Research Interests: Stem cells, developmental control, telomere biology, self-renewal and genetic instability.
Accurate replication of DNA and faithful segregation of chromosomes are essential for most forms of life. In multi-cellular organisms, the control of these processes differs markedly between various cell types and between organisms. For example, the number of cell divisions is tightly controlled in adult blood-forming stem cells, whereas embryonic stem cells are essentially immortal. Furthermore, most adult stem cells are typically quiescent, whereas embryonic stem cells turnover rapidly. The number of cell divisions adult stem cells can undergo seems much more tightly controlled in humans than in mice, perhaps because tumor growth represents a greater risk to reproductive fitness for humans (with a longer lifespan and a larger body mass) than mice. In general, it has become clear that tight control of cell cycle properties is crucial for both normal development and prevention of tumor growth.
Cell cycle checkpoints are regulatory pathways that ensure that critical events such as DNA replication and chromosome segregation are completed with high fidelity. Interestingly, murine embryonic stem (ES) cells do not appear to activate typical checkpoint responses following DNA damage, yet such cells are very efficient in maintaining a diploid genome. We previously engineered murine ES cells (1) with specific defects in the replication of chromosome ends (telomeres). Surprisingly, such cells appear to grow normally until differentiation is induced. In order to address questions about the molecular mechanisms that explain these observations, we are using these and other ES cells expressing fluorescent reporter constructs in genetic screens and live cell imaging techniques. The goal of these studies is to characterize the pathways involved in cell cycle checkpoints, telomere maintenance and DNA damage responses in ES cells before and after induction of differentiation.
In all mammals telomeres are characterized by TTAGGG repeats and associated proteins. A minimum number of telomere repeats is needed at each end in order to distinguish a normal end form a double strand break and protect chromosome ends from fusion and degradation. Critically short or truncated telomeres trigger a DNA damage response that can cause cell cycle arrest or cell death. Telomeric DNA is lost with each round of cell division via several distinct pathways (2). Lost telomere repeats are normally replenished by the enzyme telomerase, a reverse transcriptase which can add telomere repeats to the 3’ end of chromosomes using an RNA template. Telomerase levels are limiting in normal human blood forming stem cells (3,4) and normal human lymphocytes (5). We have developed novel tools to measure the telomere length in individual cells and individual chromosomes and these quantitative fluorescence in situ hybridization techniques are used to address questions about the role of telomeres in normal aging, tumor progression and specific genetic, hematological and immune disorders.