High resolution mapping of DNA replication origins in human genome

Because of the lack of information, regulation of DNA replication initiation in mammals is still poorly understood. In order to identify general rules, we have mapped replication origins along 1% of the human genome in HeLa cells. We found large gene-poor regions lacking origin and G+C rich regions containing clusters of closely spaced origins. Half of the 283 origins mapped are within or near CpG islands. The connection with gene expression is further reinforced by the observation that most origins overlap with DNAseI hypersensitive sites found at transcriptional regulatory elements. We show, however, that this association is independent of chromatin structure and transcriptional activity. Replication timing analyses coupled to our origin mapping demonstrate that origin dense regions and isolated origins are replicated at every moment in S phase. All together, our data suggest that a relatively strict origin-timing programme regulates DNA replication of the human genome. Keywords: Nascent strands, ENCODE project, HeLAS3 cells, SNS-Chip Four independent preparations of Short Nascent Strands (SNS) were performed. In order to have enough material for microarray hybridisation, we coupled the stringent preparation of SNS with the TLAD method, a technique of linear amplification that can generate several µg of amplified material from 10-20 ng of DNA (Liu et al., 2003).Two were amplified by TLAD (experiments A and B) and hybridized on DNA microarrays, and the other two (experiments C and D) were used for the validation by real-time quantitative PCR (qPCR) of results obtained on micro-arrays. We performed also a gDNA/gDNA hybridization where gDNA are also amplified by TLAD to order to do a control.