Overexpression of IRF-1 and p21WAF1/CIP1 in fanconi anemia (Fa) Cells correlates with STAT-1 phosphorylation defects but is independent of icsbp deficiency

Hematopoietic progenitor cells from FA group C (FA-C) patients are hypersensitive to IFNγ. IFNγ-inducible genes known to influence cell survival and/or cell cycle progression (IRF-1, p21 (WAF1/CIP1) and MxA) are constitutively expressed in FA-C cells while levels of the transcriptional repressor ICSBP are reduced. The FA-C protein (FANCC) interacts with Stat1 via a conserved central domain (see abstract by Pang, Q et al). Even conservative mutations in this domain result in decreased Stat1 phosphorylation in response to IFNγ. Because induction of ICSBP by IFNγ is mediated by Stat1 and activation of Stat1 is decreased in some FA cells, we sought to determine if overexpression of IRF-1 and p21 is specifically caused by deficiencies in Stat1 activation and ICSBP expression. Using real time RT-PCR and immunoblot analysis we determined that IRF-1 and p21 are upregulated in FA cells that contain a Stat1 phosphorylation defect but not in FA cells in which Stat1 activation is preserved. To determine if IRF-1 and p21 upregulation is dependent on ICSBP deficiency, we transduced Stat1 defective FA-C lymphoblasts with a retroviral vector expressing ICSBP. Constitutive expression of ICSBP failed to down-regulate IRF-1 or p21 correlates positively with Stat1 phosphorylation defects, but enforced expression of ICSBP does not suppress expression of these genes. We conclude that the capacity of the FANCC protein to facilitate Stat1 activation is linked with its capacity to functionally repress expression of IRF-1 and p21 but involves a mechanism other than ICSBP induction.