Maintenance of phenotypic heterogeneity within cell populations is an evolutionarily conserved mechanism that underlies population survival upon stressful exposures. We show that the genomes of a cancer cell subpopulation that survives treatment with otherwise lethal drugs, the drug-tolerant persisters (DTPs), exhibit a repressed chromatin state characterized by increased methylation of histone H3 lysines 9 and 27 (H3K9 and H3K27). We also show that survival of DTPs is, in part, maintained by regulators of H3K9me3-mediated heterochromatin formation and that the observed increase in H3K9me3 in DTPs is most prominent over long interspersed repeat element 1 (LINE-1). Disruption of the repressive chromatin over LINE-1 elements in DTPs results in DTP ablation, which is partially rescued by reducing LINE-1 expression or function.
Chromatin/genetics , Drug Resistance, Neoplasm/genetics , Epigenetic Repression/drug effects , Long Interspersed Nucleotide Elements/genetics , Neoplasms/pathology , Animals , Antineoplastic Agents/pharmacology , Gene Expression Regulation, Neoplastic , Genomic Instability/drug effects , Histone-Lysine N-Methyltransferase/metabolism , Histones/metabolism , Humans , Methylation , Mice , Mice, Nude , Mice, SCID , Neoplasms/drug therapy , Neoplasms/genetics , Stress, Physiological , Tumor Cells, Cultured , Xenograft Model Antitumor Assays
Maintenance of genomic stability in proliferating cells depends on a network of proteins that coordinate chromosomal replication with DNA damage responses. Human DNA helicase B (HELB or HDHB) has been implicated in chromosomal replication, but its role in this coordinated network remains undefined. Here we report that cellular exposure to UV irradiation, camptothecin, or hydroxyurea induces accumulation of HDHB on chromatin in a dose- and time-dependent manner, preferentially in S phase cells. Replication stress-induced recruitment of HDHB to chromatin is independent of checkpoint signaling but correlates with the level of replication protein A (RPA) recruited to chromatin. We show using purified proteins that HDHB physically interacts with the N-terminal domain of the RPA 70-kDa subunit (RPA70N). NMR spectroscopy and site-directed mutagenesis reveal that HDHB docks on the same RPA70N surface that recruits S phase checkpoint signaling proteins to chromatin. Consistent with this pattern of recruitment, cells depleted of HDHB display reduced recovery from replication stress.
DNA Damage/physiology , DNA Helicases/metabolism , DNA Replication/physiology , Replication Protein A/metabolism , Stress, Physiological/physiology , Amino Acid Sequence , Chromosomes/physiology , DNA Helicases/chemistry , DNA Helicases/genetics , HCT116 Cells , HeLa Cells , Humans , Molecular Sequence Data , Mutagenesis, Site-Directed , Osteosarcoma , Protein Folding , Protein Interaction Domains and Motifs/physiology , Replication Protein A/chemistry , Replication Protein A/genetics , S Phase Cell Cycle Checkpoints/physiology
