Samples of a 5-day-old Hispanic infant male (birthweight >3.00kg) were sent to the Biochemical Genetics Laboratory of Mayo Clinic to confirm an abnormal newborn screen result consisting of elevated short- to long-chain acylcarnitines (C4–C18). Plasma acylcarnitine analysis confirmed the abnormal elevations of C4-C14 acylcarnitines, and further revealed elevated glutaryl carnitine concentrations (Figure 1). In addition, urinary organic acid and acylglycine profiles indicated elevated excretions of various organic acids/glycine conjugates, of which those pertinent to this case are annotated in Figure 2. Whole exome sequencing (WES) performed in confirmation of the biochemical results revealed two compound heterozygous variants (classified as variants of unknown significance) in FLAD1. Although this patient is below the 7th percentile for growth/weight of infants his age and has visible atypical lipid accumulation in his arms, legs, torso, and face, he remains relatively asymptomatic at the current age of 12 months.
The correct answer is ...
Secondary glutaric acidemia type II
1. Ethylmalonic encephalopathy
Ethylmalonic encephalopathy is characterized as an autosomal recessive disorder caused by pathogenic variants in a mitochondrial persulfide dioxygenase encoding gene (ETHE1) required for H2S catabolism. Impaired regulation of this mechanism with the subsequent accumulation of H2S inhibits various biochemical enzymes such as cytochrome C oxidase (electron transport chain; mitochondrial dysfunction), short branched-chain acyl-CoA dehydrogenase (isoleucine metabolism; short branched-chain acyl-CoA dehydrogenase deficiency), and short-chain acyl-CoA dehydrogenase (β-oxidation; short-chain acyl-CoA dehydrogenase deficiency). Considering this, patients with ethylmalonic encephalopathy typically present with elevated concentrations of C4 and C5 acylcarnitines, ethylmalonic acid, methylsuccinic acid, isobutyrylglycine, 2-methylbutyrylglycine, and lactic acidemia.
Although strikingly similar to the presented case, ethylmalonic encephalopathy is unlikely to account for some of the other biochemical findings such as medium- and long-chain acylcarnitines, 3-hydroxyglutaric acid, and glutarylcarnitine associated with the present case. Clinically, patients with ethylmalonic encephalopathy characteristically present with developmental delay, seizures, petechiae, and orthostatic acrocyanosis early in infancy, which does not correlate with the current case.
2. Medium-chain acyl-CoA dehydrogenase deficiency (MCADD)
MCADD is a mitochondrial β-oxidation disorder that impairs the catabolism of medium-chain fatty acids (C6-C10) and the subsequent production of ketone bodies, due to biallelic pathogenic variants in the ACADM gene. These patients typically present with seizures, vomiting, lethargy, and hypoketotic hypoglycemia during fasting periods, which may lead to sudden/unexpected patient death. A potential biochemical diagnosis of MCADD should be considered in patients with elevated medium-chain plasma acylcarnitines, especially when arranged in a characteristic C6<C8>C10 pattern. Confirmatory biochemical observations include elevated hexanoylglycine, suberylglycine, and dicarboxylic aciduria.
A diagnosis of MCADD is excluded, as it doesn’t account for the elevated concentrations of short- and long-chain acylcarnitines, or the prominent excretions of ethylmalonic and methylsuccinic acids. Furthermore, no ACADM pathogenic variants were annotated during WES.
3. Glutaric acidemia type II (GA II)/multiple dehydrogenase deficiency
Patients with GA II were originally characterized as possessing biallelic pathogenic variants in flavoprotein related genes (ETF, ETFB, and/or ETFDH) responsible for transferring electrons, generated via numerous dehydrogenase reactions, to the electron transport chain. Impaired functionality of these flavoproteins thus concomitantly hamper dehydrogenase enzymes associated with fatty acid (β-oxidation), branched-chain amino acid, lysine, and tryptophan catabolism. As such, these patients present with elevated short- to long-chain acylcarnitines (C4-C18), hydroxyacids, dicarboxylic acids, and acylglycine conjugates (C5–C10 most prominent). The clinical phenotype of GA II patients is relatively heterogeneous and may manifest at various ages. Late-onset GA II may clinically present at any time after the neonatal period and exhibit signs of hypotonia, lethargy, rhabdomyolysis, lipid storage myopathy, and severe metabolic acidosis during periods of fasting.
Biochemically, GA II seems to be a likely diagnosis in the presented case. However, ethylmalonic acid, methylsuccinic acid, and 2-methylbutyrylglycine excretions in the presented case appear to be more prominently elevated than usually observed in these patients and may be more fitting with ethylmalonic encephalopathy/ethylmalonic aciduria. As further indicated, no pathogenic genetic variants were annotated in any of the aforementioned genes associated with GA II.
4. Secondary GA II
More recently, it has been reported that patients with pathogenic variants in genes (SLC52A1–3, SLC25A32, and FLAD1) associated with riboflavin transport and catabolism, exhibit biochemical abnormalities resembling GA II and ethylmalonic aciduria. In this case, two variants of unknown significance were found in one of these genes, FLAD1. Mutations in FLAD1 lead to impaired functionality of flavin adenine dinucleotide synthase (FADS), and subsequently reduce the production of FAD. The latter, in turn, could not only directly affect a variety of FAD-dependent flavoproteins (i.e., electron transferring proteins, etc.), but also cause secondary hampering of multiple acyl-CoA dehydrogenase enzymes (among others), hence the biochemical profile similar to GA II and ethylmalonic aciduria. Clinically, these patients share similarities with both the aforementioned diseases, and could reportedly present at various ages. However, very heterogeneous presentations have been described in the patients with this rare disease, the most prominent being lipid storage myopathy, reoccurring respiratory infections, weight loss, as well as speech and swallowing difficulties.
Collectively considering all the biochemical findings, molecular variants in FLAD1, and the abnormal lipid accumulation in this otherwise slender infant, the most likely diagnosis is secondary GA II, due to FADS deficiency. Since this patient remains relatively asymptomatic to date, it is anticipated that the genetic variants present in this individual may allow for some residual FADS activity that meets the current biochemical demand. Whether this may be sustainable throughout the patient’s lifespan remains unknown.
Zinandré Stander, Ph.D.
Fellow, Clinical Biochemical Genetics
Silvia Tortorelli, M.D., Ph.D.
Consultant, Biochemical Genetics
Associate Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science