Breakdance in mammalian heterochromatin!
Super-resolution image depicting DNA Double Strand Breaks (revealed by g-H2AX, the main DSB marker, in red) in centromeres (in green), localized around the DAPI-dense regions corresponding to pericentromeres.
July 7, 2016
The teams of Evi Soutoglou and Bernardo Reina-San-Martin have taken advantage of the CRISPR/Cas9 genome editing technology to identify the molecular mechanisms regulating the repair of double stranded DNA breaks (DSBs) arising in highly compacted chromatin (constitutive heterochromatin). These results are published on July 7th 2016 in Molecular Cell.
Various types of DNA damage are constantly assaulting the integrity of our genome and of its genetic information. Double Stranded DNA Breaks (DSBs) are the most lethal and dangerous, as they are the origin of chromosomal translocations and cancer. As a consequence cells have evolved multiple pathways to detect, signal and repair DSBs.
The different levels of compaction and DNA repair
Chromosomal DNA is packaged within the nucleus at different levels of compaction. Euchromatin, which is enriched in actively transcribed genes, is in an open configuration and is easily accessible to DNA repair factors. In contrast, constitutive heterochromatin is gene-poor and is in a highly compacted state. For example, centromeres and pericentromeres, which consist of tandem repetitive satellite sequences that are necessary for proper chromosomal segregation, cluster together into readily identifiable structures within the nucleus. DSBs arising in these structures represent a significant challenge that cells need to overcome in order to preserve genomic integrity. Nevertheless, little is known about the underlying mechanisms of DSB repair taking place within heterochromatin.
Technology CRISPR / Cas 9 and CDBs
To define these mechanisms, an innovative cellular system based on the CRISPR/Cas9 genome editing technology was developed in which DSBs can be specifically induced at pericentromeres or centromeres. Using this system, the teams of Evi Soutoglou and Bernardo Reina-San-Martin demonstrate that in pericentromeres, DSBs arising in the G1 phase of cell cycle are stable and are repaired by Non Homologous End Joining (NHEJ), a pathway that efficiently links broken DNA ends in the absence of a homologous template. In the S and G2 phases of the cell cycle, where the sister chromatids are present, DSBs are first processed into single strand DNA by a process named end-resection. Resected DNA ends then move to the periphery of heterochromatin to search for the sister chromatid to be repaired by Homologous Recombination (HR), an error-free pathway. Those DSBs, which fail to move, are instead repaired through highly mutagenic DNA repair pathways.
To determine whether this mechanism is common to other heterochromatin structures, the authors induced DSBs at centromeres. Interestingly, they find that contrary to pericentromeres, DSBs are repaired by NHEJ and HR throughout the cell cycle, highlighting striking differences in DNA repair between both structures.
These findings reveal that DSB mobility serves as a mechanism to prevent the activation of mutagenic DNA repair pathways within heterochromatin. Failure in this safeguard process might be at the origin of the high mutation rate and chromosomal translocations involving heterochromatin that have been described in many cancers.