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GAMT – Enzyme, Deficiency & Treatment

GAMT (guanidinoacetate methyltransferase) is an enzyme essential for creatine biosynthesis. A GAMT deficiency causes severe neurological impairment and intellectual disability.

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Things worth knowing about "GAMT"

GAMT (guanidinoacetate methyltransferase) is an enzyme essential for creatine biosynthesis. A GAMT deficiency causes severe neurological impairment and intellectual disability.

What is GAMT?

GAMT stands for guanidinoacetate methyltransferase, an enzyme that plays a central role in the biosynthesis of creatine in the human body. Creatine is a vital compound that serves as an energy buffer, particularly in muscle cells and the brain. The GAMT enzyme catalyzes the second and final step of creatine synthesis: the conversion of guanidinoacetate into creatine, using the methyl donor S-adenosylmethionine (SAM).

GAMT Deficiency

GAMT deficiency is a rare, autosomal recessive inherited metabolic disorder. It is caused by mutations in the GAMT gene on chromosome 19 and belongs to the group of creatine deficiency syndromes. Only a few hundred cases have been documented worldwide.

Causes

GAMT deficiency is caused by pathogenic variants (mutations) in the GAMT gene. Both parents must each pass on one defective copy of the gene for a child to be affected (autosomal recessive inheritance). As a result of the enzyme deficiency:

Guanidinoacetate cannot be converted into creatine and accumulates to toxic levels.
The body lacks sufficient creatine, which is essential for energy supply to the brain and muscles.

Symptoms

Symptoms of GAMT deficiency typically appear within the first months of life to the second year and include:

Intellectual disability (often severe)
Epilepsy (seizures, sometimes treatment-resistant)
Autistic behavior and behavioral problems
Muscular hypotonia (reduced muscle tone)
Speech and movement disorders
Elevated guanidinoacetate levels in urine and blood (key biochemical marker)

Diagnosis

Diagnosis involves a combination of investigations:

Metabolite analysis: Detection of elevated guanidinoacetate and reduced creatine levels in urine and plasma using mass spectrometry.
Brain MRI: Changes in the globus pallidus and a reduced creatine signal on magnetic resonance spectroscopy (MRS) are commonly observed.
Molecular genetic testing: Confirmation by identification of pathogenic variants in the GAMT gene.
Enzyme activity assay: Measurement of reduced GAMT activity in fibroblasts or liver tissue.
Newborn screening: In some countries, GAMT deficiency is included in expanded newborn screening programs.

Treatment

Early treatment is critical for improving outcomes. Therapy aims to restore creatine levels and reduce the toxic accumulation of guanidinoacetate:

Creatine supplementation: Oral administration of creatine monohydrate to replenish creatine in the brain and muscles.
Ornithine supplementation: Ornithine competes with arginine in the first step of guanidinoacetate synthesis, thereby reducing guanidinoacetate accumulation.
Arginine-restricted diet: A dietary intervention to further limit the production of guanidinoacetate.
Antiepileptic therapy: Antiepileptic medications may be used in addition to manage seizures.

When the condition is identified early -- ideally at the newborn stage -- and treated promptly, largely normal neurological development can be achieved.

References

Schulze A. - Creatine deficiency syndromes. In: Molecular and Cellular Biochemistry, 244(1-2):143-150, 2003. PubMed PMID: 12701824.
Stockler-Ipsiroglu S. et al. - Guanidinoacetate methyltransferase (GAMT) deficiency: outcomes in 48 individuals and recommendations for diagnosis, treatment and monitoring. Molecular Genetics and Metabolism, 111(1):16-25, 2014.
OMIM (Online Mendelian Inheritance in Man) - GAMT Deficiency, Entry #612736. Johns Hopkins University. Available at: https://omim.org/entry/612736

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