Molybdenum cofactor deficiency is a rare condition characterized by brain dysfunction (encephalopathy) that worsens over time. Babies with this condition appear normal at birth, but within a week they have difficulty feeding and develop seizures that do not improve with treatment (intractable seizures). Brain abnormalities, including deterioration (atrophy) of brain tissue, lead to severe developmental delay; affected individuals usually do not learn to sit unassisted or to speak. A small percentage of affected individuals have an exaggerated startle reaction (hyperekplexia) to unexpected stimuli such as loud noises. Other features of molybdenum cofactor deficiency can include a small head size (microcephaly) and facial features that are described as "coarse."

Tests reveal that affected individuals have high levels of chemicals called sulfite, S-sulfocysteine, xanthine, and hypoxanthine in the urine and low levels of a chemical called uric acid in the blood.

Because of the serious health problems caused by molybdenum cofactor deficiency, affected individuals usually do not survive past early childhood.

Molybdenum cofactor deficiency is caused by mutations in the MOCS1, MOCS2, or GPHN gene. There are three forms of the disorder, named types A, B, and C (or complementation groups A, B, and C). The forms have the same signs and symptoms but are distinguished by their genetic cause: MOCS1 gene mutations cause type A, MOCS2 gene mutations cause type B, and GPHN gene mutations cause type C. The proteins produced from each of these genes are involved in the formation (biosynthesis) of a molecule called molybdenum cofactor. Molybdenum cofactor, which contains the element molybdenum, is essential to the function of several enzymes. These enzymes help break down (metabolize) different substances in the body, some of which are toxic if not metabolized.

Mutations in the MOCS1, MOCS2, or GPHN gene reduce or eliminate the function of the associated protein, which impairs molybdenum cofactor biosynthesis. Without the cofactor, the metabolic enzymes that rely on it cannot function.

The resulting loss of enzyme activity leads to buildup of certain chemicals, including sulfite, S-sulfocysteine, xanthine, and hypoxanthine (which can be identified in urine), and low levels of uric acid in the blood. Sulfite, which is normally broken down by one of the molybdenum cofactor-dependent enzymes, is toxic, especially to the brain. Researchers suggest that damage caused by the abnormally high levels of sulfite (and possibly other chemicals) leads to encephalopathy, seizures, and the other features of molybdenum cofactor deficiency.

Normal Function

The MOCS1 gene provides instructions for making two different proteins, MOCS1A and MOCS1B. Both are involved in the formation (biosynthesis) of a molecule called molybdenum cofactor. Specifically, MOCS1A and MOCS1B perform the first of a series of reactions that produce the cofactor, although the function of MOCS1B in this process is not understood. Molybdenum cofactor, which contains the element molybdenum, is essential to the function of several enzymes called sulfite oxidase, aldehyde oxidase, xanthine dehydrogenase, and mitochondrial amidoxime reducing component (mARC). These enzymes help break down (metabolize) different substances in the body, some of which are toxic if not metabolized.

Health Conditions Related to Genetic Changes

Molybdenum cofactor deficiency

MOCS1 gene mutations cause a disorder called molybdenum cofactor deficiency. This disorder is characterized by seizures that begin early in life and brain dysfunction that worsens over time (encephalopathy); the condition is usually fatal by early childhood. At least 32 mutations in the MOCS1 gene have been found to cause a form of the disorder designated type A or complementation group A. This is the most common form of the condition, accounting for approximately two-thirds of cases.

The MOCS1 gene mutations involved in molybdenum cofactor deficiency likely eliminate the function of MOCS1A, MOCS1B, or both, although in rare cases that are less severe, some protein function may remain. Without the activity of one or both of these proteins, molybdenum cofactor biosynthesis is impaired. Loss of the cofactor impedes the function of the metabolic enzymes that rely on it.

The resulting loss of enzyme activity leads to buildup of certain chemicals, including sulfite, S-sulfocysteine, xanthine, and hypoxanthine, and low levels of another chemical called uric acid. (Testing for these chemicals can help in the diagnosis of this condition.) Sulfite, which is normally broken down by sulfite oxidase, is toxic, especially to the brain. Researchers suggest that damage caused by the abnormally high levels of sulfite (and possibly other chemicals) leads to encephalopathy, seizures, and the other features of molybdenum cofactor deficiency.

Other Names for This Gene

• cell migration-inducing gene 11 protein
• MIG11
• migration-inducing gene 11 protein
• MOCOD
• MOCODA
• MOCS1A enzyme
• molybdenum cofactor biosynthesis protein 1
• molybdenum cofactor biosynthesis protein A
• molybdenum cofactor synthesis-step 1 protein A-B

Genomic Location

Normal Function

The MOCS2 gene provides instructions for making two different proteins, MOCS2A and MOCS2B, which combine to form an enzyme called molybdopterin synthase. Molybdopterin synthase performs the second of a series of reactions in the formation (biosynthesis) of a molecule called molybdenum cofactor. Molybdenum cofactor, which contains the element molybdenum, is essential to the function of several enzymes called sulfite oxidase, aldehyde oxidase, xanthine dehydrogenase, and mitochondrial amidoxime reducing component (mARC). These enzymes help break down (metabolize) different substances in the body, some of which are toxic if not metabolized.

Health Conditions Related to Genetic Changes

Molybdenum cofactor deficiency

MOCS2 gene mutations cause a disorder called molybdenum cofactor deficiency. This disorder is characterized by seizures that begin early in life and brain dysfunction that worsens over time (encephalopathy); the condition is usually fatal by early childhood. At least a dozen mutations in the MOCS2 gene have been found to cause a form of the disorder designated type B or complementation group B.

The MOCS2 gene mutations involved in molybdenum cofactor deficiency likely eliminate the function of MOCS2A, MOCS2B, or both, although in rare cases that are less severe, some protein function may remain. Without either piece of molybdopterin synthase, molybdenum cofactor biosynthesis is impaired. Loss of the cofactor impedes the function of the metabolic enzymes that rely on it.

The resulting loss of enzyme activity leads to buildup of certain chemicals, including sulfite, S-sulfocysteine, xanthine, and hypoxanthine, and low levels of another chemical called uric acid. (Testing for these chemicals can help in the diagnosis of this condition.) Sulfite, which is normally broken down by sulfite oxidase, is toxic, especially to the brain. Researchers suggest that damage caused by the abnormally high levels of sulfite (and possibly other chemicals) leads to encephalopathy, seizures, and the other features of molybdenum cofactor deficiency.

Other Names for This Gene

• MCBPE
• MOCO1
• MOCODB
• molybdenum cofactor biosynthesis protein E
• molybdopterin synthase catalytic subunit large subunit MOCS2B
• molybdopterin synthase small and large subunit
• molybdopterin synthase sulfur carrier subunit
• molybdopterin synthase sulfur carrier subunit small subunit MOCS2A
• MPTS

Genomic Location

Normal Function

The GPHN gene provides instructions for making a protein called gephyrin, which has two major functions in the body: the protein aids in the formation (biosynthesis) of a molecule called molybdenum cofactor, and it also plays a role in communication between nerve cells (neurons).

Gephyrin performs the final two steps in molybdenum cofactor biosynthesis. Molybdenum cofactor, which contains the element molybdenum, is essential to the function of several enzymes called sulfite oxidase, aldehyde oxidase, xanthine dehydrogenase, and mitochondrial amidoxime reducing component (mARC). These enzymes help break down (metabolize) different substances in the body, some of which are toxic if not metabolized.

Gephyrin also plays an important role in neurons. Communication between neurons depends on chemicals called neurotransmitters. To relay signals, a neuron releases neurotransmitters, which attach to receptor proteins on neighboring neurons. Gephyrin anchors certain receptor proteins to the correct location in neurons so that the receptors can receive the signals relayed by neurotransmitters.

Health Conditions Related to Genetic Changes

Molybdenum cofactor deficiency

GPHN gene mutations cause a disorder called molybdenum cofactor deficiency. This disorder is characterized by seizures that begin early in life and brain dysfunction that worsens over time (encephalopathy); the condition is usually fatal by early childhood. At least two mutations in the GPHN gene have been found to cause a form of the disorder designated type C or complementation group C. This is the rarest form of the condition, affecting only a small number of individuals.

The GPHN gene mutations involved in molybdenum cofactor deficiency likely reduce or eliminate the function of gephyrin. The known mutations impair gephyrin's ability to perform one or both of the final two steps of molybdenum cofactor biosynthesis. Without the cofactor, the metabolic enzymes that rely on it cannot function.

The resulting loss of enzyme activity leads to buildup of certain chemicals, including sulfite, S-sulfocysteine, xanthine, and hypoxanthine, and low levels of another chemical called uric acid. (Testing for these chemicals can help in the diagnosis of this condition.) Sulfite, which is normally broken down by sulfite oxidase, is toxic, especially to the brain. Researchers suggest that damage caused by the abnormally high levels of sulfite (and possibly other chemicals) leads to encephalopathy, seizures, and the other features of molybdenum cofactor deficiency.

Other Names for This Gene

• GEPH
• gephyrin isoform 1
• gephyrin isoform 2
• GPH
• GPHRYN
• HKPX1
• KIAA1385
• MOCODC

Genomic Location

Molybdenum cofactor deficiency has an autosomal recessive pattern of inheritance, which means both copies of the gene in each cell have mutations. An affected individual usually inherits one altered copy of the gene from each parent. Parents of an individual with an autosomal recessive condition typically do not show signs and symptoms of the condition.

At least one individual with molybdenum cofactor deficiency inherited two mutated copies of the MOCS1 gene through a mechanism called uniparental isodisomy. In this case, an error occurred during the formation of egg or sperm cells, and the child received two copies of the mutated gene from one parent instead of one copy from each parent.

Molybdenum cofactor deficiency is a rare condition that is estimated to occur in 1 in 100,000 to 200,000 newborns worldwide. More than 100 cases have been reported in the medical literature, although it is thought that the condition is underdiagnosed, so the number of affected individuals may be higher.