3-Methylcrotonyl-CoA carboxylase deficiency (3-MCC)
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A newborn screen flags an elevated C5-hydroxyacylcarnitine (C5-OH). The baby is feeding well and examining normally. Confirmatory testing on the infant is equivocal, and testing the mother shows the same biochemical abnormality: she has 3-MCC deficiency herself, has always been asymptomatic, and was identified only because her baby was screened.
- Autosomal recessive, MCCC1 and MCCC2, the two subunits of a biotin-dependent carboxylase in leucine catabolism
- One of the most frequently detected disorders on expanded newborn screening, at roughly 1 in 30,000 to 1 in 50,000
- The great majority of people identified are and remain asymptomatic. This is the defining fact about the condition and the reason it is counselled differently from other organic acidemias
- Usually no features at all. Most are ascertained through newborn screening or, as above, through their own child's screen
- A minority decompensate under catabolic stress, typically an intercurrent illness with poor intake, with hypoglycemia, metabolic acidosis, hyperammonemia, vomiting and lethargy
- Reported in a small subset: developmental delay, seizures, hypotonia, failure to thrive. Whether these are caused by the enzyme deficiency or are coincidental in an ascertained population remains genuinely unsettled
- Maternal 3-MCC deficiency is a recognised cause of a positive newborn screen in an unaffected baby, because maternal metabolites cross the placenta
- Elevated C5-OH acylcarnitine on newborn screening
- Urine organic acids: 3-hydroxyisovaleric acid and 3-methylcrotonylglycine, the specific marker
- Plasma free carnitine is often low
- Confirm with MCCC1 and MCCC2 sequencing
- If the infant's confirmatory testing is normal or equivocal, test the mother
The C5-OH differential, which is the real teaching point
An elevated C5-OH is not specific, and two of its causes are treatable in a way 3-MCC is not:
| Cause | Distinguishing feature |
|---|---|
| 3-MCC deficiency | 3-methylcrotonylglycine; usually benign |
| Biotinidase deficiency | Low serum biotinidase enzyme activity; treat with biotin |
| Holocarboxylase synthetase deficiency | Neonatal onset, normal biotinidase activity; treat with biotin |
| 3-methylglutaconic aciduria | 3-methylglutaconic and 3-methylglutaric acids |
| HMG-CoA lyase deficiency | 3-hydroxy-3-methylglutaric acid; hypoketotic hypoglycemia |
| Beta-ketothiolase deficiency | 2-methyl-3-hydroxybutyrate, tiglylglycine |
- Avoid prolonged fasting, with an age-appropriate fasting limit and a written sick-day plan
- Emergency regimen with intravenous dextrose during illness, which is the intervention that actually prevents the rare crises
- Carnitine supplementation when free carnitine is low
- Leucine restriction is not routinely needed and is reserved for the symptomatic minority
- Counsel the family honestly: most individuals with this diagnosis never have a metabolic event, and the label carries a risk of unnecessary alarm
- Work down the C5-OH differential before settling on 3-MCC, because biotinidase and holocarboxylase synthetase deficiency are both biotin-responsive and both cause permanent harm if missed. 3-MCC is the common answer, but it is the one that matters least.
- A positive newborn screen with a well baby and equivocal confirmation should prompt testing the mother. Maternal 3-MCC deficiency is the classic explanation and is frequently how an asymptomatic adult is diagnosed.
- This is the cleanest example in newborn screening of a condition where detection outruns clinical significance, which is worth saying plainly to families.