A homozygous UBA5 pathogenic variant causes a fatal congenital neuropathy

Hypomyelinating Leukodystrophy 14 (HLD14)-Related UFC1 p.Arg23Gln Decreases Cell Morphogenesis: A Phenotype Reversable with Hesperetin

INTRODUCTION

In the central nervous system (CNS), proper interaction between neuronal and glial cells is crucial for the development of mature nervous tissue. Hypomyelinating leukodystrophies (HLDs) are a group of genetic CNS disorders characterized by hypomyelination and/or demyelination. In these conditions, genetic mutations disrupt the biological functions of oligodendroglial cells, which are responsible for wrapping neuronal axons with myelin sheaths. Among these, an amino acid mutation of the ubiquitin-fold modifier conjugating enzyme 1 (UFC1) is associated with HLD14-related disease, characterized by hypomyelination and delayed myelination in the brain. UFC1 is a critical component of the UFMylation system, functioning similarly to E2-conjugating enzymes in the ubiquitin-dependent protein degradation system.

METHODOLOGY

We describe how a missense mutation in UFC1 (p.Arg23Gln) leads to the aggregation of UFC1 primarily in lysosomes in FBD-102b cells, which are undergoing oligodendroglial cell differentiation.

RESULTS

Cells with mutated UFC1 exhibit reduced Akt kinase phosphorylation and reduced expression of differentiation and myelination marker proteins. Consistently, these cells exhibit impaired morphological differentiation with a reduced ability to extend widespread membranes. Interestingly, hesperetin, a citrus flavonoid with known neuroprotective properties, was found to restore differentiation abilities in cells with the UFC1 mutation.

CONCLUSIONS

These findings indicate that the HLD14-related mutation in UFC1 causes its lysosomal aggregation, impairing its morphological differentiation. Furthermore, the study highlights potential therapeutic insights into the pathological molecular and cellular mechanisms underlying HLD14 and suggests hesperetin as a promising candidate for treatment.

Patient derived model of UBA5-associated encephalopathy identifies defects in neurodevelopment and highlights potential therapies

UBA5 encodes for the E1 enzyme of the UFMylation cascade, which plays an essential role in ER homeostasis. The clinical phenotypes of UBA5-associated encephalopathy include developmental delays, epilepsy and intellectual disability. To date, there is no humanized neuronal model to study the cellular and molecular consequences of UBA5 pathogenic variants. We developed and characterized patient-derived cortical organoid cultures and identified defects in GABAergic interneuron development. We demonstrated aberrant neuronal firing and microcephaly phenotypes in patient-derived organoids. Mechanistically, we show that ER homeostasis is perturbed along with exacerbated unfolded protein response pathway in cells and organoids expressing UBA5 pathogenic variants. We also assessed two gene expression modalities that augmented UBA5 expression to rescue aberrant molecular and cellular phenotypes. Our study provides a novel humanized model that allows further investigations of UBA5 variants in the brain and highlights novel systemic approaches to alleviate cellular aberrations for this rare, developmental disorder.

The UFM1 system: Working principles, cellular functions, and pathophysiology

Ubiquitin-fold modifier 1 (UFM1) is a ubiquitin-like protein covalently conjugated with intracellular proteins through UFMylation, a process similar to ubiquitylation. Growing lines of evidence regarding not only the structural basis of the components essential for UFMylation but also their biological properties shed light on crucial roles of the UFM1 system in the endoplasmic reticulum (ER), such as ER-phagy and ribosome-associated quality control at the ER, although there are some functions unrelated to the ER. Mouse genetics studies also revealed the indispensable roles of this system in hematopoiesis, liver development, neurogenesis, and chondrogenesis. Of critical importance, mutations of genes encoding core components of the UFM1 system in humans cause hereditary developmental epileptic encephalopathy and Schohat-type osteochondrodysplasia of the epiphysis. Here, we provide a multidisciplinary review of our current understanding of the mechanisms and cellular functions of the UFM1 system as well as its pathophysiological roles, and discuss issues that require resolution.

Allelic strengths of encephalopathy-associated UBA5 variants correlate between in vivo and in vitro assays

Protein UFMylation downstream of the E1 enzyme UBA5 plays essential roles in development and endoplasmic reticulum stress. Variants in the UBA5 gene are associated with developmental and epileptic encephalopathy 44 (DEE44), an autosomal recessive disorder characterized by early-onset encephalopathy, movement abnormalities, global developmental delay, intellectual disability, and seizures. DEE44 is caused by at least 12 different missense variants described as loss of function (LoF), but the relationships between genotypes and molecular or clinical phenotypes remain to be established. We developed a humanized UBA5 fly model and biochemical activity assays in order to describe in vivo and in vitro genotype-phenotype relationships across the UBA5 allelic series. In vivo, we observed a broad spectrum of phenotypes in viability, developmental timing, lifespan, locomotor activity, and bang sensitivity. A range of functional effects was also observed in vitro across comprehensive biochemical assays for protein stability, ATP binding, UFM1 activation, and UFM1 transthiolation. Importantly, there is a strong correlation between in vivo and in vitro phenotypes, establishing a classification of LoF variants into mild, intermediate, and severe allelic strengths. By systemically evaluating UBA5 variants across in vivo and in vitro platforms, this study provides a foundation for more basic and translational UBA5 research, as well as a basis for evaluating current and future individuals afflicted with this rare disease.

Genetic model of UBA5 deficiency highlights the involvement of both peripheral and central nervous systems and identifies widespread mitochondrial abnormalities

Variants in UBA5 have been reported to cause neurological disease with impaired motor function, developmental delay, intellectual disability and brain pathology as recurrent clinical manifestations. UBA5 encodes a ubiquitin-activating-like enzyme that activates ufmylation, a post-translational ubiquitin-like modification pathway, which has been implicated in neurodevelopment and neuronal survival. The reason behind the variation in severity and clinical manifestations in affected individuals and the signal transduction pathways regulated by ufmylation that compromise the nervous system remains unknown. Zebrafish have emerged as a powerful model to study neurodegenerative disease due to its amenability for in vivo analysis of muscle and neuronal tissues, high-throughput examination of motor function and rapid embryonic development allowing an examination of disease progression. Using clustered regularly interspaced short palindromic repeats-associated protein 9 genome editing, we developed and characterized zebrafish mutant models to investigate disease pathophysiology. uba5 mutant zebrafish showed a significantly impaired motor function accompanied by delayed growth and reduced lifespan, reproducing key phenotypes observed in affected individuals. Our study demonstrates the suitability of zebrafish to study the pathophysiology of UBA5-related disease and as a powerful tool to identify pathways that could reduce disease progression. Furthermore, uba5 mutants exhibited widespread mitochondrial damage in both the nervous system and the skeletal muscle, suggesting that a perturbation of mitochondrial function may contribute to disease pathology.

A guide to UFMylation, an emerging posttranslational modification

Ubiquitin Fold Modifier-1 (UFM1) is a ubiquitin-like modifier (UBL) that is posttranslationally attached to lysine residues on substrates via a dedicated system of enzymes conserved in most eukaryotes. Despite the structural similarity between UFM1 and ubiquitin, the UFMylation machinery employs unique mechanisms that ensure fidelity. While physiological triggers and consequences of UFMylation are not entirely clear, its biological importance is epitomized by mutations in the UFMylation pathway in human pathophysiology including musculoskeletal and neurodevelopmental diseases. Some of these diseases can be explained by the increased endoplasmic reticulum (ER) stress and disrupted translational homeostasis observed upon loss of UFMylation. The roles of UFM1 in these processes likely stem from its function at the ER where ribosomes are UFMylated in response to translational stalling. In addition, UFMylation has been implicated in other cellular processes including DNA damage response and telomere maintenance. Hence, the study of UFM1 pathway mechanics and its biological function will reveal insights into fundamental cell biology and is likely to afford new therapeutic opportunities for the benefit of human health. To this end, we herein provide a comprehensive guide to the current state of knowledge of UFM1 biogenesis, conjugation, and function with an emphasis on the underlying mechanisms.

Allelic strengths of encephalopathy-associated UBA5 variants correlate between in vivo and in vitro assays

Protein UFMylation downstream of the E1 enzyme UBA5 plays essential roles in development and ER stress. Variants in the UBA5 gene are associated with developmental and epileptic encephalopathy 44 (DEE44), an autosomal recessive disorder characterized by early-onset encephalopathy, movement abnormalities, global developmental delay, intellectual disability, and seizures. DEE44 is caused by at least twelve different missense variants described as loss of function (LoF), but the relationships between genotypes and molecular or clinical phenotypes remains to be established. We developed a humanized UBA5 fly model and biochemical activity assays in order to describe in vivo and in vitro genotype-phenotype relationships across the UBA5 allelic series. In vivo, we observed a broad spectrum of phenotypes in viability, developmental timing, lifespan, locomotor activity, and bang sensitivity. A range of functional effects was also observed in vitro across comprehensive biochemical assays for protein stability, ATP binding, UFM1 activation, and UFM1 transthiolation. Importantly, there is a strong correlation between in vivo and in vitro phenotypes, establishing a classification of LoF variants into mild, intermediate, and severe allelic strengths. By systemically evaluating UBA5 variants across in vivo and in vitro platforms, this study provides a foundation for more basic and translational UBA5 research, as well as a basis for evaluating current and future individuals afflicted with this rare disease.

Molecular Influence of Resiniferatoxin on the Urinary Bladder Wall Based on Differential Gene Expression Profiling

Resiniferatoxin (RTX) is a potent capsaicin analog used as a drug for experimental therapy to treat neurogenic disorders associated with enhanced nociceptive transmission, including lower urinary tract symptoms. The present study, for the first time, investigated the transcriptomic profile of control and RTX-treated porcine urinary bladder walls. We applied multistep bioinformatics and discovered 129 differentially expressed genes (DEGs): 54 upregulated and 75 downregulated. Metabolic pathways analysis revealed five significant Kyoto Encyclopedia of Genes and Genomes (KEGG) items ('folate biosynthesis', 'metabolic pathways', 'sulfur relay system', 'sulfur metabolism' and 'serotonergic synapse') that were altered after RTX intravesical administration. A thorough analysis of the detected DEGs indicated that RTX treatment influenced the signaling pathways regulating nerve growth, myelination, axon specification, and elongation. Many of the revealed DEGs are involved in the nerve degeneration process; however, some of them were implicated in the initiation of neuroprotective mechanisms. Interestingly, RTX intravesical installation was followed by changes in the expression of genes involved in synaptic plasticity and neuromodulation, including 5-HT, H2S, glutamate, and GABA transmission. The obtained results suggest that the toxin may exert a therapeutic, antinociceptive effect not only by acting on TRPV1 receptors.

Deficiency of Murine UFM1-Specific E3 Ligase Causes Microcephaly and Inflammation

The UFM1 conjugation system is a Ubiquitin (Ub)-like modification system that is essential for animal development and normal physiology of multiple tissues and organs. It consists of UFM1, a Ub-like modifier, and the UFM1-specific enzymes (namely E1 enzyme UBA5, E2 enzyme UFC1 E2, and E3 ligases) that catalyze conjugation of UFM1 to its specific protein targets. Clinical studies have identified rare genetic variants in human UFM1, UBA5 and UFC1 genes that were linked to early-onset encephalopathy and defective brain development, strongly suggesting the critical role of the UFM1 system in the nervous system. Yet, the physiological function of this system in adult brain remains not defined. In this study, we investigated the role of UFM1 E3 ligase in adult mouse and found that both UFL1 and UFBP1 proteins, two components of UFM1 E3 ligase, are essential for survival of mature neurons in adult mouse. Neuron-specific deletion of either UFL1 or UFBP1 led to significant neuronal loss and elevation of inflammatory response. Interestingly, loss of one allele of UFBP1 genes caused the occurrence of seizure-like events. Our study has provided genetic evidence for the indispensable role of UFM1 E3 ligase in mature neurons and further demonstrated the importance of the UFM1 system in the nervous system.

UFMylation System: An Emerging Player in Tumorigenesis

Ubiquitin-fold modifier 1 (UFM1), a newly identified ubiquitin-like molecule (UBLs), is evolutionarily expressed in multiple species except yeast. Similarly to ubiquitin, UFM1 is covalently attached to its substrates through a well-orchestrated three-step enzymatic reaction involving E1, the UFM1-activating enzyme (ubiquitin-like modifier-activating enzyme 5, UBA5); E2, the UFM1-conjugating enzyme 1 (UFC1); and E3, the UFM1-specific ligase 1 (UFL1). To date, numerous studies have shown that UFM1 modification is implicated in various cellular processes, including endoplasmic reticulum (ER) stress, DNA damage response and erythroid development. An abnormal UFM1 cascade is closely related to a variety of diseases, especially tumors. Herein, we summarize the process and functions of UFM1 modification, illustrating the relationship and mechanisms between aberrant UFMylation and diversified tumors, aiming to provide novel diagnostic biomarkers or therapeutic targets for cancer treatments.

Emerging role of UFMylation in secretory cells involved in the endocrine system by maintaining ER proteostasis

Ubiquitin-fold modifier 1 (UFM1) is a ubiquitin-like molecule (UBL) discovered almost two decades ago, but our knowledge about the cellular and molecular mechanisms of this novel protein post-translational modification is still very fragmentary. In this review, we first summarize the core enzymes and factors involved in the UFMylation cascade, which, similar to ubiquitin, is consecutively catalyzed by UFM1-activating enzyme 5 (UBA5), UFM1-conjugating enzyme 1 (UFC1) and UFM1-specific ligase 1 (UFL1). Inspired by the substantial implications of UFM1 machinery in the secretory pathway, we next concentrate on the puzzling role of UFMylation in maintaining ER protein homeostasis, intending to illustrate the underlying mechanisms and future perspectives. At last, given a robust ER network is a hallmark of healthy endocrine secretory cells, we emphasize the function of UFM1 modification in physiology and pathology in the context of endocrine glands pancreas and female ovaries, aiming to provide precise insight into other internal glands of the endocrine system.

Homozygous UBA5 Variant Leads to Hypomyelination with Thalamic Involvement and Axonal Neuropathy

The enzyme ubiquitin-like modifier activating enzyme 5 (UBA5) plays an important role in activating ubiquitin-fold modifier 1 (UFM1) and its associated cascade. UFM1 is widely expressed and known to facilitate the post-translational modification of proteins. Variants in UBA5 and UFM1 are involved in neurodevelopmental disorders with early-onset epileptic encephalopathy as a frequently seen disease manifestation. Using whole exome sequencing, we detected a homozygous UBA5 variant (c.895C > T p. [Pro299Ser]) in a patient with severe global developmental delay and epilepsy, the latter from the age of 4 years. Magnetic resonance imaging showed hypomyelination with atrophy and T2 hyperintensity of the thalamus. Histology of the sural nerve showed axonal neuropathy with decreased myelin. Functional analyses confirmed the effect of the Pro299Ser variant on UBA5 protein function, showing 58% residual protein activity. Our findings indicate that the epilepsy currently associated with UBA5 variants may present later in life than previously thought, and that radiological signs include hypomyelination and thalamic involvement. The data also reinforce recently reported associations between UBA5 variants and peripheral neuropathy.

A description of novel variants and review of phenotypic spectrum in UBA5-related early epileptic encephalopathy

Early infantile epileptic encephalopathy-44 (EIEE44, MIM: 617132) is a previously described condition resulting from biallelic variants in UBA5, a gene involved in a ubiquitin-like post-translational modification system called UFMylation. Here we report five children from four families with biallelic pathogenic variants in UBA5 All five children presented with global developmental delay, epilepsy, axial hypotonia, appendicular hypertonia, and a movement disorder, including dystonia in four. Affected individuals in all four families have compound heterozygous pathogenic variants in UBA5 All have the recurrent mild c.1111G > A (p.Ala371Thr) variant in trans with a second UBA5 variant. One patient has the previously described c.562C > T (p. Arg188*) variant, two other unrelated patients have a novel missense variant, c.907T > C (p.Cys303Arg), and the two siblings have a novel missense variant, c.761T > C (p.Leu254Pro). Functional analyses demonstrate that both the p.Cys303Arg variant and the p.Leu254Pro variants result in a significant decrease in protein function. We also review the phenotypes and genotypes of all 15 previously reported families with biallelic UBA5 variants, of which two families have presented with distinct phenotypes, and we describe evidence for some limited genotype-phenotype correlation. The overlap of motor and developmental phenotypes noted in our cohort and literature review adds to the increasing understanding of genetic syndromes with movement disorders-epilepsy.

Highly Specialized Ubiquitin-Like Modifications: Shedding Light into the UFM1 Enigma

Post-translational modification with Ubiquitin-like proteins represents a complex signaling language regulating virtually every cellular process. Among these post-translational modifiers is Ubiquitin-fold modifier (UFM1), which is covalently attached to its substrates through the orchestrated action of a dedicated enzymatic cascade. Originally identified to be involved embryonic development, its biological function remains enigmatic. Recent research reveals that UFM1 regulates a variety of cellular events ranging from DNA repair to autophagy and ER stress response implicating its involvement in a variety of diseases. Given the contribution of UFM1 to numerous pathologies, the enzymes of the UFM1 cascade represent attractive targets for pharmacological inhibition. Here we discuss the current understanding of this cryptic post-translational modification especially its contribution to disease as well as expand on the unmet needs of developing chemical and biochemical tools to dissect its role.