Site-specific DNA hypomethylation permits expression of the IRBP gene

Interphotoreceptor retinoid binding protein (IRBP), a putative component of the visual cycle, is expressed selectively in the retina and pineal gland. This study examined whether site-specific DNA hypomethylation plays a role in this expression regulation. Southern blotting of HpaII and MspI digests of DNA from various bovine and murine tissues (whole brain, retina, pineal gland, superior colliculus, cortex, thymus, habenular nucleus, cornea, liv er, tail, and kidney) revealed that specific CpG dinucleotides in the IRBP gene promoter are hypomethylated in DNA from retinal photoreceptor cells an d pineal gland compared to DNA from other tissues. These sites are methylat ed in DNA from non-photoreceptor retinal cells. Exogenous methylation of th ese sites diminished DNA:protein binding in electrophoretic mobility shift assays. HpaII methylation of chloramphenicol acetyltransferase reporter con structs suppressed IRBP but not SV40 promoter activity in transiently trans fected primary cultures of embryonic chick retinal cells. These data indica te that specific cytosines in the bovine and murine IRBP promoters are unme thylated in photoreceptive cells but methylated in other tissues. This diff erential DNA methylation may modulate IRBP gene expression since exogenous methylation of the murine sites suppresses reporter gene transcription, app arently by inhibiting DNA:protein binding events. (C) 2000 Elsevier Science BN. All rights reserved.