Project description:Abnormal replication timing has been observed in cancer but no study has comprehensively evaluated this misregulation. We generated genome-wide replication timing profiles for pediatric leukemias from 17 patients and 3 cell lines, as well as normal B and T cells. Non-leukemic EBV-transformed lymphoblastoid cell lines displayed highly stable replication timing profiles that were more similar to normal T cells than to leukemias. Leukemias were more similar to each other than to B and T cells but were considerably more heterogeneous than non-leukemic controls. Some differences were patient-specific while others were found in all leukemic samples, potentially representing early epigenetic events. Differences encompassed large segments of chromosomes and included genes implicated in other types of cancer. Remarkably, differences that distinguished leukemias aligned in register to the boundaries of developmentally regulated replication timing domains that distinguish normal cell types. Most changes did not coincide with copy number variation or translocations. However, many of the changes that were associated with translocations in some leukemias were also shared between all leukemic samples independent of the genetic lesion, suggesting that they precede and possibly predispose chromosomes to the translocation. Altogether, our results identify sites of abnormal developmental control of DNA replication in cancer that reveal the significance of replication timing boundaries to chromosome structure and function and support the replication domain model of replication timing regulation. They also open new avenues of investigation into the chromosomal basis of cancer and provide a potential novel source of epigenetic cancer biomarkers. Four karyotypically normal B-lymphoblastoid cell types with two replicates each, one peripheral T-lymphoblast replicate, 3 leukemic cell lines with 1-3 replicates each, 17 patient samples with 1-3 replicates each (total of 40 individual replicates)
Project description:Abnormal replication timing has been observed in cancer but no study has comprehensively evaluated this misregulation. We generated genome-wide replication timing profiles for pediatric leukemias from 17 patients and 3 cell lines, as well as normal B and T cells. Non-leukemic EBV-transformed lymphoblastoid cell lines displayed highly stable replication timing profiles that were more similar to normal T cells than to leukemias. Leukemias were more similar to each other than to B and T cells but were considerably more heterogeneous than non-leukemic controls. Some differences were patient-specific while others were found in all leukemic samples, potentially representing early epigenetic events. Differences encompassed large segments of chromosomes and included genes implicated in other types of cancer. Remarkably, differences that distinguished leukemias aligned in register to the boundaries of developmentally regulated replication timing domains that distinguish normal cell types. Most changes did not coincide with copy number variation or translocations. However, many of the changes that were associated with translocations in some leukemias were also shared between all leukemic samples independent of the genetic lesion, suggesting that they precede and possibly predispose chromosomes to the translocation. Altogether, our results identify sites of abnormal developmental control of DNA replication in cancer that reveal the significance of replication timing boundaries to chromosome structure and function and support the replication domain model of replication timing regulation. They also open new avenues of investigation into the chromosomal basis of cancer and provide a potential novel source of epigenetic cancer biomarkers.
Project description:As the only surviving lineages of jawless fishes, hagfishes and lampreys provide a critical window into early vertebrate evolution. Here, we investigate the complex history, timing, and functional role of genome-wide duplications in vertebrates in the light of a chromosome-scale genome of the brown hagfish Eptatretus atami. Using robust chromosome-scale (paralogon-based) phylogenetic methods, we confirm the monophyly of cyclostomes, document an auto-tetraploidization (1RV) that predated the origin of crown group vertebrates ~517 Mya, and establish the timing of subsequent independent duplications in the gnathostome and cyclostome lineages. Some 1RV gene duplications can be linked to key vertebrate innovations, suggesting that this early genomewide event contributed to the emergence of pan-vertebrate features such as neural crest. The hagfish karyotype is derived by numerous fusions relative to the ancestral cyclostome arrangement preserved by lampreys. These genomic changes were accompanied by the loss of genes essential for organ systems (eyes, osteoclast) that are absent in hagfish, accounting in part for the simplification of the hagfish body plan; other gene family expansions account for hagfishes’ capacity to produce slime. Finally, we characterise programmed DNA elimination in somatic cells of hagfish, identifying protein-coding and repetitive elements that are deleted during development. As in lampreys, the elimination of these genes provides a mechanism for resolving genetic conflict between soma and germline by repressing germline/pluripotency functions. Reconstruction of the early genomic history of vertebrates provides a framework for further exploration of vertebrate novelties.
Project description:In multicellular organisms, developmental changes to replication timing occur in 400- 800 kb domains across half the genome. While clear examples of epigenetic control of replication timing have been described, a role for DNA sequence in mammalian replication timing has not been substantiated. To assess the role of DNA sequences in directing these changes, we profiled replication timing in mice carrying a genetically rearranged Human Chromosome 21 [Hsa21]. In two distinct mouse cell types, Hsa21 sequences maintained human-specific replication timing, except at points of Hsa21 rearrangement. Changes in replication timing at rearrangements extended up to 900 kb and consistently reconciled with the wild-type replication pattern at developmental boundaries of replication-timing domains. Our results demonstrate DNA sequencedriven regulation of Hsa21 replication timing during development and provide evidence that mammalian chromosomes consist of multiple independent units of replication timing regulation.