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MUT
methylmalonyl-CoA mutase
MUT is located on the short (p) arm of chromosome 6, at band 6p12.3. Arm ratio per GRCh38 - banding schematic.
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Overview
The MUT gene encodes methylmalonyl-CoA mutase, a mitochondrial enzyme that plays an essential role in intermediary metabolism. This enzyme catalyses the conversion of methylmalonyl-CoA to succinyl-CoA, a reaction necessary for the breakdown of specific amino acids (isoleucine, valine, methionine, threonine), odd-chain fatty acids, and cholesterol side chains. The reaction requires adenosylcobalamin, a form of vitamin B12, as a cofactor.
When both copies of the MUT gene carry pathogenic variants, the enzyme cannot function properly, causing methylmalonic acid and related toxic metabolites to accumulate in the blood and urine. This autosomal recessive condition, methylmalonic acidaemia, can present in infancy with life-threatening metabolic crises or later in childhood with developmental and neurological complications.
What the gene does
Methylmalonyl-CoA mutase is a homodimeric enzyme located in the mitochondrial matrix. Each subunit binds one molecule of adenosylcobalamin (AdoCbl), a vitamin B12 derivative that serves as a cofactor for the isomerisation reaction. The enzyme rearranges the carbon skeleton of methylmalonyl-CoA, shifting a carbonyl-CoA group from one carbon to an adjacent carbon to produce succinyl-CoA. Succinyl-CoA then enters the tricarboxylic acid cycle, contributing to cellular energy production.
The catalytic mechanism involves homolytic cleavage of the cobalt-carbon bond in AdoCbl, generating a highly reactive adenosyl radical. This radical abstracts a hydrogen atom from the substrate, initiating a series of rearrangements that ultimately yield succinyl-CoA. The enzyme's activity is crucial for processing metabolites derived from the degradation of branched-chain amino acids and propionate, which arises from gut bacterial fermentation and the oxidation of odd-chain fatty acids. Without functional methylmalonyl-CoA mutase, methylmalonic acid accumulates, disrupting acid-base balance and impairing mitochondrial function.
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Chromosome location
The MUT gene is located on the short arm of chromosome 6 at position 12.3 (6p12.3). The gene spans approximately 32 kilobases of genomic DNA and contains 13 exons. The coding sequence produces a mature protein of 750 amino acids after cleavage of a mitochondrial targeting sequence.
Protein structure
The methylmalonyl-CoA mutase protein comprises 750 amino acids organised into distinct functional regions. The N-terminal portion contains a mitochondrial targeting sequence that directs the nascent polypeptide to the mitochondrial matrix, where it is cleaved upon import. The C-terminal region includes a B12-binding domain (amino acids 614-746) that coordinates the adenosylcobalamin cofactor through interactions involving a histidine residue that forms a bond with the cobalt centre. This domain is essential for cofactor binding and stabilisation during the catalytic cycle. Proper folding and assembly of the homodimer are required for enzyme activity.
Key variants
Pathogenic variants in MUT are distributed across the gene and include missense, nonsense, frameshift, and splicing variants. Many variants cluster in regions encoding the B12-binding domain or the catalytic core, disrupting cofactor binding or substrate recognition. The severity of methylmalonic acidaemia often correlates with residual enzyme activity: variants that completely abolish function typically cause early-onset, severe disease, whereas those that retain partial activity may present with milder, later-onset forms.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Biallelic pathogenic variants in MUT cause methylmalonic acidaemia (MUT), an autosomal recessive disorder of organic acid metabolism. Affected individuals typically present in the neonatal period or early infancy with poor feeding, vomiting, lethargy, and metabolic acidosis. Acute decompensation can lead to hyperammonaemia, ketosis, and neurological complications including seizures and encephalopathy. Long-term complications may include developmental delay, chronic kidney disease, cardiomyopathy, and basal ganglia injury. Some forms respond partially to high-dose vitamin B12 supplementation, depending on the specific variants and their effect on cofactor binding.
Inheritance pattern
Conditions caused by pathogenic MUT variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
The MUT gene appears on multiple NHS Genomic Medicine Service gene panels, reflecting its clinical significance in metabolic and neurological conditions. It holds green (high-evidence) classification on the DDG2P panel, Hyperammonaemia panel, Intellectual disability panel, Ketotic hypoglycaemia panel, Likely inborn error of metabolism panel, Paediatric or syndromic cardiomyopathy panel, Structural basal ganglia disorders panel, and Undiagnosed metabolic disorders panel. This widespread inclusion underscores the gene's role in diverse presentations of inherited metabolic disease, guiding diagnostic testing for infants and children with unexplained metabolic crises or progressive neurological symptoms.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the difference between mut and cbl forms of methylmalonic acidaemia?
The mut form results from pathogenic variants in the MUT gene itself, directly impairing the methylmalonyl-CoA mutase enzyme. The cbl forms arise from variants in genes required for processing or transporting vitamin B12, preventing normal delivery of the adenosylcobalamin cofactor to an otherwise intact enzyme. Both produce elevated methylmalonic acid, but the underlying defects differ.
Can vitamin B12 supplementation help all individuals with MUT variants?
Some individuals with specific MUT variants show partial biochemical response to high-dose vitamin B12 (cobalamin) therapy, particularly those with residual enzyme activity that can be enhanced by increased cofactor availability. However, many mut variants abolish enzyme function entirely, rendering B12 supplementation ineffective. Response is determined by the nature of the genetic variants and must be assessed clinically.
Why is MUT included on cardiac and neurological gene panels?
Methylmalonic acidaemia can cause cardiomyopathy and injury to the basal ganglia, structures deep within the brain that coordinate movement and cognition. These complications arise from toxic metabolite accumulation and mitochondrial dysfunction. Inclusion on cardiac and neurological panels ensures MUT is considered when children present with these manifestations, even if metabolic disease is not initially suspected.