Cholesterol-lowering statin drugs significantly reduce cardiovascular morbidity and mortality, but a subset of statin users experience muscle pain/weakness, which limits the utility of statins in these individuals. A better understanding of the genetic predisposition to statin-induced myopathy will advance our understanding of the pathogenesis and also suggest new strategies to prevent and treat these symptoms. This application builds on our identification of a novel genetic locus that influences susceptibility to statin-induced myopathy. Using a unique genetic reference panel of 94 diverse inbred mouse strains, we assessed the adverse effects of simvastatin in more than 800 individual mice. Our integrated analysis of genome-wide association (GWAS) and expression quantitative loci (eQTL) data identified Tpmt as a high-probability casual gene that is specifically associated with statin-induced myopathy. The Tpmt gene encodes the enzyme thiopurine S-methyltransferase (TPMT), which metabolizes xenobiotics such as thiopurine drugs; the only known endogenous substrate is selenocysteine, an amino acid that is selectively incorporated into selenoproteins, which have critical roles in maintaining cellular redox state. Selenoprotein synthesis requires isopentenyl intermediates that are produced by the HMG CoA pathway for cholesterol synthesis, and which are reduced in the presence of statins. Interestingly, insufficiency of some selenoproteins causes muscle symptoms that resemble statin-induced myopathy. We hypothesize that genetic variation in Tpmt expression levels in combination with statin treatment constitutes a two-hit mechanism for the induction of statin myopathy: (1) statin limits the supply of isopentenyl groups for selenocysteine synthesis, and (2) genetic variation causing elevated TPMT levels reduces the pool of selenocysteine available for protein synthesis. We will test our hypothesis using a combination of in vivo and in vitro studies.
In Aim 1, we will determine the effect of modulating Tpmt levels in statin-treated mice through gain- and loss-of-function approaches using adeno-associated virus (AAV)-mediated gene transfer into genetic backgrounds that we have defined as myopathy-susceptible or -resistant. Conversion between myopathy sensitivity and resistance will provide strong evidence that Tpmt is a statin myopathy susceptibility gene.
In Aim 2, we will investigate each component of the statin?TPMT?selenoprotein axis for potential contributions to statin myopathy. We will first assess whether modulation of Tpmt expression levels alters selenoprotein levels in vivo and in cultured hepatocytes. We will next determine whether selenoprotein levels influence statin-induced myotoxicity in cultured myotubes. Finally, we will assess the effects of statin/selenoproteins on myocyte mitochondrial function and reactive oxygen species levels. The completion of our aims will contribute insights into the genetic susceptibility and pathogenesis of statin myopathy, and suggest strategies for personalized statin therapy.
Statin drugs are widely prescribed and effective for reducing cardiovascular disease risk, but some individuals experience side-effects, particularly muscle pain and weakness. We have identified a candidate gene that is associated with statin muscle side-effects in the mouse, and will determine how it causes them. This work may contribute to our understanding of why some individuals develop muscle side effects, and may allow prediction of adverse effects before taking statin.
