Involvement of the Fanconi's Anemia Protein FAC in a Pathway That Signals to the Cyclin B/cdc2 Kinase

Abstract

Lymphoblastoid cell lines derived from patients with the chromosomal instability disorder Fanconi's anemia (FA) are hyperresponsive to G₂ delay and apoptosis induced by cross-linking agents such as mitomycin C (MMC). Here, we investigated whether the protein defective in FA complementation group C (FA-C) cells functions in a pathway that signals to the cdc2 kinase complex, which controls mitotic progression. FA-C lymphoblasts treated with a low dose of MMC (1–5 µm, 1 h) exhibited a protracted G₂-M arrest and subsequent apoptosis by 2 days after treatment. This G₂-M arrest was mediated by persistent inactivation of the cyclin B1/cdc2 kinase complex characterized by both sustained accumulation of cyclin B1 and tyrosine phosphorylation of cdc2. In phenotypically corrected (wild-type) cells, the same treatment induced only temporal G₂-M arrest, associated with a transient inactivation of the cyclin B1/cdc2 kinase complex, after which cells resumed cycling. Treatment with higher dosages (15–30 µm, 1 h) resulted in S-phase arrest and induced a similar high level of apoptosis in FA-C and wild-type cells, accompanied by degradation of cyclin B1 and dephosphorylation of cdc2. In low-dose treated G₂-M-arrested FA-C cells, caffeine-dependent activation of cdc2 released the G₂-M block but failed to protect against apoptosis, suggesting that apoptosis was not a direct consequence of persistent cdc2 kinase inactivation. Thus, at low doses of MMC, FA-C cells exhibit a unique cyclin B1/cdc2 response that is not observed in wild-type cells treated with an equitoxic high dosage of cross-linker. Although these results do not necessarily implicate a role for FAC in regulating cyclin B/cdc2 kinase activity, available evidence suggests that the FAC protein is involved in a cross-link damage avoidance pathway that signals to this kinase complex.