Titin mutations cause centronuclear myopathy

This story first appeared in the Information Point newsletter Our World in 2013, when Sarah Foye, Lindsay Swanson, MS, CGC and Elizabeth Torosian told the The Information Point about the discovery that mutations in the Titin (TTN) gene can cause centronuclear myopathy.

As you may know, the congenital myopathies are a group of inherited disorders (passed down from parent to child) that affect the structure of muscles. The class of myopathies known as centronuclear myopathy (CNM) have historically been named due to the muscle cells appearance under the microscope. The nuclei of muscle cells affected by CNM tend to be found in the center of the cell, unlike in healthy muscles where the nucleus is found on the edge of the cell. These internalized nuclei are what give CNM it’s name, but there are several genes known to cause various forms of CNM. These genes are MTM1, DNM2, RYR1, BIN1 and TTN however some genes still remain unknown.

New gene discoveries are taking place as muscle disorder researchers learn more about CNM and more technologies become available. One new technology being utilized in the research lab of Dr Alan Beggs includes whole exome sequencing (WES) and whole genome sequencing (WGS). These new technologies, which can be less expensive than traditional genetic sequencing, allow researchers to look at larger chunks of the human genetic code.

Using this technology, the Beggs Lab, along with a group of collaborators, discovered that mutations in the Titin (TTN) gene can cause CNM. Titin, the largest known protein in the human body, is coded by the TTN gene. It is a structural protein that acts as a molecular spring within the muscle cell, as seen in the drawing to the right. Although Titin has long been known to be a structural protein within the muscle cell, examination of the gene TTN has been limited due to it’s gigantic size. However, the use of new genetic testing methods like WES and WGS have allowed researchers to understand more about Titin and the TTN gene. You can read more about this in the article below.

Beggs et al, used the new genetic testing methods to screen a group of subjects who were diagnosed with CNM by muscle biopsy but tested negative for mutations in all genes known to cause CNM. Through this process, they identified that TTN mutations were likely the genetic cause in five individuals with CNM. You can read more about this in the article below:

However, there still remains a large category of people diagnosed with CNM whose genetic cause remains unknown. With new gene discoveries and new technologies, it can be expected that people can jump from the ‘unknown’ category into a category with a known gene. New clinical testing can help ease the diagnosis by testing for many genes simultaneously. The University of Chicago Genetic Services Laboratories is now offering a state of the art test in which multiple congenital myopathy genes can all be tested at once. Known as next generation sequence genetic testing. This type of testing is faster and cheaper than prior testing methods. This panel includes the TTN gene. If you or a family member have been diagnosed with MTM or CNM but never had a genetic mutation identified or confirmed through a clinical lab, please consider discussing this with your doctor or a genetic counselor.

One final point to note is that TTN mutations are known to cause a heart problem known as adult onset dilated cardiomyopathy. This can have important clinical implications for people who may have CNM caused by a TTN mutation and may be another important reason to have the genetic testing done. Additionally, any person with heart problems and a congenital myopathy may strongly consider TTN as a possibly cause.

International collaboration

Since 2011 the Myotubular Trust has been funding a grant to Dr Jocelyn Laporte at IGBMC in France to use next generation gene sequencing technology to find some of the other genes that cause myotubular and centronuclear myopathy. Jocelyn Laporte says ‘The team in Strasbourg is supported by Myotubular Trust to identify genes linked to myotubular and centronuclear myopathies using the novel genome sequencing approach. Due to this support the lab were able to participate to an international collaborative study that culminated in the identification of mutations in Titin in patients presenting with centronuclear myopathy. Titin is the largest protein of the human body and acts as a molecular spring during muscle contraction and relaxation. Other families with centronuclear myopathy that have previously eluded genetic diagnosis may turn out to be linked to this same gene. Researchers can now use this finding to better establish diagnosis and understand how these myopathies occur’.

The discovery of the Titin protein’s role in this condition is a great example of the power of international collaboration between leading neuromuscular research teams. This is really good news for our community

Genetic testing

If you have been diagnosed with myotubular myopathy but have never had your MTM1 mutation identified of confirmed in a clinical laboratory, you may want to consider enrolling in the MTM Genetic Testing study.

For European patients, where the culprit gene has not been identified via genetic testing, retesting can be requested via a clinician, as most diagnostic laboratories in Europe are currently validating these novel sequencing technologies. In Europe, such a request for re-testing must be made via a clinician, rather than directly to a laboratory and testing is free in some countries.

Genetic mystery solved

This story first appeared in the Information Point newsletter Our World in 2012, when Sarah Foye told The Information Point how taking part in the Clarity Challenge at Boston Children’s Hospital solved her sons genetic mystery.  

“Negative.” “Normal.” “Fails to confirm the diagnosis of . . .”. “Etiology of the patient’s disease phenotype remains unknown.” These are all words that we have heard repeatedly in the past 11 years of our son’s life. In fact, negative (normal) genetic test results have been reported back to us 13 times. Each of these tests requires a painful blood draw, insurance authorization and an agonizing 4-6 week period waiting for results. That is a lot of nail biting for us as Mom and Dad.

Our son, AJ, was diagnosed with centronuclear myopathy (CNM) in infancy by muscle biopsy. The biopsy showed some elements of a centronuclear myopathy pattern, but at the time, genetic testing for CNM was limited to the XLMTM (X-linked myotubular myopathy) form, which AJ tested negative for. We have been seeking a genetic confirmation of his disorder since. As researchers worldwide have discovered addition genes responsible for CNM, AJ has been tested for them, but each time the test was negative. The mystery remained. The search continued.

We’ve wondered: what is his definitive diagnosis? What medical conditions is he at risk for? What impact could this have on future children for our extended family and us? What treatments might help our son? The possibility of finding answers to these longstanding questions is one of the many reasons we chose to participate in the CLARITY challenge at Boston Children’s Hospital.

The goal of the CLARITY (Children’s Leadership Award for the Reliable Interpretation and appropriate Transmission of Your genomic information) Challenge was to identify best methods and practices for improving the reliability and accuracy of the genomics-to-clinic pipeline spanning sequencing, analysis, interpretation and reporting – to provide the most meaningful results to patients and their families.

40 academic and commercial organizations applied to participate in the CLARITY Challenge, a contest to identify the putative disease-causing mutations in three young patients. 30 were selected and agreed to compete. 23 submitted entries by the September 30 deadline. Contestants entered from all over the world, including North America, China, India, Israel, Italy, Germany, the Netherlands, Singapore, Slovenia, Spain, Switzerland and Sweden. They had about four months to analyze genomic (DNA) data and medical histories of each of the patients and their families and submit their reports. Boston Children’s Hospital awarded $25,000 in prize money to the winning research teams, which were selected by a six person judging panel.

The contest organizers — Isaac Kohane (director of the hospital’s Informatics Program), David Margulies (executive director of the Gene Partnership at Boston Children’s Hospital) and Alan Beggs (director of the Manton Center for Orphan Disease Research at the hospital) unveiled the winners at the annual conference of the American Society of Human Genetics on November 7.

We are thrilled to share the news that using whole genome and whole exome sequencing, the contestants identified the likely genetic cause of AJ’s muscle disorder. The involved gene is called TTN, which codes for a protein called Titin. Titin is a protein located inside muscle cells, where it is crucial for proper muscle function.

Finally having this answer helps us to understand many aspects of our son’s condition. We now know the gene mutation that is responsible, and we know the protein that it impairs. We know how it was inherited and the risks for other family members. We can begin to understand the problem at the level of the muscle cell and its impact on muscle function. We are inspired to further our efforts in support of CNM research and potential future treatments. We have a greater understanding of the medical conditions that AJ is at risk for and can cross off ones that he is not at risk for. Having a genetic diagnosis is not a final destination on our medical journal, but it is a hugely important milestone along the way. We also hope that this discovery advances the current understanding of CNM overall.

Undoubtedly numerous other families with CNM that have previously eluded genetic diagnosis will turn out to have mutations on the same gene as our family. Thus, a new subcategory of CNM is born: titin myopathy. Titin myopathy now has a place right along other forms of CNM such as XLMTM (X-linked myotubular myopathy), autosomal dominant CNM (Dynamin 2, DNM2), Ryanodine Receptor 1 (RYR1) CNM, and Amphiphysin 2 (BIN1) CNM. Other forms of CNM still remain without a genetic diagnosis and research is ongoing. By learning more about the similarities and differences between the various forms of CNM, hopefully researchers can better understand why these myopathies occur and what treatments may help.

We are truly grateful to have had the opportunity to participate in the CLARITY Challenge. We are happy to have a genetic answer to our family’s questions and hope that subsequent resultant research will help all of our CNM families move further along this journey that we all share. Many thanks to Boston Children’s Hospital and the international contestants for working on our case and working to move the industry of clinical genomics forward.