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Fatty Acid Metabolism and Arrhythmias

Fatty acids are a major energy source for cardiac muscle and may play an important role in cardiovascular electrophysiology.1 Several clinical trials have shown that increased dietary intake of the n-3 (ω3) polyunsaturated fatty acids eicosapentaenoic acid and docosahexaenoic acid found in fish oils improves mortality in patients with coronary artery disease. The changes result, at least in part, from a decrease in sudden death and not from increased rates of recurrent myocardial infarction. Polyunsaturated fatty acids directly inhibit cardiac Na+ and Ca2+ channels, a finding that may underlie their beneficial effects. Abnormalities in fatty acid metabolism can lead to arrhythmias and sudden death. Refsum disease is an inherited condition caused by defects in phytanic acid (3,7,11,15tetramethylhexadecanoic acid) catabolism. Refsum disease presents with neurologic and cardiac symptoms, including retinitis pigmentosa, anosmia, polyneuropathy, ataxia, and arrhythmias.2 Cardiac arrhythmias and sudden death appear to be directly related to serum phytanic acid levels, which increase markedly during times of stress. Phytanic acid is not synthesized in humans; the major dietary source is the fats in meat and dairy products. Mutations in phytanoyl-CoA hydroxylase (PhyH), the enzyme that catalyzes conversion of phytanoyl-CoA to 2-hydroxyphytanoyl-CoA, are found in the majority of patients with Refsum disease.2,3 The downstream degradation pathway involves peroxisomal functions, and mutations in the peroxisomal matrix protein receptor PEX7 also have been found in patients with Refsum disease. Other disorders of peroxisome biogenesis, such as Zellweger syndrome and infantile Refsum disease, have related but more severe symptomatology. The mechanism by which phytanic acid accumulation leads to pathology remains unclear. Given that Refsum disease predominantly affects excitable tissues, a direct effect on membrane properties, membrane channels, and/or membrane protein trafficking has been hypothesized.3,4 Alternately, phytanic acid binds with members of the nuclear hormone family, such as the retinoid X receptor (RXR) and the peroxisome proliferator-activated receptor (PPARα). Thus, accumulation of phytanic acid could affect transcription of a large number of cellular proteins. Transgenic and gene-targeted (knockout, knockin) mice play an increasing role in the study of cardiac arrhythmias.5 Some mouse models are engineered with ion channel and connexin defects for study of mechanisms of arrhythmias