Heredity and Cancer
Cancer is such a common disease that it is no surprise that many families have at least a few members who have had cancer. Sometimes, certain types of cancer seem to run in some families. This can be caused by a number of factors. It can be because family members have certain risk factors in common, such as smoking, which can cause many types of cancer. It can also be due in part to some other factors, like obesity, that tend to run in families and influence cancer risk.
But in some cases the cancer is caused by an abnormal gene that is being passed along from generation to generation. Although this is often referred to as inherited cancer, what is inherited is the abnormal gene that can lead to cancer, not the cancer itself. Only about 5% to 10% of all cancers are inherited – resulting directly from gene defects (called mutations) inherited from a parent. This document focuses on those cancers.
DNA, Genes, and Chromosomes
Cancer is a disease of abnormal gene function. Genes are pieces of DNA (deoxyribonucleic acid). They contain the instructions on how to make the proteins the body needs to function, when to destroy damaged cells, and how to keep the cells in balance. Your genes control things such as hair color, eye color, and height. They also can affect your chance of getting certain diseases, such as cancer.
An abnormal change in a gene is called a mutation. The 2 types of mutations are inherited and acquired (somatic).
Inherited gene mutations are passed from parent to child through the egg or sperm. These mutations are in every cell in the body.
Acquired (somatic) mutations are not present in the egg or sperm. These mutations are acquired at some point in the person's life, and are more common than inherited mutations. This type of mutation occurs in one cell, and then is passed on to any new cells that are the offspring of that cell.
Genes are found on long strands of DNA called chromosomes. Humans have 23 pairs of chromosomes in each cell. We inherit one set of chromosomes from each parent. Each chromosome can contain hundreds or thousands of genes that are passed from the parents to the child. Every cell in your body has all of the genes you were born with. Although all cells have the same genes and chromosomes, different cells (or types of cells) may use different genes. For example, muscle cells use a different set of genes than skin cells use. The genes that the cell doesn't need are turned off and not used. The genes that the cell is using are activated or turned on.
Genes and cancer
Even if you were born with healthy genes, some of them can become changed (mutated) over the course of your life. These mutations are known as sporadic or somatic, meaning they are not inherited. Sporadic mutations cause most cases of cancer. These mutations may be caused by things that we are exposed to in our environment, including cigarette smoke, radiation, hormones, and diet (although in many cases there is no obvious cause). More gene mutations build up as we get older, leading to a higher risk of cancer.
When someone has inherited an abnormal copy of a gene, their cells already start out with one mutation. This makes it all the easier (and quicker) for enough mutations to build up for a cell to become cancer. That is why cancers that are inherited tend to occur earlier in life than cancers of the same type that are not inherited.
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When should I worry?
When many cases of cancer occur in a family, it is most often due to chance or because family members have been exposed to a common toxin, such as cigarette smoking. Less often, these cancers may be caused by an inherited gene mutation. (These are called family cancer syndromes.) Certain things make it more likely that an abnormal gene is causing cancers in a family, such as
Many cases of an uncommon or rare type of cancer (like kidney cancer)
Cancers occurring at younger ages than usual (like colon cancer in a 20 year old)
More than one type of cancer in a single person (like a woman with both breast and ovarian cancer)
Cancers occurring in both of a pair of organs (both eyes, both kidneys, both breasts)
More than one childhood cancer in a set of siblings (like sarcoma in both a brother and a sister)
Before you decide that cancer runs in your family, first gather some information. For each case of cancer, look at:
Who is affected? How are we related?
What type of cancer is it? Is it rare?
How old was this relative when they were diagnosed?
Did this person get more than one type of cancer?
Did they smoke or have other known risk factors?
Cancer in a close relative, like a parent or sibling (brother or sister), is more cause for concern than cancer in a more distant relative. Even if the cancer was from a gene mutation, the chance of it passing on to you gets lower with more distant relatives.
It is also important to look at each side of the family separately. Having 2 relatives with cancer is more concerning if the people are related to each other (meaning that they are both on the same side of the family).
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