Gain‐ and Loss‐of‐function Mutations in Trypsinogen

Abstract

Historically, trypsinogen has been among the most extensively studied models of protein structure and function. It has received renewed attention since the identification of a gain‐of‐function missense mutation in the cationic trypsinogen gene (PRSS1) as a cause of hereditary pancreatitis in 1996; a finding gave strong support to the then century‐old hypothesis that pancreatitis is an autodigestive disease in which prematurely activated trypsin within the pancreas was thought to play a pivotal role. Whereas gain‐of‐function PRSS1 missense mutations cause chronic pancreatitis through a negative effect on trypsin lysis and a positive effect on trypsinogen autoactivation, duplication and triplication copy number mutations of the trypsinogen locus cause the disease through a gene‐dosage effect. By contrast, loss‐of‐function variations in the PRSS1 and PRSS2 (encoding anionic trypsinogen) genes protect against chronic pancreatitis. The study of pathogenic PRSS1 mutations also shed lights on the evolution of trypsinogen genes.

Key Concepts:

  • More than 100 years ago, pancreatitis was hypothesised to be an autodigestive disease in which prematurely activated trypsin within the pancreas was thought to play a pivotal role.

  • The century‐old hypothesis was given strong support when a gain‐of‐function missense mutation (R122H) in the cationic trypsinogen gene (PRSS1) was identified as a cause of hereditary pancreatitis in 1996.

  • Gain‐of‐function PRSS1 missense mutations are characterised by a negative effect on trypsin lysis and a positive effect on trypsinogen autoactivation.

  • Duplication and triplication copy number mutations of the trypsinogen locus cause a gain of trypsin through a gene‐dosage effect.

  • Gain‐of‐function PRSS1 mutations can be generated from diverse mutational mechanisms such as CpG substitution, gene conversion, nonallelic homologous recombination (NAHR) and microhomology‐mediated replication‐dependent recombination (MMRDR).

  • Whereas gain‐of‐function PRSS1 mutations predispose to chronic pancreatitis, loss‐of‐function variations in the PRSS1 and PRSS2 (encoding anionic trypsinogen) genes protect against chronic pancreatitis.

  • The study of pathogenic PRSS1 mutations also shed lights on the evolution of trypsinogen genes.

Keywords: cationic trypsinogen; chronic pancreatitis; copy number mutation; gain‐of‐function mutation; gene conversion; hereditary pancreatitis; missense mutation; PRSS1; PRSS2; PRSS3

Figure 1.

The Group I human trypsinogen genes and proteins. Functional genes and obvious pseudogenes are highlighted in red and blue, respectively. T6, a probable expressed pseudogene, is highlighted in green. Adapted from Chen and Férec .

Figure 2.

Molecular properties of mammalian trypsin(ogen)s and naturally occurring missense mutations in the PRSS1 gene. Signal peptide is in italics, whereas activation peptide is shaded in grey. Some of the residues that are critical for trypsin structure and function and that are consequently absolutely conserved throughout vertebrate evolution are highlighted in colour: the catalytic triad residues (His63, Asp107 and Ser200) are in red; the four residues determining trypsin specificity (Asp194, Gln197, Gly217 and Gly227) are in blue and the six cysteine residues necessary to form the three disulfide bridges (48–64, 171–185 and 196–220) are in green. All of the currently reported missense mutations in the human PRSS1 gene are positioned in the sequence accordingly, with the functionally characterised ones being highlighted in bold. Exon boundaries are indicated by purple vertical lines below the aligned sequences. Adapted from Chen and Férec .

Figure 3.

Trypsinogen, a double‐edged sword. (a) Illustration of the physiological role performed by trypsin(ogen). (b) Illustration of how prematurely activated trypsinogen within the pancreas can potentially result in disease. Normally, prematurely activated trypsin within the pancreas can be inhibited by the human pancreatic secretory trypsin inhibitor and degraded by chymotrypsinogen C and trypsin itself. A defect in one or a combination of these defence mechanisms may trigger the zymogen activation cascade leading to pancreatic autodigestion. Adapted from Chen and Férec .

Figure 4.

Characterisation of the trypsinogen triplication mutation. (a) The two large triplications as revealed by array‐comparative genomic hybridisation. The left‐hand blue block corresponds to the previously described ∼605‐kb triplication (Le Maréchal et al., ), whereas the right‐hand blue block denotes the location of the newly identified ∼137‐kb triplication. The two segments are separated by ∼90 kb on chromosome 7q35. (b) Schema of the complex triplication. The two tandemly arranged composite duplication blocks are clearly indicated. Adapted from Chauvin et al., with permission from Oxford University Press.

Figure 5.

Schematic representation of how the hybrid PRSS2/PRSS1 gene was generated through nonallelic homologous recombination. Gene structure is not scaled and only the five exons of the functional PRSS1 (blue) and PRSS2 (red) genes are illustrated. Asterisk in the hybrid PRSS2/PRSS1gene highlights the key nucleotide leading to the formation of an N29I mutant cationic trypsinogen. One of the parental alleles carried the common T6/T7 deletion polymorphism. Adapted from Masson et al., with permission from Springer.

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Further Reading

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Witt H, Apte MV, Keim V and Wilson JS (2007) Chronic pancreatitis: challenges and advances in pathogenesis, genetics, diagnosis, and therapy. Gastroenterology 132: 1557–1573.

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Chen, Jian‐Min, and Férec, Claude(Feb 2011) Gain‐ and Loss‐of‐function Mutations in Trypsinogen. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021482]