Inactivation of the INK4a/ARF locus and p53 in sporadic extrahepatic bile duct cancers and bile tract cancer cell lines

Abstract

The tumor-suppressor genes p14(ARF), p16(INK4a) and Tp53 are commonly inactivated in many tumors. We investigated their role in the pathogenesis of 9 bile tract cancer cell lines and 21 primary sporadic extrahepatic bile duct carcinomas. p53 and p16 protein expression was examined by Western blot analysis and immunohistochemistry. Mutation screening of p53 was done by SSCP and direct sequencing. Inactivating mechanisms of p14 and p16 were addressed by screening for mutations, homozygous deletions, chromosomal loss of 9p21 (loss of heterozygosity [LOH] analysis) and promoter hypermethylation of the p14/p16 genes. p53 overexpression could be detected in 7 of 9 cell lines and 7 of 21 primary tumors, but mutations were found in 3 cell lines only. p16 expression was absent in all cell lines, due to homozygous deletion of the gene in 8 of 9 cell lines and hypermethylation of the p16 promoter in one cell line (CC-LP-1). p14 exon 1␤ was homozygously deleted in 6 of 9 cell lines, while retained in CC-LP-1 and 2 additional lines. No p14 promoter hypermethylation could be detected. p16 expression was lost in 11 of 21 primary tumors. p16 promoter hypermethylation was present in 9 of 21 primary tumors, all with lost p16 expression. Allelic loss at 9p21 was detected in 13 of 21 primary tumors, 10 of 11 with lost p16 expression and 8 of 9 with methylated p16 promoter. No p14 promoter hypermethylation or p14/p16 mutations could be detected. Neither Tp53 nor p16 alterations showed obvious association with histopathologic or clinical characteristics. In conclusion, inactivation of the p16 gene is a frequent event in primary sporadic extrahepatic bile duct cancers, 9p21 LOH and promoter hypermethylation being the principal inactivating mechanisms. Therefore, p16, but not p14, seems to be the primary target of inactivation at the INK4a locus in bile duct cancers. Other mechanisms than Tp53 mutations seems to be predominantly responsible for stabilization of nuclear p53 protein in bile duct cancers. © 2002 Wiley-Liss, Inc. Key words: bile tract cancer; INK4a/ARF; p53; promoter hypermethylation; histopathologyCancer of the extrahepatic bile duct (BDC) is a rare malignancy comprising less than 2% of all cancer diagnoses. 1,2 Although progress has been made in establishing the diagnosis earlier, the prognosis is still poor. Since most patients present with advanced disease, potential curative surgical therapy is applicable only to a minority of patients (Ͻ30%). 3 Palliative treatment is restricted to biliary drainage by surgery, 4 stenting 5 or photodynamic therapy. 6,7 So far, the molecular events in BDC carcinogenesis are not well understood. According to the multistep model of tumorigenesis 8,9 accumulating mutations in proto-oncogenes, DNA repair genes and tumor-suppressor genes are responsible for the malignant transformation of biliary epithelial cells. Disruption of the p53 and Rb tumor-suppressor pathways are the most common molecular events in cancer cells. 9 The most commonly altered tumor-suppressor gene in human cancers Tp5...

Two distinct pathways of p16 gene inactivation in gallbladder cancer

“…Loss of p16 expression is correlated with homozygous deletion of the p16 gene in gallbladder cancer and other malignancies. Eight of nine biliary tract cell lines with homozygous deletion of the p16 gene showed loss of p16 expression, as reported by Caca et al [26] . In our series, loss of p16 expression correlated with homozygous deletion of the p16 gene in nine of 13 tumors.…”

“…Ku et al have reported that homozygous deletion of the p16 gene was found in three of six biliary tract cell lines, but no p16 mutation was found in the remaining three biliary tract cell lines, which did not show homozygous deletion. Caca et al [26] have reported that p16 mutations were not found in three biliary tract cell lines and 21 biliary tract cancers, which did not show homozygous deletion of the p16 gene. In the present study, a p16 mutation was not found in any of the cases analyzed.…”

“…Yoshida et al [9] have reported that homozygous deletion of the p16 gene was detected in one of two gallbladder cell lines and in two biliary tract cell lines. Caca et al [26] have reported that homozygous deletion of the p16 gene was detected in eight of nine (88.8%) biliary tract cell lines, but homozygous deletion of the p16 gene was not detected in 21 biliary tract cancers. Homozygous deletion of the p16 gene has not previously been examined in gallbladder cancer.…”

Zoledronic acid determines S-phase arrest but fails to induce apoptosis in cholangiocarcinoma cells

“…Upon ZOL treatment a dose-dependent decrease in the level of p27 inhibitor was observed in both cell lines, more evident in EGI-1 cells. The p16 INK4A , which shares with pRb and p27 a regulatory role at G1/S checkpoint, is frequently mutated or deleted in a wide variety of tumors, in particular in bile duct cancers, and not expressed in the cholangiocarcinoma cell lines used in this work [31,32]. In several human malignancies, the loss, mutation or silencing of the gene encoding the CDK4 inhibitor p16 INK4a hyperactivates the kinase activity of CDK4 [33][34][35].…”

Inactivación del gen CDKN2A (p16) en cáncer de la vesícula biliar

“…Para los adenocarcinomas pancreáticos de los cuales el 98% de los tumores demuestra ausencia del p16, 48% de alteraciones genéticas en el gen CDKN2A son de lesiones homocigotas y 34% están asociadas con una pérdida de heterocigocidad (LOH), mientras el resto está relacionada con un silenciamiento mediante metilación de las áreas promotoras 25 . Más aún la pérdida del cromosoma 9p21 es común en los carcinomas del tracto de vía biliar y de la vesícula biliar 26,27 . En nuestro estudio encontramos que el 11% de los adenocarcinomas exhibían desbalance alélico en uno de los microsatélites que flanquean la región del cromosoma 9p en el cual el gen CDKN2A está localizado (Tabla 1).…”