Cellular mesoblastic nephroma: morphologic, cytogenetic and molecular links with congenital fibrosarcoma.

Congenital mesoblastic nephroma (CMN) is a rare renal tumor of early infancy with a favorable outcome after complete surgical removal. CMN consists of a heterogeneous group of spindle cell tumors subdivided into "classical", "cellular or atypical" and "mixed" forms based on histologic features. We describe a new case of cellular CMN diagnosed by antenatal ultrasonography with complete remission five years after nephrectomy. Cytogenetic study evidenced a trisomy 11, and real time RT-PCR, but not conventional karyotype, allowed for the detection of the Tel-ETV6/TrkC-NTRK3 fusion transcript as a consequence of a cryptic t(12-15)(p13;q25). As in congenital fibrosarcoma (CFS), two Tel-ETV6/ TrkC-NTRK3 fusion transcripts different by a 42 bp insert in the TrkC kinase domain were expressed. Our observations outline the close links between cellular CMN and CFS. Both tumors have the clinical presentation and histologic features as well as identical cytogenetic and molecular markers in common. Therefore, they are likely to represent the same neoplasm, but occurring at different locations.