Normal Biomarkers in an Acute Presentation in a Known Case of Medium-Chain Acyl-Coenzyme A Dehydrogenase Deficiency

Clinical History and Background

A 16-year-old male patient with medium-chain acyl-coenzyme A dehydrogenase deficiency (MCADD) diagnosed on newborn screening (NBS) presented with a history of intermittent emesis and abdominal pain. He was well-hydrated with a normal blood glucose concentration (74 mg/dL; reference interval (RI): 70 to 106 mg/dL) and a urine dipstick was negative for ketones. The liver was palpable at 1.5 cm below the right costal margin. An abdominal ultrasound revealed hepatosplenomegaly with hepatic steatosis. Salicylate-induced Reye-like syndrome was suspected, secondary to a reported history of use of an unidentified over-the-counter agent that was described as similar to bismuth salicylate. However, a serum salicylate level was undetectable and hepatic enzyme levels were normal. The department if endocrinology and Metabolism was consulted upon admission. He was started on 10% dextrose-containing intravenous fluid (IVF) and admitted for observation. A plasma acylcarnitine profile (ACP) showed normal values including C2 (5.07 µmol/L; RI: 4.21 to 20.6 µmol/L), C6 (0.08 µmol/L; RI: 0.01 to 0.22 µmol/L), C8 (0.07 µmol/L; RI: 0 to 0.5 µmol/L), C10 (0.04 µmol/L; RI: 0 to 0.9 µmol/L), and C10:1 (0.09 µmol/L; RI: 0.01 to 0.45 µmol/L). Plasma free and total carnitine levels were borderline-low at 19.5 µmol/L (RI: 17 to 59 µmol/L) and 23 µmol/L (RI: 25 to 69 µmol/L), respectively. Urine organic acid analysis (UOA) demonstrated no medium-chain dicarboxylic aciduria (MC-DCA); ion extraction for hexanoylglycine, suberylglycine and 3-phenylpropionyglycine was negative. He had not been followed in a metabolic center since diagnosis and had no prior history of hepatic dysfunction, but previously required preventative dextrose-containing IVF administration for intercurrent infections. Extensive hepatic steatosis workup was negative and the patient remained stable during the admission with no hypoglycemia; his presentation was attributed to viral infection. The family history was notable for an older brother born prior to NBS for MCADD in Pennsylvania, who had been diagnosed with MCADD via biochemical testing following the proband’s diagnosis. Molecular testing had not been performed in the family.

On a 2-month follow-up visit, he was clinically stable, and the hepatosplenomegaly resolved. A repeat ACP showed expected increases in C6 (0.22 µmol/L), C8 (1.34 µmol/L), and C10:1 (1.17 µmol/L) with normal C10 (0.52 µmol/L) and low C2 (4.06 µmol/L). Free and total carnitine concentrations improved spontaneously to 32.8 µmol/L and 45.8 µmol/L, respectively. UOA was notable for trace hexanoylglycine and suberylglycine detected by ion extraction (Fig. 1) with no evidence of 3-phenylpropionyglycine. ACADM gene sequencing revealed compound heterozygous pathogenic variants: c.199T>C/c.985A>G.

Fig. 1.

Representative UOA profile by GC-MS of the patient during his follow-up visit. The chromatogram illustrates detection of hexanoylglycine and suberylglycine by ion extraction. Urine organic acids were extracted into ethyl acetate and converted to trimethylsilyl derivatives and analyzed using a GC 7890B/MS 5977B system equipped with a HP-5MS column (Agilent).

Diagnosis and Summary

MCADD, secondary to bi-allelic variants in the ACADM gene, is the most common fatty acid oxidation disorder (FAOD) with an overall prevalence of 5.3/100 000 births. During prolonged fasting and physiological stress, the energy supply through ketogenesis is disrupted and glucose utilization is increased (Fig. 2). As a result, hypoketotic hypoglycemia and hepatopathy occur in acute decompensation and may progress to seizure and coma if untreated. Sudden death and permanent neurological sequelae are recognized complications. Detection through NBS allows early management and improves long-term outcomes.

Fig. 2.

Illustration of mitochondrial medium-chain fatty acid oxidation. Each turn of the cycle shortens the acyl-CoA by 2 carbons. The released acetyl-CoA after each cycle enters the Krebs cycle or is used for ketone synthesis in the liver. Deficiency of the MCAD enzyme will result in accumulation of acyl-CoA which is converted to acylcarnitine or acylglycine by conjugation with carnitine or glycine, respectively. The accumulated hexonylglycine, which is 6 carbons, and suberylglycine, which is 8 carbons, are subsequently excreted in the urine and serve as biomarkers for MCADD. Increased acylcarnitine concentrations can be detected in a blood or plasma sample. MCAD, medium-chain acyl-CoA dehydrogenase. M/SHADH, medium/short 3-hydroxyacyl-CoA dehydrogenase. NAD ⁺, nicotinamide adenine dinucleotide.

ACP findings include increases in medium-chain acylcarnitines (C6, C8, and C10:1), which can be masked by carnitine deficiency (1). Our patient’s ACP was normal in this acute presentation despite near-normal carnitine levels after an overnight administration of dextrose-containing fluids. Normalization of the ACP during intravenous dextrose administration has been observed in patients with long-chain FAODs and is thought to be related to the anabolic state and flushing of acylcarnitines from circulation by IVF (2).

MC-DCA out of proportion to ketonuria is a non-specific finding and usually absent in asymptomatic states (3). Urine hexanoylglycine and suberylglycine are sensitive and specific markers and observed with variable genotype and phenotype severity, and in asymptomatic patients (3, 4). The c.199T>C/c.985A>G genotype has been associated with a mild clinical and biochemical phenotype, but increased urinary excretion of hexanoylglycine is typically present and moderately high on quantitative analysis (4). However, undetectable urinary hexanoylglycine at time of diagnosis was reported in 2 cases identified by NBS (5). Interestingly, our patient’s biochemical profile including urinary acylglycines normalized after receiving high dextrose-containing IVF. This unreported observation in MCADD highlights the importance of molecular testing and repeat biochemical screening if the diagnosis is highly suspected, despite normal initial workup under supportive care; it could also be of particular concern when confirming NBS results.

Nonstandard Abbreviations:

MCADD

Medium-chain acyl-coenzyme A dehydrogenase deficiency

NBS

newborn screening

RI

reference interval

Human Genes: ACADM

acyl-CoA dehydrogenase medium chain

Author Contributions

All authors confirmed they have contributed to the intellectual content of this paper and have met the following 4 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; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved.

Authors’ Disclosures or Potential Conflicts of Interest

Upon manuscript submission, all authors completed the author disclosure form. Disclosures and/or potential conflicts of interest:

Employment or Leadership

M.J. Bennett, Clinical Chemistry, AACC.

Consultant or Advisory Role

S.R. Master, Indigo BioAutomation; R.D. Ganetzky, Minovia Therapeutics.

Stock Ownership

None declared.

Honoraria

S.R. Master, MSACL; R.D. Ganetzky, Oklahoma University and Geisinger Medical Center.

Research Funding

R.D. Ganetzky, NIDDK, NINDS, and NAMDC (North American Mitochondrial Disease Consortium).

Expert Testimony

S.R. Master, US District Attorney, Northern California.

Patents

None declared.

Other Remuneration

S.R. Master, AACC.