Differentiation-induced replication-timing changes are restricted to AT-rich/long interspersed nuclear element (LINE)-rich isochores

The replication timing of some genes is developmentally regulated, but the significance of replication timing to cellular differentiation has been difficult to substantiate. Studies have largely been restricted to the comparison of a few genes in established cell lines derived from different tissues, and most of these genes do not change replication timing. Hence, it has not been possible to predict how many or what types of genes might be subject to such control. Here, we have evaluated the replication timing of 54 tissue-specific genes in mouse embryonic stem cells before and after differentiation to neural precursors. Strikingly, genes residing within isochores rich in GC and poor in long interspersed nuclear elements (LINEs) did not change their replication timing, whereas half of genes within isochores rich in AT and long interspersed nuclear elements displayed programmed changes in replication timing that accompanied changes in gene expression. Our results provide direct evidence that differentiation-induced autosomal replication-timing changes are a significant part of mammalian development, provide a means to predict genes subject to such regulation, and suggest that replication timing may be more related to the evolution of metazoan genomes than to gene function or expression pattern.