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3-methylcrotonyl-CoA carboxylase deficiency
This condition prevents the body from breaking down a specific component of leucine, leading to a build-up of harmful substances. It can affect individuals from infancy, with varying severity.
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Overview
3-methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is a rare genetic condition that affects how the body uses a building block of protein called leucine. Leucine is one of the essential amino acids, meaning it must come from the diet. In individuals with 3-MCCD, the body lacks an enzyme called 3-methylcrotonyl-CoA carboxylase, which is vital for breaking down leucine effectively [PMID:17907261].
When this enzyme is missing or not working correctly, certain toxic by-products can accumulate in the body. The severity of 3-MCCD can vary significantly, ranging from severe symptoms appearing in infancy to milder forms that may not cause noticeable problems until adulthood, or even remain undetected.
Approximately 1 in 36,000 newborns are affected by 3-MCCD, although this can vary between populations. Newborn screening programmes in the UK often include testing for 3-MCCD, allowing for early identification and intervention.
Symptoms & clinical features
The symptoms of 3-MCCD can be quite diverse and depend on when the condition is diagnosed and its severity. In some severe cases, symptoms can appear shortly after birth or within the first few months of life. These may include poor feeding, vomiting, reduced muscle tone (hypotonia), and sluggishness or lethargy [PMID:19183492].
If left untreated, severe symptoms can progress to developmental delay, seizures, and a weak immune system. Some individuals may experience metabolic crises, which are dangerous episodes triggered by illness, fasting, or increased protein intake. During a metabolic crisis, symptoms such as extreme tiredness, low blood sugar (hypoglycaemia), and high levels of acid in the blood (metabolic acidosis) can occur and require urgent medical attention.
However, many individuals identified through newborn screening may never develop symptoms. Others might experience milder symptoms later in childhood or adulthood, often triggered by periods of stress or illness. These can include muscle weakness or fatigue.
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Affected organs
3-MCCD primarily affects the metabolic processes within the body's cells, particularly in organs that have high energy demands or are involved in detoxification. The brain is particularly vulnerable to the build-up of toxic substances, which can lead to neurological problems such as developmental delay, intellectual disability, and seizures in severe, untreated cases.
The liver also plays a crucial role in metabolism, and its function can be affected during metabolic crises. The heart muscle might also be affected, leading to issues with heart function in some individuals. Generally, a range of body systems can be impacted due to the systemic nature of metabolic imbalances.
Risks & severity
The severity of 3-MCCD varies considerably, making it challenging to predict the exact course of the condition for an individual. Some people with a genetic change causing 3-MCCD remain asymptomatic throughout their lives, never experiencing any health problems related to the condition. This means they are often only identified through newborn screening or family screening after a relative is diagnosed.
However, for those who are symptomatic, the condition can range from mild, intermittent issues to severe, life-threatening metabolic crises in infancy. These crises can lead to serious neurological damage if not promptly managed. The lifetime risk of developing severe symptoms is significantly reduced with early diagnosis and adherence to management strategies, particularly dietary modifications. The age of onset for symptoms is also variable, from early infancy to adulthood, or even never.
Genetic causes
3-MCCD is caused by pathogenic variants in either the MCCC1 or MCCC2 gene. These genes contain instructions for making the two different protein subunits that combine to form the 3-methylcrotonyl-CoA carboxylase enzyme [PMID:17907261]. This enzyme is essential for processing leucine, an amino acid obtained from protein in the diet.
When there are disruptive changes in MCCC1 or MCCC2, the enzyme does not work correctly or is not produced in sufficient amounts. This prevents the normal breakdown of 3-methylcrotonyl-CoA, an intermediate product of leucine metabolism. As a result, 3-methylcrotonyl-CoA and other related compounds can build up in the body, leading to the health problems associated with 3-MCCD. The specific variant in the MCCC1 or MCCC2 gene can influence the enzyme's residual activity, contributing to the variable severity observed.
- MCCC1 methylcrotonyl-CoA carboxylase subunit 1The *MCCC1* gene provides instructions for a crucial enzyme subunit involved in the breakdown of amino acids, particularly leucine, within the mitochondria.
- MCCC2 methylcrotonyl-CoA carboxylase subunit 2The MCCC2 gene provides instructions for a component of the 3-methylcrotonyl-CoA carboxylase enzyme, which is crucial for processing the amino acid leucine within the body.
Inheritance pattern
3-methylcrotonyl-CoA carboxylase deficiency is an autosomal recessive condition. This means that an individual must inherit two altered copies of either the MCCC1 or MCCC2 gene - one from each parent - to have the condition.
People who inherit only one altered copy of the gene are known as carriers. Carriers typically do not show symptoms of 3-MCCD themselves because their one functional copy of the gene provides enough enzyme activity. However, if two carriers of the same altered gene have children together, there is a 1 in 4 (25%) chance with each pregnancy that their child will inherit two altered copies and thus be affected by 3-MCCD. There is also a 2 in 4 (50%) chance that the child will be a carrier like the parents, and a 1 in 4 (25%) chance that the child will inherit two unaffected copies and not be a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis of 3-MCCD often begins with newborn screening, which is a routine test offered to all babies in the UK. This screening identifies abnormal levels of certain metabolites, such as 3-hydroxyisovaleric acid and 3-methylcrotonoylglycine, in blood samples taken shortly after birth [PMID:19183492].
If positive, further diagnostic tests are typically performed to confirm 3-MCCD. These include urine organic acid analysis and acylcarnitine profiling in blood. A definitive diagnosis is made through genetic testing, which looks for pathogenic variants in the MCCC1 and MCCC2 genes. This genetic testing falls under the NHS Genomic Medicine Service and can be requested by a clinical geneticist or other specialist clinicians using the appropriate R-codes, such as those for inherited metabolic disorders.
Referral to a specialist metabolic team, often via a clinical geneticist, is generally recommended for confirmation and ongoing management.
Management & lifestyle
Management for individuals with 3-MCCD primarily focuses on preventing metabolic crises and ensuring proper nutrition. A key part of this is a special low-leucine diet, which limits the amount of protein an individual consumes. This reduces the build-up of toxic by-products. In some cases, carnitine supplementation may also be recommended, as carnitine helps remove some of the accumulating harmful substances from the body.
Regular monitoring by a specialist metabolic team, including dieticians, is essential to adjust dietary plans as the individual grows and to ensure overall health. Individuals and families are also provided with an 'emergency plan' to follow during illnesses or periods of fasting, as these can trigger metabolic crises. This plan usually involves increasing fluid and carbohydrate intake to prevent the breakdown of body proteins. Adhering to these management strategies, especially for symptomatic individuals, can significantly improve outcomes and prevent serious complications.
Access to management strategies is typically facilitated through specialist metabolic centres within the NHS, often following a referral from clinical genetics.
UK care pathway
In the UK, individuals suspected of having or diagnosed with 3-MCCD are typically managed through the NHS Genomic Medicine Service. If identified through newborn screening, the family will be referred to a specialist metabolic team at an NHS centre. A clinical geneticist, or other specialist, might request genetic testing for MCCC1 and MCCC2 variants, which currently falls within the scope of R-codes for inherited metabolic disorders on the NHS National Genomic Test Directory.
Following diagnosis, care is usually coordinated by a multidisciplinary team, including metabolic consultants, dietitians, and genetic counsellors. Genetic counsellors can provide valuable information about the inheritance pattern, implications for other family members, and support for family planning.
Frequently asked questions
What is leucine, and why is it important in 3-MCCD?
Leucine is an essential amino acid, a building block of protein that the body cannot make itself and must obtain from food. In 3-MCCD, the body has difficulty breaking down leucine because a specific enzyme (3-methylcrotonyl-CoA carboxylase) isn't working correctly, leading to a build-up of harmful substances.
Can 3-MCCD be cured?
There is no cure for 3-MCCD, but it can be effectively managed. The primary treatment involves a special low-leucine diet and, in some cases, carnitine supplements. With careful management, many individuals identified early can live healthy lives with minimal or no symptoms.
If I am a carrier, will I develop symptoms of 3-MCCD?
No, carriers of 3-MCCD typically do not develop the condition's symptoms. Carriers have one functional copy of the relevant gene, which is usually enough for the body to produce sufficient enzyme for normal function. However, carriers can pass the gene variant on to their children.
Is 3-MCCD included in routine newborn screening in the UK?
Yes, 3-MCCD is part of the routine newborn bloodspot screening programme in the UK. This allows for early detection and intervention, which is crucial for preventing severe symptoms and improving long-term outcomes for affected babies.
What should I do if my child with 3-MCCD gets ill or cannot eat?
If your child with 3-MCCD becomes unwell or cannot eat, it's very important to follow a pre-arranged emergency plan provided by your metabolic team. This usually involves increasing their intake of carbohydrate drinks or other energy sources to prevent a metabolic crisis. Always contact your metabolic team or seek urgent medical advice.
References
- Grünert SC, Stucki M, Morscher RJ. 3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals. Orphanet journal of rare diseases. 2012. PMID: 22642865
- Rips J, Almashanu S, Mandel H. Primary and maternal 3-methylcrotonyl-CoA carboxylase deficiency: insights from the Israel newborn screening program. Journal of inherited metabolic disease. 2016. PMID: 26566957
- Şahin S, Yıldırım M, Bektaş Ö. Intracranial Calcification Associated with 3-Methylcrotonyl-CoA Carboxylase Deficiency. Molecular syndromology. 2021. PMID: 34899149
- Wilcken B. 3-Methylcrotonyl-CoA carboxylase deficiency: to screen or not to screen? Journal of inherited metabolic disease. 2016. PMID: 26660660
- Lin Y, Lin C, Zheng Z. Large-scale newborn screening for organic acidemias in Quanzhou, China: a 10-year retrospective observational study. Scientific reports. 2025. PMID: 40835664
- Arnold GL, Koeberl DD, Matern D. A Delphi-based consensus clinical practice protocol for the diagnosis and management of 3-methylcrotonyl CoA carboxylase deficiency. Molecular genetics and metabolism. 2008. PMID: 18155630
- Lin W, Wang K, Chen Y. Newborn screening and genetic diagnosis of 3-methylcrotonyl-CoA carboxylase deficiency in Quanzhou,China. Molecular genetics and metabolism reports. 2024. PMID: 39188588
- Cheng Y, Chen P, Yu Z. Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China. Clinica chimica acta; international journal of clinical chemistry. 2023. PMID: 36822454