A novel ACAD8 mutation in asymptomatic patients with isobutyryl-CoA dehydrogenase deficiency and a review of the ACAD8 mutation spectrum.

To the Editor : Isobutyryl-CoA dehydrogenase (IBD) deficiency is a very rare autosomal recessive metabolic disorder that is related to valine catabolism. Inherited metabolic disorders (IMDs) with an increase in C4 acylcarnitine include IBD deficiency, shortchain acyl-CoA dehydrogenase (SCAD) deficiency, ethylmalonic encephalopathy, multiple acyl-CoA dehydrogenase deficiency and formiminoglutamic aciduria. Most Korean newborns are screened by tandem mass spectrometry for differential diagnosis of various IMDs. We identified two Korean patients with IBD deficiency by newborn screening. They were asymptomatic as newborns, but had characteristic findings in acylcarnitines, organic acids, and mutation analyses. Patient 1 showed an elevated C4 acylcarnitine level (1.67μmol/l, cutoff: <0.66μmol/l) with an increased C4/C3 ratio (3.16, cutoff: <0.78) and C4/C2 ratio (0.38, cutoff: <0.13). Patient 2 also showed an elevated C4 level (2.57μmol/l) with an increased C4/C3 ratio (4.98). In urine organic acid tests, isobutyrylglycine was increased by 1.19 and 3.62 mmol/mol creatinine (cutoff: not detected) in patients 1 and 2, respectively, without elevation of ethylmalonic acid. These findings indicated that the possibility of SCAD deficiency was relatively low. In addition, ACADS sequencing analysis in two patients detected no mutations and lack of the polymorphisms p.R147W and p.G185S, known to be associated with an increased level of ethylmalonic acid excretion. In ACAD8 molecular genetic studies by PCR and sequencing, patient 1 had two mutations: c.289G>A (p.Gly97Arg) and c.1156_1158delCAG (p.Gln386del). Patient 2 had compound heterozygous mutations: c.3G>T (p.Met1Ile) and c.1156_1158delCAG (p.Gln 386del). Two missense mutations, c.289G>A and c.3G>T, have been reported in previous papers (1, 2). The novel mutation, c.1156_1158delCAG, was not found in the single nucleotide polymorphism database (dbSNP; http://www.ncbi.nlm.nih.gov/projects/SNP/) and was not detected in 110 healthy individuals. In addition, we reviewed previous studies of ACAD8 mutations in patients with IBD deficiency. To the best of our knowledge, 20 patients with IBD deficiency have been diagnosed by molecular genetic tests, and 22 ACAD8 mutations have been reported to date (Table 1) (1–7). The nomenclature was updated using Mutalyzer Name Checker (http://www.mutalyzer.nl) based on Human Genome Variation Society nomenclature version 2.0. Among the 22 known ACAD8 mutations, missense mutations are the most common (17/22), and other mutations types include one nonsense, one duplication, one deletion, one initiation codon mutation, and one splicing mutation. The c.455T>C and c.867C>A mutations are frequently observed (10%, 4/40 mutant alleles), 25% of the mutant alleles are located on exon 4, and 80% are located on exons 3, 4, 8, 9, or 10 (Figure 1). In Asia, six patients with IBD deficiency have been reported in the English literature (1, 2, 7). Three of these patients were Korean (including our cases), one was Jordanian, and the other two were East Indian siblings. None of the Asian patients had medical problems, except the Jordanian patient. The initiation codon mutation (c.3G>T, p.Met1Ile) found in the Korean patients affects the start codon, and incomplete transcription and translation might lead to the attenuation of the IBD enzyme (2). It is interesting to note that a small novel deletion of c.1156_1158del (p.Gln386del) was found in two unrelated patients. The glutamine residue at position 386 in the ACAD8 protein is highly conserved across species (http://www.h-invitational.jp/evola/). The activity and the amount of IBD protein may vary according to the ACAD8 mutation, which suggests that the phenotype of IBD deficiency may vary based on the mutation type or locus (6). Two patients with nonsense or frameshift mutations showed clinical manifestations (5, 6), but many cases with missense mutations express no symptoms (1, 2, 5, 6). It is difficult to determine the relationships between missense mutations and phenotypes, and therefore periodic follow-up is necessary