UCSF

Lung cancer is the most prevalent malignancy in the industrialized world and is responsible for ~160,000 deaths/year in the USA. Genetic analysis indicates that activation of receptor tyrosine kinase signaling, through mutation of EGFR, ALK, KRAS, BRAF or PIK3CA, is common in non-small cell lung cancer (NSCLC), the most prevalent form of the disease. The use of genetically engineered mouse models allows for the evaluation of specific stages of disease and assessing the role of effector pathways downstream of driver mutations. Expression of mutationally activated KRAS(G12D) or BRAF(V600E) leads to lung tumorigenesis in genetically engineered mouse models. We have used models for the conditional activation of KRas, a gene commonly mutated in human non-small cell lung cancer, as well as BRaf and Pik3ca, two major downstream effectors of KRAS. In a head-to-head comparison of KRAS(G12D) vs. BRAF(V600E)- induced tumorigenesis, BRAF(V600E) elicits an abundance of benign tumors that do not progress beyond low-grade adenomas while KRAS(G12D) driven tumors are fewer in number but often progress to adenocarcinoma. Despite this difference, both tumor types express the same distal lung cell type markers and remain sensitive to ERK1/2 pathway inhibition both in vivo and in vitro. In humans, BRAF and KRAS mutations are mutually exclusive in human lung cancer, but we found that activation of both oncogenes allows for the progression of tumors beyond what is observed in the KRas model. Using a conditional mouse model for the activation of Pik3ca encoding the p110(H1047R) oncogene, we found that this oncogene is unable by itself by itself is unable to induce tumorigenesis in the mouse lung. However, when combined with the BRAF(V00E) or KRAS(G12D), PIK3CA(H1047R) expression greatly increases tumor growth, resulting in a dramatic decrease in survival. This effect is dependent of signaling through AKT. Finally, in cells isolated from BRAF(V600E)- driven tumors, PIK3CA(H1047R) expressed both ectopically or endogenously promotes anchorage independent growth.

These studies demonstrate the importance of ERK1/2 activation in the establishment and maintenance of lung tumors, and mutational activation of PIK3CA(H1047R) is not sufficient to drive tumorigenesis but cooperates in a dramatic fashion with mutationally activated BRAF(V600E) or KRAS(G12D).