director of the Department of Animal Models at the PRBB
Opinion
The use of animals as models for medical research dates back to four hundred years before Christ. In the famous corpus hipocraticum, Hippocrates collected comparative anatomy studies carried out on pigs, dogs and primates.
Even in the Roman Empire, in Galen’s time during the second century before Christ, there is evidence of work on the anatomy and physiology of different organs from a variety of species.
Later, in the Middle Ages, these sciences were halted and it was only starting in the 15th century when French physiologists once again took up this research. And we mustn’t forget Mendel and his genetics research, which, although carried out with peas, was started on rats; but the bishop believed such procreation in the convent to be overly lustful.
More recently, in our modern era, Dr. C.C. Little’s development of inbred mice in 1909 was an important milestone. The first strain of genetically identical mice, called DBA (Dilute, Brown and non-Agouti), was a great step forward for research into immunology, organ transplants, predisposition to cancer and, of course, all genetic studies.
Another great discovery for these sciences was the appearance of nude mice when, in Glasgow in 1962, it was found that mice with a spontaneous genetic mutation had no hair and, additionally, had an inhibited immune system. Given the extensive information available on mouse biology and genetics for more than a decade, and thanks to the fact that it is easy to cultivate their pluripotent embryonic cells and modify them genetically using homologous recombination, mice have become the most commonly used animal model in biomedical and biotechnology research around the world. The proportion of genetically modified mice (GMM) is ever increasing in our animal facilities —at the PRBB— and in most research centers of excellence. We have more than 500 different strains of GMM in the PRBB animal facilities and the number grows exponentially each year.
When the human genome was completed at the beginning of the 21st century, the next complete genome to be obtained was that of the mouse, specifically the inbred C57BL/6J strain. When both genomes were compared, it was discovered that more than 95% of their genes are homologous. All of this has made it possible to create thousands of these animal models modified genetically to study various human diseases and those of other animals by altering the mouse genes involved in developing illnesses like cancer, Alzheimer, Parkinson, diabetes, etc. Global consortia currently exist to create these mouse strains for each of the 20,000 genes that make up the mouse genome.
Rats, a species very similar to mice, haven’t evolved as quickly as mice due to the fact that, until just two years ago, it wasn’t possible to successfully and efficiently cultivate their pluripotent embryonic cells, nor to modify them through homologous recombination, with the aim of inducing directed mutations and creating Knock-out or Knock-in rats.
Over the coming decades, this species will expand greatly, making up the 20 years of lost ground, compared to mice, in terms of applying techniques of genetic engineering.
Of course, there are other superior species like non-human primates and swine, among others, that continue to be essential to highly specific and scientifically justified areas of biomedical research. But over the past decade a new animal model has appeared on the scene, the zebrafish (Danio rerio).
In the United Kingdom in 2010, the Home Office announced a decrease in other vertebrate species traditionally used in research (guinea pigs, dogs, cats, non-human primates and pigs), but the use of mice and zebrafish has increased to nearly 23% of the total in the case of this cyprinid.
The zebrafish, given its quick development time (its organs are fully formed after just 48 hours) and transparent embryos, is frequently used in drug discovery and key aspects of developmental biology, and, above all, in research into regenerative medicine, as it is able to regenerate part of its caudal fin or, even, more than half of its heart. One of the main lines of research carried out with this animal model is that to discover the genes involved in regenerative processes. Several researchers at the PRBB have recently published work related to this topic in important scientific journals.
At the PRBB facilities, we have dozens of different strains of this fish, with 4,500 aquariums that can hold 50,000 adult fish. Hundreds of mutations of this species have already been created to study multiple human and animal illnesses, as it can be genetically modified easily and in a short time, only a few weeks, the effects of the genetic modification or chemical compound in question can be seen. As fertilization in this species is external, it isn’t necessary to euthanize the female in order to extract the embryos like in rodents.
The US FDA has calculated that up to $100 millions could be saved in drug discovery costs if the initial screening to select possible molecules was carried out on zebrafish embryos. This small fish, which measures only three or four centimeters in length at the adult stage, makes a great animal model, laying some 200 eggs per week. Its social impact is much lower as an animal model for biomedical and biotechnology research, and from a legal standpoint the use of embryos from this fish, which is less than five days old, isn’t subject to the European regulations that apply to any vertebrate used for scientific research.
Another aquatic animal that has long been used in scientific research is the frog, specifically the genus Xenopus, the most commonly used is the X. laevis, or South African frog. It has been used as an animal model since 1930 for studies in developmental biology, endocrinology, toxicology (teratogenesis), cell biology and biochemistry. It was used extensively in the 1940s, also for pregnancy tests in which the frog was injected with a woman’s urine and if this led it to lay eggs, the result was positive.
Although it isn’t a vertebrate, we must point out that an insect has greatly aided the advance of genetics and developmental biology over the past 100 years. Obviously, we are referring to the fruit fly (Drosophila melanogaster). There are currently a multitude of mutant strains of this insect, whose full genome has been sequenced. Its short gestational period –12 days– and low maintenance costs make it a popular animal model for basic research.
Therefore, over the coming decade, the number of genetically modified mice and rats will increase greatly, accompanied and complemented by a huge expansion of the zebrafish. These will be the vertebrate models most commonly used in biomedical and biotechnology research.
Dr. Juan Martín-Caballero spoke at the workshop "New pre-clinical animal models for more efficient research" organized by Biocat, on 13 October 2011, in Lleida.