How ultraviolet rays cause skin cancer

Melanoma is a cancer of skin pigment cells called melanocytes, a fraction of those melanomas come from existing moles. When melanocyte stem cells accumulate a sufficient number of genetic mutations, they can become the cells cancers originate.

Under normal conditions, ultraviolet UV radiation from the sun activates melanocytes to release melanin, a pigment that protects the skin from the sun's rays. But if melanocyte stem cells have surpassed a threshold of genetic mutations, a tumor can start to grow. The skin stem cells can be activated by exposure to sunlight.

Exposure to sunlight triggers mutations that are sufficient for melanoma. A gene known as Hgma2 was suspected to become expressed in the skin under UV radiation. When expressed, Hgma2 facilitates melanocyte stem cells to move from the base of skin hair follicles to the skin's surface-the epidermis, where the cells release melanin.

Researchers used mice engineered with melanocyte stem cell mutations. One set of mice had the mutations, while another set with the mutations had the Hgma2 gene deleted. They then gave the mice a very low dose of UV radiation, just enough to trigger a tanning response. Mice with tumor-causing mutations and the Hgma2 gene intact developed melanomas, but the mice with mutations and the deleted gene remained healthy.
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