Science & Technology
Cancer
Tet-controlled temporally defined or tissue-specific expression of oncogenes and tumor suppressor genes paved a new way to generate mouse cancer models to study tumor initiation, progression and maintenance with unprecedented precision. The possibility to shut down oncogene expression at will by an exogenous stimulus for the first time also enabled the establishment of models to study tumor regression in transgenic animals, which might be the most striking and exciting experimental approach in this research area. Such cancer models overexpressing oncogenes have been developed for Myc, H-ras, K-ras, ErbB2, Bcr-Abl1 or SV40 Tag. The main surprising conclusion from these studies was that oncogene overexpression was not only required for tumor initiation but also for tumor maintenance. This was first shown for Myc in the hematopoietic system (Felsher et al., 1999). The inactivation of the initiating oncogene led to complete regression of 90% of the tumors. Similarly, malignant melanomas which developed in an Ink4a- and Arf-tumor suppressor-deficient background underwent complete regression when the initiating oncogene H-ras was inactivated by Dox withdrawal (Chin et al., 1999). These experimental studies suggest that targeting a single oncogene could result in an effective tumor therapy, which was reduced to practice by Jain et al. (Jain et al., 2002). These experiments showed that brief inactivation of Myc expression led to sustained osteogenic sarcoma regression and differentiation of osteogenic sarcoma cells into mature osteocytes. Surprisingly, Myc reactivation failed to restore malignancy but induced apoptosis instead. Results emerging from several laboratories studying the conditional activation/inactivation of various oncogenes shed new light on mechanisms governing induction and maintenance of tumorigenesis. They suggest that many tumors are dependent on the continued activity of the initiating oncogene. This "oncogene addiction" apparently may turn tumors vulnerable by interfering with single oncogenes. However, when Myc is induced in mammary gland, only 30% of the tumors arising regress upon inactivation of the oncogene, whereas around 60% have acquired a Myc-independent but preferred secondary pathway via mutations in K-ras gene (D'Cruz et al., 2001). This finding would suggest that targeting of Myc and K-ras would result in the regression of many known tumors.
The Tet gene switch was also instrumental for a study addressing the mechanism of cancer dissemination and metastatic invasion of remote tissues by Podsypanina, Varmus and colleagues (Podsypanina et al., 2008). It has been a long held hypothesis that metastasis is a late stage event in cancer progression, resulting from an accumulation of mutations ''enabling'' the conversion to full malignancy including the potential to colonize other organs. However, the careful analysis of mouse models, patient data as well as theoretical considerations about the sequence of events in metastasis has already cast doubt on the general validity of this model. Podsypanina and colleagues established two transgenic mouse models with oncogenes under Tet control. They demonstrated the capacity of untransformed mammary cells to invade, reside, and proliferate in non-mammary tissues using a mouse line expressing rtTA under the control of the mammary-specific MMTV promoter the MYC and activated Kras oncogene under the control of the Tet promoter. The resulting tri-transgenic mice were phenotypically normal unless exposed to Dox which resulted in the rapid development of tumors. These animals were then used to bypass the step that has been widely considered to be crucial for metastasis: the escape of transformed cells into the vasculature as one bottleneck for dissemination. Podsypanina and colleagues isolated untransformed cells from rtTA-MYC/KrasD12 transgenic mice never exposed to Dox and injected them directly into the bloodstream of recipient ''wild-type'' mice. When exposed to Dox and only then the animals developed ectopic tumors. The result that untransformed cells can colonize organs like the lung was further confirmed in a second conditional model, using the same rtTA line in combination with a different oncogene, polyoma middle T -Antigen. Again, after the cell transfer tumors formed exclusively after Dox exposure. Monitoring oncogene activation at different times after cell transfer showed that non-transformed cells can reside at ectopic sites for prolonged periods of time. During this time window, they could accumulate mutations resulting in their conversion to a fully cancerous state. These findings if applicable to humans will without any doubt have profound implications for the diagnosis and treatment of human cancer.