Pathway choice in DNA double strand break repair: observations of a balancing act

Abstract

Proper repair of DNA double strand breaks (DSBs) is vital for the preservation of genomic integrity. There are two main pathways that repair DSBs, Homologous recombination (HR) and Non-homologous end-joining (NHEJ). HR is restricted to the S and G2 phases of the cell cycle due to the requirement for the sister chromatid as a template, while NHEJ is active throughout the cell cycle and does not rely on a template. The balance between both pathways is essential for genome stability and numerous assays have been developed to measure the efficiency of the two pathways. Several proteins are known to affect the balance between HR and NHEJ and the complexity of the break also plays a role. In this review we describe several repair assays to determine the efficiencies of both pathways. We discuss how disturbance of the balance between HR and NHEJ can lead to disease, but also how it can be exploited for cancer treatment.

Insulin-like growth factor binding protein-3 (IGFBP-3): Novel ligands mediate unexpected functions

“…This form of DNA damage is potentially the most hazardous to genomic integrity, in response to which cells initiate DNA repair processes but may, nevertheless, eventually undergo apoptotic death. DNA DSB damage can induce two main repair mechanisms: homologous recombination (HR), which uses a homologous DNA template to guide the repair process and is generally restricted to S and G2 phase, and non-homologous end-joining (NHEJ), more common in mammalian cells, which joins the broken DNA ends directly and can occur at any stage of the cell cycle (Brandsma and Gent 2012;Jackson and Bartek 2009). An early step in DNA DSB repair by NHEJ is the binding of the serine/threonine kinase, DNA-dependent protein kinase, catalytic subunit (DNA-PKcs) to the damaged DNA ends, followed by its autophosphorylation.…”

Repair of Accidental DNA Double-Strand Breaks in the Human Genome and Its Relevance to Vector DNA Integration

“…Both NHEJ and HR can work, however, in S to G2 phases when DNA replication has been completed and the sister chromatid is available [19]. Thus, how and which pathway is chosen for repair of a DSB(s) has been a critical issue in the DNA repair field, and there has been a debate [4]. Recent evidence suggests that Ku-bound DSBs, where end resection does not occur, are directed to NHEJ, while end-resected DSBs, to which Ku cannot bind, are channeled to HR (or alternative end-joining) [20][21][22][23][24].…”

“…Interestingly, however, it appears that cells do not always choose a proper pathway to deal with induced DSBs. In fact, absence of NHEJ gives a growth advantage to cells accumulating replication-associated DSBs [34,35], although this may simply reflect the fact that NHEJ is basically the first choice to repair any type of those DSBs that naturally allow Ku-binding [4].…”

“…DSBs, if left unrepaired or mis-repaired, lead to cell death or chromosomal aberrations [2,3]. Human cells have evolved two fundamentally different mechanisms for repairing chromosomal DSBs, homologous recombination (HR) and non-homologous end-joining (NHEJ) [4]. NHEJ not only repairs accidental (non-physiological) DSBs, but is also essential for rejoining physiological DSBs that arise in the process of V(D)J recombination in B and T lymphocytes and class switch recombination in mature B cells [3].…”

“…NHEJ (the classical NHEJ pathway) repairs broken DNA ends with little or no homology and is often associated with nucleotide loss, whereas HR allows for accurate repair of DSBs with the use of homologous DNA sequence, usually located on a sister chromatid [3,4,12]. Such difference in accuracy between the two pathways, however, does not mean that HR is superior to NHEJ in maintaining integrity of human genomes, which contain lots of repetitive DNA sequences [4]. For example, an HR reaction between Alu sequences in a cell would cause deleterious consequences and hence must be prohibited [17,18].…”