Vol. 4 No. 1 September 2005

The BioTECH Quarterly

Student Research Spotlight
Immortal DNA Strand Hypothesis in Adult Stem Cell Regeneration

Melissa Wu, a recent graduate with a degree in Biology, started working in Professor James Sherley’s lab in Summer 2004. Her project is to investigate whether immortal DNA strands exist in the germline stem cells of Drosophila ovaries. Her work is an extension of Sherley’s research on immortal DNA strands in mammalian cells to cells of another adult type and species.

By Melissa Wu '05

    Adult stem cells are rare cells in adult tissues that produce two daughters — an adult stem cell daughter and a differentiating, functional daughter — through asymmetric divisions.

    Because of this regenerative capacity, accumulated mutations in the genomes of adult stem cells can lead to cancer. Finding that the experimental rate of mutations predicted higher rates of cancer than existed, John Cairns proposed a mechanism for adult stem cells to minimize the accumulation of mutations, called the immortal DNA strand hypothesis.

    The immortal DNA strand hypothesis proposes that instead of segregating DNA strands randomly during mitosis, the adult stem cell co-segregates a template set of DNA strands to the daughter adult stem cell (Figure 1). This co-segregation allows errors from DNA replication to be passed on to the differentiating daughter.

    Segregation of DNA strands can be visualized by labeling newly synthesized DNA with bromodeoxyuridine (BrdU). Two complementary assays are being used — one incorporates BrdU into the immortal strands, the other incorporates BrdU into the non-immortal strands.

    After allowing the cells to divide in BrdU-free conditions, different patterns of BrdU labeling in the adult stem cell and differentiating daughter result if there is random segregation versus co-segregation.

    I am testing this hypothesis in fly ovarian germline stem cells, which offer the advantage of being easily identifiable. Various methods have been attempted for labeling the fly ovaries, and directly feeding the BrdU to them has worked for larvae and adult flies (Figure 2). I am currently determining when germline stem cells form to test if immortal strands can be labeled, and determining their cell cycle time for testing segregation of non-immortal strands.

    Understanding whether adult stem cells use this mechanism is important for research on cancer development and treatment. Loss of regulation of the immortal strands may result in either death of the adult stem cells or over-proliferation leading to cancer.

    Cairns suggested that weak DNA damage causes adult stem cells to undergo apoptosis rather than risk repairing and mutating immortal strands. This hypothesis suggests that DNA-damaging drugs currently used in cancer therapy also may induce death of the adult stem cells.

    The ideal of the UROP is for students to have a first hand, active experience in original research. Whether the experience progresses from only participating in a research project to conducting an independent research project depends on many factors, including the laboratory head’s philosophy, the timing of the UROP period with respect to opportunities for independent projects in the research group, and the motivation and ability of the student.

    All UROP experiences should begin with students learning and mastering new concepts and techniques that will enable their subsequent research. Thereafter, the goal is for students to advance to independently addressing an original research problem.

    There is no better way to achieve a realistic appreciation of the nature of research than to have early opportunities for significant creative input into the development and pursuit of an original research problem.

    Melissa’s research embodies all of these ideals. As her direct supervisor, I have watched her develop research skills commensurate with graduate students. She effectively designs experiments, develops and modifies required techniques, trouble-shoots technical problems that arise, critically evaluates data, and is a partner in setting the creative agenda for the project.

    Though related to my group’s core research, her project is new and reflects the input of her ideas, motivation, and talent. Melissa exemplifies the impact that UROP students can have on research at MIT.

Figure 1. Random Segregation and Immortal Strand Mechanism This schematic depicts the results of a random segregation mechanism and an immortal strand mechanism. In red are the older parental strands (denoted immortal strands in the immortal strand mechanism). Illustration by Melissa Wu.

Figure 2. BrdU Incorporation in Ovarian Cells by Feeding. A. BrdU incorporation and retention in an adult fly, after being fed BrdU as larvae. B. BrdU incorporation in an adult fly ovary, after being fed BrdU as an adult. Arrows point to germline stem cell position. Photos by Melissa Wu.

Student Research Spotlight

Why? Research is an ongoing dialogue — share your work and get feedback from faculty and peers with different backgrounds but similar interests!

How? Submit a concise and informative description of research in a BME-related field.

Interested? Contact TheBioTECH@mit.edu for more details. We hope to hear from you soon!