Heterozygosity for pathogenic variants in familial chylomicronemia syndrome genes: from carrier state to complex trait

Purpose of review Biallelic loss-of-function (LOF) variants in any of five canonical genes – LPL, GPIHBP1, APOA5, APOC2 , and LMF1 – cause familial chylomicronemia syndrome (FCS), a rare and severe Mendelian disorder. Heterozygosity for these same variants is far more common but its clinical implications have only recently come into focus. Here, the past 18months of evidence on the monoallelic carrier state across the FCS genes is synthesized. Recent findings Longitudinal studies of LPL and APOA5 heterozygotes have demonstrated that triglyceride phenotypes vary widely both between individuals and within the same individual over time, ranging from completely normal levels to severe hypertriglyceridemia. Heterozygosity contributes to 15–25% of multifactorial chylomicronemia syndrome (MCS) cases. Mechanistic data implicate dominant-negative effects for some truncating variants and saturation of compromised lipolytic reserve as unifying explanations, rather than simple haploinsufficiency. Common intermediate-effect risk alleles such as LPL p.D36N and p.N318S, and APOA5 p.S19W and p.G185C, behave differently and should not be conflated with rare LOF variants. Summary The phenotype of FCS-gene heterozygotes is best understood as a complex trait modulated by polygenic background and secondary factors. This understanding has practical implications for cascade screening, counseling, and selection of patients who may benefit from emerging apolipoprotein C-III–targeted therapies, particularly those in the refractory MCS subgroup.