Tumor escape in a Wnt1-dependent mouse breast cancer model is enabled by p19Arf/p53 pathway lesions but not p16Ink4a loss
Michael T. Debies, … , Robert D. Cardiff, Edward J. Gunther
Michael T. Debies, … , Robert D. Cardiff, Edward J. Gunther
Published December 3, 2007
Citation Information: J Clin Invest. 2008;118(1):51-63. https://doi.org/10.1172/JCI33320.
View: Text | PDF
Research Article Oncology

Tumor escape in a Wnt1-dependent mouse breast cancer model is enabled by p19Arf/p53 pathway lesions but not p16Ink4a loss

Abstract

Breast cancers frequently progress or relapse during targeted therapy, but the molecular mechanisms that enable escape remain poorly understood. We elucidated genetic determinants underlying tumor escape in a transgenic mouse model of Wnt pathway–driven breast cancer, wherein targeted therapy is simulated by abrogating doxycycline-dependent Wnt1 transgene expression within established tumors. In mice with intact tumor suppressor pathways, tumors typically circumvented doxycycline withdrawal by reactivating Wnt signaling, either via aberrant (doxycycline-independent) Wnt1 transgene expression or via acquired somatic mutations in the gene encoding β-catenin. Germline introduction of mutant tumor suppressor alleles into the model altered the timing and mode of tumor escape. Relapses occurring in the context of null Ink4a/Arf alleles (disrupting both the p16Ink4a and p19Arf tumor suppressors) arose quickly and rarely reactivated the Wnt pathway. In addition, Ink4a/Arf-deficient relapses resembled p53-deficient relapses in that both displayed morphologic and molecular hallmarks of an epithelial-to-mesenchymal transition (EMT). Notably, Ink4a/Arf deficiency promoted relapse in the absence of gross genomic instability. Moreover, Ink4a/Arf-encoded proteins differed in their capacity to suppress oncogene independence. Isolated p19Arf deficiency mirrored p53 deficiency in that both promoted rapid, EMT-associated mammary tumor escape, whereas isolated p16Ink4a deficiency failed to accelerate relapse. Thus, p19Arf/p53 pathway lesions may promote mammary cancer relapse even when inhibition of a targeted oncogenic signaling pathway remains in force.

Authors

Michael T. Debies, Shelley A. Gestl, Jessica L. Mathers, Oliver R. Mikse, Travis L. Leonard, Susan E. Moody, Lewis A. Chodosh, Robert D. Cardiff, Edward J. Gunther

×

Figure 1

Mechanisms of mammary tumor relapse in MTB/TWNT mice.

Options: View larger image (or click on image) Download as PowerPoint
Mechanisms of mammary tumor relapse in MTB/TWNT mice. (A) Mammary expres...
(A) Mammary expression of putative Wnt pathway target genes. Northern hybridization analyses are shown. Mammary gland RNA came from 5-week-old virgin female MTB/TWNT mice that were either Dox naive (Off Dox) or treated with Dox for 96 hours (On Dox). Tumor RNA came from clonally related outgrowths derived from a Dox-dependent MTB/TWNT tumor that was explanted onto the flanks of Dox-treated host mice. Paired flank explants were harvested during ongoing Dox treatment or after timed Dox withdrawal. (B) Tumor gene expression patterns. Northern hybridization analysis was performed on RNA samples from primary and relapsed MTB/TWNT mammary tumors. Three relapsed DITs expressed Wnt1 transgene in an inducer-independent manner (lanes marked T), and 3 expressed aberrant transcripts encoding activated β-catenin variants (β). (C) Molecular genetic analysis of the Ctnnb1 (β-catenin) gene. Segments of transcripts encoding the regulatory domain of β-catenin were amplified from tumor-derived RNA via RT-PCR and subjected to DNA sequencing. The coding region of mouse β-catenin is shown schematically, with arrows indicating the primers used for RT-PCR placed relative to their approximate annealing sites along the open reading frame. The blowup depicts critical aa residues encoded within exon 3; known hot spots for cancer-associated aa substitutions are in bold. Asterisks denote the residue affected by the S33Y mutation identified in 2 DITs. The upper chromatograms show detection of only the wild-type β-catenin allele in an antecedent primary tumor but additional detection of the S33Y allele in a descendant recurrent tumor. The lower panels depict RT-PCR–based detection of an aberrantly spliced β-catenin transcript lacking exon 3 within a relapsed tumor (R) and not within the antecedent primary tumor (P). (D) Tumor histology. Photomicrographs of H&E-stained sections derived from representative primary-relapse tumor pairs. The mode of tumor escape identified for each relapse is indicated. Scale bar: 50 μm.