Novel LMF1 nonsense mutation in a patient with severe hypertriglyceridemia

Abstract

Context: Lipase maturation factor 1 (LMF1) gene is a novel candidate gene in severe hypertriglyceridemia. Lmf1 is involved in the maturation of lipoprotein lipase (LPL) and hepatic lipase in endoplasmic reticulum. To date only one patient with severe hypertriglyceridemia and related disorders was found to be homozygous for a nonsense mutation in LMF1 gene (Y439X).

Objective: The objective of the study was to investigate LMF1 gene in hypertriglyceridemic patients in whom mutations in LPL, APOC2, and APOA5 genes had been excluded.

Results: The resequencing of LMF1 gene led to the discovery of a novel homozygous nonsense mutation in one patient with severe hypertriglyceridemia and recurrent episodes of pancreatitis. The mutation causes a G>A substitution in exon 9 (c.1395G>A), leading to a premature stop codon (W464X). LPL activity and mass were reduced by 76 and 50%, respectively, compared with normolipidemic controls. The proband over the years has shown a good response to treatment. The proband's son, heterozygous for the W464X, shows normal plasma triglyceride levels.

Conclusions: We identified the second novel pathogenic mutation in LMF1 gene in a patient with severe hypertriglyceridemia. LPL deficiency in our patient was milder than in the carrier of the Y439X previously described.

Figure 1

Proband’s plasma TG levels and response to treatment. *, Gemfibrozil 600 mg twice a day, EPA+DHA 3 g/d, insulin aspart 0.3 IU/kg.

Figure 2

Analysis of LMF1 gene (reference sequence NM_022773). The chromatograms show the partial sequence of exon 9 in the proband (A), the proband’s son (B), and a control subject (C). The arrow indicates the c.1395G>A mutation.

Figure 3

Plasma lipase activity and LPL mass. The postheparin LPL mass (lower panel), LPL activity, and HL activity (upper panel) were assayed in the W464X proband, 69 normolipidemic controls, and one subject affected by familial chylomicronemia due to LPL deficiency. LPL and HL activity range and LPL mass range from 69 healthy men are, respectively, 94–412 mU/liter, 40–659 mU/liter and 69–1239 ng/ml.

Figure 4

Functional analysis of Lmf1 (W464X). A, Shown are rates of secretion of LPL activity after transfection with equal amounts (5 ng) of wild-type (wt) and mutant LMF1 expression vectors. The insert shows Western blot analysis of Lmf1 in cells expressing wt and W464X proteins. B, Shown are specific activities of mutants as a percentage of wt Lmf1. Values represent the mean ± sd of three separate transfections. * and §, Significant (P < 0.05) differences from wt and W464X, respectively.

Figure 5

Comparison of wild-type Lmf1 protein and mutant Lmf1 proteins, showing predicted transmembrane domains (gray) and the conserved DUF1222 domain (black).