UBC Theses and Dissertations
Statin-induced muscle mitochondrial toxicity Schick, Brian Adam
Statins are the mainstay of cholesterol-lowering therapy and are taken by millions of people worldwide. These drugs are generally well-tolerated but can cause myopathy ranging from mild muscle pain to fatal rhabdomyolysis. The mechanism of statin-induced myopathy (SIM) is not fully understood and there is currently no convenient and reliable marker of SIM, but mitochondrial dysfunction has been implicated. We sought to investigate the effect of statins on mitochondrial DNA (mtDNA) levels in order to gain information on the mechanism of SIM and to explore the possibility of utilizing changing mtDNA levels as a marker of SIM. Several approaches were used. First, mtDNA levels were quantified in skeletal muscle biopsies collected from a previously published 8-week clinical trial of high-dose simvastatin or atorvastatin versus placebo. Forty-eight hypercholesterolemic subjects were randomly assigned to receive placebo (N=16), high dose atorvastatin 40mg/day (N=16), or high dose simvastatin 80mg/day (N=16) for 8 weeks. Muscle mtDNA content was assessed by real-time PCR atbaseline and after 8-weeks on statin treatment and found to be significantly reduced in the groupreceiving simvastatin (P=0.005) but not the other two. In addition, a significant positive correlation was observed between mtDNA and muscle ubiquinone in all groups (R=0.63, P<0.01), with the strongest association found in the simvastatin-treated subjects (R=0.75, P=0.002). Next, in an attempt to determine whether statin-induced muscle pain may be associated with muscle mtDNA depletion, archived muscle biopsies collected from statin users with muscle complaints were sought through a review of a muscle biopsy database and possible study samples were identified; however, this was put on hold as too much information was missing from the pathology reports. Third, a series of cell culture experiments were carried out in which human skeletal muscle myotubes were exposed to various concentrations of simvastatin or atorvastatin, in order to determine an appropriate dose range for subsequent mitochondrial toxicity experiments. Lastly, mtDNA content and expression was quantified in skeletal muscle biopsies collected from 10 patients with statin-induced rhabdomyolysis (SIR) and compared to 8 healthy controls to investigate whether muscle mtDNA is altered in rhabdomyolysis. No differences in mtDNA content or expression were observed between the two study groups, but this may have been be due to the SIR subjects' marked heterogeneity. Statin therapy can be associated with considerable alterations in mtDNA content, which may play a role in the aetiology of SIM. MtDNA levels alterations with statin exposure should be investigated further to explore the involvement of mitochondrial alterations in the mechanism of SIM, and determine whether these may represent a useful clinical tool for assessing statin-induced muscle toxicity.
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