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Causes of Skin Cancer: Genetic Mutations

The US Department of Health and Human Services lists ultraviolet (UV) radiation as a carcinogen or cancer-causing substance.1 Sunlight and indoor tanning beds produce UV light. UV rays can damage the DNA in skin cells. Harmful DNA changes, called genetic mutations, can lead to uncontrolled cell growth.

UV light exposure is the most important environmental risk factor for skin cancer. By limiting UV exposure, you can reduce your chance of getting skin cancer.

Why is it important to understand genetic mutations in skin cancer?

It is helpful to know about genetic mutations in order to understand how certain medications work. Some of the newer medications for skin cancer are called targeted therapies. These medications interfere with molecular “targets” that have a role in cancer cell growth and survival.2,3 Cancer cells are different from normal cells. Targeted therapies aim at the features that make the cancer cells different.

There are several targeted therapies approved for skin cancer. In general these drugs:

  • Block molecules that send signals that cause a cancer cell to grow and divide.
  • Trigger the immune system to destroy cancer cells.

The most important genetic changes in basal cell carcinoma (BCC) affect a cell signaling pathway called Sonic Hedgehog. In particular, these mutations are named:

  • PTCH1
  • SMO

Gene mutations have a role in melanoma too. The most common gene mutations in melanoma are named:4

  • BRAF
  • NRAS

Research into the specific mutations in squamous cell carcinoma is ongoing.5

What is the Sonic Hedgehog pathway?

Some medications used to treat advanced basal cell carcinoma affect a cell signaling pathway called Sonic Hedgehog. (Yes, like the video game.)6

Before you were born, the Sonic Hedgehog pathway had an important role in the development of your organs and making sure the right side of your body matches your left. This pathway is not active in most healthy adult tissue, but skin is an exception.7 In skin, Sonic Hedgehog maintains the supply of stem cells. Stem cells are flexible cells that can develop into many different cell types. The Sonic Hedgehog pathway has a role in the development of your hair follicles and glands and it helps to control skin growth.

Key players in the Sonic Hedgehog pathway are two receptors called PTCH1 (“Patched”) and SMO (“Smoothened”).7 Figure 1 illustrates how the Sonic Hedgehog pathway works. (Please note: This is a simplified explanation. Although some details are missing, it illustrates the basic way that the receptors interact with each other.) You can imagine that PTCH1 works like a door, holding SMO in a little box. The Sonic Hedgehog protein is a key to open the box (Figure 2). When Sonic Hedgehog protein opens up the PTCH1 door, SMO is free to start working (Figure 3). It starts sending signals that cause cells to grow and survive.

Figure 1. PTCH1 and SMO signaling

An arrow from PTCH1, a green squiggle of uneven size, onto SMO, an evenly sized red squiggle

PTCH1 is a receptor in the cell membrane. Without the Sonic Hedgehog protein (SHH), PTCH1 blocks SMO, another receptor, from sending any signals. When the SHH protein binds to PTCH1, the SMO travels to the cell surface. SMO begins sending signals that lead to cell growth and survival.

Figure 2. Sonic Hedgehog protein binding

A yellow ball, SSH, lands on PTCH1, a green squiggle

PTCH1 is known as a “tumor suppressor gene.” By keeping SMO inactive, PTCH1 acts as an “off switch” for the process that causes cells to divide and survive.5 But PTCH1 is mutated in about 40% of BCC tumors.8 When PTCH1 becomes mutated, the “off switch” no longer works, and SMO signals freely. The result is uncontrolled cellular growth. Two medications, Erivedge® (vismodegib) and Odomzo® (sonidegib), work by blocking the SMO receptor.

Figure 3. SMO mutation

With SSH attached to PTCH1, an arrow from SMO's red squiggle travels outward

SMO is known as an “proto-oncogene” or a type of gene that causes cells to grow, divide, and stay alive.5,9 Certain mutations to SMO cause it to stay “on” all the time, leading to cancer development. SMO is mutated in about 12% of BCC.8 When SMO is mutated, the medications that block SMO may not work.10

What genetic changes lead to melanoma?

The most common mutations in melanoma are in genes called BRAF and NRAS.4 BRAF and NRAS proteins have a role in a pathway called MAPK or ERK.11 These proteins are part of a chain of events that allow cells to grow and survive. Normally, there are mechanisms that turn each protein “on” and “off,” keeping the cell processes under control.

Figure 4. Simplified MAPK Pathway

Simplified MAPK Pathway

Certain mutations cause BRAF to stay “on,” sending continuous signals for cell growth.12 Two medications used to treat melanoma, Zelboraf® (vemurafenib) and Tafinlar® (dabrafenib), help to block the BRAF protein. They can potentially be used together with medications that block MEK, which is another protein in the chain of events. Two MEK inhibitors used for melanoma are Mekinist® (trametinib) and Cotellic® (cobimetinib).

What other mutations are common in skin cancer?

TP53 is a tumor suppressor gene that is mutated in almost 40% of BCC tumors and up to 90% of squamous cell carcinoma tumors.8,13 Inherited mutations in other tumor suppressor genes called CDKN2A and CDK4 have been seen in melanoma that runs in families.14 These mutations may be future targets for therapy.

Written by: Sarah O'Brien | Last reviewed: May 2017.
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