Melanoma, a less common form of skin cancer, is more likely to spread and invade other tissues, which significantly reduces patient survival. Previous large-scale sequencing studies have shown that melanoma has the most DNA mutations of any cancer.

In many cancers, DNA mutations are thought to be linked to environmental factors that cause tumor-specific mutations. Previously, scientists believed that DNA damage causes a mutation that is then copied into subsequent generations of cells during normal cellular replication, ultimately resulting in aberrant cancer cells.

"Cancers result from DNA mutations that allow defective cells to survive and invade other tissues," said corresponding author Gerd Pfeifer, PhD, a professor at the Van Andel Institute, in a statement. "However, in most cases, the source of these mutations is not clear, which complicates development of therapies and prevention methods."

In melanoma and other types of skin cancers, sunlight exposure -- specifically exposure to ultraviolet B (UVB) -- causes mutations in cytosine (C) to thymine (T), which causes DNA damage. UVB radiation (from sunlight exposure) hits certain sequences of bases -- CC, TT, TC, and CT -- causing them to chemically link together via double bonds and become unstable. This type of DNA damage is called cis-syn cyclobutane pyrimidine dimer (CPD).

DNA damage transforms cytosine into uracil, which is found in RNA but not in DNA. When the resulting uracil is bypassed by a polymerase with incorporation of adenine, a C-to-T mutation occurs. This change, called a "premutation," primes the DNA to mutate during normal cell replication, thereby causing alterations that underlie melanoma.

CPD lesions induce the characteristic C-to-T mutations in skin cancer. However, it is still unclear how CPD lesions cause cancer, such as melanoma. Researchers from the Van Andel Institute, led by Pfeifer, investigated the CPD deamination mechanism, with the goal of determining whether this specific pathway can be linked to the frequent C-to-T mutations found in melanoma.

First, the researchers confirmed that UVB induces CPDs in irradiated human skin cells. By mapping the lesions, the team identified that the mutations occurred at distinct tetranucleotide sequences. CPD formation occurred at high levels upstream of transcription start sites (TSSs). (Transcription factors may help enhance this process in melanoma cells.)

The work relied on a new method the team developed called circle damage sequencing (circle-damage-seq) to separately map CPDs and deaminated CPDs in human UVB-irradiated cells at the single-base resolution. The technique allows the scientists to "break" the DNA where DNA damage occurs. The DNA is then circularized and replicated by PCR and sequenced with next-generation sequencing methodologies.

The team used circle-damage-seq to determine if CPD deaminase relies on cytosine deamination and the formation of uracil. By mapping the DNA damage associated with CPD, they confirmed the presence of cytosine, thus confirming that the formation of CPDs is mediated by cytosine deamination to form uracil.

"In melanoma, we've now shown that damage from sunlight primes the DNA by creating 'premutations' that then give way to full mutations during DNA replication," Pfeifer explained.

The findings offer evidence for the specific biochemical/biophysical pathways from which DNA damage and cancer mutations occur as a result of UVB and sun exposure.

"Safe sun practices are very important. In our study, 10 to 15 minutes of exposure to UVB light was equivalent to what a person would experience at high noon, and was sufficient to cause premutations," said Pfeifer. "While our cells have built-in safeguards to repair DNA damage, this process occasionally lets something slip by. Protecting the skin is generally the best bet when it comes to melanoma prevention."

Pfeifer and his colleagues plan to use this powerful technique to investigate other types of DNA damage found in different kinds of cancer.

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