DNA Mutations Cause Cancer
Cancer Runs in Families When Children Inherit a Premade Mutation
People do not inherit cancer but instead inherit an increased risk of getting cancer.
Cancer is caused by changes in the DNA of certain genes. One set of genes, called oncogenes and tumor suppressors, control when a cell is supposed to grow or to stop growing. Mutations in these genes cause cells to grow uncontrollably.
Another set of genes that can indirectly cause cancer when mutated are DNA repair genes. These genes control the ability of a cell to fix any mistakes that happen in its DNA. When a DNA repair gene is mutated, the cell can’t fix mistakes in its DNA very well anymore. These mistakes build up until a tumor suppressor or oncogene are hit. Then the cell becomes cancerous.
DNA mutations
When people think of DNA mutations, they often think of Godzilla or giant spiders attacking San Francisco. But DNA mutations are actually much more ordinary than this. DNA mutations are pretty common and most have no effect.
DNA mutations can happen when something in the environment damages the DNA. This is why the UV light in sunlight can cause skin cancer. Or why cigarette smoke causes lung cancer. Both sunlight and cigarette smoke damage DNA and cause cancer.
Another way DNA mutations happen is when a cell makes a mistake copying its DNA. Our cells are constantly making new copies of themselves. For example, every person starts out as a single cell that eventually becomes 25 or 50 trillion cells. To get there, cells need to divide.
When a cell divides, it copies its DNA, doubles in size and then splits in two. The cellular machinery that copies the DNA is incredibly accurate. But it isn’t perfect. The occasional mistake creeps in and if that mistake is in a cancer-causing gene, then cancer can be the result.
How Cancers Run in Families
Cancers run in families when children inherit a premade mutation in one of these genes. The reason the premade mutation doesn’t cause cancer on its own is that people have two copies of most of their genes.
Most cancer-causing mutations require that both copies of a gene have the mutation (this is called a recessive mutation). When cancer runs in families, the family has one of the two copies already mutated. Now instead of needing two mutations to end up with cancer, these family members need only one. And one mutation is much more likely than two.
This is where the term “two hit hypothesis” comes from. People need a hit in both copies of a gene to end up with cancer.
An example of this is the famous breast cancer gene, BRCA1. BRCA1 is a DNA repair gene. People born with one damaged copy of this gene are 3-7 times more likely to develop breast cancer because they only need one copy to be damaged.
As with many theories like this, the two hit hypothesis is a little too simplified. The cell has too many backups to allow the mutation of a single gene to cause cancer. Usually additional mutations in other genes are required to cause a cell to go cancerous.
