FAAP100, also known as FANCX, is an essential component of the Fanconi anemia (FA) core complex. The FA core complex is central to the DNA damage response network, and FAAP100 is crucial for the stability of this complex. It directly interacts with FANCB and FANCL to form a stable subcomplex, which is important for the monoubiquitination of FANCD2 and FANCI in response to DNA damage. The FA pathway, in which FAAP100 is involved, is vital for maintaining genome stability and preventing diseases such as Fanconi anemia [2,3,4,5,6,7,8,9].

In mouse models, deletion of Faap100 leads to inactivation of the FA pathway. This results in increased transcription-replication conflicts (TRCs) and cotranscriptional R-loops, causing replication fork collapse and DNA damage. Subsequently, the activated p53 signaling pathway triggers primordial germ cell (PGC) proliferation defects, ultimately leading to insufficient establishment of the reproductive reserve in both male and female mice. Customized Faap100 -/- mice also exhibit embryonic lethality, microsomia, malformations, and gonadal atrophy, resembling mice with established FA subtypes [1,3].

In conclusion, FAAP100 is essential for resolving TRCs in PGCs to safeguard genome stability. Gene-knockout mouse models of Faap100 have been instrumental in revealing its role in Fanconi anemia-related phenotypes, such as developmental abnormalities and gonadal atrophy, highlighting its significance in maintaining genomic integrity during development and reproduction [1,3].

References:

1. Xu, Weiwei, Yang, Yajuan, Yu, Yongze, Qin, Yingying, Chen, Zi-Jiang. 2023. FAAP100 is required for the resolution of transcription-replication conflicts in primordial germ cells. In BMC biology, 21, 174. doi:10.1186/s12915-023-01676-1. https://pubmed.ncbi.nlm.nih.gov/37580696/

2. Ling, Chen, Ishiai, Masamichi, Ali, Abdullah Mahmood, Meetei, Amom Ruhikanta, Wang, Weidong. 2007. FAAP100 is essential for activation of the Fanconi anemia-associated DNA damage response pathway. In The EMBO journal, 26, 2104-14. doi:. https://pubmed.ncbi.nlm.nih.gov/17396147/

3. Kuehl, Julia, Xue, Yutong, Yuan, Fenghua, Zhang, Yanbin, Schindler, Detlev. 2025. Genetic inactivation of FAAP100 causes Fanconi anemia due to disruption of the monoubiquitin ligase core complex. In The Journal of clinical investigation, , . doi:10.1172/JCI187323. https://pubmed.ncbi.nlm.nih.gov/40232843/

4. Swuec, Paolo, Renault, Ludovic, Borg, Aaron, Deans, Andrew J, Costa, Alessandro. 2016. The FA Core Complex Contains a Homo-dimeric Catalytic Module for the Symmetric Mono-ubiquitination of FANCI-FANCD2. In Cell reports, 18, 611-623. doi:10.1016/j.celrep.2016.11.013. https://pubmed.ncbi.nlm.nih.gov/27986592/

5. Ali, Abdullah Mahmood, Singh, Thiyam Ramsing, Meetei, Amom Ruhikanta. 2009. FANCM-FAAP24 and FANCJ: FA proteins that metabolize DNA. In Mutation research, 668, 20-6. doi:10.1016/j.mrfmmm.2009.04.002. https://pubmed.ncbi.nlm.nih.gov/19379763/

6. Shakeel, Shabih, Rajendra, Eeson, Alcón, Pablo, Robinson, Carol V, Passmore, Lori A. 2019. Structure of the Fanconi anaemia monoubiquitin ligase complex. In Nature, 575, 234-237. doi:10.1038/s41586-019-1703-4. https://pubmed.ncbi.nlm.nih.gov/31666700/

7. Rajendra, Eeson, Oestergaard, Vibe H, Langevin, Frédéric, Patel, Ketan J, Passmore, Lori A. . The genetic and biochemical basis of FANCD2 monoubiquitination. In Molecular cell, 54, 858-69. doi:10.1016/j.molcel.2014.05.001. https://pubmed.ncbi.nlm.nih.gov/24905007/

8. Harrison, Benjamin A, Mizrahi-Powell, Emma, Pappas, John, Evrony, Gilad D, Smogorzewska, Agata. 2025. Deficiency of the Fanconi anemia core complex protein FAAP100 results in severe Fanconi anemia. In The Journal of clinical investigation, , . doi:10.1172/JCI185126. https://pubmed.ncbi.nlm.nih.gov/40244696/

9. Thompson, Larry H, Hinz, John M. 2009. Cellular and molecular consequences of defective Fanconi anemia proteins in replication-coupled DNA repair: mechanistic insights. In Mutation research, 668, 54-72. doi:10.1016/j.mrfmmm.2009.02.003. https://pubmed.ncbi.nlm.nih.gov/19622404/