November 2021 – Biochemical Genetics

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. 

References

1. Ersoy M, Tiranti V, Zeviani M.  Ethylmalonic encephalopathy: clinical course and therapy response in an uncommon mild case with a severe ETHE1 mutation.  Mol Genet Metab Rep. 2020; 25(2020):100641.
2. Prasun P.  Multiple acyl-CoA dehydrogenase deficiency. In: Adam MP, Ardinger HH, Pagon RA, et al, eds. GeneReviews. University of Washington, Seattle; 2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK558236/
3. Merritt JL, Chang IJ.  Medium-chain acyl-coenzyme A dehydrogenase deficiency. In: Adam MP, Ardinger HH, Pagon RA, et al, eds. GeneReviews. University of Washington, Seattle; 2019. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1424/
4. Ou M, Zhu L, Zhang Y, et al.  A novel electron transfer flavoprotein dehydrogenase (ETFDH) gene mutation identified in a newborn with glutaric acidemia type II: a case report of a Chinese family. BMC Med Genet. 2020; 21(2020):98.
5. Olsen R, Koňaříková E, Giancaspero, TA, et al. Riboflavin-responsive and non-responsive mutations in FAD synthase cause multiple acyl-CoA dehydrogenase and combined respiratory-chain deficiency. Am J Hum Genet. 2016; 98(6):1130–1145.