Newborn screening · Organic acid disorder
3-Methylcrotonyl-CoA Carboxylase Deficiency · 3-MCC
Frequently detected, often asymptomatic. The panel's overdiagnosis debate plays out here.
Description
3-Methylcrotonyl-CoA carboxylase deficiency, 3-MCC, is an autosomal recessive disorder of leucine catabolism. The enzyme 3-methylcrotonyl-CoA carboxylase is biotin-dependent and catalyzes the fourth step of leucine breakdown. The enzyme is a heterodimer of an alpha and a beta subunit encoded by MCCC1 on chromosome 3 and MCCC2 on chromosome 5. Pathogenic variants in either gene reduce or abolish enzyme activity. 3-methylcrotonyl-CoA and its derivatives accumulate, and 3-methylcrotonylglycine and 3-hydroxyisovaleric acid appear in urine.
The clinical phenotype is broad. Many people identified by newborn screening are asymptomatic through childhood and adulthood. A subset present in infancy or during catabolic illness with hypoglycemia, metabolic acidosis, hyperammonemia, vomiting, lethargy, and hypotonia. Some present later with chronic features such as developmental delay or muscle weakness. The wide phenotypic range, including a substantial fraction of biochemically affected people who never develop symptoms, is the central clinical feature.
Detection is by newborn screening on the dried blood spot, using tandem mass spectrometry to identify elevated 3-hydroxyisovalerylcarnitine, C5-OH. The same C5-OH signal flags isovaleric acidemia, holocarboxylase synthetase deficiency, biotinidase deficiency, HMG-CoA lyase deficiency, and beta-ketothiolase deficiency, and second-tier testing distinguishes them. Confirmation uses urine organic acids, plasma acylcarnitines, and MCCC1 or MCCC2 sequencing. Maternal 3-MCC is sometimes identified when an asymptomatic mother's elevated C5-OH crosses the placenta and shows on her newborn's screen; the affected person is the mother. Reported screen-detected incidence in expanded US and European newborn screening programs runs roughly 1 in 30,000 to 1 in 50,000 live births, higher than initial estimates before tandem mass spectrometry expanded the panel.
Treatments to date
There is no FDA-approved disease-specific therapy for 3-MCC. Standard of care is supportive. Symptomatic children are managed with avoidance of prolonged fasting, sick-day protocols providing intravenous dextrose during illness, and L-carnitine supplementation when secondary carnitine depletion is documented. A modest leucine or protein restriction is used in some centers for symptomatic children, and a metabolic dietitian supervises growth and protein adequacy.
Asymptomatic children identified by screening are followed without dietary intervention in many centers, with sick-day rules and clinical surveillance for the first years of life. The optimal long-term management of biochemically affected, clinically silent people is an open question in the metabolic literature, and centers vary in approach.
Acute decompensations, when they occur, are managed with intravenous dextrose, correction of acidosis, and ammonia scavengers if hyperammonemia is present. Biotin supplementation is not effective in 3-MCC because the enzymatic defect is in the carboxylase itself rather than in the cofactor pathway.