Acquired marked hypertriglyceridemia with anti-GPIHBP1 antibodies

Role of glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 in hypertriglyceridemia and diabetes

In diabetes, the impairment of insulin secretion and insulin resistance contribute to hypertriglyceridemia, as the enzymatic activity of lipoprotein lipase (LPL) depends on insulin action. The transport of LPL to endothelial cells and its enzymatic activity are maintained by the formation of lipolytic complex depending on the multiple positive (glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 [GPIHBP1], apolipoprotein C-II [APOC2], APOA5, heparan sulfate proteoglycan [HSPG], lipase maturation factor 1 [LFM1] and sel-1 suppressor of lin-12-like [SEL1L]) and negative regulators (APOC1, APOC3, angiopoietin-like proteins [ANGPTL]3, ANGPTL4 and ANGPTL8). Among the regulators, GPIHBP1 is a crucial molecule for the translocation of LPL from parenchymal cells to the luminal surface of capillary endothelial cells, and maintenance of lipolytic activity; that is, hydrolyzation of triglyceride into free fatty acids and monoglyceride, and conversion from chylomicron to chylomicron remnant in the exogenous pathway and from very low-density lipoprotein to low-density lipoprotein in the endogenous pathway. The null mutation of GPIHBP1 causes severe hypertriglyceridemia and pancreatitis, and GPIGBP1 autoantibody syndrome also causes severe hypertriglyceridemia and recurrent episodes of acute pancreatitis. In patients with type 2 diabetes, the elevated serum triglyceride levels negatively correlate with circulating LPL levels, and positively with circulating APOC1, APOC3, ANGPTL3, ANGPTL4 and ANGPTL8 levels. In contrast, circulating GPIHBP1 levels are not altered in type 2 diabetes patients with higher serum triglyceride levels, whereas they are elevated in type 2 diabetes patients with diabetic retinopathy and nephropathy. The circulating regulators of lipolytic complex might be new biomarkers for lipid and glucose metabolism, and diabetic vascular complications.

Anti-GPIHBP1 Antibody-Positive Autoimmune Hyperchylomicronemia and Immune Thrombocytopenia

Primary hyperchylomicronemia is characterized by marked hypertriglyceridemia exceeding 1,000 mg/dL. It is caused by dysfunctional mutations in specific genes, namely those for lipoprotein lipase (LPL), glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1), apolipoprotein C2 (ApoC-II), lipase maturation factor 1 (LMF1), or apolipoprotein A5 (ApoA-V). Importantly, antibodies against LPL or GPIHBP1 have also been reported to induce autoimmune hyperchylomicronemia. The patient was a 46-year-old man diagnosed with immune thrombocytopenia (ITP) at 41 years. At the time, he was administered prednisolone (PSL) and eltrombopag, a thrombopoietin receptor agonist. At 44 years, he suffered from acute myocardial infarction, and PSL was discontinued to avoid enhancing atherogenic risks. He was maintained on eltrombopag monotherapy. After discontinuing PSL, marked hypertriglyceridemia (＞3,000 mg/dL) was observed, which did not improve even after a few years of pemafibrate therapy. Upon referral to our clinic, the triglyceride (TG) level was 2,251 mg/dL, ApoC-II was 19.8 mg/dL, LPL was 11.1 ng/mL (0.02-1.5 ng/mL), GPIHBP1 was 47.7 pg/mL (740.0-1,014.0 pg/mL), and anti-GPIHBP1 antibody was detected. The patient was diagnosed to have anti-GPIHBP1 antibody-positive autoimmune hyperchylomicronemia. He was administered PSL 15 mg/day, and TG levels were controlled at approximately 200 mg/dL. Recent studies have reported that patients with anti-GPIHBP1 antibody-induced autoimmune hyperchylomicronemia had concomitant rheumatoid arthritis, systemic lupus erythematosus, Sjogren's syndrome, Hashimoto's disease, and Graves' disease. We report a rare case of anti-GPIHBP1 antibody-positive autoimmune hyperchylomicronemia with concomitant ITP, which became apparent when PSL was discontinued due to the onset of steroid-induced acute myocardial infarction.

A case of hyperchylomicronemia associated with GPIHBP1 autoantibodies and fluctuating thyroid autoimmune disease

Recent studies have reported that patients with autoimmune hyperchylomicronemia caused by glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) autoantibodies are associated with rheumatoid arthritis, systemic lupus erythematosus, Sjogren's syndrome, Hashimoto's thyroiditis, Basedow's disease, and immune thrombocytopenia. We report a rare case of hyperchylomicronemia due to GPIHBP1 autoantibodies and fluctuating thyroid autoimmune disease. A 28-year-old woman, diagnosed with Hashimoto's thyroiditis at 26 years of age, started taking 50 µg/day of levothyroxine sodium. She had an episode of acute pancreatitis at 27 years of age; her serum triglyceride (TG) level was 1291 mg/dL at that time. The patient was referred to our hospital because her hyperchylomicronemia (hypertriglyceridemia) did not improve on treatment with pemafibrate and eicosapentaenoic acid (EPA). Serum total cholesterol and TG levels were 237 mg/dL and 2535 mg/dL, respectively, while plasma pre-heparin lipoprotein lipase (LPL) mass was 15 ng/mL (26.5-105.5 ng/mL). We diagnosed her as Basedow's disease based on autoimmune antibodies and ultrasound examination. Targeted exome sequencing revealed no pathogenic variants in the LPL or GPIHBP1 genes. The serum GPIHBP1 autoantibody level was 686.0 U/mL (<58.4 U/mL) and GPIHBP1 mass was 301.9 pg/mL (570.6-1625.6 pg/mL). The patient showed hyperchylomicronemia during periods of hypothyroidism and hyperthyroidism, whereas GPIHBP1 autoantibodies were positive during episode of hyperchylomicronemia but negative during periods of normal TG levels. Based on these findings, the patient was diagnosed with hyperchylomicronemia due to GPIHBP1 autoantibodies and treated with rituximab. GPIHBP1 autoantibodies remained undetectable and TG levels were controlled at approximately 200 mg/dL.

Genetics of hypertriglyceridemia and atherosclerosis

PURPOSE OF REVIEW

The relationship between elevated triglyceride levels (i.e. hypertriglyceridemia) and risk of atherosclerotic cardiovascular disease (ASCVD) has been investigated for decades. Recent genetic studies have sought to resolve the decades-old question of a causal relationship.

RECENT FINDINGS

Genetic studies seem to demonstrate associations between elevated triglyceride levels and ASCVD risk. Mendelian randomization studies suggest this association may be causal. However, simultaneous pleiotropic effects of metabolically linked lipid variables - such as non-HDL cholesterol, apolipoprotein B and HDL cholesterol -- often go unaccounted for in these studies. Complex underlying pleiotropic interactions of triglycerides with these lipid fractions together with unmeasured intercalated nonlipid-related mechanisms, such as inflammation and coagulation, impair the ability of genetic studies to implicate a direct role for triglycerides on ASCVD risk. One potential mechanism seems largely driven by the cholesterol carried within triglyceride-rich lipoproteins and their remnants, rather than their triglyceride content.

SUMMARY

Although the exact mechanisms linking elevated triglyceride levels to ASCVD remain to be determined, new therapeutics that reduce triglyceride levels might be advantageous in certain patients. Newer investigational triglyceride-lowering therapies derived from human genetics target key proteins, such as apo C-III and ANGPTL3. Although these treatments clearly lower triglyceride levels, their efficacy in atherosclerotic risk reduction remains unproven.