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Loss of the DNA mismatch repair protein MSH3 leads to the development of a variety of tumors in mice without significantly affecting survival rates, suggesting a modulating role for the MutSβ (MSH2-MSH3) complex in late onset tumorigenesis. To better study the role of MSH3 in tumor progression, we crossed Msh3−/− mice onto a tumor predisposing p53-deficient background. Survival of Msh3/p53 mice was not reduced compared to single p53 mutant mice; however, the tumor spectrum changed significantly from lymphoma to sarcoma, indicating MSH3 as a potent modulator of p53-driven tumorigenesis. Interestingly, Msh3−/− mouse embryonic fibroblasts displayed increased chromatid breaks and persistence of γH2AX foci following ionizing radiation, indicating a defect in DNA double strand break repair. Msh3/p53 tumors showed increased loss of heterozygosity, elevated genome-wide copy number variation, and a moderate microsatellite instability phenotype compared to Msh2/p53 tumors, revealing that MSH2-MSH3 suppresses tumorigenesis by maintaining chromosomal stability. Our results show that the MSH2-MSH3 complex is important for the suppression of late onset tumors due to its role in DNA double strand break repair as well as in DNA mismatch repair. Furthermore, they demonstrate that MSH2-MSH3 suppresses chromosomal instability and modulates the tumor spectrum in p53-deficient tumorigenesis, and possibly plays a role in other chromosomally unstable tumors as well.

Background/Rationale: The non-receptor tyrosine kinase PTK6/Brk is highly expressed in the ER+/Luminal breast cancer subtypes. PTK6 expression has prognostic significance for patients with ER+ disease; higher transcript levels are associated with poorer survival. The functions of PTK6 in the context of ER+ breast cancer and sensitivity to endocrine therapy have not been explored. We sought to determine the functional roles of PTK6 in ER+ breast cancer cells, including those that are relatively resistant to current endocrine therapies. Methods/Results. We modulated the expression of PTK6 using both gain- and loss-of-function approaches. Enhanced expression of PTK6 in Tamoxifen-sensitive ER+ breast cancer cells was sufficient to confer relative Tamoxifen resistance. Furthermore, downregulation of PTK6 in ER+ breast cancer cells, including those that have acquired resistance to Tamoxifen, induced apoptosis, as evidenced by an increase in AnnexinV+ cells and increased levels of cleaved PARP. PTK6 downregulation impaired growth of Tamoxifen-resistant variants of ER+ MCF7 and T47D cells (MCF7TamR and T47DTamR) in 3D Matrigel culture, and virtually abrogated primary tumor growth of MCF7TamR xenografts. Mechanistically, p38MAPK activation is critical for PTK6 downregulation-induced apoptosis of ER+ breast cancer cells, as p38 inhibition partially rescues cells from PTK6 shRNA-associated apoptosis. Conclusions: Our studies highlight the critical role that PTK6 plays in the survival of ER+ breast cancer cells, including those that are resistant to endocrine therapy. Enhanced PTK6 expression in ER+ breast cancer cells is sufficient to promote endocrine therapy resistance, which could contribute to the poorer prognosis associated with higher PTK6 expression in ER+ patient tumors. As small molecule PTK6 inhibitors are becoming available, our studies support further evaluation of PTK6 as a candidate therapeutic target for endocrine therapy resistant ER+ breast cancers. Citation Format: Irie HY, Park SH, Katsyv I, Zhang W, Nayak A. Protein tyrosine kinase 6 (PTK6) promotes survival of endocrine therapy-resistant ER+ breast cancer cells [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-04-13.