Quantitative DSB sequencing (qDSB-Seq): a method for genome-wide accurate estimation of absolute DNA double-strand break frequencies per cell

Emerging genome-wide methods for mapping DNA double-strand breaks (DSBs) by sequencing (e.g. BLESS) are limited to measuring relative frequencies of breaks between loci. Knowing the absolute DSB frequency per cell, however, is key to understanding their physiological relevance. Here, we propose quantitative DSB sequencing (qDSB-Seq), a method to infer the absolute DSB frequency genome-wide. qDSB-Seq relies on inducing spike-in DSBs by a site-specific endonuclease and estimating the efficiency of the endonuclease cleavage by sequencing or PCR. This spike-in frequency is used to quantify DSB sequencing data. We present validation of the qDSB-Seq method and results of its application. We quantify DSBs resulting from replication stress and the collapse of replication forks on natural fork barriers in the ribosomal DNA. The qDSB-Seq approach can be used with any DSB sequencing method and allows accurate comparisons of absolute DSB frequencies across samples and precise quantification of the impact of various DSB-causing agents.