Isobutyryl-CoA dehydrogenase deficiency associated with autism in a girl without an alternative genetic diagnosis by trio whole exome sequencing: A case report

Acyl-CoA dehydrogenase substrate promiscuity: Challenges and opportunities for development of substrate reduction therapy in disorders of valine and isoleucine metabolism

Toxicity of accumulating substrates is a significant problem in several disorders of valine and isoleucine degradation notably short-chain enoyl-CoA hydratase (ECHS1 or crotonase) deficiency, 3-hydroxyisobutyryl-CoA hydrolase (HIBCH) deficiency, propionic acidemia (PA), and methylmalonic aciduria (MMA). Isobutyryl-CoA dehydrogenase (ACAD8) and short/branched-chain acyl-CoA dehydrogenase (SBCAD, ACADSB) function in the valine and isoleucine degradation pathways, respectively. Deficiencies of these acyl-CoA dehydrogenase (ACAD) enzymes are considered biochemical abnormalities with limited or no clinical consequences. We investigated whether substrate reduction therapy through inhibition of ACAD8 and SBCAD can limit the accumulation of toxic metabolic intermediates in disorders of valine and isoleucine metabolism. Using analysis of acylcarnitine isomers, we show that 2-methylenecyclopropaneacetic acid (MCPA) inhibited SBCAD, isovaleryl-CoA dehydrogenase, short-chain acyl-CoA dehydrogenase and medium-chain acyl-CoA dehydrogenase, but not ACAD8. MCPA treatment of wild-type and PA HEK-293 cells caused a pronounced decrease in C3-carnitine. Furthermore, deletion of ACADSB in HEK-293 cells led to an equally strong decrease in C3-carnitine when compared to wild-type cells. Deletion of ECHS1 in HEK-293 cells caused a defect in lipoylation of the E2 component of the pyruvate dehydrogenase complex, which was not rescued by ACAD8 deletion. MCPA was able to rescue lipoylation in ECHS1 KO cells, but only in cells with prior ACAD8 deletion. SBCAD was not the sole ACAD responsible for this compensation, which indicates substantial promiscuity of ACADs in HEK-293 cells for the isobutyryl-CoA substrate. Substrate promiscuity appeared less prominent for 2-methylbutyryl-CoA at least in HEK-293 cells. We suggest that pharmacological inhibition of SBCAD to treat PA should be investigated further.

Combined isobutyryl‐CoA and multiple acyl‐CoA dehydrogenase deficiency in a boy with altered riboflavin homeostasis

In this report, we describe the case of an 11‐year‐old boy, who came to our attention for myalgia and muscle weakness, associated with inappetence and vomiting. Hypertransaminasemia was also noted, with ultrasound evidence of hepatomegaly. Biochemical investigations revealed acylcarnitine and organic acid profiles resembling those seen in MADD, that is, multiple acyl‐CoA dehydrogenase deficiencies (OMIM #231680) a rare inherited disorder of fatty acids, amino acids, and choline metabolism. The patient carried a single pathogenetic variant in the ETFDH gene (c.524G>A, p.Arg175His) and no pathogenetic variant in the riboflavin (Rf) homeostasis related genes (SLC52A1, SLC52A2, SLC52A3, SLC25A32, FLAD1). Instead, compound heterozygosity was found in the ACAD8 gene (c.512C>G, p.Ser171Cys; c.822C>A, p.Asn274Lys), coding for isobutyryl‐CoA dehydrogenase (IBD), whose pathogenic variants are associated to IBD deficiency (OMIM #611283), a rare autosomal recessive disorder of valine catabolism. The c.822C>A was never previously described in a patient. Subsequent further analyses of Rf homeostasis showed reduced levels of flavins in plasma and altered FAD‐dependent enzymatic activities in erythrocytes, as well as a significant reduction in the level of the plasma membrane Rf transporter 2 in erythrocytes. The observed Rf/flavin scarcity in this patient, possibly associated with a decreased ETF:QO efficiency might be responsible for the observed MADD‐like phenotype. The patient's clinical picture improved after supplementation of Rf, l‐carnitine, Coenzyme Q10, and also 3OH‐butyrate. This report demonstrates that, even in the absence of genetic defects in genes involved in Rf homeostasis, further targeted molecular analysis may reveal secondary and possibly treatable biochemical alterations in this pattern.

Identification of Six Novel Variants of ACAD8 in Isobutyryl-CoA Dehydrogenase Deficiency With Increased C4 Carnitine Using Tandem Mass Spectrometry and NGS Sequencing

Isobutyryl-CoA dehydrogenase deficiency (IBDHD, MIM: #611283) is a rare autosomal recessive hereditary disease, which is caused by genetic mutations of acyl-CoA dehydrogenase (ACAD) 8 and associated with valine catabolism. Here, tandem mass spectrometry (MS/MS) was applied to screen 302,993 neonates for inherited metabolic diseases (IMD) in Ningbo of China from 2017 to 2020. The results suggest that 198 newborns (0.7‰) were initially screened positive for IBDHD with C4-Carnitine, and 27 cases (0.1‰) were re-screened positive. Genetic diagnosis was performed on 21 of the 27 cases. Seven compound heterozygous variations, three biallelic variations, and one heterozygous variation of ACAD8 were found with a pathogenicity rate of 33.3% (7/21). In addition, seven biallelic variations, one heterozygous variation of acyl-CoA dehydrogenase short chain (ACADS), and one biallelic variation of acyl-CoA dehydrogenase short/branched chain (ACADSB) was detected. Further research showed that ACAD8 mutations of 11 IBDHD cases distributed in six different exons with total 14 mutation sites. Five of which were known suspected pathogenic sites (c.286G > A, c.553C > T, c.1000C > T, c.409G > A, c.500del) and six were novel mutation sites: c.911A > T, c.904C > T, c.826G > A, c.995T > C, c.1166G > A, c.1165C > T. This finding enriched the mutation spectrum of ACAD8 in IBDHD.

Phenotype, genotype and long-term prognosis of 40 Chinese patients with isobutyryl-CoA dehydrogenase deficiency and a review of variant spectra in ACAD8

Background

Isobutyryl-CoA dehydrogenase deficiency (IBDD) is a rare autosomal recessive metabolic disorder resulting from variants in ACAD8, and is poorly understood, as only dozens of cases have been reported previously. Based on a newborn screening program, we evaluated the incidence, phenotype and genotype of IBDD as well as the prognosis. Moreover, we reviewed the variant spectrum in ACAD8 associated with IBDD.

Methods

Forty unrelated patients with IBDD were retrospectively screened for newborns between Jan 2012 and Dec 2020. Tandem mass spectrometry (MS/MS) was used to determine the concentrations of C4-acylcarnitine, C4/C2 (acetylcarnitine), and C4/C3 (propionylcarnitine). All suspected cases were genetically tested by metabolic genes panel.

Results

The incidence of IBDD here was 1: 62,599. All patients presented continuously elevated C4-acylcarnitine levels with higher ratios of C4/C2 and C4/C3. Isobutyrylglycine occurred in only 8 patients. During follow-up, four patients had a transient motor delay, and two patients had growth delay. Notably, one case harbored both ACAD8 compound heterozygous variants and a KMT2A de novo variant (c.2739del, p.E914Rfs*35), with IBDD and Wiedemann–Steiner syndrome together, had exact severe global developmental delay. All patients were regularly monitored once they were diagnosed, and each patient gradually had a normal diet after 6 months of age. After 3–108 months of follow-up, most individuals were healthy except the case harboring the KMT2A variant. A total of 16 novel variants in ACAD8, c.4_5delCT, c.109C > T, c.110–2A > T, c.236G > A, c.259G > A, c.381–14G > A, c.413delA, c.473A > G, c.500delG, c.758 T > G, c.842–1G > A, c.911A > T, c.989G > A, c.1150G > C, c.1157A > G and c.1165C > T, were identified. Along with a literature review on 51 ACAD8 variants in 81 IBDD patients, we found that the most common variant was c.286G > A (27.2%), which has been observed solely in the Chinese population to date, followed by c.1000C > T (8.6%), c.1176G > T (3.7%) and c.455 T > C (3.1%).

Conclusion

The concentration of C4-acylcarnitine in NBS plus subsequent genetic testing is necessary for IBDD diagnosis. Both the genotypes and ACAD8 variants in IBDD are highly heterogeneous, and no significant correlations between genotype and phenotype are present here in patients with IBDD. Our IBDD cohort with detaied clinical characteristics, genotypes and long-term prognosis will be helpful for the diagnosis and management of patients with IBDD in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-02018-6.