Whole-exome sequencing reveals GPIHBP1 mutations in infantile colitis with severe hypertriglyceridemia

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.

A homozygous variant in the GPIHBP1 gene in a child with severe hypertriglyceridemia and a systematic literature review

Background: Due to nonspecific symptoms, rare dyslipidaemias are frequently misdiagnosed, overlooked, and undertreated, leading to increased risk for severe cardiovascular disease, pancreatitis and/or multiple organ failures before diagnosis. Better guidelines for the recognition and early diagnosis of rare dyslipidaemias are urgently required.

Methods: Genomic DNA was isolated from blood samples of a Pakistani paediatric patient with hypertriglyceridemia, and from his parents and siblings. Next-generation sequencing (NGS) was performed, and an expanded dyslipidaemia panel was employed for genetic analysis.

Results: The NGS revealed the presence of a homozygous missense pathogenic variant c.230G>A (NM_178172.6) in exon 3 of the GPIHBP1 (glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1) gene resulting in amino acid change p.Cys77Tyr (NP_835466.2). The patient was 5.5 years old at the time of genetic diagnosis. The maximal total cholesterol and triglyceride levels were measured at the age of 10 months (850.7 mg/dl, 22.0 mmol/L and 5,137 mg/dl, 58.0 mmol/L, respectively). The patient had cholesterol deposits at the hard palate, eruptive xanthomas, lethargy, poor appetite, and mild splenomegaly. Both parents and sister were heterozygous for the familial variant in the GPIHBP1 gene. Moreover, in the systematic review, we present 62 patients with pathogenic variants in the GPIHBP1 gene and clinical findings, associated with hyperlipoproteinemia.

Conclusion: In a child with severe hypertriglyceridemia, we identified a pathogenic variant in the GPIHBP1 gene causing hyperlipoproteinemia (type 1D). In cases of severe elevations of plasma cholesterol and/or triglycerides genetic testing for rare dyslipidaemias should be performed as soon as possible for optimal therapy and patient management.

A novel GPIHBP1 mutation related to familial chylomicronemia syndrome: A series of cases

BACKGROUND AND AIMS

GPIHBP1 is an accessory protein of lipoprotein lipase (LPL) essential for its functioning. Mutations in the GPIHBP1 gene cause a deficit in the action of LPL, leading to severe hypertriglyceridemia and increased risk for acute pancreatitis.

METHODS

We describe twelve patients (nine women) with a novel homozygous mutation in intron 2 of the GPIHBP1 gene.

RESULTS

All patients were from the Northeastern region of Brazil and presented the same homozygous variant located in a highly conserved 3' splicing acceptor site of the GPIHBP1 gene. This new variant was named c.182-1G > T, according to HGVS recommendations. We verified this new GPIHBP1 variant's effect by using the Human Splicing Finder (HSF) tool. This mutation changes the GPIHBP1 pre-mRNA processing and possibly causes the skipping of the exon 3 of the GPIHBP1 gene, affecting almost 50% of the cysteine-rich Lys6 GPIHBP1 domain. Patients presented with severe hypertriglyceridemia (2351 mg/dl [885-20600]) and low HDL (18 mg/dl [5-41). Four patients (33%) had a previous history of acute pancreatitis.

CONCLUSIONS

We describe a novel GPIHBP1 pathogenic intronic mutation of patients from the Northeast region of Brazil, suggesting the occurrence of a founder effect.

GPIHBP1 and Lipoprotein Lipase, Partners in Plasma Triglyceride Metabolism

Lipoprotein lipase (LPL), identified in the 1950s, has been studied intensively by biochemists, physiologists, and clinical investigators. These efforts uncovered a central role for LPL in plasma triglyceride metabolism and identified LPL mutations as a cause of hypertriglyceridemia. By the 1990s, with an outline for plasma triglyceride metabolism established, interest in triglyceride metabolism waned. In recent years, however, interest in plasma triglyceride metabolism has awakened, in part because of the discovery of new molecules governing triglyceride metabolism. One such protein-and the focus of this review-is GPIHBP1, a protein of capillary endothelial cells. GPIHBP1 is LPL's essential partner: it binds LPL and transports it to the capillary lumen; it is essential for lipoprotein margination along capillaries, allowing lipolysis to proceed; and it preserves LPL's structure and activity. Recently, GPIHBP1 was the key to solving the structure of LPL. These developments have transformed the models for intravascular triglyceride metabolism.

Infantile Onset Intractable Inflammatory Bowel Disease Due to Novel Heterozygous Mutations in TNFAIP3 (A20)

BACKGROUND

Mutations in tumor necrosis factor alpha-induced protein 3 (TNFAIP3), a key player in the negative feedback regulation of nuclear factor-κB signaling, have recently been recognized as leading to early onset autoinflammatory and autoimmune syndrome. Here, we have reported the phenotypes of 3 infantile onset intractable inflammatory bowel disease (IBD) patients with TNFAIP3 mutations and reviewed previously reported cases to establish phenotypic features associated with TNFAIP3 monogenicity.

METHODS

From January 1, 2015, to December 31, 2017, we recruited 58 infantile-onset IBD patients. Targeted sequencing and whole-exome sequencing were performed. Sanger sequencing confirmed the variants and determined the parental origin. We followed all the patients with TNFAIP3 mutations in our cohort and analyzed their clinical data.

RESULTS

Genetic screening in all 58 patients with infantile-onset IBD revealed 44 (75.9%) cases of monogenic disorders, and 3 de novo TNFAIP3 mutations were identified, including 1 nonsense and 2 frame shift mutations. All the mutations resulted in premature stop codon. All 3 patients had multiple systemic involvements, with predominant gastrointestinal diseases.

CONCLUSIONS

Most infantile-onset IBD was associated with monogenetic mutation, and in addition to the 50 reported genes, other rare genetic variants need to be determined. TNFAIP3 may be an important candidate gene. The treatment of TNFAIP3-associated infantile-onset-IBD was challenging. 10.1093/ibd/izy165_video1izy165.video15789607089001.

Lipoprotein lipase transporter GPIHBP1 and triglyceride-rich lipoprotein metabolism

Increased plasma triglyceride serves as an independent risk factor for cardiovascular disease (CVD). Lipoprotein lipase (LPL), which hydrolyzes circulating triglyceride, plays a crucial role in normal lipid metabolism and energy balance. Hypertriglyceridemia is possibly caused by gene mutations resulting in LPL dysfunction. There are many factors that both positively and negatively interact with LPL thereby impacting TG lipolysis. Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), a newly identified factor, appears essential for transporting LPL to the luminal side of the blood vessel and offering a platform for TG hydrolysis. Numerous lines of evidence indicate that GPIHBP1 exerts distinct functions and plays diverse roles in human triglyceride-rich lipoprotein (TRL) metabolism. In this review, we discuss the GPIHBP1 gene, protein, its expression and function and subsequently focus on its regulation and provide critical evidence supporting its role in TRL metabolism. Underlying mechanisms of action are highlighted, additional studies discussed and potential therapeutic targets reviewed.

Whole-exome sequencing reveals an inherited R566X mutation of the epithelial sodium channel β-subunit in a case of early-onset phenotype of Liddle syndrome

To comprehensively evaluate a European–American child with severe hypertension, whole-exome sequencing (WES) was performed on the child and parents, which identified causal variation of the proband's early-onset disease. The proband's hypertension was resistant to treatment, requiring a multiple drug regimen including amiloride, spironolactone, and hydrochlorothiazide. We suspected a monogenic form of hypertension because of the persistent hypokalemia with low plasma levels of renin and aldosterone. To address this, we focused on rare functional variants and indels, and performed gene-based tests incorporating linkage scores and allele frequency and filtered on deleterious functional mutations. Drawing upon clinical presentation, 27 genes were selected evidenced to cause monogenic hypertension and matched to the gene-based results. This resulted in the identification of a stop-gain mutation in an epithelial sodium channel (ENaC), SCNN1B, an established Liddle syndrome gene, shared by the child and her father. Interestingly, the father also harbored a missense mutation (p.Trp552Arg) in the α-subunit of the ENaC trimer, SCNN1A, possibly pointing to pseudohypoaldosteronism type I. This case is unique in that we present the early-onset disease and treatment response caused by a canonical stop-gain mutation (p.Arg566*) as well as ENaC digenic hits in the father, emphasizing the utility of WES informing precision medicine.

Severe hypertriglyceridemia in Japan: Differences in causes and therapeutic responses

BACKGROUND

Severe hypertriglyceridemia (>1000 mg/dL) has a variety of causes and frequently leads to life-threating acute pancreatitis. However, the origins of this disorder are unclear for many patients.

OBJECTIVE

We aimed to characterize the causes of and responses to therapy in rare cases of severe hypertriglyceridemia in a group of Japanese patients.

METHODS

We enrolled 121 patients from a series of case studies that spanned 30 years. Subjects were divided into 3 groups: (1) primary (genetic causes); (2) secondary (acquired); and (3) disorders of uncertain causes. In the last group, we focused on 3 possible risks factors for hypertriglyceridemia: obesity, diabetes mellitus, and heavy alcohol intake.

RESULTS

Group A (n = 20) included 13 patients with familial lipoprotein lipase deficiency, 3 patients with apolipoprotein CII deficiency, and other genetic disorders in the rest of the group. Group B patients (n = 15) had various metabolic and endocrine diseases. In Group C (uncertain causes; n = 86), there was conspicuous gender imbalance (79 males, 3 females) and most male subjects were heavy alcohol drinkers. In addition, 18 of 105 adult patients (17%) had histories of acute pancreatitis.

CONCLUSION

The cause of severe hypertriglyceridemia is uncertain in many patients. In primary genetic forms of severe hypertriglyceridemia, genetic diversity between populations is unknown. In the acquired forms, we found fewer cases of estrogen-induced hypertriglyceridemia than in Western countries. In our clinical experience, the cause of most hypertriglyceridemia is uncertain. Our work suggests that genetic factors for plasma triglyceride sensitivity to alcohol should be explored.

Mobility of "HSPG-bound" LPL explains how LPL is able to reach GPIHBP1 on capillaries

In mice lacking glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1), the LPL secreted by adipocytes and myocytes remains bound to heparan sulfate proteoglycans (HSPGs) on all cells within tissues. That observation raises a perplexing issue: Why isn't the freshly secreted LPL in wild-type mice captured by the same HSPGs, thereby preventing LPL from reaching GPIHBP1 on capillaries? We hypothesized that LPL-HSPG interactions are transient, allowing the LPL to detach and move to GPIHBP1 on capillaries. Indeed, we found that LPL detaches from HSPGs on cultured cells and moves to: 1) soluble GPIHBP1 in the cell culture medium; 2) GPIHBP1-coated agarose beads; and 3) nearby GPIHBP1-expressing cells. Movement of HSPG-bound LPL to GPIHBP1 did not occur when GPIHBP1 contained a Ly6 domain missense mutation (W109S), but was almost normal when GPIHBP1's acidic domain was mutated. To test the mobility of HSPG-bound LPL in vivo, we injected GPIHBP1-coated agarose beads into the brown adipose tissue of GPIHBP1-deficient mice. LPL moved quickly from HSPGs on adipocytes to GPIHBP1-coated beads, thereby depleting LPL stores on the surface of adipocytes. We conclude that HSPG-bound LPL in the interstitial spaces of tissues is mobile, allowing the LPL to move to GPIHBP1 on endothelial cells.

An LPL-specific monoclonal antibody, 88B8, that abolishes the binding of LPL to GPIHBP1

LPL contains two principal domains: an amino-terminal catalytic domain (residues 1-297) and a carboxyl-terminal domain (residues 298-448) that is important for binding lipids and binding glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1) (an endothelial cell protein that shuttles LPL to the capillary lumen). The LPL sequences required for GPIHBP1 binding have not been examined in detail, but one study suggested that sequences near LPL's carboxyl terminus (residues ∼403-438) were crucial. Here, we tested the ability of LPL-specific monoclonal antibodies (mAbs) to block the binding of LPL to GPIHBP1. One antibody, 88B8, abolished LPL binding to GPIHBP1. Consistent with those results, antibody 88B8 could not bind to GPIHBP1-bound LPL on cultured cells. Antibody 88B8 bound poorly to LPL proteins with amino acid substitutions that interfered with GPIHBP1 binding (e.g., C418Y, E421K). However, the sequences near LPL's carboxyl terminus (residues ∼403-438) were not sufficient for 88B8 binding; upstream sequences (residues 298-400) were also required. Additional studies showed that these same sequences are required for LPL binding to GPIHBP1. In conclusion, we identified an LPL mAb that binds to LPL's GPIHBP1-binding domain. The binding of both antibody 88B8 and GPIHBP1 to LPL depends on large segments of LPL's carboxyl-terminal domain.

GPIHBP1 and Plasma Triglyceride Metabolism

GPIHBP1, a GPI-anchored protein in capillary endothelial cells, is crucial for the lipolytic processing of triglyceride-rich lipoproteins (TRLs). GPIHBP1 shuttles lipoprotein lipase (LPL) to its site of action in the capillary lumen and is essential for the margination of TRLs along capillaries - such that lipolytic processing can proceed. GPIHBP1 also reduces the unfolding of the LPL catalytic domain, thereby stabilizing LPL catalytic activity. Many different GPIHBP1 mutations have been identified in patients with severe hypertriglyceridemia (chylomicronemia), the majority of which interfere with folding of the protein and abolish its capacity to bind and transport LPL. The discovery of GPIHBP1 has substantially revised our understanding of intravascular triglyceride metabolism but has also raised many new questions for future research.

Clinical and genetic features of 3 patients with familial chylomicronemia due to mutations in GPIHBP1 gene

BACKGROUND

Familial chylomicronemia is a recessive disorder that may be due to mutations in lipoprotein lipase (LPL) and in other proteins such as apolipoprotein C-II and apolipoprotein A-V (activators of LPL), GPIHBP1 (the molecular platform required for LPL activity on endothelial surface), and LMF1 (a factor required for intracellular formation of active LPL).

METHODS

We sequenced the familial chylomicronemia candidate genes in 2 adult females presenting long-standing hypertriglyceridemia and a history of acute pancreatitis.

RESULTS

Both probands had plasma triglyceride >10 mmol/L but no mutations in the LPL gene. The sequence of the other candidate genes showed that one patient was homozygous for a novel missense mutation p.(Cys83Arg), and the other was homozygous for a previously reported nonsense mutation p.(Cys 89*), respectively, in GPIHBP1. Family screening showed that the hypertriglyceridemic brother of the p.(Cys83Arg) homozygote was also homozygous for this mutation. He had no history of pancreatitis. The p.(Cys83Arg) heterozygous carriers had normal triglyceride levels. The substitution of a cysteine residue in the Ly6 domain of GPIHBP1 is predicted to abolish one of the disulfide bridges required to maintain the structure of GPIHBP1. The p.(Cys89*) mutation results in a truncated protein devoid of function.

CONCLUSIONS

Both mutant GPIHBP1 proteins are expected to be incapable of transferring LPL from the subendothelial space to the endothelial surface.

Novel mutations in the GPIHBP1 gene identified in 2 patients with recurrent acute pancreatitis

BACKGROUND

Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) has been demonstrated to be essential for the in vivo function of lipoprotein lipase (LPL), the major triglyceride (TG)-hydrolyzing enzyme involved in the intravascular lipolysis of TG-rich lipoproteins. Recently, loss-of-function mutations of GPIHBP1 have been reported as the cause of type I hyperlipoproteinemia in several patients.

METHODS

Two unrelated patients were referred to our Lipid Units because of a severe hypertriglyceridemia and recurrent pancreatitis. We measured LPL activity in postheparin plasma and serum ApoCII and sequenced LPL, APOC2, and GPIHBP1.

RESULTS

The 2 patients exhibited very low LPL activity not associated with mutations in LPL gene or with ApoCII deficiency. The sequence of GPIHBP1 revealed 2 novel point mutations. One patient (proband 1) was found to be homozygous for a C>A transversion in exon 3 resulting in the conversion of threonine to lysine at position 80 (p.Thr80Lys). The other patient (proband 2) was found to be homozygous for a G>T transversion in the third base of the ATG translation initiation codon in exon 1, resulting in the conversion of methionine to isoleucine (p.Met1Ile).

CONCLUSION

In conclusion, we have identified 2 novel GPIHBP1 missense mutations in 2 unrelated patients as the cause of their severe hypertriglyceridemia.

The diagnostic approach to monogenic very early onset inflammatory bowel disease

Patients with a diverse spectrum of rare genetic disorders can present with inflammatory bowel disease (monogenic IBD). Patients with these disorders often develop symptoms during infancy or early childhood, along with endoscopic or histological features of Crohn's disease, ulcerative colitis, or IBD unclassified. Defects in interleukin-10 signaling have a Mendelian inheritance pattern with complete penetrance of intestinal inflammation. Several genetic defects that disturb intestinal epithelial barrier function or affect innate and adaptive immune function have incomplete penetrance of the IBD-like phenotype. Several of these monogenic conditions do not respond to conventional therapy and are associated with high morbidity and mortality. Due to the broad spectrum of these extremely rare diseases, a correct diagnosis is frequently a challenge and often delayed. In many cases, these diseases cannot be categorized based on standard histological and immunologic features of IBD. Genetic analysis is required to identify the cause of the disorder and offer the patient appropriate treatment options, which include medical therapy, surgery, or allogeneic hematopoietic stem cell transplantation. In addition, diagnosis based on genetic analysis can lead to genetic counseling for family members of patients. We describe key intestinal, extraintestinal, and laboratory features of 50 genetic variants associated with IBD-like intestinal inflammation. In addition, we provide approaches for identifying patients likely to have these disorders. We also discuss classic approaches to identify these variants in patients, starting with phenotypic and functional assessments that lead to analysis of candidate genes. As a complementary approach, we discuss parallel genetic screening using next-generation sequencing followed by functional confirmation of genetic defects.

A 3-day-old neonate with severe hypertriglyceridemia from novel mutations of the GPIHBP1 gene

BACKGROUND

Familial chylomicronemia is a genetic defect of the intravascular lipolysis of triglyceride (TG)-rich lipoproteins. Intravascular lipolysis involves the TG-hydrolase lipoprotein lipase (LPL) as well as other factors such as apolipoprotein CII and apolipoprotein AV (activators of LPL), GPIHBP1 (the molecular platform required for LPL activity on endothelial surface), and LMF1 (a factor required for intracellular formation of active LPL).

METHODS

We sequenced the familial chylomicronemia candidate genes in a neonate with chylomicronemia.

RESULTS

A 3-day-old newborn was found to have chylomicronemia (plasma TG 18.8 mmol/L, 1.667 mg/dL). The discontinuation of breastfeeding for 24 hours reduced plasma TG to 2.3 mmol/L (201 mg/dL), whereas its resumption induced a sharp TG increase (7.9 mmol/L, 690 mg/dL). The child was switched to a low-fat diet, which was effective in maintaining TG level below 3.5 mmol/L (294 mg/dL) during the first months of life. The child was found to be a compound heterozygous for 2 novel mutations in GPIHBP1 gene. The first mutation was a 9-bp deletion and 4-bp insertion in exon 2, causing a frameshift that abolished the canonical termination codon TGA. The predicted translation product of the mutant messenger RNA is a peptide that contains 51 amino acids of the N-terminal end of the wild-type protein followed by 252 novel amino acids. The second mutation was a nucleotide change (c.319T>C), causing an amino acid substitution p.(Ser107Pro) predicted in silico to be damaging.

CONCLUSIONS

GPIHBP1 mutations should be considered in neonates with chylomicronemia negative for mutations in LPL gene.