he third MH Molecular Genetic/Diagnostic Testing Workshop, sponsored by the Malignant Hyperthermia Association of the United States (MHAUS), was held on September 7-8, 2007, at the Children’s Hospital of Philadelphia. Attendees included representatives of seven U.S. and Canadian MH biopsy centers, three molecular genetics laboratories, the North American MH Registry, MHAUS MH Hotline consultants, and U.S. and European MH researchers. The attendees made the following consensus recommendations:

1. Multiple speakers emphasized the importance of having an accurate phenotype or clinical characterization established for an individual and his/her family prior to testing that family:
Phenotyping can be done only by obtaining a careful anesthetic and medical history of a full-blown MH event or with a diagnostic MH muscle biopsy (caffeine halothane contracture test). All North American researchers have found a low yield for genetic testing when there is a weak phenotype. To improve phenotyping, anesthesiologists, nurse anesthetists and intensivists should report adverse events to the North American MH Registry, using its downloadable “AMRA” form (PDF format) at www.mhreg.org. The detailed information contained in that form is essential for characterizing the clinical presentation of MH.

2. Patients and their families need improved access to MH diagnostic testing:
The development of a less invasive test for MH depends on continued use of the diagnostic MH muscle biopsy with contracture testing. Five U.S. centers continue to perform this test, all of which are in academic anesthesiology departments [Table 1]. These diagnostic centers depend on departmental/institutional support for their survival, as patients continue to have problems convincing their health insurers to pay for the test. In 2008, loss of support closed a busy biopsy center in Philadelphia.

In the past 3.5 years, only 147 diagnostic muscle biopsies were performed (73 positive). Roughly half of these MH-positive patients have undergone genetics testing; 10 have ryanodine receptor (RyR1) variants associated with MH, allowing less invasive testing of other family members. For the families of the 74 patients who tested negative for MH, no further testing is required, and they can be anesthetized safely with triggering agents. A negative biopsy also allows individuals to pursue careers in the military or foreign service.

3. Genetics studies can be used in specific situations:
There are two MH loci with causative mutations (chromosome 19q13.1, RyR1 gene; chromosome 1q32, CACNA1S gene). A total of 174 RyR1 DNA variants have been identified in European descendents, but only 28 are proven causative mutations. A variant is considered a causative mutation when these four criteria are met: full description of the genetic mutation, both at the DNA and protein level; co-segregation of the mutation with the disease in at least two family pedigrees; absence of the identified variant in 100 non-MH control subjects; and functional characterization of the mutation in myotubes, microsomal sarcoplasmic reticulum, lymphoblasts or MH “knock-in” animals. Gene variants vary by country and racial group. Such complexity requires great care in interpreting results for patients and their families.

In Europe, only families whose proband’s MH-susceptibility has been confirmed by diagnostic muscle biopsy are eligible for genetics studies. With this method, a variant or mutation is identified in 40 percent of families; the remaining 60 percent must rely on biopsies for MH diagnosis. In the United States, either a well-documented clinical history or a positive muscle biopsy are used to define the proband’s MH-susceptibility. There are two certified clinical genetics laboratories doing MH testing. The University of Pittsburgh did a 12 exon analysis that identified variants in 12 percent of families. The number of sequenced exons has now been increased to 16. Genetics counseling also is offered at the Center for Medical Genetics and, with consent, provides data to the MH Registry. Prevention Genetics, a private firm, analyzes 38 exons in three tiers and has identified variants in 24 percent of families. The firm does not provide counseling and gives results only to the referring physician.

A third laboratory at the Uniform Services University of the Health Sciences (USUHS) does genetics research on MH but is not a certified clinical laboratory. By analyzing 30 exons, this site has identified 45 variants in 48 percent of families. MHAUS is supporting USUHS in screening 100 well-phenotyped MH-susceptible patients. USUHS is also testing survivors of MH-associated cardiac arrest and will test individuals who had a positive muscle biopsy but negative results from clinical genetics testing.

Patient testing at USUHS is coordinated through Barbara Brandom, M.D., at the MH Registry. In addition, MHAUS is offering financial assistance for patients with a positive muscle biopsy to be tested at one of the clinical genetics laboratories. Interested patients can contact MHAUS (607) 674-7901 or the Center for Medical Genetics at the University of Pittsburgh Medical Center (800) 454-8155.

Marilyn Green Larach, M.D., F.A.A.P., is Associate Professor of Anesthesiology, Penn State College of Medicine, Hershey, Pennsylvania. She is a Senior Research Associate, North American Malignant Hyperthermia Registry of MHAUS.