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Background Metanephric adenoma (MA) is a rare benign renal neoplasm. On occasion, MA can be difficult to differentiate from renal malignancies such as papillary renal cell carcinoma in adults and Wilms̕ tumor in children. Despite recent advancements in tumor genomics, there is limited data available regarding the genetic alterations characteristic of MA. The purpose of this study is to determine the frequency of metanephric adenoma cases exhibiting cytogenetic aberration t (9;15)(p24;q24), and to investigate the association between t (9,15) and BRAF mutation in metanephric adenoma. Methods This study was conducted on 28 archival formalin fixed paraffin-embedded (FFPE) specimens from patients with pathologically confirmed MA. Tissue blocks were selected for BRAF sequencing and fluorescent in situ hybridization (FISH) analysis for chromosomal rearrangement between KANK1 on chromosome 9 (9p24.3) and NTRK3 on chromosome 15 (15q25.3), which was previously characterized and described in two MA cases. Results BRAFV600E mutation was identified in 62% of our cases, 9 (38%) cases were BRAFWT, and 4 cases were uninformative. Of the 20 tumors with FISH results, two (10%) were positive for KANK1-NTRK3 fusion. Both cases were BRAFWT suggesting mutual exclusivity of BRAFV600E and KANK1-NTRK3 fusion, the first such observation in the literature. Conclusions Our data shows that BRAF mutation in MA may not be as frequent as suggested in the literature and KANK-NTRK3 fusions may account for a subset of BRAFWT cases in younger patients. FISH analysis for KANK1-NTRK3 fusion or conventional cytogenetic analysis may be warranted to establish the diagnosis of MA in morphologically and immunohistochemically ambiguous MA cases lacking BRAF mutations.

1537 Background: Renal metanephric adenoma (MA) is a very rare benign renal tumor, which is frequently misclassified when microscopic features alone are applied. Despite the classification of adenoma as a benign tumor, it is difficult to differentiate from other renal carcinomas such as malignant papillary renal cell carcinomas and in children it can be mistaken with Wilms tumor. The correct classification of a renal tumor is critical for diagnostic, prognostic, and therapeutic purposes. Despite the advancements in cancer genomics, there is limited data available regarding the genetic alterations critical to the metanephric adenoma development. Recent data suggest that 90% of MA have BRAFV600Emutations; the genetics of the remaining 10 % are unclear. Methods: This study was conducted on 13 FFPE specimens from patients who were diagnosed with renal metanephric adenoma. H&E stained slides from all cases were reviewed by study pathologist, and representative tissue blocks were further selected for BRAFV600E sequencing and fluorescent in situ hybridization was adapted to detect chromosomal rearrangement between KANK1 on chromosome 9 (9p24.3) and NTRK3 on chromosome 15 (15q25.3). Results: In this study, we identified a novel chromosomal translocation t(9;15)(p24;q24) between KANK1 and NTRK3, and provided new insights into molecular mechanisms which might identify a subset of metanephric adenomas. Such findings imply that recurrent cytogenetic aberrations may be of prognostic significance as well. Interestingly, our data suggested mutual exclusivity of BRAFV600Eand t(9;15) aberrations. Conclusions: Molecular and cytogenetic analyses have allowed us to elucidate a genetic aberration, which may be specific to metanephric adenoma. Aberrant expression of the KANK1-NTRK3 gene fusion may be one mechanism by which functionally relevant genes are altered in the development of metanephric adenoma, and thus mark a subgroup of metanephric adenomas with particular clinicopathological features. Also, our study adds KANK1 and NTRK3 to the list of candidate genes that may play a role in the 10% of renal metanephric adenomas that lack a BRAFV600E mutation.