Solved by a verified expert:NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEIdentical Twins, Identical Fates?An Introduction to EpigeneticsbySarah A. WojiskiSchool of Arts and SciencesMassachusetts College of Pharmacy and Health Sciences, Boston, MAPart I – Coming HomeElise was excited as she boarded the bus. She had just finished her first year at college and was lookingforward to her first night back home since winter break, which would include mom’s spaghetti and meatballs andcatching up with her family. She couldn’t wait to curl up on the couch with her cat Ziggy snuggled next to her, andshe was hopeful that her sister, Shannon, would be willing to join her there, since they had talked very little sinceElise’s last visit home.As she settled in for the four-hour bus ride home, Elise pulled out her iPhone, put on some music, and started lookingthrough old photographs. She came across a few of her and her sister taken at Christmas—the last time they had seeneach other. Looking at Shannon was like looking in the mirror. After all, they were identical twins. Elise recalled allof the pranks that she and Shannon used to pull in school when they were kids. In 5th grade, they once made it all theway to lunchtime before their teachers realized that they had swapped classes and were impersonating one another!Shannon and Elise used to have so much fun together, but things had changed. Elise was worried about her sister andthe serious health troubles she had been having over the past year and a half. And she couldn’t help but wonder toherself, “Are the same troubles heading my way?”Questions1. What exactly are twins, and how do they arise? Your response should distinguish between the two different typesof twins.2. Are identical twins completely identical? Why or why not?3. What can studying twins tell us about the genetic influence on a particular trait?“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 1 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEPart II – The DiagnosisElise stared out the window of the bus at the rush-hour traffic that had befallen travelers on the other side of thehighway. She recalled that night back in November when her mother called her at school to share the fateful newsabout her sister. “Shannon has been diagnosed with schizophrenia,” was what she had said. The words had droppedinto the pit of Elise’s stomach.She had known that something was going wrong with her sister. The summer before she left for college, Elise noticedchanges in Shannon’s behavior. Despite being an avid swimmer and lifeguard, Shannon quit her highly coveted swimcamp instructor position just two weeks into the summer. She seemed withdrawn and unmotivated, and had alsounexpectedly decided not to attend college in the fall, despite Elise’s and her parent’s efforts to convince her otherwise.But Elise did not get to see the worst of Shannon’s behavior, when she began having hallucinations and couldn’t seemto carry on a coherent conversation with her parents.Elise had done some research about schizophrenia after hearing of her sister’s diagnosis. She did not like what shefound out. Apparently, schizophrenia has a tendency to run in families. In fact, studies indicate that a sibling of aschizophrenic has a 10-fold higher risk of developing schizophrenia over the general population. Elise began to worryabout her own mental health. She decided she would do some further investigation into the disease once she got homefor summer break.QuestionsYou are encouraged to consult reliable sources (such as your textbook and other online and print resources) to answersome of these questions. The review articles listed below address Question #3.1. What causes genetic variation? For example, what causes some people to have curly hair and others to not? Whatcauses some people to have a genetic disease such as cystic fibrosis and others to not?2. What does it mean when a trait or a disease “runs in families”?3. What could be some possible genetic and non-genetic causes of Shannon’s schizophrenia?Review ArticlesGejman, P.V., Sanders, A.R., and Kendler, K.S. (2011). Genetics of schizophrenia: new findings and challenges. TheAnnual Review of Genomics and Human Genetics 12: 121–44.Roth, T.L., Lubin, F.D., Sodhi, M., and Kleinman, J.E. (2009). Epigenetic mechanisms in schizophrenia. BiochimBiophys Acta 1790(9): 869–877.Rutten, B.P. and Mill, J. (2009). Epigenetic mediation of environmental influences in major psychotic disorders.Schizophrenia Bulletin 35(6): 1045–1056.“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 2 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEPart III – Just How “Identical” Are We?Elise had been home from college for a week, and she was still preoccupied with Shannon’s diagnosis and her ownpotential risk for mental illness. Elise expressed her anxiety and concerns to her mother one night after dinner. “Elise,”her mother said, “your concerns are perfectly valid, and you have every reason to want to get more information. Whydon’t we make you an appointment to consult with a psychiatrist?” Elise decided to make the appointment the next day.*****Elise left Dr. O’Brien’s office feeling that some of the weight had been lifted from her shoulders. On the car ride home,she thought about the things that Dr. O’Brien had said to her during their consultation.“It was good of you to come in to see me, Elise. You are absolutely right to have concerns for yourself when youridentical twin has been diagnosed with schizophrenia. Research shows that schizophrenia is almost 50% heritable, andsince you share nearly identical DNA with your sister, that puts you at a higher risk for developing this disease as well.”“Fifty percent may sound like a scary number, but remember that schizophrenia is a very complex disease, and 50% ofwhat causes schizophrenia is due to things other than your DNA.”“Well, like what? What else could be contributing to Shannon’s schizophrenia that wouldn’t necessarily affect me?” Eliseasked.Dr. O’Brien replied, “There are many, many environmental influences that seem to play a role in the developmentof this disease, such as increased stresses and anxiety, or difficult relationships with other people. Interestingly, thereis some groundbreaking research that is going on that suggests that the environment itself might even play a role atinfluencing one’s DNA at the molecular level. This concept is called epigenetics. An example of epigenetics in nature isthe calico cat. Each calico cat has a unique orange and black fur color pattern because of alterations, called epigeneticchanges, which occur within the cells that produce coat color during the cat’s development. Research in the field ofepigenetics suggests that individuals with schizophrenia appear to have some of these epigenetic changes to their DNAthat are due to environmental influences, and that these alterations could be contributing to their development ofmental illness.”“But wouldn’t I also have these ‘epigenetic alterations’ in my DNA?” Elise asked.“Not necessarily, because you and Shannon have not experienced completely identical environments throughoutyour lives. For example, you and Shannon have had different teachers and jobs throughout high school. And I alsounderstand that you spent many childhood summers with a friend and her family out in the Grand Canyon, whileyour sister was off at swim camps. If you are interested, I can give you some literature to read about this subject.”Elise was definitely interested. She took the articles and headed home.Questions1. Briefly describe what you know about the structure of DNA and how DNA is packaged in a cell.2. At the molecular level, speculate on some ways that the environment might have an influence on DNA and itspackaging.“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 3 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEPart IV – What Really is “Epigenetics”?Despite being three weeks into her summer break, Elise felt like she was back in school. The more she read about thetopic of epigenetics, the more fascinated she became, and she found herself spending most of her days on the Internetdoing research. Elise had learned about genetics in her general biology class and thought she had a pretty good idea ofhow the Laws of Mendel worked, but this whole field of epigenetics seemed to take the idea of inheritance to anotherlevel. She was particularly fascinated by an article that Dr. O’Brien had given her regarding epigenetic differencesbetween identical twins. The article suggested that during one’s lifetime epigenetic changes occur to one’s DNA thatcan affect gene expression, and therefore whether or not one will express a certain trait. These epigenetic changes areinfluenced by one’s environment and behaviors, so despite having identical DNA, identical twins will not always havethe same epigenetic changes, and therefore, will not always express the same traits.In this article, researchers examined a particular type of epigenetic modification called DNA methylation, whereby acytosine base becomes methylated through the action of an enzyme called a DNA methyltransferase. The reaction isshown below in Figure 1.Figure 1. Methylation of cytosine to form 5-methylcytosine. SAM(S-adenosyl methionine) serves as the source of the methyl group, givingSAH (S-adenosyl-L-homocysteine) as a by-product.The researchers examined genome-wide methylation patterns in several twin pairs of various ages. Representative datafrom their analysis is shown below in Figures 2 and 3:Figure 2. Differential DNA methylation between two sets of monozygotic twins, oneset at age 3 (left), one set at age 50 (right) using AIMS (amplification of intermethylated sites). Different bands, corresponding to sibling-specific changes ofDNA methylation, are indicated with arrows. (Panel A of Figure 2 in Fraga et al.,2005. Epigenetic differences arise during the lifetime of monozygotic twins. PNAS102:10604–10609. Copyright 2005 National Academy of Sciences, USA.)“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 4 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEElise read this article several times, and began to feel a littlebit better about her “genetic future.” According to thisarticle, it seemed that as time went on she and Shannonwould become more and more epigenetically dissimilar,even though they did carry the same genes. Perhaps shewould not share the same fate as her sister after all.QuestionsYou are encouraged to consult reliable sources (such as yourtextbook and other online and print resources) to answersome of these questions. The article by Singh et al. (2003)referenced below is a useful resource for Question #2.1. Examine the data shown in Figures 2 and 3.Carefully compare the DNA methylation profilesfrom the 3-year-old twins versus the 50-year-oldtwins and summarize your observations. Which setof twins (3-year-old or 50-year-old) have the mostsimilar DNA methylation profiles? Provide a briefexplanation of your observations.2. What types of environmental factors can influenceDNA methylation?3. Aside from DNA methylation, what other typesof epigenetic modifications can occur within thegenome to influence gene expression?4. Do you think Elise needs to be worried about herown mental health? Why or why not? If you were ahealth-care professional, what would you advise Eliseto do?ReferenceSingh, S.M., Murphy, B., and O’Reilly, R.L. (2003).Involvement of gene-diet/drug interaction in DNAmethylation and its contribution to complex diseases:from cancer to schizophrenia. Clinical Genetics 64(6):451–460.“Identical Twins, Identical Fates?” by Sarah A. WojiskiFigure 3. Mapping of chromosomal regions with differentialDNA methylation in monozygotic twins using comparativegenomic hybridization for methylated DNA. Fourrepresentative chromosomes pairs are shown. MethylatedDNA from one twin was labeled with a red fluorescentdye, while methylated DNA from the other twin in thepair was labeled with a green dye. Both sets of twin DNAwere hybridized to normal metaphase chromosomes. Theyellow color represents equal amounts of red and greendye hybridizing to the chromosomes, indicative of similarlevels of DNA methylation at those particular chromosomallocations. (Figure 3 of Fraga et al., 2005. Epigeneticdifferences arise during the lifetime of monozygotic twins.PNAS 102:10604–10609. Copyright 2005 NationalAcademy of Sciences, USA.)Page 5 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEPart V – What Does the Research Say?Dr. O’Brien took off her reading glasses and rubbed her temples. It had been a long day of office visits, rounds atthe hospital, and reviewing the latest literature on schizophrenia at her desk. Her recent office visit with Elise, whoseidentical twin sister had been diagnosed with schizophrenia, had prompted her to revisit some studies that had beenconducted showing a link between epigenetic modifications and schizophrenia.In particular, certain studies demonstrated that the DNA methylation patterns of select genes were abnormal in thebrains of patients with schizophrenia as opposed to non-psychotic control subjects. The abnormal methylation patternsled to the abnormal expression of these genes. One such gene encodes REELIN, a glycoprotein secreted by GABAergicinterneurons, which activates signaling pathways important for many neurological processes responsible for braindevelopment and adult brain functioning. Figure 4 shows a schematic diagram of the REELIN signaling pathway.Figure 4. Schematic diagram of REELIN signaling. Reelin is a secreted glycoproteincapable of binding to several receptors, including apolipoprotein E receptor 2(ApoE2) and very low density lipoprotein receptor (VLDLR). Binding of reelinto these receptors leads to phosphorylation of the intracellular adaptor proteindisabled-1 (Dab-1), which is then capable of activating many downstream signalingpathways important in neurological function.It seemed clear to Dr. O’Brien that epigenetics played an important role in the clinical course of schizophrenia. Butwhat epigenetic modifications were the most critical? And, more importantly, could these epigenetic changes somehowbe reversed pharmalogically as a form of therapy for patients with this disease?Dr. O’Brien put on her reading glasses again. She decided to review the REELIN papers one more time before finallyheading home.Figures 5–7 represent some of the data Dr. O’Brien reviewed that evening at her desk:“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 6 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEFigure 5. Mean number of REELIN (RELN) positive neurons detected by immunohistochemistryin prefrontal cortex layers I-VI in non-psychiatric control subjects and patients with schizophrenia.(Figure 3 of Impagnatiello et al., 1998. A decrease of reelin expression as a putative vulnerability factorin schizophrenia. PNAS 95:15718–15723. Copyright 1998 National Academy of Sciences., USA.)Figure 6. Levels of methylation of the REELIN promoter in sections of the prefrontal cortex takenpost-mortem from non-psychotic control brains (panel A) or schizophrenic brains (panel B). Levelsof 5-methylcytosine are mapped against specific positions along the REELIN promoter, as shownin panel C. (Figure 1 of Grayson, et al., 2005. Reelin promoter hypermethylation in schizophrenia.PNAS 102: 9341–9346. Copyright 2005 National Academy of Sciences, USA.)“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 7 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEFigure 7. Measurements of REELIN (RELN) expression by RT-PCR analysis. NT-2neuronal precursor cells were grown for 48 hours in the presence of varying concentrations(0-250nM) of the DNA methyltransferase inhibitor Doxorubicin (DOXO). G3PDHexpression serves as a control. (Portion of Figure 1A from Marija Kundakovic, YingChen, Erminio Costa, and Dennis R. Grayson, DNA Methyltransferase InhibitorsCoordinately Induce Expression of the Human Reelin and Glutamic Acid Decarboxylase67 Genes, Molecular Pharmacology March 2007 71: 644–653. Used with permission.)QuestionsYou are encouraged to consult reliable sources (such as your textbook and other online and print resources) toanswer some of these questions. The NCBI’s Online Mendelian Inheritance in Man (OMIM) is a useful resource forexamining the role of reelin in the brain.1. What is the role of reelin in the brain? According to Figure 5, how does the expression of reelin in the prefrontalcortex of schizophrenic patients differ from reelin expression in non-psychotic subjects?2. Study the promoter methylation data shown in Figure 6. How does the overall level of methylation of the reelinpromoter in schizophrenic brains compare to the methylation of the reelin promoter in non-psychotic controlbrains? What would be the most probable effect of this methylation pattern on the expression of reelin inpatients with schizophrenia?3. What is the enzyme responsible for methylating DNA? How does a drug like doxorubicin affect DNAmethylation?4. In reference to Figure 7, what is the effect of doxorubicin treatment on the expression of reelin in NT-2 cells?How does increasing amounts of doxorubicin affect reelin expression in these cells?5. Based on these data, might a drug like doxorubicin be a potential treatment for schizophrenia? Why or why not?What additional experiments should be performed before a drug like doxorubicin goes into clinical trials?“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 8 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCEReferencesFraga, M.F., Ballestar, E., Paz, M.F., Ropero, S., Setien, F., Ballestar, M.L., Heine-Suner, D., Cigudosa, J.C., Urioste,M., Benitez, J., Boix-Chornet, M., Sanchez-Aguilera, A., Ling, C., Carlsson, E., Poulsen, P., Vaag, A., Stephan,Z., Spector, T.D., Wu, Y-Z., Plass, C., and Esteller, M. (2005). Epigenetic differences arise during the lifetime ofmonozygotic twins. Proceedings of the National Academy of Sciences 102(30): 10604–10609.Gejman, P.V., Sanders, A.R., and Kendler, K.S. (2011). Genetics of schizophrenia: new findings and challenges. TheAnnual Review of Genomics and Human Genetics 12: 121–144.Grayson, D.R., Kundakovic, M., and Sharma, R.P. (2010). Is there a future for histone deacetylase inhibitors in thepharmacotherapy of psychiatric disorders? Molecular Pharmacology 77(2): 126–135.Impagnatiello, F., Guidotti, A.R., Pesold, C., Dwivedi, Y., Caruncho, H., Pisu, M.G., Uzunov, D.P., Smalheiser, N.R.,Davis, J.M., Pandey, G.N., Pappas, G.D., Tueting, P., Sharma, R.P., and Costa, E. (1998). A decrease of reelinexpression as a putative vulnerability factor in schizophrenia. PNAS. 95: 15718–15723.Kundakovic, M., Chen, Y., Costa, E., and Grayson, D. (2007). DNA methyltransferase inhibitors coordinately induceexpression of the human reelin and glutamic acid decarboxylase 67 genes. Molecular Pharmacology 71: 644–653.Maiti, S., Kumar, K.H.B.G., Casetllani, C.A., O’Reilly, R., and Singh, S.M. (2011). Ontogenetic de novo copynumber variations (CNVs) as a course of genetic individuality: studies on two families with MZD twins forschizophrenia. PLoS One 6(3): e17125.Roth, T.L., Lubin, F.D., Sodhi, M., and Kleinman, J.E. (2009). Epigenetic mechanisms in schizophrenia. BiochimBiophys Acta 1790(9): 869–877.Rutten, B.P. and Mill, J. (2009). Epigenetic mediation of environmental influences in major psychotic disorders.Schizophrenia Bulletin 35(6): 1045–1056.Singh, S.M., Murphy, B., and O’Reilly, R.L. (2003). Involvement of gene-diet/drug interaction in DNA methylationand its contribution to complex diseases: from cancer to schizophrenia. Clinical Genetics 64(6): 451–460.•Credit: Licensed image in title block © Hipering | Dreamstime.com, id: 22094413. Case copyright held by the National Centerfor Case Study Teaching in Science, University at Buffalo, State University of New York. Originally published October 4, 2012.Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.“Identical Twins, Identical Fates?” by Sarah A. WojiskiPage 9
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