Case Report: Compound Heterozygous Variants in MOCS3 Identified in a Chinese Infant With Molybdenum Cofactor Deficiency

A case report of MoCD etiology in a neonate: A novel homozygous MoCS2 variant

Key Clinical Message

Molybdenum cofactor deficiency is a rare and fatal genetic disorder. Due to recurrence in the family, the etiological diagnosis could have impacted family planning and alertness to future offspring.

Abstract

Molybdenum cofactor deficiency (MoCD) is a rare and fatal genetic disorder that impairs molybdenum-dependent enzymes, resulting in conspicuous elevated urine sulfite levels and lowered serum uric acid levels. The disorder may be early-onset, causing high fatality in neonates due to secondary complications, or late-onset, manifesting in the first 2 years of life. Severe seizures, progressive neurological degeneration, motor abnormalities, and feeding difficulties are hallmarks of MoCD. Due to the similarity of clinical findings with those of sulfite oxidase deficiency and its neurological findings with hypoxic-ischemic encephalopathy, determining the true etiology remains challenging in MoCD patients. This case report presents a neonate in the first week of life with early onset refractory seizures, motor abnormalities, hypoactivity, and poor feeding behavior. Administering anti-epileptic drugs did not improve the patient's condition, who started decompensating further. Nevertheless, a thorough screening for metabolic disorders revealed low serum uric acid and high sulfite levels in the urine, indicating potential MoCD. A whole exome sequencing (WES) was thus consulted for confirmatory diagnosis. Unfortunately, the patient's WES results were received after his demise, revealing MoCD caused by a novel variant of the MoCS2 gene that has not yet been reported to the best of our knowledge. This case emphasizes the need to disseminate crucial information regarding MoCD and its etiologies for prompt molecular diagnosis to reduce morbidity and mortality.

Consensus guidelines for the diagnosis and management of isolated sulfite oxidase deficiency and molybdenum cofactor deficiencies

Sulfite intoxication is the hallmark of four ultrarare disorders that are caused by impaired sulfite oxidase activity due to genetic defects in the synthesis of the molybdenum cofactor or of the apoenzyme sulfite oxidase. Delays on the diagnosis of these disorders are common and have been caused by their unspecific presentation of acute neonatal encephalopathy with high early mortality, followed by the evolution of dystonic cerebral palsy and also by the lack of easily available and reliable diagnostic tests. There is significant variation in survival and in the quality of symptomatic management of affected children. One of the four disorders, molybdenum cofactor deficiency type A (MoCD-A) has recently become amenable to causal treatment with synthetic cPMP (fosdenopterin). The evidence base for the rational use of cPMP is very limited. This prompted the formulation of these clinical guidelines to facilitate diagnosis and support the management of patients. The guidelines were developed by experts in diagnosis and treatment of sulfite intoxication disorders. It reflects expert consensus opinion and evidence from a systematic literature search.

A case report of molybdenum cofactor deficiency type A: the first case diagnosed in Syria

Introduction

Molybdenum cofactor deficiency (MoCD) type A, a rare mitochondrial disorder with characteristic clinical presentation and imaging findings, is one of the forms of molybdenum cofactor deficiency. It presents with seizures, psychomotor delay, and breastfeeding difficulties. Seizures are especially prominent in patients with MoCD.

Case presentation

A 3-month-old girl presented with refractory generalized tonic-clonic seizures since the third day of life. Her parents were third-degree relatives. On physical examination, she demonstrated psychomotor delay, breastfeeding difficulties, seizures, doll-like facial features, and other neurological abnormalities. Her brain MRI scan revealed cortical and white matter atrophy of the cerebral hemispheres. Metabolic workup revealed elevated levels of liver enzymes, lactic acid, and ammonia. These results were inconclusive. She received anticonvulsants and vitamin therapy to manage her seizures. Based on a suspicion of mitochondrial disease, genetic analysis was performed, revealing a homozygous variant of uncertain significance in the MOCS1 gene associated with autosomal recessive molybdenum cofactor deficiency type A.

Conclusion

MoCD is a rare disease. Early diagnosis should be considered based on the patient's medical history and MRI findings, after excluding other possible diagnoses. The definitive diagnosis relies on genetic testing results.

Rhodanese-Fold Containing Proteins in Humans: Not Just Key Players in Sulfur Trafficking

The Rhodanese-fold is a ubiquitous structural domain present in various protein subfamilies associated with different physiological functions or pathophysiological conditions in humans. Proteins harboring a Rhodanese domain are diverse in terms of domain architecture, with some representatives exhibiting one or several Rhodanese domains, fused or not to other structural domains. The most famous Rhodanese domains are catalytically active, thanks to an active-site loop containing an essential cysteine residue which allows for catalyzing sulfur transfer reactions involved in sulfur trafficking, hydrogen sulfide metabolism, biosynthesis of molybdenum cofactor, thio-modification of tRNAs or protein urmylation. In addition, they also catalyse phosphatase reactions linked to cell cycle regulation, and recent advances proposed a new role into tRNA hydroxylation, illustrating the catalytic versatility of Rhodanese domain. To date, no exhaustive analysis of Rhodanese containing protein equipment from humans is available. In this review, we focus on structural and biochemical properties of human-active Rhodanese-containing proteins, in order to provide a picture of their established or putative key roles in many essential biological functions.

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.

Essential metals in health and disease

In total, twenty elements appear to be essential for the correct functioning of the human body, half of which are metals and half are non-metals. Among those metals that are currently considered to be essential for normal biological functioning are four main group elements, sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca), and six d-block transition metal elements, manganese (Mn), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn) and molybdenum (Mo). Cells have developed various metallo-regulatory mechanisms for maintaining a necessary homeostasis of metal-ions for diverse cellular processes, most importantly in the central nervous system. Since redox active transition metals (for example Fe and Cu) may participate in electron transfer reactions, their homeostasis must be carefully controlled. The catalytic behaviour of redox metals which have escaped control, e.g. via the Fenton reaction, results in the formation of reactive hydroxyl radicals, which may cause damage to DNA, proteins and membranes. Transition metals are integral parts of the active centers of numerous enzymes (e.g. Cu,Zn-SOD, Mn-SOD, Catalase) which catalyze chemical reactions at physiologically compatible rates. Either a deficiency, or an excess of essential metals may result in various disease states arising in an organism. Some typical ailments that are characterized by a disturbed homeostasis of redox active metals include neurological disorders (Alzheimer's, Parkinson's and Huntington's disorders), mental health problems, cardiovascular diseases, cancer, and diabetes. To comprehend more deeply the mechanisms by which essential metals, acting either alone or in combination, and/or through their interaction with non-essential metals (e.g. chromium) function in biological systems will require the application of a broader, more interdisciplinary approach than has mainly been used so far. It is clear that a stronger cooperation between bioinorganic chemists and biophysicists - who have already achieved great success in understanding the structure and role of metalloenzymes in living systems - with biologists, will access new avenues of research in the systems biology of metal ions. With this in mind, the present paper reviews selected chemical and biological aspects of metal ions and their possible interactions in living systems under normal and pathological conditions.

Fosdenopterin: a First-in-class Synthetic Cyclic Pyranopterin Monophosphate for the Treatment of Molybdenum Cofactor Deficiency Type A

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