Harvard University - Department of Molecular & Cellular Biology


by Thomas Torello

June 3rd, 2009

(L-R) Kevin Peterson, Kaloyan Tsanov, and Yuichi Nishi

Kaloyan Tsanov has been awarded the Lawrence J. Henderson Prize for his thesis, An Embryonic Stem Cell Based System for Rapid Analysis of Developmentally Regulated Enhancer Elements. The Henderson Prize is awarded annually to the undergraduate who submits the most meritorious thesis to the Board of Tutors in Biochemical Sciences. Kaloyan will receive a $350 book award, a framed certificate, and a copy of the book “Fitness of the Environment” by Lawrence Henderson. The Henderson Prize honors the work of Professor Henderson (1878-1942) who began teaching at Harvard in 1905 and is best known for his research on blood biochemistry.

Kaloyan performed this work in Professor Andy McMahon's lab with mentoring and guidance from postdoctoral fellows Yuichi Nishi and Kevin Peterson. His project involved the development of a new system to analyze the regulation of developmental gene expression. “Though there are many different cell types in the body, all of those cells contain the same genetic information,” explains Kaloyan. “My research addresses a fundamental question in developmental biology: How is the expression of genes - particularly genes that give cells specific identities - regulated during development?”

“Enhancer elements” are segments of DNA found to regulate gene expression; these have been uncovered by whole genome searches using bio-computational or experimental approaches. Determining how enhancers function in the developing embryo, however, generally involves creating transgenic animals - a costly and time consuming endeavor. Kaloyan aimed to develop a more high-throughput experimental strategy to examine the role that particular enhancers play in developmental gene regulation. Instead of using a transgenic approach, he decided to perform his experiments in embryonic stem cells, which can be grown in the laboratory as undifferentiated cells or can be coaxed to differentiate into different cell types (such as neurons, muscle and connective tissue) by altering the culture conditions.

Kaloyan decided to test the feasibility of his approach using a well-characterized enhancer element. He chose an enhancer that is activated by the Sonic hedgehog pathway (Sonic hedgehog is a signaling molecule involved in neuronal differentiation) and regulates expression of a gene in a subset of neuronal progenitors. He prepared a DNA construct in which the enhancer drives expression of a beta-galactosidase reporter and introduced that construct into a specific genomic site in embryonic stem cells using a molecular genetic trick called “recombinase-mediated cassette exchange“ (RMCE). Kaloyan hypothesized that he could examine the activity of the enhancer by looking for expression of the beta-galactosidase reporter in his modified cells after exposure to the Sonic hedgehog. Indeed, Kaloyan found beta-galactosidase in a subset of the cells induced to form neuronal progenitors, and importantly, expression of the reporter overlapped with expression of the endogenous gene. To further validate the system, he performed the same experiment with reporters containing specific modified versions of the enhancer, and as expected, the modified enhancers functioned differently than the normal enhancer.

In principle, the strategy Kaloyan developed could be used to characterize any enhancer element. “Kaloyan has shown that the idea has merit, validated an example and generated a scalable platform, one that we can now use to launch a systematic effort to analyze regulatory domains,” says McMahon. “His thesis is outstanding, and I don't use the 'o-word' liberally.”

Post-doctoral fellow Yuichi Nishi agrees. “Kaloyan is an extremely hard worker, and beautifully demonstrated the experimental system he developed faithfully reports transcriptional enhancer activity in a developmental context. We currently assess such activity in mice, which is time consuming, labor intensive, and costly. His rapid, inexpensive and scalable system provides a very effective approach complementary to the conventional mouse analysis.”

Kaloyan was born and raised in Sofia, Bulgaria, and as a high school student was a two-time medalist in the International Biology Olympiad. He transferred to Harvard from the University of Sofia in the Fall of 2006 and began working in the McMahon lab in the summer of 2007. He will continue working on his project in the McMahon lab next year while applying to graduate school. He intends to enroll in a graduate program in which he can become involved in translational research, as he is interested in the intersection of developmental and cancer biology.