XAF-1 exists as a single gene with eight exons, which localizes to chromosome 17p13.2, a region just telomeric to the p53 gene. Although XAF-1 has been shown to have a potent proapoptotic activity and to act as a tumor suppressor in several solid tumors by inhibiting X-linked inhibitor of apoptosis at the mitochondrial level, its potential role in hematological malignancies has not been addressed. As 17p- deletion has been associated with poor prognosis in patients affected by B chronic lymphocytic leukemia (B-CLL), we have analyzed the steady-state messenger RNA (mRNA) levels of XAF-1 in a cohort of B-CLL samples, characterized for CLL-associated chromosomal aberrations using fluorescence in situ hybridization, Zap70 levels and p53 status. When assayed ex vivo for cytotoxic response to 10 μM of the non-genotoxic activator of the p53 pathway Nutlin-3, B-CLL samples showed a variable susceptibility to Nutlin-3 cytotoxicity upon 48 h of treatment. Our data suggest that XAF-1 might represent an important determinant in modulating the cytotoxic activity of Nutlin-3 and probably also of other pro-apoptotic drugs in B-CLL cells. In fact, p53del/mut B-CLL samples, which are less responsive to chemotherapeutic or Nutlin-3 treatment than p53wt B-CLL samples, showed low levels of XAF-1 mRNA. Moreover, modulating the steady-state mRNA levels of XAF-1 through IFN-β treatment or silencing the XAF-1 gene significantly affected the Nutlin-3-mediated cytotoxicity in selected samples of primary p53wt and p53del/mut B-CLL as well as in leukemic cell lines.
