Chimerism

Chimerism is a rare condition that occurs when you have two or more different sets of DNA in your body. This means that some of your cells came from one fertilized egg, and other cells came from another. In simple terms, one body contains two genetic makeups from different sources.

You may also hear the terms genetic chimera or human chimerism. The medical term comes from Greek mythology. Chimera was a fire-breathing creature with a lion’s head, a goat’s body and a snake’s tail. But in healthcare, it simply describes mixed DNA in one person.

Chimerism in humans is uncommon. It may happen naturally before birth (for example, during twin development), or when you’re pregnant. It can also happen through medical treatments like bone marrow or organ transplants. Most people with chimerism don’t know they have it unless genetic testing shows unexpected DNA results.

Types of this condition

There are several types of chimerism in humans:

• Microchimerism: When you’re pregnant, a very small number of cells can pass between you and the fetus. These cells can stay in your body for years. This is also called fetal maternal chimerism. It’s the most common type and usually harmless.
• Twin chimerism: This can happen when fraternal twins share the same blood supply in the uterus. It can also occur when one twin stops developing very early (like in vanishing twin syndrome). Cells from one twin remain in the other.
• Tetragametic chimerism: This rare form happens when two fertilized eggs fuse very early in development and form one embryo. The fetus grows with two sets of DNA from the start.
• Transplant-related chimerism: People who receive an organ, bone marrow or stem cell transplant will have a mix of DNA in their blood cells. They’ll have their own, and they’ll have the donor’s. Your healthcare provider will monitor this after a transplant.

Chimerism symptoms

Most people with chimerism have no symptoms. When symptoms do appear, they depend on the type and how the DNA is mixed in your body. Possible signs may include:

• Patches of a different skin color
• Eyes that are different colors
• Differences in blood type
• Differences in reproductive or sex traits (rare)

Many people discover chimerism only after unusual DNA test results. This could be during paternity testing or an organ transplant evaluation.

Chimerism causes

Chimerism happens when cells from different fertilized eggs combine in one body. This can occur:

• When two embryos fuse early in pregnancy
• When twins share blood circulation
• When fetal and maternal cells mix during pregnancy
• After an organ, bone marrow or stem cell transplant

Risk factors

Risk factors depend on the type. They include:

• Having a twin
• Being pregnant (for fetal maternal cell exchange)
• Receiving an organ, bone marrow or stem cell transplant

Complications of this condition

Most people with chimerism live healthy lives. Possible complications may include:

• Graft vs. host disease (after stem cell transplant)
• Transplant rejection (in transplant-related chimerism)
• Rare autoimmune issues

In extremely rare cases, mixed reproductive cells can affect fertility testing or DNA results.

How doctors diagnose chimerism

Healthcare providers usually find chimerism by accident during genetic testing. Your provider may suspect it if:

• Blood tests show two different blood types.
• DNA tests don’t match expected family patterns.
• Transplant monitoring shows changes in donor cells.

Tests that are used

Tests may include:

• DNA testing from blood, saliva or tissue
• Polymerase chain reaction (PCR) testing
• Bone marrow testing (after transplant)
• Whole genome testing

If you’re getting a transplant, your provider will regularly measure how much donor DNA you have in your body. This helps them track your recovery and find complications early.

How is chimerism treated?

You usually don’t need to treat natural types of chimerism. That’s because healthcare providers don’t consider it a disease by itself. If you have transplant-related chimerism, treatment may include:

• Medications that suppress your immune system
• Additional stem cell or bone marrow transplant procedures
• Close blood monitoring

But keep in mind, these treatments manage the transplant, not the mixed DNA itself.

When should I see my healthcare provider?

You should talk to your provider if you:

• Receive unusual or confusing DNA test results
• Have unexpected transplant test findings
• Notice unexplained differences in blood typing

Most of the time, chimerism doesn’t cause symptoms. But it’s important to discuss any surprising genetic results with your healthcare provider.

What is the outlook (prognosis) for someone with chimerism?

Most people with human chimerism live normal, healthy lives. Many never know they have it. When chimerism is related to a transplant, your outlook depends on the success of the transplant.