How Are Cancer Cells Different From Normal Cells?

Key takeaways

Here’s what to know about cancer cell differences and advances in treatment:

• Learn how cancer starts. DNA mistakes can cause cells to grow and divide out of control.
• Recognize why cancer cells are different. They can evade the immune system and adapt over time.
• Understand how cancer can spread. Some cancer cells can break away, travel through the body, and grow in new places.
• Get the latest information on how research is changing cancer care. Mass General Brigham Cancer Institute experts use advances in immunotherapy and targeted treatment to help the immune system find and fight cancer.
• Know when expertise matters. Early detection, personalized treatment, and ongoing research help cancer specialists better understand risk, slow cancer growth, and improve care.

Cancer cells start out just like all the other cells in your body. So what goes wrong? What makes them the harmful, devastating cells we’re working so hard to eliminate?

“Normally, cells follow a process where they multiply to form new cells as needed and die when they become old or damaged. Cancer cells override this process and start to multiply and grow uncontrollably in the body,” says Ryan Sullivan, MD, program director for melanoma at the Mass General Brigham Cancer Institute.

In this article, Dr. Sullivan explains how cancer starts and why cancer cells differ from healthy cells.

How does cancer start?

The cells in our bodies normally grow and divide to create new cells when our body needs it. Cancer starts when cells in the body begin to grow out of control.

All the cells in our body have genetic information called DNA (deoxyribonucleic acid). A mistake in the DNA can cause a cell to grow and multiply too much. They crowd out normal cells around them and may lead to cancer.

The American Cancer Society suggests thinking of a cell as a car to help explain how cell growth works. Like a car, there are parts of a cell that help control its movement. It has a gas pedal to speed up growth when needed, and brakes to slow down and keep growth under control. A mistake in the DNA may affect how these parts do their job. The gas pedal may get stuck down, or the brakes may no longer work, causing the cell to grow out of control.

How do cancer cells differ from normal cells?

Researchers think everything starts with those DNA mistakes, also known as mutations. A cancer cell may have just one or two mutations, or it can have multiple changes that make it different from normal cells. More advanced cancers develop when lots of mutations occur.

“These mutations make cancer cells act differently from healthy cells,” Dr. Sullivan explains. “Normal cells stop growing when there’s no signal to continue and when they run into other cells. They also don’t spread to other areas of the body.”

In addition to these mutations, other abnormalities can happen within many parts of a cell. These changes may include what genes get turned on and off, or what proteins get made.

At the DNA level, abnormalities are broken down into two categories:

• Driver mutations: Changes that cause a cancer cell to form and grow out of control
• Passenger mutations: Changes that happen over time once the cell is already abnormal

Driver gene mutations

There are two broad types of driver mutations:

1. Oncogenes: These genes mutate from proto-oncogenes, which help cells grow and divide properly. When an oncogene develops, it turns on signals for cancer cells to just keep multiplying.
2. Tumor suppressor genes: These genes keep cell growth in check, telling cells to grow more slowly and prompting cell death when needed. They even fix mistakes in DNA to help prevent cancer. But mutations prevent them from doing their job.

Some cancer cells arise from changes to oncogenes, while others happen from problems with tumor suppressor genes. If both these mutations occur, they often lead to fast-growing cancers.

An ever-evolving structure

Cancer cells keep developing new abilities that help them adapt and survive. These changes can be thought of as a version of evolution.

“The body’s immune system has the capacity to identify and get rid of cancer cells before they become a problem,” says Dr. Sullivan. “But the cells in tumors that ultimately develop have adapted in ways that help the cells evade the immune system and keep growing. Then the cancer cells can start to cause problems, like forming a tumor that pushes against other organs and spreading to other sites in the body.”

How do cancer cells evade the immune system?

Even though cancers get more aggressive and abnormal as they progress, they still escape detection and/or destruction. They use several ways to do this. One way is to take advantage of the immune system’s checkpoints by tiring out immune cells. Another is that cancer cells can trick the immune system into protecting them by communicating with immune cells that block the arrival and/or function of immune cells that can attack the cancer cells.

“The good news is, researchers have figured out ways to use these behaviors against them,” explains Dr. Sullivan. “We have immunotherapy treatments to help your body produce more or stronger immune cells to better find and fight cancer. Immune checkpoint inhibitor therapy encourages your immune system to keep fighting against cancer cells, even when they pretend a tumor needs to be protected instead of destroyed.”

The damaging nature of cancer cells

All these characteristics allow cancer cells to act differently than normal cells. They take up space, pushing against blood vessels, nerves, and organs. Their growth causes pain and damage, keeping the body from working properly.

A tumor (mass of cancer cells) near the eye, for example, can grow to impair vision. A tumor in the colon can block the piping and interfere with digestion. Cancer cells in the blood set up shop in the bone marrow and keep healthy blood cells from being made.

Solid tumors and metastatic cancer

Some cancers, such as squamous cell or basal cell skin cancer, typically grow within just one area. They may form a solid tumor in one location, like the breast or colon. Even though they don’t spread, they can create a lot of devastation.

Other cancers leave their original spot and spread to invade new parts of the body. This type of cancer is called metastatic cancer. For example, colorectal cancer that starts as a tumor in the colon may spread to the liver, making it a metastatic colorectal cancer.

How does cancer spread?

Picture a mass of cancer cells: the ball keeps getting bigger as the cells grow and divide into new ones. The new cells continue to develop more abnormal changes and proteins. Eventually, some cells have enough strength to pull off and move away from the ball.

These cancer cells can get into the bloodstream, which is like a roadmap throughout the entire body. They use these roads to travel to other organs or invade nearby lymph nodes. Only a fraction of the cells manages to survive the trip, but those that do can thrive in their new environment.

Dr. Sullivan describes the ‘seed and soil’ theory to help explain why cancer is sometimes able to spread to another part of the body. “Cancer cells that break off are like seeds, and the organs in your body are like soil,” Dr. Sullivan explains. “Most often, the seeds travel to different soils and aren’t able to grow. But sometimes, a seed finds just the right soil that’s perfect for it to thrive.”

The theory explains why some cancers commonly spread to specific areas of the body. For example, cancer that starts in the colon often spreads to the liver because it naturally follows the bloodstream, and is then able to thrive in that new environment.

Does everyone have cancer cells?

Cells develop mutations all the time, but the immune and DNA repair systems work to keep abnormalities under control. But some cancer cells get past the gatekeepers and start wreaking havoc. Whether this happens depends on many risk factors, things that increase a person’s chance of cancer.

“If you have a family history of cancer, you may have DNA mutations that were passed down to you and increase your risk of cancer. But environmental and lifestyle factors also play a role in cancer risk,” says Dr. Sullivan. “I feel encouraged by how much we’ve learned regarding what factors increase risk. It helps us detect cancer cells earlier, so we can stop them before they spread.”