A Well Newborn With Elevated C5-OH: Is the Baby or the Mother Affected?
A term male newborn is well at discharge. Newborn screening at 36 hours shows C5-OH acylcarnitine 1.8 µmol/L (reference <0.8) and C5:1 0.2. Repeat filter card at 10 days: C5-OH 1.6. Urine organic acids show increased 3-hydroxyisovaleric acid and 3-methylcrotonylglycine; plasma free carnitine is 12 µmol/L (low). The infant is breastfeeding well and gaining weight. The mother, 29, is asymptomatic but recalls that her own “carnitine was low” during a prior pregnancy. The family asks whether the baby has a metabolic disease and whether treatment is urgent. The local laboratory can sequence MCCC1 and MCCC2 but results will take 3 weeks.
1. What is the differential diagnosis, and what is the first additional test you would order? Explain why the mother is central.
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The profile is classic for 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency, an autosomal recessive disorder caused by MCCC1 or MCCC2 variants. The differential includes other causes of elevated C5-OH, such as biotinidase deficiency, holocarboxylase synthetase deficiency, and maternal 3-MCC deficiency transferring metabolites across the placenta. The first additional test should be mother’s acylcarnitine profile and urine organic acids, because maternal 3-MCC deficiency is a well-recognized cause of a positive newborn screen in an unaffected infant. Many affected mothers are asymptomatic. If the mother has the same biochemical profile, the infant may be a carrier rather than affected, and the positive screen may reflect maternal metabolites. MCCC1/MCCC2 sequencing in both mother and infant will clarify genotype.
2. If the mother has the same biochemical profile, how does that change management?
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It changes the interpretation substantially. If the mother is biochemically and genetically affected with 3-MCC deficiency, the infant is an obligate carrier if the father is unaffected. The infant’s positive screen could then be due to placental transfer of maternal metabolites, and the infant may not require treatment. The infant should still have confirmatory testing, but invasive or urgent treatment should be avoided until the genotype is known. The mother needs counseling about her own condition, including potential carnitine supplementation, monitoring during future pregnancies, and family testing. Asymptomatic 3-MCC deficiency in adults is common and usually benign, so over-treatment should be avoided.
3. If the infant is confirmed affected, what is the treatment and follow-up?
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Management should be individualized because many children with 3-MCC deficiency remain asymptomatic. Avoid immediate severe protein restriction. Provide carnitine supplementation if free carnitine is low or symptoms develop, and monitor growth, development, and metabolic status. A leucine-restricted diet is reserved for symptomatic individuals or those with recurrent metabolic decompensation. Emergency protocols should be provided for illness, and the family should receive education about avoiding prolonged fasting. Follow-up should include a metabolic dietitian and genetic counseling. Over-treatment can cause growth faltering, so the plan should be reassessed as the child develops.