FANCL

The Fanconi Anemia (FA) DNA repair pathway is essential for the recognition and repair of DNA interstrand crosslinks (ICL). A critical step in the pathway is the monoubiquitination of FANCD2 by the RING E3 ligase FANCL. FANCL comprises three domains: a RING domain that interacts with E2 conjugating enzymes, a central domain required for substrate interaction, and an N-terminal E2-like fold (ELF) domain that interacts with FANCB. The ELF domain of FANCL is also required to mediate a non-covalent interaction between FANCL and ubiquitin. The ELF domain promotes efficient DNA damage-induced FANCD2 monoubiquitination in vertebrate cells, suggesting an important function of FANCB and ubiquitin binding by FANCL in vivo.

A nuclear complex containing FANCL (as well as FANCA, FANCB, FANCC, FANCE, FANCF, FANCG and FANCM) is essential for the activation of FANCD2 to the mono-ubiquitinated isoform. In normal, non-mutant cells FANCD2 is mono-ubiquitinated in response to DNA damage. Activated FANCD2 protein co-localizes with BRCA1 at ionizing radiation-induced foci and in synaptonemal complexes of meiotic chromosomes.

ATM (ATM) is a protein kinase that is recruited and activated by DNA double-strand breaks. DNA double-strand damages also activate the Fanconi anemia core complex (FANCA/B/C/E/F/G/L/M). The FA core complex monoubiquitinates the downstream targets FANCD2 and FANCI. ATM activates (phosphorylates) CHEK2 and FANCD2. CHEK2 phosphorylates BRCA1. Ubiquitinated FANCD2 complexes with BRCA1 and RAD51.

The PALB2 protein acts as a hub, bringing together BRCA1, BRCA2 and RAD51 at the site of a DNA double-strand break, and also binds to RAD51C, a member of the RAD51 paralog complex RAD51B-RAD51C-RAD51D-XRCC2 (BCDX2). The BCDX2 complex is responsible for RAD51 recruitment or stabilization at damage sites. RAD51 plays a major role in homologous recombinational repair of DNA during double-strand break repair. In this process, an ATP-dependent DNA strand exchange takes place in which a single strand invades base-paired strands of homologous DNA molecules. RAD51 is involved in the search for homology and strand pairing stages of the process.

The clinical phenotype of mutational defects in all Fanconi anemia (FA) complementation groups is similar. This phenotype is characterized by progressive bone marrow failure, cancer proneness and typical birth defects. The main cellular phenotype is hypersensitivity to DNA damage, particularly inter-strand DNA crosslinks. The FA proteins interact through a multi-protein pathway. DNA interstrand crosslinks are highly deleterious damages that are repaired by homologous recombination involving coordination of FA proteins and breast cancer susceptibility gene 1 (BRCA1).