Reduced activity of two LPL C-terminal variants, p.Glu396Val and novel p.Trp417Cys: a clinical, biochemical and...

Lipoprotein lipase (LPL) is essential for plasma triglyceride (TG) hydrolysis, and biallelic LPL loss-of-function variants cause familial chylomicronemia syndrome. However, the clinical and functional significance of heterozygous LPL variants remains incompletely characterized. We investigated two heterozygous LPL missense variants, c.1187A>T (p.Glu396Val) and the novel c.1251G>C (p.Trp417Cys), in patients with severe hypertriglyceridemia (HTG).

Two probands underwent comprehensive clinical, biochemical, and imaging assessments. Genetic analysis involved whole-exome sequencing and Sanger confirmation. Variant interpretation incorporated in silico prediction, evolutionary conservation, AlphaFold-based residue mapping, LPL-GPIHBP1 structural-context analysis, and NetCGlyc prediction. Functional effects were assessed in HEK293T cells transfected with wild-type or mutant LPL plasmids, measuring mRNA/protein expression and enzymatic activity.

Clinically, Proband 1 exhibited recurrent pancreatitis with moderate hyperglycemia, whereas Proband 2 had extreme HTG without pancreatitis but with severe insulin resistance, reflecting variable expressivity. Two LPL missense variants, c.1187A>T (p.Glu396Val) and the novel c.1251G>C (p.Trp417Cys), both located in exon 8. Both affected residues were highly conserved and localized to the C-terminal domain by AlphaFold-based mapping. NetCGlyc predicted Trp417 as a potential C-mannosylation-related residue. Mutant LPL mRNA and protein expression were comparable to wild-type, whereas enzymatic activity was significantly reduced in cell lysates and culture medium.

These findings provide functional evidence that p.Glu396Val and the novel p.Trp417Cys impair LPL enzymatic activity despite preserved protein abundance, supporting a qualitative functional defect. Our findings highlight the value of functional validation and metabolic assessment in interpreting heterozygous LPL variants in HTG.