Molybdenum cofactor deficiency

OVERVIEW: What every practitioner needs to know

Are you sure your patient has molybdenum cofactor deficiency? What are the typical findings for this disease?

Molybdenum cofactor deficiency is a rare autosomal recessive disorder most often presenting with severe neonatal seizures. It should be suspected in any infant with progressive neurologic decline in which asphyxia is suspected but there is no clear documentation of an hypoxic-ischemic event. The neonatal seizures are difficult to treat with standard anticonvulsants. The neurologic decline may present as initial worsening hypotonia with neonatal myoclonic or other exaggerated responses to stimuli. There may be increasing frequency of neonatal stiffening episodes or frank opisthotonus.

Molybdenum cofactor deficiency is often a fatal disease. Most infants who survive demonstrate severe developmental delay. Occasionally the initial presentation is of an infantile/early childhood static encephalopathy with microcephaly. Rarely it is an etiology of psychomotor retardation of childhood. Lens dislocation outside of the neonatal period is a common finding but of variable onset.

Many of the infants demonstrate similar dysmorphic features, including progressive microcephaly with hypertelorism, puffy cheeks, small nose, elongated philtrum and thickened lips. Milder later onset presentations of molybdenum cofactor deficiency may exist.

What other disease/condition shares some of these symptoms?

Several groups of disorders present as neonatal seizures nonresponsive to standard anticonvulsant therapy. Occasionally severe brain malformations with disorders of neuronal migration and severe perinatal asphyxia will present with similarly difficult to treat neonatal seizures. Both molybdenum cofactor deficiency (MCD) and nonketotic hyperglycinemia (NKH) present in the neonatal period with EEG evidence of burst suppression and/or severe seizure acitivty patterns.

Disorder of the glucose transporter (GLUT1) may similarly present with difficult to control neonatal seizures, but severely depressed CSF glucose levels with normal serum levels should be evident.

Vitamin B6 (pyridoxine) dependent seizures also present as difficult to manage neonatal seizures.

Inborn errors of metabolism such as urea cycle defects and organic acidemias (disorders of amino acid catabolism and fatty acid oxidation) usually are not symptomatic on day one of life, typically presenting on day 2-3 or later. These types of disorders will show metabolic acidosis, ketosis, and/or hyperammonemia at time of mental status change, in contrast to molybdenum cofactor deficiciency.

Other metabolic disorders may present in the neonatal period with seizures but often with additional findings. Several of these disorders, such as mitochondrial disorders, those affecting carbohydrate utilization or producing hypoglycemia, congenital disorders of glycosylation, and peroxisomal disorders, more typically will show additional involvement of other organs, in contrast to MCD, and may show different dysmorphic features than MCD.

Mutations in the gene for sulfite oxidase (SUOX) produce a similar phenotype to that produced by molybdenum cofactor deficiency (MCD). The activity of SUOX depends on the presence of active molybdenum cofactor. Therefore the phenotype observed in MCD is in fact due in part to the clinical effects of deficiency of SUOX.

Isolated SUOX deficiency presents with intractable seizures, similar dysmorphic features including progressive microcephaly and lens dislocation, and if infants with this condition survive, subsequent developmental delay. Some of the metabolites used to initially suggest MCD are due to the deficiency of SUOX activity (urinary sulfites and thiosulfites, and blood and urine S-sulfocysteine). The frequency of isolated SUOX deficiency appears to be rarer than that of MCD, both likely less than 1/100-200,000, and should be easily distinguished by laboratory patterns of accumulated metabolites.

What caused this disease to develop at this time?

Molybdenum cofactor deficiency (MCD) is a group of autosomal recessive disorders due to mutations affecting the genes required for molybdenum cofactor biosynthesis. Multiple complementation groups (A,B,C) of individuals with molybdenum cofactor deficiency reflect deficiencies in different individual sequential steps which prepare molybdenum for biologic activity. All three origins of deficiency (complementation groups A, B or C), reflecting mutations in three different genes (MOCS1, MOCS2, GEPH), present with similar clinical and laboratory findings called molybdenum cofactor deficiency (MCD).

It is a rare set of disorders, likely less than 1/100-200,000 frequency. It has been observed in all ethnic groups but its rarity so far prevents clear delineation of epidemiologic patterns.

Molybdenum is a trace element which is biologically modified to function as a cofactor for three enzymatic activities - sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase. Of these, only deficiency of sulfite oxidase (SUOX) appears most clinically significant, associated with neurologic dysfunction. Sulfite oxidase deficiency, either due to molybdenum cofactor deficiency or isolated sulfite oxidase deficiency, is associated with seizure disorder, feeding difficulties, mental retardation and lens dislocation.

Some of the clinical effects observed in both disorders (MCD and isolated SUOX) are felt to be due in part to the effect of SUOX deficiency on total homocysteine and cysteine. Both of these compounds are involved in the maintenance of crosslinking between and within protein structures but it is also felt that direct sulfate/sulfite toxicities may be involved in producing the CNS manifestations. It is unclear why approximately 10% of MCD presents as later onset CNS disease.

Because it is well absorbed in the gut, dietary deficiency as a cause of clinical presentation is extremely rare (once reported with TPN deficiency).

Molybdenum cofactor deficiency also causes loss of activity of xanthine dehydrogenase, which functions in purine degradation (converting hypoxanthine to xanthine and then to uric acid). This leads to decreased levels of both serum and urinary uric acid. Elevated excretion of xanthine may ultimately lead to renal stones in survivors of MCD.

What laboratory studies should you request to help confirm the diagnosis? How should you interpret the results?

Diagnostic laboratory testing for both molybdenum cofactor deficiency (MCD) and isolated sulfite oxidase deficiency involves urine analysis for S-sulfocysteine. The finding of low or nondetectable serum uric acid levels may suggest the diagnosis of MCD but is not always a reliable indicator. Hypouricemia occasionally is not seen in affected patients during the neonatal period, even when symptomatic with intractable seizures.

Colorimetric dip stick tests for urine sulfites may be available at some locations. These urinary screening tests for sulfites by sulfite dipsticks may be offered, but fresh urine must be used to avoid false negative results due to oxidation of the sulfites to sulfates. This test may therefore be unreliable.

Urine thiosulfites may be measured but one needs to be aware of the presence of cross-reacting antibiotics, ie false positive results.

The most stable diagnostic marker is the urine S-sulfocysteine, often offered as part of quantitative urinary amino acid analysis, but best detected by tandem mass spectrometry. The receiving lab should be notified of request for S-sulfocysteine identification if not offered as a specific urinary test.

Complete laboratory workup for diagnosis for MCD therefore includes plasma amino acids, serum uric acid, urine amino acids and total homocysteine (urine and plasma). Be sure to mention to the laboratory that you are looking for S-sulfocysteine, since this may be performed as a separate test.

For molybdenum cofactor deficiency (MCD), one should see decreased uric acid in blood (and urine if measured), decreased cysteine and homocysteine in plasma, and decreased sulfate and homocysteine in urine. Increases in urinary S-sulfocysteine, thiosulfite, sulfite, hypoxanthine, and taurine may be seen. Increases in blood S-sulfocysteine, taurine, xanthine may be noted. These results are then followed by specific enzymatic testing in cultured fibroblasts. This can then be followed by DNA sequencing studies looking for mutations in the appropriate genes.

Would imaging studies be helpful? If so, which ones?

Brain abnormalities are commonly identifiable on computed tomography (CT) scan and especially magnetic resonance imaging (MRI) in patients with molybdenum cofactor deficiency (MCD). However, all of the observed abnormalities are nonspecific and may be observed in a number of other disorders.

White matter and basal ganglia abnormalities, consistent with damage causing cerebral volume loss, is the general theme of either neuroimaging or pathologic examination of patients with MCD. This will be manifested as cerebral atrophy with dilated ventricles (hydrocephalus ex vacuo), white matter hypodensity, areas of neuronal loss or cystic lesions in basal ganglia and/or cerebellum, as well as cerebral and/or basal ganglia calcifications. Cerebral edema may be present. Additionally, abnormal patterns of cerebral gyration and/or neuronal migration are common. On autopsy, severe brain atrophy is often a dominant finding.

Confirming the diagnosis

Neonates presenting with progressive neurologic deterioration, manifested by refractory seizures, progressive hypotonia, abnormal reflexive myoclonus and/or deteriorating mental status often have initial routine examinations of serum and CSF chemistries and EEG performed that may suggest an inborn error of metabolism. Burst suppression pattern on EEG should always prompt an investigation for an inborn error of metabolism, especially for nonketotic hyperglycinemia (NKH) and molybdenum cofactor deficiency (MCD), and any etiology for severe hyperammonemia.

In consideration of MCD, check blood uric acid (hypouricemia) and send blood and urine specifically for S-sulfocysteine. Low CSF glucose but normal serum glucose should prompt investigation for possible GLUT1 (glucose transporter) deficiency. Elevated lactate in blood, and especially CSF, may prompt investigation for mitochondrial disease. Serum acidosis and hypoglycemia might suggest inborn errors of amino acid catabolism or fatty acid oxidation, which may also produce a secondary defect in the urea cycle associated with hyperammonemia at times.

Hyperammonemia without acidosis or hypoglycemia would be consistent with a primary urea cycle defect. This type of clinical presentation should always prompt quantitative plasma amino acid analysis and urine for organic acids, which are good screening tests for many of the above inborn errors of metabolism.

EEG burst suppression pattern should prompt sending plasma and urine amino acids specifically looking for S-sulfocysteine as an indication of MCD. Any plasma amino acid analysis should also be compared to simultaneously obtained CSF amino acid analysis. NKH will demonstrate an abnormally high ratio of CSF glycine compared to plasma glycine but be normal in MCD.

Plasma amino acids may reveal a number of inborn errors of metabolism but specifically request S-sulfocysteine looking for molybdenum cofactor deficiency (or less common subtype sulfite oxidase deficiency). Similarly, urine amino acids should cite the request to determine the presence of S-sulfocysteine. Its presence may then prompt other studies specifically examining for urine and/or plasma purines and/or sulfated compounds (i.e., total homocysteine, cysteine, homocysteine, etc.).

Elevated S-sulfocysteine (blood and urine) with decreased serum uric acid and elevated urine xanthine and hypoxanthine suggest molybdenum cofactor deficiency. Normal serum uric acid and normal urine xanthine and hypoxanthine with elevated S-sulfocysteine suggest sulfite oxidase deficiency. Followup DNA sequence testing for gene mutations may then confirm the diagnosis of MCD if a likely pathogenic mutation is identified.

If you are able to confirm that the patient has molybdenum cofactor deficiency, what treatment should be initiated?

Molybdenum cofactor deficiency (MOCD) with severe neonatal symptoms has very few therapeutic options and overall the outcome is usually poor. Many of the infants require intubation for supportive ventilation due to the seizure activity and/or poor mental status. For the most part, the approach is to inform parents and caregivers of the extremely poor prognosis and offer options of abstaining from aggressive resucitative medical interventions and/or withdrawing life supportive measures.

Most survivors have some combination of seizures, feeding difficulties, mental retardation and visual dysfunction, with lens dislocation in many. Anticonvulsant therapies are usually initiated by consulting pediatric neurologists but often are not entirely successful in the neonatal presentations. Some successes with vigabatrin have been reported. Dietary restriction of sulfur containing amino acids (cysteine and methionine) has been associated with positive outcome in two patients with mild sulfite oxidase deficiency and may be translatable to mild molybdenum cofactor deficiency (older presenting patients).

For later onset and milder cases, it is unclear what the outcomes may be, and treatment is individualized for severity of neurologic symptoms, including seizures, disorders of tone and movement. Dietary restriction is similarly adjusted based on ongoing laboratory surveillance of amino acids and levels of sulfocysteine.

Recently, an experimental therapeutic approach in one neonatal patient was reported. Daily injection of cyclic pyranopterin monophosphate (cPMP) in a patient with MOCD type A was reported to be successful. The infant became more alert within 2 days, took full bottle feeds within 1 week, demonstrated improvement of EEG by day 12, with decreased frequency but continued daily seizure/twitching events. At 18 months, the infant was reported to be thriving, clinically free of seizures but demonstrated signs of quadriplegic cerebral palsy with developmental delay.

If the efficacy of cPMP injections can be substantiated, immediate treatment of patients with likely diagnoses of this disorder will be encouraged. Otherwise, currently for neonatal presentations, no medical treatments that improve neurologic outcome have been demonstrated.

What are the adverse effects associated with each treatment option?

Individual adverse effects of anticonvulsants used must be specifically addressed, usually with direct input or counseling by the pediatric neurologist. Restricted dietary therapies may be difficult to adhere to and/or difficult to maintain due to availabilities of substitute food items and/or costs of specialized formulas. Various nutritional deficiencies may be incurred if not properly monitored. Close interaction with an experienced metabolic nutritionist is mandated.

Use of cPMP is experimental and will be available in only a few centers worldwide in the immediate future. Adverse effects will need to be determined following its use in several more patients (currently only one is reported).

What are the possible outcomes of molybdenum cofactor deficiency?

Inform the family of the likely extremely poor neurologic prognosis if neonatal presentation of condition. Most survivors have some combination of seizures, feeding difficulties, mental retardation, visual dysfunction with lens dislocation, and quadriplegic cerebral palsy type condition. This is usually in conjunction with severe defects observed in brain imaging.

These outcomes may change if cPMP therapy becomes approved and standardized, but that will be several years in the future. Be supportive of decisions not to resuscitate, and discuss possible option of withdrawal of life supporting measures. Most other current therapies may change the biochemical profiles without any effect on the severely poor neurologic outcome.

For later onset and milder cases, it is unclear what the outcomes may be. There are a few cases of positive neurologic development in mild sulfite oxidase deficiency and there may be similar few cases of mild molybdenum cofactor deficiency, but it is not clear that a true mild or partial form of molybdenum cofactor deficiency has been identified.

What causes this disease and how frequent is it?

Molybdenum cofactor deficiency (MOCD) is considered a rare but not extremely rare disorder (on the order of 100-200 patients reported worldwide). Estimates of incidence range from 1/1-200,000 but this is at best a rough guess. It may be an underreported condition of neonatal death and/or childhood developmental delay with or without epilepsy.

The pathology is caused by the inability of the patients to prepare molybdenum for its biological role as an enzyme cofactor. This results in deficiencies of three enzymatic activites, ie., sulfite oxidase, xanthine dehydrogenase, and aldehyde dehydrogenase. Probably deficiency of only the first two have clinical consequence, and deficiency of sulfite oxidase is by far more severe neurologically.

Mutations in the genes encoding the enzymes preparing molybdenum for cofactor activity are autosomal recessive. There are basically three steps encoded by what are referred to as complementation groups, MOCS 1, 2, and 3.

MOCS 1 and 2 are encoded by two separate gene regions, both encoding polycistronic mRNAs, and thus each is labeled as two separate gene sequences MOCS1A and MOCS1B, and MOCS2A and MOCS2B (molybdopterin synthase). The third complementation group C (MOCS3, molybdopterin synthase sulfurylase) is encoded by GEPH (gephyrin protein) which somehow functions to allow creation of the intact molybdenum/molybdopterin complex that inserts molybdenum into the three enzymes (XDH, SUOX, ALDH) for activity. This latter process in humans in poorly understood. Deficiencies in any of the three complementation groups produces the phenotype of molybdenum cofactor deficiency.

How do these pathogens/genes/exposures cause the disease?

The actual pathogenesis of the CNS disease is unclear although GEPH is described as a receptor clustering neuroprotein. It appears to be an essential membrane protein involved in postsynaptic localization of receptors for the neurotransmitters glycine and GABAA. Because of GEPH dysfunction from the influence of several potential mutations in the genes responsible for molybdenum cofactor biosynthesis, it would be expected to generate neurologic signs and symptoms as described for the MOCDs. This would also explain its ability to complement mutations earlier in the pathway.

What complications might you expect from the disease or treatment of the disease?

Described above

Are additional laboratory studies available; even some that are not widely available?

As discussed, DNA testing for mutations in the genes responsible for molybdenum cofactor deficiency (currently three groupings of MOCS1A/MOCS2A, MOCS1B/MOCS2B, and GEPH) may be available to aid diagnosis.

How can molybdenum cofactor deficiency be prevented?

Because the condition is due to mutations of autosomal recessive genes, risk of recurrence with the same parents would be predicted to be 25%. Prenatal diagnosis may be available if the specific familial DNA mutation is identified PRIOR to pregnancy. It may take several weeks to months to identify the mutation for clinical use at the current time. For that reason, DNA sequence analysis of probands in families should be obtained as early as possible. Prenatal analysis of amniotic fluid for S-sulfocyteine and amniotic cell enzymatic activity measurements of sulfite oxidase are also available. These tests, but especially DNA mutation analysis of amniocytes, allow the most effective prenatal counseling and appropriate discussion regarding reproductive options, including possible termination of affected fetuses.

Because of the initial success of the first patient with molybdenum cofactor deficiency to be experimentally treated with cPMP, this therapy may be available in the future to give to prenatally diagnosed patients at the time of birth to prevent or at least ameliorate the severe predicted outcome.

What is the evidence?

Veldman, A, Santamaria-Araujo, JA, Sollazzo, A, Pitt, J, Gianello, R, Yaplito-lee, J. "Successful Treatment of Molybdenum Cofactory Deficiency Type A with cPMP". Pediatrics. vol. 125. 2010. pp. e1249-e1254.

(The first report of a successful approach to treatment, still in the research therapeutics stage.)

Johnson, JL, Duran, M, Valle, D, Beaudet, A, Vogelstein, B, Kinzler, K, Antonarakis, T, Ballabio, A. "Chapter 128: Molybdenum Cofactor Deficiency and Isolated Sulfite Oxidase Deficiency, in Scriver’s OMMBD The Online Metabolic and Molecular Bases of Inherited Disease".

(A comprehensive chapter on the biochemical, molecular, clinical and genetic aspects of this set of disorders.)

Koeller, DM, Sarafoglou, K, Hoffmann, GF, Roth, KS. "Pediatric Endocrinology and Inborn Errors of Metabolism". 2009. pp. 674-676.

(An excellent short review of all clinical and biochemical aspects of molybdenum cofactor deficiency with useful tables for quick reference.)

(An excellent review of the clinical and genetic aspects of the disorder, including genetic testing for the disorder.)

Reiss, J, Gross-Hardt, S, Christensen, E, Schmidt, P, Mendel, RR, Schwarz, G. "A Mutation in the Gene for the Neurotransmitter Receptor-Clustering Protein Gephyrin Causes a Novel Form of Molybdenum Cofactor Deficiency". Am J Hum Genet. vol. 68. pp. 208-213.

(Includes an excellent review of the molecular basis and pathophysiology causing the clinical disease.)
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