Streamlined quantitative analysis of histone modification abundance at nucleosome-scale resolution with siQ-ChIP version 2.0

Abstract

We recently introduced an absolute and physical quantitative scale for chromatin immunoprecipitation followed by sequencing (ChIP-seq). The scale itself was determined directly from measurements routinely made on sequencing samples without additional reagents or spike-ins. We called this approach sans spike-in quantitative ChIP, or siQ-ChIP. Herein, we extend those results in several ways. First, we simplified the calculations defining the quantitative scale, reducing practitioner burden. Second, we reveal a normalization constraint implied by the quantitative scale and introduce a new scheme for generating ‘tracks’. The constraint requires that tracks are probability distributions so that quantified ChIP-seq is analogous to a mass distribution. Third, we introduce some whole-genome analyses that allow us, for example, to project the IP mass (immunoprecipitated mass) onto the genome to evaluate how much of any genomic interval was captured in the IP. We applied siQ-ChIP to p300/CBP inhibition and compare our results to those of others. We detail how the same data-level observations are misinterpreted in the literature when tracks are not understood as probability densities and are compared without correct quantitative scaling, and we offer new interpretations of p300/CBP inhibition outcomes.