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A rare genetic variant of the ryanodine receptor in a suspected malignant hyperthermia susceptible patient
Journal of Clinical Anesthesia (2016) 33, 144–146
Case report
A rare genetic variant of the ryanodine receptor in a suspected malignant hyperthermia susceptible patient☆,☆☆,★,★★ Emily Jane MacKay DO (Cardiothoracic Anesthesia and Surgical Critical Care Fellow)⁎,1 , Carlos Wilkerson MD (Instructor of Anesthesiology), Natalia Kraeva PhD (Research Associate)2 , Henry Rosenberg MD (Professor of Anesthesiology)3 , Tara Kennedy MD (Assistant Professor of Anesthesiology) Thomas Jefferson University Hospital, Philadelphia, PA Received 30 December 2015; revised 13 February 2016; accepted 18 February 2016
Keywords: Malignant hyperthermia; Ryanodine receptor; Genetic variation; Genetic testing in MH; Dantrolene; Case report
Abstract Malignant hyperthermia (MH) remains a diagnostic challenge. This case report describes the anesthetic management of a suspected intraoperative MH episode and the subsequent, genetic sequence analysis of 3 genes associated with MH. The results of the molecular genetic testing revealed heterozygosity for a rare variant, c.12553G N A (p.Ala4185Thr), in the RYR1 gene encoding the ryanodine receptor. Although the RYR1 gene has previously been implicated in the pathogenesis of MH, (1) this particular variant has only been reported in one other case of MH; (2) the role for diagnostic genetic testing in the diagnosis of MH will be examined. © 2016 Elsevier Inc. All rights reserved.
☆ For Editorial Office: This report was previously presented, in part, at the IARS, March, 2015 ☆☆ The author states that the report describes the care of one or more patients. The patient consented to publication of the report. This is described in the report. ★ Funding: No outside funding was obtained for this study. ★★ All authors reported no conflicts of interest. ⁎ Corresponding author at: Thomas Jefferson University Hospital, 111 South 11th Street, Philadelphia, PA 19107. Tel.: + 1 267 693 7855, + 1 412 728 8793 (mobile). E-mail addresses: [email protected], [email protected] (E.J. MacKay), [email protected] (C. Wilkerson), [email protected] (N. Kraeva), [email protected] (H. Rosenberg), [email protected] (T. Kennedy). 1 Current Affiliation: The Hospital of the University of Pennsylvania, Department of Anesthesiology and Critical Care, Dripps Library, 5 Dulles Building, 3400 Spruce Street Philadelphia, PA 19104. 2 Current Affiliation: MH Investigation Unit, Toronto General Hospital, UHN, Toronto ON, Canada. 3 Current Affiliation: Saint Barnabas Medical Center, Livingston, NJ.
http://dx.doi.org/10.1016/j.jclinane.2016.02.038 0952-8180/© 2016 Elsevier Inc. All rights reserved.
1. Introduction Written permission was obtained from the patient for the publication of this case report. The case report describes the anesthetic management of a suspected intraoperative malignant hyperthermia (MH) episode and subsequent sequence analysis of three genes that are associated with MH (RYR1, CACNA1S, and STAC3). The molecular genetics analysis revealed heterozygosity for a rare variant, c.12553G N A (p.Ala4185Thr), in the RYR1 gene encoding the ryanodine receptor; well established in the pathogenesis of MH [1]. Bioinformatics analysis indicated a possible functional impact of this variant. This variant has only been reported once before, in another MH susceptible patient [2].
145
2. Case report A healthy, athletic, 82 kg 17-year-old man presented for a cervical spine fusion to repair a traumatic C5 vertebral body burst fracture, which had been initially managed with a halo brace. The patient was scheduled for an anterior C5 corpectomy with anterior and posterior C4 to C6 fusion, utilizing sensory and motor evoked potential monitoring. The patient was fiberoptically intubated awake, using titrated doses of midazolam, glycopyrrolate, and remifentanil. Vascular access was achieved with 2 16 g peripheral intravenous catheters and a radial arterial catheter. Intraoperatively, total intravenous anesthetic (TIVA) was administered, consisting of propofol and sufentanil infusions. Once neuromonitoring concluded, TIVA infusions were discontinued and an inhalational anesthetic agent initiated. Over the following 45 minute time period, the end-tidal CO2 (ETCO2) concentration had steadily increased from 30 to 60 mm Hg, the heart had increased to over 90 bpm, and the minute ventilation increased from 3 liters/minute (L/m) on pressure support ventilation to greater than 14 L/m on spontaneous ventilation. Core esophageal temperature at that time was 36.9 °C. In preparation for extubation, the patient was placed supine and the head released from the Mayfield pins. While still intubated, the patient started to have severe rigors, heart rate and blood pressure increased to 130 bpm and 190 mmHg systolic respectively. Spontaneous minute ventilation increased to greater than 20 L/m on 100% high flow oxygen despite an ETCO2 greater than 70 mm Hg and the temperature increased to 38.4 °C. The differential diagnosis included shivering during emergence from general anesthesia, myoclonus, and malignant hyperthermia. Given the continued and rapidly worsening hemodynamics, rising ETCO2 and rising temperature, the decision was made to empirically treat for MH. An initial, diagnostic arterial blood gas (ABG) was sent, MH kit with dantrolene, code cart, and additional personnel were called to the operating room. Resuscitation was initiated with rapid infusion of crystalloid, 500 mg of calcium chloride, 45 mEq of sodium bicarbonate, and dantrolene. Charcoal filters were placed on the machine, the CO2 absorbent and machine circuit were replaced, an external cooling blanket was set to 32 °C, ice packs were applied to the groin and axillae, a defibrillator was applied, and the MH hotline was consulted. The patient improved dramatically after the initiation of dantrolene; total dose administered was 225 (2.75 mg/kg for the 82 kg patient). The heart rate decreased to 80 bpm, systolic blood pressure decreased from 208 to 150 mm Hg, spontaneous minute ventilation decreased to 13 L/m, and the rigors stopped. The results of the diagnostic ABG, sent prior to dantrolene administration, returned as follows: pH: 7.25, PCO2 57 mmHg, PaO2 332 mmHg (100% FIO2), HCO3− 24 mmol/L, base excess − 1.9 mmol/L, O2Sat 100%, K+ 4.2 mmol/L. A repeat ABG along with comprehensive laboratory studies were sent 15 minutes following the initial ABG. The
complete blood count, PT, PTT, fibrinogen, D-dimer, chemistry and liver panel, thyroid stimulating hormone and creatinine kinase all returned normal results. The serum myoglobin returned elevated at 184 ng/mL (RR, 1-100 ng/mL), lactate returned elevated at 3.1 mmol/L (RR, 0.5-2.2 mmol/L). Repeat arterial blood gas returned as follows: pH 7.39, PCO2 42 mm Hg, PaO2 410 mm Hg (100% FIO2), HCO3− 25 mmol/L, base excess + 0.6 mmol/L, O2 saturation 100%, K + 4.7 mmol/L. The patient remained intubated and was transferred to the surgical intensive care unit. Serial creatinine kinases drawn 2 and 4 hours following the end of the case returned 202 IU/L and 264 IU/L respectively (RR, 25–215 IU/L). A lactate, drawn 2 hours following the end of the case returned normalized at 1.2 mmol/L (RR, 0.5-2.2 mmol/L). The patient was extubated 6 hours following the end of the case, had a stable hospital course, and was discharged from the hospital on post-operative day 2 in excellent condition. There were multiple discussions with the patient's parents during the hospitalization and after discharge stressing the importance of obtaining a Caffeine Halothane Contracture Test (CHCT), as it remains the gold standard for the diagnosis of MH. Despite the information provided, the parents elected instead for DNA testing. The genes sequenced were RYR1, CACNA1S, and STAC3. The molecular genetics analysis revealed heterozygosity in exon 90 of the RYR1 gene for a sequence variant designated c.12553G N A (p.Ala4185Thr).
3. Discussion There are two diagnostic tests available for the diagnosis of MH: CHCT, and genetic testing that is often limited to the sequence analysis of the RYR1 gene. CHCT is the gold standard, although there are limitations in its specificity [3], as well as variation among testing centers [4]. It requires a skeletal muscle biopsy from the thigh, which can be performed under general or neuraxial anesthesia, at a designated MH muscle biopsy center. Sensitivity is close to 100%, and false negatives are rare [3]. Specificity is about 80%, with a 20% false positive rate [3]. Genetic testing is newly emerging as a viable alternative to the caffeine halothane contracture test in certain cases. Via DNA samples, genetic testing aims to find mutations in the genes responsible for contraction-excitation coupling and calcium regulation in skeletal muscle, namely RYR1, CACNA1S, and STAC3, that have been associated with MH [5]. Recently novel RYR1 mutations have been reported by genetic testing and confirmed by CHCT; eg, Met.4230.Arg [6] RYR1 codes for the ryanodine receptor type 1 (RYR1), which is responsible for calcium release in the sarcoplasmic reticulum, and has been implicated in 50–70% of MH susceptibility cases, but their role in the pathogenicity of MH has not been determined. More than 300 RYR1 variants have been identified so far, but their role in MH has not been yet clarified; several of the
146 RYR1 variants have also been associated with central core disease [2,7]. A 2013 study published in Anesthesiology, 2013 investigated the genetic penetrance of RYR1 and CACNA1S variants in a random, 870-cohort population [1]. The findings demonstrated that the RYR1 p.Ala4185Thr variant was not found to be penetrant in a random selection [1]. The study cites the variant as a rare variant of unknown significance; indicating that further investigation is warranted for correlation with MH susceptibility [1]. The CACNA1S variant, which codes for the α1 subunit of dihydropyridine voltage-gated calcium channel, account for only 1% of MH cases. Lastly, STAC3 has been recently added to the MH genetic testing panel, particularly after being linked with Native American myopathy [8]. Genetic testing is often favored in patients whose MH susceptibility status is confirmed, or whose relatives are confirmed with the CHCT [9]. In addition, genetic testing may benefit patients for whom the CHCT may be contraindicated. Genetic testing confers the advantage of eliminating invasive surgery for an open muscle biopsy at specific MH testing centers, and being cost-effective. The main drawback of the genetic testing is low sensitivity due to the heterogeneity of the condition. Only about 50% of MHS patients will have a mutation in one of the known MH candidate genes. The complex genetic nature of MH is the reason why negative genetic results cannot rule out a diagnosis of MH susceptibility. In this case report, a RYR1 variant of unknown significance, c.12553G N A (p.Ala4185Thr), was identified in a young male patient who experienced an MH event with Clinical Grading Score of 53; indicating he should be tested for MH susceptibility [10]. This variant is quite rare: it was observed only once in the sample population of 1088 people, ie, its minor allele frequency is 0.0002 (rs151119428, NCBI dbSNP) [2]. The variant has been reported previously in a MH susceptible individual who carried a second, potentially causative RYR1 variant and, thus, significance of the p.Ala4185Thr variant remains unclear [1,2]. Bioinformatics evaluation of the variant pathogenic potential using 4 software tools, PolyPhen-2 [11], SIFT [12], Provean [13], and MutationTaster [14] gave ambiguous prediction results: p.Ala4185Thr was found to be probably deleterious and damaging by MutationTaster and SIFT, and possibly damaging and neutral by PolyPhen-2 and Provean. Functional studies are needed to fully establish the p.Ala4185Thr variant's effect on the RYR1 function as well as its potential role in the pathogenesis of MH.
4. Conclusion Malignant hyperthermia remains a diagnostic challenge, and anesthesiologists must have a high index of suspicion in the presence of intraoperative nonspecific signs such as tachy-
E.J. MacKay et al. cardia or rising end-tidal carbon dioxide. This patient had a suspected malignant hyperthermia episode intraoperatively with a Clinical Grading Score of 53 [10], which indicates an “almost certain” MH episode. The genetic testing revealed heterozygosity for a rare variant in the gene that encodes the RYR1 receptor, of which there is evidence to the involvement in the pathogenesis of MH. However, in the setting of a clinically suspicious MH episode in the presence of a RYR1 genetic variant of unknown significance, the patient must be considered MH susceptible.
References [1] Gonsalves SG, Ng D, Johnston JJ, Teer JK, Stenson PD, Cooper DN, et al. Using exome data to identify malignant hyperthermia susceptibility mutations. Anesthesiology 2013;119:1043-53. [2] Kraeva N, Riazi S, Loke J, Frodis W, Crossan ML, Nolan K, et al. Ryanodine receptor type 1 gene mutations found in the Canadian malignant hyperthermia population. Can J Anaesth 2011;58:504-13. [3] Allen GC, Larach MG, Kunselman AR. The sensitivity and specificity of the caffeine-halothane contracture test: a report from the North American malignant hyperthermia registry. the North American malignant hyperthermia registry of MHAUS. Anesthesiology 1998;88:579-88. [4] Ording H, Islander G, Bendixen D, Ranklev-Twetman E. Betweencenter variability of results of the in vitro contracture test for malignant hyperthermia susceptibility. Anesth Analg 2000; 91:452-7. [5] Rosenberg H, Sambuughin N, Riazi S, Dirksen R. In: Pagon RA, Adam MP, Ardinger HH, Wallace SE, Amemiya A, Bean LJH, et al, editors. Malignant Hyperthermia Susceptibility. Seattle (WA): GeneReviews(R); 1993. [6] Brandom BW, Bina S, Wong CA, Wallace T, Visoiu M, Isackson PJ, et al. Ryanodine receptor type 1 gene variants in the malignant hyperthermia-susceptible population of the United States. Anesth Analg 2013;116:1078-86. [7] Rosenberg H, Rueffert H. Clinical utility gene card for: malignant hyperthermia. Eur J Hum Genet 2011:19. [8] Horstick EJ, Linsley JW, Dowling JJ, Hauser MA, McDonald KK, Ashley-Koch A, et al. Stac3 is a component of the excitation-contraction coupling machinery and mutated in native American myopathy. Nat Commun 2013;4:1952. [9] Toppin PJ, Chandy TT, Ghanekar A, Kraeva N, Beattie WS, Riazi S. A report of fulminant malignant hyperthermia in a patient with a novel mutation of the CACNA1S gene. Can J Anaesth 2010; 57:689-93. [10] Larach MG, Localio AR, Allen GC, Denborough MA, Ellis FR, Gronert GA, et al. A clinical grading scale to predict malignant hyperthermia susceptibility. Anesthesiology 1994;80:771-9. [11] Adzhubei IA, Schmidt S, Peshkin L, Ramensky VE, Gerasimova A, Bork P, et al. A method and server for predicting damaging missense mutations. Nat Methods 2010;7:248-9. [12] Kumar P, Henikoff S, Ng PC. Predicting the effects of coding nonsynonymous variants on protein function using the SIFT algorithm. Nat Protoc 2009;4:1073-81. [13] Choi Y, Sims GE, Murphy S, Miller JR, Chan AP. Predicting the functional effect of amino acid substitutions and indels. PLoS One 2012;7: e46688. [14] Schwarz JM, Rodelsperger C, Schuelke M, Seelow D. MutationTaster evaluates disease-causing potential of sequence alterations. Nat Methods 2010;7:575-6.
Case report
A rare genetic variant of the ryanodine receptor in a suspected malignant hyperthermia susceptible patient☆,☆☆,★,★★ Emily Jane MacKay DO (Cardiothoracic Anesthesia and Surgical Critical Care Fellow)⁎,1 , Carlos Wilkerson MD (Instructor of Anesthesiology), Natalia Kraeva PhD (Research Associate)2 , Henry Rosenberg MD (Professor of Anesthesiology)3 , Tara Kennedy MD (Assistant Professor of Anesthesiology) Thomas Jefferson University Hospital, Philadelphia, PA Received 30 December 2015; revised 13 February 2016; accepted 18 February 2016
Keywords: Malignant hyperthermia; Ryanodine receptor; Genetic variation; Genetic testing in MH; Dantrolene; Case report
Abstract Malignant hyperthermia (MH) remains a diagnostic challenge. This case report describes the anesthetic management of a suspected intraoperative MH episode and the subsequent, genetic sequence analysis of 3 genes associated with MH. The results of the molecular genetic testing revealed heterozygosity for a rare variant, c.12553G N A (p.Ala4185Thr), in the RYR1 gene encoding the ryanodine receptor. Although the RYR1 gene has previously been implicated in the pathogenesis of MH, (1) this particular variant has only been reported in one other case of MH; (2) the role for diagnostic genetic testing in the diagnosis of MH will be examined. © 2016 Elsevier Inc. All rights reserved.
☆ For Editorial Office: This report was previously presented, in part, at the IARS, March, 2015 ☆☆ The author states that the report describes the care of one or more patients. The patient consented to publication of the report. This is described in the report. ★ Funding: No outside funding was obtained for this study. ★★ All authors reported no conflicts of interest. ⁎ Corresponding author at: Thomas Jefferson University Hospital, 111 South 11th Street, Philadelphia, PA 19107. Tel.: + 1 267 693 7855, + 1 412 728 8793 (mobile). E-mail addresses: [email protected], [email protected] (E.J. MacKay), [email protected] (C. Wilkerson), [email protected] (N. Kraeva), [email protected] (H. Rosenberg), [email protected] (T. Kennedy). 1 Current Affiliation: The Hospital of the University of Pennsylvania, Department of Anesthesiology and Critical Care, Dripps Library, 5 Dulles Building, 3400 Spruce Street Philadelphia, PA 19104. 2 Current Affiliation: MH Investigation Unit, Toronto General Hospital, UHN, Toronto ON, Canada. 3 Current Affiliation: Saint Barnabas Medical Center, Livingston, NJ.
http://dx.doi.org/10.1016/j.jclinane.2016.02.038 0952-8180/© 2016 Elsevier Inc. All rights reserved.
1. Introduction Written permission was obtained from the patient for the publication of this case report. The case report describes the anesthetic management of a suspected intraoperative malignant hyperthermia (MH) episode and subsequent sequence analysis of three genes that are associated with MH (RYR1, CACNA1S, and STAC3). The molecular genetics analysis revealed heterozygosity for a rare variant, c.12553G N A (p.Ala4185Thr), in the RYR1 gene encoding the ryanodine receptor; well established in the pathogenesis of MH [1]. Bioinformatics analysis indicated a possible functional impact of this variant. This variant has only been reported once before, in another MH susceptible patient [2].
145
2. Case report A healthy, athletic, 82 kg 17-year-old man presented for a cervical spine fusion to repair a traumatic C5 vertebral body burst fracture, which had been initially managed with a halo brace. The patient was scheduled for an anterior C5 corpectomy with anterior and posterior C4 to C6 fusion, utilizing sensory and motor evoked potential monitoring. The patient was fiberoptically intubated awake, using titrated doses of midazolam, glycopyrrolate, and remifentanil. Vascular access was achieved with 2 16 g peripheral intravenous catheters and a radial arterial catheter. Intraoperatively, total intravenous anesthetic (TIVA) was administered, consisting of propofol and sufentanil infusions. Once neuromonitoring concluded, TIVA infusions were discontinued and an inhalational anesthetic agent initiated. Over the following 45 minute time period, the end-tidal CO2 (ETCO2) concentration had steadily increased from 30 to 60 mm Hg, the heart had increased to over 90 bpm, and the minute ventilation increased from 3 liters/minute (L/m) on pressure support ventilation to greater than 14 L/m on spontaneous ventilation. Core esophageal temperature at that time was 36.9 °C. In preparation for extubation, the patient was placed supine and the head released from the Mayfield pins. While still intubated, the patient started to have severe rigors, heart rate and blood pressure increased to 130 bpm and 190 mmHg systolic respectively. Spontaneous minute ventilation increased to greater than 20 L/m on 100% high flow oxygen despite an ETCO2 greater than 70 mm Hg and the temperature increased to 38.4 °C. The differential diagnosis included shivering during emergence from general anesthesia, myoclonus, and malignant hyperthermia. Given the continued and rapidly worsening hemodynamics, rising ETCO2 and rising temperature, the decision was made to empirically treat for MH. An initial, diagnostic arterial blood gas (ABG) was sent, MH kit with dantrolene, code cart, and additional personnel were called to the operating room. Resuscitation was initiated with rapid infusion of crystalloid, 500 mg of calcium chloride, 45 mEq of sodium bicarbonate, and dantrolene. Charcoal filters were placed on the machine, the CO2 absorbent and machine circuit were replaced, an external cooling blanket was set to 32 °C, ice packs were applied to the groin and axillae, a defibrillator was applied, and the MH hotline was consulted. The patient improved dramatically after the initiation of dantrolene; total dose administered was 225 (2.75 mg/kg for the 82 kg patient). The heart rate decreased to 80 bpm, systolic blood pressure decreased from 208 to 150 mm Hg, spontaneous minute ventilation decreased to 13 L/m, and the rigors stopped. The results of the diagnostic ABG, sent prior to dantrolene administration, returned as follows: pH: 7.25, PCO2 57 mmHg, PaO2 332 mmHg (100% FIO2), HCO3− 24 mmol/L, base excess − 1.9 mmol/L, O2Sat 100%, K+ 4.2 mmol/L. A repeat ABG along with comprehensive laboratory studies were sent 15 minutes following the initial ABG. The
complete blood count, PT, PTT, fibrinogen, D-dimer, chemistry and liver panel, thyroid stimulating hormone and creatinine kinase all returned normal results. The serum myoglobin returned elevated at 184 ng/mL (RR, 1-100 ng/mL), lactate returned elevated at 3.1 mmol/L (RR, 0.5-2.2 mmol/L). Repeat arterial blood gas returned as follows: pH 7.39, PCO2 42 mm Hg, PaO2 410 mm Hg (100% FIO2), HCO3− 25 mmol/L, base excess + 0.6 mmol/L, O2 saturation 100%, K + 4.7 mmol/L. The patient remained intubated and was transferred to the surgical intensive care unit. Serial creatinine kinases drawn 2 and 4 hours following the end of the case returned 202 IU/L and 264 IU/L respectively (RR, 25–215 IU/L). A lactate, drawn 2 hours following the end of the case returned normalized at 1.2 mmol/L (RR, 0.5-2.2 mmol/L). The patient was extubated 6 hours following the end of the case, had a stable hospital course, and was discharged from the hospital on post-operative day 2 in excellent condition. There were multiple discussions with the patient's parents during the hospitalization and after discharge stressing the importance of obtaining a Caffeine Halothane Contracture Test (CHCT), as it remains the gold standard for the diagnosis of MH. Despite the information provided, the parents elected instead for DNA testing. The genes sequenced were RYR1, CACNA1S, and STAC3. The molecular genetics analysis revealed heterozygosity in exon 90 of the RYR1 gene for a sequence variant designated c.12553G N A (p.Ala4185Thr).
3. Discussion There are two diagnostic tests available for the diagnosis of MH: CHCT, and genetic testing that is often limited to the sequence analysis of the RYR1 gene. CHCT is the gold standard, although there are limitations in its specificity [3], as well as variation among testing centers [4]. It requires a skeletal muscle biopsy from the thigh, which can be performed under general or neuraxial anesthesia, at a designated MH muscle biopsy center. Sensitivity is close to 100%, and false negatives are rare [3]. Specificity is about 80%, with a 20% false positive rate [3]. Genetic testing is newly emerging as a viable alternative to the caffeine halothane contracture test in certain cases. Via DNA samples, genetic testing aims to find mutations in the genes responsible for contraction-excitation coupling and calcium regulation in skeletal muscle, namely RYR1, CACNA1S, and STAC3, that have been associated with MH [5]. Recently novel RYR1 mutations have been reported by genetic testing and confirmed by CHCT; eg, Met.4230.Arg [6] RYR1 codes for the ryanodine receptor type 1 (RYR1), which is responsible for calcium release in the sarcoplasmic reticulum, and has been implicated in 50–70% of MH susceptibility cases, but their role in the pathogenicity of MH has not been determined. More than 300 RYR1 variants have been identified so far, but their role in MH has not been yet clarified; several of the
146 RYR1 variants have also been associated with central core disease [2,7]. A 2013 study published in Anesthesiology, 2013 investigated the genetic penetrance of RYR1 and CACNA1S variants in a random, 870-cohort population [1]. The findings demonstrated that the RYR1 p.Ala4185Thr variant was not found to be penetrant in a random selection [1]. The study cites the variant as a rare variant of unknown significance; indicating that further investigation is warranted for correlation with MH susceptibility [1]. The CACNA1S variant, which codes for the α1 subunit of dihydropyridine voltage-gated calcium channel, account for only 1% of MH cases. Lastly, STAC3 has been recently added to the MH genetic testing panel, particularly after being linked with Native American myopathy [8]. Genetic testing is often favored in patients whose MH susceptibility status is confirmed, or whose relatives are confirmed with the CHCT [9]. In addition, genetic testing may benefit patients for whom the CHCT may be contraindicated. Genetic testing confers the advantage of eliminating invasive surgery for an open muscle biopsy at specific MH testing centers, and being cost-effective. The main drawback of the genetic testing is low sensitivity due to the heterogeneity of the condition. Only about 50% of MHS patients will have a mutation in one of the known MH candidate genes. The complex genetic nature of MH is the reason why negative genetic results cannot rule out a diagnosis of MH susceptibility. In this case report, a RYR1 variant of unknown significance, c.12553G N A (p.Ala4185Thr), was identified in a young male patient who experienced an MH event with Clinical Grading Score of 53; indicating he should be tested for MH susceptibility [10]. This variant is quite rare: it was observed only once in the sample population of 1088 people, ie, its minor allele frequency is 0.0002 (rs151119428, NCBI dbSNP) [2]. The variant has been reported previously in a MH susceptible individual who carried a second, potentially causative RYR1 variant and, thus, significance of the p.Ala4185Thr variant remains unclear [1,2]. Bioinformatics evaluation of the variant pathogenic potential using 4 software tools, PolyPhen-2 [11], SIFT [12], Provean [13], and MutationTaster [14] gave ambiguous prediction results: p.Ala4185Thr was found to be probably deleterious and damaging by MutationTaster and SIFT, and possibly damaging and neutral by PolyPhen-2 and Provean. Functional studies are needed to fully establish the p.Ala4185Thr variant's effect on the RYR1 function as well as its potential role in the pathogenesis of MH.
4. Conclusion Malignant hyperthermia remains a diagnostic challenge, and anesthesiologists must have a high index of suspicion in the presence of intraoperative nonspecific signs such as tachy-
E.J. MacKay et al. cardia or rising end-tidal carbon dioxide. This patient had a suspected malignant hyperthermia episode intraoperatively with a Clinical Grading Score of 53 [10], which indicates an “almost certain” MH episode. The genetic testing revealed heterozygosity for a rare variant in the gene that encodes the RYR1 receptor, of which there is evidence to the involvement in the pathogenesis of MH. However, in the setting of a clinically suspicious MH episode in the presence of a RYR1 genetic variant of unknown significance, the patient must be considered MH susceptible.
References [1] Gonsalves SG, Ng D, Johnston JJ, Teer JK, Stenson PD, Cooper DN, et al. Using exome data to identify malignant hyperthermia susceptibility mutations. Anesthesiology 2013;119:1043-53. [2] Kraeva N, Riazi S, Loke J, Frodis W, Crossan ML, Nolan K, et al. Ryanodine receptor type 1 gene mutations found in the Canadian malignant hyperthermia population. Can J Anaesth 2011;58:504-13. [3] Allen GC, Larach MG, Kunselman AR. The sensitivity and specificity of the caffeine-halothane contracture test: a report from the North American malignant hyperthermia registry. the North American malignant hyperthermia registry of MHAUS. Anesthesiology 1998;88:579-88. [4] Ording H, Islander G, Bendixen D, Ranklev-Twetman E. Betweencenter variability of results of the in vitro contracture test for malignant hyperthermia susceptibility. Anesth Analg 2000; 91:452-7. [5] Rosenberg H, Sambuughin N, Riazi S, Dirksen R. In: Pagon RA, Adam MP, Ardinger HH, Wallace SE, Amemiya A, Bean LJH, et al, editors. Malignant Hyperthermia Susceptibility. Seattle (WA): GeneReviews(R); 1993. [6] Brandom BW, Bina S, Wong CA, Wallace T, Visoiu M, Isackson PJ, et al. Ryanodine receptor type 1 gene variants in the malignant hyperthermia-susceptible population of the United States. Anesth Analg 2013;116:1078-86. [7] Rosenberg H, Rueffert H. Clinical utility gene card for: malignant hyperthermia. Eur J Hum Genet 2011:19. [8] Horstick EJ, Linsley JW, Dowling JJ, Hauser MA, McDonald KK, Ashley-Koch A, et al. Stac3 is a component of the excitation-contraction coupling machinery and mutated in native American myopathy. Nat Commun 2013;4:1952. [9] Toppin PJ, Chandy TT, Ghanekar A, Kraeva N, Beattie WS, Riazi S. A report of fulminant malignant hyperthermia in a patient with a novel mutation of the CACNA1S gene. Can J Anaesth 2010; 57:689-93. [10] Larach MG, Localio AR, Allen GC, Denborough MA, Ellis FR, Gronert GA, et al. A clinical grading scale to predict malignant hyperthermia susceptibility. Anesthesiology 1994;80:771-9. [11] Adzhubei IA, Schmidt S, Peshkin L, Ramensky VE, Gerasimova A, Bork P, et al. A method and server for predicting damaging missense mutations. Nat Methods 2010;7:248-9. [12] Kumar P, Henikoff S, Ng PC. Predicting the effects of coding nonsynonymous variants on protein function using the SIFT algorithm. Nat Protoc 2009;4:1073-81. [13] Choi Y, Sims GE, Murphy S, Miller JR, Chan AP. Predicting the functional effect of amino acid substitutions and indels. PLoS One 2012;7: e46688. [14] Schwarz JM, Rodelsperger C, Schuelke M, Seelow D. MutationTaster evaluates disease-causing potential of sequence alterations. Nat Methods 2010;7:575-6.