FANCD2

Recombinational repair of DNA double-strand damage - some key steps. 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).[6] The FA core complex monoubiquitinates the downstream targets FANCD2 and FANCI.[7] ATM activates (phosphorylates) CHEK2 and FANCD2[8] CHEK2 phosphorylates BRCA1.[9] Ubiquinated FANCD2 complexes with BRCA1 and RAD51.[10] The PALB2 protein acts as a hub,[11] 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.[12] 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.

Fanconi anemia is a disorder with a recessive Mendelian pattern of inheritance characterized by chromosomal instability, hypersensitivity to DNA crosslinking agents, increased chromosomal breakage, and defective DNA repair. The members of the Fanconi anemia complementation group do not share sequence similarity; they are related by their assembly into a common nuclear protein complex. This gene encodes the protein for complementation group D2. This protein is monoubiquitinated in response to DNA damage, resulting in its localization to nuclear foci with other proteins (BRCA1 and BRCA2) involved in homology-directed DNA repair (see Figure: Recombinational repair of DNA double-strand damages). A nuclear complex containing FANCA, [Fanconi anemia, complementation group A], FANCB, FANCC, FANCE, FANCF, FANCL and FANCG proteins is required for the activation of the FANCD2 protein to the mono-ubiquitinated isoform.[13]

Mono-ubiquination of FANCD2 is essential for repairing DNA interstrand crosslinks, and clamps the protein on DNA together with its partner protein FANCI. The monoubiquitinated FANCD2:FANCI complex coats DNA in a filament-like array, potentially as a way to protect DNA associated with stalled replication.[14]

Mono-ubiquitination is also required for interaction with the nuclease FAN1. FAN1 recruitment and its consequent activity restrain DNA replication fork progression and prevent chromosome abnormalities from occurring when DNA replication forks stall.[15]

Humans with a FANCD deficiency display hypogonadism, male infertility, impaired spermatogenesis, and reduced female fertility. Similarly, mice deficient in FANCD2 show hypogonadism, impaired fertility and impaired gametogenesis.[16]

In the non-mutant mouse, FANCD2 is expressed in spermatogonia, pre-leptotene spermatocytes, and in spermatocytes in the leptotene, zygotene and early pachytene stages of meiosis.[17] In synaptonemal complexes of meiotic chromosomes, activated FANCD2 protein co-localizes with BRCA1 (breast cancer susceptibility protein).[13] FANCD2 mutant mice exhibit chromosome mis-pairing during the pachytene stage of meiosis and germ cell loss.[18] Activated FANCD2 protein may normally function prior to the initiation of meiotic recombination, perhaps to prepare chromosomes for synapsis, or to regulate subsequent recombination events.[13]

Tobacco smoke suppresses the expression of FANCD2, which codes for a DNA damage "caretaker" or repair mechanism.[19]

FANCD2 has been shown to interact with:

• FANCI[22][23]
• Ataxia telangiectasia mutated,[24][25]
• BARD1,[26]
• BRCA1.[25][26]
• BRCA2,[27][28][29]
• FANCE,[28][30][31]
• HTATIP,[29] and
• MEN1.[32]