This week we profile a recent publication in PNAS from the
laboratory of Dr. Masaoki Kawasumi (pictured, far right) at UW.
Can you provide a brief overview of your lab’s current research focus?
Our research is to elucidate molecular, genetic, and epigenetic mechanisms of UV-induced skin carcinogenesis with a goal of developing novel means to inhibit skin cancer (https://kawasumilab.org). UV radiation from the sun damages DNA and generates DNA ‘lesions’ which may become genetic mutations and cause cancer (Lee et al., Photochem Photobiol 2020 PMID: 32119110). Cells respond to these deleterious lesions by activating a DNA damage response pathway called the ATR-Chk1 signaling pathway. ATR is an essential kinase that senses UV-induced DNA damage and regulates the cell cycle and cell survival. Thus, we have been addressing a long-standing, important question: How exactly is the ATR pathway activated?
What is the significance of the findings in this publication?
UV generates two major types of DNA lesions: cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). One hour of UV leads to 100,000 DNA lesions per cell, and CPDs are 8-fold more abundant than 6-4PPs. It has been difficult to know which type of DNA lesions activate the rapid ATR signaling pathway because UV instantaneously and simultaneously generates both CPDs and 6-4PPs. To solve this problem, we developed a novel approach to generate and isolate a particular type of cells by using flow cytometry and ‘photolyase’ (the present study: Hung et al. PNAS 2020 PMID: 32444488). Photolyase is a unique enzyme that can remove either CPDs or 6-4PPs, but this enzyme is not present in placental mammals. We introduced CPD-photolyase or 6-4PP-photolyase derived from marsupials and plants into human cells in order to generate cells with only one type of DNA lesion. Using this novel system, we found that ATR is activated in cells with 6-4PPs, but not in cells with CPDs. This is the first study that reveals differential effects of CPDs and 6-4PPs on ATR activation. This is surprising because CPDs are much more abundant than 6-4PPs but in fact have no effect on ATR activation. We conclude that 6-4PPs are the trigger of UV-induced ATR activation.
What are the next steps for this research?
We would like to use these findings to prevent skin cancer, the most prevalent cancer in the United States. UV-induced 6-4PPs are known to be more ‘mutagenic’ than CPDs, thus more likely to cause genetic mutations and cancer. Because 6-4PPs activate ATR which promotes survival of DNA-damaged cells, targeting ATR-activated cells may eliminate 6-4PP-containing cells that are ‘precancerous’, leading to suppression of skin cancer development. Indeed, our previous study showed that ATR inhibition kills UV-damaged cells and suppresses UV-induced skin cancer development in a mouse model (Kawasumi et al. PNAS 2011 PMID: 21844338). Intriguingly, caffeine is a nonspecific ATR inhibitor, and previous human and mouse studies showed that caffeine prevents UV-induced skin cancer (Conney et al. Front Oncol 2013 PMID: 23785666). In human studies, each daily cup of caffeinated coffee intake is associated with a 5% reduced risk of developing skin cancer. This cancer-preventive effect of caffeine is dose-dependent; people who drink 6 cups of caffeinated coffee per day showed 30% reduced risk of developing skin cancer. Therefore, we would like to propose and test a model that may explain how caffeine prevents skin cancer: caffeine inhibits the ATR kinase, thereby eliminating 6-4PP-containing cells and preventing cancer development.
This work was funded by:
National Institutes of Health and Dermatology Foundation.
