The acyl-CoA dehydrogenases (ACADs) are a family of closely related mitochondrial enzymes that catalyzes the removal of hydrogen atoms from the α and β positions (also known as β-oxidation) of straight or branched chain acyl-CoA esters in the metabolism of amino acids and fatty acids, transferring electrons to electron transferring flavoprotein [1
]. At least 9 members of this family have been described so far. Short chain acyl-CoA dehydrogenase (SCAD), medium chain acyl-CoA dehydrogenase (MCAD), long chain acyl-CoA dehydrogenase (LCAD), very long chain acyl-CoA dehydrogenase (VLCAD), and acyl-CoA dehydrogenase 9 (ACAD9) all catalyze oxidation of acyl-CoAs in mitochondrial fatty acid β-oxidation. Human ACAD9 plays a unique role in the mitochondrial β-oxidation of unsaturated fatty acids [9
]. Glutaryl-CoA dehydrogenase is active in the lysine degradation, while isovaleryl-CoA dehydrogenase, short branched chain acyl-CoA dehydrogenase (SBCAD, also known as 2-methylbutyryl-CoA dehydrogenase), and isobutyryl-CoA dehydrogenase (IBD, also designated ACAD8) function in the catabolism of leucine, isoleucine, and valine intermediates, respectively [5
]. SBCAD, like all ACADs except VLCAD and ACAD9, is a homotetrameric mitochondrial enzyme catalyzing alpha and beta dehydrogenation of acyl-CoA derivatives with greatest activity toward straight/branched short chain acyl-CoA metabolites and specifically toward 2-methybutyryl-CoA, catalyzing the third step of the isoleucine catabolic pathway [13
Deficiencies in all of these enzymes except LCAD have been associated with human disease [7
]. SBCAD deficiency (OMIM 600301/610006) has only recently been described. The first two patients were identified due to significant medical problems and were shown to have SBCAD deficiency [16
]. Subsequently, this disorder has been shown, through population and newborn screening studies, to be common in individuals of Hmong ancestry [26
.] A minority of patients in the Hmong population diagnosed through newborn screening was reported to have transient muscle hypotonia, but the Hmong patients have otherwise been well. Additional non-Hmong patients with SBCAD deficiency have also been identified through newborn screening and remain well, as have others diagnosed through metabolic evaluations for clinical symptoms [26
]. One patient was reported to have autism and mental retardation, but causation could not be established [31
]. Thus, the significance of SBCAD deficiency remains unclear.
In this study we describe 11 patients from various medical centers identified by newborn screening because of elevated C5-acylcarnitine levels. An additional patient who was born before acylcarnitine analysis was added to the respective State’s newborn screening program was identified during an evaluation for clinical symptoms. Further diagnostic testing, including plasma acylcarnitines, plasma amino acids, urine organic acids, urine acylglycines, and fibroblast acylcarnitine profiling, substantiated SBCAD deficiency in all 12 patients. Sequencing of genomic DNA from 5 patient fibroblasts identified mutations predicted to be deleterious in four of the patients identified by newborn screening and in the symptomatic patient. In vitro mutagenesis and expression studies of point mutations in E. coli confirmed that they lead to an inactive or unstable SBCAD protein. Individuals detected through newborn screening programs remain well with the oldest being 4 years old, suggesting that this deficiency may be a biochemical phenotype rather than a disease.