Crespo wins NSF CAREER award to study DNA sensitizers

Carlos CrespoCarlos E. Crespo-Hernández, an assistant professor of chemistry, won a National Science Foundation CAREER award to investigate how compounds found in human cells and medicines trigger DNA mutations that can damage cells and lead to skin cancer when exposed to the sun’s ultraviolet rays.

Crespo, who joined the Case Western Reserve University faculty in 2007, will use the $584,200 Faculty Early Career Development award to make movies of chemical reactions in action.

“Our expertise in spectroscopy will allow us to visualize how these molecules absorb radiation and what they do with it,” he said.

The radiation in question is the form of ultraviolet light called UVA, which is the most common ultraviolet light to reach Earth. Once thought to be benign to humans, scientists have found differently.

“There is more and more evidence that UVA radiation, which is absorbed only slightly by DNA, can still lead to DNA damage and skin cancer through indirect means,” Crespo said.

Skin cancer development is complicated with many reaction mechanisms. Crespo’s lab will study two families of compounds that other researchers have shown to be key links in reactions connecting UVA to skin disorders.

Thiobases are found in medicines used to treat acute lymphoblastic leukemia, inflammatory bowel disease, skin disorders, superficial tumors and bladder cancer, and to prevent rejection of transplanted tissues. But they can turn toxic in sunlight, prompting warnings for patients to avoid the sun.

Pterins (pronounced TARE-ins), are a group of compounds that include folic acid, a member of the vitamin B complex found in living cells. They are involved in the synthesis of amino and nucleic acids, the metabolism of nitric oxide and the activation of cell-mediated immune responses.

Folic acid is essential for human nutrition and is widely prescribed in pregnant women and for cancer prevention, neural tube defects and other disorders. But recent studies have linked excess intake of folic acid with DNA damage by UVA light.

Pterins and UVA exposure are also linked to skin cancers among patients who suffer from a disease called vitiligo, which causes skin to lose the pigment melanin and affects about one in 100 Americans.

“Thiobases and pterins are DNA sensitizers, which means they absorb UVA light and can transfer the energy to DNA, damaging the DNA and, thereby, causing genetic mutations that can eventually lead to skin cancer,” Crespo said. “The energy often decays away harmlessly, but occasionally it triggers a chemical reaction that alters the DNA’s molecular structure. What we want to know is how these molecules damage DNA. To accomplish this we will make ‘molecular movies’ to measure the time scales on which these reactions occur and to learn how the high-energy (excited) states in these molecules interact with DNA.”

First, the researchers will put thiobase or pterin molecules in a solution that mimics physiological conditions. Two laser beams will pulse light through the solution one after the other.

The first pulse will expose the molecules to UVA, which elevates the molecules into excited states or can create highly reactive chemical species. Either can be harmful to DNA.

The second is a white light pulse, which is composed of a broad spectrum of wavelengths and used to detect and characterize electronic changes in the molecules. Each different excited state and reactive species absorbs light from the second pulse in a different region of the electromagnetic spectrum, rendering distinct images.

The laser pulses last as little as a few hundred femtoseconds (one femtosecond is one-quadrillionth of a second). By precisely controlling the time differences between the white light probe pulse relative to the UVA excitation pulse, the researchers can study how the excited states and reactive species that are formed evolve in time, thus effectively creating a ‘molecular movie’ of the energy transformation processes.

Initially, the researchers will do their experiments with thiobase and pterin molecules alone, in order to understand the mechanisms that form the excited states and reactive species. Next, the lab will perform identical experiments, but in the presence of DNA to see how the species react.

The researchers will then piece the images together into movies, much like time-lapse photography.

“We’ll see what excited states are created when these molecules absorbs UVA-light, what reactive species they forms, and how these interact with DNA, leading to DNA damage or genetic mutations,” Crespo said.

As they experiment at the Center for Chemical Dynamics in the department of chemistry at Case Western Reserve, Crespo and fellow researchers will also model the processes on the computer, to help them better understand the mechanisms involved.

“If we can understand the molecular mechanisms by which thiobases damage DNA, then other scientists can use this information to make modifications to medicines that would maintain the therapeutic benefits but minimize the side effects,” Crespo said. “Understanding how pterins damage DNA will increase awareness of the ‘yin and yang’ nature of these biomolecules. It can also allow scientists working to understand vitiligo to find ways to block the unwanted reactions and minimize the problems associated with the disorder.”