What's a BRCA Gene Anyway?

The first thing you need to know about the BRCA gene is that you have it.

Don't panic. Everyone does. In fact, we all have two of them -- the BRCA1 and 2 genes. They are normal genes that "have an important function in the cell. They are involved in repairing DNA damage," explained Dr. Robert Nussbaum, a medical geneticist at UCSF. "When they're functioning normally, they do a good job for us."

The problem is what happens when they don't function normally. We'll get to that in a minute. But first, in our call, Nussbaum gave me a helpful primer in basic genetics.

For starters, we all have two copies of each of the BRCA genes. Men, too. We get one copy from each parent.  These genes are "like sentences," Nussbaum said. "They are made up of words." When they're spelled right, all is well.

But "you can have all kinds of misspellings," Nussbaum said. "Red becomes reed. All kinds of things can happen that will alter the meaning of that sentence."

If you've ever texted or sent an email or written anything on a computer, you know that there are misspellings that don't change the meaning of what you're trying to say. Nussbaum used the example of "gray" and "grey". The change in spelling does not change the meaning.

With the BRCA genes, there are "thousands" of ways the gene can be misspelled, Nussbaum said. But just like gray/grey doesn't matter, neither do many of those BRCA misspellings or what Angelina Jolie correctly called a mutation.

Then there are the ones that do matter. Sadly, they matter way too much. On Tuesday Angelina Jolie said she had a mutation that gave her an 87 percent risk of developing breast cancer, which is why she decided to undergo a preventive double mastectomy.

In general, women who have inherited the harmful mutations have about 60 percent risk of developing breast cancer over their lifetimes, according to the National Cancer Institute. They are also at much higher risk of ovarian cancer. And men who inherit the gene have a dramatically increased risk of male breast cancer as well as pancreatic and prostate cancers.

Still, only about 10 percent of all breast cancers are associated with these mutations. Or, to put it the opposite way, roughly 90 percent of all breast cancers are not related to BRCA mutations. They just happen, for reasons doctors and scientists cannot fully explain.

I just googled "Angelina Jolie" and "mastectomy" and got 736 million results. It's probably safe to say countless women are thinking about BRCA testing right now. But before you run to a lab, Nussbaum said they start with an "accurate and complete" family history, including on the father's side. Remember that you get one copy of all your genes, including the BRCA1 and 2 genes, from your dad.

While "thousands" of mutations cause harm and "thousands" more have no effect at all, Nussbaum then pointed to a third category, "variance of uncertain significance," he said.  Doctors see the mutation, but cannot determine if it damages the gene's function or not. "So there is no such thing as 'the BRCA1 mutation.' There are thousands. Some are deleterious. Some are not. Some we don't know."