Mutations in LPL, APOC2, APOA5, GPIHBP1 and LMF1 in patients with severe hypertriglyceridaemia

Abstract

Objectives: The severe forms of hypertriglyceridaemia (HTG) are caused by mutations in genes that lead to the loss of function of lipoprotein lipase (LPL). In most patients with severe HTG (TG > 10 mmol L(-1) ), it is a challenge to define the underlying cause. We investigated the molecular basis of severe HTG in patients referred to the Lipid Clinic at the Academic Medical Center Amsterdam.

Methods: The coding regions of LPL, APOC2, APOA5 and two novel genes, lipase maturation factor 1 (LMF1) and GPI-anchored high-density lipoprotein (HDL)-binding protein 1 (GPIHBP1), were sequenced in 86 patients with type 1 and type 5 HTG and 327 controls.

Results: In 46 patients (54%), rare DNA sequence variants were identified, comprising variants in LPL (n = 19), APOC2 (n = 1), APOA5 (n = 2), GPIHBP1 (n = 3) and LMF1 (n = 8). In 22 patients (26%), only common variants in LPL (p.Asp36Asn, p.Asn318Ser and p.Ser474Ter) and APOA5 (p.Ser19Trp) could be identified, whereas no mutations were found in 18 patients (21%). In vitro validation revealed that the mutations in LMF1 were not associated with compromised LPL function. Consistent with this, five of the eight LMF1 variants were also found in controls and therefore cannot account for the observed phenotype.

Conclusions: The prevalence of mutations in LPL was 34% and mostly restricted to patients with type 1 HTG. Mutations in GPIHBP1 (n = 3), APOC2 (n = 1) and APOA5 (n = 2) were rare but the associated clinical phenotype was severe. Routine sequencing of candidate genes in severe HTG has improved our understanding of the molecular basis of this phenotype associated with acute pancreatitis and may help to guide future individualized therapeutic strategies.

Figure 1. Functional analysis of LMF1 mutants

LMF1 function was tested in vitro in cld-mutant hepatocytes co-expressing LPL and LMF1. Wild-type LMF1 protein shows release of LPL activity in the medium. This activity level was set as 1. LMF1-W464X and LMF1-Y439X result in loss of LMF1 function and thus severely impaired release of active LPL. All newly diagnosed LMF1 variants show a normal release of active LPL and thus do not affect LMF1 function. Means ± SE are shown; *P<0.05.

Figure 2. The distribution of genetic variants in candidate genes for severe HTG type 1 and type 5

Among patients with type 1 HTG, 51% carry a mutation in LPL (13 have a homozygous mutation, two are compound heterozygote, four are heterozygous and three are carriers of a novel mutation). Seven type 1 HTG patients (16%) are carriers of a rare mutation in APOC2, APOA5 and GPIHBP1. Nine patients carry a common variant in LPL or APOA5 and five patients did not have any mutation in the candidate genes. Among type 5 patients, only 19% were carriers of a mutation in LPL (two compound heterozygous, three heterozygous for one mutation and three were carriers of a novel mutation). Only one carrier was identified with a heterozygous mutation in APOA5. There were two carriers with a mutation in LMF1. Sixteen patients (37%) had a common genetic variation in LPL or APOA5 and 17 patients (40%) did not have any genetic abnormality in the candidate genes.