This web page was produced as an assignment for Genetics 564, an undergraduate capstone course at UW-Madison
Introduction
What are Model Organisms?
nEssentially, a model organism can be thought of as a "stand-in" for another organism, due to their ability to cheaply and effectively model a certain biological processes [1]. In the context of human disease, model organisms are invaluable for human genetics, as it would be highly unethical to introduce genetic alterations in humans to study how a certain gene functions in a given biological process. Luckily, there are countless examples of model organisms that share fundamental similarities to certain aspects of human biology, such as mice, rats, fruit flies, zebrafish and even worms (c. elegans).
Furthermore, one of the main advantages of using a model organism is the speed of development in the organism (2). For example, neurodegenerative diseases take decades to manifest in humans, but these diseases can be modeled within months in a rat or mouse model (2). Likewise, model organisms are useful because their genetic makeup can be altered to study certain diseases and select cellular mechanisms (2). Thanks to high-throughput sequencing, researchers now have access to the entire genomes of countless model organisms. With this knowledge, researchers can create gene knockouts, that turn off a gene's function permanently, allowing researchers to better understand a given gene's function [2].
Furthermore, one of the main advantages of using a model organism is the speed of development in the organism (2). For example, neurodegenerative diseases take decades to manifest in humans, but these diseases can be modeled within months in a rat or mouse model (2). Likewise, model organisms are useful because their genetic makeup can be altered to study certain diseases and select cellular mechanisms (2). Thanks to high-throughput sequencing, researchers now have access to the entire genomes of countless model organisms. With this knowledge, researchers can create gene knockouts, that turn off a gene's function permanently, allowing researchers to better understand a given gene's function [2].
How to Choose a Model Organism
Choosing a model organism entirely depends on the research question that is proposed for a body of work. For example, the single-celled eukaryote, Saccharomyces cerevisiae (Yeast) can be a great model for understanding cell cycle regulation and division, as well as cellular signaling mechanisms in eukaryotes. However, it would be more appropriate to use the model Arabidopsis thaliana if a researcher wanted to analyze how the plant hormone auxin, regulates growth in plant species. Additionally, neither of these organisms will be informative for neurological development or function, making Drosophila melanogaster a better choice within the context of neurological study. Additionally, if genetic research is being conducted, a homolog must be present in the species of interest. Overall, researchers make informed decisions as to what model organism they will use based on their research question and cost constraints.
Results
What Organisms Are Useful Models for Ovarian Cancer?
In order to determine an appropriate model organism for elucidating RAD51D's role in ovarian cancer development, the following criteria were desirable in a model organism:
Given the above criteria E. coli, C. elegans, and Saccharomyces cerevisiae were not considered since they lack a true RAD51D ortholog. Likewise, Arabidopsis thaliana will not be used due to the lack of similarity to human ovaries and ovarian hormones. Furthermore, the identification of the ssDNA binding N-terminal domain being of interest for RAD51D function, Drosophila melanogaster was not considered since its RAD51D protein lacked ssDNA binding domain. Therefore, only the vertebrate model organisms remained, which are discussed below.
- The organism has ovaries and produces certain ovarian hormones.
- A RAD51D ortholog is present in the organism's genome.
- The physiology and microenvironment of the organism's ovaries are similar to that of mammals.
Given the above criteria E. coli, C. elegans, and Saccharomyces cerevisiae were not considered since they lack a true RAD51D ortholog. Likewise, Arabidopsis thaliana will not be used due to the lack of similarity to human ovaries and ovarian hormones. Furthermore, the identification of the ssDNA binding N-terminal domain being of interest for RAD51D function, Drosophila melanogaster was not considered since its RAD51D protein lacked ssDNA binding domain. Therefore, only the vertebrate model organisms remained, which are discussed below.
Mus musculus (Mouse) [3] : Cancer research has often relied on mouse models due to their similarities to human anatomy and physiology. In fact, ovarian cancer research has been especially useful in mice for this reason. One of the advantages to using a mouse model is the ability to xenograft patient tumor cells, or cultured human cell lines directly into the mouses ovary. By doing so, one can study the en-vivo physiological interactions between tumor-cells as well as neighboring ovarian tissue, along with an endocrine function similar to that of our own. Furthermore, there are already a number of transgenic mice lines with altered RAD51D function (can be found at the file at the bottom of the page. Additionally, mice can be genetically engineered to knockout, or "turn off" RAD51D in certain tissues and at predetermined developmental periods using Cre-LoxP transgenic tools. This is important because homozygous RAD51D loss of function mutations (both copies of RAD51D dysfunctional) are embryonic lethal, thus killing the organism during development [3].
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Danio rerio (Zebrafish) [4,5]: Like mice, zebrafish have been previously used to model various cancers, including those originating in the ovaries. Likewise, since zebrafish have conserved ovarian steroid hormone signal transduction pathways similar to humans, the ovarian microenvironment of zebrafish is comparable to humans' [4]. Additionally, there are countless benefits for using zebrafish over other organisms. For one, zebrafish are cheaper to maintain than their mice counterparts [5]. Likewise, their translucent body parts, allow for the use of fluorescent reporters that are invaluable in genetic screens [5]. For these reasons, zebrafish will be used for my research project.
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Resources Available for Mice Models
Figure 1: Results of the phenotypes associated with the RAD51D protein obtained from the Mouse Genome Informatics Database using the search query "RAD51D".
Figure 2: Expression of the RAD51D protein in different mice tissues, obtained from Mouse Genome Informatics Database using the search query "RAD51D".
Inducible Knockout for RAD51D in Ovarian Tissues.
Figure 3: Results depicting mice transgenic constructs identified using Mouse Genome Informatics Database's recombinases tab . Search queries and resulting constructs can be viewed in the above slideshow.
Conclusions
Both mice and zebrafish will be used to model RAD51D's role in regulating DNA repair and the development of ovarian cancer. Additionally, the ability to perform xenografts of human tumor cells, make zebrafish and mice attractive candidates for studying human cell tumors in a natural environment. Furthermore, the available inducible CRE-LoxP lines available for mice reproductive tissues will allow for knockout mutations that would otherwise be impossible to conduct in the germline of the organism. However, its interesting to note RAD51D has mutant phenotypes that are exclusive to mice such as neurological and cardiovascular disease. Recalling back to the transcriptomic results, RAD51D is coexpressed with genes involved in neurological regulation during ovarian tumor development. It would be interesting to examine RAD51D function in different tissues to better understand its role in mouse vs human systems.
References:
- Maximino, C., Silva, R. X., da Silva, S., Rodrigues, L., Barbosa, H., de Carvalho, T. S., … Herculano, A. M. (2015). Non-mammalian models in behavioral neuroscience: consequences for biological psychiatry. Frontiers in behavioral neuroscience, 9, 233. doi:10.3389/fnbeh.2015.00233
- National Institute of General Medical Sciences. Using Research Organisms to Study Health and Disease (2017). Retrieved from https://www.nigms.nih.gov/education/Pages/modelorg_factsheet.aspx
- Zhang, W., Moore, L., & Ji, P. (2011). Mouse models for cancer research. Chinese journal of cancer, 30(3), 149-52.
- Gorelick, D. A., Halpern, M. E. (2011). Visualization of Estrogen Receptor Transcriptional Activation in Zebrafish. Endocrinology, 152(7), 2690-2703. https://doi.org/10.1210/en.2010-1257
- Facts:Why use the zebrafish in research? (2014). Retrieved from https://www.yourgenome.org/facts/why-use-the-zebrafish-in-research
References - Images
Figure 1: http://www.informatics.jax.org/marker/key/36541
Figure 2: http://www.informatics.jax.org/marker/key/36541
Figure 3: http://www.informatics.jax.org/marker/key/36541 , http://www.informatics.jax.org/home/recombinase
Header: https://elifesciences.org/collections/8de90445/the-natural-history-of-model-organisms
Figure 2: http://www.informatics.jax.org/marker/key/36541
Figure 3: http://www.informatics.jax.org/marker/key/36541 , http://www.informatics.jax.org/home/recombinase
Header: https://elifesciences.org/collections/8de90445/the-natural-history-of-model-organisms
mutant_mouse_lines_rad51d.txt | |
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File Type: | txt |