What Causes Basal and Squamous Cell Skin Cancers?

While many risk factors for basal and squamous cell skin cancers have been found, it’s not always clear exactly how these factors might cause cancer.

Most basal cell and squamous cell skin cancers are caused by repeated and unprotected skin exposure to ultraviolet (UV) rays from sunlight, as well as from man-made sources such as tanning beds.

UV rays can damage the DNA inside skin cells. DNA is the chemical in each of our cells that makes up our genes, which control how our cells function. We usually look like our parents because they are the source of our DNA. But DNA affects more than just how we look.

Some genes help control when our cells grow, divide into new cells, and die:

  • Genes that help cells grow, divide, and stay alive are called oncogenes.
  • Genes that keep cell growth in check by slowing down cell division or causing cells to die at the right time are called tumor suppressor genes.

Cancers can be caused by DNA changes that turn on oncogenes or turn off tumor suppressor genes. Changes in several different genes are usually needed for a cell to become cancer.

Researchers don’t yet know all of the DNA changes that result in skin cancer, but they have found that in many skin cancers the cells have changes in tumor suppressor genes.

The gene most often altered in squamous cell cancers is called TP53. This tumor suppressor gene normally causes cells with damaged DNA to die. When TP53 is altered, these abnormal cells may live longer and perhaps go on to become cancerous.

A gene often mutated in basal cell cancers is the PTCH1 gene, which is part of the “hedgehog” signaling pathway inside cells. PTCH1 is a tumor suppressor gene that normally helps keep cell growth in check, so changes in this gene can allow cells to grow out of control. People who have basal cell nevus syndrome (Gorlin syndrome), which is often inherited from a parent and results in getting many basal cell cancers, have an altered PTCH1 gene in all the cells of their body.

These are not the only gene changes that play a role in the development of skin cancer. There are likely to be many others as well.

People with xeroderma pigmentosum (XP) have a high risk for skin cancer. XP is a rare, inherited condition resulting from a defect in an enzyme that repairs DNA damage. Because people with XP are less able to repair DNA damage caused by sunlight, they often develop many cancers on sun-exposed areas of their skin.

The link between squamous cell skin cancer and infection with some types of the human papilloma virus (HPV) also involves DNA and genes. These viruses have genes that affect growth-regulating proteins of infected skin cells. This can cause skin cells to grow too much and to not die when they’re supposed to.

Scientists are studying other links between DNA changes and skin cancer. A better understanding of how damaged DNA leads to skin cancer might be used to design treatments to overcome or repair that damage.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: April 1, 2016 Last Revised: May 10, 2016

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