Smith-Lemli-Opitz Syndrome reveals requirement for sterol biosynthesis in the innate immune response

FARE Award Winner

Authors

• KA Gabor
• CA Wassif
• PR Bushel
• MW Henderson
• FD Porter
• MB Fessler

Abstract

Smith-Lemli-Opitz Syndrome (SLOS) is a rare developmental disorder caused by mutation of 7-dehydrocholesterol (7DHC) reductase (DHCR7), a terminal enzyme in the cholesterol biosynthesis pathway. SLOS is associated with cholesterol deficit and 7DHC excess. SLOS patients have increased incidence of infections, but no mechanism of immunosuppression has been identified. Toll-like Receptors (TLRs), key pathogen-recognition receptors of the innate immune system, localize to cholesterol-rich lipid raft microdomains. We predicted that SLOS cells would have attenuated TLR signaling due to deficient raft cholesterol. Primary dermal fibroblasts from SLOS patients were used to assess the TLR4 response to lipopolysaccharide (LPS). Upon stimulation, significantly lower NF-kB activation and interleukin (IL)-6 and IL-8 production were observed in SLOS cells versus controls. Across patients, a direct relationship between residual DHCR7 enzyme activity and IL-6 production was observed, together with an inverse relation between clinical severity scores and IL-8 production. Microarray analysis revealed several pro-inflammatory chemokines that were less robustly induced by LPS in SLOS fibroblasts. Deficient LPS-induced cytokines were also observed in both macrophages from Dhcr7 mutant mice and a pharmacological model of SLOS that we generated using RAW264.7 macrophages treated with DHCR7 inhibitors. Mechanistically, confocal microscopy revealed that TLR4 colocalization with lipid rafts is decreased in SLOS macrophages. Taken together, these findings indicate that the SLOS mutation confers an abnormal TLR4 response that may arise from deficient raft cholesterol. We propose that deficient innate immunity may be a clinically relevant contributor to SLOS pathogenesis, and that SLOS highlights the importance of cholesterol biosynthesis to innate immunity.

Scientific Focus Area: Immunology

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