Impact of Pcsk9 deficiency on cardiac function and mitochondrial metabolism. Pcsk9 deficiency is associated with increased heart left ventricular thickness and reduced running performance independently of skeletal muscle alterations. Electron microscopy analysis showed increased cardiac accumulation of lipid droplets associated with a reduced density of mitochondrial cristae; this proffunctionally translated into impaired oxidative phosphorylation and mitochondrial metabolism in PCSK9 KO hearts.