HomeGeneticsCondition3-methylcrotonyl-CoA carboxylase deficiency
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3-methylcrotonyl-CoA carboxylase deficiency
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

3-methylcrotonyl-CoA carboxylase deficiency

3-methylcrotonyl-CoA carboxylase deficiency (MCC deficiency) is an inherited metabolic disorder preventing the body from properly processing the amino acid leucine, with variable symptom presentation.

Overview

3-methylcrotonyl-CoA carboxylase deficiency (also called MCC deficiency) is an inherited disorder in which the body is unable to process certain proteins. People with this disorder have a shortage of an enzyme that helps break down proteins that contain a particular building block (amino acid) called leucine.

The signs and symptoms of MCC deficiency can vary among individuals, even among individuals in the same family. Some people with the genetic changes that cause MCC deficiency will not develop symptoms until adulthood, while many will never develop signs or symptoms.

Some affected individuals develop signs and symptoms in infancy or early childhood after an event such as an infection, a long period without food, or the introduction of a high-protein diet. Features of MCC deficiency may include feeding difficulties, delayed development, vomiting, excessive tiredness (lethargy), and weak muscle tone (hypotonia). If untreated, MCC deficiency can lead to seizures; breathing difficulties; and comas, which can be life-threatening.

Frequency

As many as 1 in 36,000 newborns may receive a diagnosis of MCC deficiency.

Causes

Variants (also called mutations) in the MCCC1 or MCCC2 gene cause 3-methylcrotonyl-CoA carboxylase deficiency. The condition is sometimes referred to as 3-methylcrotonyl-CoA carboxylase 1 deficiency or 3-methylcrotonyl-CoA carboxylase 2 deficiency depending on the specific gene that is affected. These two genes provide instructions for making different parts (subunits) of an enzyme called 3-methylcrotonyl-CoA carboxylase (MCC). This enzyme plays a critical role in breaking down proteins obtained from food. Specifically, MCC is responsible for the fourth step in the breakdown of leucine, an essential amino acid that is part of many proteins.

Variants in the MCCC1 or MCCC2 gene reduce or eliminate the activity of MCC, preventing the body from processing leucine properly. In some people, these variants do not cause signs or symptoms. In others, toxic byproducts of leucine processing build up to harmful levels, which can damage the nervous system. This damage underlies the signs and symptoms of MCC deficiency.

Learn More About the Associated Genes

  • MCCC1
  • MCCC2

Inheritance

This condition is inherited in an autosomal recessive pattern, which means both copies of either the MCCC1 gene or the MCCC2 gene in each cell must have a variant to cause the disorder. These gene variants increase the risk of developing the signs and symptoms of MCC deficiency. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Other Names

  • 3-MCC deficiency
  • 3-methylcrotonylglycinuria
  • BMCC deficiency
  • Deficiency of methylcrotonoyl-CoA carboxylase
  • MCC deficiency
  • MCCD
  • Methylcrotonyl-CoA carboxylase deficiency
  • Methylcrotonyl-coenzyme A carboxylase deficiency

Frequently Asked Questions

Parents should seek immediate medical attention if an infant or child with MCC deficiency shows signs of severe lethargy, persistent vomiting, poor feeding, unusual irritability, seizures, or difficulty breathing. These can indicate a metabolic crisis that, if untreated, can lead to coma and be life-threatening.
MCC deficiency is inherited in an autosomal recessive pattern. This means a child must inherit two copies of a mutated MCCC1 or MCCC2 gene (one from each parent) to develop the condition. If both parents are carriers, there is a 25% chance with each pregnancy that their child will have MCC deficiency, a 50% chance the child will be a carrier, and a 25% chance the child will not have the condition and not be a carrier. Genetic counseling can help families understand their specific risks.
MCC deficiency is often detected through newborn screening programs, which test for elevated levels of specific metabolites in the blood. If the screening is positive, further diagnostic tests like urine organic acid analysis and genetic testing for MCCC1 or MCCC2 gene variants are performed to confirm the diagnosis.
Management primarily involves a special diet that restricts the intake of leucine, the amino acid the body struggles to process. This usually means a low-protein diet, often supplemented with medical foods or formulas that are leucine-free. Regular monitoring by a metabolic specialist and dietitian is crucial to ensure proper nutrition and prevent metabolic crises.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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