A Brief Genetics Lesson as it Relates to BRCA

I know I’ve spent a lot of time on genetics, genetic testing, and family history.  The reason for that is because my official diagnosis is Hereditary Breast and Ovarian Cancer Syndrome (HBOC).  Therefore, it is extremely important to understand how heredity works, how this gene is passed on to a child, and whether it can be prevented.  So let’s go back to high school biology.

When someone has a BRCA mutation, each of their children has a 50% chance of inheriting that mutation.  This holds regardless of whether the mutation is passed from the mother or the father, as well as whether the child is male or female.  The diagram below shows how this works.

Notice that the diagram does not show Father or Mother; but rather Parent 1 and Parent 2. Men inherit and pass on a mutated gene at the same rate as women.  However, the incidence of BRCA-related cancers is much lower in men.  The lifetime incidence of male breast cancer and prostate cancer for someone with a BRCA1 mutation is 1.2% and 16%, respectively. 

This means that approximately 83% of men who carry a BRCA1 mutation will never develop a BRCA-related cancer.  And many of the 16% who develop prostate cancer will never connect their diagnosis with their genetic mutation.  Yet their daughters, for whom the genetic mutation is much more deadly, have a 50% chance of inheriting this mutation.  These women will not necessarily have a family history of breast cancer, because the mutation did not pass through their maternal genes. And this is a problem.

As I mentioned in an earlier blog post, I inherited my mutation from my father.   My father’s mother died of ovarian cancer (so I never answered YES to all those screening questions about breast cancer).  Her two sisters—my great-aunts—died of breast cancer at young ages.  Between the three of them, they had seven biological children.  Six of the seven were sons.  At least four of these six sons—and possibly at least one more—inherited this mutation and some passed it on to their daughters.  In a nutshell, the manifestation, and therefore our awareness, of the mutation skipped a generation, because it was passed on through men who never developed cancer.

BRCA mutations can now be detected in fetuses and embryos through amniocentesis in pregnant women, and preimplantation genetic diagnosis (PGD) in couples undergoing IVF.  If BRCA is in your family, a fertility specialist can provide more information.

I ask everyone to take a close look at your family history, especially through your paternal line.  If your paternal grandmother, or a paternal aunt, great aunt, niece, or cousin has or had breast or ovarian cancer, consider contacting a genetic counselor.  It may save your life.