis, which in turn activated aerobic metabolism and enhanced oxidative stress resistance. We showed that Ptc7p phosphatase specifically activated coenzyme Q6 biosynthesis through the dephosphorylation of the demethoxy-Q6 hydroxylase Coq7p. The current findings revealed that Ptc7p is a regulator of mitochondrial metabolism that is essential to maintain proper function of the mitochondria by regulating energy metabolism and oxidative stress resistance.

BTN1, the Saccharomyces cerevisiae homolog to the human Batten disease gene, is involved in phospholipid distribution.

BTN1, the yeast homolog to human CLN3 (which is defective in Batten disease), has been implicated in the regulation of vacuolar pH, potentially by modulating vacuolar-type H(+)-ATPase (V-ATPase) activity. However, we report that Btn1p and the V-ATPase complex do not physically interact, suggesting that any influence that Btn1p has on V-ATPase is indirect. Because membrane lipid environment plays a crucial role in the activity and function of membrane proteins, we investigated whether cells lacking BTN1 have altered membrane phospholipid content. Deletion of BTN1 (btn1-Δ) led to a decreased level of phosphatidylethanolamine (PtdEtn) in both mitochondrial and vacuolar membranes. In yeast there are two phosphatidylserine (PtdSer) decarboxylases, Psd1p and Psd2p, and these proteins are responsible for the synthesis of PtdEtn in mitochondria and Golgi-endosome, respectively. Deletion of both BTN1 and PSD1 (btn1-Δ psd1-Δ) led to a further decrease in levels of PtdEtn in ER membranes associated to mitochondria (MAMs), with a parallel increase in PtdSer. Fluorescent-labeled PtdSer (NBD-PtdSer) transport assays demonstrated that transport of NBD-PtdSer from the ER to both mitochondria and endosomes and/or vacuole is affected in btn1-Δ cells. Moreover, btn1-Δ affects the synthesis of PtdEtn by the Kennedy pathway and impairs the ability of psd1-Δ cells to restore PtdEtn to normal levels in mitochondria and vacuoles by ethanolamine addition. In summary, lack of Btn1p alters phospholipid levels and might play a role in regulating their subcellular distribution.

Comment on "Deletion of btn1, an orthologue of CLN3, increases glycolysis and perturbs amino acid metabolism in the fission yeast model of Batten disease".

We provide a comment on the paper of Pears et al. (Mol. BioSyst., 2010, 6, 1093-1102).

pH-dependent localization of Btn1p in the yeast model for Batten disease.

Btn1p the yeast homolog of human CLN3, which is associated with juvenile Batten disease has been implicated in several cellular pathways. Yeast cells lacking BTN1 are unable to couple ATP hydrolysis and proton pumping activities by the vacuolar ATPase (V-ATPase). In this work, we demonstrate that changes in extracellular pH result in altered transcription of BTN1, as well as a change in the glycosylation state and localization of Btn1p. At high pH, Btn1p expression was increased and the protein was mainly located in vacuolar membranes. However, low pH decreased Btn1p expression and changed its location to undefined punctate membranes. Moreover, our results suggest that differential Btn1p localization may be regulated by its glycosylation state. Underlying pathogenic implications for Batten disease of altered cellular distribution of CLN3 are discussed.

Interaction between Sdo1p and Btn1p in the Saccharomyces cerevisiae model for Batten disease.

Juvenile Batten disease is an autosomal recessive pediatric neurodegenerative disorder caused by mutations in the CLN3 gene. The CLN3 protein primarily resides in the lysosomal membrane, but its function is unknown. We demonstrate that CLN3 interacts with SBDS, the protein mutated in Shwachman-Bodian-Diamond syndrome patients. We demonstrate that this protein-protein interaction is conserved between Btn1p and Sdo1p, the respective yeast Saccharomyces cerevisiae orthologs of CLN3 and SBDS. It was previously shown that deletion of BTN1 results in alterations in vacuolar pH and vacuolar (H(+))-ATPase (V-ATPase)-dependent H(+) transport and ATP hydrolysis. Here, we report that an SDO1 deletion strain has decreased vacuolar pH and V-ATPase-dependent H(+) transport and ATP hydrolysis. These alterations result from decreased V-ATPase subunit expression. Overexpression of BTN1 or the presence of ionophore carbonyl cyanide m-chlorophenil hydrazone (CCCP) causes decreased growth in yeast lacking SDO1. In fact, in normal cells, overexpression of BTN1 mirrors the effect of CCCP, with both resulting in increased vacuolar pH due to alterations in the coupling of V-ATPase-dependent H(+) transport and ATP hydrolysis. Thus, we propose that Sdo1p and SBDS work to regulate Btn1p and CLN3, respectively. This report highlights a novel mechanism for controlling vacuole/lysosome homeostasis by the ribosome maturation pathway that may contribute to the cellular abnormalities associated with juvenile Batten disease and Shwachman-Bodian-Diamond syndrome.

Genetic evidence for the requirement of the endocytic pathway in the uptake of coenzyme Q6 in Saccharomyces cerevisiae.

Coenzyme Q is an isoprenylated benzoquinone lipid that functions in respiratory electron transport and as a lipid antioxidant. Dietary supplementation with Q is increasingly used as a therapeutic for treatment of mitochondrial and neurodegenerative diseases, yet little is known regarding the mechanism of its uptake. As opposed to other yeast backgrounds, EG103 strains are unable to import exogenous Q(6) to the mitochondria. Furthermore, the distribution of exogenous Q(6) among endomembranes suggests an impairment of the membrane traffic at the level of the endocytic pathway. This fact was confirmed after the detection of defects in the incorporation of FM4-64 marker and CPY delivery to the vacuole. A similar effect was demonstrated in double mutant strains in Q(6) synthesis and several steps of endocytic process; those cells are unable to uptake exogenous Q(6) to the mitochondria and restore the growth on non-fermentable carbon sources. Additional data about the positive effect of peptone presence for exogenous Q(6) uptake support the hypothesis that Q(6) is transported to mitochondria through an endocytic-based system.

Nitric oxide signaling is disrupted in the yeast model for Batten disease.

The juvenile form of neuronal ceroid lipofuscinoses (JNCLs), or Batten disease, results from mutations in the CLN3 gene, and it is characterized by the accumulation of lipopigments in the lysosomes of several cell types and by extensive neuronal death. We report that the yeast model for JNCL (btn1-Delta) that lacks BTN1, the homologue to human CLN3, has increased resistance to menadione-generated oxidative stress. Expression of human CLN3 complemented the btn1-Delta phenotype, and equivalent Btn1p/Cln3 mutations correlated with JNCL severity. We show that the previously reported decreased levels of L-arginine in btn1-Delta limit the synthesis of nitric oxide (.NO) in both physiological and oxidative stress conditions. This defect in .NO synthesis seems to suppress the signaling required for yeast menadione-induced apoptosis, thus explaining btn1-Delta phenotype of increased resistance. We propose that in JNCL, a limited capacity to synthesize .NO directly caused by the absence of Cln3 function may contribute to the pathology of the disease.