In silico modeling of epigenetic-induced changes in photoreceptor cis-regulatory elements

DNA methylation is a well-characterized epigenetic repressor of mRNA transcription in many plant and vertebrate systems. However, the mechanism of this repression is not fully understood. The process of transcription is controlled by proteins that regulate recruitment and activity of RNA polymerase by binding to specific cis-regulatory sequences. Cone-rod homeobox (CRX) is a well-characterized mammalian transcription factor that controls photoreceptor cell-specific gene expression. Although much is known about the functions and DNA binding specificity of CRX, little is known about how DNA methylation modulates CRX binding affinity to genomic cis-regulatory elements.

We used bisulfite pyrosequencing of human ocular tissues to measure DNA methylation levels of the regulatory regions of RHO, PDE6B, PAX6, and LINE1 retrotransposon repeats. To describe the molecular mechanism of repression, we used molecular modeling to illustrate the effect of DNA methylation on human RHO regulatory sequences.

In this study, we demonstrate an inverse correlation between DNA methylation in regulatory regions adjacent to the human RHO and PDE6B genes and their subsequent transcription in human ocular tissues. Docking of CRX to the DNA models shows that CRX interacts with the grooves of these sequences, suggesting changes in groove structure could regulate binding. Molecular dynamics simulations of the RHO promoter and enhancer regions show changes in the flexibility and groove width upon epigenetic modification. Models also demonstrate changes in the local dynamics of CRX binding sites within RHO regulatory sequences which may account for the repression of CRX-dependent transcription.

Collectively, these data demonstrate epigenetic regulation of CRX binding sites in human retinal tissue and provide insight into the mechanism of this mode of epigenetic regulation to be tested in future experiments.