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Renal cell carcinoma (RCC) is the 7th most common cancer in the United States. Annually, 74,000 Americans are estimated to be newly diagnosed, accounting for more than 14,000 deaths (Clear-cell histological subtype, or ccRCC, is the most common subtype, accounting for approximately 80% of RCC cases. Patients with localized malignancy are treated with nephrectomy and have a favorable 5-year survival rate of 73%1. However, 25%-30% of patients develop distant metastases to vital organs such as the lungs, resulting in a poor mean survival of 13 months and 5-year survival rate of only 11%1,2,3. Further understanding of the metastatic mechanism is needed to improve the deadly outcome for metastatic ccRCC.
The loss of the VHL tumor suppressor gene is a hallmark genetic lesion observed in a majority of human ccRCC cases4,5,6,7. However, the precise oncogenic mechanism of VHL loss in ccRCC is unknown. Also, VHL expression status is not predictive of outcome in ccRCC8. Notably, despite numerous attempts at renal-epithelial-targeted VHL knockout, scientists have failed to generate renal abnormality beyond the preneoplastic cystic lesions observed in mice9, even when combined with deletion of other tumor suppressors such as PTEN and p5310. These findings support the idea that VHL loss alone is insufficient for tumorigenesis or the subsequent spontaneous metastasis.
Recently, our laboratory created a new VHL knockout (VHL-KO) cell line using CRISPR/Cas9 mediated deletion of the VHL gene in the murine VHL+ ccRCC cell line (RENCA, or VHL-WT)11,12. We showed that VHL-KO is not only mesenchymal, but also promotes epithelial to mesenchymal transition (EMT) of VHL-WT cells12. EMT is known to play an important role in the metastatic process13. Our work further showed that distant lung metastasis occurs only with co-implantation of VHL-KO and VHL-WT cells in the kidney, supporting a cooperative mechanism of metastasis. Importantly, our orthotopically implanted VHL-KO and VHL-WT model leads to robust lung metastases, recapitulating the clinical ccRCC cases. This spontaneous metastatic ccRCC model compensates for the lack of a transgenic metastatic mouse model, especially in the development of novel anti-metastasis drugs. This protocol demonstrates the renal capsule implantation of the heterogeneous cell populations of genetic engineered RENCA cells.
Chicken CAM models have a long history in research for angiogenesis and tumor biology due to their numerous advantages, as summarized in Table 114,15,16,17,18. Briefly, the time window for CAM tumor growth is short, allowing a maximum of 11 days until the CAM is destroyed upon hatching of the chicken16. Despite the short growth time, the rich nutrition supply and immunodeficient state of the chicken embryo enable very efficient tumor engraftment16,19,20,21. Finally, the cost of each fertilized egg is ~$1, compared to over $100 for a SCID mouse. Together, the CAM model can serve as a valuable alternative animal model in establishing new PDXs at a great saving in time and cost in comparison to the mouse. In this protocol, we assessed whether the model was able to recapitulate the biology of metastatic ccRCC observed in the mouse orthotopic model.
| (SCID) Mouse | CAM | Note |
| Cost | >$100 each | ~$1 each | Viability ranging from 50-75% |
| Need for barrier housing | Yes | No | Further reduces cost & simplifies serial monitoring of the tumors |
| Tumor directly visible | No | Yes | Figure 3A |
| Time to first engraftment (RENCA) | 2 weeks | 2-4 days | ref 14, 15 |
| Endpoint of growth (RENCA) | 3-6 weeks | 10 days | ref 14, 15 |
| Metastasis (RENCA) observed | Yes | Yes in chicks | Figure 3D |
| Serial passages | Yes | Yes | ref 16-18 |
| Passage to mice (RENCA) | Yes | Yes | Hu, J., et al. under review (2019) |
| Maintain tumor heterogeneity | Yes | Yes | Hu, J., et al. under review (2019) |
Table 1: Advantages and limitations of the mouse and CAM models. This table compares the two models for their advantages and limitations in terms of required time, cost, labor, as well as the biology. The CAM model has advantages in efficiency, but it also has its own unique limitations due to the different morphology between birds and mammals. Therefore, it is important to confirm that the model can retain the biology of the xenografts.