Enoyl-CoA hydratase deficiency: identification of a new type of D-bifunctional protein deficiency

D-bifunctional protein is involved in the peroxisomal beta-oxidation of ver y long chain fatty acids, branched chain fatty acids and bile acid intermed iates. In line with the central role of D-bifunctional protein in the beta- oxidation of these three types of fatty acids, all patients with D-bifuncti onal protein deficiency so far reported in the literature show elevated lev els of very long chain fatty acids, branched chain fatty acids and bile aci d intermediates. In contrast, we now report two novel patients with D-bifun ctional protein deficiency who both have normal levels of bile acid interme diates. Complementation analysis and D-bifunctional protein activity measur ements revealed that both patients had an isolated defect in the enoyl-CoA hydratase domain of D-bifunctional protein. Subsequent mutation analysis sh owed that both patients are homozygous for a missense mutation (N457Y), whi ch is located in the enoyl-CoA hydratase coding part of the D-bifunctional protein gene. Expression of the mutant protein in the yeast Saccharomyces c erevisiae confirmed that the N457Y mutation is the disease-causing mutation . Immunoblot analysis of patient fibroblast homogenates showed that the pro tein levels of full-length D-bifunctional protein were strongly reduced whi le the enoyl-CoA hydratase component produced after processing within the p eroxisome was undetectable, which indicates that the mutation leads to an u nstable protein.