Digenic Inheritance and Gene-Environment Interaction in a Patient With Hypertriglyceridemia and Acute Pancreatitis

Abstract

The etiology of hypertriglyceridemia (HTG) and acute pancreatitis (AP) is complex. Herein, we dissected the underlying etiology in a patient with HTG and AP. The patient had a 20-year history of heavy alcohol consumption and an 8-year history of mild HTG. He was hospitalized for alcohol-triggered AP, with a plasma triglyceride (TG) level up to 21.4 mmol/L. A temporary rise in post-heparin LPL concentration (1.5-2.5 times of controls) was noted during the early days of AP whilst LPL activity was consistently low (50∼70% of controls). His TG level rapidly decreased to normal in response to treatment, and remained normal to borderline high during a ∼3-year follow-up period during which he had abstained completely from alcohol. Sequencing of the five primary HTG genes (i.e., LPL, APOC2, APOA5, GPIHBP1 and LMF1) identified two heterozygous variants. One was the common APOA5 c.553G > T (p.Gly185Cys) variant, which has been previously associated with altered TG levels as well as HTG-induced acute pancreatitis (HTG-AP). The other was a rare variant in the LPL gene, c.756T > G (p.Ile252Met), which was predicted to be likely pathogenic and found experimentally to cause a 40% loss of LPL activity without affecting either protein synthesis or secretion. We provide evidence that both a gene-gene interaction (between the common APOA5 variant and the rare LPL variant) and a gene-environment interaction (between alcohol and digenic inheritance) might have contributed to the development of mild HTG and alcohol-triggered AP in the patient, thereby improving our understanding of the complex etiology of HTG and HTG-AP.

Keywords: APOA5 gene; LPL gene; acute pancreatitis; alcohol-induced hypertriglyceridemia; gene-environment interaction.

FIGURE 1

Dynamic changes of the fasting plasma TG levels, post-heparin LPL mass and LPL activity during hospitalization and follow-up periods of the patient with alcohol-triggered HTG-AP. Day 3 (D3), numbered by reference to the time of disease onset, corresponds to the date when the patient was transferred to our service. See Table 1 for the precise values of the three parameters. HTG-AP, hypertriglyceridemia-induced acute pancreatitis; LPL, lipoprotein lipase; ICU, intensive care unit.

FIGURE 2

Abdominal computed tomography images of the patient showing the affected pancreas. The left image was taken on the third day of onset of acute pancreatitis, showing peripancreatic exudation (arrow) and the enlarged pancreas with adjacent water density shadow. The right image was taken on the eighteenth day of disease onset, showing the occurrence of infected pancreatic necrosis. The box indicates a partial necrotic pancreas whilst the arrows indicate infective necrosis.

FIGURE 3

Genetic variants detected in the patient and the family tree. (A) Sanger sequencing electropherogram showing the novel LPL c.756T > G (p.Ile252Met) variant and the known common APOA5 c.553G > T (p.Glu185Cys) variant identified in the patient. The variants are indicated by arrows. (B) Arrow denotes the patient. Genotype status with respect to the APOA5 c.553G > T and LPL c.756T > G variants, age, BMI and TG levels are provided for each subject. (C) Alignment of partial LPL amino acid sequences spanning the p.252 site. LPL, lipoprotein lipase; APOA5, apolipoprotein A5; BMI, body mass index; het, heterozygous; wt, wild-type; TG, triglyceride; NA, not analyzed.

FIGURE 4

Western blot analysis of LPL expression in transfected HEK293T cells without heparin treatment. (A) Results were the average taken from three independent experiments. (B) LPL, lipoprotein lipase; WT, wild-type; EV, empty vector.

FIGURE 5

Western blot analysis of post-heparin LPL expression in cell lysates (A) and media (B) of transfected HEK293T cells. Results were the average taken from three independent experiments. LPL, lipoprotein lipase; WT, wild-type; EV, empty vector.

FIGURE 6

Post-heparin LPL activity in transfected HER293T cell lysates (A) and medium (B). Results were expressed as mean ± SD from three independent transfections, with each transfection being performed in five replicates. LPL, lipoprotein lipase; WT, wild-type; EV, empty vector. *p < 0.05; **p < 0.01; ***p < 0.001.