In the course of our studies on the molecular mechanisms involved in peroxisome biogenesis, we have isolated several mutants of the methylotrophic yeast Hansenula polymorpha impaired in the import of peroxisomal matrix proteins. These mutants are characterized by the presence of few small intact peroxisomes, while the bulk of the peroxisomal matrix protein is not imported and resides in the cytosol (Pim-phenotype). Genetic analysis of back-crossed mutants revealed five different complementation groups, which were designated PERI-PER5. Mapping studies to determine the linkage relationships indicated that the observed Pim-phenotypes were determined by single recessive nuclear mutations.The different mutants had comparable phenotypes: (i) they were impaired to utilize methanol as the sole source of carbon and energy but grew well on various other compounds, including nitrogen sources, the metabolism of which is known to be mediated by peroxisome-borne enzymes in wild-type cells; (ii) all peroxisomal enzymes tested were induced, assembled and activated as in wild-type cells although their activities varied between the different representative mutants; (iii) all peroxisomal proteins, whether constitutive or inducible, were found both in the cytosol and in the small peroxisomes. These results suggest that a general, major import mechanism is affected in all mutants.

SummarySmith-Lemli-Opitz syndrome, a severe developmental disorder associated with multiple congenital anomalies, is caused by a defect of cholesterol biosynthesis. Low cholesterol and high concentrations of its direct precursor, 7-dehydrocholesterol, in plasma and tissues are the diagnostic biochemical hallmarks of the syndrome. The plasma sterol concentrations correlate with severity and disease outcome. Mutations in the DHCR7 gene lead to deficient activity of 7-dehydrocholesterol reductase (DHCR7), the final enzyme of the cholesterol biosynthetic pathway. The human DHCR7 gene is localised on chromosome 11q13 and its structure has been characterized. Ninetyone different mutations in the DHCR7 gene have been published to date. This paper is a review of the clinical, biochemical and molecular genetic aspects.

For many decades, cholesterol has been considered an important structural component of cellular membranes and myelin, and a precursor of steroid hormones and bile acids. Moreover, the recognition that high cholesterol levels (hypercholesterolemia) are a major risk factor for the development of heart disease and atherosclerosis has gained enormous attention not only in medicine, medical and pharmacological research, but also from the general public. The discovery of a crucial role of cholesterol in human embryogenesis and the recent identification of a number of inherited disorders of cholesterol biosynthesis also show that low cholesterol levels (hypocholesterolemia) may have severe consequences for human health and development. In the past few years, seven distinct inherited disorders have been linked to different enzyme defects in the cholesterol biosynthetic pathway by the finding of abnormally increased levels of intermediate metabolites in patients followed by the demonstration of disease-causing mutations in genes encoding the implicated enzymes. Patients afflicted with these disorders are characterized by multiple morphogenic and congenital anomalies including internal organ, skeletal and/or skin abnormalities.

Smith–Lemli–Opitz syndrome (SLOS) is caused by mutations in the DHCR7 gene leading to deficient activity of 7‐dehydrocholesterol reductase (DHCR7; EC 1.3.1.21), the final enzyme of the cholesterol biosynthetic pathway, resulting in low cholesterol and high concentrations of its direct precursor 7‐dehydrocholesterol in plasma and tissues. We here report mutations identified in the DHCR7 gene of 13 children diagnosed with SLOS by clinical and biochemical criteria. We found a high frequency of the previously described IVS8–1 G > C splice acceptor site mutation (two homozygotes, eight compound heterozygotes). In addition, 13 missense mutations and one splice acceptor mutation were detected in eleven patients with a mild to moderate SLOS‐phenotype. The mutations include three novel missense mutations (W182L, C183Y, F255L) and one novel splice acceptor site mutation (IVS8–1 G > T). Two patients, homozygous for the IVS8–1 G > C mutation, presented with a severe clinical phenotype and died shortly after birth. Seven patients with a mild to moderate SLOS‐phenotype disclosed compound heterozygosity of the IVS8–1 G > C mutation in combination with different novel and known missense mutations.