Identification and functional analysis of novel homozygous LMF1 variants in severe hypertriglyceridemia

BACKGROUND: The genetic basis of hypertriglyceridemia (HTG) is complex and includes variants in lipase maturation factor 1 (LMF1), an endoplasmic reticulum (ER)-chaperone involved in the post-translational activation of lipoprotein lipase (LPL). OBJECTIVE: The objective of this study was to identify and functionally characterize biallelic LMF1 variants in patients with HTG. METHODS: Genomic DNA sequencing was used to identify biallelic LMF1 variants in HTG patients without deleterious variants in LPL, apolipoprotein C-II (APOC2), glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) or apolipoprotein A-V (APOA5). LMF1 variants were functionally evaluated by in silico analyses and assessing their impact on LPL activity, LMF1 protein expression, and specific activity in transiently transfected HEK293 cells. RESULTS: We identified four homozygous LMF1 variants in patients with severe HTG: two novel rare variants (p.Asn147Lys and p.Pro246Arg) and two low-frequency variants (p.Arg354Trp and p.Arg364Gln) previously reported at heterozygosity. We demonstrate that all four variants reduce the secretion of enzymatically active LPL by impairing the specific activity of LMF1, whereas p.Asn147Lys also diminishes LMF1 protein expression. CONCLUSION: This study extends the role of LMF1 as a genetic determinant in severe HTG and demonstrates that rare and low-frequency LMF1 variants can underlie this condition through distinct molecular mechanisms. The clinical phenotype of patients affected by partial loss of LMF1 function is consistent with multifactorial chylomicronemia syndrome (MCS) and suggests that secondary factors and additional genetic determinants contribute to HTG in these subjects.