Child Neurology: Molybdenum cofactor deficiency

A milder form of molybdenum cofactor deficiency type A presenting as Leigh's syndrome-like phenotype highlighting the secondary mitochondrial dysfunction: a case report

Background

Molybdenum cofactor deficiency (MoCD) (OMIM# 252150) is an autosomal-recessive disorder caused by mutations in four genes involved in the molybdenum cofactor (MOCO) biosynthesis pathway.

Objectives

We report a milder phenotype in a patient with MOCS1 gene mutation who presented with a Leigh-like presentation.

Case report

We present the case of a 10-year-old boy who was symptomatic at the age of 5 months with sudden onset of dyskinesia, nystagmus, and extrapyramidal signs following a febrile illness. Initial biochemical, radiological, and histopathological findings a Leigh syndrome-like phenotype; however, whole-exome sequencing detected compound heterozygous mutations in MOCS1 gene, c.1133 G>C and c.217C>T, confirming an underlying MoCD. This was biochemically supported by low uric acid level of 80 (110-282 mmol/L) and low cystine level of 0 (3-49), and a urine S-sulfocysteine at 116 (0-15) mmol/mol creatinine. The patient was administered methionine- and cystine-free formulas. The patient has remained stable, with residual intellectual, speech, and motor sequelae.

Conclusion

This presentation expands the phenotypic variability of late-onset MoCD A and highlights the role of secondary mitochondrial dysfunction in its pathogenesis.

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.

Precision Medicine Approaches for Infantile-Onset Developmental and Epileptic Encephalopathies

Epilepsy is an etiologically heterogeneous condition; however, genetic factors are thought to play a role in most patients. For those with infantile-onset developmental and epileptic encephalopathy (DEE), a genetic diagnosis is now obtained in more than 50% of patients. There is considerable motivation to utilize these molecular diagnostic data to help guide treatment, as children with DEEs often have drug-resistant seizures as well as developmental impairment related to cerebral epileptiform activity. Precision medicine approaches have the potential to dramatically improve the quality of life for these children and their families. At present, treatment can be targeted for patients with diagnoses in many genetic causes of infantile-onset DEE, including genes encoding sodium or potassium channel subunits, tuberous sclerosis, and congenital metabolic diseases. Precision medicine may refer to more intelligent choices of conventional antiseizure medications, repurposed agents previously used for other indications, novel compounds, enzyme replacement, or gene therapy approaches. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

[Molybdenum cofactor deficiency caused by MOCS1 gene mutation: a case report]

A boy attended the hospital at the age of 1 month due to left hand tremor for 1 week. A blood test showed a reduction in serum uric acid and a cranial MRI showed encephalomalacia, atrophy, and cystic changes. The boy had microcephalus, unusual facial features (long face, long forehead, protruded forehead, long philtrum, low nasal bridge, facial swelling, and thick lower lip), hypertonia of lower extremities, and severe global developmental delay. Whole-exome sequencing performed for the boy detected a homozygous mutation, c.217C > T(p.R73W), in the MOCS1 gene, which came from his parents and was determined as "possibly pathogenic". The boy was diagnosed with molybdenum cofactor deficiency type A based on clinical manifestations and gene test results. This disease is reported for the first time in China.

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.