Original contributionFrameshift mutation of UVRAG, an autophagy-related gene, in gastric carcinomas with microsatellite instability☆
Introduction
Autophagy, a cellular homeostatic mechanism responsible for the removal or turnover of cytoplasmic components, induces type II programmed cell death [1], [2], [3]. Compared with apoptosis, data connecting cancer pathogenesis with autophagic cell death are limited, and most of them were related to Beclin1 [4], [5], [6], [7], [8], [9]. Beclin1, a mammalian orthologue of yeast Atg6, plays a crucial role in the vesicle nucleation process of autophagy [4], [5]. In breast cancer cell lines, beclin-1 gene is deleted mono-allelically [6]. Experimentally, Beclin1-deficient mice suffer from a high incidence of tumors, suggesting Beclin1 is a haploinsufficient tumor suppressor gene [8], [9]. However, our recent study revealed that Beclin1 point mutation is rare in common human cancers, including breast, colorectal, gastric, and lung cancers [10], suggesting that there may be another inactivating mechanism of Beclin1-mediated autophagy in cancers.
The ultraviolet (UV) radiation resistance–associated gene (UVRAG) was initially identified as a gene that is responsible for partial complement of UV sensitivity in xeroderma pigmentosum cells [11]. Recently, Liang et al [12] found that UVRAG binds with Beclin1/PI(3)kinase III (PI(3)KC3) complex and induces autophagy. UVRAG-induced activation of autophagy formation suppresses the tumorigenic activity of cancer cells [12]. Also, Bif-1, a Bax activator, interacts with Beclin-1 through UVRAG and regulates autophagy and tumorigenesis [13]. UVRAG may act as a tumor suppressor by activating autophagic cell death and might be inactivated in cancer cells. In exon 8 of UVRAG there is a polyadenine repeat (A10), and an earlier study detected a frameshift mutation in the A10 in 18% (2/11) of microsatellite instability (MSI)–positive colorectal cancers [14]. Because the frameshift mutation would result in the production of truncated UVRAG protein, the autophagy activity of UVRAG would be inactivated in the cancer cells with the frameshift mutation. The aim of this study was to determine whether the UVRAG mutation occurs in gastric cancers as well.
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Materials and methods
Methacarn-fixed tissues of 137 gastric adenocarcinomas were used for this study. All of the patients with cancers were Asians (Koreans). Approval for this study was obtained from the Catholic University of Korea, College of Medicine's institutional review board. The gastric carcinomas consisted of 32 MSI-high (MSI-H), 13 MSI-low (MSI-L), and 92 microsatellite-stable (MSS) cancers according to the National Cancer Institute criteria [15]. The gastric carcinomas consisted of 55 diffuse-type, 52
Results
Genomic DNAs isolated from normal and tumor tissues of the 137 gastric carcinomas through the microdissection were analyzed for detection of mutation in UVRAG gene (exon 8) by the PCR-SSCP analysis. The PCR-SSCP analysis identified aberrant bands in 3 (2.2%) of the 137 gastric carcinomas (Fig. 1). DNA from normal tissues from the same patients showed no evidence of the mutation in SSCP, indicating the mutations had risen somatically (Fig. 1A). DNA sequence analysis of the aberrantly migrating
Discussion
Frequent alterations of cell death–related genes and their products in cancers [16], [17], [18], [19] led us to analyze the somatic mutation of UVRAG gene in gastric cancer tissues. Because mononucleotide repeats are frequent targets for somatic mutation in gastric carcinomas with MSI, we focused our analysis within the polyadenine tract of UVRAG gene. Because a previous study showed a moderate frequency (17.1%) of UVRAG exon 8 mutation in colorectal carcinomas with MSI [14], we expected to
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This work was supported by funding from the Korea Research Foundation made in the program year of 2007 (E00042).