Advances in Diagnostic Molecular Genetics and the Implications for Malignant Hyperthermia

Very recently, the Institute of Medicine, an independent, nonprofit, non-governmental prestigious "think tank" composed of eminent scholars published the proceedings of a roundtable discussion on "Translating Genomic-Based Research for Health".  In other words, "How do physicians and scientists make sense of the large amount of information that is derived from the ability to analyze the DNA from people, animals and even plants"?  The report focused on humans, of course, and was generated because the technical capabilities of "sequencing" DNA has improved to such a point that it will soon be possible to display the genetic code in detail from any person for about $1,000.  This is truly remarkable because only a decade ago, the cost of taking apart the complete sequence of a human’s DNA was tens of millions of dollars.  As you all probably know, DNA is made up of a very long string of only four specific chemicals compounds that form the template for a cell’s ability to make proteins. Proteins are the basic structure of all cellular elements. The DNA is housed on 46 chromosomes in each person’s cell.  The DNA is divided in such a way as to make up about 30,000 genes.  However, much of a person’s DNA is not involved in the structure of those genes.  The role  of the remainder of the DNA that does not make up active genes is still unclear.  The portion of the DNA that makes up the genes is called the "exome."

Analyzing just that part of DNA that forms the template for proteins, i.e., the exome, is termed whole exome sequencing. It is also possible to analyze, or sequence, the entire DNA structure (called the genome) of a human, and this is called whole genome sequencing. 

This is a wonderful accomplishment, but the chief problem at this time is understanding the clinical significance of the mass of DNA data that is being produced, i.e., how does the DNA relate to a patient’s physical characteristics in health and disease.  A whole new medical discipline has developed to try to make sense of the data derived from sequencing DNA, called bioinformatics.

The study of DNA and its interpretation is advancing so rapidly that many physicians are not able to keep up with the science and have difficulty interpreting a genetic test for an individual or indeed know what tests to order. It is for that reason, and in order to help patients concerned about MH that members of our staff, professional advisory council, hotline consultants, MH-susceptible family members, and others interested in MH have put together a "roadmap" to try and make sense of how to use the standard muscle biopsy contracture test as well as genetic tests for assessing susceptibility to MH.

The genetics of MH are complex because the gene most often associated with MH susceptibility, the ryanodine receptor type 1 gene, is extremely large and there are hundreds of DNA variations that have been related to patients who are MH susceptible. Not all the DNA variations are meaningful in terms of producing an altered protein and a risk for MH.  Some changes are quite benign.  Sorting through the many variations is complex.  A group of geneticists and MH experts in Europe have outlined strict criteria to determine whether a change in the DNA of the ryanodine receptor truly predisposes the patient to developing MH. (See the European MH Group web site, www.emhg.org).   One specific criterion is that the DNA change should be identified in several families who manifest an MH reaction but are not related to one another.  Another is that the DNA variant, when implanted in a cell system,  results in changes in calcium characteristic of MH upon exposure to agents such as caffeine or halothane.   As a result, only 31 DNA changes have been determined to be causal for MH.  (That number will certainly increase over time).  When a DNA variant is found in the ryanodine receptor gene that meets these criteria, then the person can be identified as being at risk for MH.  If not found, we still cannot be sure that the patient is NOT at risk for MH, because he or she may harbor one or more DNA variants whose significance is not yet known. If a DNA variant is found that is not yet been shown to be "causal "for MH then, again, no diagnostic conclusion can be drawn.

In order to clarify the clinical significance of changes in the DNA in the two genes associated with MH susceptibility, a database consisting of both genetic information and clinical information regarding a confirmed diagnosis of MH is needed (the phenotype).  The North American MH Registry of MHAUS contains detailed information concerning the clinical manifestations of MH in several thousand individuals, many of whom have been confirmed to have experienced an MH episode by the standard caffeine halothane contracture test.  Over one hundred have also had genetic testing done and many demonstrate the known "causative" mutations found in the ryanodine receptor gene.

 However, many have a DNA variant or mutation that has not been characterized fully.  In addition, many of the patients have not had their DNA analyzed at all. 

In order to make sense of the meaning of these changes in the DNA, it would be necessary to collect and compare data from many more patients and their family members.  By collecting such information, it may be possible to expand the list of ryanodine receptor DNA changes that can be determined to be "causal" (that is, to lead to) MH and progressively increase the applicability of DNA testing to more reliably help determine who is at risk for MH. 

Although there are many databases that list DNA changes in specific genes such as ClinVar (a product of the NIH) or the open Leiden database or the Human Genome Mutation Database, the weak link is that to the correlation with the clinical syndrome is missing.  This is the case because the vast majority of those at risk for MH only demonstrate a problem when exposed to anesthetic agents or perhaps on exposure to high environmental temperatures in conjunction with exercise and are otherwise asymptomatic.

Linking the clinical presentations of MH to the genetics is not necessarily a technically challenging feat. It just requires time and expertise, namely money.  For example, there are several clinical laboratories in the US performing molecular genetic testing by sequencing the ryanodine receptor gene and there are also several insurance companies that are paying for the test.  However, because of privacy issues, it is not possible, without the patient’s permission, to know how the genetic test relates to the clinical problems a particular patient experienced.  What some of us have found is that those who have undergone genetic testing would be willing to share the reasons for the test if their identity and that of their family were held in confidence.  I think that a detailed analysis of several hundred families who have had genetic testing for MH and where a family member experienced an MH episode, would improve the diagnostic applicability of the genetic test.  Over the next several months our Board of Directors and members of the hotline and the Professional Advisory Committee will begin a discussion of how to move this effort forward.  

The improvement of genetic testing accuracy has been accomplished for several disorders such as Cystic Fibrosis where genetic screening is recommended prior to considering having children.  In general, the clinical recognition of the disease state is much easier to recognize than MH.  I am hopeful that through the efforts of MHAUS, the North American MH Registry of MHAUS and other organizations such as the European MH group, great progress can be made in enhancing the accuracy and applicability of molecular genetic diagnosis of MH.

References/suggested reading

1. Assessing Genomic Sequencing Information for Health Care Decision Making.

A Roundtable on Translating Genomic-Based  Research for Health. The National Academies Press, Washington, D.C. www.nap.edu

2. Lu JT, Campeasu PM, Lee BH. Genotype-Phenotype Correlation-Promiscuity in the Era of Next-Generation Sequencing.  New England J. of Medicine. 371:593-596, 2014

 Copyright 2014.