Scientists have known for decades that smoking cigarettes causes DNA damage, which leads to lung cancer.

Now, for the first time, UNC School of Medicine scientists created a method for effectively mapping that DNA damage at high resolution across the genome.

The innovation comes from the laboratory of Nobel laureate Aziz Sancar, MD, PhD, the Sarah Graham Kenan Professor of Biochemistry and Biophysics at UNC's School of Medicine.

In a study published in the Proceedings of the National Academy of Sciences, Sancar and his team developed a useful technique for mapping sites on the genome that are undergoing repair following a common type of DNA damage.

They then used that technique to map all damage caused by the major chemical carcinogen - benzo[α]pyrene.

"This is a carcinogen that accounts for about 30 percent of the cancer deaths in the United States, and we now have a genome-wide map of the damage it causes," Sancar said.

Maps like these will help scientists better understand how smoking-induced cancers originate, why some people are more vulnerable or resistant to cancers, and how these cancers might be prevented.

Sancar also hopes that providing such stark and specific evidence of smoking's harm at the cellular level might induce some smokers to kick the habit.

"It would be good if this helps raise awareness of how harmful smoking can be," he said. "It also would be helpful to drug developers if we knew exactly how DNA damage is repaired throughout the entire genome."

Benzo[α]pyrene (BaP) is a member of a family of simple, hardy, carbon-rich hydrocarbons - polycyclic aromatic hydrocarbons - that can form even in outer space.

It's a byproduct of burning organic compounds, such as tobacco plants, and is enzymatically reduced in human blood to a compound called benzo[α]pyrene diol epoxide (BPDE), which turns out to be worse than BaP itself.

BPDE reacts chemically with DNA, forming a very tight bond at the nucleobase guanine.

This bond, or adduct, means that the genes can no longer make proper proteins and DNA can't be duplicated properly during cell division, which can result in disease.

"If a BPDE adduct occurs in a tumour suppressor gene and isn't repaired in a timely manner, it can lead to a permanent mutation that turns a cell cancerous," said Wentao Li, PhD, a postdoctoral researcher and lead author of the study.

There is no doubt about the basic carcinogenicity of chemical reaction - paint a moderate dose of BaP on the skin of a lab mouse, and tumours are almost certain to erupt.

Sancar's new method for mapping BaP-induced DNA damage enables scientists to identify the sites on the genome where cells are trying to repair the damage.

Sancar won a share of the 2015 Nobel Prize for Chemistry for teasing apart the detailed workings of this biochemical repair process.

Known as nucleotide excision repair, it involves the recruitment of special proteins that perform DNA surgery to snip out the affected strand of DNA.

If all goes well, DNA-synthesising enzymes then reconstruct the missing section of DNA from another unaffected strand.

This is possible because all cell-based life forms on Earth have two complementary strands of DNA.

Meanwhile, the snipped-out damaged section of DNA floats free until garbage-disposal molecules eventually degrade it.

With the new method, scientists can tag and collect these cast-off snippets, sequence them, and then fit together their sequences - like tiny pieces of a giant puzzle - to create a map of the genome.

Given the effort and expense required for DNA sequencing, the initial, proof-of-principle map published by Sancar, Li and colleagues doesn't have the highest resolution possible. 

But it points the way towards the routine scientific use of such maps, especially as costs drop, to better understand how DNA-damaging events lead to disease and death.

This mapping technique should help answer several questions, such as:

• What dose of a toxin is needed to overwhelm the average person's nucleotide excision repair capacity?
• Which variations - and in which genes - give people more or less capacity to repair such DNA damage?
• Are there certain spots on the genome where successful repairs are inherently less likely?

Even with their initial, medium-resolution map, Sancar and colleagues were able to show that repairs of BPDE damage tend to occur more often when the BPDE-burdened guanine (G) is next to a cytosine (C) rather than a thymine (T) or adenine (A).

"Understanding this bias in repair should help us better understand why exposures to toxins such as BaP tend to cause certain gene mutations," Li said.

The new technique employs "translesional" enzymes with dimensions that allow it to keep reading a strand of DNA even when a bulky BPDE adduct is present.

"This new method can be applied to any type of DNA damage that involves nucleotide excision repair," Sancar said. "I'm certain that all this information will lead to a better understanding of why certain people are predisposed to cancer, and which smoking-related mutations lead to lung cancer specifically."

And that, in turn, could have implications for the development of more targeting therapies down the line.

Source: University of North Carolina