Familial hypercholesterolemia concealed by a protein-truncating variant of PCSK9

Familial hypercholesterolemia (FH) is one of the most common inherited dyslipidemias and a major risk factor for premature coronary artery disease. Statins are the primary lipid-lowering therapy for FH but are usually insufficient for reducing low-density lipoprotein cholesterol to normal levels, necessitating additional medications such as proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors. However, the safety of long-term PCSK9 inhibition is unclear. Here, we report an extremely rare family where FH phenotypes are mitigated by co-existing familial hypobetalipoproteinemia caused by a protein-truncating variant of PCSK9. This case suggests that long-term PCSK9 inhibitor treatment may be safe and effective for patients with FH.