Invasive urothelial carcinoma with chordoid and myxoid features shows increased RAS/RAF pathway alterations

Invasive urothelial carcinoma (UCa) with chordoid and myxoid features is an uncommon but previously described subtype of UCa composed of distinctive elongated nests and cords of tumor cells floating within a prominent myxoid stroma. The molecular features of chordoid and myxoid UCa are not known. We identified 9 cases of UCa with chordoid and myxoid features and performed a comprehensive next generation sequencing assay, targeting the chordoid and myxoid component. The cases included 7 TURBT and 2 nephroureterectomies in which the chordoid/myxoid component ranged from 10 to 80 % (mean 47 %). Available immunohistochemistry in the chordoid and myxoid component showed p63+ (9/9) and GATA3+ (8/9), supporting UCa. All cases were largely negative for SOX10 and CDX2 (one case showed weak CDX2 staining). On molecular analysis, chordoid and myxoid UCa showed a molecular profile typical of conventional UCa, with recurrent alterations in TERTp (7/9), chromatin remodeling genes (8/9) and cell cycle pathway genes (8/9). A high rate of DNA damage repair gene alterations was also seen (6/9). Interestingly, 5/9 cases demonstrated recurring alterations in RAS/RAF pathway genes, a finding typically more frequently seen in primary adenocarcinomas of the urinary bladder and less common in conventional UCa, and also suggesting possible alternative therapies for these patients. In conclusion, chordoid and myxoid UCa shows a genomic profile with both urothelial and adenocarcinoma features. Furthermore, the molecular profile reveals potentially actionable therapeutic targets in the RAS/RAF and DNA damage repair pathways and high tumor mutational burden.