No abstract

Background: A substantial group of patients with cholestatic liver disease in infancy excrete, as the major urinary bile acids, the glycine and taurine conjugates of 7a-hydroxy-3-oxo-4-cholenoic acid and 7a,12a-dihydroxy-3-oxo-4-cholenoic acid. It has been proposed that some (but not all) of these have mutations in the gene encoding D 4 -3-oxosteroid 5b-reductase (SRD5B1; AKR1D1, OMIM 604741). Aims: Our aim was to identify mutations in the SRD5B1 gene in patients in whom chenodeoxycholic acid and cholic acid were absent or present at low concentrations in plasma and urine, as these seemed strong candidates for genetic 5b-reductase deficiency. Patients and subjects: We studied three patients with neonatal onset cholestatic liver disease and normal c-glutamyl transpeptidase in whom 3-oxo-D 4 bile acids were the major bile acids in urine and plasma and saturated bile acids were at low concentration or undetectable. Any base changes detected in SRD5B1 were sought in the parents and siblings and in 50 ethnically matched control subjects. Methods: DNA was extracted from blood and the nine exons of SRD5B1 were amplified and sequenced. Restriction enzymes were used to screen the DNA of parents, siblings, and controls. Results: Mutations in the SRD5B1 gene were identified in all three children. Patient MS was homozygous for a missense mutation (662 C.T) causing a Pro198Leu amino acid substitution; patient BH was homozygous for a single base deletion (511 delT) causing a frame shift and a premature stop codon in exon 5; and patient RM was homozygous for a missense mutation (385 C.T) causing a Leu106Phe amino acid substitution. All had liver biopsies showing a giant cell hepatitis; in two, prominent extramedullary haemopoiesis was noted. MS was cured by treatment with chenodeoxycholic acid and cholic acid; BH showed initial improvement but then deteriorated and required liver transplantation; RM had advanced liver disease when treatment was started and also progressed to liver failure. Conclusions: Analysis of blood samples for SRD5B1 mutations can be used to diagnose genetic 5b-reductase deficiency and distinguish these patients from those who have another cause of 3-oxo-D 4 bile aciduria, for example, severe liver damage. Patients with genetic 5b-reductase deficiency may respond well to treatment with chenodeoxycholic acid and cholic acid if liver disease is not too advanced.

Peroxisomes play an essential role in a number of different metabolic pathways, including the ␤-oxidation of a distinct set of fatty acids and fatty acid derivatives. The importance of the peroxisomal ␤-oxidation system in humans is made apparent by the existence of a group of inherited diseases in which peroxisomal ␤-oxidation is impaired. This includes X-linked adrenoleukodystrophy and other disorders with a defined defect. On the other hand, many patients have been described with a defect in peroxisomal ␤-oxidation of unknown etiology. Resolution of the defects in these patients requires the elucidation of the enzymatic organization of the peroxisomal ␤-oxidation system. Importantly, a new peroxisomal ␤-oxidation enzyme was recently described called Dbifunctional protein with enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activity primarily reacting with ␣-methyl fatty acids like pristanic acid and di-and trihydroxycholestanoic acid. In this patient we describe the first case of D-bifunctional protein deficiency as resolved by enzyme activity measurements and mutation analysis. The mutation found (Gly 16 Ser) is in the dehydrogenase coding part of the gene in an important loop of the Rossman fold forming the NAD ؉ -binding site. The results show that the newly identified D-bifunctional protein plays an essential role in the peroxisomal ␤-oxidation pathway that cannot be compensated for by the L-specific bifunctional protein.