Frequent alteration of XAF1 in human colorectal cancers:: Implication for tumor cell resistance to apoptotic stresses

Background & Aims: X-linked inhibitor of apoptosis protein-associated factor 1 (XAF1) is a candidate tumor suppressor located at the chromosome 17p13 region, but the molecular basis underlying its inactivation in human tumors and growth-inhibiting function has not been well defined. We explored the candidacy of XAF1 as a suppressor in colorectal tumorigenesis. Methods: XAF1 expression was characterized by polymerase chain reaction-based cloning, isoform-specific polymerase chain reaction, ribonuclease protection, and immunoblot assays. Allelic loss of the gene was evaluated by loss of heterozygosity (LOH) assay, and promoter CG dinucleotide (CpG) site methylation was determined using bisulfite sequencing. The effect of XAF1 on tumor growth was examined using flow cytometry, terminal deoxynucleotidyl transferase-mectiated deoxyuridine triphosphate nick-end labeling, colony formation, and viability assays. Results: Expression of 5 XAF1 variants including 2 novel transcripts was down-regulated concomitantly in 11 of 20 (55%) cell lines and 26 of 65 (40%) primary tumors. XAF1 reduction was tumor-specific and showed a correlation with advanced stage and high grade of tumor. LOH of the gene was found in 12 of 33 (36%) tumors. Promoter CpG site methylation was observed frequently in both cell lines and tumor tissues including many LOH tumors, suggesting that biallelic inactivation of XAF1 might be common in colorectal cancers. XAF1 expression suppressed tumor cell growth and enhanced cellular response to various apoptotic stimuli, such as 5-fluorouracil, etoposide, H2O2, gamma-irradiation, ultraviolet, and tumor necrosis factor-a, whereas knockdown of its expression protected cells from the stresses. Conclusions: Genetic and epigenetic alteration of XAF1 is a common event in colorectal tumorigenesis and contributes to the malignant tumor progression by providing survival advantages for tumor cells under various stress conditions.