Mouse model for molybdenum cofactor deficiency type B recapitulates the phenotype observed in molybdenum cofactor deficient patients

A zebrafish gephyrinb mutant distinguishes synaptic and enzymatic functions of Gephyrin

Gephyrin is thought to play a critical role in clustering glycine receptors at synapses within the central nervous system (CNS). The main in vivo evidence for this comes from Gephyrin (Gphn)-null mice, where glycine receptors are depleted from synaptic regions. However, these mice die at birth, possibly due to impaired molybdenum cofactor (MoCo) synthesis, an essential role Gephyrin assumes throughout an animal. This complicates the interpretation of synaptic phenotypes in Gphn-null mice and raises the question whether the synaptic and enzymatic functions of Gephyrin can be investigated separately. Here, we generated a gephyrinb zebrafish mutant, vo84, that almost entirely lacks Gephyrin staining in the spinal cord. gephyrinbvo84 mutants exhibit normal gross morphology at both larval and adult stages. In contrast to Gphn-null mice, gephyrinbvo84 mutants exhibit normal motor activity and MoCo-dependent enzyme activity. Instead, gephyrinbvo84 mutants display impaired rheotaxis and increased mortality in late development. To investigate what may mediate these defects in gephyrinbvo84 mutants, we examined the cell density of neurons and myelin in the spinal cord and found no obvious changes. Surprisingly, in gephyrinbvo84 mutants, glycine receptors are still present in the synaptic regions. However, their abundance is reduced, potentially contributing to the observed defects. These findings challenge the notion that Gephyrin is absolutely required to cluster glycine receptors at synapses and reveals a new role of Gephyrin in regulating glycine receptor abundance and rheotaxis. They also establish a powerful new model for studying the mechanisms underlying synaptic, rather than enzymatic, functions of Gephyrin.

Parkinsonism in Genetic Neurodevelopmental Disorders: A Systematic Review

Background

With advances in clinical genetic testing, associations between genetic neurodevelopmental disorders and parkinsonism are increasingly recognized. In this review, we aimed to provide a comprehensive overview of reports on parkinsonism in genetic neurodevelopmental disorders and summarize findings related to genetic diagnosis, clinical features and proposed disease mechanisms.

Methods

A systematic literature review was conducted in PubMed and Embase on June 15, 2021. Search terms for parkinsonism and genetic neurodevelopmental disorders, using generic terms and the Human Phenotype Ontology, were combined. Study characteristics and descriptive data were extracted from the articles using a modified version of the Cochrane Consumers and Communication Review Group's data extraction template. The protocol was registered in PROSPERO (CRD42020191035).

Results

The literature search yielded 208 reports for data-extraction, describing 69 genetic disorders in 422 patients. The five most reported from most to least frequent were: 22q11.2 deletion syndrome, beta-propeller protein-associated neurodegeneration, Down syndrome, cerebrotendinous xanthomatosis, and Rett syndrome. Notable findings were an almost equal male to female ratio, an early median age of motor onset (26 years old) and rigidity being more common than rest tremor. Results of dopaminergic imaging and response to antiparkinsonian medication often supported the neurodegenerative nature of parkinsonism. Moreover, neuropathology results showed neuronal loss in the majority of cases. Proposed disease mechanisms included aberrant mitochondrial function and disruptions in neurotransmitter metabolism, endosomal trafficking, and the autophagic-lysosomal and ubiquitin-proteasome system.

Conclusion

Parkinsonism has been reported in many GNDs. Findings from this study may provide clues for further research and improve management of patients with GNDs and/or parkinsonism.

Molybdenum Cofactor Deficiency in Humans

Molybdenum cofactor (Moco) deficiency (MoCD) is characterized by neonatal-onset myoclonic epileptic encephalopathy and dystonia with cerebral MRI changes similar to hypoxic-ischemic lesions. The molecular cause of the disease is the loss of sulfite oxidase (SOX) activity, one of four Moco-dependent enzymes in men. Accumulating toxic sulfite causes a secondary increase of metabolites such as S-sulfocysteine and thiosulfate as well as a decrease in cysteine and its oxidized form, cystine. Moco is synthesized by a three-step biosynthetic pathway that involves the gene products of MOCS1, MOCS2, MOCS3, and GPHN. Depending on which synthetic step is impaired, MoCD is classified as type A, B, or C. This distinction is relevant for patient management because the metabolic block in MoCD type A can be circumvented by administering cyclic pyranopterin monophosphate (cPMP). Substitution therapy with cPMP is highly effective in reducing sulfite toxicity and restoring biochemical homeostasis, while the clinical outcome critically depends on the degree of brain injury prior to the start of treatment. In the absence of a specific treatment for MoCD type B/C and SOX deficiency, we summarize recent progress in our understanding of the underlying metabolic changes in cysteine homeostasis and propose novel therapeutic interventions to circumvent those pathological changes.

A systematic analysis of diet-induced nephroprotection reveals overlapping changes in cysteine catabolism

Caloric Restriction (CR) extends lifespan and augments cellular stress-resistance from yeast to primates, making CR an attractive strategy for organ protection in the clinic. Translation of CR to patients is complex, due to problems regarding adherence, feasibility, and safety concerns in frail patients. Novel tailored dietary regimens, which modulate the dietary composition of macro- and micronutrients rather than reducing calorie intake promise similar protective effects and increased translatability. However, a direct head-to-head comparison to identify the most potent approach for organ protection, as well as overlapping metabolic consequences have not been performed. We systematically analyzed six dietary preconditioning protocols - fasting mimicking diet (FMD), ketogenic diet (KD), dietary restriction of branched chained amino acids (BCAA), two dietary regimens restricting sulfur-containing amino acids (SR80/100) and CR - in a rodent model of renal ischemia-reperfusion injury (IRI) to quantify diet-induced resilience in kidneys. Of the administered diets, FMD, SR80/100 and CR efficiently protect from kidney damage after IRI. Interestingly, these approaches show overlapping changes in oxidative and hydrogen sulfide (H₂S)-dependent cysteine catabolism as a potential common mechanism of organ protection.

Impaired mitochondrial maturation of sulfite oxidase in a patient with severe sulfite oxidase deficiency

Sulfite oxidase (SO) is encoded by the nuclear SUOX gene and catalyzes the final step in cysteine catabolism thereby oxidizing sulfite to sulfate. Oxidation of sulfite is dependent on two cofactors within SO, a heme and the molybdenum cofactor (Moco), the latter forming the catalytic site of sulfite oxidation. SO localizes to the intermembrane space of mitochondria where both—pre-SO processing and cofactor insertion—are essential steps during SO maturation. Isolated SO deficiency (iSOD) is a rare inborn error of metabolism caused by mutations in the SUOX gene that lead to non-functional SO. ISOD is characterized by rapidly progressive neurodegeneration and death in early infancy. We diagnosed an iSOD patient with homozygous mutation of SUOX at c.1084G>A replacing Gly362 to serine. To understand the mechanism of disease, we expressed patient-derived G362S SO in Escherichia coli and surprisingly found full catalytic activity, while in patient fibroblasts no SO activity was detected, suggesting differences between bacterial and human expression. Moco reconstitution of apo-G362S SO was found to be approximately 90-fold reduced in comparison to apo-WT SO in vitro. In line, levels of SO-bound Moco in cells overexpressing G362S SO were significantly reduced compared to cells expressing WT SO providing evidence for compromised maturation of G362S SO in cellulo. Addition of molybdate to culture medium partially rescued impaired Moco binding of G362S SO and restored SO activity in patient fibroblasts. Thus, this study demonstrates the importance of the orchestrated maturation of SO and provides a first case of Moco-responsive iSOD.

S-Sulfocysteine Induces Seizure-Like Behaviors in Zebrafish

Sulfite is a neurotoxin, which is detoxified by the molybdenum cofactor (Moco)-dependent enzyme sulfite oxidase (SOX). In humans, SOX deficiency causes the formation of the glutamate analog S-Sulfocysteine (SSC) resulting in a constant overstimulation of ionotropic glutamatergic receptors. Overstimulation leads to seizures, severe brain damage, and early childhood death. SOX deficiency may be caused either by a mutated sox gene or by mutations in one of the genes of the multi-step Moco biosynthesis pathway. While patients affected in the first step of Moco biosynthesis can be treated by a substitution therapy, no therapy is available for patients affected either in the second or third step of Moco biosynthesis or with isolated SOX deficiency. In the present study, we used a combination of behavior analysis and vital dye staining to show that SSC induces increased swimming, seizure-like movements, and increased cell death in the central nervous system of zebrafish larvae. Seizure-like movements were fully revertible upon removal of SSC or could be alleviated by a glutamatergic receptor antagonist. We conclude that in zebrafish SSC can chemically induce phenotypic characteristics comparable to the disease condition of human patients lacking SOX activity.

Molybdenum cofactor deficiency type B knock-in mouse models carrying patient-identical mutations and their rescue by singular AAV injections

Molybdenum cofactor deficiency is an autosomal, recessively inherited metabolic disorder, which, in the absence of an effective therapy, leads to early childhood death due to neurological deterioration. In type A of the disease, cyclic pyranopterin monophosphate (cPMP) is missing, the first intermediate in the biosynthesis of the cofactor, and a biochemical substitution therapy using cPMP has been developed. A comparable approach for type B of the disease with a defect in the second step of the synthesis, formation of molybdopterin, so far has been hampered by the extreme instability of the corresponding metabolites. To explore avenues for a successful and safe gene therapy, knock-in mouse models were created carrying the mutations c.88C>T (p.Q30X) and c.726_727delAA, which are also found in human patients. Recombinant adeno-associated viruses (rAAVs) were constructed and used for postnatal intrahepatic injections of MoCo-deficient mice in a proof-of-concept approach. Singular administration of an appropriate virus dose in 60 animals prevented the otherwise devastating phenotype to a variable extent. While untreated mice did not survive for more than 2 weeks, some of the treated mice grew up to adulthood in both sexes.

Severe cystic degeneration and intractable seizures in a newborn with molybdenum cofactor deficiency type B

Newborns with cystic degeneration with or without intractable seizures should be investigated for inborn errors of metabolism, including molybdenum cofactor deficiency (MoCoD). MoCoD may present with non-specific hypoxic ischemic injury in the neonatal period with MRI showing extensive prenatally acquired cystic encephalomalacia involving grey and white matter. Most newborns with MoCoD will present with normal head size and brain appearance at birth and postnatally rapidly develop cystic encephalomalacia. A significant minority will present with signs of prenatal brain injury or malformation. It is important to consider the diagnosis in both scenarios. Low plasma urate and homocysteine may help direct the diagnostic evaluation. Herein, we describe the clinical, radiological and biochemical features of a newborn with MoCoD that was initially suspected of having the condition on biochemical screening and confirmed on rapid whole exome sequencing.

Metabolic etiologies in West syndrome

West syndrome (WS) is an early life epileptic encephalopathy associated with infantile spasms, interictal electroencephalography (EEG) abnormalities including high amplitude, disorganized background with multifocal epileptic spikes (hypsarrhythmia), and often neurodevelopmental impairments. Approximately 64% of the patients have structural, metabolic, genetic, or infectious etiologies and, in the rest, the etiology is unknown. Here we review the contribution of etiologies due to various metabolic disorders in the pathology of WS. These may include metabolic errors in organic molecules involved in amino acid and glucose metabolism, fatty acid oxidation, metal metabolism, pyridoxine deficiency or dependency, or acidurias in organelles such as mitochondria and lysosomes. We discuss the biochemical, clinical, and EEG features of these disorders as well as the evidence of how they may be implicated in the pathogenesis and treatment of WS. The early recognition of these etiologies in some cases may permit early interventions that may improve the course of the disease.

Development of a rapid UPLC–MS/MS determination of urine sulfocysteine for diagnosis of sulfocysteinuria and molybdenum co-factor deficiencies

Aim: Molybdenum co-factor deficiencies and isolated sulfite oxidase deficiency are rare autosomal recessively inherited diseases characterized by severe psychomotor impairment, intractable seizures, dislocated lens and dysmorphic facial features. The biochemical diagnosis of these diseases requires the determination of urine sulfocysteine. Materials & methods: Urine sulfocysteine was quantified by an ultra-high performance liquid chromatography–MS/MS assay. The method was validated for linearity, accuracy, precision, recovery and stability. Results & conclusion: Total imprecision of accuracy was less than 6%. Intra-assay and inter-assay precisions were less than 5%. The recovery was higher than 98%. The method is inexpensive, fast, accurate and has been successfully used for identifying five molybdenum co-factor deficient and six sulfite oxidase deficient patients since deployed.