Sequencing of the complex CTRB1-CTRB2 locus in chronic pancreatitis

Genetic determinants of pancreatitis: relevance in severe hypertriglyceridemia

PURPOSE OF REVIEW

Not all patients with severe hypertriglyceridemia develop acute pancreatitis. We surveyed recent literature on inter-individual genetic variation in susceptibility to pancreatitis.

RECENT FINDINGS

Genetic determinants of pancreatitis include: rare Mendelian disorders caused by highly penetrant pathogenic variants in genes involved in trypsinogen activation; uncommon susceptibility variants in genes involved in trypsinogen activation, protein misfolding as well as calcium metabolism and cystic fibrosis, that have variable penetrance and show a range of odds ratios for pancreatitis; and common polymorphisms in many of the same genes that have only a small effect on risk. The role of these genetic variants in modulating pancreatitis risk in hypertriglyceridemia is unclear. However, among genetic determinants of plasma triglycerides, those predisposing to more severe hypertriglyceridemia associated with chylomicronemia appear to have higher pancreatitis risk.

SUMMARY

Currently, among patients with severe hypertriglyceridemia, the most consistent predictor of pancreatitis risk is the triglyceride level. Furthermore, pancreatitis risk appears to be modulated by a higher genetic burden of factors associated with greater magnitude of triglyceride elevation. The role of common and rare genetic determinants of pancreatitis itself in this metabolic context is unclear.

Proteome-Wide Mendelian Randomization Identifies Causal Links Between Blood Proteins and Acute Pancreatitis

BACKGROUND & AIMS

Acute pancreatitis (AP) is a complex disease and the leading cause of gastrointestinal disease-related hospital admissions. Few therapeutic options exist for AP prevention. Blood proteins with causal evidence may represent promising drug targets, but few have been causally linked with AP. Our objective was to identify blood proteins linked with AP by combining genome-wide association meta-analysis and proteome-wide Mendelian randomization (MR) studies.

METHODS

We performed a genome-wide association meta-analysis totalling 10,630 patients with AP and 844,679 controls and a series of inverse-variance weighted MR analyses using cis-acting variants on 4719 blood proteins from the deCODE study (n = 35,559) and 4979 blood proteins from the Fenland study (n = 10,708).

RESULTS

The meta-analysis identified genome-wide significant variants (P <5 × 10^-8) at 5 loci (ABCG5/8, TWIST2, SPINK1, PRSS2 and MORC4). The proteome-wide MR analyses identified 68 unique blood proteins that may causally be associated with AP, including 29 proteins validated in both data sets. Functional annotation of these proteins confirmed expression of many proteins in metabolic tissues responsible for digestion and energy metabolism, such as the esophagus, adipose tissue, and liver as well as acinar cells of the pancreas. Genetic colocalization and investigations into the druggable genome also identified potential drug targets for AP.

CONCLUSIONS

This large genome-wide association study meta-analysis for AP identified new variants linked with AP as well as several blood proteins that may be causally associated with AP. This study provides new information on the genetic architecture of this disease and identified pathways related to AP, which may be further explored as possible therapeutic targets for AP.

Arg236 in human chymotrypsin B2 (CTRB2) is a key determinant of high enzyme activity, trypsinogen degradation capacity, and protection against pancreatitis

Pancreatic chymotrypsins (CTRs) are digestive proteases that in humans include CTRB1, CTRB2, CTRC, and CTRL. The highly similar CTRB1 and CTRB2 are the products of gene duplication. A common inversion at the CTRB1-CTRB2 locus reverses the expression ratio of these isoforms in favor of CTRB2. Carriers of the inversion allele are protected against the inflammatory disorder pancreatitis presumably via their increased capacity for CTRB2-mediated degradation of harmful trypsinogen. To reveal the protective molecular determinants of CTRB2, we compared enzymatic properties of CTRB1, CTRB2, and bovine CTRA (bCTRA). By evolving substrate-like Schistocerca gregaria proteinase inhibitor 2 (SGPI-2) inhibitory loop variants against the chymotrypsins, we found that the substrate binding groove of the three enzymes had overlapping specificities. Based on the selected sequences, we produced eight SGPI-2 variants. Remarkably, CTRB2 and bCTRA bound these inhibitors with significantly higher affinity than CTRB1. Moreover, digestion of peptide substrates, beta casein, and human anionic trypsinogen unequivocally confirmed that CTRB2 is a generally better enzyme than CTRB1 while the potency of bCTRA lies between those of the human isoforms. Unexpectedly, mutation D236R alone converted CTRB1 to a CTRB2-like high activity protease. Modeling indicated that in CTRB1 Met210 partially obstructed the substrate binding groove, which was relieved by the D236R mutation. Taken together, we identify CTRB2 Arg236 as a key positive determinant, while CTRB1 Asp236 as a negative determinant for chymotrypsin activity. These findings strongly support the concept that in carriers of the CTRB1-CTRB2 inversion allele, the superior trypsinogen degradation capacity of CTRB2 protects against pancreatitis.

Colocalization analysis of pancreas eQTLs with risk loci from alcoholic and novel non-alcoholic chronic pancreatitis GWAS suggests potential disease causing mechanisms

BACKGROUND

Previous genome-wide association studies (GWAS) identified genome-wide significant risk loci in chronic pancreatitis and investigated underlying disease causing mechanisms by simple overlaps with expression quantitative trait loci (eQTLs), a procedure which may often result in false positive conclusions.

METHODS

We conducted a GWAS in 584 non-alcoholic chronic pancreatitis (NACP) patients and 6040 healthy controls. Next, we applied Bayesian colocalization analysis of identified genome-wide significant risk loci from both, our recently published alcoholic chronic pancreatitis (ACP) and the novel NACP dataset, with pancreas eQTLs from the GTEx V8 European cohort to prioritize candidate causal genes and extracted credible sets of shared causal variants.

RESULTS

Variants at the CTRC (p = 1.22 × 10^-21) and SPINK1 (p = 6.59 × 10^-47) risk loci reached genome-wide significance in NACP. CTRC risk variants colocalized with CTRC eQTLs in ACP (PP4 = 0.99, PP4/PP3 = 95.51) and NACP (PP4 = 0.99, PP4/PP3 = 95.46). For both diseases, the 95% credible set of shared causal variants consisted of rs497078 and rs545634. CLDN2-MORC4 risk variants colocalized with CLDN2 eQTLs in ACP (PP4 = 0.98, PP4/PP3 = 42.20) and NACP (PP4 = 0.67, PP4/PP3 = 7.18), probably driven by the shared causal variant rs12688220.

CONCLUSIONS

A shared causal CTRC risk variant might unfold its pathogenic effect in ACP and NACP by reducing CTRC expression, while the CLDN2-MORC4 shared causal variant rs12688220 may modify ACP and NACP risk by increasing CLDN2 expression.

[Pathogenesis of chronic pancreatitis]

Long-term alcohol consumption and gene mutations are the most important causes of chronic pancreatitis. In addition to mutations in acinar genes, such as digestive enzymes and their inhibitors, defects in genes that primarily or exclusively affect the duct cells have also been described in recent years. Genetic changes are found not only in patients with a positive family history (hereditary pancreatitis) but also in so-called idiopathic and, to a lesser extent, in alcoholic chronic pancreatitis. The coming years will likely show that there are very complex interactions between environmental influences and numerous genetic factors.