DNA CROSS-LINKER-INDUCED G2 M ARREST IN GROUP-C FANCONI-ANEMIA LYMPHOBLASTS REFLECTS NORMAL CHECKPOINT FUNCTION/

Cells from individuals with Fanconi anemia (FA) arrest excessively in the G2/M cell cycle compartment after exposure to low doses of DNA cro ss linking agents. The relationship of this abnormality to the fundame ntal genetic defect in such cells is unknown, but many investigators h ave speculated that the various FA genes directly regulate cell cycle checkpoints. We tested the hypothesis that the protein encoded by the FA group C complementing gene (FAG) functions to control a cell cycle checkpoint and that cells from group C patients (FA[C]) have abnormali ties of cell cycle regulation directly related to the genetic mutation . We found that retroviral transduction of FA(C) lymphoblasts with wil d-type FAC cDNA resulted in normalization of the cell cycle response t o low-dose mitomycin C (MMC). However, when DNA damage was quantified in terms of cytogenetic damage or cellular cytotoxicity, we found simi lar degrees of G2/M arrest in response to equitoxic amounts of MMC in FA(C) cells as well as in normal lymphoblasts. Similar results were ob tained using isogenic pairs of uncorrected, FAC or mock-corrected (neo only) FA(C) cell lines. To test the function of other checkpoints we examined the effects of hydroxyurea (HU) and ionizing radiation on cel l cycle kinetics of FA(C) and normal lymphoblasts as well as with isog enic pairs of uncorrected, FAG-corrected, or mock-corrected FA(C) cell lines. In all cases the cell cycle response of FA(C) and normal lymph oblasts to these two agents were identical. Based on these studies we conclude that the aberrant G2/M arrest that typifies the response of F A(C) cells to low doses of cross-linking agents does not represent an abnormal cell cycle response but instead represents a normal cellular response to the excessive DNA damage that results in FA(C) cells follo wing exposure to low doses of cross-linking agents. (C) 1998 by The Am erican Society of Hematology.