Furthermore, a recent study in PCSK9 KO mice [63] showed that PCSK9 deficiency, of most likely locally produced (in the heart muscle), not circulating, PCSK9 rewires heart metabolism and contributes to the development of HF with preserved ejection fraction — a puzzling observation, especially since PCSK9 is expressed at extremely low levels in mouse hearts [64]. This evidence concerns the gene PCSK9 and hydrops fetalis.