Abstract LB-88: The role of PBRM1 as tumor suppressor gene in renal cell carcinomas

Abstract

Abstract Objective: The polybromodomain containing gene PBRM1 is thought to be the second major tumor suppressor gene in clear cell renal cell carinomas (ccRCCs) next to the VHL gene. The PBRM1 protein BAF180 belongs to the SWI/SNF complex and provides functional specificity to this class of chromatin remodelers by binding to acetylated histones via its bromodomains and to nucleosomal DNA by its HMG domain. Our aim was to study the expression of PBRM1 in RCC in comparison to the mutational status of PBRM1 and VHL as well as the expression of VHL. Methods: To assess the contribution of somatic mutations for PBRM1 downregulation we resequenced 31 exons of 96 tumor tissues from consenting patients who underwent radical tumor nephrectomy. In addition, the 3 exons of VHL were resequenced as well. Expression of PBRM1 and VHL was measured in 96 cases of paired normal and tumor tissue samples of RCC patients by Q-PCR using TaqMan specific probes. By immunohistochemistry BAF180 could be detected using a rabbit anti-BAF180 antibody. Results: Specific truncating mutations in the PBRM1 gene were detected in 13 (13.5%) of 96 tumor samples (15.5% of 84 ccRCC cases). No mutations could be observed in 12 cases of papillary tumors or tumors of the chromophobe type. Mutations in the VHL gene could be detected in 28/84 (33.3%) of the ccRCC samples. A marked downregulation of PBRM1 expression (> 2fold) was measured in 41/81 ccRCC cases (50.6%) whereas in 3/81 ccRCC cases (3.7%) PBRM1 was upregulated (>2fold). In 37/81 cases (45.7%) no expression differences between normal and tumor tissue could be seen. However, all cases with mutations in the PBRM1 gene showed a clear decrease of PBRM1 mRNA expression. Interestingly, in papillary carcinomas PBRM1 was strongly upregulated in comparison to the normal tissue showing a completely different expression behaviour. In addition, there was a significant difference in PBRM1 expression depending on the site of the tumor within the kidney (p=0.001). PBRM1 codes for different major transcripts. In our experiments five different transcripts could be observed although two transcripts gave only faint bands in PCR. Concurrent expression of three major transcripts (var.1, 2, and 4) is seen in all matched normal and tumor samples analyzed so far. In 22/46 samples (47.8%) we detected a dominance of var.1 (NM_018165) in normal tissue when compared to the corresponding tumor sample, that preferentially expresses var.4 (NM_181042) that might alter the properties of the HMG domain. Conclusions: Understanding the contribution of PBRM1 changes, existing at the genomic (mutation) and/or mRNA (differential splicing) and/or protein (cellular localization) level, to clinical disease progression and outcome in kidney cancer is an absolute prerequisite for the better understanding of renal carcinogenesis and the putative development of new therapeutic approaches. Citation Format: Burkhard Jandrig, Odiljon Ikromov, Anica Hoegner, Johann J. Wendler, Martin Schostak, Hans Krause. The role of PBRM1 as tumor suppressor gene in renal cell carcinomas. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-88. doi:10.1158/1538-7445.AM2014-LB-88