The
Fanconi anemia (FA) molecular network consists of 15 "
FANC" proteins, of which 13 are associated with mutations in patients with this
cancer-prone
chromosome instability disorder. Whereas historically the common phenotype associated with FA mutations is marked sensitivity to
DNA interstrand crosslinking agents, the literature supports a more global role for
FANC proteins in coping with diverse stresses encountered by replicative polymerases. We have attempted to reconcile and integrate numerous observations into a model in which
FANC proteins coordinate the following physiological events during
DNA crosslink repair: (a) activating a FANCM-ATR-dependent S-phase checkpoint, (b) mediating enzymatic replication-fork breakage and crosslink unhooking, (c) filling the resulting gap by translesion synthesis (TLS) by error-prone polymerase(s), and (d) restoring the resulting one-ended double-strand break by homologous recombination repair (HRR). The FANC core subcomplex (FANCA, B, C, E, F, G, L, FAAP100) promotes TLS for both crosslink and non-crosslink damage such as spontaneous oxidative base damage, UV-C photoproducts, and alkylated bases. TLS likely helps prevent stalled replication forks from breaking, thereby maintaining chromosome continuity. Diverse
DNA damages and replication inhibitors result in monoubiquitination of the FANCD2-FANCI complex by the FANCL
ubiquitin ligase activity of the core subcomplex upon its recruitment to
chromatin by the FANCM-FAAP24 heterodimeric translocase. We speculate that this translocase activity acts as the primary damage sensor and helps remodel blocked replication forks to facilitate checkpoint activation and repair. Monoubiquitination of FANCD2-FANCI is needed for promoting HRR, in which the
FANCD1/BRCA2 and FANCN/PALB2
proteins act at an early step. We conclude that the core subcomplex is required for both TLS and HRR occurring separately for non-crosslink damages and for both events during crosslink repair. The FANCJ/BRIP1/BACH1 helicase functions in association with BRCA1 and may remove structural barriers to replication, such as
guanine quadruplex structures, and/or assist in crosslink unhooking.