2-Methylbutyryl-CoA dehydrogenase (2-MBCDase) deficiency: a new inborn error of L-isoleucine metabolism

With the exception of beta-kelothiolase (BKT) deficiency, there are no reported inborn errors of human metabolism specific to L-isoleucine degradation. EA presented at three days of age with lethargy, hypoglycemia and apnea. MRI demonstrated ischemic changes bilaterally in parietal and occipital lobes and abnormal EEG. Acylcarnitines in plasma were increased (range 8-22 uM, n=7; nl, 3-10). Tandem mass spectrometry (MS/MS) analysis revealed elevated plasma C5-acylcarnitine (1.4-2.4 μM, nl < 0.4), verified as 2-methylbutyrylcarnitine by gas chromatography-mass speclrometry. Urine 2-methylbutyrylglycine (2-MBG) was consistently elevated (range 8-30 mg/g creatinine, n=6; nl < 8). Intact fibroblast conversion of U-14C-isoleucine to 14CO2 was decreased to 26% of control, comparable to that of BKT deficient fibroblasts. Conversion of 13C6-leucine and 13C5-valine in patient's fibroblasts incubated with L-carnitine revealed no abnormal accumulation of acylcarnitines; in the same cells, there was a 10-fold accumulation of C5-acylcarnitine in comparison to control (consistent with impaired 2-MBCDase activity) when incubated with 13C6-isoleucine and L-carnitine. Western-blot analysis of fibroblasts from EA revealed greatly reduced 2-MBCDase cross-reactive material. Mutation screening in patient fibroblasts revealed a C778T substitution in the 2-MBCDase gene, substituting leucine for phenylalanine at amino acid 222 (L222F); the patient's mother also carried this mutation. In a subsequent pregnancy, the concentration of 2-MBG in amniotic fluid obtained at 15 weeks gestation was 0.27 umol/L (nl < 0.04, n=5), while total C5-acylcarnitine concentration was 1.9 umol/L (nl < 0.7, n=27), suggesting an affected fetus. Cultured amniocytes accumulated excess (10-fold) C5-acylcarnitine when incubated with 13C6-isoleucine and L-carnitine. At 1 year of age EA carries the diagnosis of athetoid cerebral palsy, and continues to manifest impaired visual, motor and cognitive skills. Our studies reveal 1) the first documented case of isolated 2-MBCDase deficiency; 2) a novel mutation in the 2-MBCDase gene; and 3) the first prenatal diagnosis of an affected fetus with 2-MBCDase deficiency, a new inborn error of L-isoleucine metabolism in humans.