by ecancer reporter Clare Sansom

Malignant melanoma is the most deadly form of skin cancer; although it is rarer than other types of skin cancer, about 75% of deaths from skin cancer are from this form.

It is well known that the most important risk factor for melanoma development is exposure to ultra-violet radiation (UVR) from the sun, particularly in light-skinned individuals.

The molecular mechanisms through which UVR exposure drive melanoma development are still largely unclear.

About 60% of melanomas carry a mutation in the tumour suppressor gene BRAF that causes a change from valine to glutamic acid (V600E) in the protein BRAF, and melanomas bearing this mutation are sensitive to BRAF inhibitors such as vemurafenib.

A group of researchers led by Richard Marais of the Cancer Research UK Manchester Institute, University of Manchester, UK have investigated the mechanistic link between UVR and melanoma development using a mouse model.

The BRAF protein bearing the V600E mutation was expressed in the melanocytes of two-month-old mice.

One month later, the backs of the mice were shaved; half of the back of each mouse was protected with a cloth and the other half exposed to a low dose of UVR to mimic mild sunburn in humans.

The gene Trp53, which encodes the tumour suppressor protein TRP53 (equivalent to human P53) was over-expressed in the UVR-exposed mouse skin only 24 hours after exposure.

Seven days after exposure, the exposed skin had thickened and developed mild erythema, and had more abundant and larger naevi.

The skin of mice with wild type BRAF did not exhibit these changes when exposed to low doses of UVR, indicating that BRAF(V600E) melanocytes are particularly susceptible to radiation doses equivalent to mild sunburn.

About 70% of all mice bearing the BRAF mutation in their melanocytes develop melanomas, with a median latency of about 12 months.

The process of melanogenesis was accelerated in the mice that had been exposed to UVR; all UVR-exposed mice carrying this mutation developed melanomas within seven months, with 98% of the tumours occurring within the exposed area.

Wild type mice that had been exposed to UVR did not develop melanomas.

The researchers then treated the backs of BRAF(V600E) mice with a powerful sunscreen (UVA superior, UVB sun protection factor (SPF) 50) before exposing them to the same dose of UVR.

Mouse skin that had been pre-treated with sunscreen did not develop erythema or skin thickening, Trp53 was not over-expressed, and the naevi that developed on sunscreen-protected skin were indistinguishable from those on cloth-protected skin.

This proved that sunscreen protected the mouse skin from the immediate effects of UVA and UVB; nevertheless, all these mice developed melanomas within 15 months of exposure.

Whole-exome sequencing showed that the melanomas that developed after UVR exposure had significantly more single nucleotide variations with a particularly high proportion of C to T transitions, which provided direct evidence for the DNA damage known to be caused by UV radiation.

Approximately 40% (6/15) of the UVR-induced melanomas carried missense mutations in the Trp53 gene, with most of the amino acids involved in the DNA-binding domain of the protein; mutations in the equivalent amino acids had been observed in P53 in human melanomas.

Mice heterozygous for a conditionally inducible mutation in the DNA-binding domain of Trp53, which do not spontaneously develop melanoma, were crossed with BRAF(V600E) mice, and all the offspring were seen to have developed melanomas by 3.5 months of age.

This finding also showed that mutations in Trp53 accelerate melanomagenesis in mice bearing BRAF mutations.

Finally, the researchers analysed whole exome sequencing data in several human tumour databases to show that primary and metastatic melanomas bearing mutations in TP53 carry a high proportion of C to T transitions, which is a signature of UVR-induced DNA damage.

Taken together, these results indicate that UVR induces mutations in Trp53 (human equivalent TP53) that accelerate melanoma development in mice and humans carrying the BRAF(V600E) mutation, although they do not establish a molecular mechanism for this process.

Marais and his colleagues conclude by recommending that sunscreens, which delay but do not prevent melanoma development, should be used by individuals at risk of developing this form of cancer in combination with other sun avoidance strategies.

Reference

Viros, A., Sanchez-Laorden, B., Pedersen, M. and 8 others (2014). Ultraviolet radiation accelerates BRAF-driven melanomagenesis by targeting TP53. Nature, published online ahead of print 11 June 2014.