Genetic and genomic testing

Genetics vs. genomics

Although commonly used interchangeably, the terms “genetics” and “genomics” are not synonyms. Both involve the study of genetic material and both are derived from the Greek word gen, which means birth or origin. But the similarities largely end there. Though genetics and genomics are each complex topics, the difference between them is much simpler: One (genetics) refers to a person’s genetic makeup, and the other (genomics) is typically used in reference to a tumor’s molecular composition. You can also think about genetics in terms of inherited traits and genomics in terms of cancer-specific mutations.

What is genetics?

Genetics is the study of the genes people inherit at birth, passed on from their family through the generations. Every cell in the human body has a complete strand of DNA, and each strand is packed with genes, which carry instructions for certain traits, such as blue eyes, red hair—or, perhaps, a stronger likelihood of certain cancers. Genetic tests may help identify a person’s risk of cancer and other diseases.

What is genomics?

Genomics generally refers to the study of mutations in genes that may drive various cancer behaviors, from how aggressive it is to whether it spreads to distant locations in the body. Each cell in the human body contains tens of thousands of genes, but mutations in just a single gene may cause cells to grow out of control and lead to tumor growth. Often, the cells in a tumor change over time because their genes continue to mutate. So a genomic test may vary widely over time, even when conducted on the same tumor. When the tumor’s molecular structure changes, the patient may develop resistance to a certain treatment.

What the tests show

Both tests—those mapping a person’s DNA profile and those analyzing a tumor’s genomic abnormalities—may be helpful in treating cancer, though genetic and genomic testing are used in very different ways and in very different circumstances. In the cancer world, genetic testing looks for genetic mutations that the patient may have inherited through his or her family, which is why it’s often recommended for people who have a family history of a certain type of cancer. Those who test positive for the BRCA1 gene mutation, for example, have a higher risk of developing breast and ovarian cancer.

Genomic testing is used to identify mutations that have nothing to do with heredity but instead occur within the cancer cell itself, either due to an external cause, such as tobacco use or sun exposure, or an internal factor, such as a random molecular change within the cell. These changes may determine why the tumor behaves the way it does—why it grows or spreads, for example. If the mutation matches a known abnormality, the oncologist may recommend a certain targeted therapy designed to attack that mutation. This form of testing may be recommended for people whose cancer stopped responding or didn’t respond well enough to conventional treatments like chemotherapy and radiation.

Advanced genomic testing for cancer

In advanced genomic testing, a biopsy is taken of the patient’s tumor, cancer cells are isolated and extracted from the biopsy sample, and the cancer cells’ DNA is sequenced in the lab. Then, sophisticated equipment is used to scan the sequenced genetic profile for abnormalities that dictate how the tumor functions.

If identifiable abnormalities are found, they are analyzed to determine whether they match known mutations that may have responded to particular therapies or where evidence suggests there may be a potential treatment option not previously considered. If there’s a match, doctors may be able to use the results to suggest treatments that have been used in the past to target the same mutations.

Although conventional genomic testing is standard-of-care for some patients, advanced assessments are not recommended or available to all patients. Gene-mapping tests may be appropriate for patients with rare, unusual or hard-to-treat cancers, and for patients whose tumors did not respond adequately to conventional therapies.

Learn more about advanced genomic testing

Genetic testing for cancer

Genetic testing consists of a mouthwash or blood test. Analysis of the sample may determine a person’s likelihood of developing a certain type of cancer, whether a gene mutation contributed to an existing cancer diagnosis, and whether a person is at a greater risk of developing the same cancer again or developing another type of cancer.

A positive result on a genetic test does not always mean the person will develop cancer. It only means he or she carries one or more gene mutations. Similarly, a negative result on the test doesn’t mean a person will not develop cancer.

Learn more about genetic testing

Gene mutations for cancer

Gene mutations are permanent abnormal changes in the DNA of a gene. These mutations may increase the risk of developing cancer and other diseases. There are two types of gene mutations:

Acquired mutations occur when cells are damaged during replication, by viruses or by exposure to carcinogens, such as tobacco smoke or radiation.

Hereditary or germline mutations are passed down from generation to generation.

Not everyone with a gene mutation develops cancer, but either type of gene mutation may cause cells to grow out of control and form tumors. Acquired mutations are the most common causes of cancer. Hereditary mutations account for about 10 percent of all cancers.

Gene mutations that may play a role in the development of cancer include:

BRCA1 and BRCA2

Mutations in the BRCA1 and BRCA2 (breast cancer 1 and breast cancer 2) genes may increase the risk of breast and/or ovarian cancer. BRCA mutations may be inherited and are associated with an increased risk of developing cancer. The BRCA gene test is typically recommended for people with a personal or family history of cancer, or for those with a specific type of breast cancer. The test generally isn’t performed on those with an average risk for breast or ovarian cancer.

Microsatellite instability and mismatch repair deficiency

Cancer is sometimes caused when the DNA in normal cells becomes damaged. But the body often fixes this damage before cancer forms, using mechanisms found in normal cells that look for repair flaws in DNA. One of those mechanisms is called mismatch repair, which corrects errors that occur when cells divide. When that function breaks down, the cells have what’s referred to as a mismatch repair deficiency (dMMR). Cells with dMMR can create an even more unstable cellular environment that causes microsatellites, or the repetitive stretches of DNA in a cell, to form mismatches, instead of exact duplicates of the DNA profile. This volatility is called microsatellite instability (MSI-H), and it sometimes causes cells to mutate and grow out of control, forming tumors. 

MSI-H and dMMR are found in a variety of cancers. For instance, MSI-H has been detected in 15 percent of all colorectal tumors, as well as in some uterine, bladder, breast, prostate and thyroid cancers. The defect has also been found in 90 percent of colorectal cancer patients who also have Lynch syndrome, a genetic condition that elevates the risk for some cancers. Not all cells with dMMR develop MSI-H. And, in rare cases, MSI-H can occur in cells without dMMR. Immunotherapy drugs called checkpoint inhibitors have been approved to treat cancers with dMMR or MSI-H.

PD-L1

Programmed cell death receptor 1, or PD-1, is a protein receptor found on T-cells, which are immune cells that protect the body from disease and infections. PD-1 is called a checkpoint protein because it seeks out cells and checks to see if they are harmful. When the PD-1 on an immune cell interacts with PD-L1 on another cell, it shuts down an immune response. Some cancer cells may have many PD-L1 receptors, allowing them to disguise themselves as healthy cells when “checked” by the immune system. Checkpoint inhibitors are designed to block the interaction of PD-1 and PD-L1 receptors and allow the immune system to better recognize cancer cells and attack them. These drugs have been approved to treat a variety of cancers, including melanoma, lung and bladder cancers.

Other gene mutations that may be found in cancer include: