Abstract
INTRODUCTION
Coronary artery disease (CHD) is the most common cause of death globally with atherosclerosis, an inflammatory process, as the underlying mechanism [1]. Statins have been widely used in clinical practice to lower cholesterol by inhibiting the rate limiting enzyme HMG-CoA reductase in the cholesterol synthesis pathway. Meanwhile, the anti-inflammatory properties of statins have been shown to have a protective effect in patients with cardiovascular disease [1, 2], consequently contributing to the prevention of heart attacks and strokes [3]. It is also well known that statins have adverse side effects, the most significant of which are myopathic symptoms including, muscle pain, cramps, and/or weakness. In severe cases of statin-induced myopathy (SM), symptoms may lead to rhabdomyolysis, a severe form of muscle damage usually associated with very high creatine kinase (CK) levels (>10X the upper limit of normal) with the release of myoglobin into the bloodstream (myoglobinemia) and urine (myoglobinuria) [4, 5]. Approximately 7–29% (2.8–11.6 million) of people taking statins develop muscle symptoms such as pain and/or weakness that are often reversible with drug or dosage changes [6–9]. Meta-analysis has shown that there is a higher incidence of statin-induced muscle symptoms in clinical practice than in clinical trials. There may be several explanations, e.g., clinical trials are randomized and controlled, study participants are eliminated if they have early symptoms during the run-in period or have comorbidities that would predispose to increased risk for muscle symptoms, and trials often do not query for muscle complaints among participants [4, 10]. These exclusion criteria do not represent cases that come to attention in clinical practice. While serious adverse reactions to statin therapy are considered rare, as many as 1 per 1000 to 1 per 10 000 people on standard statin doses may be at risk for severe, life-threatening muscle symptoms [4]; in extreme cases, liver damage, kidney failure and death may occur [5].
Biochemical and molecular evidence has accumulated in retrospective studies of statin myopathy patients for genetic risk underlying disorders of muscle metabolism such as carnitine palmitoyltransferase (CPT) II deficiency, McArdle disease, and exertional rhabdomyolysis associated with pathogenic variants in the RYR1 (ryanodine receptor 1) gene that are known to cause malignant hyperthermia [11, 12]. Variants in the SLCO1B1 gene, which encodes the organic anion-transporting polypeptide OATP1B1 known to regulate the hepatic uptake of statins, have been shown to be strongly associated with an 18% increased risk for SM with high doses (80 mgs) of simvastatin; the association of risk with other statins has not been consistently reproduced [13, 14]. Additional molecular genetic studies have revealed evidence for underlying heritable muscle disorders in >30% of cases that are triggered by statins [15].
Mitochondrial disease is one of the most common groups of heritable disorders known with a prevalence of 1 in 4,300 [16]. While multi-system involvement is the most evident characteristic, neurologic manifestations are the most prominent including neuromuscular symptoms. Mitochondrial disease per se, is a collective designation that includes genetically and clinically heterogeneous disorders leading to defects in mitochondrial oxidative phosphorylation, mitochondrial DNA (mtDNA) replication, and structural and transport defects in mitochondria; genes encoded by both the nuclear (nDNA) and mtDNA genomes may be involved. The exact mechanism(s) underlying SM are unknown, however, some studies suggest that the toxic effect of statins is associated with mitochondrial dysfunction by decreasing energy production and altering muscle protein degradation [4, 17]. Statins have also been associated with high oxidative stress in skeletal muscle in human and animal studies and this is responsible for transcriptional deactivation of mitochondrial biogenesis together with mitochondrial dysfunction [18].
Histochemical evidence for the presence of mitochondrial disease, has not been studied systematically in a large cohort of SM cases. Here, we undertook a retrospective analysis of 279 patient records stored in an Access database derived from patients with severe SM. We proposed that a proportion of individuals with SM will have underlying histopathologic evidence for mitochondrial dysfunction leading to further quantitative biochemical analysis of respiratory chain complexes to identify underlying defects. We expect that the majority of cases with histopathologic evidence for mitochondrial disease will not have primary biochemical defects in respiratory chain enzymes but may have enough evidence for mitochondrial abnormalities to consider additional diagnostic studies such as nDNA and mtDNA analysis for specific molecular etiologies.
MATERIAL AND METHOD
Subjects
A total of 459 subjects were included in this retrospective study. All information used was derived from abbreviated patient records logged into an Access database. In all cases, skeletal muscle biopsies (primarily from large muscle groups, e.g., the gastrocnemius or vastus lateralis muscles) were surgically obtained and snap frozen in liquid nitrogen or on dry ice and sent to the Robert Guthrie Biochemical & Molecular Genetics Laboratory, Kaleida Health Laboratories, Buffalo, NY for biochemical assessment for specific metabolic myopathies. The laboratory provides CLIA-approved diagnostic services and has held a permit for more than 30 years from the NYSDOH to perform quantitative biochemical analyses on frozen muscle biopsies. The majority of patient records were obtained between the late 1990’s, when statin myopathy referrals began, through 2015. All biopsies were sent from physician specialists at major medical centers in the United States and Canada (see Acknowledgments). Referring physicians were specialists in neuromuscular pathology who had reviewed biopsies ordered by neurologists specializing in neuromuscular disease or metabolic specialists from centralized genetic services. A clinical summary and a histopathology report were required for each specimen as with all referrals to the laboratory. Subsequently, the laboratory’s director logged information from the reports into a customized Access database populated with key words representing clinical symptoms, personal and family history, histopathologic findings and electron microscopic results. Features were primarily noted as present or absent as judged by the referring expert physicians. There were no quantitative measures recorded for the extent of clinical symptoms such as specific muscle groups affected with degrees of pain or weakness or the details of electromyography (EMG) findings; histopathologic findings entered in the database were qualitative, e.g., presence or absence of cytochrome c oxidase (COX) (–) fibers taking into account appropriateness for age. In other words, if COX (–) fibers were listed as present, the pathologist would state that the number exceeded what was expected for age and was, therefore, considered pathologic. Cases were only used if the COX (–) fibers present exceeded what was expected for age. While the lack of available details could be viewed as a limitation, this was beyond the scope of the present study as the expertise of the referring physicians was relied upon to provide the presence or absence of key clinical and laboratory features.
A total of 279 (198M/71F; 56+/–12.3 yrs) patients were categorized as having severe SM defined as muscle pain and/or weakness often accompanied by rhabdomyolysis, myolgobinuria and/or significantly elevated plasma CK (4–10 times the upper limit of normal; ULN). The symptoms may begin immediately or within weeks or months after starting therapy and are directly related to the use of statin therapy; symptoms may be incapacitating and permanently disabling in severe cases. Combinations of these features constituted the reason for muscle biopsy and referral to our laboratory; the characteristics of severe SM were previously published in detail [12]. The need for muscle biopsy and analysis was deemed necessary by expert referring physicians for all patients evaluated in the study. The remaining 10 muscle biopsies from SM patients were referred directly to the histopathology laboratory by physicians at the Buffalo General Medical Center, Kaleida Health Laboratories, Buffalo, NY. Unfortunately, a limitation of the study was that in most cases the type and dose of statin that resulted in myopathic symptoms was not provided by referring physicians. We were unable to include a study of muscle biopsies from statin-tolerant controls as this would be beyond the purview of this records review study of abnormal cases. A recent relatively small retrospective study performed in statin-tolerant control volunteers showed no evidence for pathologic changes in single muscle fiber morphology or mechanics or ultrastructural changes in mitochondria from intact skeletal muscle fiber bundles [19].
Two adult myopathic control groups without statin exposure were included for comparison of features with the intent to note similarities with or differences from characteristics of the SM group. The control groups consisted of 94 individuals (54M/40F; 51+/–6.7 yrs) with evidence for idiopathic mitochondrial myopathies (MMP) such as the excessive presence of ragged-red fibers in muscle biopsies, abnormally increased oxidative staining, electron microscopic evidence for increased mitochondrial content or abnormal mitochondrial structure but lacking a biochemical diagnosis in our laboratory; they had similar myopathic features as those of the SM group. Eighty-six individuals (49M/37F; 67+/–3.7 yrs) comprised the unknown metabolic myopathy (UMP) group, without substantial evidence for mitochondrial disease, and without a biochemical diagnosis. Members of this control group had been referred to rule out suspected metabolic myopathies, they were not taking statins, and had the largest number of biochemical profiles performed in seeking a diagnosis compared to the test and MMP groups. There was minimal histopathologic evidence for mitochondrial abnormalities. Members of the UMP group had similar myopathic symptoms as those of the SM and MMP groups. The rationale for using abnormal control groups was to determine how closely the SM group shared characteristics with the MMP group versus the UMP group. The SM and MMP patient groups were within the same age range while the overall age of the UMP group was somewhat higher.
Methods
A retrospective analysis of the Guthrie laboratory’s clinical database containing available abbreviated medical and laboratory records was performed for all subjects in the study. The database was queried for clinical, histopathologic and biochemical features of muscle disease. Fifty-two percent of biopsies from participants with severe statin myopathy underwent at least one metabolic profile offered by the laboratory and 34% had ≥2 profiles performed. An additional 13% had individual biochemical tests performed that did not constitute a complete profile; usually due to inadequate tissue for completion. The primary profiles performed most commonly were the Mitochondrial Myopathy Profile and the Myoglobinuria Profile, and less often the Fatty Acid Oxidation Profile. Details of the individual tests performed in each profile are described at http://www.rgbmgl.org/Diagnostic-Tests/Test-Profiles-Gene-Sequence-Analysis. Ninety-four percent of the MMP control group had Mitochondrial Myopathy Profiles performed on muscle and 21% had at least 1 additional profile performed. The remainder had other combinations of individual biochemical tests performed. To be categorized as MMP, a combination of clinical, histopathologic or biochemical evidence leading to a suspected diagnosis of mitochondrial myopathy was obtained but with no specific biochemical defect identified; the laboratory was not provided with any mutation analysis data that may have been obtained subsequent to our studies that could have led to a molecular diagnosis. In the UMP control group, 65% had at least one profile performed in muscle, 30% had ≥2 profiles performed and 5% had limited individual tests performed; none led to a final biochemical diagnosis.
The study was performed in accordance with the ethical standards of the Humans Subjects Institutional Review Board at the University at Buffalo in which the studies were done or in accord with the Helsinki Declaration of 1975.
Histopathologic, biochemical and electron microscopic (EM) analysis of muscle biopsies
The majority of specimens (n = 449) sent to the Guthrie Laboratory for biochemical analysis were accompanied by histopathology reports of assessments performed by neuromuscular specialists at referring institutions; the remaining 10 muscle biopsies from SM patients were analyzed by a neuromuscular specialist and co-author (Dr. Reid Heffner) in the histopathology laboratory at the Buffalo General Medical Center, Kaleida Health Laboratories, Buffalo, NY. The local subset of 10 muscle biopsies was submitted to histochemical analysis using the following stains: hematoxylin and eosin (H&E), modified Gomori trichrome, tetrazolium-NADH reductase, myosin-ATPase at pH 9.4 and 4.6, COX, succinate dehydrogenase, myoadenylate deaminase, phosphorylase, acid and alkaline phosphatase, nonspecific esterase, oil-red-O (O-R-O) and periodic acid Schiff (PAS) with and without diastase using standard methodology [20]. A portion of each muscle biopsy was processed for paraffin embedding or EM and a portion was snap frozen in liquid nitrogen for biochemical analysis [21]. Assays used for the quantitation of enzyme activities in frozen muscle biopsies were performed on whole homogenates as previously described [22]. Abnormal results leading to a biochemical diagnosis of an enzyme deficiency in skeletal muscle had to have met certain criteria. Individual enzymes quantified in frozen muscle and considered to have significant partial deficiencies were at least 2 standard deviations below the normal reference mean. When tissue was available, additional enzymes were quantified to assist in determining certain aspects of the quality of the specimen. For example, phosphofructokinase, a relatively labile enzyme, was quantified for tissue quality. Citrate synthase, as a marker for mitochondrial content, was used for normalization of respiratory chain enzyme data to reveal potential underlying deficiencies. Inclusion of actual enzyme activities for individual muscle biopsies or corresponding normalized data was beyond the scope of this study; the final biochemical diagnoses were provided for those in whom a respiratory chain enzyme deficiency was identified.
Statistical analysis
Data were analyzed for individual groups and compared using Chi-Square with 95% confidence intervals. Mean values in groups of subjects with different genetic findings were compared by analysis of variance followed by Student-Newman-Keuls with the nominal statistical significance level set at p < 0.05.
RESULTS
Patient information and clinical features
A number of clinical and laboratory features were shared among the SM, MMP, UMP and groups including muscle pain, cramps or weakness (62–74%), abnormal EMG (61–66%), elevated plasma CK (52–79%), abnormal neurologic exam (39–45%), fatigue (27–38%), and a progressive course (16–25%). Significantly more patients with MMP had gait abnormalities and ptosis than the SM or UMP groups (Table 1). The ratio of males to females in the statin myopathy group was nearly 3 : 1; a characteristic not found in either control group (p < 0.01). A statistically significant increase in the number of cases with elevated plasma CK was found in the SM group compared to either control group (p < 0.01).
Histopathologic and EM findings
Twenty-two percent of muscle biopsies in the SM group had either increased trichrome staining or ragged-red fibers, and 19% had excessive COX (–) fibers. All three features are histopathologic indicators for mitochondrial abnormalities (Table 2). Excessive COX (–) fibers were determined by the referring neuromuscular pathologist to be beyond the acceptable range for age and, therefore, this finding was not explained by normal aging. The percentage of biopsies in the SM group that had ragged-red fibers (12%) or elevated trichrome staining (10%) was similar to those of the UMP group. The findings in the SM group suggest that the presence of increased trichrome staining or COX (–) fibers alone were not discriminatory from the UMP group to be indicative of mitochondrial disease. Significantly more biopsies in the MMP group had ragged-red fibers and elevated trichrome staining than in the SM group (60%; p < 0.01). The MMP group also had a significant increase in COX (–) fibers (34%) compared to biopsies in the SM group (19%) (p < 0.01). Only 29 MMP biopsies were evaluated by EM and of these, 23 (79%) showed abnormal numbers and/or structure of mitochondria. This percentage was significantly lower in the SM group (24 of 55; 44%, p < 0.01). Nevertheless, the relatively high yield of ultrastructural abnormalities in both groups suggests that EM should be performed more routinely in biopsies from adult statin myopathy patients and in suspected mitochondrial myopathy cases for potentially more diagnostic information. Unfortunately, only 20% of all SM and 31% of all MMP biopsies were evaluated by EM. Due to the limited number of cases in the UMP group assessed by EM, the percentage of total was not calculated for this group.
Among 24 SM biopsies with abnormal numbers and/or structure of mitochondria by EM, only 6 (25%) had ragged-red fibers, and 9 (38%) had increased trichrome staining (63% of total) Similarly, in the MMP control group, only 31% of those with abnormal mitochondria by EM had ragged-red fibers and 56% had increased trichrome staining (87% of total). These findings highlight the importance of EM in detecting more qualitative and quantitative mitochondrial abnormalities than can be detected histochemically.
The number of patients with fiber atrophy and type 1 fiber predominance in the SM group was about 1.5 times higher than in the MMP group (not statistically significant). Other findings, including fiber size variation, presence of a denervating process, glycogen and lipid storage, were all more prominent in the statin myopathy group than the MMP group, although without statistical significance. Inflammatory infiltrates were present in 4% of SM, 3% of MMP and 5% of UMP suggesting that there were no significant difference between the groups. Nevertheless, consideration should be given to measuring anti-HMG-CoA reductase antibodies in the serum of SM patients with inflammatory infiltrates to determine if they have statin-induced immune-mediated necrotizing myopathy (IMNM) [23].This condition was first recognized to be prominent in a subset of statin-induced myositis in 2010 [24].
Respiratory chain defects, Coenzyme Q10, and mitochondrial abnormalities
Among biopsies from SM patients, 29 of 279 (10%) were confirmed to have respiratory chain defects by quantitative biochemical analysis using previously described methods (21); all had deficiencies in one or more individual respiratory chain enzyme activities that were at least 2 standard deviations below the mean. There were significantly more males than females in this group (p < 0.05) (Table 3), but this result was not unexpected since there were three times as many males as females in the starting SM group. Interestingly, the majority of those with respiratory chain defects did not have corresponding histopathologic changes suggestive of mitochondrial disease (Table 4). Four of 21 (19%) had ragged-red fibers and 4 had increased trichrome staining (19%); 38% of total. Ten of 29 patients with respiratory chain defects were evaluated by EM, and four of them had abnormalities in number and/or structure of mitochondria. Table 5 lists detailed descriptions of these four patients. Three had a diagnosis of complex (CM) II, III (succinate cytochrome c reductase) deficiency, and one had a diagnosis of CM I, III (NADH cytochrome c reductase) deficiency. The remaining 25 patients without EM evaluation had the following deficiency diagnoses: complex I (2 cases), complex I-III (2 cases), complex II-III (8 cases), complex II+IV (3 cases), complex II-IV (2 cases), complex IV (7 cases) and complex I-IV (1 case). Biopsies from a majority of patients with respiratory chain defects did not have histopathologic evidence for mitochondrial disease demonstrating that histopathologic evidence alone should not be the only criterion used to proceed with quantitative biochemical analysis in skeletal muscle.
Coenzyme Q10 (CoQ10) was quantified in the muscle of 60 SM patients in which 10 were found to have values that were >2 standard deviations below the mean and considered significant partial deficiencies; however, 6 of these also had low citrate synthase, the marker for mitochondrial content. Normalization of the data against citrate synthase placed CoQ10 for these 6 individuals within normal range. Therefore, only 4 of 60 (7%) had significant partial reductions in CoQ10; 2 of these had rhabdomyolysis with myoglobinuria as features. Among the controls, no biopsies from the UMP group and only 4 in the MMP group had CoQ10 quantified; all 4 MMP had normal COQ10 activity.
Biochemical evidence for additional metabolic muscle disorders identified in the SM group included: significantly reduced myophosphorylase activity (McArdle disease carriers; 8 cases); reduced CPT activity (14 possible CPT deficiency carriers); secondary carnitine deficiency (1 case), phosphorylase b kinase deficiency (1 case), and myoadenylate deaminase deficiency (5 cases). In summary, an additional 29 patients had biochemical evidence for significantly reduced enzyme activities or deficiencies for other metabolic myopathies. Unfortunately, not all members of the SM group had evaluations for all possible enzymes relevant to metabolic muscle disorders offered in the laboratory due to either the absence of specific orders or inadequate biopsy material to perform all testing requested.
DISCUSSION
The clinical and laboratory presentations of SM vary from idiopathic hyperCKemia to severe rhabdomyolysis including muscle pain with or without weakness and potential acute kidney injury [25]. While there have been conflicting reports on the actual incidence of SM, it is well-documented that up to 29% of patients on statin therapy acquire muscle aches, pain, and/or weakness attributed to the initiation of therapy [4] and that symptoms usually begin within 4 to 6 weeks after starting statin therapy yet may not occur until years of treatment [26]. Most clinical findings in the SM group were similar to the MMP and UMP groups with the exception of ptosis and gait abnormalities which were significantly increased in MMP patients and known to be features of certain mitochondrial myopathies [27]. More SM patients had significantly elevated plasma CK than in the other two groups (p < 0.01); elevated CK is not unexpected and commonly used for the classification of statin-induced myopathy [4, 28]. While CK is used as a marker for the severity of statin myotoxicity, it should be noted that not all patients with statin myopathy have elevated CK [29, 30]; the correlation between CK and clinical symptoms can be inconsistent [14]. Nevertheless, it is important to measure CK for both prognosis and treatment management; of the 72% of SM patients in whom CK was measured, 79% had abnormal elevations. An elevation of CK above 4 times the upper limit of normal is a clear indication that statin therapy should be discontinued at least temporarily with efforts made to find alternative therapies [4]. A limitation of this study was that the actual levels of CK were not always provided in the records studied. Presently, CK measurement is only indicated for symptomatic patients and CK screening is not recommended routinely in patients taking statins [29, 30]; however, at least some clinicians are now measuring CK prior to prescribing statin drugs for a baseline level and because of increased awareness that asymptomatic elevation of plasma CK is relatively common in the general population [30].
The proportion of patients with histochemical and/or EM changes in the UMP group was comparable to those of the SM group while, as expected, the MMP group had significantly more evidence for mitochondrial abnormalities than the SM or UMP groups. Since more mitochondrial abnormalities were found by EM than by histochemical analysis in our study, histochemical staining results should not be the only criterion used by pathologists to determine whether to add EM evaluation for mitochondrial changes or to refer biopsies for quantitative biochemical analysis of respiratory chain enzymes.
Only 10% of SM patients in our study actually had respiratory chain enzyme abnormalities and a majority of these did not have histopathologic or EM evidence for mitochondrial disease further demonstrating that clinical criteria should also be used for pursuing quantitative biochemical analysis in skeletal muscle. It is important to recognize that although pathologic findings in muscle biopsies are helpful when present, they are often absent and therefore the importance of muscle biopsy is in its potential to assess mitochondrial structure and function using multiple technologies [31]. The presence of histopathologic evidence for mitochondrial abnormalities without impaired respiratory chain enzyme activities may suggest that secondary toxic effects of statins on mitochondrial structure and/or function could be present without being accompanied by biochemical abnormalities that were quantifiable [32–34]. In cases with histopathologic evidence for mitochondrial abnormalities, with or without biochemical abnormalities, molecular analysis may be considered for possible added diagnostic information. The yield of molecular diagnoses for mitochondrial disease has increased from the early yields at the 5.4% level [35] thanks to the advent of next generation sequencing and recognition that over 1,000 nDNA genes are involved in mitochondrial structure and function in addition to the 37 mtDNA genes; only 15% of all known mitochondrial disease is caused by mtDNA-encoded defects [31]. Nevertheless, the yield from molecular testing in statin-induced myopathy in patients who were not symptomatic prior to taking statins is likely to be very low.
Additionally, those patients with evidence for inflammatory infiltrates should be followed up for autoantibodies to HMG-CoA reductase causative for IMNM. Cases of IMNM account for 2-3/100,000 persons per year with severe statin-induced myopathy with muscle weakness and elevated CK [36]. Patients with IMNM often do not obtain relief from their symptoms with the withdrawal of statin therapy and immune-mediated necrotizing myopathy ensues [23]. It is important to withdraw statin use in these patients to reverse the progression of the myopathy, however, treatment with immune modulations such as IV immunoglobulin may also be required [23].
The question arises as to whether mitochondrial dysfunction is an underlying cause or a consequence of SM and this remains to be resolved [4]. A wide range of mitochondrial abnormalities have been reported including histopathologic evidence for mitochondrial dysfunction, e.g., ragged-red fibers, COX (–) fibers, lipid storage and ultrastructural changes [18, 37] and reductions in muscle CoQ10 [11]. The role of mitochondria in SM has also been repeatedly implicated in recent cell culture and animal models [38]. However, most clinical studies have been limited by small sample size (<50 cases), lack of well-matched controls, and inadequate data. We have described a large retrospective analysis of 279 patients with SM as compared to MMP and UMP control groups. While our results showed that mitochondrial dysfunction was present histopathologically in a subset of SM patients, only a small portion of patients had respiratory chain defects. Therefore, it is important to consider a combination of clinical, histopathologic, biochemical and molecular approaches to the diagnosis of statin myopathy for the most complete characterization and understanding of the mechanistic etiologies. There is a large number of factors, including environmental exposures, drug interactions, personal or family history of muscle disease, and comorbidities that contribute to the pathology of SM [23].
Several mechanisms have been proposed for statin associated mitochondrial damage. While CoQ10 has been reported consistently to be decreased in the circulation of SM patients, reductions in skeletal muscle CoQ10 are inconsistent [23]; reductions in plasma CoQ10 may be explained by the simultaneous decrease in LDL cholesterol which is a circulatory carrier of CoQ10 [39]. The present study has shown that CoQ10 deficiency in skeletal muscle is not a prominent finding represented in only a few cases (7%). Also, CoQ10 supplementation has led to contradictory effects in preventing or improving SM [40–45]. A recent meta-analysis of randomized controlled trials from the Mayo Clinic showed that CoQ10 had no significant effect on muscle pain or plasma CK level in statin myopathy [46]. Other alterations associated with mitochondrial dysfunction have also been observed after statin treatment including, decreased citrate synthase [47], increased reactive oxygen species production [18] and prolonged metabolic recovery time [48].
Additionally, one study reported reductions in mtDNA in the muscle of patients treated with high-dose (80 mgs) simvastatin [33]. A limitation of our study was that molecular studies were either not performed or not reported to the laboratory by referring physicians. Disorders involving mtDNA may be present in only a very few cases in spite of the absence of respiratory chain abnormalities. The introduction of next generation sequencing has led to the identification of many nDNA genes relating to mtDNA synthesis and maintenance that contain pathogenic variants causing mitochondrial disease in adults [16]. Although some pathogenic variants may be linked to the primary function of the mutated protein, others lead to mtDNA depletion [33, 34], or the accumulation of secondary mtDNA deletions and point mutations which occur with time in adults [16]; it is possible that statin exposure may enhance the occurrence of these secondary abnormalities.
We proposed that a subset of individuals with SM would have underlying histopathologic evidence for mitochondrial dysfunction suggesting further exploration via quantitative biochemical analysis of respiratory chain complexes to identify underlying defects. We found that EM identified more cases with mitochondrial abnormalities than histochemical studies. We described evidence for respiratory chain defects in 10% of cases. Although we could not determine whether mitochondrial dysfunction was an underlying cause or a consequence of SM, we did demonstrate that in a large cohort of patients afflicted with statin myopathy, a substantial number of patients had either histopathologic or biochemical evidence or both for mitochondrial dysfunction and that the medical management of SM patients should take into account the importance of mitochondrial abnormalities whether they are primary or secondary effects of statin therapy.
CONFLICT OF INTEREST
The authors have no conflicts of interest to report.
Footnotes
ACKNOWLEDGMENTS
The authors wish to thank Dr. Edward Fine, Department of Neurology, Jacobs School of Medicine & Biomedical Sciences, University at Buffalo, Buffalo, NY for his helpful comments in the preparation of the manuscript. The authors also wish to thank all of the physician specialists and their medical centers who referred patient muscle biopsies to our laboratories for evaluation: Hennepin County Medical Center, Minneapolis, MN; Henry Ford Hospital, Detroit, MI; Hershey Medical Center, Hershey, PA; Froedert Hospital, medical College of Wisconsin, Milwaukee, WI; Memorial Hospital, Colorado Springs, CO; William Beaumont Medical Center, Royal Oak, MI; Vanderbilt Medical Center, Nashville, TN; National Institutes of Health, Bethesda, MD; Dent Neurologic Institute, Buffalo, NY; Upstate Medical Center, Syracuse, NY; St. John Health System, Tulsa, OK; Health Network Laboratories, Allentown, PA; Veterans Administration Hospital, Buffalo, NY; Alaska Medical Center, Anchorage, AK; Stanford University Medical Center, Stanford, CA; Hillcrest Medical Center, Tulsa, OK; Athena Diagnostics, Inc., Marlborough, MA; University of Michigan Health System, Ann Arbor, MI; Yale University Medical Center, New Haven, CT; Medical College of Georgia, Augusta, GA; Regional Medical Laboratory, Tulsa, OK; All Children’s Hospital, St. Petersburg, FL; McMaster University Medical Centre, Hamilton, ON, Canada; Mayo Clinic Medical Laboratories, Rochester, MN; UC San Diego Medical Center, San Diego, CA; Oregon Health Science Center, Portland, OR; Allegheny General Hospital, Pittsburgh, PA; Parkview Medical Center, Pueblo, CO; Albany Medical Center, Albany, NY; Harborview Medical Center, Seattle, WA; East Tennessee Baptist Hospital, Knoxville, TN; Cleveland Clinic, Cleveland, OH; Buffalo General Medical Center, Buffalo, NY; Medical University of South Carolina, Charleston, SC; Bradenton Neurology, Inc., Bradenton, FL; Baylor College of Medicine, Houston, TX; Scripps Mercy Hospital, San Diego, CA; Strong Memorial Hospital, University of Rochester Medical Center, Rochester, NY; UNC Hospitals, Chapel Hill, NC; Willford Hall Medical Center, Lackland AFB, TX; Dartmouth Hitchcock Medical Center, Lebanon, NH; University of Mississippi Medical Center, Jackson, MS; North Shore Medical Center, Salem, MA; Clarian Health Partners, Indianapolis, IN; Dell Children’s Medical Center, Austin, TX; Kaiser Permanente- Sacramento, CA and North Hollywood, CA; University of Pittsburgh Medical Center, Pittsburgh, PA; Carolinas Healthcare System, Charlotte, NC; UC Los Angeles Medical Center, Los Angeles, CA; Indiana University Health, Indianapolis, IN.