Epigenetics involves heritable changes in gene expression that do not involve alterations in the DNA sequence. I developed an active-learning approach to convey this topic to students in a college genetics course. I posted a brief summary of the topic before class to stimulate exchange in cooperative groups. During class, we discussed the genotypic and phenotypic differences between monozygotic twins and the role of epigenetic mechanisms in these differences. I also presented the molecular mechanisms that lead to these epigenetic changes as well as techniques used to study them. Students were particularly interested in pondering the relationships between environmental interactions, epigenetic changes, and phenotypic consequences, including human behavior.
Students are often exposed to the subject of epigenetics through scientific literature and the national media. The epigenetic phenomenon is characterized by a change in phenotype that is heritable but does not involve a DNA mutation (Allis et al., 2007). I used an active-learning, inquiry-based approach to present this topic to students in an introductory genetics course. Before class, I used the Blackboard virtual platform to post a summary of the topic, with the textbook reading assignment and a set of five questions to assess students' preparation.
In class, I gave a brief introduction on regulation of eukaryotic gene expression, and then I asked: What do you expect to find from a study that used hybridization techniques to analyze condensed chromosomes of samples of identical twins? Students' answers spanned various topics, from the techniques used to visualize condensed chromosomes, to molecules that help regulate gene expression, to whether structures or the molecules associated with the condensed chromosomes were identical in the monozygotic twin samples. After listening to different opinions and considering them, I presented a result described by Fraga et al. (2005), who compared DNA methylation events in monozygotic twins of different ages. The students examined figure 3 from the article, which showed color-coded methylated DNA in condensed chromosomes from a 3-year-old pair and a 50-year-old pair of twins. Similar patterns of DNA methylation between twins resulted in yellow staining, whereas different patterns yielded red and green stains in the condensed chromosomes.
The students could see that in one pair of monozygotic twins there was a very similar pattern (mostly yellow staining), whereas the other pair showed very distinct staining patterns. I then asked (1) which pair of chromosomes corresponded to the 50-year-old twins and (2) which molecule did they think was being stained (labeled) in this experiment. Most responded correctly to the first question, recognizing that the greater degree of difference in DNA methylation occurred in the older pair of twins. In considering the second question, the students discussed the different epigenetic mechanisms that they had read about before class, including DNA methylation and histone modifications.
After discussing group outcomes, I described the comparative hybridization technique and the epigenetic modifications of cytosine methylation and histone acetylation and deacetylation. Then the class continued with another problem that allowed the students to apply their knowledge on methylation and gene expression. The gene expression of the glucocorticoid receptor gene is known to be regulated by DNA methylation at the promoter region (McGowan et al., 2009). Expression of this gene tends to inhibit hormonal responses associated with stress, and methylation tends to silence gene expression. I asked: If you could measure the state of methylation in the promoter, would you expect to see a difference in the methylation pattern between stressed and relaxed individuals? Why?
This topic ignited a very interesting discussion. Students showed great interest and started to make connections between epigenetic changes, gene expression, and environmental influences. They really connected the topic to their everyday life. It was an inspiring and exciting class for them, and for me too!