Detection of defective 3β‐hydroxysterol Δ7‐reductase activity in cultured human fibroblasts: a method for the diagnosis of Smith‐Lemli‐Opitz syndrome

Summary Patients with the autosomal recessive disorder Smith‐Lemli‐Opitz syndrome (SLO) have recently been shown to have markedly increased tissue levels of certain cholesterol biosynthesis intermediates, most notably 7‐dehydrocholesterol. The findings strongly suggest a block in the step that catalyses reduction of 7‐dehydrocholesterol to cholesterol. The accumulation of 7‐dehydrocholesterol can generally easily be detected in serum by gas chromatography‐mass spectrometry. However, it could not be totally ruled out that SLO patients with less severe enzyme defects could escape detection by this method. A more direct way of diagnosing a defect in 7‐dehydrocholesterol reduction would be to assay the conversion of 7‐dehydrocholesterol to cholesterol in cultured fibroblasts from patients with suspected SLO. In the present work, an assay for the conversion of [3H]lathosterol to [3H]‐cholesterol in cultured human fibroblasts is described. Lathosterol is the immediate precursor of 7‐dehydrocholesterol in the cholesterol biosynthetic pathway and was chosen for the assay instead of 7‐dehydrocholesterol owing to the difficulty in preparation and handling of the latter compound. Fibroblasts from control subjects converted [3H]lathosterol to [3H]cholesterol efficiently, whereas in fibroblasts from SLO patients the conversion did not go beyond 7‐dehydrocholesterol. It is concluded that the present method is useful for the diagnosis of SLO.