ReCREating BRAF-driven thyroid and lung cancer in mice

Abstract

Papillary thyroid carcinoma (PTC) is the most common type of thyroid cancer with a 3:1 female/male incidence. PTC is caused by oncogenic BRAF mutation encoding BRAFV600E in 50% of cases. Prognosis is mostly excellent post- surgery and radioiodine therapy, but 15% of PTC patients with clinical tumors die of the disease. Transgenic mouse models are invaluable tools in dissecting the mechanisms of thyroid tumor progression, and to monitor novel targeted drug treatments in vivo. Conditional expression of BRAFV600E using a thyroid specific Cre driver e.g., the thyroglobulin (Tg) promoter, can be used to activate mutant Braf (BrafCA) specifically in the thyroid and with temporal control using tamoxifen inducible Cre. However, an inborn problem with this procedure is that nearly all thyroid cells are synchronously oncogene-activated. This causes hypothyroidism and unphysiologically high levels of circulating TSH that is goitrogenic, making it difficult to investigate tumor clonality confounded by reactive hyperplasia. In paper I, we developed a new PTC model based on stochastic BRAF activation (due to spontaneous Cre activity in the absence of tamoxifen) by which tumors developed in a normal microenvironment and with maintained systemic thyroid function. Originating from a single follicle, individual tumors had different histologic phenotypes and were initially oligoclonal identified by lineage tracing. We applied this model in paper III to evaluate drug responses to a BRAF-inhibitor (a vemurafenib analog) and found that female mutant mice recovered poorer in thyroid gene expression (Slc5a5 and Tshr) than males and developed larger tumors that progressed more with long-term drug treatment. Analysis of cytokine expression in paper IV revealed differential cytokine expression indicating tumor heterogeneity distinguished by level of inflammation, and that the tumor cells themselves secreted cytokines (IL-1b, IL-6 and TNF-a) in early tumor development. Finally, we confirmed that targeted oncogene activation without induction can generate sporadic tumorigenesis in other tissues. In paper II, using Nkx2.1, a transcription factor shared by thyroid and lung, as Cre driver, mutant BRAF independently caused both thyroid and non- small cell lung carcinomas with different growth and progression features consistent with modulation of oncogene activity in an organ-specific fashion. This represents the first mouse model in which lung adenomas progress to adenocarcinomas due to BRAF mutation.