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.

Whole exome sequencing identified a homozygous novel mutation in SUOX gene causes extremely rare autosomal recessive isolated sulfite oxidase deficiency.

BACKGROUND: Isolated sulfite oxidase deficiency (ISOD) is a rare type of life-threatening neurometabolic disorders characterized by neonatal intractable seizures and severe developmental delay with an autosomal recessive mode of inheritance. Germline mutation in SUOX gene causes ISOD. Till date, only 32 mutations of SUOX gene have been identified and reported to be associated with ISOD. METHODS: Here, we investigated a 5-days old Chinese female child, presented with intermittent tremor or seizures of limbs, neonatal encephalopathy, subarachnoid cyst and haemorrhage, dysplasia of corpus callosum, neonatal convulsion, hyperlactatemia, severe metabolic acidosis, hyperglycemia, and hyperkalemia. RESULTS: Whole exome sequencing identified a novel homozygous transition (c.1227G > A) in exon 6 of the SUOX gene in the proband. This novel homozygous variant leads to the formation of a truncated sulfite oxidase (p.Trp409*) of 408 amino acids. This variant causes partial loss of the dimerization domain of sulfite oxidase. Hence, it is a loss-of-function variant. Proband's father and mother is carrying this novel variant in a heterozygous state. This variant was not found in 200 ethnically matched normal healthy control individuals. CONCLUSIONS: Our study not only expanded the mutational spectrum of SUOX gene associated with ISOD, but also strongly suggested the significance of whole exome sequencing for identifying candidate genes and novel disease-causing variants.

A defect in molybdenum cofactor binding causes an attenuated form of sulfite oxidase deficiency.

Isolated sulfite oxidase deficiency (ISOD) is a rare recessive and infantile lethal metabolic disorder, which is caused by functional loss of sulfite oxidase (SO) due to mutations of the SUOX gene. SO is a mitochondrially localized molybdenum cofactor (Moco)- and heme-dependent enzyme, which catalyzes the vital oxidation of toxic sulfite to sulfate. Accumulation of sulfite and sulfite-related metabolites such as S-sulfocysteine (SSC) are drivers of severe neurodegeneration leading to early childhood death in the majority of ISOD patients. Full functionality of SO is dependent on correct insertion of the heme cofactor and Moco, which is controlled by a highly orchestrated maturation process. This maturation involves the translation in the cytosol, import into the intermembrane space (IMS) of mitochondria, cleavage of the mitochondrial targeting sequence, and insertion of both cofactors. Moco insertion has proven as the crucial step in this maturation process, which enables the correct folding of the homodimer and traps SO in the IMS. Here, we report on a novel ISOD patient presented at 17 months of age carrying the homozygous mutation NM_001032386.2 (SUOX):c.1097G > A, which results in the expression of SO variant R366H. Our studies show that histidine substitution of Arg366, which is involved in coordination of the Moco-phosphate, causes a severe reduction in Moco insertion efficacy in vitro and in vivo. Expression of R366H in HEK SUOX-/- cells mimics the phenotype of patient's fibroblasts, representing a loss of SO expression and specific activity. Our studies disclose a general paradigm for a kinetic defect in Moco insertion into SO caused by residues involved in Moco coordination resulting in the case of R366H in an attenuated form of ISOD.

Postmortem whole-genome sequencing on a dried blood spot identifies a novel homozygous SUOX variant causing isolated sulfite oxidase deficiency.

Rapid whole-genome sequencing (rWGS) has shown that genetic diseases are a common cause of infant mortality in neonatal intensive care units. Dried blood spots collected for newborn screening allow investigation of causes of infant mortality that were not diagnosed during life. Here, we present a neonate who developed seizures and encephalopathy on the third day of life that was refractory to antiepileptic medications. The patient died on day of life 16 after progressive respiratory failure and sepsis. The parents had lost two prior children after similar presentations, neither of whom had a definitive diagnosis. Postmortem rWGS of a dried blood spot identified a pathogenic homozygous frameshift variant in the SUOX gene associated with isolated sulfite oxidase deficiency (c.1390_1391del, p.Leu464GlyfsTer10). This case highlights that early, accurate molecular diagnosis has the potential to influence prenatal counseling and guide management in rare, genetic disorders and has added importance in cases of a strong family history and risk factors such as consanguinity.

Identification of a novel SUOX pathogenic variants as the cause of isolated sulfite oxidase deficiency in a Chinese pedigree.

BACKGROUND: Isolated sulfite oxidase deficiency (ISOD) is a life-threatening rare autosomal recessive disorder caused by pathogenic variants in SUOX (OMIM 606887) gene. The aim of our study was to establish a comprehensive genetic diagnosis strategy for the pathogenicity analysis of the SUOX gene within a limited time and to lay the foundation for precise genetic counseling, prenatal diagnosis, and preimplantation genetic diagnosis. METHODS: Two offspring from one set of parents were studied. Next-generation sequencing (NGS) was used to screen for disease-causing gene variants in a family with ISOD. Then, Sanger sequencing was performed to verify the presence of candidate variants. Sulfite, homocysteine and uric acid levels were detected in the patients. According to the ACMG/AMP guidelines, the pathogenicity level of novel variants was annotated. RESULTS: The nonsense pathogenic variant (c.1200C > G (p.Y400*)) and a duplication (c.1549_1574dup (p.I525 Mfs*102)) were found in the SUOX gene in the proband. The nonsense mutation (c.1200C > G (p.Y400*), pathogenic, isolated sulfite oxidase deficiency, autosomal recessive) has been reported as pathogenic and the duplication (c.1549_1574dup (p.I525 Mfs*102), pathogenic, isolated sulfite oxidase deficiency, autosomal recessive) was novel, which was classified as pathogenic according to the ACMG/AMP Standards and Guidelines. CONCLUSION: We established the pathogenicity assessment in ISOD patients based on ACMG/AMP Standards and Guidelines and this is the first ISOD patient reported in mainland China. We also discovered that ISOD is caused by SUOX gene duplication mutation, which enriches the spectrum of SUOX pathogenic variants.

The mitochondrial-targeted reactive species scavenger JP4-039 prevents sulfite-induced alterations in antioxidant defenses, energy transfer, and cell death signaling in striatum of rats.

Sulfite oxidase (SO) deficiency is a disorder caused either by isolated deficiency of SO or by defects in the synthesis of its molybdenum cofactor. It is characterized biochemically by tissue sulfite accumulation. Patients present with seizures, progressive neurological damage, and basal ganglia abnormalities, the pathogenesis of which is not fully established. Treatment is supportive and largely ineffective. To address the pathophysiology of sulfite toxicity, we examined the effects of intrastriatal administration of sulfite in rats on antioxidant defenses, energy transfer, and mitogen-activated protein kinases (MAPK) and apoptosis pathways in rat striatum. Sulfite administration decreased glutathione (GSH) concentration and glutathione peroxidase, glucose-6-phosphate dehydrogenase, glutathione S-transferase, and glutathione reductase activities in striatal tissue. Creatine kinase (CK) activity, a crucial enzyme for cell energy transfer, was also decreased by sulfite. Superoxide dismutase-1 (SOD1) and catalase (CAT) proteins were increased, while heme oxygenase-1 (HO-1) was decreased. Additionally, sulfite altered phosphorylation of MAPK by decreasing of p38 and increasing of ERK. Sulfite further augmented the content of GSK-3ß, Bok, and cleaved caspase-3, indicating increased apoptosis. JP4-039 is a mitochondrial-targeted antioxidant that reaches higher intramitochondrial levels than other traditional antioxidants. Intraperitoneal injection of JP4-039 before sulfite administration preserved activity of antioxidant enzymes and CK. It also prevented or attenuated alterations in SOD1, CAT, and HO-1 protein content, as well as changes in p38, ERK, and apoptosis markers. In sum, oxidative stress and apoptosis induced by sulfite injection are prevented by JP4-039, identifying this molecule as a promising candidate for pharmacological treatment of SO-deficient patients.

Cholestatic Jaundice in Sulphite Oxidase Deficiency - An Unusual Association.

Sulphite oxidase deficiency is an extremely rare inborn error of metabolism of sulphur containing amino acids. There are no reports of liver involvement in this condition. The authors describe a 9-y-old boy with known sulphite oxidase deficiency who presented with worsening cholestatic hepatitis which may be possibly related to underlying metabolic disorder. Although there is no current evidence that treating liver disease and ensuring normal hepatic function in sulphite oxidase deficiency would likely benefit patients, this could potentially contribute to optimising growth and development as well as improving the overall prognosis.

Isolated sulfite oxidase deficiency: a founder mutation.

Isolated sulfite oxidase deficiency is a rare autosomal recessive inborn error of sulfur metabolism. Clinical features generally include devastating neurologic dysfunction, ectopia lentis, and increased urinary excretion of sulfite, thiosulfate, and S-sulfocysteine. Missed diagnosis is not unusual because of variability in the sensitivity of the urinary sulfite and thiosulfate screening test. We present clinical, biochemical, and molecular data on two unrelated patients with isolated sulfite oxidase deficiency. The two patients belong to an Indigenous genetic isolate in Manitoba, Canada. Both patients (one male and one female, both now deceased) developed neonatal seizures and demonstrated progressive neurodevelopmental delay. Based on increased urinary excretion of sulfite, thiosulfate, and S-sulfocysteine and normal serum uric acid levels, sulfite oxidase deficiency was suspected. Both patients have a homozygous 4-bp deletion, 1347-1350delTTGT in the sulfite oxidase gene (SUOX), predicting a premature termination of the sulfite oxidase protein leading to absence of the carboxy-terminal third portion of the protein. This domain contains most of the contact sites essential for enzyme dimerization. This deletion mutation resulted in sulfite oxidase deficiency with early-onset severe clinical phenotype.

Biological applications of a turn-on bioluminescent probe for monitoring sulfite oxidase deficiency in vivo.

Sulfites are widely used as preservative and antioxidant additives in food and drug. A non-invasive method for in vivo imaging of sulfite represents a powerful tool for estimating its potential effects in living organisms. Herein, we report the design, development, and application of sulfite bioluminescent probes (SBPs) for the analyte-specific detection of sulfite through sulfite-mediated intramolecular cleavage. Among them, SBP-1 exhibited the excellent responsivity, high selectivity and sensitivity. By taking advantage of this probe, the first in vivo imaging of sulfate was successfully carried out, not only to trace exogenous sulfite level in living animal, but also to investigate endogenous sulfite in a sulfite oxidase deficiency model.

Oxygen and nitrite reduction by heme-deficient sulphite oxidase in a patient with mild sulphite oxidase deficiency.

Isolated sulphite oxidase deficiency (iSOD) is an autosomal recessive inborn error in metabolism characterised by accumulation of sulphite, which leads to death in early infancy. Sulphite oxidase (SO) is encoded by the SUOX gene and forms a heme- and molybdenum-cofactor-dependent enzyme localised in the intermembrane space of mitochondria. Within SO, both cofactors are embedded in two separated domains, which are linked via a flexible 11 residue tether. The two-electron oxidation of sulphite to sulphate occurs at the molybdenum active site. From there, electrons are transferred via two intramolecular electron transfer steps (IETs) via the heme cofactor and to the physiologic electron acceptor cytochrome c. Previously, we reported nitrite and oxygen to serve as alternative electron acceptors at the Moco active site, thereby overcoming IET within SO. Here, we present evidence for these reactions to occur in an iSOD patient with an unusual mild disease representation. In the patient, a homozygous c.427C>A mutation within the SUOX gene leads to replacement of the highly conserved His143 to Asn. The affected His143 is one of two heme-iron-coordinating residues within SO. We demonstrate, that the H143N SO variant fails to bind heme in vivo leading to the elimination of SO-dependent cytochrome c reduction in mitochondria. We show, that sulphite oxidation at the Moco domain is unaffected in His143Asn SO variant and demonstrate that nitrite and oxygen are able to serve as electron acceptors for sulphite-derived electrons in cellulo. As result, the patient H143N SO variant retains residual sulphite oxidising activity thus ameliorating iSOD progression.

Metabolic crisis after trivial head trauma in late-onset isolated sulfite oxidase deficiency: Report of two new cases and review of published patients.

BACKGROUND: Isolated sulfite oxidase deficiency (ISOD) is a rare autosomal recessively inherited inborn error of metabolism, caused by mutation in SUOX gene. ISOD has two kind of presentation; early and late-onset. The late-onset form is extremely rare and only 10 cases have been reported. METHODS: We report two new cases of late-onset ISOD with biochemical and genetic confirmation. We did a review of the previously published cases of late-onset ISOD. RESULTS: Together with the presented two cases, 12 cases were available for analysis. The median age at symptom onset and at diagnosis was 8.5 and 23 months respectively. Almost all children had acute regression of milestones followed by slow recovery. The common presenting signs and symptoms were movement disorders, seizures, ectopia lentis and hypertonia. Five children had antecedent events. Trivial trauma precipitating the metabolic crisis was unique to the two cases we report. The most common MRI feature was globus pallidi changes followed by cerebellar white matter changes, vermian hypoplasia and thinned out corpus callosum. Diffusion weighted sequence was performed in 3 children and all had diffusion restriction in the affected area. CONCLUSION: Trivial trauma can precipitate metabolic crisis in late-onset ISOD. Low plasma homocysteine and involvement of globus pallidi with diffusion restriction on the MRI are important diagnostic clues. Early diagnosis and intervention with special diet may be effective in preventing long term neurodisability.

Stable clinical course in three siblings with late-onset isolated sulfite oxidase deficiency: a case series and literature review.

BACKGROUND: Isolated sulfite oxidase deficiency (ISOD) is an autosomal recessive disorder caused by a deficiency of sulfite oxidase, which is encoded by the sulfite oxidase gene (SUOX). Clinically, the disorder is classified as one of two forms: the late-onset mild form or the classic early-onset form. The latter is life-threatening and always leads to death during early childhood. Mild ISOD cases are rare and may benefit from dietary therapy. To date, no cases of ISOD have been reported to recover spontaneously. Here, we present three mild ISOD cases in one family, each with a stable clinical course and spontaneous recovery. CASE PRESENTATION: All three siblings had two novel compound heterozygous mutations in the SUOX gene (NM_000456; c.1096C > T [p.R366C] and c.1376G > A [p.R459Q]). The siblings included two males and one female with late ages of onset (12-16 months) and presented with specific neuroimaging abnormalities limited to the bilateral globus pallidus and substantia nigra. The three patients had decreased plasma homocysteine levels. They exhibited a monophasic clinical course continuing up to 8.5 years even without dietary therapy. CONCLUSION: This is the first report of mild ISOD cases with a stable clinical course and spontaneous recovery without dietary therapy. Our study provides an expansion for the clinical spectrum of ISOD. Furthermore, we highlight the importance of including ISOD in the differential diagnosis for patients presenting with late-onset symptoms, bilaterally symmetric regions of abnormal intensities in the basal ganglia, and decreased plasma homocysteine levels.

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.

The Role of Oxidative Stress and Bioenergetic Dysfunction in Sulfite Oxidase Deficiency: Insights from Animal Models.

Sulfite oxidase (SO) deficiency is an autosomal recessive inherited neurometabolic disease caused by deficient activity of SO. It is biochemically characterized by tissue accumulation and high urinary excretion of sulfite, thiosulfate, and S-sulfocysteine. Severe neurological symptoms, including neonatal seizures, encephalopathy, and psychomotor retardation, are commonly observed in the affected patients, but the pathogenesis of the neurologic dysfunction is still poorly understood. In this minireview, we will briefly summarize the knowledge obtained from in vivo and in vitro findings from animal studies indicating that oxidative stress and mitochondrial dysfunction are involved in the pathophysiology of the brain damage in this disease. Recent reports have shown that sulfite induces free radical generation, impairs brain antioxidant defenses, and disturbs mitochondrial energy metabolism and biogenesis. Moreover, it has been evidenced that free radical scavengers and the pan-PPAR agonist bezafibrate are able to prevent most deleterious effects elicited by sulfite on the brain. These promising data offer new perspectives for potential therapeutic strategies for this condition, which may include the early use of appropriate antioxidants and PPAR agonists in addition to the available treatment.

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.

Isolated sulfite oxidase deficiency.

Isolated sulfite oxidase deficiency (ISOD) is a life-threatening, autosomal recessive disease characterized by severe neurological impairment. As no long-term effective treatment is available, distinction from other treatable diseases, such as molybdenum cofactor deficiency (MoCD) type A, should be made. We reviewed 47 patients (45 previously reported in the literature). Cases were reviewed for consanguinity, sex, age at onset, death, clinical findings (including spasticity, seizures, psychomotor retardation, feeding difficulties, ectopia lentis, microcephaly), laboratory findings [urinary sulfite, S-sulfocysteine (in plasma and urine), plasma cystine, total homocysteine, uric acid, and oxypurines in urine] and radiological findings (including cerebral/cerebellar atrophy, cystic white matter changes, ventriculomegaly). We also aligned the published SUOX gene mutations to the reference sequence NM_000456.2. Onset occurred mostly during the first 72 h of life (57%) and within the first year of life in all but two patients (96%). All patients presented with neurological abnormalities, such as neonatal axial hypotonia and/or peripheral hypertonia (100%), (pharmacoresistant) seizures (84%), or developmental delay (97%). Feeding problems were also common. As found in our review, measurement of homocysteine in plasma, amino acids in plasma/urine, and sulfite in fresh urine supports the diagnosis of ISOD. Analysis of uric acid (plasma) and oxypurines (urine) is useful to rule out MoCD. In all patients in whom brain magnetic resonance imaging/computed tomography (MRI/CT) was performed, brain abnormalities were found. The purpose of this literature review is to provide a thorough overview of clinical, neuroimaging, biochemical, and genetic findings of patients with ISOD.