To the editor
Fanconi anemia (FA) cells have a well-known sensitivity to oxygen1,2,3, although the molecular basis of this sensitivity remains unknown. The article by Cumming et al.4 in the July 2001 issue of Nature Medicine proposes that one of the six cloned FA proteins, FA group C protein (FANCC), regulates glutathione S-transferase P1-1 (GSTP1), a cytoplasmic enzyme involved in detoxifying reactive oxygen species (ROS) and by-products of oxidative stress. The authors show the following: 1) a GST–FANCC fusion protein interacts with GSTP1; 2) ectopic overexpression of human FANCC and human GSTP1 in a murine IL-3–dependent cell line (expressing endogenous murine FANCC and GSTP1) protects these transfected cells from apoptosis during cytokine withdrawal; and 3) overexpressed FANCC increases GSTP1 activity. Although no genetic evidence is provided to support this interaction, the study suggests a regulatory role of FANCC in the detoxification of ROS and electrophilic metabolites in the cytoplasm.
Although this model is attractive, its validity depends on the localization of endogenous FANCC to the cytoplasm. There is considerable disagreement on this issue. Some studies5,6, using overexpression of FANCC in transfected cells, suggest a cytoplasmic localization, whereas other studies7,8,9, examining endogenous FANCC, localize the protein to the nucleus. Moreover, the functional form of FANCC appears to be nuclear10.
I wish to propose an alternative model for the ROS sensitivity of FA cells, based on the localization of FANCC to the nucleus. Many studies have demonstrated that FANCC is a subunit of a nuclear complex containing several other cloned FA proteins (A, E, F and G)7,8,9,11. In response to DNA damage, this FA protein complex becomes 'activated', leading to the monoubiquitination of the downstream FANCD2 (the Fanconi anemia subtype D2 protein) and its targeting to DNA repair foci, including the BRCA1 (the breast cancer susceptibility protein)12,13. In FA-C (Fanconi anemia subtype C cells), which lack functional FANCC, the nuclear FA complex does not assemble properly8 and the FANCD2 protein is not activated, leading to a defect in DNA repair13. Moreover, ROS generated by hydrogen peroxide can activate the monoubiquitination of FANCD2 in normal cells, but does not activate the FA pathway in cells from FA complementation groups A, C, F or G (unpublished observation). Hence, FA cells are sensitive to ROS, not because of a failure to detoxify ROS, but because of a failure to respond to ROS-mediated DNA damage.
Additional studies will be required to distinguish between these two models. For instance, the measurement of intracellular ROS levels and oxidative DNA damage in FA cells versus functionally complemented FA cells may determine whether the FA defect occurs before or after DNA repair.
Finally, FANCC may have two discrete functions: one as a cytoplasmic regulator of GSTP1 and one as a subunit of a nuclear complex regulating DNA repair. However, this seems unlikely based on genetic arguments. Disruption of the Fancc gene in mice14,15 generates a cellular and organismal phenotype which is indistinguishable from genetic disruption of the Fanca (ref. 16) or Fancg (ref. 17) gene. Accordingly, FANCC appears to function primarily as a critical subunit of the nuclear FA protein complex (A,C,E,F,G complex) and is less likely to have additional cellular functions outside of this pathway.
See Reply to ‘Cellular function of the Fanconi anemia pathway’ by Cumming and Buchwald.
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D'Andrea, A. Cellular function of the Fanconi anemia pathway. Nat Med 7, 1259 (2001). https://doi.org/10.1038/nm1201-1259a
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DOI: https://doi.org/10.1038/nm1201-1259a
