Polyglucosan body myopathy caused by defective ubiquitin ligase RBCK1

A novel likely pathogenic homozygous RBCK1 variant in dilated cardiomyopathy with muscle weakness

AIMS

Polyglucosan body myopathy 1 (PGBM1) is a type of glycogen storage disease where polyglucosan accumulation leads to cardiomyopathy and skeletal muscle myopathy. Variants of RBCK1 is related with PGBM1. We present a newly discovered pathogenic RBCK1 variant resulting in dilated cardiomyopathy (DCM) and a comprehensive literature review.

METHODS AND RESULTS

Whole-exome sequencing (WES) was utilized to detect genetic variations in a 7-year-old girl considered the proband. Sanger sequencing was performed to validate the variant in the patient and all the available family members, whether affected or unaffected. The variant's pathogenicity was assessed by conducting a cosegregation analysis within the family with in silico predictive software. WES showed that the proband's RBCK1 gene contained a missense likely pathogenic homozygous nucleotide variant, c.598_599insT: p.His200LeufsTer14 (NM_001323956.1), in exon 8. The computational analysis supported the variant's pathogenicity. The variant was identified in a heterozygous form among all the healthy members of the family. Variants with changes in N-terminal part of the protein were more likely to manifest immunodeficiency and auto-inflammation than those with C-terminal protein modifications according to prior variations of RBCK1 reported in the literature.

CONCLUSIONS

Our study offers novel findings indicating an RBCK1 variant in individuals of Iranian ancestry presenting with DCM leading to heart transplantation and myopathy without immunodeficiency or auto-inflammation.

HOIL-1L deficiency induces cell cycle alteration which causes immaturity of skeletal muscle and cardiomyocytes

HOIL-1L deficiency was recently reported to be one of the causes of myopathy and dilated cardiomyopathy (DCM). However, the mechanisms by which myopathy and DCM develop have not been clearly elucidated. Here, we sought to elucidate these mechanisms using the murine myoblast cell line C2C12 and disease-specific human induced pluripotent stem cells (hiPSCs). Myotubes differentiated from HOIL-1L-KO C2C12 cells exhibited deteriorated differentiation and mitotic cell accumulation. CMs differentiated from patient-derived hiPSCs had an abnormal morphology with a larger size and were excessively multinucleated compared with CMs differentiated from control hiPSCs. Further analysis of hiPSC-derived CMs showed that HOIL-1L deficiency caused cell cycle alteration and mitotic cell accumulation. These results demonstrate that abnormal cell maturation possibly contribute to the development of myopathy and DCM. In conclusion, HOIL-1L is an important intrinsic regulator of cell cycle-related myotube and CM maturation and cell proliferation.

A case of polyglucosan body myopathy caused by an RBCK1 gene variant and literature review

OBJECTIVE

To analyze the clinical and genetic characteristics of a patient with Polyglucosan body myopathy 1 (PGBM1) caused by a novel compound heterozygous variant in the RBCK1 gene.

METHODS

The clinical data of the patient were collected, next-generation sequencing technology was used to determine the exome sequence of the patient, and the suspected pathogenic locus was verified by Sanger sequencing.

RESULTS

Through whole-exome sequencing, we found that there were c.919G>T; p. (Glu307*) and c.723_730dup; p. (Glu244fs) variants of the RBCK1 gene in the patient, inherited from his parents, constituting a compound heterozygous variation. According to the guidelines of the American College of Medical Genetics and Genomics (ACMG), the two variants were rated as pathogenic, but there were no comparable cases. Previous literature reported 24 patients with RBCK1 gene variants, involving a total of 20 myocardial and 18 skeletal muscle cases.

CONCLUSIONS

The patient was twice diagnosed with cardiac insufficiency, neglecting the usual manifestations of muscle weakness, resulting in misdiagnosis. Later, novel variants in the RBCK1 gene were discovered through whole-exome sequencing, and symptomatic treatment was given after diagnosis. The importance of whole-exome sequencing technology in disease diagnosis and genetic counseling was emphasized.

Unraveling a history of overlap: A phenotypic comparison of RBCK1-related disease and glycogen storage disease type IV

RBCK1-related disease is a rare, multisystemic disorder for which our current understanding of the natural history is limited. A number of individuals initially carried clinical diagnoses of glycogen storage disease IV (GSD IV), but were later found to harbor RBCK1 pathogenic variants, demonstrating challenges of correctly diagnosing RBCK1-related disease. This study carried out a phenotypic comparison between RBCK1-related disease and GSD IV to identify features that clinically differentiate these diagnoses. Literature review and retrospective chart review identified 25 individuals with RBCK1-related disease and 36 with the neuromuscular subtype of GSD IV. Clinical features were evaluated to assess for statistically significant differences between the conditions. At a system level, any cardiac, autoinflammation, immunodeficiency, growth, or dermatologic involvement were suggestive of RBCK1, whereas any respiratory involvement suggested GSD IV. Several features warrant further exploration as predictors of RBCK1, such as generalized weakness, heart transplant, and recurrent infections, among others. Distinguishing RBCK1-related disease will facilitate correct diagnoses and pave the way for accurately identifying affected individuals, as well as for developing management recommendations, treatment, and an enhanced understanding of the natural history. This knowledge may also inform which individuals thought to have GSD IV should undergo reevaluation for RBCK1.

Expanding the phenotype of RBCK1-associated polyglucosan body myopathy type 1

Polyglucosan body myopathy-1 (PGBM1) is an extremely rare glycogen storage diseases that leads to muscle weakness and cardiomyopathy due to the accumulation of polyglucosan bodies. The clinical presentation appears to be partially dependent on the genetic mutation, but no clear genotype/phenotype correlation is currently possible. We describe a 7 year old patient, who initially presented with recurrent vomiting and respiratory infections until her first year of life. Diagnostic workup revealed an achalasia and the whole exome sequencing revealed an homozygous RBCK1 (RANBP2-type and C3HC4-type zinc finger containing 1) variant (c.896_899delAGTG) located in exon 7 (mid-domain), which has also been described in 4 patients with PGBM1. The unusual presentation with gastrointestinal and respiratory symptoms before the development of progressive muscle weakness expands the phenotype of this disease.

Myofiber-type-dependent 'boulder' or 'multitudinous pebble' formations across distinct amylopectinoses

At least five enzymes including three E3 ubiquitin ligases are dedicated to glycogen's spherical structure. Absence of any reverts glycogen to a structure resembling amylopectin of the plant kingdom. This amylopectinosis (polyglucosan body formation) causes fatal neurological diseases including adult polyglucosan body disease (APBD) due to glycogen branching enzyme deficiency, Lafora disease (LD) due to deficiencies of the laforin glycogen phosphatase or the malin E3 ubiquitin ligase and type 1 polyglucosan body myopathy (PGBM1) due to RBCK1 E3 ubiquitin ligase deficiency. Little is known about these enzymes' functions in glycogen structuring. Toward understanding these functions, we undertake a comparative murine study of the amylopectinoses of APBD, LD and PGBM1. We discover that in skeletal muscle, polyglucosan bodies form as two main types, small and multitudinous ('pebbles') or giant and single ('boulders'), and that this is primarily determined by the myofiber types in which they form, 'pebbles' in glycolytic and 'boulders' in oxidative fibers. This pattern recapitulates what is known in the brain in LD, innumerable dust-like in astrocytes and single giant sized in neurons. We also show that oxidative myofibers are relatively protected against amylopectinosis, in part through highly increased glycogen branching enzyme expression. We present evidence of polyglucosan body size-dependent cell necrosis. We show that sex influences amylopectinosis in genotype, brain region and myofiber-type-specific fashion. RBCK1 is a component of the linear ubiquitin chain assembly complex (LUBAC), the only known cellular machinery for head-to-tail linear ubiquitination critical to numerous cellular pathways. We show that the amylopectinosis of RBCK1 deficiency is not due to loss of linear ubiquitination, and that another function of RBCK1 or LUBAC must exist and operate in the shaping of glycogen. This work opens multiple new avenues toward understanding the structural determinants of the mammalian carbohydrate reservoir critical to neurologic and neuromuscular function and disease.

RBCK1 overexpression is associated with immune cell infiltration and poor prognosis in hepatocellular carcinoma

RBCK1 is an important E3 ubiquitin ligase, which plays an important role in many major diseases. However, the function and mechanism of RBCK1 in pan-cancer and its association with immune cell infiltration have not been reported. The purpose of this study is to find out the expression of RBCK1 in cancer, and to explore the relationship between RBCK1 and the prognosis of patients. Our results show that the expression of RBCK1 is up-regulated in a variety of malignant tumors, and is closely related to the prognosis of patients. Further studies have shown that RBCK1 regulates protein expression in the nucleus and plays an important role in ribosome and valine, leucine, and isoleucine degradation. Genetic variation analysis showed that RBCK1 was mainly involved in missense mutations in multiple tumors, and mutated patients showed poor prognoses. Further studies showed that RBCK1 may be interacted with proteins such as RNRPB, MCRS1, TRIB3, MKKS and ARPC3. Through protein interaction analysis, we found 43 proteins interacting with RBCK1 in liver cancer. We also analyzed immune cell infiltration and found that RBCK1 expression was positively correlated with T cells and macrophages, while it was negatively correlated with neutrophils, NK cells, and DCs in liver cancer. Finally, we confirmed experimentally that RBCK1 can significantly inhibit the apoptosis and invasion of HCC. Therefore, we speculate that RBCK1 plays an important regulatory role in the occurrence and development of HCC.

Mechanistic insights into the homo-dimerization of HOIL-1L and SHARPIN

HOIL-1L and SHARPIN are two essential regulatory subunits of the linear ubiquitin chain assembly complex (LUBAC), which is the only known E3 ligase complex generating linear ubiquitin chains. In addition to their LUBAC-dependent functions, HOIL-1L and SHARPIN alone play crucial roles in many LUBAC-independent cellular processes. Importantly, deficiency of HOIL-1L or SHARPIN leads to severe disorders in humans or mice. However, the mechanistic bases underlying the multi-functions of HOIL-1L and SHARPIN are still largely unknown. Here, we uncover that HOIL-1L and SHARPIN alone can form homo-dimers through their LTM motifs. We solve two crystal structures of the dimeric LTM motifs of HOIL-1L and SHARPIN, which not only elucidate the detailed molecular mechanism underpinning the dimer formations of HOIL-1L and SHARPIN, but also reveal a general mode shared by the LTM motifs of HOIL-1L and SHARPIN for forming homo-dimer or hetero-dimer. Furthermore, we elucidate that the polyglucosan body myopathy-associated HOIL-1L A18P mutation disturbs the structural folding of HOIL-1L LTM, and disrupts the dimer formation of HOIL-1L. In summary, our study provides mechanistic insights into the homo-dimerization of HOIL-1L and SHARPIN mediated by their LTM motifs, and expands our understandings of the multi-functions of HOIL-1L and SHARPIN as well as the etiology of relevant human disease caused by defective HOIL-1L.

A gene network database for the identification of key genes for diagnosis, prognosis, and treatment in sepsis

Sepsis and sepsis-related diseases cause a high rate of mortality worldwide. The molecular and cellular mechanisms of sepsis are still unclear. We aim to identify key genes in sepsis and reveal potential disease mechanisms. Six sepsis-related blood transcriptome datasets were collected and analyzed by weighted gene co-expression network analysis (WGCNA). Functional annotation was performed in the gProfiler tool. DSigDB was used for drug signature enrichment analysis. The proportion of immune cells was estimated by the CIBERSORT tool. The relationships between modules, immune cells, and survival were identified by correlation analysis and survival analysis. A total of 37 stable co-expressed gene modules were identified. These modules were associated with the critical biology process in sepsis. Four modules can independently separate patients with long and short survival. Three modules can recurrently separate sepsis and normal patients with high accuracy. Some modules can separate bacterial pneumonia, influenza pneumonia, mixed bacterial and influenza A pneumonia, and non-infective systemic inflammatory response syndrome (SIRS). Drug signature analysis identified drugs associated with sepsis, such as testosterone, phytoestrogens, ibuprofen, urea, dichlorvos, potassium persulfate, and vitamin B12. Finally, a gene co-expression network database was constructed ( https://liqs.shinyapps.io/sepsis/ ). The recurrent modules in sepsis may facilitate disease diagnosis, prognosis, and treatment.

Mechanisms underlying linear ubiquitination and implications in tumorigenesis and drug discovery

Linear ubiquitination is a distinct type of ubiquitination that involves attaching a head-to-tail polyubiquitin chain to a substrate protein. Early studies found that linear ubiquitin chains are essential for the TNFα- and IL-1-mediated NF-κB signaling pathways. However, recent studies have discovered at least sixteen linear ubiquitination substrates, which exhibit a broader activity than expected and mediate many other signaling pathways beyond NF-κB signaling. Dysregulation of linear ubiquitination in these pathways has been linked to many types of cancers, such as lymphoma, liver cancer, and breast cancer. Since the discovery of linear ubiquitin, extensive effort has been made to delineate the molecular mechanisms of how dysregulation of linear ubiquitination causes tumorigenesis and cancer development. In this review, we highlight newly discovered linear ubiquitination-mediated signaling pathways, recent advances in the role of linear ubiquitin in different types of cancers, and the development of linear ubiquitin inhibitors. Video Abstract.

How to Build a Fire: The Genetics of Autoinflammatory Diseases

Systemic autoinflammatory diseases (SAIDs) are a heterogeneous group of disorders caused by excess activation of the innate immune system in an antigen-independent manner. Starting with the discovery of the causal gene for familial Mediterranean fever, more than 50 monogenic SAIDs have been described. These discoveries, paired with advances in immunology and genomics, have allowed our understanding of these diseases to improve drastically in the last decade. The genetic causes of SAIDs are complex and include both germline and somatic pathogenic variants that affect various inflammatory signaling pathways. We provide an overview of the acquired SAIDs from a genetic perspective and summarize the clinical phenotypes and mechanism(s) of inflammation, aiming to provide a comprehensive understanding of the pathogenesis of autoinflammatory diseases.

Solution structure of the HOIL-1L NZF domain reveals a conformational switch regulating linear ubiquitin affinity

Attachment of polyubiquitin (poly-Ub) chains to proteins is a major posttranslational modification in eukaryotes. Linear ubiquitin chain assembly complex, consisting of HOIP (HOIL-1-interacting protein), HOIL-1L (heme-oxidized IRP2 Ub ligase 1), and SHARPIN (Shank-associated RH domain-interacting protein), specifically synthesizes "head-to-tail" poly-Ub chains, which are linked via the N-terminal methionine α-amino and C-terminal carboxylate of adjacent Ub units and are thus commonly called "linear" poly-Ub chains. Linear ubiquitin chain assembly complex-assembled linear poly-Ub chains play key roles in immune signaling and suppression of cell death and have been associated with immune diseases and cancer; HOIL-1L is one of the proteins known to selectively bind linear poly-Ub via its Npl4 zinc finger (NZF) domain. Although the structure of the bound form of the HOIL-1L NZF domain with linear di-Ub is known, several aspects of the recognition specificity remain unexplained. Here, we show using NMR and orthogonal biophysical methods, how the NZF domain evolves from a free to the specific linear di-Ub-bound state while rejecting other potential Ub species after weak initial binding. The solution structure of the free NZF domain revealed changes in conformational stability upon linear Ub binding, and interactions between the NZF core and tail revealed conserved electrostatic contacts, which were sensitive to charge modulation at a reported phosphorylation site: threonine-207. Phosphomimetic mutations reduced linear Ub affinity by weakening the integrity of the linear di-Ub-bound conformation. The described molecular determinants of linear di-Ub binding provide insight into the dynamic aspects of the Ub code and the NZF domain's role in full-length HOIL-1L.

NGS-Based Genetic Analysis in a Cohort of Italian Patients with Suspected Inherited Myopathies and/or HyperCKemia

Introduction/Aims HyperCKemia is considered a hallmark of neuromuscular diseases. It can be either isolated or associated with cramps, myalgia, weakness, myoglobinuria, or rhabdomyolysis, suggesting a metabolic myopathy. The aim of this work was to investigate possible genetic causes in order to help diagnose patients with recurrent hyperCKemia or clinical suspicion of inherited metabolic myopathy. Methods A cohort of 139 patients (90 adults and 49 children) was analyzed using a custom panel containing 54 genes associated with hyperCKemia. Results A definite genetic diagnosis was obtained in 15.1% of cases, while candidate variants or variants of uncertain significance were found in a further 39.5%. Similar percentages were obtained in patients with infantile or adult onset, with some different causative genes. RYR1 was the gene most frequently identified, either with single or compound heterozygous variants, while ETFDH variants were the most common cause for recessive cases. In one patient, mRNA analysis allowed identifying a large LPIN1 deletion missed by DNA sequencing, leading to a certain diagnosis. Conclusion These data confirm the high genetic heterogeneity of hyperCKemia and metabolic myopathies. The reduced diagnostic yield suggests the existence of additional genes associated with this condition but also allows speculation that a significant number of cases presenting with hyperCKemia or muscle symptoms are due to extrinsic, not genetic, factors.

Metabolic Cardiomyopathies and Cardiac Defects in Inherited Disorders of Carbohydrate Metabolism: A Systematic Review

Heart failure (HF) is a progressive chronic disease that remains a primary cause of death worldwide, affecting over 64 million patients. HF can be caused by cardiomyopathies and congenital cardiac defects with monogenic etiology. The number of genes and monogenic disorders linked to development of cardiac defects is constantly growing and includes inherited metabolic disorders (IMDs). Several IMDs affecting various metabolic pathways have been reported presenting cardiomyopathies and cardiac defects. Considering the pivotal role of sugar metabolism in cardiac tissue, including energy production, nucleic acid synthesis and glycosylation, it is not surprising that an increasing number of IMDs linked to carbohydrate metabolism are described with cardiac manifestations. In this systematic review, we offer a comprehensive overview of IMDs linked to carbohydrate metabolism presenting that present with cardiomyopathies, arrhythmogenic disorders and/or structural cardiac defects. We identified 58 IMDs presenting with cardiac complications: 3 defects of sugar/sugar-linked transporters (GLUT3, GLUT10, THTR1); 2 disorders of the pentose phosphate pathway (G6PDH, TALDO); 9 diseases of glycogen metabolism (GAA, GBE1, GDE, GYG1, GYS1, LAMP2, RBCK1, PRKAG2, G6PT1); 29 congenital disorders of glycosylation (ALG3, ALG6, ALG9, ALG12, ATP6V1A, ATP6V1E1, B3GALTL, B3GAT3, COG1, COG7, DOLK, DPM3, FKRP, FKTN, GMPPB, MPDU1, NPL, PGM1, PIGA, PIGL, PIGN, PIGO, PIGT, PIGV, PMM2, POMT1, POMT2, SRD5A3, XYLT2); 15 carbohydrate-linked lysosomal storage diseases (CTSA, GBA1, GLA, GLB1, HEXB, IDUA, IDS, SGSH, NAGLU, HGSNAT, GNS, GALNS, ARSB, GUSB, ARSK). With this systematic review we aim to raise awareness about the cardiac presentations in carbohydrate-linked IMDs and draw attention to carbohydrate-linked pathogenic mechanisms that may underlie cardiac complications.

Role of Astrocytes in the Pathophysiology of Lafora Disease and Other Glycogen Storage Disorders

Lafora disease is a rare disorder caused by loss of function mutations in either the EPM2A or NHLRC1 gene. The initial symptoms of this condition are most commonly epileptic seizures, but the disease progresses rapidly with dementia, neuropsychiatric symptoms, and cognitive deterioration and has a fatal outcome within 5-10 years after onset. The hallmark of the disease is the accumulation of poorly branched glycogen in the form of aggregates known as Lafora bodies in the brain and other tissues. Several reports have demonstrated that the accumulation of this abnormal glycogen underlies all the pathologic traits of the disease. For decades, Lafora bodies were thought to accumulate exclusively in neurons. However, it was recently identified that most of these glycogen aggregates are present in astrocytes. Importantly, astrocytic Lafora bodies have been shown to contribute to pathology in Lafora disease. These results identify a primary role of astrocytes in the pathophysiology of Lafora disease and have important implications for other conditions in which glycogen abnormally accumulates in astrocytes, such as Adult Polyglucosan Body disease and the buildup of Corpora amylacea in aged brains.

Genetic Insights from Consanguineous Cardiomyopathy Families

Inherited cardiomyopathies are a prevalent cause of heart failure and sudden cardiac death. Both hypertrophic (HCM) and dilated cardiomyopathy (DCM) are genetically heterogeneous and typically present with an autosomal dominant mode of transmission. Whole exome sequencing and autozygosity mapping was carried out in eight un-related probands from consanguineous Middle Eastern families presenting with HCM/DCM followed by bioinformatic and co-segregation analysis to predict the potential pathogenicity of candidate variants. We identified homozygous missense variants in TNNI3K, DSP, and RBCK1 linked with a dilated phenotype, in NRAP linked with a mixed phenotype of dilated/hypertrophic, and in KLHL24 linked with a mixed phenotype of dilated/hypertrophic and non-compaction features. Co-segregation analysis in family members confirmed autosomal recessive inheritance presenting in early childhood/early adulthood. Our findings add to the mutational spectrum of recessive cardiomyopathies, supporting inclusion of KLHL24, NRAP and RBCK1 as disease-causing genes. We also provide evidence for novel (recessive) modes of inheritance of a well-established gene TNNI3K and expand our knowledge of the clinical heterogeneity of cardiomyopathies. A greater understanding of the genetic causes of recessive cardiomyopathies has major implications for diagnosis and screening, particularly in underrepresented populations, such as those of the Middle East.

Laforin targets malin to glycogen in Lafora progressive myoclonus epilepsy

Glycogen is the largest cytosolic macromolecule and is kept in solution through a regular system of short branches allowing hydration. This structure was thought to solely require balanced glycogen synthase and branching enzyme activities. Deposition of overlong branched glycogen in the fatal epilepsy Lafora disease (LD) indicated involvement of the LD gene products laforin and the E3 ubiquitin ligase malin in regulating glycogen structure. Laforin binds glycogen, and LD-causing mutations disrupt this binding, laforin-malin interactions and malin's ligase activity, all indicating a critical role for malin. Neither malin's endogenous function nor location had previously been studied due to lack of suitable antibodies. Here, we generated a mouse in which the native malin gene is tagged with the FLAG sequence. We show that the tagged gene expresses physiologically, malin localizes to glycogen, laforin and malin indeed interact, at glycogen, and malin's presence at glycogen depends on laforin. These results, and mice, open the way to understanding unknown mechanisms of glycogen synthesis critical to LD and potentially other much more common diseases due to incompletely understood defects in glycogen metabolism.

Non-lysine ubiquitylation: Doing things differently

The post-translational modification of proteins with ubiquitin plays a central role in nearly all aspects of eukaryotic biology. Historically, studies have focused on the conjugation of ubiquitin to lysine residues in substrates, but it is now clear that ubiquitylation can also occur on cysteine, serine, and threonine residues, as well as on the N-terminal amino group of proteins. Paradigm-shifting reports of non-proteinaceous substrates have further extended the reach of ubiquitylation beyond the proteome to include intracellular lipids and sugars. Additionally, results from bacteria have revealed novel ways to ubiquitylate (and deubiquitylate) substrates without the need for any of the enzymatic components of the canonical ubiquitylation cascade. Focusing mainly upon recent findings, this review aims to outline the current understanding of non-lysine ubiquitylation and speculate upon the molecular mechanisms and physiological importance of this non-canonical modification.

A new dawn beyond lysine ubiquitination

The ubiquitin system has become synonymous with the modification of lysine residues. However, the substrate scope and diversity of the conjugation machinery have been underappreciated, bringing us to an epoch in ubiquitin system research. The striking discoveries of metazoan enzymes dedicated toward serine and threonine ubiquitination have revealed the important role of nonlysine ubiquitination in endoplasmic reticulum-associated degradation, immune signaling and neuronal processes, while reports of nonproteinaceous substrates have extended ubiquitination beyond the proteome. Bacterial effectors that bypass the canonical ubiquitination machinery and form unprecedented linkage chemistry further redefine long-standing dogma. While chemical biology approaches have advanced our understanding of the canonical ubiquitin system, further study of noncanonical ubiquitination has been hampered by a lack of suitable tools. This Perspective aims to consolidate and contextualize recent discoveries and to propose potential applications of chemical biology, which will be instrumental in unraveling this new frontier of ubiquitin research.

HOIL-1 ubiquitin ligase activity targets unbranched glucosaccharides and is required to prevent polyglucosan accumulation

HOIL-1, a component of the linear ubiquitin chain assembly complex (LUBAC), ubiquitylates serine and threonine residues in proteins by esterification. Here, we report that mice expressing an E3 ligase-inactive HOIL-1[C458S] mutant accumulate polyglucosan in brain, heart and other organs, indicating that HOIL-1's E3 ligase activity is essential to prevent these toxic polysaccharide deposits from accumulating. We found that HOIL-1 monoubiquitylates glycogen and α1:4-linked maltoheptaose in vitro and identify the C6 hydroxyl moiety of glucose as the site of ester-linked ubiquitylation. The monoubiquitylation of maltoheptaose was accelerated > 100-fold by the interaction of Met1-linked or Lys63-linked ubiquitin oligomers with the RBR domain of HOIL-1. HOIL-1 also transferred pre-formed ubiquitin oligomers to maltoheptaose en bloc, producing polyubiquitylated maltoheptaose in one catalytic step. The Sharpin and HOIP components of LUBAC, but not HOIL-1, bound to unbranched and infrequently branched glucose polymers in vitro, but not to highly branched mammalian glycogen, suggesting a potential function in targeting HOIL-1 to unbranched glucosaccharides in cells. We suggest that monoubiquitylation of unbranched glucosaccharides may initiate their removal from cells, preventing precipitation as polyglucosan.

AAV-Mediated Artificial miRNA Reduces Pathogenic Polyglucosan Bodies and Neuroinflammation in Adult Polyglucosan Body and Lafora Disease Mouse Models

Adult polyglucosan body disease (APBD) and Lafora disease (LD) are autosomal recessive glycogen storage neurological disorders. APBD is caused by mutations in the glycogen branching enzyme (GBE1) gene and is characterized by progressive upper and lower motor neuron dysfunction and premature death. LD is a fatal progressive myoclonus epilepsy caused by loss of function mutations in the EPM2A or EPM2B gene. These clinically distinct neurogenetic diseases share a common pathology. This consists of time-dependent formation, precipitation, and accumulation of an abnormal form of glycogen (polyglucosan) into gradually enlarging inclusions, polyglucosan bodies (PBs) in ever-increasing numbers of neurons and astrocytes. The growth and spread of PBs are followed by astrogliosis, microgliosis, and neurodegeneration. The key defect in polyglucosans is that their glucan branches are longer than those of normal glycogen, which prevents them from remaining in solution. Since the lengths of glycogen branches are determined by the enzyme glycogen synthase, we hypothesized that downregulating this enzyme could prevent or hinder the generation of the pathogenic PBs. Here, we pursued an adeno-associated virus vector (AAV) mediated RNA-interference (RNAi) strategy. This approach resulted in approximately 15% reduction of glycogen synthase mRNA and an approximately 40% reduction of PBs across the brain in the APBD and both LD mouse models. This was accompanied by improvements in early neuroinflammatory markers of disease. This work represents proof of principle toward developing a single lifetime dose therapy for two fatal neurological diseases: APBD and LD. The approach is likely applicable to other severe and common diseases of glycogen storage.

Linear ubiquitination in immune and neurodegenerative diseases, and beyond

Ubiquitin regulates numerous aspects of biology via a complex ubiquitin code. The linear ubiquitin chain is an atypical code that forms a unique structure, with the C-terminal tail of the distal ubiquitin linked to the N-terminal Met1 of the proximal ubiquitin. Thus far, LUBAC is the only known ubiquitin ligase complex that specifically generates linear ubiquitin chains. LUBAC-induced linear ubiquitin chains regulate inflammatory responses, cell death and immunity. Genetically modified mouse models and cellular assays have revealed that LUBAC is also involved in embryonic development in mice. LUBAC dysfunction is associated with autoimmune diseases, myopathy, and neurodegenerative diseases in humans, but the underlying mechanisms are poorly understood. In this review, we focus on the roles of linear ubiquitin chains and LUBAC in immune and neurodegenerative diseases. We further discuss LUBAC inhibitors and their potential as therapeutics for these diseases.

Structures, functions, and inhibitors of LUBAC and its related diseases

Ubiquitination is a reversible posttranslational modification in which ubiquitin is covalently attached to substrates at catalysis by E1, E2, and E3 enzymes. As the only E3 ligase for assembling linear ubiquitin chains in animals, the LUBAC complex exerts an essential role in the wide variety of cellular activities. Recent advances in the LUBAC complex, including structure, physiology, and correlation with malignant diseases, have enabled the discovery of potent inhibitors to treat immune-related diseases and cancer brought by LUBAC complex dysfunction. In this review, we summarize the current progress on the structures, physiologic functions, inhibitors of LUBAC, and its potential role in immune diseases, tumors, and other diseases, providing the theoretical basis for therapy of related diseases targeting the LUBAC complex.

Compound heterozygous variants in OTULIN are associated with fulminant atypical late-onset ORAS

Autoinflammatory diseases are a heterogenous group of disorders defined by fever and systemic inflammation suggesting involvement of genes regulating innate immune responses. Patients with homozygous loss-of-function variants in the OTU-deubiquitinase OTULIN suffer from neonatal-onset OTULIN-related autoinflammatory syndrome (ORAS) characterized by fever, panniculitis, diarrhea, and arthritis. Here, we describe an atypical form of ORAS with distinct clinical manifestation of the disease caused by two new compound heterozygous variants (c.258G>A (p.M86I)/c.500G>C (p.W167S)) in the OTULIN gene in a 7-year-old affected by a life-threatening autoinflammatory episode with sterile abscess formation. On the molecular level, we find binding of OTULIN to linear ubiquitin to be compromised by both variants; however, protein stability and catalytic activity is most affected by OTULIN variant p.W167S. These molecular changes together lead to increased levels of linear ubiquitin linkages in patient-derived cells triggering the disease. Our data indicate that the spectrum of ORAS patients is more diverse than previously thought and, thus, supposedly asymptomatic individuals might also be affected. Based on our results, we propose to subdivide the ORAS into classical and atypical entities.

Scaling skin and failing heart: the cardio-cutaneous connection

Ichthyosis refers to a comparatively rare group of skin disorders which may present with associated cardiomyopathy. We report a case of an 11-year-old female child who presented with ichthyosis and associated dilated cardiomyopathy. Genetic testing revealed mutation in the RBCK1 gene. She was successfully managed with heart transplantation. The purpose of the case report is to embark on the association between the skin and heart, the role of desmosomes, and the cutaneous manifestations of life-threatening cardiac disease. Cutaneous manifestations should not be escaped, as some of which could be a marker for sudden cardiac death and appropriate corrective actions can potentially save life.

A novel variant of RBCK1 gene causes mild polyglucosan myopathy

Homozygous or compound heterozygous pathogenic variants of the RBCK1 gene can result in a systemic disorder characterized by the accumulation of complex carbohydrate molecules, namely polyglucosan bodies in the muscular tissues. The role of this gene in the pathophysiology of the disorder at the molecular level remains unclear. Being a very rare disorder, the medical knowledge is based on just a few reported cases. Here we report a 7-year-old girl who presented with exercise intolerance and hepatosplenomegaly. Her liver profile was constantly raised. The genetic investigation has revealed a variant of the RBCK1 gene of unknown significance, which has later been confirmed as pathogenic via a variety of clinical, genetic, and histopathological approaches. More importantly, it is evident that the availability of sophisticated genetic testing, such as whole-exome sequencing, has significantly improved the knowledge of and diagnosis of many rare metabolic disorders.

Lack of p62 Impairs Glycogen Aggregation and Exacerbates Pathology in a Mouse Model of Myoclonic Epilepsy of Lafora

Lafora disease (LD) is a fatal childhood-onset dementia characterized by the extensive accumulation of glycogen aggregates—the so-called Lafora Bodies (LBs)—in several organs. The accumulation of LBs in the brain underlies the neurological phenotype of the disease. LBs are composed of abnormal glycogen and various associated proteins, including p62, an autophagy adaptor that participates in the aggregation and clearance of misfolded proteins. To study the role of p62 in the formation of LBs and its participation in the pathology of LD, we generated a mouse model of the disease (malin^KO) lacking p62. Deletion of p62 prevented LB accumulation in skeletal muscle and cardiac tissue. In the brain, the absence of p62 altered LB morphology and increased susceptibility to epilepsy. These results demonstrate that p62 participates in the formation of LBs and suggest that the sequestration of abnormal glycogen into LBs is a protective mechanism through which it reduces the deleterious consequences of its accumulation in the brain.

LUBAC: a new player in polyglucosan body disease

Altered protein ubiquitination is associated with the pathobiology of numerous diseases; however, its involvement in glycogen metabolism and associated polyglucosan body (PB) disease has not been investigated in depth. In PB disease, excessively long and less branched glycogen chains (polyglucosan bodies, PBs) are formed, which precipitate in different tissues causing myopathy, cardiomyopathy and/or neurodegeneration. Linear ubiquitin chain assembly complex (LUBAC) is a multi-protein complex composed of two E3 ubiquitin ligases HOIL-1L and HOIP and an adaptor protein SHARPIN. Together they are responsible for M1-linked ubiquitination of substrates primarily related to immune signaling and cell death pathways. Consequently, severe immunodeficiency is a hallmark of many LUBAC deficient patients. Remarkably, all HOIL-1L deficient patients exhibit accumulation of PBs in different organs especially skeletal and cardiac muscle resulting in myopathy and cardiomyopathy with heart failure. This emphasizes LUBAC's important role in glycogen metabolism. To date, neither a glycogen metabolism-related LUBAC substrate nor the molecular mechanism are known. Hence, current reviews on LUBAC's involvement in glycogen metabolism are lacking. Here, we aim to fill this gap by describing LUBAC's involvement in PB disease. We present a comprehensive review of LUBAC structure, its role in M1-linked and other types of atypical ubiquitination, PB pathology in human patients and findings in new mouse models to study the disease. We conclude the review with recent drug developments and near-future gene-based therapeutic approaches to treat LUBAC related PB disease.

Biochemistry, Pathophysiology, and Regulation of Linear Ubiquitination: Intricate Regulation by Coordinated Functions of the Associated Ligase and Deubiquitinase

The ubiquitin system modulates protein functions by decorating target proteins with ubiquitin chains in most cases. Several types of ubiquitin chains exist, and chain type determines the mode of regulation of conjugated proteins. LUBAC is a ubiquitin ligase complex that specifically generates N-terminally Met1-linked linear ubiquitin chains. Although linear ubiquitin chains are much less abundant than other types of ubiquitin chains, they play pivotal roles in cell survival, proliferation, the immune response, and elimination of bacteria by selective autophagy. Because linear ubiquitin chains regulate inflammatory responses by controlling the proinflammatory transcription factor NF-κB and programmed cell death (including apoptosis and necroptosis), abnormal generation of linear chains can result in pathogenesis. LUBAC consists of HOIP, HOIL-1L, and SHARPIN; HOIP is the catalytic center for linear ubiquitination. LUBAC is unique in that it contains two different ubiquitin ligases, HOIP and HOIL-1L, in the same ligase complex. Furthermore, LUBAC constitutively interacts with the deubiquitinating enzymes (DUBs) OTULIN and CYLD, which cleave linear ubiquitin chains generated by LUBAC. In this review, we summarize the current status of linear ubiquitination research, and we discuss the intricate regulation of LUBAC-mediated linear ubiquitination by coordinate function of the HOIP and HOIL-1L ligases and OTULIN. Furthermore, we discuss therapeutic approaches to targeting LUBAC-mediated linear ubiquitin chains.

Proteomic characterisation of polyglucosan bodies in skeletal muscle in RBCK1 deficiency

AIMS

Several neurodegenerative and neuromuscular disorders are characterised by storage of polyglucosan, consisting of proteins and amylopectin-like polysaccharides, which are less branched than in normal glycogen. Such diseases include Lafora disease, branching enzyme deficiency, glycogenin-1 deficiency, polyglucosan body myopathy type 1 (PGBM1) due to RBCK1 deficiency and others. The protein composition of polyglucosan bodies is largely unknown.

METHODS

We combined quantitative mass spectrometry, immunohistochemical and western blot analyses to identify the principal protein components of polyglucosan bodies in PGBM1. Histologically stained tissue sections of skeletal muscle from four patients were used to isolate polyglucosan deposits and control regions by laser microdissection. Prior to mass spectrometry, samples were labelled with tandem mass tags that enable quantitative comparison and multiplexed analysis of dissected samples. To study the distribution and expression of the accumulated proteins, immunohistochemical and western blot analyses were performed.

RESULTS

Accumulated proteins were mainly components of glycogen metabolism and protein quality control pathways. The majority of fibres showed depletion of glycogen and redistribution of key enzymes of glycogen metabolism to the polyglucosan bodies. The polyglucosan bodies also showed accumulation of proteins involved in the ubiquitin-proteasome and autophagocytosis systems and protein chaperones.

CONCLUSIONS

The sequestration of key enzymes of glycogen metabolism to the polyglucosan bodies may explain the glycogen depletion in the fibres and muscle function impairment. The accumulation of components of the protein quality control systems and other proteins frequently found in protein aggregate disorders indicates that protein aggregation may be an essential part of the pathobiology of polyglucosan storage.

Expression, solubility monitoring, and purification of the co-folded LUBAC LTM domain by structure-guided tandem folding in autoinducing cultures

The linear ubiquitin chain assembly complex tethering motif (LUBAC-LTM) domain is composed of two different accessory LUBAC components (HOIL-1L and SHARPIN) but folds as a single globular domain. Targeted disruption of the intricate LTM-LTM interaction destabilizes LUBAC in lymphoma cells, thereby attenuating LUBAC stability, which highlights that targeting the interaction between the two LTM motifs is a promising strategy for the development of new agents against cancers that depend on LUBAC activity for their survival. To further screen for small-molecule inhibitors that can selectively disrupt the LTM-LTM interaction, it is necessary to obtain high-purity samples of the LTM domain. Ideally, such a sample would not contain any components other than the LTM itself, so that false positives (molecules binding to other parts of LUBAC) could be eliminated from the screening process. Here we report a simple strategy that enabled successful bacterial production of the isolated LUBAC LTM domain in high yield and at high purity. The strategy combines (1) structural analysis highlighting the possibility of tandem expression in the SHARPIN^L™ to HOIL-1L^L™ direction; (2) bacterial expression downstream of EGFP to efficiently monitor expression and solubility; (3) gentle low-temperature folding using autoinduction. Formation of stably folded LTM was verified by size-exclusion chromatography and heteronuclear NMR spectroscopy. From 200-ml cultures sufficient quantities (∼7 mg) of high-purity protein for structural studies could be obtained. The presented strategy will be beneficial for LUBAC LTM-based drug-screening efforts and likely serve as a useful primer for similar cases, i.e., whenever a smaller folded fragment is to be isolated from a larger protein complex for site-specific downstream applications.

Lafora disease: Current biology and therapeutic approaches

The ubiquitin system impacts most cellular processes and is altered in numerous neurodegenerative diseases. However, little is known about its role in neurodegenerative diseases due to disturbances of glycogen metabolism such as Lafora disease (LD). In LD, insufficiently branched and long-chained glycogen forms and precipitates into insoluble polyglucosan bodies (Lafora bodies), which drive neuroinflammation, neurodegeneration and epilepsy. LD is caused by mutations in the gene encoding the glycogen phosphatase laforin or the gene coding for the laforin interacting partner ubiquitin E3 ligase malin. The role of the malin-laforin complex in regulating glycogen structure remains with full of gaps. In this review we bring together the disparate body of data on these two proteins and propose a mechanistic hypothesis of the disease in which malin-laforin's role to monitor and prevent over-elongation of glycogen branch chains, which drive glycogen molecules to precipitate and accumulate into Lafora bodies. We also review proposed connections between Lafora bodies and the ensuing neuroinflammation, neurodegeneration and intractable epilepsy. Finally, we review the exciting activities in developing therapies for Lafora disease based on replacing the missing genes, slowing the enzyme - glycogen synthase - that over-elongates glycogen branches, and introducing enzymes that can digest Lafora bodies. Much more work is needed to fill the gaps in glycogen metabolism in which laforin and malin operate. However, knowledge appears already adequate to advance disease course altering therapies for this catastrophic fatal disease.

The Nuclear Factor Kappa B (NF-kB) signaling in cancer development and immune diseases

The nuclear factor kappa B (NF-kB) family of transcription factors plays an essential role as stressors in the cellular environment, and controls the expression of important regulatory genes such as immunity, inflammation, death, and cell proliferation. NF-kB protein is located in the cytoplasm, and can be activated by various cellular stimuli. There are two pathways for NF-kB activation, as the canonical and non-canonical pathways, which require complex molecular interactions with adapter proteins and phosphorylation and ubiquitinase enzymes. Accordingly, this increases NF-kB translocation in the nucleus and regulates gene expression. In this study, the concepts that emerge in different cellular systems allow the design of NF-kB function in humans. This would not only allow the development for rare diseases associated with NF-kB, but would also be used as a source of useful information to eliminate widespread consequences such as cancer or inflammatory/immune diseases.

GBE1-related disorders: Adult polyglucosan body disease and its neuromuscular phenotypes

Adult polyglucosan body disease (APBD) represents a complex autosomal recessive inherited neurometabolic disorder due to homozygous or compound heterozygous pathogenic variants in GBE1 gene, resulting in deficiency of glycogen-branching enzyme and secondary storage of glycogen in the form of polyglucosan bodies, involving the skeletal muscle, diaphragm, peripheral nerve (including autonomic fibers), brain white matter, spinal cord, nerve roots, cerebellum, brainstem and to a lesser extent heart, lung, kidney, and liver cells. The diversity of new clinical presentations regarding neuromuscular involvement is astonishing and transformed APBD in a key differential diagnosis of completely different clinical conditions, including axonal and demyelinating sensorimotor polyneuropathy, progressive spastic paraparesis, motor neuronopathy presentations, autonomic disturbances, leukodystrophies or even pure myopathic involvement with limb-girdle pattern of weakness. This review article aims to summarize the main clinical, biochemical, genetic, and diagnostic aspects regarding APBD with special focus on neuromuscular presentations.

Met1-linked ubiquitin signalling in health and disease: inflammation, immunity, cancer, and beyond

Post-translational modification of proteins with ubiquitin (ubiquitination) provides a rapid and versatile mechanism for regulating cellular signalling systems. Met1-linked (or 'linear') ubiquitin chains have emerged as a key regulatory signal that controls cell death, immune signalling, and other vital cellular functions. The molecular machinery that assembles, senses, and disassembles Met1-linked ubiquitin chains is highly specific. In recent years, the thorough biochemical and genetic characterisation of the enzymes and proteins of the Met1-linked ubiquitin signalling machinery has paved the way for substantial advances in our understanding of how Met1-linked ubiquitin chains control cell signalling and biology. Here, we review current knowledge and recent insights into the role of Met1-linked ubiquitin chains in cell signalling with an emphasis on their role in disease biology. Met1-linked ubiquitin has potent regulatory functions in immune signalling, NF-κB transcription factor activation, and cell death. Importantly, mounting evidence shows that dysregulation of Met1-linked ubiquitin signalling is associated with multiple human diseases, including immune disorders, cancer, and neurodegeneration. We discuss the latest evidence on the cellular function of Met1-linked ubiquitin in the context of its associated diseases and highlight new emerging roles of Met1-linked ubiquitin chains in cell signalling, including regulation of protein quality control and metabolism.

Polyglucosan body myopathy 1 may cause cognitive impairment: a case report from China

Background

Polyglucosan body myopathy 1 (PGBM1) is a type of glycogen storage disease that can cause skeletal muscle myopathy and cardiomyopathy with or without immunodeficiency due to a pathogenic mutation in the RBCK1 gene. PGBM1 has been reported in only 14 European and American families, and no cognitive impairment phenotype was reported. Its prevalence in Asia is unknown.

Case presentation:

We report a Chinese boy with teenage onset of skeletal muscle myopathy and mild cognitive impairment. Whole-exome sequencing analysis identified a homozygous missense mutation in RBCK1 (c.1411G > A:p.Glu471Lys). A muscle biopsy indicated the accumulation of periodic acid-Schiff-positive material, which could be ubiquitinated by immunohistochemistry with an anti-ubiquitin antibody. In skeletal muscle tissue, HOIL-1 and HOIP protein levels were lower than those in the control, confirming the phenotype of an RBCK1 mutation. MRI revealed abnormal cerebral white matter signals. Immune system and cardiac examination found no abnormalities. The patient was diagnosed with PGBM1 with no effective treatment.

Conclusions

This case from China with a novel homozygous missense mutation in RBCK1 extends the phenotypic spectrum and geographical distribution of PGBM 1, which may cause cerebral white matter changes and cognitive impairment.

LUBAC-mediated linear ubiquitination: a crucial regulator of immune signaling

Ubiquitination is a reversible post-translational modification in which ubiquitin chains are conjugated to target proteins to modulate protein function. The type of ubiquitin chain determines the mode of protein regulation. It has been shown that ubiquitin chains are formed via one of seven Lys residues in ubiquitin, and several types of ubiquitin chains are found in cells. We identified a new type of linear ubiquitin chain linked through the N-terminal Met of ubiquitin and assembled by the linear ubiquitin chain assembly complex (LUBAC), which is specific for linear chains. The discovery of linear ubiquitin chains and LUBAC is considered as a paradigm shift in ubiquitin research because linear ubiquitination is exclusive to animals, despite the existence of ubiquitination throughout eukaryotic kingdoms. Linear ubiquitination plays a critical role in immune signaling and cell death regulation. Dysregulation of LUBAC-mediated linear ubiquitination underlies various human diseases, including autoinflammation, autoimmunity, infection, and malignant tumors. This review summarizes the current status of linear ubiquitination research.

RNA-sequencing reveals transcriptional signature of pathological remodeling in the diaphragm of rats after myocardial infarction

The diaphragm is the main inspiratory muscle, and the chronic phase post-myocardial infarction (MI) is characterized by diaphragm morphological, contractile, and metabolic abnormalities. However, the mechanisms of diaphragm weakness are not fully understood. In the current study, we aimed to identify the transcriptome changes associated with diaphragm abnormalities in the chronic stage MI. We ligated the left coronary artery to cause MI in rats and performed RNA-sequencing (RNA-Seq) in diaphragm samples 16 weeks post-surgery. The sham group underwent thoracotomy and pericardiotomy but no artery ligation. We identified 112 differentially expressed genes (DEGs) out of a total of 9664 genes. Myocardial infarction upregulated and downregulated 42 and 70 genes, respectively. Analysis of DEGs in the framework of skeletal muscle-specific biological networks suggest remodeling in the neuromuscular junction, extracellular matrix, sarcomere, cytoskeleton, and changes in metabolism and iron homeostasis. Overall, the data are consistent with pathological remodeling of the diaphragm and reveal potential biological targets to prevent diaphragm weakness in the chronic stage MI.

RBCK1-related disease: A rare multisystem disorder with polyglucosan storage, auto-inflammation, recurrent infections, skeletal, and cardiac myopathy-Four additional patients and a review of the current literature

In this article, we report four new patients, from three kindreds, with pathogenic variants in RBCK1 and a multisystem disorder characterised by widespread polyglucosan storage. We describe the clinical presentation of progressive skeletal and cardiac myopathy, combined immunodeficiencies and auto-inflammation, illustrate in detail the histopathological findings in multiple tissue types, and report muscle MRI findings.

Keratinocyte-specific deletion of SHARPIN induces atopic dermatitis-like inflammation in mice

Spontaneous mutations in the SHANK-associated RH domain interacting protein (Sharpin) resulted in a severe autoinflammatory type of chronic proliferative dermatitis, inflammation in other organs, and lymphoid organ defects. To determine whether cell-type restricted loss of Sharpin causes similar lesions, a conditional null mutant was created. Ubiquitously expressing cre-recombinase recapitulated the phenotype seen in spontaneous mutant mice. Limiting expression to keratinocytes (using a Krt14-cre) induced a chronic eosinophilic dermatitis, but no inflammation in other organs or lymphoid organ defects. The dermatitis was associated with a markedly increased concentration of serum IgE and IL18. Crosses with S100a4-cre resulted in milder skin lesions and moderate to severe arthritis. This conditional null mutant will enable more detailed studies on the role of SHARPIN in regulating NFkB and inflammation, while the Krt14-Sharpin-/- provides a new model to study atopic dermatitis.

Mendelian diseases of dysregulated canonical NF-κB signaling: From immunodeficiency to inflammation

NF-κB is a master transcription factor that activates the expression of target genes in response to various stimulatory signals. Activated NF-κB mediates a plethora of diverse functions including innate and adaptive immune responses, inflammation, cell proliferation, and NF-κB is regulated through interactions with IκB inhibitory proteins, which are in turn regulated by the inhibitor of κB kinase (IKK) complex. Together, these 3 components form the core of the NF-κB signalosomes that have cell-specific functions which are dependent on the interactions with other signaling molecules and pathways. The activity of NF-κB pathway is also regulated by a variety of post-translational modifications including phosphorylation and ubiquitination by Lys63, Met1, and Lys48 ubiquitin chains. The physiologic role of NF-κB is best studied in the immune system due to discovery of many human diseases caused by pathogenic variants in various proteins that constitute the NF-κB pathway. These disease-causing variants can act either as gain-of-function (GoF) or loss-of-function (LoF) and depending on the function of mutated protein, can cause either immunodeficiency or systemic inflammation. Typically, pathogenic missense variants act as GoF and they lead to increased activity in the pathway. LoF variants can be inherited as recessive or dominant alleles and can cause either a decrease or an increase in pathway activity. Dominantly inherited LoF variants often result in haploinsufficiency of inhibitory proteins. Here, we review human Mendelian immunologic diseases, which results from mutations in different molecules in the canonical NF-κB pathway and surprisingly present with a continuum of clinical features including immunodeficiency, atopy, autoimmunity, and autoinflammation.

The PANoptosome: A Deadly Protein Complex Driving Pyroptosis, Apoptosis, and Necroptosis (PANoptosis)

Programmed cell death is regulated by evolutionarily conserved pathways that play critical roles in development and the immune response. A newly recognized pathway for proinflammatory programmed cell death called PANoptosis is controlled by a recently identified cytoplasmic multimeric protein complex named the PANoptosome. The PANoptosome can engage, in parallel, three key modes of programmed cell death—pyroptosis, apoptosis, and necroptosis. The PANoptosome components have been implicated in a wide array of human diseases including autoinflammatory diseases, neurodegenerative diseases, cancer, microbial infections, and metabolic diseases. Here, we review putative components of the PANoptosome and present a phylogenetic analysis of their molecular domains and interaction motifs that support complex assembly. We also discuss genetic data that suggest PANoptosis is coordinated by scaffolding and catalytic functions of the complex components and propose mechanistic models for PANoptosome assembly. Overall, this review presents potential mechanisms governing PANoptosis based on evolutionary analysis of the PANoptosome components.

The Balance of TNF Mediated Pathways Regulates Inflammatory Cell Death Signaling in Healthy and Diseased Tissues

Tumor necrosis factor alpha (TNF; TNFα) is a critical regulator of immune responses in healthy organisms and in disease. TNF is involved in the development and proper functioning of the immune system by mediating cell survival and cell death inducing signaling. TNF stimulated signaling pathways are tightly regulated by a series of phosphorylation and ubiquitination events, which enable timely association of TNF receptors-associated intracellular signaling complexes. Disruption of these signaling events can disturb the balance and the composition of signaling complexes, potentially resulting in severe inflammatory diseases.

The HOIL-1L ligase modulates immune signalling and cell death via monoubiquitination of LUBAC

The linear ubiquitin chain assembly complex (LUBAC), which consists of HOIP, SHARPIN and HOIL-1L, promotes NF-κB activation and protects against cell death by generating linear ubiquitin chains. LUBAC contains two RING-IBR-RING (RBR) ubiquitin ligases (E3), and the HOIP RBR is responsible for catalysing linear ubiquitination. We found that HOIL-1L RBR plays a crucial role in regulating LUBAC. HOIL-1L RBR conjugates monoubiquitin onto all LUBAC subunits, followed by HOIP-mediated conjugation of linear chains onto monoubiquitin, and these linear chains attenuate the functions of LUBAC. The introduction of E3-defective HOIL-1L mutants into cells augmented linear ubiquitination, which protected the cells against Salmonella infection and cured dermatitis caused by reduced LUBAC levels due to SHARPIN loss. Our results reveal a regulatory mode of E3 ligases in which the accessory E3 in LUBAC downregulates the main E3 by providing preferred substrates for autolinear ubiquitination. Thus, inhibition of HOIL-1L E3 represents a promising strategy for treating severe infections or immunodeficiency.

Linear Ubiquitin Code: Its Writer, Erasers, Decoders, Inhibitors, and Implications in Disorders

The linear ubiquitin chain assembly complex (LUBAC) is a ubiquitin ligase composed of the Heme-oxidized IRP2 ubiquitin ligase-1L (HOIL-1L), HOIL-1L-interacting protein (HOIP), and Shank-associated RH domain interactor (SHARPIN) subunits. LUBAC specifically generates the N-terminal Met1-linked linear ubiquitin chain and regulates acquired and innate immune responses, such as the canonical nuclear factor-κB (NF-κB) and interferon antiviral pathways. Deubiquitinating enzymes, OTULIN and CYLD, physiologically bind to HOIP and control its function by hydrolyzing the linear ubiquitin chain. Moreover, proteins containing linear ubiquitin-specific binding domains, such as NF-κB-essential modulator (NEMO), optineurin, A20-binding inhibitors of NF-κB (ABINs), and A20, modulate the functions of LUBAC, and the dysregulation of the LUBAC-mediated linear ubiquitination pathway induces cancer and inflammatory, autoimmune, and neurodegenerative diseases. Therefore, inhibitors of LUBAC would be valuable to facilitate investigations of the molecular and cellular bases for LUBAC-mediated linear ubiquitination and signal transduction, and for potential therapeutic purposes. We identified and characterized α,β-unsaturated carbonyl-containing chemicals, named HOIPINs (HOIP inhibitors), as LUBAC inhibitors. We summarize recent advances in elucidations of the pathophysiological functions of LUBAC-mediated linear ubiquitination and identifications of its regulators, toward the development of LUBAC inhibitors.

OTULIN deficiency in ORAS causes cell type-specific LUBAC degradation, dysregulated TNF signalling and cell death

The deubiquitinase OTULIN removes methionine‐1 (M1)‐linked polyubiquitin signals conjugated by the linear ubiquitin chain assembly complex (LUBAC) and is critical for preventing TNF‐driven inflammation in OTULIN‐related autoinflammatory syndrome (ORAS). Five ORAS patients have been reported, but how dysregulated M1‐linked polyubiquitin signalling causes their symptoms is unclear. Here, we report a new case of ORAS in which an OTULIN‐Gly281Arg mutation leads to reduced activity and stability in vitro and in cells. In contrast to OTULIN‐deficient monocytes, in which TNF signalling and NF‐κB activation are increased, loss of OTULIN in patient‐derived fibroblasts leads to a reduction in LUBAC levels and an impaired response to TNF. Interestingly, both patient‐derived fibroblasts and OTULIN‐deficient monocytes are sensitised to certain types of TNF‐induced death, and apoptotic cells are evident in ORAS patient skin lesions. Remarkably, haematopoietic stem cell transplantation leads to complete resolution of inflammatory symptoms, including fevers, panniculitis and diarrhoea. Therefore, haematopoietic cells are necessary for clinical manifestation of ORAS. Together, our data suggest that ORAS pathogenesis involves hyper‐inflammatory immune cells and TNF‐induced death of both leukocytes and non‐haematopoietic cells.

Update Review about Metabolic Myopathies

The aim of this review is to summarize and discuss recent findings and new insights in the etiology and phenotype of metabolic myopathies. The review relies on a systematic literature review of recent publications. Metabolic myopathies are a heterogeneous group of disorders characterized by mostly inherited defects of enzymatic pathways involved in muscle cell metabolism. Metabolic myopathies present with either permanent (fixed) or episodic abnormalities, such as weakness, wasting, exercise-intolerance, myalgia, or an increase of muscle breakdown products (creatine-kinase, myoglobin) during exercise. Though limb and respiratory muscles are most frequently affected, facial, extra-ocular, and axial muscles may be occasionally also involved. Age at onset and prognosis vary considerably. There are multiple disease mechanisms and the pathophysiology is complex. Genes most recently related to metabolic myopathy include PGM1, GYG1, RBCK1, VMA21, MTO1, KARS, and ISCA2. The number of metabolic myopathies is steadily increasing. There is limited evidence from the literature that could guide diagnosis and treatment of metabolic myopathies. Treatment is limited to mainly non-invasive or invasive symptomatic measures. In conclusion, the field of metabolic myopathies is evolving with the more widespread availability and application of next generation sequencing technologies worldwide. This will broaden the knowledge about pathophysiology and putative therapeutic strategies for this group of neuromuscular disorders.

The 5th International Lafora Epilepsy Workshop: Basic science elucidating therapeutic options and preparing for therapies in the clinic

Lafora disease (LD) is both a fatal childhood epilepsy and a glycogen storage disease caused by recessive mutations in either the Epilepsy progressive myoclonus 2A (EPM2A) or EPM2B genes. Hallmarks of LD are aberrant, cytoplasmic carbohydrate aggregates called Lafora bodies (LBs) that are a disease driver. The 5th International Lafora Epilepsy Workshop was recently held in Alcala de Henares, Spain. The workshop brought together nearly 100 clinicians, academic and industry scientists, trainees, National Institutes of Health (NIH) representation, and friends and family members of patients with LD. The workshop covered aspects of LD ranging from defining basic scientific mechanisms to elucidating a LD therapy or cure and a recently launched LD natural history study.

The genetic basis of pneumococcal and staphylococcal infections: inborn errors of human TLR and IL-1R immunity

Many bacteria can cause pyogenic lesions in humans. Most of these bacteria are harmless in most individuals, but they, nevertheless, cause significant morbidity and mortality worldwide. The inherited and acquired immunodeficiencies underlying these pyogenic infections differ between bacteria. This short review focuses on two emblematic pyogenic bacteria: pneumococcus (Streptococcus pneumoniae) and Staphylococcus, both of which are Gram-positive encapsulated bacteria. We will discuss the contribution of human genetic studies to the identification of germline mutations of the TLR and IL-1R pathways.