Monogenic and polygenic causes of low and extremely low LDL-C levels in patients referred to specialty lipid clinics: Genetics of low LDL-C

A case of hypocholesterolemia under study

Primary hypocholesterolemia (or hypobetalipoproteinemia) is a rare disorder of lipoprotein metabolism that may be due to a polygenic predisposition or a monogenic disease. Among these, it is possible to differentiate between symptomatic and asymptomatic forms, in which, in the absence of secondary causes, the initial clinical suspicion is plasma ApoB levels below the 5th percentile of the distribution by age and sex. Here we describe the differential diagnosis of a case of asymptomatic hypocholesterolemia. We studied proband's clinical data, the lipid profile of the proband and her relatives and the clinical data of the family relevant to carry out the differential diagnosis. We performed a genetic study as the diagnostic test. The information obtained from the differential diagnosis suggested a heterozygous hypobetalipoproteinemia due to PCSK9 loss-of-function variants. The diagnostic test revealed, in the proband, the presence of a heterozygous PCSK9 frame-shift variant of a maternal origin. Plasma levels of LDL cholesterol and PCSK9 of the patient and her relatives were compatible with the segregation of the variant revealed. In conclusion, the diagnostic test performed confirmed the suspected diagnosis of the proband as asymptomatic familial hypobetalipoproteinemia due to a loss-of-function variant in the PCSK9 gene.

LDL Cholesterol-How Low Can We Go?

Combinations of lipid-lowering agents can often bring LDL cholesterol down to around 40 mg/dL (1 mmol/L). Randomized controlled trials indicate that this reduces the risk of atherosclerotic vascular events with minimal adverse effects. This has raised the question of whether there is any concentration of LDL cholesterol below which further lowering is futile and/or a source of new adverse effects. This article examines several lines of evidence that lead to the conclusion that there is no known threshold below which lowering LDL cholesterol is harmful, but reduction of LDL cholesterol below 25 mg/dL may provide little if any further benefit.