Structure-function analysis of the fanconi anemia complementation group c (Fancc) Gene reveals different requirements for STAT-1 activation and for resistance to cross-linking agents

The FA group C gene product (FANCC) is required for resisting alkylating agents and for optimal activation of Stat-1 in normal cytokine-stimulated cells. Seeking to isolate structural determinants of these two functions of FANCC, we transduced cells obtained from children with FA-C with retroviral vectors expressing six alanine mutants of FANCC cDNA and tested cell survival in mitomycin C (MMC) and Stat-1 activation in response to IFNγ. All mutants effectively complemented the alkylator hypersensitive phenotype, but two mutations, S249A and E251A, failed to correct defective Stat-1 activation in the FA-C lymphoblast cell line HSC536N. We next asked whether this domain might be functionally preserved in some milder cases of FA. In the FA-C cell line PD4 which expresses an N-terminally truncated mutant 55 kDa protein (M55) with normal S249 and E251, we determined that Stat-1 is normally phosphorylated. Moreover, M55 cDNA complemented the Stat-1 activation defect in HSC536 cells without reversing MMC sensitivity. Thus, a central domain of the FANCC protein is required for functional interaction of FANCC with Stat-1 and differs from structural elements required for MMC protection which do not tolerate N-terminal truncation. Functional preservation of this domain may explain the less severe hematologic phenotype in selected children with FA-C.