A 5-day-old male infant with an increased dried blood spot propionylcarnitine (C3-carnitine) value of 7.93 μmol/L (cutoff <6.79 μmol/L) was identified by the New Jersey state newborn screening program. C3-carnitine is used as a screening tool for methylmalonic and propionic acidemias, potentially fatal but treatable inborn errors of metabolism. The initial screen values provided a calculated C3:C2 carnitine ratio of 0.23 (cutoff <0.32, mean 0.074) and a C3:C16 ratio of 2.51 (cutoff <4.16, mean 0.96). The child was an inpatient at an outlying neonatal intensive care unit. He was born at 35 weeks estimated gestational age, required continuous positive airway pressure for a short time after birth, and transitioned quickly to room air. He was taking regular feedings with a cow’s milk protein–based formula.
On day of life 6, the patient developed a mild acidosis (pH 7.24 on arterial blood gas testing). Because methylmalonic and propionic acidemia could not be excluded while confirmatory test results were pending, feedings were discontinued, intravenous hydration with glucose-containing fluids was initiated, and the infant was transferred to our institution. On arrival the child appeared well, was alert, and had normal growth parameters and no tachypnea. He had good tone and normal reflexes, and laboratory studies showed no acidosis. We allowed normal feedings and proceeded with the diagnostic evaluation.
Although C3-carnitine appears in the blood, the active metabolite within the mitochondrion is propionyl-CoA. Propionyl-CoA is an intermediate in the degradation of several amino acids. It can also appear as an intermediate of odd-chain fatty acid metabolism and exogenously as a derivative of propionate that is generated by gastrointestinal flora. Normally propionyl-CoA is metabolized to methylmalonyl-CoA by the action of propionyl-CoA carboxylase (PCC), but if the metabolite is in excess the propionyl species is released from the mitochondrion after conversion by carnitine palmitoyl transferase II to the corresponding acylcarnitine (Fig. 1⇓ ).
The differential diagnosis for increased C3-carnitine in a newborn includes inborn errors of metabolism, vitamin B12–deficiency, and false-positive results (1). The associated inborn errors of metabolism include PCC defects that cause propionic acidemia. Children with this condition potentially have the greatest elevations in C3-carnitine because of an immediate backup of metabolic flux resulting in increased concentrations of propionyl-CoA. Defects in processing of the cofactor for PCC, biotin, could in theory lead to C3-carnitine elevation, but isolated elevations of C3-carnitine in patients with biotinidase deficiency or holocarboxylase synthetase deficiency have not been reported.
The largest group of defects associated with C3-carnitine elevation involves the downstream enzyme methylmalonyl-CoA mutase (MMM). MMM converts methylmalonyl-CoA to succinyl-CoA, an intermediate in the Krebs cycle. This enzyme is one of 2 in the body that uses vitamin B12 as a cofactor. C3-carnitine is the newborn screen metabolite used for detection of MMM deficiency (known as methylmalonic acidemia) because C4-dicarboxylcarnitine elevations (MMA-carnitine or succinyl-carnitine) are not consistently or sufficiently increased to enable differentiation of patients from those who are unaffected; the backup to propionyl-CoA and C3-carnitine is more readily detectable. A broad variety of defects in vitamin B12 processing, known as cobalaminopathies, can lead to disorders with a biochemical overlap with methylmalonic acidemia.
Maternal vitamin B12 deficiency, and vertical transmission of this deficiency, is a known cause of C3-carnitine elevation (2)(3). This defect is not isolated to the newborn period; breast-fed infants of vegan mothers with B12 deficiency have been reported with neurological impairment and methylmalonate excretion (4). Commercially available formulas and term breast-milk from mothers with normal B12 metabolism have adequate concentrations of B12 to avoid such complications.
The use of diagnostic laboratory evaluations can help to differentiate the causes of C3-carnitine elevations (Table 1⇓ ). The acylcarnitine profile of our patient on day of life 5 did not detect C3-acylcarnitine, nor did a repeat acylcarnitine analysis on day of life 6. There was no increased methylmalonate, the diagnostic species of methylmalonic acidemia, in urine organic acids measured by GC-MS. Homocysteine, often increased in B12 deficiency and some defects of cobalamin metabolism, was not detected. Methylcitrate and 3-hydroxy propionate, additional markers of propionic acidemia, were absent from urine organic acids, effectively ruling out defects in PCC. A serum B12 concentration obtained on day of life 5 was in the lower end of the normal range (3300 pg/L, normal reference interval 2930–12080 pg/L).
On further review of the history, we learned that the mother was diagnosed with anemia during the pregnancy. On closer questioning, she disclosed that 3 years before the pregnancy she had undergone gastric bypass for the purpose of weight loss. She could not recall having received supplemental vitamin B12 following the procedure.
The diagnosis was vitamin B12 deficiency due to maternal vitamin B12 deficiency after gastric bypass.
resolution of case
Vertical transmission of vitamin B12 deficiency caused a metabolic disturbance in the child on the second day of life, at the time of newborn screening. The B12 deficiency in our patient, however, had been corrected by the time of transfer to our institution. After 5 days of enteral feeding, provision of dietary vitamin B12 in infant formula had corrected both the intramitochondrial defect (as determined by C3-carnitine concentrations) and plasma concentrations of B12. The cause of the patient’s transient acidosis on day of life 6 was unclear, but metabolic studies confirmed that it was unrelated to defects in branched-chain amino acid metabolism. The mother was told to inform her primary care physician that she had B12 deficiency and would require lifelong supplementation.
Newborn screening is designed to detect inborn errors of metabolism in a time course that improves prospects for treatment and survival. Newborn screening was initially developed for detection of phenylketonuria, but has expanded in most states to include other conditions such as fatty acid oxidation defects, amino acidopathies, and organic acidopathies. Increased C3-carnitine presents a diagnostic challenge because of the wide range of possible causes, including false-positive results, vitamin deficiency, and life-threatening disorders such as methylmalonic or propionic acidopathy. Severe disease was unlikely in this case, because the typical infant with methylmalonic or propionic acidopathy presents with dehydration, moderate hepatomegaly, increased ammonia, ketoacidosis, poor feeding, drowsiness, axial hypotonia, and limb hypertonia.
Detection of maternal pathology (in this case B12 deficiency) through the newborn screen is not unique to this condition. Elevations in C5-OH acylcarnitine are indicative of a variety of pathologies, including 3-methylcrotonyl-CoA carboxylase deficiency, and have frequently resulted in a diagnosis of this deficiency in the mother rather than in the newborn (5). The mechanism in these patients is probably distinct, with 3-methylcrotonyl-CoA carboxylase elevations due to a direct maternal transfer of the acylcarnitine species. Defects in the maternal carnitine transporter have also been detected by low concentrations of carnitine on a newborn screen (6).
The mechanism of B12 deficiency in the patient’s mother is also not uncommon. The number of gastric bypasses performed on women of reproductive age has increased as roux-en-Y gastric bypasses in the general population have increased from 16 000 per year in the early 1990s to 103 000 in 2003 (7). Patients who have had gastric bypass are at significant risk for B12 deficiency because of the loss of intrinsic-factor secretion that is required for absorption. Patients should receive 1 mg intramuscular B12 every 3 months or 500 μg intranasal B12 weekly following gastric bypass (8).
This case illustrates an unusual mechanism leading to the elevation of a diagnostic metabolite. The patient’s benign presentation in combination with family history and laboratory studies revealed the cause before any form of treatment was instituted in the infant. In this case, the mother elected to feed her child commercial infant formula, which corrected the vitamin deficiency. Had the infant been breastfed, the deficiency would have continued and acidosis resulting from impaired MMM function could have resulted. As a result of the newborn screening program, the maternal deficiency in B12 was detected and treated before the emergence of neurological symptoms, although she had already developed anemia. This unusual route of diagnosis, although it may provoke anxiety in the clinician and family, can be considered an unexpected benefit of the newborn screening program. History, examination, and metabolic laboratory studies are sufficient to expeditiously separate cases of methylmalonic and propionic acidemia from false-positive or nutritional causes of C3-carnitine elevation on newborn screening.
POINTS TO REMEMBER
The diagnosis of a metabolic disease cannot be made exclusively on the basis of newborn screening. An abnormal result must be confirmed by additional testing pursued in consideration of the clinical presentation and family history.
Elevations in C3-acylcarnitines are used in the diagnosis of methylmalonic aciduria, although this metabolite is several steps removed from the metabolic defect.
Newborn screen findings can reveal maternal defects in the case of several inborn errors of metabolism, including 3-methylcrotonyl-CoA carboxylase deficiency and carnitine transport defect, and also can reveal nutritional deficits in the mother.
Newborn screening is designed to accept an increased false-positive rate to have an excellent sensitivity and negative predictive value.
Author Contributions: Each author confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors’ Disclosures of Potential Conflicts of Interest: Upon manuscript submission, all authors completed the Disclosures of Potential Conflict of Interest form. Potential conflicts of interest:
Employment or Leadership: M. J. Bennett, Board of Directors, AACC, and member of the editorial board of Clinical Chemistry.
Consultant or Advisory Role: None declared.
Stock Ownership: None declared.
Honoraria: None declared.
Research Funding: K. A. Chapman is the recipient of grant 2T32GM008638, NIH, National Institute of General Medical Sciences; and N. Sondheimer is the recipient of grant 2K12HD043245, NIH, National Institute of Child Health and Human Development.
Expert Testimony: None declared.
Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.
- © 2008 The American Association for Clinical Chemistry