Do sharks get cancer?
Despite the widely touted myth that sharks do not develop cancer, fish of all species do occasionally develop spontaneous tumours. This is of course also true for the most common of laboratory fish, the zebrafish. In this article, I will give you a brief overview of how the unique properties of the zebrafish have been exploited by scientists to generate very useful models to study the molecular basis of various cancers.
The use of zebrafish in cancer biology goes right back to when scientists first started using them in the lab, at which point it was noticed that they spontaneously develop various kinds of tumours. However, using these naturally occurring malignancies to study cancer development is rather impractical – not only would you need a lot of fish due to the rarity of these cancers, but there would also be a lot of heterogeneity as to what kinds of tumours develop. This is clearly not ideal if you want to study the molecular basis and treatment options of one particular cancer.
From disease to model
Subsequently, carcinogenic chemicals were used to speed up the onset of cancer development. However, similar to using naturally occurring tumours, this strategy is not terribly useful for studying one particular kind of cancer, as the resulting tumours can still be very diverse(although some substances tend to always cause the same type). This approach is mostly used to identify cancer-causing chemicals during human and environmental safety testing.
To study one specific cancer type in detail, scientists started to create zebrafish carrying particular loss of function mutations (i.e. genes that lose activity due to a change), or overexpressing certain cancer-causing oncogenes (i.e. genes that cause cancer when they are overly active). Usually, this leads to the early development of only one – or at most a few – types of cancer. The first of these more specific models were acute lymphoblastic leukaemia (ALL) models, but nowadays there are models for cancers of various tissues, ranging from the brain to the pancreas.
Most of these mutant models were originally created using mutagenizing drugs followed by screening for a phenotype, but recently the research community has shifted to more targeted techniques. These make use of novel genome editing tools, such as the CRISPR-Cas9 system to switch off certain genes. The overexpression of specific genes on the other hand was usually achieved using proteins called transposases to integrate novel genetic information, but very recently the CRISPR-Cas9 system has also been tweaked to do the same.
Why study cancer in fish?
So why would anyone bother to go through this effort and do all this in fish, if we can just use the more closely related mice or rats? Apart from the lower expense and easier generation of large numbers of fish, the main reason why fish are used is that visualizing particular cells is much easier than in other organisms. This is mainly due to two factors: the existence of various transgenic fish lines in which a particular cell type is labelled, and the existence of transparent adult fish (the casper fish, as below).
The ease of labelling specific cell types has been exploited elegantly for studying the clonal expansion of cancer cells that drives tumor growth in vivo as it happens, as well for the study of cancer metastases. Now that adult transparent zebrafish have enabled even easier in vivo imaging, the approach has been used successfully to visualize the process by which metastases arise and cancer cells distribute throughout the body.
Understanding the origins of melanoma
A recent paper from Charles Kaufman of the Harvard Stem Cell Institute and colleagues nicely illustrates how these advantages can be very powerful indeed. In this paper published in Science magazine, the researchers used a zebrafish melanoma model that they had developed a few years earlier which expresses gene variants associated with the cancer in humans, and combined this with a newly developed transgenic zebrafish line, in which cells expressing a gene known as Crestin, which is involved in early neural development , are labelled in green. The Crestin gene is normally not expressed in adult humans, but is switched on again in melanomas. This is also why this combination is interesting; emerging melanoma cells will re-express the normally silent gene and be labelled fluorescently.
This method allowed the researchers to track melanoma development from the very first tumour cell to the macroscopically visible tumour comprised of millions of cells. The very early changes that have to occur for cancer to develop can now be studied at much greater detail than before, as these very early tumorigenic cells are extremely hard (or completely impossible) to distinguish from normal cells if they are not labelled. In this specific case the researchers identified the activation of several gene pathways that are usually involved in neural crest development in the embryo as key events in the initiation of melanoma, and believe that their findings could lead to a new genetic test for suspicious moles in patients. Their work suggests a model of cancer development where normal tissue becomes primed for cancer when oncogenes are activated and tumour suppressor genes are silenced or lost, but where cancer develops only when a cell in the tissue reverts to a more primitive, embryonic state and starts dividing.
This paper increased our understanding of the underlying biology of the very early stages of tumour development, and a detailed understanding of these early steps might be very important when developing preventative or therapeutic drugs.
In summary, the field of zebrafish cancer biology has made great advances in the last decade and will continue to do so with the increasing popularity of genome editing techniques. The easy visualization of particular cell types leads to distinct advantages of using zebrafish, particularly for the study of metastases and the very early stages of cancer development.
Kaufman, C.K., Mosimann, C., Fan, Z.P., Yang, S., Thomas, A.J., Ablain, J., Tan, J.L., Fogley, R.D., van Rooijen, E., Hagedorn, E.J. and Ciarlo, C., 2016. A zebrafish melanoma model reveals emergence of neural crest identity during melanoma initiation. Science, 351(6272), p.aad2197. DOI: 10.1126/science.aad2197