False negative thiosulphate screening test in a case of molybdenum cofactor deficiency

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.

Clinical and Laboratory Barriers to the Timely Diagnosis of Sulphite Oxidase Deficiency

Isolated sulphite oxidase deficiency (SOD) is a rare genetic neurometabolic disorder characterised by intractable seizures and progressive severe neurological dysfunction. Affected children commonly present at or soon after birth with seizures and neurological deficits. In the newborn period, diagnosis may be missed as clinical features may mimic perinatal hypoxia-ischaemia, a more common entity. We report an infant with SOD whose clinical presentation of seizures and lethargy on the 4th day of life had suggested hypoxic-ischaemic encephalopathy, and discuss the challenges in the diagnostic process. The infant's progressive severe neurological deterioration suggested a metabolic disorder. His brain magnetic resonance imaging at 10 weeks showed severe cerebral atrophy and cystic changes. Biochemical findings of a very low plasma cystine concentration (<5μmol/L, reference range 24–100) and elevated urinary sulphocysteine in the presence of a normal plasma uric acid pointed towards SOD. The diagnosis was confirmed by mutational analysis. Isolated SOD can be difficult to diagnose on the basis of clinical features and routine metabolic tests. Nonetheless heightened awareness of the SOD entity coupled with selective biochemical testing will help towards a more accurate diagnosis. The disorder should be considered in all infants with unexplained hypoxic-ischaemic encephalopathy or progressive neurological dysfunction.

Isolated sulphite oxidase deficiency mimics the features of hypoxic ischaemic encephalopathy

Isolated sulphite oxidase deficiency (ISOD) is a rare autosomal recessive inborn error of metabolism, which may present at birth with intractable seizures (often of prenatal onset) and severe neurological abnormalities. In infants who survive, lens dislocation may occur from 8 weeks of age. The neuropathological findings in ISOD are similar to those seen in severe perinatal asphyxia. We describe two siblings with ISOD born to healthy non-consanguineous parents. The first child presented within 48 h of birth with poor feeding and seizures. He died from septicaemia on day 20 of life. The clinical presentation, neuroradiology and autopsy suggested a diagnosis of severe hypoxic ischaemic encephalopathy with a low recurrence risk. The second child presented with seizures within an hour of birth. She lived for 16 months during which time she failed to make developmental progress and continued to experience intractable seizures. Her neuroradiology was similar to her brother's. A diagnosis of ISOD was suggested from high urinary S-sulphocysteine in the second child and confirmed by the absence of sulphite oxidase activity in skin fibroblast culture. The diagnosis has enabled the couple to access prenatal testing in a subsequent pregnancy.

CONCLUSION

Isolated sulphite oxidase deficiency is an autosomal recessive condition which may mimic ischaemic encephalophathy. The disorder should be considered in all cases of intrauterine seizures, intractable seizures in the newborn period and infants with clinical and radiological features of ischaemic encephalophathy, especially when no obvious insult can be determined.

Determination of urinary S-sulphocysteine, xanthine and hypoxanthine by liquid chromatography-electrospray tandem mass spectrometry

Molybdenum cofactor and isolated sulphite oxidase deficiencies are two related rare autosomal recessive diseases characterized by severe neurological abnormalities, dislocated lens and mental retardation. Determination of three biochemical markers S-sulphocysteine (SSC), xanthine (XAN) and hypoxanthine (HXAN) in urine is essential for a definitive diagnosis and identification of the exact defect. We developed a rapid liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the analysis of SSC, XAN and HXAN in urine. The analysis was carried out in the negative-ion selected-reaction monitoring mode. The turnaround time for the assay was 7 min. Linear calibration curves for the three biomarkers were obtained in the range of 12-480 micromol/L. The intra- and inter-day assay variations were <2.5%. Mean recoveries of SSC, XAN and HXAN added to urine at two significantly different concentrations were in the range 94.3-107.3%. At a normal SSC urine excretion value of 3.2 micromol/mmol creatinine, the signal-to-noise ratio was 337:1. This stable isotope dilution LC-MS/MS method is specific, rapid and simple, and provides definitive diagnosis for molybdenum cofactor and isolated sulphite oxidase deficiencies in very small volumes of urine. We have identified seven new cases of isolated sulphite oxidase deficiency from four Saudi families and one Sudanese family.