Why these aquatic creatures are redefining how we study cancer development
The zebrafish (Danio rerio) is a freshwater fish with unique characteristics that allow it to develop fast and share a close homology to humans. As such, it is an ideal candidate for a drug development platform.
Since the 1960´s different zebrafish-based platforms were used to examine the biology of different diseases. In cancer research, the zebrafish model has advantages over traditional cell culture assays as well as the common mouse models—the workhorse of most in vivo studies. Compared to rodent models, zebrafish have an extremely large progeny. The size of the group enables the discovery of small and perhaps important differences between different treatment arms.
They are also less expensive to care for and breed, and some biological processes can be observed directly in the living animal due to its transparency. Moreover, human and zebrafish share a high grade of similarity: 71% of human proteins and 82% of disease-causing human proteins have genes also found in zebrafish.
Different zebrafish models
There are different methods that can be used to generate oncology models in zebrafish. One way is to develop mutant and transgenic lines. Another is to transplant human tumor cells into the zebrafish. Each methodology has its pros and cons. In the first scenario the influence of single genes on tumor development can be monitored very closely. In the second case a broad range of different tumor types can be examined in parallel.
Another critical point in tumor model development is the selection of the zebrafish stage in which experimentation should be carried out. For obvious reasons embryos are most commonly used as they can be cultivated in a 96-well format. Zebrafish embryos are also still transparent, enabling the in-life visualization of the biological process. In addition, cancer develops more rapidly in embryos, showing tumor formation within two days after induction.
Nevertheless, adults offer a more realistic in vivo model, as the organs and the immune systems are fully developed. The downsides are longer incubation times and the need for a more sophisticated husbandry system. In contrast, adults require immune system ablation—the destruction of the immune system to prepare the fish for transplantation of human tissue—to avoid engraftment rejection.
Scientists at BioReperia, a contract research organization, pushed the larvae-based platform to a new level, making it interesting for large-scale drug testing. It is now possible to implant patient-derived xenograft (PDX) tissue in zebrafish larvae cultivated in a 96-well plate. After a three day incubation time, it is not only possible to determine the activity on the primary tumor but also observe the metastases within individual larvae. The quantification of the results is done using fluorescent labeling. The fast turnaround time opens up new opportunities for combination treatments and screening PDX models.
In a collaborative study we compared the sensitivity towards different standard–of-care treatments in non-small cell lung cancer (NSCLC) PDX models in zebrafish and mice. The results of this study will be presented at the virtual AACR meeting 2020 this week. The predictivity of the zebrafish platform will facilitate the development of innovative cancer treatments by filtering the most promising candidates faster and cheaper than it now takes with mice.
So it seems that this tiny in vivo model provides many advantages in cancer research in comparison with the broadly used traditional in vitro or rodent in vivo model. Due to its low maintenance costs, fast turnaround time, and the ease in establishing metastases models, zebrafish have the potential to bridge in vitro cell-based assays and more time intensive PDX based rodent in vivo models.