Expanding the molecular spectrum of congenital mesoblastic nephroma: A rarely described KIAA1549::BRAF fusion in a classic variant

Congenital mesoblastic nephroma (CMN) are low grade spindle cell neoplasms with myofibroblastic differentiation and classified morphologically as classic, cellular, or mixed. Recurrent genetic alterations have been identified – EGFR internal tandem duplication is associated classic morphology, ETV6::NTRK3 fusion the cellular subtype. We report a case of variant CMN rarely described KIAA1549::BRAF fusion. A 45 mm left renal tumour was antenatally resected in 5-day-old boy. The unencapsulated demonstrated typical morphology infiltration into adjacent parenchyma extrarenal extension. Small foci intra-tumoural cartilage were seen. showed widespread cytoplasmic staining for vimentin WT1, while there nuclear WT1 islands cells pale cytoplasm. SMA desmin patchy. RNA sequencing using targeted 53 gene panel 5' to 3' in-frame KIAA1549 exon 13 BRAF 9, result 1.9 Mb recurrent within chromosome 7 band q34. While other rearrangements CMN, these mixed variants. To our knowledge this first identifying congenital nephroma.