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Table S1 has the RNA-Seq sample names and raw counts.

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∼MAML3 fusions and all were sporadic cases with metastatic disease. Fusion-positive tumors had intense MAML3 nuclear staining and increased β-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 BON-1 (all P < 0.05). Cotransfection with FL or dEx1 MAML3 and β-catenin increased TCF/LEF promoter activation by luciferase activity and coimmunoprecipitation 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.

MAML3 overexpression has no effect on proliferation or apoptotic signaling. SK-N-SH, QGP-1 and BON-1 transfected with FL or dEx1 MAML3 and showed A) no increase in proliferation by BrdU assay when grown in full serum media for 72 hours and B) no increase in total or cleaved PARP or Caspase 3 when grown in serum free media for 48 or 72 hours. No cleaved caspase 3 was seen at all. The % of cleaved PARP was calculated after normalizing to beta tubulin and no significant differences were found at either time point for any cell line. Experiment performed N=3 with representative blots shown. T-PARP= total PARP; C-PARP=cleaved PARP; T-Caspase 3=total Caspase 3; C-Caspase 3=cleaved Caspase 3