Immunophenotype-Genotype Correlations in Clear Cell Sarcoma of Kidney-An Evaluation of Diagnostic Ancillary Studies.

Introduction The purpose of this study was to establish a reliable panel of antibodies for immunohistochemical corroboration of a diagnosis of clear cell sarcoma of kidney (CCSK), taking into consideration the various genotypic subsets of CCSK. Methods We conducted full genotypic analysis for evidence of YWHAE-NUTM2, BCOR internal tandem duplication (ITD), and BCOR-CCNB3 in 68 archival cases of CCSK and then immunostained all cases for CCND1, TLE1, and BCOR along with 63 control samples representing tumor types that may enter into the differential diagnosis of CCSK, including 7 congenital mesoblastic nephromas, 2 desmoplastic small round cell tumors, 13 malignant rhabdoid tumors, 9 Ewing sarcomas/primitive neuroectodermal tumor, 5 synovial sarcomas, and 27 Wilms' tumors. Results Molecular assays showed that 54 CCSKs harbored a BCOR-ITD, 1 case expressed a YWHAE-NUTM2 fusion transcript while none expressed the BCOR-CCNB3 fusion. The remaining 13 CCSKs were designated "triple-negative" based on the molecular findings. CCND1 showed positive immunoreactivity across all subgroups. TLE1 was positive in 94% of cases, including 1 YWHAE-NUTM2 fusion-positive case. Three BCOR-ITD-positive tumors were TLE1-negative. BCOR immunostaining was most variable among subgroups, with triple-negative tumors showing the weakest staining. In all, 10/68 (15%) tumors did not stain for BCOR, of which 4 were triple-negative (4/13 = 31%) and 6 were BCOR-ITD-positive (6/54 = 11%). The single YWHAE-NUTM2-positive tumor showed strong staining for all 3 markers. No single case was negative for all 3 stains; however, 3 cases showed no reactivity for either BCOR or TLE1 of which 1 was triple-negative and 2 BCOR-ITD-positive. Conclusion Having completed the first comprehensive evaluation of immunostaining of 68 fully genotyped CCSK tumors, we show herein that there is a rationale for the use of a small panel of antibodies to assist in the diagnosis of CCSK regardless of genotype, and we demonstrate that in combination CCND1, TLE1, and BCOR are compelling markers in aiding CCSK diagnosis.