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 immunohistochemistry 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~MAML3 fusions and all were sporadic cases with metastatic disease. Fusion-positive tumors had intense MAML3 nuclear staining and increased β-catenin by immunohistochemistry 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 BON-1 (all p<0.05). Co-transfection with FL or dEx1 MAML3 and β-catenin increased TCF/LEF promoter activation by luciferase activity and co-immunoprecipitation confirmed interaction between MAML3 and β-catenin. These data suggest MAML3 is involved in WNT signaling pathway activation. In summary, UBTF~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. Implications: MAML3 increases tumorigenicity and invasion in neuroendocrine tumor cells and may be a prognostic marker for aggressive disease.