Profiling methyl-CpG specific determinants on transcriptionally silent chromatin.

Transcriptional activity is closely associated with DNA methylation and chromatin remodelling. Evidence is emerging that a family of methylation specific (methyl-CpG binding domain, MBD) proteins have the capacity to bind to methylated sequences and repress transcription. Recent advances in this area reveal that many of the MBD proteins are associated with histone deacetylase (HDAC) dependant repression. The capacity of MBD association to repress transcription would largely be defined by promoter structure and this is best explained by the position and density of DNA methylation. The mechanism of specific targeting of MBD family members to methylated sequences remains largely unknown. In order to understand the mechanistic details of silencing the current challenge is to identify and map these molecular determinants assembled on native chromatin in model systems of human development and disease. Downstream targets such as the methylated Fragile X Mental Retardation gene 1 (FMR1) gene and tumour suppressor genes are likely candidates. In this article, we describe a powerful strategy that involves the immunoprecipitation of in vivo formaldehyde fixed chromatin to identify MBD binding complexes directly isolated from the natural chromosomal environment. We demonstrate the methylated human Multidrug Resistance gene 1 (MDR1) is enriched with transcriptional repressors that belong to the MBD family and this would account for transcriptional silencing.