Genomic Screening for Malignant Hyperthermia Susceptibility

Accurate phenotyping is essential in the genetic investigation of any trait. Singly, none of the clinical signs of an MH reaction is specific, but a nascent reaction can be recognized by an astute clinician, and the management imperative is to abort a reaction as soon as it is suspected. It is now rare for a reaction to reach such a fulminant stage that the diagnosis is unequivocal. Even when the proband’s diagnosis could be made on the basis of their clinical reaction, clinical phenotyping for other family members is challenging. Scientific advances in the genetics of MH have substantially been founded on the use of the MH susceptibility phenotype determined using contracture testing. Indeed, the original identification of the RYR1 and CACNA1S susceptibility loci, and many other large genetic studies of MH have come from countries where contracture testing of affected families is quality-controlled and practicable.

While it might be ideal that contracture testing is universally available, there are numerous barriers to this goal that are beyond the scope of this commentary. Therefore, the development of a physiologic diagnostic confirmation test that is analytically robust, but which can use tissue that can be sampled locally (ideally less invasively) and transported to the testing center, would greatly improve accessibility to MH testing. The challenge here is daunting—although we would be eager to work toward an alternative clinical phenotyping test, there are no existing data to our knowledge that point to a ready path to such an assay.

Genomic technologies are rapidly advancing, primarily due to chip-based DNA testing platforms¹³  and next generation sequencing.¹⁴  Whereas Sanger sequencing of RYR1 and CACNA1S has been, and remains, expensive, next generation sequencing panel tests that include these genes are now available at costs well below that of Sanger sequencing. Next generation exome and genome sequencing are increasingly available in many countries and becoming an affordable part of research and health care. These rapid advances and falling costs enable both research and clinical genomic testing that were inconceivable just a few years ago. They enable rapid identification of sequence variants in individuals with putative inherited diseases. However, these variants may number several thousand in each sample and predicting which variant(s) is(are) implicated in the disease can be challenging. In MH susceptibility where a single missense variant may be all that is required, once variants in RYR1, CACNA1S, and STAC3 have been excluded, this approach has proved fruitless to date. So far, relatively few samples from MH-susceptible individuals have undergone exome or genome sequencing. If a larger number can be sequenced, we will more likely be able to identify rare recurrent variants or genes that have an increased burden of rare variants. We propose that there should be a coordinated program of clinical and research testing such that every individual with an MH reaction or positive contracture test is evaluated by next generation sequencing to increase the chances of identifying the causative variant(s). This should be a mix of both clinical testing and clinical research testing. Clinical sequencing of known MH susceptibility–associated genes is available from a number of laboratories (see the appendix). Deidentified data from all who are sequenced and found to harbor a pathogenic, or likely pathogenic variant (determined as per Richards et al. and Harrison et al.^15,16 ), should be deposited in a public repository, such as ClinVar or a dedicated MH database so that all can benefit from this knowledge. Individuals who are not found to have an unambiguously pathogenic variant should be referred to a clinical research program to be further evaluated to better understand the genetic basis of this disease. The pooling and organization of these cases and data will add immeasurably to efforts to fully catalog genetic variation associated with MH susceptibility.

Escalation to next generation sequencing may also prove useful in cases where MH susceptibility is apparently more genetically complex,^12,17  especially if combinations of rare variants are involved. However, if the genetic model involves combinations of several more common variants, sample sizes will need to be even larger and single nucleotide polymorphism chip genotyping is likely to be more cost-effective.

Efforts are underway to comprehensively characterize the pathogenicity of reported variants in RYR1 and CACNA1S through the ClinGen Variant Curation Expert Panel process (https://www.clinicalgenome.org/affiliation/50038/, accessed August 25, 2020). This effort is initially focused on the variants proposed by the European Malignant Hyperthermia Group (https://www.emhg.org/, accessed August 25, 2020), with an adaptation of the American College of Medical Genetics and Genomics variant pathogenicity standards.¹⁵  These standards, which must be adapted to take into account knowledge of the biology of RYR1 and malignant hyperthermia susceptibility, comprise 27 criteria including observations of inheritance, case-control studies, functional studies, and in silico predictors, and conforms to the current international standard for variant pathogenicity assertions. A major deficit in being able to assign pathogenic status is the small number of variants that have been robustly functionally characterized in relevant model systems. Current testing is robust, but low throughput. A recent revolution in functional genomics heralds a realistic prospect of overcoming this bottleneck. Prime editing, an adaptation of CRISPR technology,¹⁸  coupled with strategies for high-throughput functional assays¹⁹  have the potential to support highly robust functional assessments of all variants, even before they are detected in a human. If these technologies can be adapted to RYR1 and other genes implicated in MH susceptibility, they have the potential to provide for high-throughput, low-cost, functional in vitro assays for every potential variant. This task is not trivial, but it should be feasible. Even achieving a goal of assessing the pathogenicity of variants that can account for 80% of known cases of MH susceptibility would create a set of pathogenic variants that could be employed for clinical research to test the practicality of preoperative screening.

A future is coming where large numbers of individuals undergo genome-wide screening that encompasses many disease and pharmacogenetic susceptibilities: it is essential to develop evidence to support this approach on a disease-by-disease basis. We propose that a trial of preoperative screening for MH susceptibility would serve as a proof of principle to test the applicability and utility of extracting MH-associated variants from genomic or exomic data. Once a suitable set of pathogenic variants is identified, genomic screening for MH susceptibility could be piloted on a population of individuals scheduled for elective surgery. The size and power analysis of such a study will require more accurate estimates of prevalence and the diagnostic yield of a given set of pathogenic variants. We propose that this could be fruitful, even without a clear understanding of penetrance of the variants, because one could eliminate MH reactions if every person with an at-risk genotype was administered a nontriggering agent. While general population screening for MH susceptibility will likely not be practical for some time, an ever-increasing number of individuals with variants in RYR1 and CACNA1S are being identified through secondary findings from exome and genome sequencing.^20,21  These individuals provide opportunities to study and pilot approaches to presymptomatic diagnosis. When MH-susceptible individuals are identified through preoperative screening or secondary findings, and there is no personal or family history of suspected MH, the presence of the variant represents the only known risk factor for MH susceptibility in the family. Identifying an individual with MH susceptibility is an opportunity to classify all members within a family, where the risk of having MH (50% for first-degree relatives) is orders of magnitude greater than the general population. Prospective determination of risk of relatives can therefore be made by testing for the single variant, which is simpler to perform and interpret than is exome, panel, or even full gene testing as the laboratory does not need to interpret other variants.

A genetic test, even with physiologic confirmation is not enough: these individuals also need access to a knowledgeable provider (most likely anesthesiologist, neurologist specializing in myopathy, or a clinical geneticist) to engage with the affected individual to analyze the test results, make the clinical–molecular diagnosis, and educate the patient, their family, and care provider about their disorder. The affected individual is a key part of the puzzle—it will be critical that they accept and understand their diagnosis and its implications to maximize the likelihood that the information is used to their benefit. Support groups such as Malignant Hyperthermia Association of the United States (in North America) can be helpful to identify experts and provide information (see links in the appendix).

There must be support also for care providers unfamiliar with incorporating genomic test information into anesthetic management decisions. Taking the data from the advances we propose into account, professional bodies (such as the American Society of Anesthesiologists) will need to develop policies and practice standards that are based on the risk stratification of genomic predictive testing. An analogous approach has been adopted in obstetrics, where the highly complex noninvasive prenatal genomic screening test has been rapidly taken up, with clear guidelines and risk determinations. Such guidelines are no guarantee of good care, nor are they a perfect shield from liability, but they give providers clear guidance and substantially lessen risks.

In North America, the Malignant Hyperthermia Association of the United States provides 24/7 hotline support for clinicians managing patients with a known or suspected MH reaction (similar resources are available in other countries). These valuable resources should be universally recognized and readily used, but additional resources for the identification and management of MH should be developed. Artificial intelligence–driven²²  patient monitoring and clinical decision support tools²³  within the electronic health record could be developed to support preoperative decision-making regarding test results, facilitate real-time early recognition of an MH event, and other decisions. Finally, information on MH susceptibility should be readily portable so that patients can benefit from it no matter where they receive their care.

None of these approaches alone will accomplish our objective. Instead, we recognize that it will be essential that research, screening, education, and management are integrated into a functional whole systems–based approach to end deaths from MH. This proposal is centered on a genomics-centered approach to MH susceptibility. This is not to say that a major, disruptive advance in muscle physiologic testing could not occur that would change this assessment entirely—disruptive advances are by their nature unpredictable. We propose a model for organizing the necessary genetic and anesthetic activities needed to build an integrated system that will capture all events, maximize knowledge, and reduce death and disability from this disease. We propose a flow diagram that incorporates all of these activities and builds data and expertise into the future (fig. 1).

Population screening for genetic disease is not risk-free. There will be false-positive and false-negative results. The risk of false positives would be that individuals would be offered nontriggering agents unnecessarily. False negatives could lead to very rare occurrences of MH. There is also a risk that by reducing the incidence of MH, anesthesiologists would be become less familiar with the recognition and management of a reaction. While the United States’ Genetic Information Nondiscrimination Act should be protective in most cases, it is possible that some individuals who are diagnosed by screening (true or false positive) could be, for example, denied entry to the Armed Forces.

We recognize that these goals are grand and challenging. We also recognize, and indeed hope, that others will debate and help us to refine our proposals and weigh in with different approaches that could help us work toward the goal of ending deaths from MH.

This work was supported by National Institutes of Health (Bethesda, Maryland) grant No. AR053349 (to Dr. Dirksen) and grant Nos. HG200359 and HG200387 (to Dr. Biesecker).

Dr. Weber is the CEO and part owner of Prevention Genetics (Marshfield, Wisconsin). Dr. Biesecker reports research support from ArQule (now wholly owned by Merck, Inc., Kenilworth, New Jersey), Pfizer, Inc. (New York, New York), and honoraria from Cold Spring Harbor Laboratory (Cold Spring Harbor, New York). Dr. Riazi reports research support from Norgine Pharmaceuticals (Amsterdam, the Netherlands). The remaining authors declare no competing interests.

Genetic Test Registry for RYR1 testing: https://www.ncbi.nlm.nih.gov/gtr/all/tests/?term=6261%5Bgeneid%5D. Accessed August 25, 2020.

Genetic Test Registry for CACNA1S testing: https://www.ncbi.nlm.nih.gov/gtr/all/tests/?term=779%5Bgeneid%5D. Accessed August 25, 2020.

Malignant Hyperthermia Association of the United States (MHAUS): https://www.mhaus.org. Accessed August 25, 2020.

The North American Malignant Hyperthermia Registry of MHAUS: https://anest.ufl.edu/namhr. Accessed August 25, 2020.

European Malignant Hyperthermia Group (EMHG): https://www.emhg.org. Accessed August 25, 2020.