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

Abstract

Abstract Smith-Lemli-Opitz Syndrome (SLOS) is a rare neurodevelopmental disorder caused by mutation of 7-dehydrocholesterol (7DHC) reductase (DHCR7), a terminal enzyme in cholesterol biosynthesis. SLOS patients have increased incidence of infections, but no mechanism of immunosuppression has been identified. As cholesterol is a key component of membrane raft microdomains and rafts have been implicated in Toll-like receptor (TLR) signaling, we examined whether TLR responsiveness is altered in SLOS. Primary fibroblasts from SLOS patients with varying degrees of deficit in DHCR7 activity were used to assess the TLR4 response. Upon stimulation, significantly lower NF-kB activation and IL-6 and IL-8 production were observed in SLOS cells than in controls. Across patients, a direct relation 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 that several pro-inflammatory chemokines are less responsive to LPS in SLOS. Deficient LPS-induced cytokines were also observed in macrophages from Dhcr7 mutant mice and in RAW264.7 macrophages treated with DHCR7 inhibitors. Supporting cholesterol deficit as the cause of the TLR4 impairment, the LPS response of SLOS macrophages was partially rescued by cholesterol repletion. Mechanistically, confocal microscopy revealed that TLR4 localization to rafts is decreased in SLOS macrophages. Taken together, these findings indicate that the SLOS mutation confers an abnormal TLR4 response. Deficient innate immunity may be a clinically relevant contributor to SLOS pathogenesis, and SLOS highlights the importance of cholesterol biosynthesis to innate immunity.