Genetics of Chronic Pancreatitis

Approximately 50 years ago, it was recognized that chronic pancreatitis (CP) may cluster in selected families, suggesting an inherited disease in these patients (1). However, the underlying genetic defect remained obscure for more than four decades. As stated in the first report on inherited CP, “hereditary chronic relapsing pancreatitis does not present earmarks which distinguish it from nonhereditary chronic relapsing pancreatitis”(1). In many children with CP without affected family members, no predisposing factors are identified. These children with so-called idiopathic CP may also have a genetic basis for their condition. This review summarizes the recent breakthroughs in understanding the molecular basis of hereditary and idiopathic CP. Because growing evidence suggests that both forms of CP are genetic disorders, we classify CP as “primary CP” (with or without a family history) or “secondary CP” (caused by toxic, metabolic, anatomic factors, or systemic disease). CHRONIC PANCREATITIS Definition Chronic pancreatitis in adults is defined as a relapsing or continuing inflammatory disease of the pancreas characterized by irreversible morphologic changes, typically causing pain and, in some patients, permanent impairment of exocrine function, endocrine function, or both (2). During acute episodes, mild edematous to severe necrotizing inflammation of the pancreas may occur. Morphologically, the pancreas shows irregular sclerosis with focal, segmental, or diffuse destruction of the parenchyma. Abnormalities of the pancreatic duct system, such as dilatations, strictures, or intraductal plugs containing protein or calculi, are frequent findings. In children, CP generally starts with relapsing attacks of acute pancreatitis with initially reversible morphologic and biochemical alterations. Pancreatic insufficiency is a rare feature in this age group. Symptoms and Complications The clinical picture of CP in childhood is characterized by recurrent episodes of abdominal pain often accompanied by nausea, vomiting, and anorexia. The pain is usually located diffusely in the upper abdomen or in the periumbilical area, but may on some occasions radiate to the back or to the lower quadrants. The pain is typically sudden in onset, but may be absent in rare cases. In contrast with adults, chronic pain in childhood is unusual. Because recurrent abdominal pain is a frequent finding in children, diagnosis of CP is often delayed for many years. During the acute attack, abdominal tenderness, often in combination with decreased bowel sounds, is the most common physical finding. During the course of the disease, some patients develop pancreatic insufficiency with bulky and greasy stools, weight loss, and insulin-dependent diabetes mellitus. The formation of pancreatic pseudocysts is a frequent complication of pancreatic inflammation. Pseudocysts may rupture, become infected, or cause local mechanical effects such as biliary or duodenal obstruction by compressing adjacent structures. Other complications of CP include calcifications and necroses, with the risk of subsequent pancreatic infection. The development of pancreatic cancer is a late complication (3). Compared with the general population, the risk of pancreatic cancer in patients with hereditary CP increases 50 to 60 times (4). Epidemiology and Etiology The incidence and prevalence of CP in childhood and the relative frequency of the predisposing factors have not yet been precisely determined. This disorder probably is more frequent in children than previously believed. In industrialized countries, the estimated incidence of CP in adults is 3.5 to 10 cases per 100,000 (5,6). However, 70% to 80% of adult patients have alcohol-induced CP (7,8). Because alcohol abuse as a predisposing condition is uncommon in children, the epidemiologic data published are not applicable to pediatric patients. Some well-recognized causative factors are anatomic anomalies, metabolic disorders, trauma, cystic fibrosis, and inflammatory bowel disease (9). In many patients, no etiologic factor can be identified, and therefore these patients are classified as having idiopathic CP, or in the presence of a family history of CP, as having hereditary CP. Comfort and Steinberg (1) first described hereditary pancreatitis in 1952. We estimate that of childhood cases, idiopathic CP accounts for approximately 40% to 60% and hereditary CP accounts for approximately 20% to 25%. Pathogenesis More than a century ago, Chiari (10) hypothesized that pancreatitis results from pancreatic autodigestion. An inappropriate conversion of pancreatic zymogens to active enzymes within the pancreatic parenchyma may initiate the inflammatory process. A key role has been attributed to the activation of trypsinogen to trypsin, converting all proteolytic proenzymes to their active form (11). Identification of disease-causing mutations in a trypsinogen gene confirmed the significance of the pancreatic protease system in CP (12). CATIONIC TRYPSINOGEN General Aspects and Biologic Function Cationic trypsinogen (OMIM 276000) (13), also referred to as serine protease 1 (PRSS1), is one of the most abundant secretory proteins synthesized by the pancreas (14,15). Three different trypsinogens have been described in human pancreatic juice. According to their electrophoretic mobility in isoelectric focusing, these trypsinogens have been designated as cationic trypsinogen, anionic trypsinogen, and mesotrypsinogen (15). The ratio of the cationic to the anionic form is about 1:1.5 (15). Compared with the anionic isoenzyme, the cationic trypsinogen autoactivates more easily and is more resistant to autolysis (16,17). The human cationic trypsinogen gene is approximately 3.6 kilobases (kb) long, is separated into 5 exons, and is located on the long arm of chromosome 7 (7q35) (18). The preproprotein comprises 247 amino acids, including a 15–amino acid signal peptide and an 8–amino acid activation peptide. Human cationic trypsinogen gene is embedded in a cluster of eight trypsinogen genes within the human β T-cell receptor locus. Three nonfunctioning genes are located at the 5′ end of the locus, and five tandemly arrayed genes, including the cationic and the anionic trypsinogen, are located at the 3′ end (18). The mesotrypsinogen gene is located on the short arm of chromosome 9 (9p13). Trypsin has a key position in the pancreatic protease system. Serine protease is capable of activating itself and all other proteolytic pancreatic proenzymes. Trypsin is synthesized in the pancreas as inactive trypsinogen and is secreted into the pancreatic juice. In the intestine, enteropeptidase (enterokinase) activates trypsinogen to trypsin by cleavage of the activation peptide. Several mechanisms protect the pancreas from autodigestion by activation of the pancreatic digestive cascade (19): 1) the synthesis of digestive enzymes as inactive proenzymes (zymogens), which prevents intrapancreatic tryptic activity; 2) localizing the activating enzyme enteropeptidase outside the pancreas; 3) and by low calcium concentration. Physiologically, small amounts of trypsinogen are hydrolyzed to active trypsin in the pancreatic parenchyma, but this tryptic activity is blocked by cosynthesized protease inhibitors, such as serum protease inhibitor Kazal type 1 (SPINK1), and by trypsinlike enzymes, such as mesotrypsin, readily degrading trypsinogen and other zymogens. Identification of PRSS1 as a Pancreatitis Gene In 1996, several groups mapped a gene for primary CP to chromosome 7q35 (20–22). In the same year, Whitcomb et al. (12) identified a mutation in the cationic trypsinogen gene as the underlying defect. In five families with CP, substitution of arginine with histidine at residue 122 (R122H) in exon 3 was found in affected subjects. In the following years, several other PRSS1 mutations were described: −28delTCC, A16V, D22G, K23R, and N29I (23–27) (Table 1). These mutations may have lead to increased trypsin activity within the pancreatic parenchyma, resulting in autodigestion and inflammation of the pancreas.TABLE 1: Genes and mutations investigated in chronic pancreatitisNomenclature Considerable confusion exists concerning the nomenclature of the identified trypsinogen mutations. In the literature, two different systems have been used in parallel: the chymotrypsin system and the trypsinogen system. The first two descriptions for trypsinogen mutations (R122H and N29I) used the chymotrypsin nomenclature (R117H and N21I) (12,23), whereas other mutations were reported in the trypsinogen system (25–27). The chymotrypsinogen numbering system is based on the alignment of the amino acid sequences at serine 195 of the chymotrypsin molecule, because serine 195 is the catalytic residue of all serine proteases. This nomenclature for cationic trypsinogen mutations has several drawbacks: First, the start codon is not codon 1. Second, the amino acids of the signal peptide as well as five amino acids in the active enzyme are not numbered. Mutations of these amino acids cannot be referred to adequately. Third, there are 11 numbers without corresponding amino acids. The nomenclature of mutations must be accurate and unambiguous. Therefore, we use the generally accepted system recommended by the Nomenclature Working Group (28) and also used by the Human Gene to be the most common PRSS1 mutation a and CP the underlying mechanisms are the basis of molecular and protein it was that the mutation trypsin resistant to (12). In that arginine at position 122 is the primary of trypsin by trypsin itself and other trypsinlike This is a trypsin in the the trypsin and activates pancreatic zymogens. This was by that the trypsin catalytic but increased However, a recent that increased is the of human cationic trypsinogen that both of the two mutations (R122H and N29I) in but not autolysis of the enzyme intrapancreatic trypsinogen activation may be the common in pancreatitis by PRSS1 whereas of trypsin, as in the of may be an N29I families without two groups found an in exon to substitution of with at codon This mutation has been identified in many patients was that the N29I mutation trypsinogen the of trypsin to trypsin, or trypsin by a that the of the to trypsin and trypsinlike enzymes The finding that N29I trypsinogen and in the However, with human trypsinogen, the same increased of the N29I protein and no decreased of pediatric patients with primary CP a in exon to of an with at codon Three of these patients no family history of CP, one in all cases inherited the one of of patients with was low of this This with the reported for the N29I and mutations Other groups confirmed the significance of the mutation The mutation the first amino acid of the trypsinogen and the cleavage of the signal peptide 1). other mutations in the small acid activation and K23R, have been described the significance of this data on the of may that because of a of activation of trypsinogen results in cleavage of the activation peptide amino acid of the cationic trypsinogen in one the position of amino acids at the published The first the amino acid of the and the the from et al. PRSS1 Mutations Several other PRSS1 mutations of significance have been In a mutation of families with CP, an in exon resulting in an substitution at position was in two affected in one family In the same of in the 5′ of PRSS1 at position was reported The that this increases the of trypsinogen This was found in one affected of a family with CP. Other groups have described In one CP an in exon resulting in substitution of acid with at codon was reported The gene was found in the in the and in the affected The and the also of recurrent upper abdominal no from these two was of of the of PRSS1 corresponding to the the and the an increased of the two that and may cleavage of the activation peptide and of of pancreatitis is defined as an disease with a of The clinical of most families with the or the N29I mutation In shows low and is typically found in patients without family history of CP, that PRSS1 mutations cause CP not in an trypsinogen mutations of 1 General Aspects and Biologic Function The serine protease inhibitor Kazal type 1 (13), also as pancreatic secretory trypsin inhibitor or as trypsin inhibitor is a protease inhibitor to be a factor of intrapancreatic trypsin In Kazal et al. first in the has been found in the pancreatic of all The of is not to the has been identified in human including and and is in the does as an intrapancreatic protease it also is synthesized by the as an and may be in the from and in Human is approximately long, is separated into exons, and is located on chromosome 5 The gene of amino acids, which include a acid signal peptide. The peptide amino acids, has 3 and is in an acid The comprises a residue at position and an residue at position corresponding to and of the as a for trypsin of of trypsin and the inhibitor a with a the catalytic serine residue of the enzyme and the of the of However, of the trypsin activity The that the itself as a for trypsin this of the inhibitor results in of the trypsin Because many patients with primary CP not a PRSS1 it was that genetic in other genes be in CP. was identified by a gene as a pancreatitis gene In of pediatric patients with primary CP, a mutation was In patients, an resulting in a substitution of with serine at codon in exon 3 was found patients were for this and patients were of the and or were by the sequences have confirmed the frequency of in in patients with primary CP without family 7 were and were A with 7 and 5 by patients CP but no family history is in with four other and 2) This finding that is an no have the of The mutation located to the of may lead to decreased However, the at position is not may protein resulting in of the inhibitor within the as with other inherited such as cystic or However, may be an and one of the four may the mutations are frequent in such as cystic or this in the of the and found in patients with chronic are by of with the from et al. was found in one are in with is found in patients without family history of to 40% of patients with so-called idiopathic CP on one or on both Approximately of the general is for is the of not have CP. may that the combination of and other genetic or factors results in CP. However, in two on the of CP, approximately of patients with idiopathic CP were for According to an frequency of about the frequency of be 1 in that the estimated prevalence of idiopathic CP is about 1 in the disease in be at 25%. Other Mutations Several other have been but their In one family with affected members, a mutation the start codon was The mutation was found in the in the and in the affected the CP. The mutation was not found in suggesting that the not a this mutation This suggests a with mutation the at position of the which is in was in several patients with CP. 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More than mutations have been identified. 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We genetic of pediatric patients with primary CP for the two most common and a and not an of patients with CP were four patients were for the and 3 patients were for the In of a was were for and for or was or for these mutations. The frequency in both groups was with the reported frequency in in patients with primary CP with or without a family history of CP not from in These data that is not in the of CP. data the results of a of patients were In this no of pancreatitis was In a genetic of a without a mutation of the cationic trypsinogen a chronic pancreatitis was found on the short arm of chromosome However, the underlying genetic defect has not yet been identified. and Because mutations in the cationic trypsinogen in genetic of the two other trypsinogen genes in the pancreatic may also cause et al. the of anionic trypsinogen and mesotrypsinogen in CP by of families with CP that not have a PRSS1 However, a small of families and no patients without a family history were Other The pancreas a of digestive enzymes, including and genetic must one or more of these are in the of CP. Pancreatic pancreatic protein also as or as is the of the protein of in patients with chronic pancreatitis The of in pancreatic to of suggests that it an role in exocrine pancreatic Pancreatic is with calcium and in that of decreased pancreatic and in the pancreatic of patients with chronic pancreatitis to the that CP may be to by However, have the of in in patients with primary CP the role of in inherited CP. intrapancreatic activation of zymogens by trypsin probably results in autodigestion and different mechanisms activation of the pancreatic digestive enzyme First, to 20% of trypsin activity Second, trypsin itself activates trypsinlike enzymes such as mesotrypsin, readily degrading trypsinogen and other zymogens 3 Pancreatitis from an of and their within the pancreatic parenchyma 3 Whitcomb et al. (12) first that a mutation in a digestive enzyme is with CP. trypsinogen activation may be the common of pancreatitis by these mutations. The of mutations the of the protease inhibitor system in the of CP mutations in the cationic trypsinogen that in of the enzyme or mutations in that in decreased in a the intrapancreatic of and their of chronic in the trypsin resulting from of trypsinogen within the pancreatic parenchyma is by serine protease inhibitor Kazal type 1 and in the by or This prevents the pancreas from activating the pancreatic enzyme cascade and autodigestion. in chronic mutations in cationic trypsinogen or in lead to an of and their within the pancreatic parenchyma, resulting in inappropriate conversion of pancreatic zymogens to active enzymes with autodigestion and inflammation. Mutations in cystic may this by intrapancreatic or by of and may the intrapancreatic activation of digestive of genes, from et al. activation of a mutation may on of the in which this mutation protein As with one may classify PRSS1 and mutations as severe or PRSS1 mutations that in disease may or may such as trypsin Other PRSS1 such as A16V, may and the is in combination with other genetic or As with the of mutations may on the of pancreatic Mutations that a such as may intrapancreatic to a than does of the general and most not develop CP. The frequency of as well as of is increased in patients with CP. In the may cause of but may initiate pancreatitis in the In a combination with other factors that with the is The be such a Pancreatic in cystic may from a decreased of the and may lead to of of or to of and of trypsinogen is In is low and is at a and more with a 5 and A of function, as for the may for CP by the intrapancreatic activation of digestive Pancreatitis may from an of and their within the pancreatic parenchyma. The of cationic trypsinogen mutations in families of patients with CP was the first in understanding the underlying genetic The recent of and PRSS1 mutations in patients with CP without a family history have the of idiopathic CP as a disorder and the hereditary and idiopathic CP. data that CP may be inherited as or as a of mutations in the or yet Therefore, we hereditary and idiopathic CP and CP as “primary CP” (with or without a family history) or “secondary CP” (caused by toxic, metabolic, anatomic factors, or systemic disease). of patients with chronic pancreatitis include genetic for mutations in the genes in the of family history of