We’re making progress, but there’s a lot left to do.

Thymidine kinase 2 deficiency, or TK2 deficiency (TK2d), is a very rare genetic disease with about 100 known patients. It is also called Thymidine kinase 2–related mitochondrial DNA depletion syndrome.

  • TK2d was first diagnosed in children in 2001
  • It was just in 2013 that improved genetic testing discovered an adult form of TK2d

Because the genetic testing that finds TK2d is so new, and there are many other mitochondrial depletion syndromes (MDS) with symptoms like those of TK2d, it is believed that there are many more people who have TK2 who have not been diagnosed.

Let’s work together

Whether you’re living with an MDS, have been identified with TK2d, or are a parent, caregiver, or HCP who wants to learn more about MDS and TK2d, join the registry so we can all take on TK2d together.

Patients and caregivers, continue reading. HCPs, learn more HERE.

An introduction to mitochondrial diseases


Inside each of our cells are mitochondria, or the powerhouses of the cell. Their job is to make the energy the body needs to function properly, including actions like lifting your arm or inflating your lungs with each breath. This energy is referred to as adenosine triphosphate, or ATP. 


You may have heard of DNA before and know that it’s stored inside the nucleus of our cells. That is called nuclear DNA. But did you know that we also have mitochondrial DNA (mtDNA)?

To make ATP, the mitochondria need enzymes and proteins from both our nuclear DNA and our mtDNA. When there is an error, or mutation, in either kind of DNA, this mutation can cause a mitochondrial disease. Ultimately, mitochondrial diseases result in less ATP being made than the body needs to work right. 

There are many kinds of mitochondrial disease that affect people in very different ways. In fact, 2 people with the same mutation may experience their disease completely differently. This is one of the reasons it is difficult to diagnose mitochondrial diseases or predict how a person may experience it. Mitochondrial diseases may affect the brain, heart, muscles, eyes, gastrointestinal tract, and many other systems. 


One class of mitochondrial diseases is mitochondrial DNA depletion syndromes (MDS or MDDS). Many people have a diagnosis of MDS, but getting a specific genetic diagnosis—like TK2d—is hard. 

With MDS, your cells are not making enough mtDNA to create ATP. There are many MDS disorders, and TK2d is one of them, primarily affecting muscles, including those that impact the function of the lungs and eyes. 


TK2 deficiency basics


TK2d is an enzyme deficiency. It is a genetic disease that is defined by muscle weakness (myopathy), with effects like difficulty breathing, droopy or saggy eyelids, or trouble chewing and swallowing. It can take a long time before a person is diagnosed with  TK2d. Why? Because TK2d is such a rare disease, doctors are just beginning to learn about it. Also, muscle weakness is a sign of many diseases, so there are many disorders for doctors to think about and check for.


What does the TK2 gene do?


Simply put, a fully functional TK2 gene is needed for normal function of mtDNA. Fully supported mtDNA is necessary for proper cell function.



What happens with TK2d?

With TK2d, less mtDNA is being made. The result is lower ATP production, which means muscles don’t have enough energy and cannot function right. The more mtDNA depletion you have, the worse your TK2d will be.