3-Methylcrotonyl-CoA carboxylase deficiency is an organic aciduria caused by the deficiency of 3-methylcrotonyl-CoA carboxylase enzyme (3-MCC) that plays an important role in the metabolism of leucine. Patients with this enzyme defect appear normal at birth, with the first signs and symptoms of the condition appearing in infancy or early childhood. These signs include poor appetite, lethargy, extreme sleepiness, irritable mood, hypotonia, nausea, vomiting, feeding difficulties, and behavior changes. If left untreated, the condition can progress to delayed development, breathing problems, seizures, liver failure, and coma, sometimes even leading to death. The severity of the symptoms differs from patient to patient. Some children with the enzyme deficiency remain asymptomatic, and do not require any treatment or management.
3-Methylcrotonyl-CoA carboxylase deficiency type I (MCC1D) is caused by mutations in MCCC1 gene, which encodes the alpha subunit of 3-MCC enzyme.