The FANC/BRCA Pathway Releases Replication Blockades by Eliminating DNA Interstrand Cross-Links

Abstract

DNA interstrand cross-links (ICLs) represent a major barrier blocking DNA replication fork progression. ICL accumulation results in growth arrest and cell death—particularly in cell populations undergoing high replicative activity, such as cancer and leukemic cells. For this reason, agents able to induce DNA ICLs are widely used as chemotherapeutic drugs. However, ICLs are also generated in cells as byproducts of normal metabolic activities. Therefore, every cell must be capable of rescuing lCL-stalled replication forks while maintaining the genetic stability of the daughter cells in order to survive, replicate DNA and segregate chromosomes at mitosis. Inactivation of the Fanconi anemia/breast cancer-associated (FANC/BRCA) pathway by inherited mutations leads to Fanconi anemia (FA), a rare developmental, cancer-predisposing and chromosome-fragility syndrome. FANC/BRCA is the key hub for a complex and wide network of proteins that—upon rescuing ICL-stalled DNA replication forks—allows cell survival. Understanding how cells cope with ICLs is mandatory to ameliorate ICL-based anticancer therapies and provide the molecular basis to prevent or bypass cancer drug resistance. Here, we review our state-of-the-art understanding of the mechanisms involved in ICL resolution during DNA synthesis, with a major focus on how the FANC/BRCA pathway ensures DNA strand opening and prevents genomic instability.

Chromosome Instability in Fanconi Anemia: From Breaks to Phenotypic Consequences

“…The FA/BRCA pathway is activated for repairing ICLs during the S phase of the cell cycle, when the replisome finds an ICL and two convergent replication forks become stalled [11]. The ICL repair process can be divided in modules of activity of the FA/BRCA pathway [12] (Figure 1). 1) Lesion recognition.…”

“…After ICLs repair the activity of the replication fork is restarted, the last module includes the deubiquitination of FANCD2/FANCI activated complex, leading to the re-start of the DNA synthesis by the canonical DNA polymerases. The deubiquitination is performed by the USP1-UAF1 complex resulting in the release of the ID2 complex from the chromatin to complete the ICL repair cycle [12]. partners recognize ICLs during the convergence of two replication forks and promote ATR activation; the CMG helicase complex is unloaded to allow the approach of the leading strands to the ICL.…”

Causes and Consequences of Chromosomal Instability in Fanconi Anemia. Alterations at the Cellular and Organism Level

“…5) After ICLs repair the activity of the replication fork is restarted, the finisher module includes the deubiquitination of FANCD2/FANCI activated complex, leading to the re-start of the DNA synthesis by the canonical DNA polymerases. The deubiquitination is performed by the USP1-UAF1 complex resulting in the release of the ID2 complex from the chromatin to finish the ICL repair cycle [10]. partners recognize ICLs during the convergence of two replication forks and promote ATR activation; the CMG helicase complex is unloaded to allow the approach of the leading strands to the ICL.…”

“…Importantly, HR preferentially uses the sister chromatid as a template due to its perfect homology and close proximity, though the use of the homologous chromosome is also possible, however this alternative is less efficient and is prone to generate regions of homozygosity in the next cell generation. Once the DSB is repaired, FANCD2-Ub have to be extracted from the lesion by the deubiquitinase complex USP1-UAF1 and p97 to finish HR repair [10,28].…”

“…The FA/BRCA pathway, via FANCD2, restrains the accumulation of 53BP1 by regulating the activity of TIP60, an acetylase of the histone H4 that increases the presence of H4K16ac and H2AK15ac in the site of DNA damage and avoids the accumulation of H4K20me (the docking site for 53BP1). Failure of the FA/BRCA pathway leads to 53BP1 accumulation and favors NHEJ, leading to chromosomal aberrations [10].…”