A genome is a living thing’s complete set of DNA (deoxyribonucleic acid), including all of its genes. Genes are particles in cells, contained in chromosomes, and made of DNA. DNA contains the instructions for building proteins. And proteins control the structure and function of all the cells that make up your body.
DNA can change or be damaged over time. These abnormal changes in DNA are called genetic mutations. Some DNA changes are harmless, but others can cause disease. Cancer cells are “born” when abnormal changes in DNA tell cells to grow faster and behave differently than they should. As these cancer cells multiply to form a tumor, they continue to change – becoming more and more different from each other. As a cancer grows, new and different types of breast cancer cells are created within that same cancer. The mixture of cells that builds up over time becomes more and more complex. So even though every cell of a cancer is related to the same original "parent" cell, all the cells that make up a cancer are not the same. The idea that different kinds of cells make up one cancer is called "tumor heterogeneity."
By the time a breast cancer tumor is 1 centimeter (less than half an inch) in size, the millions of cells that make up the lump are very different from each other. And each cancer has its own genetic identity, or fingerprint, created by the DNA in its cells. So two people with breast cancer who are the same age, height, weight, and ethnicity, and who have similar medical histories, almost surely have two very different cancers. The only thing the cancers have in common is that they started from a breast tissue cell.
Genomic analysis is a relatively new way to look for unique genetic mutations in cancer tumors. Genomic analysis means researchers map out (called sequencing) the genome of a cancer tumor to look for mutations that may allow a cancer to be treated with a medicine that specifically targets that mutation – a targeted therapy medicine.
For example, some breast cancers are HER2-positive. This is because of a mutation that makes too many copies of the HER2 gene. The targeted therapy Herceptin (chemical name: trastuzumab) works by attaching to HER2 receptors and blocking them from receiving growth signals. Herceptin is not effective on HER2-negative cancers – cancers that don’t have that particular mutation.
A study strongly suggests that cancer tumor genomes should be compared to genomes from noncancerous tissue from the patient so doctors can be sure any mutations found are unique to the cancer.
The study was published in the April 15, 2015 issue of Science Translational Medicine. Read the abstract of “Personalized genomic analyses for cancer mutation discovery and interpretation.”
"Increasingly, hospitals and companies are beginning to sequence patients' tumors in an attempt to personalize therapy," said Victor Velculescu, M.D., Ph.D., professor of oncology and pathology and co-director of the Cancer Biology Program at the Johns Hopkins University School of Medicine, who was the lead author of the study. “However, many are not sequencing each person's normal tissue to filter out non-cancer-related changes and to really understand what is occurring in the tumor.”
In the study, the researchers analyzed the genomes of both cancer tumor and normal tissue from 815 people with 15 different types of cancer. When they looked only at genomic results from the cancer tumors, they found a number of mutations. But when they compared the genomic results from the cancer tumors to the genomic results from the normal tissue, they found that many of the mutations were also in the normal tissue. This means that these mutations are not suitable for targeted therapy treatment.
Not all genetic mutations in a cancer tumor are directly related to the cancer. Some mutations are what researchers call germline changes. Germline mutations are changes in genes inherited from your parents and are in all your DNA (your entire genome). These germline changes differ from person to person and are part of what makes each person unique. Other genetic mutations aren’t inherited and can happen during your life. Some mutations happen during cell division, when DNA gets duplicated. Other mutations are caused when DNA gets damaged by environmental factors, including UV radiation and chemicals.
Most genetic mutations aren’t harmful, but some can cause diseases, such as cancer.
The researchers said that only by comparing the genome of a person’s cancer tumor with the person’s normal tissue can doctors know which genetic changes are more likely to be cancer-related as well as which targeted therapies are likely to work.
Right now, genomic analysis isn’t commonly done on all cancer tumors. But as the technology becomes more available and less expensive, it’s likely that more doctors will use it. Stay tuned to Breastcancer.org for the latest news on genomic sequencing and how it may affect breast cancer treatment decisions.