μChIP -: a rapid micro chromatin immunoprecipitation assay for small cell samples and biopsies

Chromatin immunoprecipitation (ChIP) is a powerful technique for studying proteinDNA interactions. Drawbacks of current ChIP assays however are a requirement for large cell numbers, which limits applicability of ChIP to rare cell samples, and/or lengthy procedures with limited applications. There are to date no protocols for fast and parallel ChIPs of post-translationally modified histones from small cell numbers or biopsies, and importantly, no protocol allowing for investigations of transcription factor binding in small cell numbers. We report here the development of a micro (mu) ChIP assay suitable for up to nine parallel quantitative ChIPs of modified histones or RNA polymerase II from a single batch of 1000 cells. mu ChIP can also be downscaled to monitor the association of one protein with multiple genomic sites in as few as 100 cells. mu ChIP is applicable to small fresh tissue biopsies, and a cross-link-while-thawing procedure makes the assay suitable for frozen biopsies. Using mu ChIP, we characterize transcriptionally permissive and repressive histone H3 modifications on developmentally regulated promoters in human embryonal carcinoma cells and in osteosarcoma biopsies. mu ChIP creates possibilities for multiple parallel and rapid transcription factor binding and epigenetic analyses of rare cell and tissue samples.