Disruption of p16 and Activation of Kras in Pancreas Increase Ductal Adenocarcinoma Formation and Metastasis in vivo

Abstract

Inactivation of tumor suppressor gene p16/INK4A and oncogenic activation of KRAS occur in almost all pancreatic cancers. To better understand the roles of p16 in pancreatic tumorigenesis, we created a conditional p16 knockout mouse line (p16flox/flox), in which p16 is specifically disrupted in a tissue-specific manner without affecting p19/ARF expression. p16flox/flox; LSL-KrasG12D; Pdx1-Cre mice developed the full spectrum of pancreatic intraepithelial neoplasia (mPanIN) lesions, pancreatic ductal adenocarcinoma (PDA), and metastases were observed in all the mice. Here we report a mouse model that simulates human pancreatic tumorigenesis at both genetic and histologic levels and is ideal for studies of metastasis. During the progression from primary tumors to metastases, the wild-type allele of Kras was progressively lost (loss of heterozygosity at Kras or LOH at Kras) in p16flox/flox; LSL- KrasG12D; Pdx1-Cre mice. These observations suggest a role for Kras beyond tumor initiation. In vitro assays performed with cancer cell lines derived from primary pancreatic tumors of these mice showed that cancer cells with LOH at Kras exhibited more aggressive phenotypes than those retained the wild-type Kras allele, indicating that LOH at Kras can provide cancer cells functional growth advantages and promote metastasis. Increased LOH at KRAS was also observed in progression of human pancreatic primary tumors to metastases, again supporting a role for the KRAS gene in cancer metastasis. This finding has potential translational implications- future KRAS target therapies may need to consider targeting oncogenic KRAS specifically without inhibiting wild-type KRAS function.

Pancreatic cancer–associated retinoblastoma 1 dysfunction enables TGF-β to promote proliferation

“…Oncogenic KRAS is the initiating molecular alteration in PDAC in humans (hPDAC) and mice (mPDAC) (14)(15)(16)(17)(18)(19). KC (which stands for Kras;Pdx1-Cre recombinase) mice carry an oncogenic Kras (Kras G12D ) allele that is silenced by an upstream LoxP-Stop-LoxP (LSL) element, but activated following Cre-mediated recombination (14).…”

“…By 10 months, KC mice develop mPDAC at moderate penetrance (14). PanIN are an important feature of PDAC initiation in both humans and GEMs, and PanIN progression to mPDAC is accelerated by deletion of the p53 (16), p16 Ink4a /p19 Arf (Cdkn2a) (17), Smad4 (15), p16 Ink4a (19), and RB (18) tumor suppressors. However, the RB1 gene is rarely mutated in hPDAC (20), and given the high frequency of KRAS and CDKN2A mutations occurring in conjunction with the overexpression of multiple tyrosine kinase receptors and increased cyclin D1 levels (6), the loss of RB function in PDAC presumably does not drive its pathobiology.…”

Genetics and biology of pancreatic ductal adenocarcinoma

“…The wild-type KRAS allele may serve to counter the function of the oncogenic KRAS allele (Zhang et al 2001;Staffas et al 2015). This concept derives from LOH of the wild-type KRAS allele in lung cancer and PDAC with oncogenic KRAS mutation (Li et al 2003;Qiu et al 2011) and is consistent with the selective amplification of the oncogenic Kras allele in the GEMM of PDAC (Bardeesy et al 2006a). In contrast, wild-type HRAS and NRAS play a supportive role in tumor growth by keeping the oncogenic KRAS-induced DNA damage response in check (Grabocka et al 2014).…”

The emerging role of the TGFβ tumor suppressor pathway in pancreatic cancer

“…The most frequently mutated oncogene by far is KRAS, which plays a role in tumor initiation but also during tumor progression. 8 Pancreatic tumors also show frequent amplification of the transcription factor gene GATA6. 9 Other amplified oncogenes are AKT2, CCND3, CDK4, MDM2 and MYC.…”