If you’ve been diagnosed with desmoid tumors, a rare condition involving the body’s connective tissue, your doctor may suggest that you undergo genetic testing. But what do these tests involve, and what does genetics have to do with desmoid tumors (also called aggressive fibromatosis)?

This article will help you understand what genetic testing is, why your doctor may recommend it, and what you can expect from genetic testing with desmoid tumors.

What Are Genes?

Genes are made up of DNA, and you inherit them from your parents. Your genes are like an instruction manual that tells your cells what to do. Some genes direct the production of proteins that help your cells function, and others help control other genes in a cell.

When your cells divide, the DNA is copied exactly, so both cells have the same set of instructions. However, sometimes mistakes happen when the DNA in your genes is copied — these are called mutations (variants). These changes in genes that code for proteins may result in proteins that don’t work correctly. Some mutations can cause diseases like cancer.

What Is Genetic Testing?

Genetic testing is a way to see if your genes have any variants that may increase your risk of illness. The testing may be done for many reasons, such as before or during pregnancy to look for any disease-causing genetic mutations that could be passed on to a child.

Sometimes genetic testing is used to diagnose a genetic condition in children or adults. Genetic testing may be recommended for people who show symptoms of a genetic condition or have a family history of the disease. You may undergo genetic testing to help diagnose desmoid tumors or direct your treatment.

Why May Genetic Testing Be Part of Desmoid Tumor Care?

Your health care provider may recommend genetic testing as part of your desmoid tumor care because variations in two different genes have been found to cause desmoid tumors. Most desmoid tumors are caused by genetic mutations that happen after you’re born. Everyone has variants in their DNA, but only some will cause disease.

CTNNB1 Gene

In 85 percent to 90 percent of cases, desmoid tumors are caused by mutations in a gene called CTNNB1. These genetic changes lead to an overproduction of a protein called beta-catenin, resulting in uncontrolled cell growth and sporadic desmoid tumors — tumors that form spontaneously, with no clear cause.

Mutations in the CTNNB1 gene are somatic, meaning they happen after birth in cells other than eggs or sperm. If your desmoid tumor is caused by a somatic CTNNB1 mutation, it won’t be passed on to your children. The variation will be present only in your tumor cells, not throughout your body. During your diagnosis, your provider may perform a biopsy (removing a bit of tumor tissue to be tested for CTNNB1 mutations).

APC Gene

The second gene that can be involved in desmoid tumors is APC. Harmful variants in the APC gene result in a genetic condition called familial adenomatous polyposis, or FAP. FAP is caused by a germline mutation — it may be inherited from your parents and can be passed down to your children. It also means that the mutation is in all your body’s cells, not just the tumor cells. Genetic testing of your blood can show if you have a germline APC mutation.

This APC mutation causes people with FAP to develop hundreds or thousands of growths called polyps in their intestines, starting during childhood. By the time a person with FAP is in their 40s, they’re likely to have colorectal cancer. People with FAP are also at increased risk of developing other cancers, including:

• Duodenal cancer (affecting the small intestine)
• Thyroid cancer
• Hepatoblastoma (a type of liver cancer)
• Stomach cancer
• Brain cancer

FAP can also lead to growths in other parts of the body. In people without FAP, the APC protein helps get rid of beta-catenin that isn’t needed. However, the genetic mutation found in people with FAP results in a faulty APC protein that can’t interact with beta-catenin. This leads to a buildup of beta-catenin, uncontrolled cell growth, and the formation of desmoid tumors. About 15 percent of people with FAP will develop desmoid tumors, typically in the abdomen (belly).

Because a germline mutation to the APC gene also means an increased risk of cancer, your health care provider will need to know whether your desmoid tumor was caused by a variation in the APC or the CTNNB1 gene.

What Can You Expect During Genetic Testing?

When undergoing genetic testing, you’ll likely work with a genetic counselor. These health care providers have specialized education in medical genetics. They’re trained to understand genetic conditions and can talk to you about the risks and benefits of these tests.

Before starting genetic testing, your genetic counselor will talk with you about the type of test you’ll receive, what the results might tell you, and whether insurance will cover testing. If you agree to be tested, a doctor or nurse will take a sample from you, usually in the form of blood drawn from your vein.

The sample will be sent to a lab to extract the DNA from your blood. The lab will sequence your DNA — figure out the exact order of its building blocks — and computers will compare your DNA to other human DNA. This will help find any genetic mutations you may have. It’s important to know that everyone has variations in their DNA, but only some of these will cause disease. Genetic counselors and other health care providers will analyze the changes in your DNA and determine if any are likely to have caused your desmoid tumors, like an APC gene mutation.

After your DNA has been analyzed, your genetic counselor will tell you the results of the test. They’ll also help you understand what the results mean for you and your family and answer any questions you may have.

What Happens After Genetic Testing?

After genetic testing, your doctor will discuss treatment options. If your desmoid tumors aren’t painful or likely to cause other health problems, your doctor will probably want to observe them over time.

If your desmoid tumors are interfering with your life, your doctor may consider other treatment options like surgery. However, surgery is less common because desmoid tumors have a very high rate of recurrence (returning after treatment). Research shows that 20 percent to 30 percent of desmoid tumors come back after surgery.

The results of your genetic testing can also affect the decision to do surgery and what to expect afterward. For instance, up to 84 percent of people with FAP who have abdominal surgery will develop desmoid tumors within five years. People with FAP also have a higher risk of their desmoid tumors returning after surgical removal than people who have sporadic tumors.

If you’re diagnosed with FAP during genetic testing, your health care provider will probably refer you to other specialists who provide desmoid tumor care. Most people with FAP will have surgery to remove parts of their intestines and reduce their high risk of colon cancer. After surgery, doctors will carefully monitor people with FAP to lower their risk of cancer.

If you’re diagnosed with FAP, your doctor might also recommend that your family members get genetic testing for this hereditary condition. Your health care team will help you understand more about what a diagnosis of FAP means for you and your family.

Talk With Others Who Understand

MyDesmoidTumorTeam is the social network for people with desmoid tumors and their loved ones. On MyDesmoidTumorTeam, members come together to ask questions, give advice, and share their stories with others who understand life with desmoid tumors.

Have you had genetic testing after being diagnosed with desmoid tumors? What was the process like for you? Share your experiences in the comments below, or start a conversation by posting on your Activities page.