Dipeptidyl peptidases in gastrointestinal health and disease

Author: Simone Jaenisch

Jaenisch, Simone, 2018 Dipeptidyl peptidases in gastrointestinal health and disease, Flinders University, College of Medicine and Public Health

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Current methods for diagnosis and monitoring of gastrointestinal health and disease are highly invasive, expensive and represent a major burden to patients, the economy and the health care system. New serological, tissue and non-invasive biomarkers that can better reflect and/or predict the physiological changes that occur in the gastrointestinal tract during disease would reduce the burden of disease and improve disease management. Dipeptidyl peptidases (DPPs) have roles in gastrointestinal health and immunity and have been proposed as potential biomarkers for gastrointestinal conditions including inflammatory bowel disease (IBD) and Barrett’s oesophagus. This thesis aimed to better understand the roles of DPPs in gastrointestinal health and disease and explore their potential as disease biomarkers by characterising the expression and activity profiles of DPPs during health and gastrointestinal disease.

The mRNA expression and enzyme activity profiles of DPP enzymes throughout the gastrointestinal tract was initially characterised in mucosal biopsies from healthy donors. Low levels of DPP9 mRNA were found throughout the gastrointestinal tract. In comparison, higher levels of DPP8 mRNA was found in the gastrointestinal tissues, which may suggest a more prominent role for DPP8 in gastrointestinal health. Low but detectable levels of DPP4 mRNA was found in colonic and gastric tissues, though the highest levels of DPP4 mRNA were found within ileal tissues. Consistent with this, high levels of DPP4 enzyme activity were found within the ileum. In comparison, DPP8/9 enzyme activity was found at more consistent levels in the different gastrointestinal tissues.

Initial analysis of enzyme activity levels during oesophageal and ileal disease identified differences in DPP4 enzyme activity. Higher levels of DPP4 enzyme activity were found in biopsies taken from lesions of Barrett’s oesophagus compared to oesophageal tissue from healthy individuals. Though in a patient with Crohn’s Disease (CD), lower levels of DPP4 enzyme activity were found within affected ileal tissue compared to matched inactive tissue. These results may suggest utility of mucosal DPP4 enzyme activity as a biomarker of gastrointestinal health and integrity throughout the gastrointestinal tract, in various disease paradigms.

Breath testing represents a novel, non-invasive technique that has clinical applications in diagnosis and monitoring of different conditions. Our laboratory previously developed and validated a 13C-based non-invasive assay to quantify mucosal DPP4 enzyme activity in vitro. Application of this novel 13C-DPP4 assay in oesophageal cancer cell lines found differential 13CO2 levels that approximately corresponded to the DPP4 enzyme activity levels detected using the colorimetric assay, suggesting that the 13C-DPP4 assay can selectively detect mucosal DPP4 activity. While further validation studies are necessary, a 13C-DPP4 breath test that can detect and quantify DPP4 enzyme activity throughout the gastrointestinal tract, could have clinical applications in the diagnosis and monitoring of Barrett’s oesophagus and IBD.

The utility of DPPs as plasma and tissue biomarkers in human IBD was investigated further in two separate observational studies (Chapter 3 & 4). Lower levels of plasma DPP enzyme activity were found in patients with IBD, compared to non-IBD individuals. Further analysis also found that plasma DPP enzyme activity and plasma FAP enzyme activity were lower in patients with evidence of active inflammation, which may suggest utility for plasma DPP enzyme activity in the diagnosis and monitoring of IBD.

Differences in DPP mRNA expression and DPP enzyme activity were also found within the colorectal tissue of IBD patients. Higher levels of DPP4, FAP and DPP8 mRNA were found in the colorectal tissues from patients with IBD compared to controls. Increased DPP enzyme activity was also found in inflamed colorectal tissues compared to matched inactive tissue from IBD patients, suggesting that changes to DPP mRNA expression and enzyme activity may be associated with gastrointestinal damage or inflammation.

In summary, the significant original contributions to knowledge achieved in this thesis are firstly, an addition to the current understanding of DPP mRNA expression and enzyme activities in gastrointestinal health and disease. Secondly, this thesis provides further clinically relevant evidence for the usefulness of DPPs as biomarkers to diagnose and monitor premalignant and inflammatory conditions. The ability for DPP4 activity to be detected using a non-invasive 13C-based approach is novel and enhances the potential clinical utility of DPP4. It is envisaged that future studies may be able to develop and validate the 13C-DPP4 assay that will provide accurate real-time information relating to DPP4 enzyme activity and expression in the gastrointestinal tract that can be used to diagnose and monitor disease states.

Keywords: Biomarker, Inflammatory Bowel Disease, Ulcerative Colitis, Crohns Disease, Dipeptidyl peptidase

Subject: Medical Science thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Medicine and Public Health
Supervisor: Dr Roger Yazbek