Ann L. Beyer, PhD
Ribosome biogenesis and its up-regulation in cancer; action of camptothecin on rRNA synthesis
The Beyer lab uses the Miller chromatin spreading method for electron microscopic visualization of transcriptionally active cellular chromatin. Using electron microscopy in combination with genetic and molecular biology approaches; efforts are focused on understanding the regulation of ribosomal RNA transcription and ribosome biogenesis. Ribosome synthesis is highly regulated and very tightly coupled to cell growth. It is typically up-regulated in cancer cells and thus is a target of some chemotherapies. Recent findings from the lab have included identification of the key regulatory determinant of ribosomal RNA transcription, as well as identifying this same step within transcription initiation as being the target of the Tor signaling pathway. In ongoing efforts to understand transcriptional mechanisms in rRNA synthesis, cells deficient in topoisomerase activity were visualized. Topoisomerase I, which provides the most potent relaxing activity in eukaryotic cells, is concentrated in the nucleolus at the site of rRNA synthesis. Camptothecin, a widely used chemotherapeutic agent, is a specific topoisomerase I poison. It has been known for 25 years that mitotic recombination is significantly increased in the absence of topoisomerase I action, but the molecular basis for this phenomenon has remained a mystery. Using electron microscopic visualization of chromatin from cells deficient in topoisomerase I, two highly unusual DNA structures have been seen within active genes, both of which are potential recombinogenic intermediates. These recent findings have led to a major new initiative to test this hypothesis and thus to understand the lesions leading to recombination. In addition, cells treated with camptothecin will be analyzed in a similar way. The EM approach promises to be an important new in vivo tool to elucidate molecular mechanisms of genomic instability, which is a contributing factor in many cancers.