Mastermind Like Transcriptional Coactivator 3 (Maml3) Drives Neuroendocrine Tumor Progression

Metastatic disease in pheochromocytomas and paragangliomas (PCC/PGL) is not well-understood. The Cancer Genome Atlas discovered recurrent MAML3 fusion genes in a subset of tumors that lacked known germline or somatic driver mutations and were associated with aggressive disease. Here, we aimed to investigate the role of MAML3 in tumorigenesis. Human PCC/ PGLs were used for IHC and genetic analysis. Three neuroendocrine tumor cell lines, SK-N-SH, QGP-1, and BON-1, were transiently transfected with MAML3 (FL) or exon 1 deleted MAML3 (dEx1; mimicking the fusion), and biologic effects of overexpression were examined in vitro. We found 7% (4/55) of human PCC/PGL have UBTF similar to MAML3 fusions and all were sporadic cases with metastatic disease. Fusion-positive tumors had intense MAML3 nuclear staining and increased b-catenin by IHC and showed increased WNT4 expression. In vitro, overexpression of FL and dEx1 MAML3 increased invasion in SK-N-SH, QGP-1, and BON-1 (all P < 0.05) and increased soft-agar colony formation in QGP-1 and BON1 (all P < 0.05). Cotransfection with FL or dEx1 MAML3 and b-catenin increased TCF/LEF promoter activation by luciferase activity and coimmunoprecipitation confirmed interaction between MAML3 and beta-catenin. These data suggest MAML3 is involved in WNT signaling pathway activation. In summary, UBTF similar to MAML3 fusions are present in a subset of PCC/ PGL and associated with metastatic disease without other known drivers. MAML3 overexpression led to increased tumorigenicity in neuroendocrine tumor cells and the mechanism of action may involve WNT signaling pathways.