Natural history of TANGO2 deficiency disorder: Baseline assessment of 73 patients

Mitochondrial mayhem: Disrupting conserved N-terminal motifs in TANGO2 impacts its localization and function

TANGO2 deficiency in humans leads to progressive neurological impairment, punctuated by life-threatening metabolic crises. In this issue, Lujan and colleagues demonstrate that TANGO2 localizes within the mitochondrial lumen and binds acyl-CoA species, potentially implicating it as a lipid trafficking protein.

Vitamin B5 Monotherapy Improves Symptoms in a 7-Year-Old Girl With TANGO2 Deficiency Disorder

TANGO2 deficiency disorder is often underdiagnosed and lacks an optimal treatment strategy. A 7-year-old Chinese girl presented with epilepsy, developmental delay, neuroregression, and episodes of dyskinesia. Additionally, she lapsed into a comatose state following her the last generalized tonic-clonic seizure. Trio whole-exome sequencing revealed compound heterozygous variants of the TANGO2 gene. Eventually, her clinical signs and symptoms significantly improved following treatment with vitamin B5. TANGO2 deficiency disorder is a severe neurodegenerative condition that can be diagnosed via trio whole-exome sequencing. This report highlights the potential therapeutic effects of vitamin B5 against this disease and suggests that high-dose vitamin B5 administration may be safe for the treatment of TANGO2 deficiency disorder.

TANGO-2: A Rare Genetic Condition With Severe Clinical Presentation of Encephalopathy, Rhabdomyolysis, and Cardiac Rhythm Disorders in 2 Children

Biallelic pathogenic or likely pathogenic variants in Transport and Golgi Organization 2 (TANGO-2) are associated with a spectrum of clinical features including encephalopathy, rhabdomyolysis, cardiac rhythm disorders, and neurologic regression. We are reporting on 2 unrelated children with biallelic TANGO-2 pathogenic variants. These variants were identified through a Next Generation Sequencing (NGS) panel of genes associated with hereditary rhabdomyolysis. Both children had a history of developmental delay, especially in their motor milestones. They also experienced episodic transient weakness with acute illness. One of the children's siblings had similar complaints and died at an early age. During their illness, both children developed extreme lethargy with very high CPK levels, lactic acidosis, rising trends of transaminases, and recurrent hypoglycemia. Both patients developed ventricular tachyarrhythmias, and the echocardiogram showed cardiomyopathy. Despite intensive symptomatic management, both patients died of cardiac failure because of fatal ventricular arrhythmia. Genetic testing revealed the presence of biallelic pathogenic variants TANGO-2. This rare genetic condition should be suspected in any patient with episodic recurrent weakness, rhabdomyolysis, abdominal pain, and cardiac arrhythmias, because of its diverse clinical presentation. However, early diagnosis is challenging because there are no specific biochemical markers for the disease. There is strong evidence that vitamin B supplementation can significantly reduce the number of metabolic crises in these children. Although this is not a targeted therapy, it can be a potentially life-saving treatment for these patients.

Emergency Management of Intoxication-Type Inherited Metabolic Disorders

In many intoxication-type inherited metabolic disorders, the accumulation of the toxic chemical can cause acute life-threatening emergencies. Sometimes this is the inevitable consequence of a severe metabolic defect, but it is often triggered by catabolism. In this article, we consider the acute management when these conditions cause encephalopathy, seizures, stroke-like episodes, thromboses, liver failure, cardiac failure, arrhythmias and rhabdomyolysis. Treatment is available for most intoxication-type disorders, though it is seldom entirely satisfactory. The emergency management involves general measures for the immediate problem (such as liver failure, thrombosis or an arrhythmia) and specific treatment for the metabolic disorder. The latter usually aims to reduce the accumulation of the toxic small molecule. Often this involves preventing or reversing catabolism. Sometimes the formation of the toxic chemical can be reduced by removing dietary precursors, by diverting precursors to alternative pathways, or by inhibiting an earlier step in the affected pathway. Another strategy is to remove the toxic chemical by binding it to a drug or by extracorporeal blood purification. Occasionally, the block in the pathway can be ameliorated and some disorders, specific treatment may prevent the consequences of the accumulating chemical. Despite all these treatment strategies, outcomes are often disappointing, particularly if an intoxication disorder first presents as an emergency. Newborn screening has greatly improved the prognosis for some disorders. For others, outcomes can only be improved by earlier recognition and treatment.

Cellular mechanisms of acute rhabdomyolysis in inherited metabolic diseases

Acute rhabdomyolysis (RM) constitutes a life-threatening emergency resulting from the (acute) breakdown of skeletal myofibers, characterized by a plasma creatine kinase (CK) level exceeding 1000 IU/L in response to a precipitating factor. Genetic predisposition, particularly inherited metabolic diseases, often underlie RM, contributing to recurrent episodes. Both sporadic and congenital forms of RM share common triggers. Considering the skeletal muscle's urgent need to rapidly adjust to environmental cues, sustaining sufficient energy levels and functional autophagy and mitophagy processes are vital for its preservation and response to stressors. Crucially, the composition of membrane lipids, along with lipid and calcium transport, and the availability of adenosine triphosphate (ATP), influence membrane biophysical properties, membrane curvature in skeletal muscle, calcium channel signaling regulation, and determine the characteristics of autophagic organelles. Consequently, a genetic defect involving ATP depletion, aberrant calcium release, abnormal lipid metabolism and/or lipid or calcium transport, and/or impaired anterograde trafficking may disrupt autophagy resulting in RM. The complex composition of lipid membranes also alters Toll-like receptor signaling and viral replication. In response, infections, recognized triggers of RM, stimulate increased levels of inflammatory cytokines, affecting skeletal muscle integrity, energy metabolism, and cellular trafficking, while elevated temperatures can reduce the activity of thermolabile enzymes. Overall, several mechanisms can account for RMs and may be associated in the same disease-causing RM.

Current global vitamin and cofactor prescribing practices for primary mitochondrial diseases: Results of a European reference network survey

Primary mitochondrial diseases (PMD) account for a group of approximately 400 different genetic disorders with diverse clinical presentations and pathomechanisms. Although each individual disorder is rare, collectively they represent one of the largest groups in the field of inherited metabolic disorders. The complexity of PMD results in a continued lack of therapeutic options, necessitating a predominantly symptomatic treatment approach for affected patients. While a subset of diseases responds exceptionally well to treatment with specific vitamins or cofactors, for most PMD systematic reviews were not able to show significant benefit. This is in discrepancy to their continued frequent use among specialists. To gain further insight into the current clinical practice of vitamin and cofactor supplementation among clinicians treating children and adults affected by PMD, we conducted a worldwide cross-sectional questionnaire study exploring the choice of substances and the specific diseases where they are applied. To our knowledge, this is the first global study exploring this topic and featuring a high response rate from paediatricians. The vast majority (95%, 106/112) of responding specialists recommended the use of vitamins and cofactors, either in an agnostic approach irrespective of the specific PMD or directed to the treatment of specific diseases or phenotypes. Our study highlights significant regional and specialty-specific differences in supplementation practices. We provide some preliminary insights into specialist-based opinions regarding the use of vitamins and cofactors in PMD and highlight the need for more rigorous clinical and preclinical investigations and/or clear consensus statements.

Heme's relevance genuine? Re-visiting the roles of TANGO2 homologs including HRG-9 and HRG-10 in C. elegans

Mutations in the TANGO2 gene cause severe illness in humans, including life-threatening metabolic crises. However, the function of TANGO2 protein remains unknown. Using Caenorhabditis elegans and other models, it has recently been proposed that TANGO2 transports heme within and between cells, from areas with high heme concentrations to those with lower concentrations. Here, we demonstrate that heme-related observations in nematodes may be better explained by a previously unreported metabolic phenotype in these worms, characterized by reduced feeding, decreased lifespan and brood sizes, and poor motility. We also show that several genes not implicated in heme transport are upregulated in the low heme state and conversely demonstrate that hrg-9 in particular is highly responsive to oxidative stress, independent of heme status. Collectively, these data implicate bioenergetic failure and oxidative stress as key factors in the pathophysiology of TANGO2 deficiency, in alignment with observations from human patients. Our group performed several experiments in yeast and zebrafish deficient in TANGO2 homologs and was unable to replicate prior findings from these models. Overall, we believe there is insufficient evidence to support heme transport as the primary function for TANGO2.

Multicenter appraisal of comorbid TANGO2 deficiency disorder in patients with 22q11.2 deletion syndrome

TANGO2 deficiency disorder (TDD) is a rare, autosomal recessive condition caused by pathogenic variants in TANGO2, a gene residing within the region commonly deleted in 22q11.2 deletion syndrome (22q11.2DS). Although patients with 22q11.2DS are at substantially higher risk for comorbid TDD, it remains underdiagnosed within 22q11.2DS, likely due to overlapping symptomatology and a lack of knowledge about TDD. Initiation of B-vitamin supplementation may provide therapeutic benefit in TDD, highlighting the need for effective screening methods to improve diagnosis rates in this at-risk group. In this retrospective, multicenter study, we evaluated two cohorts of patients with 22q11.2DS (total N = 435) for possible comorbid TDD using two different symptom-based screening methods (free text-mining and manual chart review versus manual chart review alone). The methodology of the cohort 1 screening method successfully identified a known 22q11.2DS patient with TDD. Combined, these two cohorts identified 21 living patients meeting the consensus recommendation for TANGO2 testing for suspected comorbid TDD. Of the nine patients undergoing TANGO2 sequencing with del/dup analysis, none were ultimately diagnosed with TDD. Of the 12 deaths in the suspected comorbid TDD cohort, some of these patients exhibited symptoms (rhabdomyolysis, cardiac arrhythmia, or metabolic crisis) suspicious of comorbid TDD contributing to their death. Collectively, these findings highlight the need for robust prospective screening tools for diagnosing comorbid TDD in patients with 22q11.2DS.

Clinical case report of intractable paroxysmal sympathetic hyperactivity in TANGO2 deficiency disorder

TANGO2 deficiency disorder (TDD) is a neurodegenerative disease characterized by a broad and variable spectrum of clinical manifestations, even among individuals sharing the same pathogenic variants. Here, we report a severely affected individual with TDD presenting with intractable paroxysmal sympathetic hyperactivity (PSH). While progressive brain atrophy has been observed in TDD, PSH has not been reported. Despite comprehensive workup for an acute trigger, no definite cause was identified, and pharmacological interventions were ineffective to treat PSH. Ultimately care was redirected to comfort measures. This article expands the clinical phenotype of patients with TDD, highlights the possibility of PSH in these patients, and the need for continued research for better treatments of TDD.

Impaired coenzyme A homeostasis in cardiac dysfunction and benefits of boosting coenzyme A production with vitamin B5 and its derivatives in the management of heart failure

Coenzyme A (CoA) is an essential cofactor required for over a hundred metabolic reactions in the human body. This cofactor is synthesized de novo in our cells from vitamin B5, also known as pantothenic acid, a water-soluble vitamin abundantly present in vegetables and animal-based foods. Neurodegenerative disorders, cancer, and infectious diseases have been linked to defects in de novo CoA biosynthesis or reduced levels of this coenzyme. There is now accumulating evidence that CoA limitation is a critical pathomechanism in cardiac dysfunction too. In the current review, we will summarize our current knowledge on CoA and heart failure, with emphasis on two primary cardiomyopathies, phosphopantothenoylcysteine synthetase and phosphopantothenoylcysteine decarboxylase deficiency disorders biochemically characterized by a decreased level of CoA in patients' samples. Hence, we will discuss the potential benefits of CoA restoration in these diseases and, more generally, in heart failure, by vitamin B5 and its derivatives pantethine and 4'-phosphopantetheine.

Lipidomic analysis of human TANGO2-deficient cells suggests a lipid imbalance as a cause of TANGO2 deficiency disease

TANGO2 deficiency disease (TDD) is a multisystem disease caused by variants in the TANGO2 gene. Symptoms include neurodevelopmental delays, seizures and potentially lethal metabolic crises and cardiac arrhythmias. While the function of TANGO2 remains elusive, vitamin B5/pantothenic acid supplementation has been shown to alleviate symptoms in a fruit fly model and has also been used with success to treat individuals suffering from TDD. Since vitamin B5 is the precursor to the lipid activator coenzyme A (CoA), we hypothesized that TANGO2-deficient cells would display changes in the lipid profile compared to control and that these changes would be rescued by vitamin B5 supplementation. In addition, the specific changes seen might point to a pathway in which TANGO2 functions. Indeed, we found profound changes in the lipid profile of human TANGO2-deficient cells as well as an increased pool of free fatty acids in both human cells devoid of TANGO2 and Drosophila harboring a previously described TANGO2 loss of function allele. All these changes were reversed upon vitamin B5 supplementation. Pathway analysis showed significant increases in triglyceride as well as in lysophospholipid levels as the top enriched pathways in the absence of TANGO2. Consistent with a defect in triglyceride metabolism, we found changes in lipid droplet numbers and sizes in the absence of TANGO2 compared to control. Our data will allow for comparison between other model systems of TDD and the homing in on critical lipid imbalances that lead to the disease state.

Folate as a potential treatment for lethal ventricular arrhythmias in TANGO2-deficiency disorder

TANGO2-deficiency disorder (TDD) is an autosomal-recessive genetic disease caused by biallelic loss-of-function variants in the TANGO2 gene. TDD-associated cardiac arrhythmias are recalcitrant to standard antiarrhythmic medications and constitute the leading cause of death. Disease modeling for TDD has been primarily carried out using human dermal fibroblast and, more recently, in Drosophila by multiple research groups. No human cardiomyocyte system has been reported, which greatly hinders the investigation and understanding of TDD-associated arrhythmias. Here, we established potentially novel patient-derived induced pluripotent stem cell differentiated cardiomyocyte (iPSC-CM) models that recapitulate key electrophysiological abnormalities in TDD. These electrophysiological abnormalities were rescued in iPSC-CMs with either adenoviral expression of WT-TANGO2 or correction of the pathogenic variant using CRISPR editing. Our natural history study in patients with TDD suggests that the intake of multivitamin/B complex greatly diminished the risk of cardiac crises in patients with TDD. In agreement with the clinical findings, we demonstrated that high-dose folate (vitamin B9) virtually abolishes arrhythmias in TDD iPSC-CMs and that folate's effect was blocked by the dihydrofolate reductase inhibitor methotrexate, supporting the need for intracellular folate to mediate antiarrhythmic effects. In summary, data from TDD iPSC-CM models together with clinical observations support the use of B vitamins to mitigate cardiac crises in patients with TDD, providing potentially life-saving treatment strategies during life-threatening events.

TANGO2 deficiency disease is predominantly caused by a lipid imbalance

TANGO2 deficiency disease (TDD) is a rare genetic disorder estimated to affect ∼8000 individuals worldwide. It causes neurodegeneration often accompanied by potentially lethal metabolic crises that are triggered by diet or illness. Recent work has demonstrated distinct lipid imbalances in multiple model systems either depleted for or devoid of the TANGO2 protein, including human cells, fruit flies and zebrafish. Importantly, vitamin B5 supplementation has been shown to rescue TANGO2 deficiency-associated defects in flies and human cells. The notion that vitamin B5 is needed for synthesis of the lipid precursor coenzyme A (CoA) corroborates the hypothesis that key aspects of TDD pathology may be caused by lipid imbalance. A natural history study of 73 individuals with TDD reported that either multivitamin or vitamin B complex supplementation prevented the metabolic crises, suggesting this as a potentially life-saving treatment. Although recently published work supports this notion, much remains unknown about TANGO2 function, the pathological mechanism of TDD and the possible downsides of sustained vitamin supplementation in children and young adults. In this Perspective, we discuss these recent findings and highlight areas for immediate scientific attention.

Management of acute metabolic crisis in TANGO2 deficiency: a case report

OBJECTIVES

TANGO2 deficiency is a rare inborn error of metabolism, with distinct clinical features. The clinical presentations of TANGO2 deficiency are developmental delay, speech difficulties, intellectual disability, non-life-threatening paroxysmal neurologic episodes (TANGO2 spells), acute metabolic crises, cardiac crises, seizures and hypothyroidism. Patients may die in acute metabolic crises. Here we report our experience in the management of an acute metabolic crisis in TANGO2 deficiency.

CASE PRESENTATION

A 9-year-old patient diagnosed with TANGO2 deficiency was admitted with fever, fatigue, unable to walk. In follow up, encephalopathy, rhabdomyolysis and arrhythmia were detected. Vitamin B-complex was started. Our patient's mental status and rhabdomyolysis improved dramatically, and cardiac crises ended without Torsades de pointes, ventricular tachycardia and/or fibrillation or myocardial dysfunction.

CONCLUSIONS

With this report, we aimed to show the effectiveness of vitamin B-complex in the management of acute metabolic crises.

TANGO2 Deficiency Disorder: Two Cases of Developmental Delay Preceding Metabolic Crisis

BACKGROUND

TANGO2 deficiency disorder is a rare genetic disease caused by biallelic defects in TANGO2 gene.

METHODS

We report the clinical phenotype of two children with TANGO2 deficiency disorder.

RESULTS

Patient 1 is a female child presenting with developmental delay and microcephaly during the second year of life, who evolved with severe cognitive impairment, facial dysmorphisms, spastic paraparesis, and atonic seizures. At age 13 years, she was hospitalized due to an episode of rhabdomyolysis complicated with cardiac arrhythmia and hypothyroidism. Patient 2 is a female child with dysmorphic facial features, cleft palate, and developmental delay who was diagnosed with DiGeorge syndrome. At age three years, she presented with an acute episode of severe rhabdomyolysis in the context of human herpesvirus 6 infection. After the resolution of this acute episode, she maintained recurrent muscle weakness with axial hypotonia and progressive spasticity of the lower extremities. In both patients, diagnosis of TANGO2 deficiency disorder was only confirmed after an acute metabolic crisis.

CONCLUSIONS

A high index of suspicion for TANGO2 deficiency disorder is needed in patients with developmental delay or other neurological symptoms and episodic rhabdomyolysis.

Early initiation of B-vitamin supplementation may reduce symptoms and explain intrafamilial variability: Insights from two sibling pairs from the TANGO2 natural history study

TANGO2-deficiency disorder (TDD) is an autosomal recessive condition arising from pathogenic biallelic variants in the TANGO2 gene. TDD is characterized by symptoms typically beginning in late infancy including delayed developmental milestones, cognitive impairment, dysarthria, expressive language deficits, and gait abnormalities. There is wide phenotypic variability where some are severely affected while others have mild symptoms. This variability has been documented even among sibling pairs who share the same genotype, but reasons for this variability have not been well understood. Emerging data suggest a potential link between B-complex or multivitamin supplementation and decreased metabolic crises in TDD. In this report, we describe two sibling pairs from unreladiagnosed with TDD with marked differences in symptoms. In both families, the older siblings suffered multiple metabolic crises and are clinically more affected than their younger siblings who have very mild to no symptoms; they are the least impaired among 70 other patients in our ongoing international natural history study. Unlike their older siblings, the two younger siblings started taking B-complex vitamins early between 9 and 16 months. This report delineates the mildest presentation of TDD in two families. These data may support a role for early diagnosis and initiation of vitamin supplementation to not only prevent metabolic crises but also improve neurologic outcomes in this life-threatening disorder.

Notes from the Underground: Heme Homeostasis in C. elegans

Heme is an iron-containing tetrapyrrole that plays a critical role in various biological processes, including oxygen transport, electron transport, signal transduction, and catalysis. However, free heme is hydrophobic and potentially toxic to cells. Organisms have evolved specific pathways to safely transport this essential but toxic macrocycle within and between cells. The bacterivorous soil-dwelling nematode Caenorhabditis elegans is a powerful animal model for studying heme-trafficking pathways, as it lacks the ability to synthesize heme but instead relies on specialized trafficking pathways to acquire, distribute, and utilize heme. Over the past 15 years, studies on this microscopic animal have led to the identification of a number of heme-trafficking proteins, with corresponding functional homologs in vertebrates. In this review, we provide a comprehensive overview of the heme-trafficking proteins identified in C. elegans and their corresponding homologs in related organisms.

Vitamin B5, a coenzyme A precursor, rescues TANGO2 deficiency disease-associated defects in Drosophila and human cells

Mutations in the Transport and Golgi Organization 2 (TANGO2) gene are associated with intellectual deficit, neurodevelopmental delay and regression. Individuals can also present with an acute metabolic crisis that includes rhabdomyolysis, cardiomyopathy, and cardiac arrhythmias, the latter of which are potentially lethal. While preventing metabolic crises has the potential to reduce mortality, no treatments currently exist for this condition. The function of TANGO2 remains unknown but is suspected to be involved in some aspect of lipid metabolism. Here, we describe a model of TANGO2-related disease in the fruit fly Drosophila melanogaster that recapitulates crucial disease traits. Pairing a new fly model with human cells, we examined the effects of vitamin B5, a coenzyme A (CoA) precursor, on alleviating the cellular and organismal defects associated with TANGO2 deficiency. We demonstrate that vitamin B5 specifically improves multiple defects associated with TANGO2 loss-of-function in Drosophila and rescues membrane trafficking defects in human cells. We also observed a partial rescue of one of the fly defects by vitamin B3, though to a lesser extent than vitamin B5. Our data suggest that a B complex supplement containing vitamin B5/pantothenate may have therapeutic benefits in individuals with TANGO2-deficiency disease. Possible mechanisms for the rescue are discussed that may include restoration of lipid homeostasis.