Susceptibility pathways in Fanconi's anemia and breast cancer

Abstract

The study of rare genetic diseases can lead to insights into the cause and treatment of common diseases. An example is the rare chromosomal instability disorder, Fanconi Anemia (FA). Studies of this disease have elucidated general mechanisms of bone marrow failure, cancer pathogenesis, and resistance to chemotherapy. The principal features of FA are aplastic anemia in childhood, susceptibility to cancer or leukemia, and hypersensitivity of FA cells to DNA cross-linking agents. There are thirteen FA genes, and one of these genes is identical to the well known breast cancer susceptibility gene, BRCA2. The corresponding FA proteins cooperate in the recognition and repair of damaged DNA. Inactivation of FA genes occurs not only in FA patients but also in a variety of cancers in the general population. These findings have broad implications for predicting the sensitivity and resistance of tumors to conventional anti-cancer agents, to inhibitors of poly-ADP ribose polymerase 1, an enzyme involved in DNA repair, and to other inhibitors of DNA repair.

Figure 1. Schematic of the Fanconi anemia DNA repair pathway

After DNA damage, the Ataxia Telangiectasia and Rad3-related kinase (ATR) activates FA core complex (FANCA/B/C/E/F/G/L/M). The FA core complex then functions as an E3 ubiquitin ligase and monoubiquitinates FANCD2 and FANCI. The Monoubiquitinated FANCD2/FANCI complex is then targeted to chromatin where it forms complex with additional FA proteins and other DNA repair proteins. The USP1 and UAF1 protein complex then deubiquitinates FANCD2/FANCI complex, allowing their release from chromatin. USP1 is also required for localization of FANCD2/FANCI into DNA repair foci.

Figure 2. A simplified model of FA-mediated crosslink repair involving nucleotide excision repair (NER), translesion synthesis (TLS), and homologous recombination (HR)

1) According to this model, the replication fork arrests at a DNA crosslink. The stalled fork is detected by the FA complex, thereby activating the assembly of the monoubiquitinated FANCD2/FANCI heterodimer. The heterodimer stabilizes the arrested fork. Recent studies also indicate that the heterodimer allows DNA strand incision and lesion bypass by the replication machinery 2) NER incises one side of the crosslink leaving a single strand DNA break on one strand and a monoadduct on the opposite strand. 3) The heterodimer recruits a TLS polymerase which bypasses the remaining monoadduct. 4) NER excises the monoadduct. 5) FA pathway mediated HR finally repairs the double strand DNA break. 6) Intermediate DNA cross over structures are resolved and the replication fork is reestablished, a process that may require FA pathway-associated DNA helicases.

Figure 3. The downstream FA proteins, FANCD1, FANCN, and FANCJ interact in a common protein complex with BRCA1 to coordinate Homologous Recombination (HR) Repair

According to this model, the D2/I complex is monoubiquitinated by the FA core complex (not shown). The Ubiquitinated complex then recruits three downstream FA proteins (D1, N. J), which are also breast cancer susceptibility proteins. These three FA proteins form a complex with BRCA1. Recent studies indicate that the PALB2/FANCN protein provides interaction surfaces with both D1/BRCA2 and BRCA1, as drawn (see references and 19).

Figure 4. Methylation of the FANCF gene during ovarian tumor progression

According to this model, early in the course of ovarian cancer, the FANCF gene is methylated, leading to chromosome instability. Chromosome instability allows for the inactivation of other tumor suppressor genes, by loss of heterozygosity (LOH), and the generation of new fusion oncogenes. The tumor is hypersensitive to cisplatin, but, after cisplatin exposure, the tumor reemerges as a cisplatin resistant tumor with unmethylated FANCF. Theoretically, an inhibitor of the FA/BRCA pathway may re-sensitize the relapsed tumor to cisplatin.