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Cholesterol plays an essential role in many cellular and developmental processes. In addition to its role as a membrane lipid, it is the precursor to numerous molecules that play an important role in cell growth and differentiation, protein glycosylation, and signaling pathways. The biosynthesis of cholesterol and its subsequent conversion to other essential compounds is complex, involving a number of intermediates and enzymes. Disorders that result from a deficiency of these enzymes lead to an accumulation of specific intermediates and inhibit the formation of important biomolecules. Clinical findings common to cholesterol biosynthesis disorders include congenital skeletal malformations, dysmorphic facial features, psychomotor retardation, and failure to thrive.
Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive disorder caused by mutations in the DHCR7 gene leading to a deficiency of the 7-dehydrocholesterol reductase enzyme. It is characterized by markedly increased plasma concentrations of 7-dehydrocholesterol (7-DHC) and 8-DHC levels. Clinical features can include microcephaly, growth retardation, developmental delay, dysmorphic facial features, cleft palate, limb abnormalities (especially 2-3 syndactyly of the toes and postaxial polydactyly), and heart and kidney malformations. Severity of SLOS ranges from severe to mild. Some mildly affected individuals may only have 2 to 3 toe syndactyly and mild cognitive impairment. The reported incidence is between 1 in 10,000 and 1 in 60,000, but it may be more prevalent due to underdiagnosis of mildly affected individuals.
Other disorders of cholesterol biosynthesis, including desmosterolosis (desmosterol reductase deficiency) and sitosterolemia, may present with similar manifestations. These disorders can be detected biochemically by performing a quantitative profile of plasma sterols (STER / Sterols, Plasma).
Diagnosis of Smith-Lemli-Opitz syndrome (7-dehydrocholesterol reductase deficiency)
Elevated plasma concentrations of 7-dehydrocholesterol (7-DHC) and 8-DHC are highly suggestive of a biochemical diagnosis of Smith-Lemli-Opitz (SLO).
Mild elevations of these cholesterol precursors can be detected in patients with hypercholesterolemia and patients treated with haloperidol. However, the 7-DHC to cholesterol ratio is only elevated in SLO patients.
The enzymatic tests for measuring plasma cholesterol quantitate all 3 hydroxysterols and are unreliable for diagnosis for Smith-Lemli-Opitz.
Negative (reported as positive or negative)
Quantitative results are provided when positive.
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2. Nowaczyk MJM: Smith-Lemli-Opitz Syndrome. In GeneReviews. Edited by RA Pagon, MP Adam, TD Bird, et al. Seattle, WA, University of Washington. 1993-2014. Available from: http://www.ncbi.nlm.nih.gov/books/NBK1143/
3. Hall P, Michels V, Gavrilov D, et al: Aripiprazole and trazodone cause elevations of 7-dehydrocholesterol in the absence of Smith-Lemli-Opitz Syndrome. Mol Genet Metab 2013 Sep-Oct;110(1-2):176-178