Journal Article

Cell-type-specific replication initiation programs set fragility of the FRA3B fragile site

Hansen, R. Scott, Vogt, Nicolas, Lachagès, Anne-Marie, Malfoy, Bernard, Millot, Gaël A., Debatisse, Michelle, Brison, Olivier, Koundrioukoff, Stéphane, Letessier, Anne

2011

Overview

The origins of fragility Some chromosomal locations, known as common fragile sites, are predisposed to breakage. These sites have pathological relevance because they are often associated with chromosomal translocations. An analysis of the replication dynamics along a 1.6-Mb region of FRA3B , a common fragile site in human lymphocytes that hosts the FHIT tumour suppressor gene, shows that, rather than breakage being due to replication stalling, FRA3B site fragility results from an unusually low density of replication origins. Surprisingly, fragility is cell-type-specific, which may have implications for current models of translocations and tumorigenesis. Some chromosomal locations, known as common fragile sites, are predisposed to breakage. These sites have pathogenic relevance as they are frequently associated with chromosomal translocations. Here, it is found that rather than breakage being due to replication stalling, the fragility of site FRA3B results from an unusually low density of replication origins in this region. Unexpectedly, fragility is found to be cell-type-specific, which may have implications for current models of translocations. Common fragile sites have long been identified by cytogeneticists as chromosomal regions prone to breakage upon replication stress 1 . They are increasingly recognized to be preferential targets for oncogene-induced DNA damage in pre-neoplastic lesions 2 and hotspots for chromosomal rearrangements in various cancers 3 . Common fragile site instability was attributed to the fact that they contain sequences prone to form secondary structures that may impair replication fork movement, possibly leading to fork collapse resulting in DNA breaks 4 . Here we show, in contrast to this view, that the fragility of FRA3B —the most active common fragile site in human lymphocytes—does not rely on fork slowing or stalling but on a paucity of initiation events. Indeed, in lymphoblastoid cells, but not in fibroblasts, initiation events are excluded from a FRA3B core extending approximately 700 kilobases, which forces forks coming from flanking regions to cover long distances in order to complete replication. We also show that origins of the flanking regions fire in mid-S phase, leaving the site incompletely replicated upon fork slowing. Notably, FRA3B instability is specific to cells showing this particular initiation pattern. The fact that both origin setting 5 , 6 and replication timing are highly plastic 7 , 8 in mammalian cells explains the tissue specificity of common fragile site instability we observed. Thus, we propose that common fragile sites correspond to the latest initiation-poor regions to complete replication in a given cell type. For historical reasons, common fragile sites have been essentially mapped in lymphocytes 1 . Therefore, common fragile site contribution to chromosomal rearrangements in tumours should be reassessed after mapping fragile sites in the cell type from which each tumour originates.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: Abnormalities / Acid Anhydride Hydrolases / Acid Anhydride Hydrolases - genetics / Artificial chromosomes / Asymmetry / Biological and medical sciences / Cancer / Cell Line / Chromosome Breakage / Chromosome Fragile Sites / Chromosome Fragile Sites - genetics / Chromosome Fragility / Chromosome Fragility - genetics / Chromosome Fragility - physiology / Deoxyribonucleic acid / DNA / DNA Replication / DNA Replication - genetics / DNA Replication - physiology / Fibroblasts / Fundamental and applied biological sciences. Psychology / Genes, Tumor Suppressor / Genetic aspects / Genetic Loci / Genetic Loci - genetics / Health aspects / Humanities and Social Sciences / Humans / letter / Life Sciences / Lymphocytes / Lymphocytes - metabolism / Models, Biological / Molecular and cellular biology / Molecular genetics / multidisciplinary / Neoplasm Proteins / Neoplasm Proteins - genetics / Organ Specificity / Replication / Replication Origin / Replication Origin - genetics / Science / Science (multidisciplinary)