Educational Level:

Roadmap Objectives:

• • Article: Pearse, A-M, & Swift, K. (2006). Transmission of devil facial-tumour disease, Nature, 439, 549.
  • Content area/major concepts: This paper was the first to investigate the mechanism underlying DFTD. It is a very simple paper, consisting of one large figure. The figure is deceptively simple. It requires a lot of analysis and will make a wonderful exercise for the annotation. It’s easy to miss the point if the analysis is not done in enough depth. However, once students get it, they will understand that it is a paradigm shift: it provides evidence that cancer CAN be transmissible between individuals!
    
    Karyotype, chromosome anatomy (and chromosomal mutations), chromosome staining, heterochromatin versus euchromatin
    Cancer biology (causes, characteristics, how cell function is affected)
    Concepts of microbiology (what are typical infective agents and their characteristics, Koch’s postulates) would be an asset during the discussions (since any transmissible disease is typically presumed to have a microbial cause)
    
  • Methods or technology used to obtain data: Karyotype analysis, chromosome staining (G banding) Students will need to know karyotype analysis. They should understand what a chromosome is, how they are studied, how to stain them (G staining in particular), what the banding patterns represent
  • How the CREATE strategy was used:
  • Biggest teaching challenge: The hypothesis contains some jargon “allography theory” that may require students to research what it means. Students will have to spend a good deal of time analyzing the figure to extract its meaning. They have to read the caption closely to realize that ALL cancer cells had the karyotype shown at the bottom of Figure 1. That’s really the key finding that supports the hypothesis. I typically direct my students to one passage (middle column, middle of the page) in the text that is very telling: “Further support for the allograft theory of disease transmission derives from the serendipitous observation of a pericentric inversion of chromosome 5 in the constitutional karyotype of one animal. This constitutional anomaly was found in all cultures of that devil’s normal tissues, but was not present in either of the chromosomes 5 in his facial-tumour cells, where it would have been found had the neoplasm arisen from his own tissue.” Students seem to miss the importance of this observation and it is important to draw attention to it.
• • Article: Siddle, H. V., Kreiss, A., Eldridge, M. D. B., Nooman, E., Clarke, C. J., Pyecroft, S., Woods, G. M., & Belov, K. (2007). Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial. PNAS, 104(41), 16221–26. [Instructors may wish to note that Supplementary Information on the methods of this paper are available from the PNAS website]
  • Content area/major concepts: In this paper, the authors first confirmed, using molecular data, the results of Pearse & Swift. Then, they tested alternative hypotheses that could explain why the devil immune system is not responding to the foreign cancer cells. The three hypotheses (inspired by the mechanism by which little known transmissible cancers occur in hamsters and in dogs) are that 1) the MHC proteins on cancer cells are down regulated, thereby becoming “invisible” to the immune system of the host, 2) that the immune system of the devil is not working, and 3) that the immune system of the devils is working except to recognize cells from other devils (presumably because their MHC proteins are too similar across individuals). This paper is longer, and requires more knowledge of molecular techniques.
    
    Immunology (MHC class I and II, how lymphocytes recognize and react to foreign antigens).
    Genetic diversity in a population (how measured, how it arises, how it is reduced, implications for the species).
  • Methods or technology used to obtain data: PCR, Gel electrophoresis, Microsatellite analysis, RT-PCR, Single Strand Conformation Polymorphism (SSCP), DNA sequencing, Mixed lymphocyte response assay, Amino acid polymorphism in a population
  • How the CREATE strategy was used:
  • Biggest teaching challenge: This is a challenging paper for my GenEd students, because it has many molecular techniques and terminology. Here are a few particular challenges I have encountered. Table 1 is challenging, because it contains the results of several experiments in one place. It is important to separate out each experiment and its purpose. Also, the genotypes that are identified are labeled (0,2) etc, and the students don’t understand what these numbers correspond to (in particular, that the numbers represent an overall banding pattern (i.e. a genotype, not a specific band) for a cell). In figure 1, there is a critique that many students may miss. The experiment was an RT-PCR, and therefore looked at the RNA produced for MHC class I and II. Most students are satisfied that these are expressed in the cell and discount the hypothesis that MHC are downregulated. However, the authors never test whether the RNA is translated into proteins, or whether the protein is stable or functional, or whether the protein is brought to the cell surface. Figure 2: The mixed lymphocyte assay. Many students will miss the hypothesis being tested here, i.e. whether the immune system of the devil is active, and whether it can react to the cells of conspecific. Cartooning this experiment is critical, as they have a hard time understanding what is being measured (or what it represents), as well as why the different treatments were done (what information is gleamed from each one). Table 2 and Figure 3. I have taken out these two figures from the paper for my class. These figures are sidebars that don’t directly address the central question of the paper, and that would require me to spend too much time on new methods.

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