The Fanconi anemia complementation group C gene product: structural evidence of multifunctionality

The Fanconi anemia (FA) group C gene product (FANCC) functions to protect c ells from cytotoxic and genotoxic effects of cross-linking agents. FANCC is also required for optimal activation of STAT1 in response to cytokine and growth factors and for suppressing cytokine-induced apoptosis by modulating the activity of double-stranded RNA-dependent protein kinase. Because not all FANCC mutations affect STAT1 activation, the hypothesis was considered that cross-linker resistance function of FANCC depends on structural elemen ts that differ from those required for the cytokine signaling functions of FANCC. Structure-function studies were designed to test this notion. Six se parate alanine-substituted mutations were generated In 3 highly conserved m otifs of FANCC. All mutants complemented mitomycin C (MMC) hypersensitive p henotype of FA-C cells and corrected aberrant posttranslational activation of FANCD2 in FA-C mutant cells. However, 2 of the mutants, S249A and E251 A , failed to correct defective STAT1 activation. FA-C lymphoblasts carrying these 2 mutants demonstrated a defect In recruitment of STAT1 to the interf eron gamma (IFN-gamma) receptor and GST-fusion proteins bearing S249A and E 251A mutations were less efficient binding partners for STAT1 in stimulated lymphoblasts. These same mutations failed to complement the characteristic hypersensitive apoptotic responses of FA-C cells to tumor necrosis factor- alpha (TNF-alpha) and IFN-alpha. Cells bearing a naturally occurring FANCC mutation (322delG) that preserves this conserved region showed normal STAT1 activation but remained hypersensitive to MMC. The conclusion Is that a ce ntral highly conserved domain of FANCC Is required for functional interacti on with STAT1 and that structural elements required for STAT1-related funct ions differ from those required for genotoxic responses to cross-linking ag ents. Preservation of signaling capacity of cells bearing the del322G mutat ion may account for the reduced severity and later onset of bone marrow fai lure associated with this mutation. (C) 2001 by The American Society of Hem atology.