Malignant Hyperthermia in North America: Genetic Screening of the Three Hot Spots in the Type I Ryanodine Receptor Gene

We studied 124 unrelated patients with a diagnosis of MHS. Diagnosis of MHS was provided according to North American MH CHCT.3Studies were approved by the Institutional Review Board of participating biopsy centers (Uniformed Services University of the Health Sciences, Bethesda, Maryland; University of California Davis, Davis, California; Thomas Jefferson University-Jefferson Medical College, Philadelphia, Pennsylvania; Wake Forest University, Winston Salem, North Carolina), and informed consent was obtained from each patient.16 

Genomic DNA was extracted from muscle tissue, peripheral blood, or buccal cells using a DNA purification kit (Qiagen, Valencia, CA) or the Wizard Genomic DNA Purification Kit (Promega, Madison, WI). Amplification was performed under standard polymerase chain reaction (PCR) conditions: denaturation at 95°C for 3 min followed by 45 cycles of 94°C for 30 s, annealing at primer specific temperatures (table 1) for 30 s, and 72°C for 30 s, with a final 7-min extension at 72°C. The PCR mixture contained 100 ng genomic DNA, 1.5–3.0 mm MgCl², Perkin Elmer PCR buffer (10 mm Tris-HCl [pH 8.3], 50 mm KCl), 20 pmol of each primer, 250 μm of each deoxyribonucleoside triphosphate, and 0.3 U AmpliTaq (Perkin Elmer, Foster City, CA) in a final volume of 20–30 μl. For PCR, we also used Master Amp PCR Pre-Mix D (Epicenter Technologies, Madison, WI) and Expand long PCR System (Boehringer Mannheim, Indianapolis, IN). Primers used in the study have been published or designed and synthesized in our laboratory (table 1).

Screening for mutations was performed by restriction fragment length polymorphism, single-strand conformation polymorphism (SSCP), or DHPLC analysis to test for the presence of known mutations in MH/CCD1 (exons 2, 6, 9, 11, 12, and 17), MH/CCD2 (exons 39, 40, 44, 45, and 46), and MH/CCD3 (exons 95, 100, 101, and 102) regions. PCR fragments were digested with 3–4 U restriction enzyme (New England Biolabs, Beverly, MA) according to manufacturer instructions. The digested products were resolved in a 3% NuSieve GTG® low-melting agarose gel (FMC BioProducts, Rockland, ME) or 20% polyacrylamide Novex TBE gel (Invitrogen, Carlsbad, CA) depending on their sizes. For SSCP analysis, PCR products were mixed with formamide in a ratio of 1:3 and, after denaturing for 5 min, were loaded onto a 20% polyacrylamide gel. The samples were to run overnight at 45 V and at 4°C. Denatured single-stranded DNA bands were visualized by staining with 1 μg/ml ethidium bromide. DHPLC analysis was performed using the Transgenomic WAVE DNA fragment analysis system (Transgenomic, Omaha, NE). Direct sequencing of PCR product was performed when changes in restriction site, single-strand DNA bands, or heteroduplex were detected. After purification using a QIAquick PCR Purification Kit (Qiagen), the PCR products were directly sequenced using the Big DyeTerminator Cycle Sequencing kit and analyzed on an automated fluorescence sequence apparatus (ABI 310 Genetic Analyzer; Perkin Elmer).

Relative changes in [Ca²⁺]ⁱin B cells were derived from changes in the fluorescence intensity of fluo-3 in peripheral mononuclear cells stained with phycoerythrin-conjugated anti-CD19 mAB. The measurements were performed using FACScan (Becton-Dickinson, San Jose, CA) as previously described.17The percentage of fluo-3⁺cells relative to unstimulated baseline was calculated and analyzed using the Consort 30 program (Becton-Dickinson).

One hundred twenty-four unrelated MHS patients were screened for the presence of mutations in MH/CCD1 (exons 2, 6, 9, 11, 12, and 17) and MH/CCD2 (exons 39, 40, 44, 45, and 46) regions using restriction fragment length polymorphism, SSCP, or DHPLC methods. A total of 13 RYR1 mutations (12 known and 1 novel) were detected in 28 patients in these two regions (table 1). The identified known mutations were Arg163Cys, Gly248Arg, Arg614Cyt, Arg2163Cys, Val2168Met, Thr2206Met, Val2214Ile, Ala2350Thr, Ala2367Thr, Asp2431Asn, Gly2434Arg, and Arg2454His (table 2). The most common mutation was Gly2434Arg, occurring among 4.7% of the studied patients, followed by Arg2454His (3.1%) and Arg614Cys (3.1%). One patient was found to have a novel nucleotide substitution, C7301T, in exon 45 (table 2). To determine whether this change was a normal polymorphism, we examined 108 chromosomes from healthy individuals by DHPLC and found that the mutation was absent. Direct sequencing of the patient sample in both 5′-to-3′ and 3′-to-5′ directions confirmed nucleotide substitution C7310T, which resulted in Ala2437Val amino acid change (fig. 1). This Ala at 2437 is evolutionally conserved, as shown in Drosophila melanoga  and Caenorhabditis elegans  (fig. 1C). The Ala2437Val mutation was found in a patient who, in his fifties, experienced a reaction suggestive of MH during surgery on his wrist during general anesthesia. The anesthetic agent was isoflurane following use of succinylcholine for tracheal intubation. After 1 h of anesthesia, there was an unexplained hypercapnia. The creatine kinase concentration was 40,000 U after surgery. CHCT conducted 10 yr after the episode recorded a contracture of more than 3.0 g in response to 3% halothane. The muscle histology test did not reveal any specific abnormalities.

We used a DHPLC method to test 124 MHS samples for the presence of mutations in exons 95, 100, 101, and 102. The DHPLC screening and direct DNA sequencing confirmed three different nucleotide substitutions: C-to-A substitution within intron 99 (−47 from the first nucleotide of exon 100); C-to-T substitution at nucleotide 14589, which preserves Phe at amino acid 4863; and T-to-C transition at nucleotide 14471, which causes a Leu4824Pro substitution (fig. 2). Absence of Leu4824Pro was confirmed in 108 normal chromosomes by screening exon 100 using DHPLC (fig. 2A) followed by DNA sequencing (fig. 2B). The substituted residue is also evolutionally conserved (fig. 2C). The patient, who carried Leu4824Pro, was a muscular 34-yr-old white man who had a maternal male first cousin who died of MH. The first cousin’s brother also had a positive diagnosis by CHCT. The CHCT results in the patient showed contracture tensions of 11.8, 12.2, and 8.9 g in three separate muscle strips after 3% halothane and 4.1, 4.6, and 5.2 g after 2 mm caffeine. Histopathologic examination from the muscle biopsy indicated central core myopathy. Because the B lymphocytes have been shown to express the functional RyR1 channel, we examined Ca²⁺response using B lymphocytes from this patient (US53). Measurement of Ca²⁺responses to the RyR1 agonists caffeine and 4-chloro-m-cresol (4-CmC) indicated that the Ca²⁺release was significantly greater as compared with B lymphocytes from control subjects (fig. 3A).

Using restriction fragment length polymorphism, SSCP, or DHPLC methods, we screened three selected MH/CCD regions for the RYR1 mutations, which included 15 exons and at least 25 known mutations. A total of 14 RYR1  mutations (12 known and 2 novel) were detected in 29 patients, estimating an incidence of 23% (table 2). Overall, 20 of the 29 mutations (69%), including a novel mutation, Ala 2437Val, originated from the MH/CCD2 region. In 8 patients (28%), mutations were identified in MH/CCD1 region. Screening the MH/CCD3 region yielded a novel mutation, Leu4824Pro, in a single patient with a diagnosis of CCD.

Numerous genetic studies have identified more than 40 missense mutations and several deletions linked to MH susceptibility. The mutations are clustered in three regions: the N-terminus MH/CCD1, the central MH/CCD2, and the C-terminus MH/CCD3.8In all three MH regions, mutations are also associated with CCD, a congenital myopathy that usually predisposes MHS.12Until recently, seven mutations have been identified in the N-terminal and the central region of the gene as being causative of CCD.18–21However, the C-terminal MH/CCD3 region has now been found as the major hot spot for CCD-causing mutations; to date, 15 mutations have been identified in this MH3 region.12–15As a result, the number of mutations in the MH/CCD3 region is approximately one third of all the mutations identified in the RYR1 . We studied the MH/CCD3 region because frequency of mutations in the C-terminus region has not been extensively studied for the MHS population. Among 124 unrelated MHS patients, we have found only one MH/CCD3 mutation in a single MHS patient. This Leu4824Pro mutation in exon 100 is in close proximity to Arg4825Cys that is reported to be associated with CCD.

There was evidence to suggest functional abnormalities in the RyR1 of patient US53, who carried the Leu4824Pro. As we previously reported, the functional RyR1 channels are expressed in B lymphocytes22and have been used to study phenotypic function14,17,23as well as genotype24,25of the RYR1  for MH and CCD. We observed that the intracellular concentrations of Ca²⁺in the B cells of patient US53 were significantly higher than those in B cells from controls in response to caffeine and 4CmC (fig. 3A). This augmented Ca²⁺response was similar to those reported for MH mutations in the MH/CCD1 and 2 regions, such as Arg614Cys and Val2168Met, respectively,17but in contrast to the Ca²⁺response found in some CCD mutations in the MH/CCD3 region. Four MH/CCD3 mutations, Arg4861His, Arg4893Trp, Ile4898Thr, and Gly4899Arg, have been all shown to cause spontaneous leakage from the RYR1 Ca²⁺pool and reduced Ca²⁺release by 4CmC in Epstein-Barr Virus–transformed B-cell lines.14It has also been recently shown that the two latter mutations cause excitation–contraction uncoupling.26Because contractile force in normal excitation–contraction coupling is proportional to myoplasmic Ca²⁺concentrations, reduced RyR1-mediated Ca²⁺release due to leaky Ca²⁺stores, excitation–contraction uncoupling, or both may explain muscle weakness in patients with CCD. Unlike with these four mutations, we found no spontaneous leakage of Ca²⁺but enhanced response to 4CmC in B cells from patient US53. Consistent with this finding, patient US53 exhibited no muscle weakness and had large contractile responses to the RyR1 agonists. Therefore, there may be different functional abnormalities in the RYR1  mutants that are associated with different subsets of CCD pathologies. Interestingly, the mutations, which have been found to cause leaky Ca²⁺, are localized in the sarcoplasmic luminal domain between M8 and M10, according to the new transmembrane model proposed by Du et al.  27(fig. 3B). Additional genetic linkage and gene transfection studies are needed to further define causality of this mutation for MH susceptibility and CCD pathology.

The detection rate of the screened RyR1 mutations is only 23% in our study population. Similar frequency to ours has recently been obtained in a study of 105 German MHS families.26It is currently unknown whether there are additional mutations in less investigated regions of the RYR1 or whether they are located in other genes. Determining this requires a sequence analysis of the entire RyR1 gene for every patient in whom a known mutation is not identified. We have previously reported that there were differences in frequencies for common MHS mutations between Western Europe and North America. Gly314Arg mutation, which is common in Europe,8,9was not detected in our population. The Arg614Cys, which is common in Germany and other central European countries,8,28was relatively less common in our MHS population (4%). In contrast, 4% of the North American MHS population had Arg2454His, which has been identified in only a single family in Europe.9As a result, frequencies for mutations in the MH/CCD1 region were relatively higher in Europe than in North America. The status of mutations in the MH/CCD3 region in Europe has not been well studied. In the North American MHS population, using the current screening protocol, only 1 in 124 patients was found to have a mutation in MH/CCD3, and this was associated with CCD. This is consistent with the fact that most of the mutations in the MH/CCD3 region have been associated with CCD.12However, it should be noted that inclusion of more exons for screening may lead to identification of novel mutations, as shown in a recent report by Galli et al.  29Classifying new mutations in the screening panel is a constant challenge for developing genetic testing for MH.

Although discovery of new mutations may change our statistics, using our current protocol, mutations in the MH/CCD2 region, within exons 39, 40, 44, 45, and 46, accounts for 72% of the MHS patients with positive mutations. Therefore, results from the current study suggest that initial screening for mutations in the MH/CCD2 region may be the most time- and cost-effective strategy for the North American MHS population at this time. We found the DHPLC method highly effective for screening mutations in the RYR1 gene, especially for the six exons in the MH/CCD2 regions. Further, the DHPLC method produced more accurate results and was more time efficient to perform than SSCP in screening the MH/CCD2 region. However, more investigations are needed to discuss sensitivity and specificity of DHPLC.

The MH genetics working group in North America encourages use of genetic information from the RYR1  gene to develop genetic testing for MH and CCD.11Considering the complex etiology of MH, use of genetic information for MH cannot replace CHCT as the standard diagnostic test for MH in the near future. However, in those families in which a familial causative mutation is identified, genetic screening of other members of the family can be beneficial. For this reason, the North American MH genetics working group recommends continuous and collaborative research efforts to advance genetic screening and to make appropriate revisions/recommendations to the mutation panel for molecular genetic studies of MH as well as CCD.