How Is Congenital Adrenal Hyperplasia Inherited?

If you or your child has been diagnosed with congenital adrenal hyperplasia (CAH), you may wonder: How did that happen? Especially if no one in your family has CAH.

CAH is a genetic disorder, which means it runs in families.

You didn’t do anything to cause it.

But the way CAH is passed down can be complicated. So, we asked endocrinologist Pratibha Rao, MD, to explain how it happens.

CAH and your genes

CAH follows an autosomal recessive inheritance pattern.

If that means anything to you — congratulations, you’ve made your high school biology teacher incredibly proud!

For the rest of us, Dr.Rao explains it like this: “You must inherit two mutated copies of a gene — one from each parent — to develop CAH.” In about 95% of people with CAH, the gene in question is CYP21A2.

A mutation (also called a “pathogenic variant) in CYP21A2 changes how the gene works — it can’t do its job properly.

The variant in CYP21A2 blocks the production of an enzyme called 21-hydroxylase. Without it, your body can’t effectively make the hormones cortisol or aldosterone. That’s what leads to the effects of CAH that you’re probably all too familiar with.

Inheriting CAH

Now, let’s get into the nitty-gritty of how you wound up with CAH. It can be surprising to hear that it’s a genetic condition. Especially considering that a lot of people living with CAH had no idea that it ran in their family.

Your genes are the instruction manual for how your body works. Genes live in threads called chromosomes. Half of your chromosomes come from your biological mother. The other half comes from your biological father.

Some traits, like having a cleft chin, are controlled by a dominant gene. If you get one dominant gene for a particular trait, you’ll show it.

Easy enough.

Other genes are recessive. The mutation in CYP21A2 that causes CAH falls into that category.

You need two copies of a recessive gene for it to affect you.

Let’s say neither of your parents has CAH. But deep in their genetic code, they have a mutated CYP21A2 gene. On the other related chromosome, they have a healthy copy of CYP21A2. That makes them carriers of CAH.

“The non-mutated gene is dominant, so it masks the effects of the faulty one,” Dr. Rao explains.

The mutated gene lurks around in the shadows. It doesn’t do much, if anything, to affect them. But they can still pass it on.

“CAH carriers often don’t show symptoms. They can pass it on for generations without knowing it,” she continues. “It’s only when a CAH carrier has children with another carrier that CAH shows up in the family.”

When your parents made you, you inherited one set of chromosomes from each of them. Through the luck of the biological lottery, they each passed to you the chromosome with the mutated CYP21A2 gene.

Let’s look at the odds:

Can I be tested for CAH risk?

It can be scary to learn that CAH can be passed along silently through your family. But today’s genetic tools can give people more clarity and control.

“Genetic counseling and genetic testing before pregnancy can determine if you’re a carrier,” Dr. Rao shares. “That can help you to understand your risk for passing on CAH and other conditions to a child.”

If you and your partner are found to be carriers of CAH, a healthcare provider can discuss options with you to lower the risk of passing it on. That might include treatments like in vitro fertilization (IVF). This way, doctors can test embryos for CAH markers before implantation.

Your genes have a big say in who you are and how your body works. Learning that CAH is part of your genetic makeup can feel overwhelming.

But knowledge is power. And understanding how CAH is inherited can help you make informed decisions about your health, your family and your future.

Remember: You’re not alone. With support, treatment and expert guidance, people with CAH can lead full, healthy lives.