Current Diagnosis and Management of Primary Chylomicronemia.

1. Definition of Chylomicronemia

Chylomicrons (CMs) are intestine-derived lipoproteins that transport dietary fat to peripheral tissues 1) . CMs are usually quickly cleared from plasma after an overnight fast. The hallmark of chylomicronemia is persistent elevation of CMs in the fasting state (＞12h). Plasma triglyceride (TG) levels in chylomicronemia typically range from 1,000 to 15,000 mg/dL or higher 2) . As CMs start to accumulate in plasma when TG levels increase to more than 500 - 1,000 mg/dL 1) , it is practical to screen and suspect chylomicronemia if plasma TGs ＞1,000 mg/dL 2 , 3) .

2. Metabolism of Chylomicrons

CMs are produced by the intestine after a meal and secreted into the circulation 1) . Their synthesis requires apoB-48 protein, which is transcribed from the edited mRNA of APOB , and microsomal triglyceride transfer protein (MTTP), which incorporates dietary TGs into CMs. More than 90% of lipids in CMs are TGs (derived from dietary fatty acids and TGs), which are hydrolyzed by lipoprotein lipase (LPL) on the endothelial cell surface of peripheral tissues (muscle, heart, adipose tissue, etc.) to liberate free fatty acids (FFAs). FFAs are used as an energy source in peripheral tissues, or stored as TGs in adipose tissues, or re-esterified and secreted as very low-density lipoprotein (VLDL)-TG by the liver. CMs are converted to CM remnants after TG hydrolysis by LPL and then cleared from plasma through endocytosis mainly by the liver.

3. Molecular Basis of Chylomicronemia

Plasma levels of CMs are affected by primary factors as well as secondary factors 2 , 4 , 5) . Primary factors consist of defects in the proteins that metabolize CMs, such as LPL and its related proteins (apolipoprotein(apo)C-II, apoA-V, glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1), lipase maturation factor 1 (LMF1)) ( Fig.1 ) . Secondary factors include conditions that impair CM metabolism, such as poor control of diabetes, excessive alcohol intake, and pregnancy. In primary chylomicronemia (PCM), chylomicronemia usually persists even after management of secondary factors. PCM is caused by defects in LPL pathway genes ( LPL, APOC2, APOA5, GPIHBP1, LMF1 ) due to genetic mutations or presence of autoantibodies against LPL pathway proteins (GPIHBP1, LPL, and apoC-II) 6 - 9) .

4. Complications of Chylomicronemia

Chylomicronemia is a risk factor for acute pancreatitis 10) . Although the underlying mechanisms are not fully understood, the widely accepted hypothesis is that an excess of FFAs derived from lipolysis of chylomicron-TG by pancreatic lipase damages pancreatic acinar cells or endothelial cells of pancreatic capillaries, leading to activation of inflammatory processes 11) . In addition, high levels of plasma CM may increase the viscosity of pancreatic microcirculation and impair pancreatic blood flow, further exacerbating inflammation. Genetic predisposition may also contribute to the pathogenesis 12) .

Based on this hypothesis, it is generally believed that chylomicronemia per se induces pancreatitis and, therefore, reducing plasma CM levels should lower the risk of pancreatitis 10) . The widely accepted treatment goal is to reduce plasma TG levels to at least less than 1,000 mg/dL 2 , 4 , 5 , 12 - 14) .

5. Primary Factors of Chylomicronemia

Five genes ( LPL , LMF1 , GPIHBP1 , APOC2 , APOA5 ) have been identified as the causative genes of monogenic chylomicronemia ( Fig.1 ) 4 , 5) . All of them are required for the normal function of LPL. Homozygous, compound heterozygous, or double heterozygous loss-of-function mutations of these LPL pathway genes usually cause monogenic chylomicronemia with an autosomal recessive mode of inheritance 15) . Heterozygous mutations or pathological variants in HTG-susceptible genes, including those for hepatic lipase (HL) and apolipoprotein E, may also predispose to chylomicronemia 3 , 5 , 16 - 18) .

Loss-of-function mutations in LPL-pathway genes have been identified in only less than 30-40% of patients suspected of monogenic chylomicronemia 19 , 20) . In some cases, autoantibodies against LPL pathway proteins can cause PCM. Additional genetic factors or unknown causes may also underlie PCM.

• LPL (LPL deficiency, OMIM: 238600): LPL is the most common causative gene of monogenic chylomicronemia. More than 220 pathogenic variants in LPL have been described (missense, nonsense, splicing variants, insertion/deletion of nucleotide(s), deletion/duplication/insertion/rearrangement of exon(s)) 2 , 4 , 15 , 20 - 22) . A list of mutations in LPL has been comprehensively summarized in exon-intron diagrams by Rabacchi C et al. and Rodrigues R et al. 20 , 22) . LPL activity is usually tested in post-heparin plasma taken 10-15 min after iv. heparin injection (10-60 IU/kg body weight) 4 , 23) .

• LMF1 (LMF1 deficiency, OMIM: 246650): LMF1 encodes LMF1, a transmembrane chaperone protein of parenchymal cells (muscle, adipose tissues, etc.). LMF1 is localized to the endoplasmic reticulum and is required for proper synthesis and secretion of LPL and HL. In addition to nonsense mutations that were originally reported, several other mutations, including missense loss-of-function mutations, have been identified in LMF1 24 - 29) .

• GPIHBP1 (GPIHBP1 deficiency, OMIM: 615947): GPIHBP1 encodes GPIHBP1, a transmembrane protein that tethers LPL on the endothelial cell surface to provide a platform for TG hydrolysis 30 , 31) . GPIHBP1 captures LPL in the subendothelial (interstitial) space of peripheral tissues (skeletal muscles, heart, adipose tissues, etc.), transports LPL from the subendothelial surface to the luminal surface of endothelial cells by transcytosis, and anchors LPL on the luminal surface facing the bloodstream to facilitate lipolysis. So far, at least 23 pathogenic mutations in GPIHBP1 have been reported in severe HTG patients, including those causing total GPIHBP1 deficiency 21 , 32 - 36) . Mutations in GPIHBP1 are comprehensively summarized in the exon-intron diagram by Rabacchi et al. 35) . GPIHBP1 deficiency may be differentiated from LPL deficiency in that LPL activity or protein in post-heparin plasma is not totally lacking in GPIHBP1 deficiency 37) .

• APOC2 (apoC-II deficiency, OMIM: 207750): APOC2 encodes apoC-II, a co-factor required for LPL activity. APOC2 is the second most common causative gene of monogenic chylomicronemia. At least 24 mutations have been reported worldwide 2 , 4 , 5 , 38 - 41) , and are comprehensively summarized in the exon-intron diagram by Wolska et al. 41) . In some cases, plasma levels of apoC-II have been severely decreased but are detected with apparently normal electrophoretic patterns (so-called hypoapoC-II) 42 - 45) . In apoC-II deficiency, symptoms often develop at an older age (13-60 years) than in LPL deficiency 1 , 4) . As apoC-II deficient patients tend to be subjected to strict fat restriction at an older age, they may have poorer dietary adherence and more frequent episodes of acute pancreatitis in adulthood than LPL deficient patients, often accompanied by high VLDL levels 2) .

• APOA5 (apoA-V deficiency, OMIM: 144650): APOA5 encodes apoA-V, a co-factor of LPL that enhances interaction between LPL and TG-rich lipoproteins (TGRLs) 46 , 47) . ApoA-V has dual binding affinity for both TGRL and GPIHBP1, thereby forming the TGRL-apoA-V-GPIHBP1-LPL complex to facilitate lipolysis 31 , 48 , 49) . So far, at least 21 mutations in APOA5 have been reported, which are comprehensively summarized in exon-intron diagrams by Albers et al. 50 - 53) . In most cases, severe HTG develops later in life due to the combinatorial effects of primary (genetic) and secondary factors (aging, diabetes, pregnancy, HIV therapy, etc.) 53) . The underlying molecular mechanisms of apoA-V deficient HTG in terms of gene-environmental interactions has only begun to be elucidated 54 , 55) .

• Autoantibodies against LPL pathway proteins : Autoimmune chylomicronemia due to autoantibodies against LPL pathway proteins (GPIHBP1, LPL, apoC-II) has been reported 6 - 9) . Autoimmune chylomicronemia may be complicated by other autoimmune diseases and be ameliorated by steroid or immune-suppressive therapy.

6. Secondary Factors of Chylomicronemia

The following factors have been reported to induce chylomicronemia 3 - 5 , 13 , 35 , 56 - 61) . Among them, excess alcohol consumption and diabetes mellitus are the two most common factors associated with severe HTG 18 , 62 , 63) .

Pregnancy is another important factor that can induce chylomicronemia. Hormonal changes during pregnancy result in hyperlipidemia even in healthy subjects, as a physiological response to ensure nutrient supply to the fetus 64 - 66) . Plasma TG levels progressively increase in healthy subjects by 2- to 4-fold 67) , and markedly increase in subjects with PCM 68 - 74) . Pregnancy increases TGRL (VLDL) production and decreases TGRL clearance 75 - 78) .

• Lifestyle-related factors : Dietary factors such as excessive alcohol intake, high-fat diet, high-carbohydrate diet rich in fructose and other simple sugars. Less exercise and excessive intake of total calories, particularly in subjects who are overweight, have metabolic syndrome, or diabetes mellitus.

• Pathophysiological conditions : Pregnancy, obesity, metabolic syndrome, anorexia nervosa, glucose intolerance, diabetes mellitus with insulin resistance or insulin deficiency, endocrine diseases such as hypothyroidism, acromegaly, Cushing’s syndrome, Nelson’s syndrome, glycogen storage diseases, amyloidosis, renal disease (nephrotic syndrome, proteinuria, uremia, glomerulonephritis, etc.), liver disease, autoimmune disorders, lipodystrophies, Weber-Christian disease, multiple myeloma, paraproteinemia, and lymphoproliferative disorders, etc.

• Medications : Glucocorticoids, oral estrogens (contraceptives, postmenopausal replacement therapies), clomiphene, tamoxifen, exogenous testosterone; retinoids such as isotretinoin, bexarotene; immunosuppressants and anticancer drugs such as sirolimus, cyclosporine A, tacrolimus, capecitabine, cyclophosphamide, asparaginase; antihypertensives such as thiazides, loop diuretics, non-selective β-blockers; bile acid-sequestrants; antiviral drugs such as entecavir, ritonavir and other antiretroviral protease inhibitors; second-generation antipsychotics (atypical antipsychotics), such as dozapine, clozapine, olanzapine, risperidone, quetiapine; antidepressants such as mirtazapine, venlafaxine; selective serotonin reuptake inhibitors (sertraline, etc.); anticonvulsants, such as valproate; anesthetic drugs, such as propofol.

7. Classification of Chylomicronemia

Chylomicronemia was formerly classified as type 1 and 5 hyperlipoproteinemia (HLP) in the Fredrickson classification (WHO classification) 5 , 79) , where increased plasma lipoproteins are CM for type 1 and CM+VLDL for type 5 HLP. However, differentiation of type 1 and type 5 HLP is often difficult because of considerable overlaps in phenotypes and genetic backgrounds 3 , 5 , 17) .

Differentiation of monogenic from polygenic chylomicronemia is also difficult 80) . Even when monogenic chylomicronemia is suspected, causative mutations have been identified in less than 30-40% 19 , 20) .

For practical reasons, we prefer to use the term PCM, which is defined as a condition with persistent elevation of TG ＞1,000 mg/dL even after management of secondary factors. The term familial chylomicronemia syndrome (FCS) is not used here, as monogenic chylomicronemia is typically autosomal recessive, and most cases are sporadic, without a family history. The term “familial” may confuse both patients and doctors, and may lead to underdiagnosis 3 , 81) .

8. Prevalence of Chylomicronemia

The prevalence of severe HTG (TG ＞10 mmol/L (885 mg/dL)) is estimated at ~1 in 600 in North America 5) . In a population of 440,240 subjects in the US, severe HTG (886-2,000 mg/dL) and very severe HTG (＞2,000 mg/dL) were observed in 0.15% and 0.014%, respectively 36) .

The monogenic type of chylomicronemia is very rare and its frequency is estimated at 1 to 2 per million in the general population 1 , 4 , 5) . The most common causative gene is LPL , followed by other LPL pathway genes ( APOC2 , GPIHBP1 , APOA5 , and LMF1 ) 5 , 15 , 53) .

Severe HTG is more often due to polygenic causes 3 , 82) . Dron et al. reported that the etiology of severe HTG (TG ＞10 mmol/L (885 mg/dL)) is monogenic in 1.1%, polygenic in 46%, and genetically unidentified in the rest of cases 83) . Another study identified monogenic causes in 0.96% of patients with very severe HTG (TG ＞20 mmol/L (1770 mg/dL)) recorded at least once during regular medical care 22) .

9. Prevalence of HTG-Induced Acute Pancreatitis

Regardless of the underlying etiology of HTG, the risk of acute pancreatitis increases as TG levels increase, particularly when they exceed 1,000 - 2,000 mg/dL 84) .

HTG is the third leading cause (5-38%) of acute pancreatitis after alcohol and gallstones 1 , 12 , 85 - 87) . HTG is the leading cause (25~50%) of acute pancreatitis during pregnancy 12 , 74 , 88) , which is most often seen in the third trimester (19%, 26%, 53% in the 1 st , 2 nd , and 3 rd trimester, and 2% in postpartum period) 89) . The frequency of HTG-induced acute pancreatitis in pregnancy is estimated as 1 in 1000-12,000 pregnancies 14 , 73 , 74) .

Risk of acute pancreatitis increases by ~4% for every 100 mg/dL increase in plasma TG 56 , 84 , 87 , 90 , 91) . According to the 2010 Endocrine Society guidelines, HTG can be classified into mild HTG (150-199 mg/dL), moderate HTG (200-999 mg/dL), severe HTG (sHTG, 1000-1999 mg/dL), or very severe HTG (vsHTG, ＞2000 mg/dL). The prevalence of pancreatitis in sHTG and vsHTG is about 10% and 20%, respectively 5 , 13 , 14 , 87 , 92) .

Clinically, it remains difficult to predict if a HTG patient will develop acute pancreatitis or not. Some patients do not develop acute pancreatitis even at TG levels higher than 30,000 mg/dL 1 , 2) . Conversely, patients can develop acute pancreatitis at TG levels of 5-10 mmol/L (442-885 mg/dL) 93) . The underlying etiology of monogenic chylomicronemia seems to confer a potent risk of acute pancreatitis 53 , 94 - 96) . Compared to normolipidemic individuals, the risk of acute pancreatitis increased by 16-fold, 56-fold, and 361-fold in HTG (5-9 mmol/L (442-796 mg/dL)) without LPL deficiency, HTG (＞9 mmol/L (796 mg/dL)) without LPL deficiency, and HTG (＞9 mmol/L (796 mg/dL)) with LPL deficiency (genetically-confirmed), respectively 94) . Greater cumulative exposure of HTG to the pancreas due to genetic causes may impose higher pancreatitis risk 53 , 94 - 97) (See sections 12 and 16(B) for other suggested risk factors of acute pancreatitis among HTG subjects).

10. Signs, Symptoms, and Complications of Chylomicronemia

Depending on the severity of the mutation, signs, symptoms, and complications of chylomicronemia may manifest as early as in infancy, in childhood, or later in life 4 , 95) .

• Abdominal pain, fat intolerance, failure to thrive : Abdominal pain affects ~60% of monogenic chylomicronemia patients and can be mild to incapacitating 4 , 95 , 96) . Recurrent episodes of abdominal pain may lead to dietary fat intolerance and failure to thrive. Body weight may be lower because of restricted food intake.

• Pancreatitis : Acute pancreatitis can be severe, recurrent, and life-threatening 1 , 4 , 95) . Acute pancreatitis may lead to chronic pancreatitis and diabetes. Irrespective of its etiology, severe HTG should be carefully monitored for the possible complications of pancreatitis. Compared to polygenic chylomicronemia, monogenic chylomicronemia is associated with a higher risk of acute pancreatitis and tends to manifest severe phenotypes 86 , 94 , 95) . According to a survey of lipidologists, ~67% of monogenic patients were hospitalized for acute pancreatitis vs. ~14% of polygenic patients 95 , 96) . It should be noted that high levels of plasma TG may interfere with assays of plasma pancreatic enzymes (lipase, amylase), resulting in falsely low levels 2 , 98 - 100) . HTG-induced acute pancreatitis should not be misdiagnosed due to apparently low serum levels of pancreatic enzymes.

• Lipemic plasma : Lipemic plasma (milky-looking plasma) is characterized by a creamy CM layer floating above the bottom layer, after leaving serum standing overnight in a refrigerator (the “refrigerator” test) 1) . In type 5 HLP, the bottom layer contains VLDLs and has a lactescent appearance. In type 1 HLP, the bottom layer is clear without apparent VLDL accumulation. Type 1 HLP is more frequently observed than type 5 in monogenic chylomicronemia 19) . (See reference by Yuan G et al. for a photograph of lipemic plasma 56) .)

• Eruptive xanthomas : Eruptive xanthomas affect 10-50% of the monogenic type 95 , 96) . They appear when plasma TG levels increase to more than 2,000 mg/dL as yellow papules on the skin of the trunk, buttocks, and extremities (extensor surfaces of the arms and knees) as a result of TG uptake by macrophages (see references by Nayak KR et al. and Yuan G et al. for photographs) 2 , 4 , 56 , 101) . Along with reduction in plasma TG levels, they gradually disappear over several weeks to a few months 2) .

• Lipemia retinalis : Lipemia retinalis affects ~40% of the monogenic type 95 , 96) and is characterized by retinal blood vessels with a milky appearance (a pale pink color) on fundoscopy (see references by Kumar J et al. and Yuan G et al. for photographs) 56 , 102) . Vision is not impaired. It is usually visible when plasma TG levels increase to more than 4,000 mg/dL 2) .

• Hepatomegaly and splenomegaly : Hepatomegaly and splenomegaly affect 10-50% of the monogenic type 95 , 96) as a result of TG uptake by macrophages and other cell types in these tissues 1 , 4) . Both conditions are reversible and rapidly improve within a week, along with reduction in plasma TG levels 2) .

• Other symptoms (fatigue, dyspnea, and neurological symptoms) : Other clinical symptoms include fatigue, dyspnea, and neurological symptoms 1) such as memory impairment (transient memory loss), cognitive impairment, mild dementia, cloudy thought, brain fog, neurosis, irritability, anxiety, and depression 4 , 95 , 103 - 107) . Neurological symptoms affect ~8% of the monogenic type 95) . Although little is known about the underlying mechanisms of these symptoms, large surveillance studies among chylomicronemia patients have shown that they dominantly and adversely affect patients’ quality of life and increase the burden of the disease 104 - 107) .

11. Diagnosis of Chylomicronemia

Undertreatment and underdiagnosis of chylomicronemia are one of the major risks for acute pancreatitis 84 , 87) . In order to achieve early diagnosis and treatment for chylomicronemia, simple diagnostic criteria that can be readily translated into general practice are required.

Based on TG levels that lead to suspicion of chylomicronemia (TG ＞1,000 mg/dL), clinical manifestations and the available data on diagnostic tests, here we propose diagnostic criteria for PCM ( Table 1 ) .

It should be noted that PCM is genetically confirmed in less than 30-40% of patients suspected of monogenic chylomicronemia 19 , 20) . Therefore, we have set three categories (definite, probable, and possible) so that PCM patients will not be missed even without a genetic diagnosis.

To achieve timely diagnosis and improve the diagnostic rate of PCM, screening of plasma TG levels in the following settings will be helpful. Clinicians across multiple disciplines, such as primary care physicians, gastroenterologists, gynecologists, and other doctors who have occasionally encountered severe HTG patients, should consult lipidologists concerning further diagnostic tests for chylomicronemia.

• Health checkup or opportunistic blood test : All patients who have TG levels of more than 1,000 mg/dL in universal lipid screening 108) or a routine clinic visit should be suspected of chylomicronemia 96 , 109) .

• Acute abdomen (including pancreatitis) : Those who have acute abdomen or are suspected of pancreatitis should have their plasma TG levels measured 96) . Plasma TG should be measured as early as possible after the onset of abdominal pain, as TG levels rapidly decrease within 24-48 hours of onset 110) .

• Pregnancy : Many cases of monogenic chylomicronemia have been discovered in the third trimester of pregnancy 111 - 118) . HTG-induced acute pancreatitis in pregnancy can be lethal to both mother and fetus 119) . Gestational HTG may also increase the risk of hyperviscosity syndrome 120) , pre-eclampsia 121) , fetal macrosomia, and fetal pancreatitis-related complications (in-utero fetal death, preterm labor, and prematurity) 66) . Pregnant women who are suspected of pancreatitis should be tested for plasma TG. Pregnant women at high-risk for HTG-induced acute pancreatitis may benefit from plasma TG screening and monitoring on a weekly basis 122) . Such patients include: those with HTG or pancreatitis prior to or during pregnancy; high predisposition for HTG-induced acute pancreatitis due to diabetes mellitus, obesity, hypertension, hypothyroidism, renal disease, liver disease, family history of HTG, alcohol consumption, and medications that cause HTG; HTG with abdominal pain or other symptoms typical to chylomicronemia 84 , 87 , 122) (See sections 12 and 16(B) for suggested risk factors of acute pancreatitis among HTG patients).

• Family members of chylomicronemia patients : Evaluation of plasma TG levels in family members is beneficial for early diagnosis and management of possible complications 96) .

12. Features for Suspecting Monogenic Chylomicronemia

Clinical features that lead to suspicion of monogenic chylomicronemia have been suggested by previous studies and in expert opinions, but further validation in various cohorts is required 2 - 5 , 15 , 18 , 53 , 80 , 96 , 109 , 123 , 124) . These features may be useful not only for indicating the likelihood of monogenic chylomicronemia but also for predicting a higher risk of pancreatitis 53 , 86 , 94 - 96) :

On the other hand, the polygenic type of chylomicronemia is more frequently associated with secondary/environmental factors, such as high-alcohol intake, diabetes mellitus, hypertension, and obesity 2 , 18 , 96) .

13. Treatment of Chylomicronemia

The treatment goal of chylomicronemia is to lower plasma TG levels enough to reduce the risk of pancreatitis. Data from large healthcare databases suggest that sustained HTG (＞500 mg/dL) increases the risk of pancreatitis (hazard ratio 1.79 [CI 95%: 1.10-1.28]) 125) and lowering TG from ＞500 mg/dL to less than 200 mg/dL can reduce the incidence of acute pancreatitis from 1.1 to 0.4 per 100 person-year (adjusted OR 0.45 [CI 95%: 0.34-0.60]) 126) . Due to the rareness of the disease, there have been no randomized control trials (RCTs) to determine treatment TG targets for prevention of pancreatitis. Mainly based on clinical experience, the opinion of experts is to recommend maintaining plasma TG levels below 500-1,000 mg/dL to prevent pancreatitis 2 , 4 , 5 , 12 - 14) .

A) Control of Secondary Factors

Comorbid conditions that aggravate chylomicronemia should be thoroughly evaluated in order to rule them out. If any are present, they should be managed adequately. Bodyweight reduction, reduced calorie intake, and increased energy expenditure through regular physical exercise may help reduce plasma TG levels, particularly in overweight subjects. Regular physical exercise may also help in the non-obese 93 , 127) . Bodyweight should be carefully controlled as rebound weight gain might elicit pancreatitis 13) .

B) Dietary Therapy

Strict dietary control is currently the primary treatment modality for chylomicronemia, although it is often insufficient and difficult to maintain in the long term 128 , 129) . Children and adolescents should be carefully monitored to ensure proper growth and development. Adjustment of social life might be a challenge throughout life 104 - 107) .

• Fat restriction : The mainstay of dietary treatment is a low-fat diet. Restriction of total dietary fat to ＜15-20 g per day (＜10-15 % of total energy intake) is usually required to reduce plasma CMs and prevent pancreatitis 128 , 129) . Under fat restriction, adequate intake of essential fatty acids (EFA; 2-4% of daily calories) and fat-soluble vitamins (A, D, E, and K) should be ensured to avoid deficiency. Signs and symptoms typical of EFA deficiency include: inadequate growth in pediatric patients, dry or dull hair, dry or scaly skin, skin lesions, particularly raised bumps on the skin, soft and brittle nails, and impaired wound healing 129) . Food sources of EFAs include soybeans, tofu, flaxseeds, walnuts, and chia seeds for alpha-linolenic acid (ALA) and whole grains for linoleic acid (LA).

• Medium-chain triglycerides (MCTs) : In a very-low-fat diet, MCTs containing fatty acids of ≦10 carbon atoms in length may be used to provide sufficient calories in meals or infant formula 4 , 36 , 128 , 129) . MCTs are absorbed directly into the circulation via the CM-independent pathway. MCTs may help reduce plasma TG levels 130) . In order to avoid possible adverse effects (diarrhea, abdominal pain, etc.), MCTs should be introduced slowly. The safety of long-term MCTs is not established and patients should be carefully monitored for possible complications such as hepatotoxicity. MCTs should not be confused with coconut oil, which contains lauric acid (C12) and other long-chain fatty acids 129) .

• Carbohydrate restriction : Restriction of carbohydrates, particularly fructose and other simple and refined carbohydrates, is advisable for patients with increased VLDL levels such as those with diabetes mellitus, metabolic syndrome, and obesity. As both CMs and VLDLs are substrates for LPL, reduced production of VLDLs due to carbohydrate restriction enhances the catabolism of CMs by LPL 131) . In cases where carbohydrate intake needs to be adequate, such as in pregnancy-associated HTG, carbohydrate iv may be a better therapeutic choice, as oral carbohydrate intake may produce a greater rise in plasma TG than carbohydrate iv 122 , 132) .

• Alcohol restriction : Alcohol intake should be restricted 4 , 128 , 129) .

C) Lipid-Lowering Medications

Current lipid-lowering medications (fibrates, n-3 polyunsaturated fatty acids (PUFAs), niacin, etc.) generally have little to no TG-lowering effects in patients with PCM, as they lower plasma TGs mainly by enhancing the LPL pathway and reducing VLDL levels. Treatment with n-3 PUFA or fish oil may be useful for lowering TG and preventing pancreatitis as suggested in patients with APOA5 mutations 21) . However, the effect of n-3 PUFAs needs to be monitored carefully, as their effectiveness has only been suggested by small studies without controls 3 , 5) . Fish oil supplements may increase the production of chylomicrons and are contraindicated according to an expert opinion 4 ) . In patients with autoantibodies against LPL pathway proteins (GPIHBP1, LPL, apoC-II), immune-suppressive agents may ameliorate HTG as well as the comorbid autoimmune diseases 7) .

14. Treatment of HTG-Induced Acute Pancreatitis

The clinical course of HTG-induced acute pancreatitis may be more severe than acute pancreatitis due to other causes in terms of complications and mortality rates 12 , 14 , 133 , 134) , but more controlled studies are required to produce firm evidence. Meta analysis of acute pancreatitis is difficult due to the heterogeneity of scoring systems for its severity 86) .

A) Standard Care for Acute Pancreatitis

Treatment of HTG-induced acute pancreatitis is based on standard care, including cessation of oral food intake, admission to hospital, intravenous hydration, hypocaloric parenteral nutrition avoiding excess calories and glucose infusions, pain management, prophylactic antibiotics, and protease inhibitors 12 , 130 , 135) . Any precipitating factors should be treated appropriately (e.g., insulin treatment for diabetes). Patients should be carefully monitored for development of pancreatic complications (necrosis, abscesses, etc.).

B) Specific Therapy for HTG-Induced Acute Pancreatitis

When patients can tolerate, oral TG-lowering medications (fibrates, n-3 PUFAs, niacin, etc.) may be administered 12 , 130 , 135) . MCTs may help reduce plasma TG levels as well as the risk of pancreatitis 130) . With a few exceptions, current TG-lowering medications are not based on the etiology of chylomicronemia. In apoC-II deficiency, infusion of normal human plasma containing apoC-II can greatly reduce plasma TG levels, and plasmapheresis has been suggested as a treatment of choice for pancreatitis due to apoC-II deficiency 4 , 58) .

C) Management of Chronic Pancreatitis

As acute pancreatitis can lead to chronic complications, patients with a history of it are better monitored for complications such as chronic pancreatitis, pancreatic pseudocysts, pancreatic insufficiency, steatorrhea, and insulin-dependent diabetes mellitus 4 , 86) . Although chronic complications are not invariably associated with HTG-induced acute pancreatitis 136) , they are not uncommon despite modern medical care 95) .

D) Other Therapeutic Options for HTG-Induced Acute Pancreatitis

• Insulin (should be individualized) : Insulin therapy is advised in patients with diabetes mellitus. Insulin stimulates LPL activity, thereby reducing plasma TG levels. Administration of insulin or insulin plus glucose may be considered in non-diabetic patients in the case of severe HTG-induced acute pancreatitis 14 , 137 - 139) . Detailed protocols for insulin/glucose administration have been summarized elsewhere 137 , 139) .

• Heparin (not usually recommended) : Heparin infusion has been used as a therapeutic option but is not usually recommended as a monotherapy in treatment guides by experts 12 , 14) . Heparin transiently increases plasma LPL levels by releasing LPL from the endothelial cell surface, which temporarily reduces plasma TG levels. However, heparin can also deplete LPL, causing a rebound increase in plasma TGs 140) . Heparin may increase the risk of pancreatic hemorrhage when pancreatic necrosis is present 66) .

• Heparin plus insulin (should be individualized) : Combination of heparin and insulin may be a therapeutic option for severe HTG-induced acute pancreatitis 14) . Although heparin infusion alone is usually not advised, a recent study has suggested that combination of heparin and insulin may be effective 14) . Evaluation in RCTs is awaited.

• Apheresis for HTG-induced acute pancreatitis (should be individualized) : In the acute setting, apheresis (lipoprotein apheresis (LA), plasmapheresis, or plasma exchange (PEX), etc.) 141 , 142) can rapidly reduce plasma TG levels (40-80%) by directly removing TGRLs, as reported in case reports, case series, and multi-center studies 12 , 14 , 36 , 57) . However, it is not proven whether rapid TG reduction by apheresis leads to better clinical outcomes than other therapeutic modalities in terms of pancreatic complications and mortality 59) . Plasmapheresis is costly, has only a transient TG- lowering effect (usually for a day), and may have adverse reactions (e.g., allergic reactions, anaphylactic shock, infusion-related infections, thromboses, etc.) 14) . A recent systematic review and case-control studies indicated that while plasmapheresis decreased plasma TG, it did not conclusively affect the morbidity or mortality of acute pancreatitis 14 , 86 ). A recent RCT, the first one in HTG-induced acute pancreatitis, has demonstrated that although plasma apheresis lowers plasma TG more efficiently than insulin plus heparin, it is costly and does not lead to better clinical outcomes 143) . In the guideline of the American Society for Apheresis (ASFA), plasmapheresis is a category III indication with Grade 2C recommendation (“optimum role of apheresis therapy is not established. Decision making should be individualized”; “Weak recommendation, low-quality or very low-quality evidence due to observational studies or case series”) 144) , and generally not recommended by experts in treatment guides for chylomicronemia 4 , 5) . Plasmapheresis may be a therapeutic option for: 1) severe HTG-induced acute pancreatitis with persistent TG elevation past the first 48-72h with no other therapeutic choice 57 , 59 , 86 , 120) ; 2) HTG-induced acute pancreatitis in pregnancy or postpartum with no other therapeutic choice 14 , 36 , 74 , 120 , 145 - 149) ; or 3) severe HTG-induced acute pancreatitis with high levels of serum lipase, hypocalcemia, lactic acidosis, worsening inflammation or organ dysfunction 149 , 150) . However, such advice is experience-based, not evidence-based.

• Prophylactic apheresis to prevent HTG-induced acute pancreatitis (should be individualized) : Prophylactic apheresis may be a therapeutic choice for preventing severe recurrent HTG-induced acute pancreatitis but evidence for it is limited to several case reports of HTG-induced acute pancreatitis 12 , 14 , 57 , 151 - 154) and gestational HTG-induced acute pancreatitis 155 , 156) .

15. Treatment of HTG and HTG-induced Acute Pancreatitis in Pregnancy

There are currently no formal guidelines for gestational HTG and HTG-induced acute pancreatitis due to the rarity of these conditions and insufficient evidence. The treatment approach for gestational HTG and HTG-induced acute pancreatitis is well summarized by Wong et al. 66) .

• Dietary therapy (restriction of dietary fat, MCTs, n-3 PUFA) : There have been reports of successful management of HTG and prevention of HTG-induced acute pancreatitis during pregnancy through early intervention with a low-fat or very-low fat diet, MCTs, and n-3 fatty acids 4 , 66 , 113 , 122 , 149 , 157 - 162) . For pregnant women at high-risk of pregnancy-associated pancreatitis, extreme fat restriction to ＜2 g/day may be required during the 2 nd and 3 rd trimesters for successful delivery 1 , 4 , 122 , 159) . Topical application of sunflower oil or corn oil in pregnancy with an extra-low-fat diet may help prevent EFA deficiency 122 , 129 , 159) . In pregnancy-induced HTG, carbohydrate may need to be restricted, but an adequate amount should be taken. Carbohydrate iv may be a better therapeutic choice during carbohydrate restriction, as carbohydrate per os may produce a greater rise in plasma TG than carbohydrate iv 122 , 132) . The risk of MCTs to the fetus is thought to be low 66) . The safety of maternal n-3 fatty acid supplementation (DHA 2.2 g and EPA 1.1 g/day) for the mother and the fetus has been confirmed through a RCT 66) .

• TG-lowering medications (some are contraindicated in Japan) : There have been several reports on the use of niacin, or fibrates (gemfibrozil and fenofibrate) for pregnancy-associated HTG 66 , 120 , 122 , 159 , 163 - 166) . However, the safety of niacin or fibrate use during pregnancy has not been established, and the use of fibrates during pregnancy is contraindicated in Japan.

• Admission to hospital : For gestational HTG, admission to hospital may be advised in the following cases: suspected pancreatitis, persistent abdominal pain, steep increase in plasma TG in the 3 rd trimester, or TG ＞40 mmol/L (3540 mg/dL) 66 , 122 , 159) . Gestational acute pancreatitis is managed through standard care, which has been extensively reviewed by Papadakis EP et al. 73) .

• Other therapeutic options : When uncontrollable, further management may include: insulin, insulin pus glucose, insulin plus heparin, or plasmapheresis 36 , 66 , 119 , 120 , 137 - 139 , 149 , 156 , 161) . In a treatment guide for gestational HTG, the use of insulin is recommended only for hyperglycemic pregnant women, and the use of heparin is not recommended due to the paucity of clinical evidence 66) .

16. Unanswered Questions

A) Molecular Basis and New Therapeutic Modalities

Unraveling the molecular basis of chylomicronemia may lead to the development of new therapeutic modalities 167) .

Emerging therapeutic targets for chylomicronemia include: a microsomal triglyceride transfer protein (MTTP) inhibitor (lomitapide) 168 - 170) ; an APOB antisense oligonucleotide (ASO) inhibitor (mipomersen) 171 - 173) ; APOC3 ASO inhibitors, e.g., volanesorsen 174) , which has been approved by the EMA for genetically confirmed chylomicronemia at high-risk for pancreatitis 93) ; diacylglycerol O-acyltransferase 1 (DGAT1) inhibitors (AZD7687, LCQ908 (Pradigastat)) 175 - 177) ; angiopoietin-like protein 3 (ANGPTL3) inhibitors, e.g., ANGPTL3 ASOs (IONIS-ANGPTL3-L Rx ) 178) and ANGPTL3 antibody (evinacumab) 179 - 182) .

For these new therapies, potential adverse effects need to be carefully evaluated, including fatty liver associated diseases for lomitapide and mipomersen 168 - 170 , 172) as a consequence of their inhibition of VLDL secretion; thrombocytopenia for volanesorsen 174) ; gastrointestinal adverse effects (diarrhea, nausea, etc.) for DGAT inhibitors 175 - 177) , consistent with the fact that homozygous loss-of-function mutations of DGAT1 cause a congenital diarrheal disorder (OMIM: 615863) 183) .

Other agents under development include CAT-2003, a niacin-eicosapentaenoic acid conjugate that blocks sterol regulatory element-binding protein (SREBP). Inhibition of SREBP-1c has ameliorated environment-induced severe HTG in mouse models of hyperlipidemia, including apoA-V deficient mice, by blocking secretion of large-sized VLDL particles 54 , 55) .

Orlistat, an inhibitor of intestinal lipase, may help reduce TG levels in PCM 21 , 184 , 185) but may have adverse effects such as oily stools and fat-soluble vitamin insufficiency 128) .

Treatment that targets a specific genetic cause of PCM is not available. A gene therapy for LPL (alipogene tiparvovec) was approved by the EMA in 2012, but is costly and has been withdrawn from the market 94 , 186) .

B) Risk and Management of CM-Related Complications

Not all patients with severe HTG manifest pancreatitis. Suggested risk factors of acute pancreatitis among HTG patients include underlying genetic etiology of monogenic type, underdiagnosis and undertreatment, younger age, higher baseline TG levels, prior history of acute pancreatitis, male, alcohol use, obesity, diabetes, hypertension, renal disease, liver disease, and hypothyroidism 84 , 87 , 94 , 95) . Further elucidation and validation of the risk factors and genetic predisposition for HTG-induced acute pancreatitis is awaited 187 - 191) . Understanding the molecular basis of HTG-induced acute pancreatitis is necessary for developing diagnostic markers as well as effective therapeutic modalities.

There has been controversy as to the atherogenicity of HTG, including chylomicronemia 136 , 192 , 193) . Although it remains uncertain whether HTG is a causal factor or a mere marker of atherosclerosis 3 , 91 , 194 - 196) , recent mendelian randomization studies have indicated an association between risk of cardiovascular diseases and variants in HTG-related genes, including causative genes of chylomicronemia ( APOA5 , LPL , etc.) 197 - 199) . Chylomicronemia, particularly when it is polygenic, may be associated with higher risk of cardiovascular diseases 53 , 96 , 200) , which warrants further studies.

C) Genotype-Phenotype Relationship of PCM

The genetic etiology of chylomicronemia may influence the risk of complications such as pancreatitis and atherosclerotic diseases 86 , 94 - 96) . The benefit, risk, and cost-effectiveness of genetic testing for chylomicronemia should be carefully evaluated 15 , 109 , 201 - 203) . Some expert reviews, which include the Consensus Panel report of the European Atherosclerosis Society, do not recommend routine genetic testing for severe HTG 3 , 204) .

D) Underdiagnosis and Undertreatment

Underdiagnosis of chylomicronemia is one of the major risks for pancreatitis 84 , 87) . A web-based patient survey reported that patients with chylomicronemia typically visit 5 physicians (range, 1-30) on average before receiving a final diagnosis of chylomicronemia 106 , 107) . Owing to the variety of symptoms and complications, patients with chylomicronemia may visit not only lipidologists and endocrinologists but also other diverse specialists, such as primary care physicians, pediatricians, obstetricians, emergency physicians, gastroenterologists, pancreatologists, and psychologists. Simple diagnostic criteria as well as cooperation among different medical specialists will be necessary to achieve timely diagnosis and treatment.

E) Unmet Needs and Burden of Disease

Due to the rarity of the disease, the clinical experience of each doctor is limited. Large registry studies 205 , 206) as well as patient-oriented observational studies 104 - 107) from the patient’s perspective are useful for understanding the unmet needs and burden of the disease from the physical, psychological, social, and financial viewpoints 104 - 107) .

A recent web-based patient survey revealed physical, emotional, and cognitive symptoms that are relevant to patient’s quality of life but have not been recognized by physicians 207 , 208) , including abdominal pain (41%), fatigue (23%), feeling sad/down/blue/depressed (18%), difficulty in concentrating (16%), impaired judgment (11%), brain fog (8%), forgetfulness (8%), and recent memory loss (5%) 106 , 107) . This survey also revealed actual handicaps felt at school, in society, and work, and family-related issues 106 , 107) .

Self-monitoring of plasma TG may be an unmet need that could help patients with the long-term management of the disease. It may enable patients to individualize their low-fat diets, hopefully leading to fewer episodes of acute pancreatitis 209) .

F) Support for Patients

The mainstay of the current treatment for chylomicronemia is dietary interventions. Supporting information and materials for patients on diets will help develop recipes and menu plans that would be more enjoyable and sustainable. Information and support for patients can be found at FCS Foundation (www.livingwithfcs.org; www.facebook.com/livingwithfcs), FCS Focus (fcsfocus.com), LPLD Alliance (UK) (www.lpldalliance.org), the National Organization for Rare Disorders (NORD) (https://rarediseases.org), and the Japan Intractable Diseases Information Center (https://www.nanbyou.or.jp/entry/4883). Supportive care from other healthcare professionals, such as medical social workers and mental health professionals, will be necessary to reduce the burden of the disease as well as to improve the quality of life of patients with chylomicronemia 105 - 107) .

Acknowledgments and Notice of Grant Support

This work has been supported by Health, Labour and Welfare Sciences Research Grant for Research on Rare and Intractable Diseases (H30-nanji-ippan-003).

Conflicts of Interest

Atsushi Nohara has nothing to disclose. Hayato Tada has nothing to disclose. Masatsune Ogura has received honoraria from Amgen Inc., Astellas Pharma Inc. Sachiko Okazaki has received scholarship grants from Minophagen Pharmaceutical Co., Ltd., Kowa Company, Ltd. Koh Ono has nothing to disclose. Hitoshi Shimano has nothing to disclose. Hiroyuki Daida has received honoraria from Amgen Inc., Daiichi-Sankyo Co., Ltd., Kowa Co., Ltd., and MSD K.K., Novartis Pharma K.K., Bayer Yakuhin, Ltd. and received clinical research funding from Canon Medical Systems Corporation, Philips Japan, Ltd., Toho Holdings Co., Ltd., Asahi Kasei Corporation, and Inter Reha Co., Ltd. HD has also received scholarship grants from Nippon Boehringer Ingelheim Co., Ltd., Otsuka Pharmaceutical Co., Ltd., Sanofi K.K., MSD K.K., Daiichi-Sankyo Co., Ltd., Pfizer Co., Ltd., Mitsubishi Tanabe Pharma Corp., Astellas Pharma Inc., Takeda Pharmaceutical Co., Ltd., Teijin Pharma, Ltd., Shionogi & Co., Ltd., Actelion Pharmaceuticals, Ltd., Actelion Ltd., Kowa Co., Ltd., Bayer Yakuhin, Ltd. HD has also courses endowed by companies, including Philips Japan, Ltd., ResMed, Fukuda Denshi Co., Ltd., and Paramount Bed Co., Ltd. Kazushige Dobashi has nothing to disclose. Toshio Hayashi has nothing to disclose. Mika Hori has nothing to disclose. Kota Matsuki has nothing to disclose. Tetsuo Minamino has nothing to disclose. Shinji Yokoyama has nothing to disclose. Mariko Harada-Shiba has received stock holdings or options from Liid Pharma, honoraria from Amgen Inc., Astellas Pharma Inc., Sanofi, and scholarship grants from Aegerion Pharmaceuticals, Inc., Recordati Rare Diseases Japan, and Kaneka Corporation. Katsunori Ikewaki has nothing to disclose. Yasushi Ishigaki has nothing to disclose. Shun Ishibashi has received honoraria from Kowa Co., Ltd., and a scholarship grant from Ono Pharmaceutical Co., Ltd. Kyoko Inagaki has nothing to disclose. Hirotoshi Ohmura has nothing to disclose. Hiroaki Okazaki has received scholarship grants from Minophagen Pharmaceutical Co., Ltd., Kowa Company, Ltd. Masa-aki Kawashiri has nothing to disclose. Masayuki Kuroda has nothing to disclose. Masahiro Koseki has received clinical research funding from Kowa Company, Ltd., Rohto Pharmaceutical Co., Ltd. Takanari Gotoda has nothing to disclose. Shingo Koyama has nothing to disclose. Yoshiki Sekijima has nothing to disclose. Manabu Takahashi has nothing to disclose. Yasuo Takeuchi has nothing to disclose. Misa Takegami has nothing to disclose. Kazuhisa Tsukamoto has received honoraria from Bayer Yakuhin, Ltd., MSD Ltd., Takeda Pharmaceutical Company Ltd., and scholarship grants from Mitsubishi Tanabe Pharma Corporation., Bayer Yakuhin, Ltd., Sanofi K.K. Atsuko Nakatsuka has nothing to disclose. Kimitoshi Nakamura has nothing to disclose. Satoshi Hirayama has nothing to disclose. Hideaki Bujo has nothing to disclose. Daisaku Masuda has received clinical research funding from MSD K.K., Ono Pharmaceutical Co., Ltd., Takeda Pharmaceutical Co., Ltd., Kowa Co., Ltd. Takashi Miida has nothing to disclose. Yoshihiro Miyamoto has nothing to disclose. Takeyoshi Murano has nothing to disclose. Takashi Yamaguchi has nothing to disclose. Shizuya Yamashita has received honoraria from Kowa Company, Ltd., MSD K.K. Masashi Yamamoto has nothing to disclose. Koutaro Yokote has received honoraria from Kowa Company, Ltd., MSD K.K., Astellas Pharma Inc., Mitsubishi Tanabe Pharma Corp., Amgen K.K., Takeda Pharmaceutical Company Limited, Sanofi K.K., Ono Pharmaceutical Co., Ltd., AstraZeneca K.K., Daiichi-Sankyo Co., Ltd., Novartis Pharma K.K., Sumitomo Dainippon Pharma Co., Ltd., Kyowa Kirin Co., Ltd., Pfizer Japan Inc., Novo Nordisk Pharma Ltd., Nippon Boehringer Ingelheim Co., Ltd., Eli Lilly Japan K.K., Taisho Pharmaceutical Co., Ltd., Janssen Pharmaceutical K.K., and received clinical research funding from Taisho Pharmaceutical Co., Ltd. KY has also received scholarship grants from Mitsubishi Tanabe Pharma Corp., Takeda Pharmaceutical Co., Ltd., MSD K.K., Pfizer Japan Inc., Novo Nordisk Pharma Ltd., Taisho Pharmaceutical Co., Ltd., Kao Corporation, Ono Pharmaceutical Co., Ltd., Eli Lilly Japan K.K., Sumitomo Dainippon Pharma Co., Ltd., Nippon Boehringer Ingelheim Co., Ltd., Daiichi-Sankyo Co., Ltd., Teijin Pharma, Ltd., Shionogi Co., Ltd., Bayer Yakuhin, Ltd. Jun Wada has nothing to disclose.

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