Douglas E Brash PhD
Senior Research Scientist in Therapeutic Radiology and in Dermatology and Clinical Professor of Therapeutic Radiology
Ultraviolet Light; Skin Cancer
Cancer begins as an encounter between a carcinogen and a gene. We are pinpointing these early events, which occur decades before the appearance of a tumor. Our past work on sunlight-induced mutations in tumor suppressor genes has led us to three current topics: 1) Exploring how UVB-induced apoptosis and UV-driven cell fate decisions drive a single mutant cell to clonally expand. 2) Determining rates of DNA photoproduct formation and repair across the genome, as dosimeters of a person's past UV exposure. 3) The interaction of UV and melanin in causing melanoma.
Extensive Research Description
The story thus far, from photons up to cells: UV leads to mutations at the site of DNA photoproducts (rather than elevating genomic instability); the important photoproducts are cyclobutane dimers and (6-4) photoproducts, which join adjacent cytosines or thymines; only the cytosine mutates; these unique properties create a characteristic "mutation signature" for UV that can be seen in tumors decades later; sunlight mutates the P53 and PTCH genes in non-melanoma skin cancer; P53 is required for UV-induced apoptosis, which prevents mutations; apoptosis is signaled by DNA photoproducts in actively transcribed genes and by a product of UV-irradiated melanin; another cause of apoptosis is exposure of melanin to sunlight, particularly the melanin found in blonde and red hair; and our sun-exposed skin carries about 60,000 tiny clones of P53-mutant keratinocytes. Expansion of single mutant cells into clones is due to physiology rather than a 2nd mutation: UV-induced apoptosis deletes normal progenitor cells while sparing the mutant ones. UV also tilts the progenitor cell's fate decision toward self-renewal rather than differentiation. Now, the lab is in the midst of some "functional genomics" questions:
- How does a single mutant keratinocyte expand into a clone? We are attempting to track the clonal expansion of mutant stem cells in a living mouse.
- What DNA regions are the most important UV targets? In addition to actively-transcribed genes, we find that telomeres are UV-sensitive and fail to repair this damage. We've developed a method to measure UV photoproducts and repair in specific genomic regions such as telomeres, and across the entire genome.
- Is cancer development in melanoma unique because of its melanin? We are looking at how reactive oxygen and nitrogen species triggered by UV exposure and reacting with melanin affect genetic and epigenetic fidelity, as well as whether protection against these events varies between individuals due to DNA polymorphisms.