Family Tree
What Causes HAE?
HAE is caused by a problem in a person’s genetic code for a single protein, known as C1 -esterase (C1 - ESS•ter•ace) inhibitor, or C1 -INH. An abnormal code can lead to a deficiency in C1 -INH (type I HAE) or an inability of C1 -INH to function properly (type II HAE).
C1-INH Deficiency
In a healthy person, C1 -INH regulates chemical responses in the immune system (the system that deals with foreign invaders, such as bacteria and viruses) and the formation of blood clots. Without adequate, functioning C1 -INH, the person with HAE can have a chemical response that is out of proportion with the body’s demand.
A Dominant Gene
As you might recall from health class, people receive half their genetic code from each parent. In some diseases, such as cystic fibrosis or sickle cell anemia, a child must receive the disease-causing gene from both parents.
 This is not the case with HAE. In HAE, just one problem gene produces the disease. Biologists call this an autosomal (AW•toe•ZOAM•ul) dominant gene. As a result, children born with just one disease-causing gene will have HAE — although, like other people with HAE, they may or may not have attacks due to HAE. The good news is that children born without the abnormal gene will effectively prune the occurrence of HAE from that branch of the family tree. All successive generations (that child’s children and grandchildren) will be free of HAE.
But Nobody in My Family Has HAE
Sometimes people with a new diagnosis of HAE are not aware of the disease in other family members.
In hindsight, however, they will discover that a parent, grandparent, or other relative did indeed have symptoms that sound very much like HAE.
Occasionally, HAE occurs in people without a previous family history of the disease. This is due to a spontaneous change in the gene of the sperm or egg that formed the developing person. Not only will these individuals have the disease, but they will also have the potential to pass this gene (and HAE) to their children.
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