Judith
Crohnsforum Science Advisor
Crohn’s Disease is Often Unpredictable
Patients with Crohn’s Disease (CD), a subtype of Inflammatory Bowel Disease (IBD), have non-uniform treatment outcomes and variable disease progression between patients. This unpredictable nature commonly seen in Crohn’s disease is frustrating for both patient and physician alike. Genetically related individuals can exhibit vastly different symptoms and disease progression. Disease treatment approaches differ as well. Some physicians opt for a traditional treatment regimen, while others employ a “Top-Down” approach, or even offer “Alternative” treatments. Treatments are often based on symptom severity and approached in a trial-and-error manner, as patients can be (or become) nonresponsive and treatment outcomes are highly variable. A major factor in Crohn’s disease unpredictability lies in our lack of understanding the full biological mechanism of Crohn’s disease. Although Crohn’s disease has been well studied by researchers, the precise factor(s) that cause Crohn’s disease have yet to be described.
The Biological Mechanism of Crohn’s Disease is Extremely Complex
The root of the unpredictable nature of Crohn’s disease is largely due to the complexity of the Gastrointestinal (GI) Tract and the myriad of factors that can affect it. The GI tract is constantly exposed to external factors which it must either remove from the body as waste or incorporate as nutrients. The delicate balance between the GI Tract’s “Exclude” versus “Include” programs necessitates a very complicated network between the intestinal mucosa, immune system function, and microbes.
This delicate balance in the GI Tract can easily be upset by many factors, such as: Environment, Microbial Load/Microbial Balance, Gene Mutations, Immune Function, Treatment (i.e. Antibiotics), etc. Many factors have been shown to influence, to a greater or lesser extent, whether a person will exhibit Crohn’s disease symptoms and disease severity. Arguably one of the most heavily weighted factors in Crohn’s disease is the genetic background of the patient.
Gene Mutations Differ in Crohn’s Disease Effects
Crohn’s is known to be associated with multiple different gene mutations. Although the mechanism is not well-understood, it appears that gene mutational effects interact with effects of other gene mutations in addition to interactions with environmental factors. Each of these factors is individually weighted in terms of how much it will affect whether a person will exhibit Crohn’s disease symptoms and the severity of disease. To complicate this “Crohn’s Disease Equation”, some gene mutations have actually been described as having protective effects against Crohn’s disease. Furthermore, the biochemical pathways involved in GI Tract immunology are complex and intertwined- a small change in one factor in one pathway can have widespread effects on the flux in multiple other pathways. These effects may contribute to Crohn’s disease pathology in significant or insignificant ways.
Crohn’s Disease-Associated Gene Mutations Vary in Functionality
There are multiple gene mutations that are associated with an increased likelihood of Crohn’s disease. A major factor in the variability in Crohn’s disease between patients is that these Crohn’s disease-associated genes act within very different biochemical pathways. These disrupted pathways can feed into other pathways, giving rise to additional symptoms, diseases, or differing treatment responses between patients.
IBD Associated Genes
Innate Immune System Microbial Recognition Genes
The NOD2 / CARD15 (Nucleotide-binding Oligomerization Domain containing 2 / CAspase Re-cruitment Domain 15) acts as a bacterial sensor by binding a major component found in bacterial cell walls, MDP. NOD2/CARD15 is expressed in Macrophages (Macs), Paneth Cells (PC), Dendritic Cells (DC), and the Mucosal Epithelial Cells of the GI Tract. MDP binding to NOD2/CARD15 results in three major effects.
First, NOD2/CARD15 binding induces the bacterial phagocytic, or “cell eating” pathways. Bacteria are engulfed and fused with digestive enzymes within the cell. After bacteria are digested, small portions of the bacteria are returned to the surface of the cell for Antigen Presentation. Through Antigen Presentation, the infectious agent is “shown” to immune response cells so they may recognize and clear the infectious agent.
Secondly, NOD2/CARD15 detection of MDP causes secretion of alpha-Defensins, HD5 and HD6, which are antimicrobial factors that can aid in infectious agent killing. In the NOD2/CARD15 gene mutation 3020insC, alpha-defensin levels are about 3X less than alpha-defensin levels seen in other NOD2 mutations.
Third, MDP binding to NOD2/CARD15 activates multiple downstream effects which causes the transcription factor, NFkB, to turn on multiple pro-inflammatory genes. These pro-inflammatory factors assist in the clearance of infectious microbes in multiple ways including, recruiting immune cells to the site of infection, increasing the permeability (leakiness) of tissues so immune cells can easily access the site of infection, and by stimulating systemic anti-microbial immune responses (i.e. fever).
NFkB activation induces feedback inhibition pathways which reduces MDP-NOD2/CARD15 signaling. As a result, the NOD2/CARD15 pathway effective self-dampens its own signal which is important for tolerance of commensal microbes. For example, activation of NFkB causes ITCH- mediated pathway inhibition. A reduction in NOD2/CARD15 pathway signaling can reduce activation of NFkB and ITCH. Reduced ITCH activity can lead to the exacerbated inflammatory reactions typically seen in Crohn’s disease.
NOD2/CARD15 gene mutations are associated with Crohn’s disease and impaired MDP-dependent bacterial killing. NOD2/CARD15 mutations typically correlate with an earlier age of onset and more severe form of the disease.
NOD2/CARD15 mutations are also associated with ileal location, fistulas, and strictures in Crohn’s disease. In addition to Crohn’s disease, NOD2/CARD15 mutations are associated with Blau Syndrome, Uveitis, Iritis, Graft-Versus Host Disease (GVHD), and Early Onset Sarcoidosis (EOS).
Autophagy / Xenophagy Genes
Phagocytosis, or “Cell Eating”, is important for recycling or degradation of old cell components, or during cell-starvation to obtain energy or nutrients. This process is known as Autophagy, or in the case of breakdown of infectious microbes, Xenophagy. AuTophaGy-related 16-Like 1 (ATG16L1) is important in effective targeting of digestive membranes and secretion of alpha-defensins from Paneth Cells (PC)
The ATG16L1 gene mutation T300A, impairs the autophagic process and antimicrobial secretion from PCs, and correlates with increased risk of Crohn’s disease. This gene mutation is also associated with early onset of symptoms in Crohn’s disease.
Anti-Inflammatory Immune Responses
Anti-inflammatory immune responses are known to downregulate microbe-induced pro-inflammation pathway effects. Interleukin-10 (IL-10) is upregulated in T-cells in response to interaction with commensal bacteria and is implicated in both, UC and CD symptoms. Patients who have mutations in both copies of the IL-10 receptor (IL-10R) have extremely severe, early onset IBD symptoms due to intestinal hyperinflammation. IL-10 mutations have also been seen in Diabetes Type-1, Systemic Lupus Erythematosus (SLE), Bechet’s Disease, and Multiple Sclerosis (MS).
Epithelial Barrier and Tight Junction Gene Mutations
Mutations in genes which code for Epithelial Barrier Functional proteins and Tight Junction proteins often correlate with Ulcerative Colitis symptoms. Gene mutations which disrupt the integrity of the intestinal epithelium, for example in genes such as, Cadherin-1 (CDH1) or Laminin-beta 1 (LAMB-1) decrease the mucosal layer’s resistance to injury, and are more susceptible to invasion by intestinal microbes.
Conclusion
These are just a few examples of genes, when mutated, can increase the risk of Inflammatory Bowel Disease. Gene mutations can affect multiple pathways including: microbial sensing, up- or down- regulation of the inflammatory response, antimicrobial compound secretion, lysosomal targeting, mucosal barrier, etc. Each mutation is correlated with a different expected symptom severity, disease course, response to treatment and presence of extraintestinal manifestations. The further understanding of how these gene mutations affect molecular pathways involved in IBD pathology will increase options for discovery of effective therapeutic treatments of IBD and IBD-related diseases.
References
1. Cho JH and Brant SR. Recent Insights Into the Genetics of Inflammatory Bowel Disease. Gastroenterology. 2011;140:1704-1712. http://download.journals.elsevierhealth.com/pdfs/journals/0016-5085/PIIS0016508511002009.pdf
2. Triantafillidis JK, Merikas E, and Georgopoulos F. Current and emerging drugs for the treatment of inflammatory bowel disease. Drug Design, Development and Therapy; 2011:5 185–210. http://www.dovepress.com/getfile.php?fileID=9609
3. Meggyesi N, Kiss LS, Koszarska M, et al. NKX2-3 and IRGM variants are associated with disease susceptibility to IBD in Eastern European patients. World J Gastroenterol. 2010; 16(41): 5233–5240. http://www.wjgnet.com/1007-9327/pdf/v16/i41/5233.pdf
4. Helminth Therapy. Trichuris suis Ova (TSO©). OvaMed. GmbH. Accessed May 2012. http://www.ovamed.org
5. Cario E. Toll-like Receptors in Inflammatory Bowel Diseases: A Decade Later. Inflamm Bowel Dis. 2010;16:1583-1597. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958454/pdf/ibd0016-1583.pdf
6. Willer Y, Muller B, and Bumann D. Intestinal Inflammation Responds to Microbial Tissue Load Independent of Pathogen/Non-Pathogen Discrimination. PLoS One. 2012; 7(5): http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3346762/pdf/pone.0035992.pdf
7. Choi AJS and Ryter SW. Autophagy in Inflammatory Diseases. International Journal of Cell Biology. 2011. http://downloads.hindawi.com/journals/ijcb/2011/732798.pdf
8. Beisner ,J Stange EF and Wehkamp J. Paneth cell function. Implications in pediatric Crohn disease. Gut Microbes. 2011;2(1): 47-51. http://www.landesbioscience.com/journals/gutmicrobes/BeisnerGMIC2-1.pdf
9. Fritz T, Niederreiter L, Adolph T, et al. Crohn’s disease: NOD2, autophagy and ER stress converge. 2011;Gut: 1580-1588. http://gut.bmj.com/content/60/11/1580.full.pdf+html?sid=fe20932d-d9a5-4df0-b9b3-37eff91eaf3a
10. Khor B, Gardet A, and Xavier RJ. Genetics and pathogenesis of inflammatory bowel disease. Nature. 2011; 474: 307-317. http://www.nature.com/nature/journal/v474/n7351/pdf/nature10209.pdf
11. Lovato P, Brender C, Agnholt J et al. Constitutive STAT3 Activation in Intestinal T Cells from Patients with Crohn's Disease. Journal Biol. Chem. 2003;278: 16777-16781. http://www.jbc.org/content/278/19/16777.full.pdf+html
12. Biswas A, Petnicki-Ocwieja T, and Kobayshi KS. Nod2: a key regulator linking microbiota to intestinal mucosal immunity. J. Mol. Med (Berl). 2012; 90(1): 15-24. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3263373/pdf/nihms347685
Patients with Crohn’s Disease (CD), a subtype of Inflammatory Bowel Disease (IBD), have non-uniform treatment outcomes and variable disease progression between patients. This unpredictable nature commonly seen in Crohn’s disease is frustrating for both patient and physician alike. Genetically related individuals can exhibit vastly different symptoms and disease progression. Disease treatment approaches differ as well. Some physicians opt for a traditional treatment regimen, while others employ a “Top-Down” approach, or even offer “Alternative” treatments. Treatments are often based on symptom severity and approached in a trial-and-error manner, as patients can be (or become) nonresponsive and treatment outcomes are highly variable. A major factor in Crohn’s disease unpredictability lies in our lack of understanding the full biological mechanism of Crohn’s disease. Although Crohn’s disease has been well studied by researchers, the precise factor(s) that cause Crohn’s disease have yet to be described.
The Biological Mechanism of Crohn’s Disease is Extremely Complex
The root of the unpredictable nature of Crohn’s disease is largely due to the complexity of the Gastrointestinal (GI) Tract and the myriad of factors that can affect it. The GI tract is constantly exposed to external factors which it must either remove from the body as waste or incorporate as nutrients. The delicate balance between the GI Tract’s “Exclude” versus “Include” programs necessitates a very complicated network between the intestinal mucosa, immune system function, and microbes.
This delicate balance in the GI Tract can easily be upset by many factors, such as: Environment, Microbial Load/Microbial Balance, Gene Mutations, Immune Function, Treatment (i.e. Antibiotics), etc. Many factors have been shown to influence, to a greater or lesser extent, whether a person will exhibit Crohn’s disease symptoms and disease severity. Arguably one of the most heavily weighted factors in Crohn’s disease is the genetic background of the patient.
Gene Mutations Differ in Crohn’s Disease Effects
Crohn’s is known to be associated with multiple different gene mutations. Although the mechanism is not well-understood, it appears that gene mutational effects interact with effects of other gene mutations in addition to interactions with environmental factors. Each of these factors is individually weighted in terms of how much it will affect whether a person will exhibit Crohn’s disease symptoms and the severity of disease. To complicate this “Crohn’s Disease Equation”, some gene mutations have actually been described as having protective effects against Crohn’s disease. Furthermore, the biochemical pathways involved in GI Tract immunology are complex and intertwined- a small change in one factor in one pathway can have widespread effects on the flux in multiple other pathways. These effects may contribute to Crohn’s disease pathology in significant or insignificant ways.
Crohn’s Disease-Associated Gene Mutations Vary in Functionality
There are multiple gene mutations that are associated with an increased likelihood of Crohn’s disease. A major factor in the variability in Crohn’s disease between patients is that these Crohn’s disease-associated genes act within very different biochemical pathways. These disrupted pathways can feed into other pathways, giving rise to additional symptoms, diseases, or differing treatment responses between patients.
IBD Associated Genes
Innate Immune System Microbial Recognition Genes
The NOD2 / CARD15 (Nucleotide-binding Oligomerization Domain containing 2 / CAspase Re-cruitment Domain 15) acts as a bacterial sensor by binding a major component found in bacterial cell walls, MDP. NOD2/CARD15 is expressed in Macrophages (Macs), Paneth Cells (PC), Dendritic Cells (DC), and the Mucosal Epithelial Cells of the GI Tract. MDP binding to NOD2/CARD15 results in three major effects.
First, NOD2/CARD15 binding induces the bacterial phagocytic, or “cell eating” pathways. Bacteria are engulfed and fused with digestive enzymes within the cell. After bacteria are digested, small portions of the bacteria are returned to the surface of the cell for Antigen Presentation. Through Antigen Presentation, the infectious agent is “shown” to immune response cells so they may recognize and clear the infectious agent.
Secondly, NOD2/CARD15 detection of MDP causes secretion of alpha-Defensins, HD5 and HD6, which are antimicrobial factors that can aid in infectious agent killing. In the NOD2/CARD15 gene mutation 3020insC, alpha-defensin levels are about 3X less than alpha-defensin levels seen in other NOD2 mutations.
Third, MDP binding to NOD2/CARD15 activates multiple downstream effects which causes the transcription factor, NFkB, to turn on multiple pro-inflammatory genes. These pro-inflammatory factors assist in the clearance of infectious microbes in multiple ways including, recruiting immune cells to the site of infection, increasing the permeability (leakiness) of tissues so immune cells can easily access the site of infection, and by stimulating systemic anti-microbial immune responses (i.e. fever).
NFkB activation induces feedback inhibition pathways which reduces MDP-NOD2/CARD15 signaling. As a result, the NOD2/CARD15 pathway effective self-dampens its own signal which is important for tolerance of commensal microbes. For example, activation of NFkB causes ITCH- mediated pathway inhibition. A reduction in NOD2/CARD15 pathway signaling can reduce activation of NFkB and ITCH. Reduced ITCH activity can lead to the exacerbated inflammatory reactions typically seen in Crohn’s disease.
NOD2/CARD15 gene mutations are associated with Crohn’s disease and impaired MDP-dependent bacterial killing. NOD2/CARD15 mutations typically correlate with an earlier age of onset and more severe form of the disease.
NOD2/CARD15 mutations are also associated with ileal location, fistulas, and strictures in Crohn’s disease. In addition to Crohn’s disease, NOD2/CARD15 mutations are associated with Blau Syndrome, Uveitis, Iritis, Graft-Versus Host Disease (GVHD), and Early Onset Sarcoidosis (EOS).
Autophagy / Xenophagy Genes
Phagocytosis, or “Cell Eating”, is important for recycling or degradation of old cell components, or during cell-starvation to obtain energy or nutrients. This process is known as Autophagy, or in the case of breakdown of infectious microbes, Xenophagy. AuTophaGy-related 16-Like 1 (ATG16L1) is important in effective targeting of digestive membranes and secretion of alpha-defensins from Paneth Cells (PC)
The ATG16L1 gene mutation T300A, impairs the autophagic process and antimicrobial secretion from PCs, and correlates with increased risk of Crohn’s disease. This gene mutation is also associated with early onset of symptoms in Crohn’s disease.
Anti-Inflammatory Immune Responses
Anti-inflammatory immune responses are known to downregulate microbe-induced pro-inflammation pathway effects. Interleukin-10 (IL-10) is upregulated in T-cells in response to interaction with commensal bacteria and is implicated in both, UC and CD symptoms. Patients who have mutations in both copies of the IL-10 receptor (IL-10R) have extremely severe, early onset IBD symptoms due to intestinal hyperinflammation. IL-10 mutations have also been seen in Diabetes Type-1, Systemic Lupus Erythematosus (SLE), Bechet’s Disease, and Multiple Sclerosis (MS).
Epithelial Barrier and Tight Junction Gene Mutations
Mutations in genes which code for Epithelial Barrier Functional proteins and Tight Junction proteins often correlate with Ulcerative Colitis symptoms. Gene mutations which disrupt the integrity of the intestinal epithelium, for example in genes such as, Cadherin-1 (CDH1) or Laminin-beta 1 (LAMB-1) decrease the mucosal layer’s resistance to injury, and are more susceptible to invasion by intestinal microbes.
Conclusion
These are just a few examples of genes, when mutated, can increase the risk of Inflammatory Bowel Disease. Gene mutations can affect multiple pathways including: microbial sensing, up- or down- regulation of the inflammatory response, antimicrobial compound secretion, lysosomal targeting, mucosal barrier, etc. Each mutation is correlated with a different expected symptom severity, disease course, response to treatment and presence of extraintestinal manifestations. The further understanding of how these gene mutations affect molecular pathways involved in IBD pathology will increase options for discovery of effective therapeutic treatments of IBD and IBD-related diseases.
References
1. Cho JH and Brant SR. Recent Insights Into the Genetics of Inflammatory Bowel Disease. Gastroenterology. 2011;140:1704-1712. http://download.journals.elsevierhealth.com/pdfs/journals/0016-5085/PIIS0016508511002009.pdf
2. Triantafillidis JK, Merikas E, and Georgopoulos F. Current and emerging drugs for the treatment of inflammatory bowel disease. Drug Design, Development and Therapy; 2011:5 185–210. http://www.dovepress.com/getfile.php?fileID=9609
3. Meggyesi N, Kiss LS, Koszarska M, et al. NKX2-3 and IRGM variants are associated with disease susceptibility to IBD in Eastern European patients. World J Gastroenterol. 2010; 16(41): 5233–5240. http://www.wjgnet.com/1007-9327/pdf/v16/i41/5233.pdf
4. Helminth Therapy. Trichuris suis Ova (TSO©). OvaMed. GmbH. Accessed May 2012. http://www.ovamed.org
5. Cario E. Toll-like Receptors in Inflammatory Bowel Diseases: A Decade Later. Inflamm Bowel Dis. 2010;16:1583-1597. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958454/pdf/ibd0016-1583.pdf
6. Willer Y, Muller B, and Bumann D. Intestinal Inflammation Responds to Microbial Tissue Load Independent of Pathogen/Non-Pathogen Discrimination. PLoS One. 2012; 7(5): http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3346762/pdf/pone.0035992.pdf
7. Choi AJS and Ryter SW. Autophagy in Inflammatory Diseases. International Journal of Cell Biology. 2011. http://downloads.hindawi.com/journals/ijcb/2011/732798.pdf
8. Beisner ,J Stange EF and Wehkamp J. Paneth cell function. Implications in pediatric Crohn disease. Gut Microbes. 2011;2(1): 47-51. http://www.landesbioscience.com/journals/gutmicrobes/BeisnerGMIC2-1.pdf
9. Fritz T, Niederreiter L, Adolph T, et al. Crohn’s disease: NOD2, autophagy and ER stress converge. 2011;Gut: 1580-1588. http://gut.bmj.com/content/60/11/1580.full.pdf+html?sid=fe20932d-d9a5-4df0-b9b3-37eff91eaf3a
10. Khor B, Gardet A, and Xavier RJ. Genetics and pathogenesis of inflammatory bowel disease. Nature. 2011; 474: 307-317. http://www.nature.com/nature/journal/v474/n7351/pdf/nature10209.pdf
11. Lovato P, Brender C, Agnholt J et al. Constitutive STAT3 Activation in Intestinal T Cells from Patients with Crohn's Disease. Journal Biol. Chem. 2003;278: 16777-16781. http://www.jbc.org/content/278/19/16777.full.pdf+html
12. Biswas A, Petnicki-Ocwieja T, and Kobayshi KS. Nod2: a key regulator linking microbiota to intestinal mucosal immunity. J. Mol. Med (Berl). 2012; 90(1): 15-24. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3263373/pdf/nihms347685
