7-DHC and SLOS

7-DHC and SLOS


3β-Hydroxysterol-Δ7-reductase (DHCR7; EC catalyzes one of the two parallel last steps of the cholesterol biosynthesis pathway. Defects in DHCR7 lead to greatly elevated levels of 7-dehydrocholesterol (7-DHC) and decreased levels of cholesterol in individuals affected with Smith-Lemli-Opitz syndrome (SLOS). The level of 8-DHC is also elevated in SLOS patients due to the functioning of 3β-hydroxysterol-Δ87-isomerase (EBP; EC, which catalyzes the equilibration between the Δ8– and the Δ7-double bond. Therapeutic intervention toward SLOS has been focused on cholesterol supplementation, but this approach gives inconsistent results and does not benefit neurological defects. Our recent finds suggest that 7-DHC is prone to free radical oxidation, leading to the formation of over a dozen oxysterol products, many of which have been observed in vivo (see Figure below) [1,2]. Some of these oxysterols are highly cytotoxic and lead to changes in gene expression and cell differentiation.


7-DHC also serves as an unusual substrate for cytochrome P450 (CYP) 7A1, leading to the cytotoxic 7-ketocholesterol without going through an epoxide [3]. This is a novel pathway of formation for 7-ketocholesterol as cholesterol is the only known precursor to this oxysterol  (mostly via free radical mechanism) before this finding. Recently, 7-DHC was also found to be a substrate of CYP 46A1, leading to the expected 24-OH-7-DHC and the unusual 25-OH-7-DHC (the latter being the favored product) [4]. In addition, 4α-OH- and 4β-OH-7-DHC may also be enzymatic products of 7-DHC, and the responsible enzyme(s) are still under investigation [5].


We aim to elucidate the biological actions of these 7-DHC-derived oxysterols by examining the transcriptome and lipidome as oxysterols play central roles in lipid metabolism. We use a combination of neurobiology, mass spectrometry, transcriptomics, and organic synthesis approaches to accomplish the Aims in this project.

The ultimate goal of this project is to develop therapies that target 7-DHC and/or its oxysterols, ameliorating or eliminating their detrimental effects. Recently, through collaboration with Prof. Steven Fliesler at SUNY-Buffalo, we demonstrated that supplementation of an antioxidant mix (containing vitamin E, vitamin C, and selenite) completely prevented retinal degeneration in AY9944-treated rats, a SLOS model (Fliesler et al. Sci. Rep. 2018). To our knowledge, this is the first time that an intervention has completely prevented the display of a phenotype associated with SLOS.


1. Xu, L., Davis, T. A., and Porter, N. A. (2009) Rate Constants for Peroxidation of Polyunsaturated Fatty Acids and Sterols in Solution and in Liposomes, J. Am. Chem. Soc. 131, 13037-13044.

2. Xu, L.,* Korade, Z., Rosado, D. A., Mirnics, K., Porter, N. A.* (2013) Metabolism of oxysterols derived from nonenzymatic oxidation of 7-dehydrocholesterol in cells, J. Lipid Res. 54, 1135-1143.

3. Shinkyo, R., Xu, L., Tallman, K. A., Cheng, Q., Porter, N. A., and Guengerich, F. P. (2011) Conversion of 7-dehydrocholesterol to 7-ketocholesterol is catalyzed by human cytochrome P450 7A1 and occurs by direct oxidation without an epoxide intermediate, J. Biol. Chem. 286, 33021-33028.

4. Goyal, S., Xiao, Y., Porter, N. A., Xu, L.,* Guengerich, F. P.* (2014) Oxidation of 7-Dehydrocholesterol and Desmosterol by Human Cytochrome P450 46A1 J. Lipid Res. 55, 1933-1943.

5. Xu, L., Liu, W., Sheflin, L. G., Fliesler, S. J., and Porter, N. A. (2011) Novel oxysterols observed in tissues and fluids of AY9944-treated rats – a model for Smith-Lemli-Opitz Syndrome, J. Lipid Res. 52, 1810-1820.

6. Fliesler, S. J.*; Peachey, N. S.; Herron, J.; Hines, K. M.; Weinstock, N. I.; Ramachandra Rao, S.; and Xu, L.* (2018) Prevention of Retinal Degeneration in a Rat Model of Smith-Lemli-Opitz Syndrome, Sci. Rep., 8, 1286.