title: Masking epistasis between MYC and TGF-β pathways in antiangiogenesis-mediated colon cancer suppression.
abstract: BACKGROUND
  The c-Myc oncoprotein is activated in the majority of colorectal cancers (CRCs), whereas the TGF-β pathway is frequently affected by loss-of-function mutations, for example in SMAD2/3/4 genes. The canonical model places Myc downstream of inhibitory TGF-β signaling. However, we previously demonstrated that Myc also inhibits TGF-β signaling through the miR-17~92 microRNA cluster, raising the question about functional relationships between these two pathways.
  
  
  METHODS
  We engineered a series of genetically complex murine and human CRC cell lines in which Myc and TGF-β activities could be manipulated simultaneously. This was achieved through retroviral expression of the Myc-estrogen receptor fusion protein and through Smad4 short hairpin RNA knockdown. Cell lines thus modified were injected subcutaneously in immunocompromised mice, and the resultant tumors (n = 5-10 per treatment group) were analyzed for overall growth and neovascularization. Additionally, the distribution of MYC and TGF-β pathway mutations was analyzed in previously profiled human CRC samples.
  
  
  RESULTS
  In kras-mutated/trp53-deleted murine colonocytes, either Myc activation or TGF-β inactivation increased tumor sizes and microvascular densities approximately 1.5- to 2.5-fold, chiefly through downregulation of thrombospondin-1 and related type I repeat-containing proteins. Combining Myc activation with TGF-β inactivation did not further accelerate tumorigenesis. This redundancy and the negative effect of TGF-β signaling on angiogenesis were also demonstrated using xenografts of human CRC cell lines. Furthermore, the analysis of the Cancer Genome Atlas data revealed that in CRC without microsatellite instability, overexpression of Myc and inactivation of Smads (including acquired mutations in SMAD2) are mutually exclusive, with odds ratio less than 0.1.
  
  
  CONCLUSIONS
  In human CRC, gain-of-function alterations in Myc and loss-of-function alterations in TGF-β exhibit a masking epistatic interaction and are functionally redundant.
authors.affiliation: Affiliations of authors: Division of Cancer Pathobiology, Center for Childhood Cancer Research (MD, GST, SH, AT-T) and Center for Biomedical Informatics (PR), Children's Hospital of Philadelphia, Philadelphia, PA; Cancer Biology Graduate Program (JC, EKD, AT-T) and Department of Pathology & Laboratory Medicine (AT-T), Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA; Division of Experimental Pathology, Beth Israel Deaconess Medical Center (JL) and Department of Medical Oncology, Dana-Farber Cancer Institute (AB), Harvard Medical School, Boston, MA.
authors.forename: Michael
authors.initials: M
authors.lastname: Dews
authors.forename: Grace S
authors.initials: GS
authors.lastname: Tan
authors.forename: Stacy
authors.initials: S
authors.lastname: Hultine
authors.forename: Pichai
authors.initials: P
authors.lastname: Raman
authors.forename: Jaewoo
authors.initials: J
authors.lastname: Choi
authors.forename: Elizabeth K
authors.initials: EK
authors.lastname: Duperret
authors.forename: Jack
authors.initials: J
authors.lastname: Lawler
authors.forename: Adam
authors.initials: A
authors.lastname: Bass
authors.forename: Andrei
authors.initials: A
authors.lastname: Thomas-Tikhonenko
mesh_terms: D020533:Angiogenesis Inhibitors / Q000494:pharmacology*; D000818:Animals; D015153:Blotting, Western; D045744:Cell Line, Tumor; D015179:Colorectal Neoplasms / Q000235:genetics / Q000378:metabolism*; D004843:Epistasis, Genetic; D064593:Heterografts; D006801:Humans; D007150:Immunohistochemistry; D051379:Mice; D009154:Mutation; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D060888:Real-Time Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism*
chemical_list: D020533:Angiogenesis Inhibitors; C489427:MYC protein, human; C489428:Myc protein, mouse; D016271:Proto-Oncogene Proteins c-myc; D016212:Transforming Growth Factor beta