[Frequent epigenetic inactivation of XAF1 by promotor hypermethylation in human colon cancers].
OBJECTIVE
X-linked inhibitor of apoptosis (XIAP) is the most potent member of the IAP family that exerts antiapoptotic effects. Recently, XIAP-associated factor 1 (XAF1) and two mitochondrial proteins, Smac/ DIABLO and HtrA2, have been identified to negatively regulate the caspase-inhibiting activity of XIAP. We explored the candidacy of XAF1, Smac/DIABLO and HtrA2 as a tumor suppressor in colonic carcinogenesis.


METHODS
Expression and mutation status of the genes in 10 colorectal carcinoma cell lines and 40 primary tumors were examined by quantitative PCR analysis.


RESULTS
XAF1 transcript was not expressed or present at extremely low levels in 60% (6/10) of cancer cell lines whereas Smac/DIABLO and HtrA2 are normally expressed in all cell lines examined. Tumor-specific loss or reduction of XAF1 was also found in 35% (14/40) of matched tissue sets obtained from the same patients. XAF1 transcript was reactivated in all the low expressor cell lines by treatment with the demethylating agent 5-aza-2'-deoxycytidine. Moreover, bisulfite DNA sequencing analysis for 34 CpG sites in the promoter region revealed a strong association between hypermethylation and gene silencing. Restoration of XAF1 expression resulted in enhanced apoptotic response to etoposide and 5-flurouracil, whereas knockdown of XAF1 expression by siRNA transfection significantly inhibited chemotherapeutic drug-induced apoptosis.


CONCLUSIONS
XAF1 undergoes epigenetic gene silencing in a considerable proportion of human colon cancers by aberrant promoter hypermethylation, suggesting that XAF1 inactivation might be implicated in colonic tumorigenesis.
Jang
Jae Young
JY
Department of Internal Medicine, Kyung Hee University College of Medicine, Seoul 130-702, Korea.
Kim
Hyo Jong
HJ
Chi
Sung-Gil
SG
Lee
Kil Yeon
KY
Nam
Ki Deuk
KD
Kim
Nam Hoon
NH
Lee
Sang Kil
SK
Joo
Kwang Ro
KR
Dong
Seok Ho
SH
Kim
Byung-Ho
BH
Chang
Young Woon
YW
Lee
Joung Il
JI
Chang
Rin
R
D048868:Adaptor Proteins, Signal Transducing; D051017:Apoptosis Regulatory Proteins; D045744:Cell Line, Tumor; D003110:Colonic Neoplasms / Q000235:genetics*; D019175:DNA Methylation; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D000074923:High-Temperature Requirement A Serine Peptidase 2; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics; D024101:Mitochondrial Proteins / Q000235:genetics; D009363:Neoplasm Proteins / Q000235:genetics*; D011401:Promoter Regions, Genetic; D012697:Serine Endopeptidases / Q000235:genetics
D048868:Adaptor Proteins, Signal Transducing; D051017:Apoptosis Regulatory Proteins; C412403:DIABLO protein, human; D047908:Intracellular Signaling Peptides and Proteins; D024101:Mitochondrial Proteins; D009363:Neoplasm Proteins; C417551:XAF1 protein, human; D012697:Serine Endopeptidases; C000615914:HTRA2 protein, human; D000074923:High-Temperature Requirement A Serine Peptidase 2