Clear cell sarcoma of the kidney: clinical and molecular characterization

Abstract

Clear cell sarcoma of the kidney (CCSK) is a rare pediatric renal tumor comprising 2-5% of all primary renal tumors in children. A review of all available literature revealed that, with current intensive treatment schedules, outcome of CCSK patients has improved significantly. In addition, it demonstrated that a specific molecular signature of CCSK was not yet identified. We subsequently analyzed prospectively collected clinical data of 191 CCSK patients treated according to recent SIOP guidelines. This showed that most CCSK patients (93%) present with localized disease. Using highly intensive treatment schedules five-year event-free and overall survival rates were 78% and 86%, respectively. Stage IV disease and age younger than 12 months were identified as significant adverse prognostic factors. We then evaluated clinical data of 37 relapsed CCSK patients treated according to recent SIOP and AIEOP protocols. This showed that most relapses were metastatic (95%). Brain relapses were more common than previously recognized. Five-year event-free survival after relapse was 18% and five-year overall survival was 26%. We proceeded by studying the prognostic value of t(10;17)(q22;p13) which gives rise to the YWHAE-NUTM2 fusion transcript. We reported that, although the number of fusion-transcript positive CCSK cases turned out to be too small to permit reliable statistical analysis, the clinical phenotype was not overtly different between patients with and without the fusion transcript. We then analyzed CCSK samples using various genome-wide sequencing and array-based methods. This showed that the genome of CCSKs seems to be rather stable. Integrated analysis of expression and methylation data identified promoter hypermethylation and low expression of the tumor suppressor gene TCF21 in all CCSKs, except one case harboring t(10;17)(q22;p13). We functionally verified the tumorigenic role of hypermethylation of TCF21 using a clear-cell renal cell carcinoma (cRCC) cell line (786-O). 786-O cells transfected with pBABE-TCF21 showed significantly decreased clonogenic proliferation and migration, suggesting a tumor suppressor function for TCF21 in ccRCC, which needs to be verified in CCSK. Recently, BCOR internal tandem duplications (ITDs) have been identified as a recurrent aberration in CCSKs. We identified the absence of BCOR ITDs in a CCSK case bearing the YWHAE-NUTM2 transcript, together with previously reported cases indicating that BCOR ITDs and the YWHAE-NUTM2 transcript are mutually exclusive events in CCSK. We additionally showed that CCSKs harboring BCOR ITDs show hypermethylation of TCF21, while CCSKs bearing the YWHAE-NUTM2 transcript show significant lower methylation of TCF21, suggesting that BCOR ITDs may be (in)directly responsible for TCF21 hypermethylation. Results of this thesis have guided the development of renewed treatment recommendations for CCSK patients, which will be included in the UMBRELLA SIOP-RTSG 2016 protocol. The combination of alkylating agents ifosfamide and cyclophosphamide in an alternating setting and the adaptation of the abdominal radiotherapy dose of 10.8 Gy as used by the Children’s Oncology Group to limit toxicity are hallmarks of this (based on consensus) best available treatment regimen. Altogether, the results of this thesis added knowledge to the existing clinical picture, started to unravel the molecular signature and provided potential targets for new treatments for CCSK in children.