A Quick Guide to Understanding the Genetics of Cancer

Cancer is a genetic disease. Even when due to lifestyle or environment, cancer can develop when genes experience damage. Some people inherit genetic mutations that increase their cancer risk, but most mutations are spontaneous.

Your genes contain the blueprint for almost everything about you. You have between 20,000 and 25,000 genes , typically organized on 23 pairs of chromosomes. These genes each contain a fundamental building block of life — DNA.

Each of the 37 trillion cells in your body contains more than 2 meters (6 feet) of tightly coiled DNA. This code tells each type of cell what to do and when to grow, divide, and die.

Cancer results from damage to this DNA code. In rare cases, people can pass these DNA changes (mutations) to the next generation, causing cancer to run in families.

Keep reading to discover what factors can contribute to cancer development and how it can carry on through generations.

To understand the origins of cancer, we need first to understand what can cause damage to your DNA. Mutations can result from:

• Environment: Environmental factors, like UV radiation or chemical exposure, can damage your DNA. Agents that cause DNA mutations that lead to cancer are called carcinogens.
• Metabolism: Metabolism is when your body converts food into energy for cells to use. During this process, your cells can damage their DNA, so we accumulate a certain amount of DNA damage over time. Experts estimate that this occurs about 70,000 times each day.
• Spontaneous mutations: Your cells divide multiple trillions of times during your life. This process is usually accurate, but spontaneous errors (mutations) occur once every 100,000 to 100 million times.

Most of the time, your body corrects these errors. Even when your body can’t fix the errors, most of these mutations don’t cause any changes. But if a mutation occurs in a specific type of gene, it may lead to cancer.

A 2017 study showed that as many as two-thirds of cancers occurred due to random gene changes during cell replication.

What percent of cancer is genetic and what percent is environmental?

Technically, all cancer is genetic, since it results from DNA damage that causes genetic mutations. But if you’re wondering how much cancer is due purely to genes and not the environment, the answer is a bit different.

According to a 2017 study, only about 23% of all cancers were due to environmental factors. The rest were due to genetic factors — inherited mutations or spontaneous errors in DNA replication.

But these percentages differed, depending on the type of cancer. For example, the above study found that environmental factors contributed to 15% or less of prostate, brain, and breast cancers but more than 60% of lung and skin cancers.

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Cancer isn’t usually due to just any mutation in any gene. Your cells undergo trillions of point mutations daily, most of which don’t cause any changes. But mutations in specific types of genes can cause cancer.

Let’s take a look at them.

Oncogenes

Proto-oncogenes help a cell grow typically. A mutation in this gene may cause it to become an oncogene. When activated, oncogenes cause cells to grow and divide out of control.

Most mutations that cause the activation of oncogenes are not inherited.

Tumor suppressor genes

Tumor suppressor genes typically slow down cell growth and division. If a mutation causes a tumor suppression gene to turn off, it can no longer control cell growth. This allows cells to grow and divide out of control.

Like with oncogenes, mutations that affect tumor suppressor genes are usually not inherited.

DNA repair genes

DNA repair genes help fix the spontaneous mistakes that occur during DNA replication. If they can’t fix them, they cause the cell to die, so it can’t affect you. But mutations that affect DNA repair genes can prevent this process from happening.

The BRCA1 and BRCA2 genes are famous examples of DNA repair genes. Inherited mutations in these genes can often lead to breast or ovarian cancer.

If a condition is hereditary, it means one generation can pass it down to the next. That’s not entirely true for cancer, but it’s not entirely false either. This can happen in cancer, but not in most cases.

Most cancers are known as somatic variants. DNA damage in a somatic cell causes these variants. Somatic cells can be any cell that isn’t a sperm or egg cell. People cannot pass these mutations on to each other.

But about 5% to 10% of cancers are germline variants. DNA mutation in a sperm or egg cell causes these variants, and a parent can pass them to their offspring. The mutation then becomes part of the DNA of every cell in the offspring.

Even so, that doesn’t mean that the offspring will have cancer. But it does mean they will have a much higher risk of developing cancer.

What percent of cancer is hereditary?

According to the National Cancer Institute, up to 10% of cancers are hereditary. That involves inheriting a genetic mutation that increases your likelihood of developing cancer.

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Inherited gene mutations tend to run in families. When this happens, it is known as a family cancer syndrome or hereditary cancer syndrome.

Family cancer syndromes are rare. Cancer will affect one-third of people in the United States, which means you’re likely to have multiple family members with cancer. Family cancer syndromes tend to have specific qualities:

• The same cancer shows up in multiple generations, especially if it’s a rare cancer.
• Family members tend to develop cancer at a younger age than usual.
• Some family members have multiple types of cancers.
• Cancer appears in a pair of organs, like both breasts or kidneys.

Examples of family cancer syndromes include:

• Familial adenomatous polyposis (FAP):FAP occurs due to an inherited mutation in the APC gene and increases your risk of colorectal cancer.
• Hereditary breast and ovarian cancer syndrome (HBOC):HBOC results from an inherited mutation in the BRCA1 or BRCA2 genes, increasing your risk of breast and ovarian cancers.
• Li-Fraumeni syndrome (LFS): LFS results from an inherited mutation in the tumor-suppressing TP53 gene. People who inherit the mutation have a 70% to 90% risk of various cancers, like breast cancer or central nervous system tumors, depending on their sex assigned at birth.
• Cowden syndrome: Cowden syndrome results from an inherited mutation in the PTEN gene. It increases your risk of skin, breast, thyroid, and kidney cancer, among others.
• Lynch syndrome:Lynch syndrome affects one of five possible DNA repair genes and increases your risk of several cancers, including colorectal, stomach, and uterine cancers.

Familial vs. hereditary cancers

Some cancers, such as breast and prostate cancer, tend to run in families. But not all cancers that run in families are due to inherited gene mutations or family cancer syndromes. They could also be due to similar environmental factors.

Hereditary cancers tend to show up earlier in life, before people turn 50, and are more likely to include rare cancers or cancer in multiple sites, such as both breast and ovarian cancer. They also tend to appear in multiple generations on the same side of the family.