The importance of genetic testing

This article first appeared in The Information Point newsletter Our World in 2012 when Sarah Foye with contributions by Dr. Alan Beggs and Lindsay Swanson, MS, CGC told us about the importance of genetic testing.

The centronuclear myopathies (CNMs) are a group of rare inherited muscle disorders that are caused by genetic changes in any one of several different genes. CNMs are considered to be an inherited group of disorders, meaning that the genetic change can be passed from parent to child. However, there are rare cases where neither parent is found to share the genetic change, meaning that the child has a new or ‘sporadic’ mutation.

In any case of muscle disorder, families are often left wondering, where did this come from, what caused this problem, what are the risks for other family members? For many families the answers to these questions can be found by testing the affected individual for the genetic mutations known to cause CNM.

Diagnosing a muscle disorder like CNM is usually made by reviewing a person’s symptoms and their family medical history and by having a physical examination by a doctor who is often a specialist. Once a doctor suspects a muscle disorder, they will often do further testing like a blood test, a nerve test, or a muscle biopsy.

During a muscle biopsy, a small portion of muscle is removed and examined closely. Different muscle disorders often have a unique appearance under a microscope. Examining the features of the muscle helps doctors to diagnose a specific muscle disorder. The muscle biopsy is considered the gold standard method of diagnosing CNM today. However, there are limitations to this method. Unfortunately, there are many problems with diagnosing CNM by muscle biopsy. Certain features of a muscle biopsy may be ambiguous or misleading because these findings may be present in many different forms of muscle disease. For example, centrally located nuclei can be present in myotubular myopathy but also with muscular dystrophy, myotonic dystrophy and many other muscle disorders.

Although there are difficulties with muscle biopsies, the discovery of new genes and the invention of new gene-testing technologies have opened new doors for diagnosing CNM and other related muscle disorders. Testing for genetic mutations known to cause CNM is one way of answering families’ questions as well as confirming a diagnosis. The genes known to be associated with CNM include the following: MTM1, DNM2, BIN1 and RYR1. Once a doctor suspects that a person has CNM, they can send a sample of DNA to test for one of these known genes.

Genetic testing can be arranged by a pediatrician, primary doctor, or a doctor specializing in genetics (geneticist). Because genetic testing for CNM can be complex, families with CNM may want to consider seeing a geneticist along with a genetic counselor when pursing a genetic diagnosis. Further information on how to arrange genetic testing can be found on the genetic testing pages of the Information Point website.

Why bother with genetic testing?

There are many benefits to pursing a genetic diagnosis. We already touched on how genetic testing can confirm a specific diagnosis. Some other benefits include:

Family planning

Knowing the genetic diagnosis can help people to understand how the disorder is passed within a family. It can also inform people about the risks of having an affected child. This information can help people make decisions about having children and can provide health risks for family members.

Medical problems

Identifying the specific gene can help to predict medical problems that may be associated with the gene. For example, liver problems can be associated with mutations in the MTM1 gene and malignant hypothermia can be associated with RYR1 mutations. By knowing that there are risks for certain medical problems, patients and their doctors can decide together what type of monitoring, treatments, or preventative actions they may need.

Research

As clinical trials get underway for CNM, researchers will want to enroll people in research based on their genetic diagnosis. Thus, genetic confirmation may be a condition to participate in future research.

Treatment

Knowing the specific gene mutation can help doctors and scientists to understand the specific cause of the disorder in the body. By identifying the specific mechanism which is causing the deficit, it can help to target potential treatments.

Please keep in mind that genetic test results can sometimes be surprising, upsetting, and lead to more questions. Please work with your healthcare professionals and genetic counselors to guide you through this process. If you might be interested in learning more about whole exome sequencing for your rare or unknown condition, please contact the Manton Center for Orphan Disease Research at Boston Children’s Hospital or contact Meghan Connolly by email or by telephone one 617-919-4287.

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.