Loss of Faap20 Causes Hematopoietic Stem and Progenitor Cell Depletion in Mice Under Genotoxic Stress

Gene loss in Antarctic icefish: evolutionary adaptations mimicking Fanconi Anemia?

BACKGROUND

The white-blooded Antarctic icefishes is a representative organism that survive under the stenothermal conditions of the Southern Ocean without the hemoglobin genes. To compensate for inefficient oxygen transport, distinct features such as increased heart size, greater blood volume, and reduced hematocrit density enhance the amount of dissolved oxygen and the velocity of blood flow.

RESULTS

Here, we investigated these unique characteristics by comparing high-quality genomic data between white-blooded and red-blooded fishes and identified the loss of FAAP20, which is implicated in anemia. Although the gene region containing FAAP20 is conserved in notothenioids as shown through collinear analysis, only remnants of FAAP20 persist in several icefish species. Additionally, we observed the loss of SOAT1, which plays a pivotal role in cholesterol metabolism, providing a clue for further investigations into the unique mitochondrial form of the icefish.

CONCLUSIONS

The loss of FAAP20, which is known to reduce erythrocyte counts under stress conditions in mice and humans, may provide a clue to understanding the genomic characteristics related to oxygen supply, such as low hematocrit, in Antarctic icefishes.

Fanconi anemia associated protein 20 (FAAP20) plays an essential role in homology-directed repair of DNA double-strand breaks

FAAP20 is a Fanconi anemia (FA) protein that associates with the FA core complex to promote FANCD2/FANCI monoubiquitination and activate the damage response to interstrand crosslink damage. Here, we report that FAAP20 has a marked role in homologous recombination at a DNA double-strand break not associated with an ICL and separable from its binding partner FANCA. While FAAP20's role in homologous recombination is not dependent on FANCA, we found that FAAP20 stimulates FANCA's biochemical activity in vitro and participates in the single-strand annealing pathway of double-strand break repair in a FANCA-dependent manner. This indicates that FAAP20 has roles in several homology-directed repair pathways. Like other homology-directed repair factors, FAAP20 loss causes a reduction in nuclear RAD51 Irradiation-induced foci; and sensitizes cancer cells to ionizing radiation and PARP inhibition. In summary, FAAP20 participates in DNA double strand break repair by supporting homologous recombination in a non-redundant manner to FANCA, and single-strand annealing repair via FANCA-mediated strand annealing activity.

Hypomorphic Brca2 and Rad51c double mutant mice display Fanconi anemia, cancer and polygenic replication stress

The prototypic cancer-predisposition disease Fanconi Anemia (FA) is identified by biallelic mutations in any one of twenty-three FANC genes. Puzzlingly, inactivation of one Fanc gene alone in mice fails to faithfully model the pleiotropic human disease without additional external stress. Here we find that FA patients frequently display FANC co-mutations. Combining exemplary homozygous hypomorphic Brca2/Fancd1 and Rad51c/Fanco mutations in mice phenocopies human FA with bone marrow failure, rapid death by cancer, cellular cancer-drug hypersensitivity and severe replication instability. These grave phenotypes contrast the unremarkable phenotypes seen in mice with single gene-function inactivation, revealing an unexpected synergism between Fanc mutations. Beyond FA, breast cancer-genome analysis confirms that polygenic FANC tumor-mutations correlate with lower survival, expanding our understanding of FANC genes beyond an epistatic FA-pathway. Collectively, the data establish a polygenic replication stress concept as a testable principle, whereby co-occurrence of a distinct second gene mutation amplifies and drives endogenous replication stress, genome instability and disease.

PrimPol-mediated repriming facilitates replication traverse of DNA interstrand crosslinks

DNA interstrand crosslinks (ICLs) induced by endogenous aldehydes or chemotherapeutic agents interfere with essential processes such as replication and transcription. ICL recognition and repair by the Fanconi Anemia pathway require the formation of an X‐shaped DNA structure that may arise from convergence of two replication forks at the crosslink or traversing of the lesion by a single replication fork. Here, we report that ICL traverse strictly requires DNA repriming events downstream of the lesion, which are carried out by PrimPol, the second primase‐polymerase identified in mammalian cells after Polα/Primase. The recruitment of PrimPol to the vicinity of ICLs depends on its interaction with RPA, but not on FANCM translocase or the BLM/TOP3A/RMI1‐2 (BTR) complex that also participate in ICL traverse. Genetic ablation of PRIMPOL makes cells more dependent on the fork convergence mechanism to initiate ICL repair, and PRIMPOL KO cells and mice display hypersensitivity to ICL‐inducing drugs. These results open the possibility of targeting PrimPol activity to enhance the efficacy of chemotherapy based on DNA crosslinking agents.

The ubiquitin family meets the Fanconi anemia proteins

Fanconi anaemia (FA) is a hereditary disorder characterized by bone marrow failure, developmental defects, predisposition to cancer and chromosomal abnormalities. FA is caused by biallelic mutations that inactivate genes encoding proteins involved in replication stress-associated DNA damage responses. The 20 FANC proteins identified to date constitute the FANC pathway. A key event in this pathway involves the monoubiquitination of the FANCD2-FANCI heterodimer by the collective action of at least 10 different proteins assembled in the FANC core complex. The FANC core complex-mediated monoubiquitination of FANCD2-FANCI is essential to assemble the heterodimer in subnuclear, chromatin-associated, foci and to regulate the process of DNA repair as well as the rescue of stalled replication forks. Several recent works have demonstrated that the activity of the FANC pathway is linked to several other protein post-translational modifications from the ubiquitin-like family, including SUMO and NEDD8. These modifications are related to DNA damage responses but may also affect other cellular functions potentially related to the clinical phenotypes of the syndrome. This review summarizes the interplay between the ubiquitin and ubiquitin-like proteins and the FANC proteins that constitute a major pathway for the surveillance of the genomic integrity and addresses the implications of their interactions in maintaining genome stability.