Genetic determinants of disease characteristics of inflammatory bowel disease and diverticulitis
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Background: Inflammatory Bowel Disease (IBD) is a complex, multigenic disease in which both the innate and adaptive immune systems play a strong role. Although over 40 microbes including bacteria, viruses and yeasts and over 100 genes have been implicated in the pathophysiology of the disease, no one factor alone causes inflammatory bowel disease. The results from a multitude of immunological and physiological studies on gut permeability/tight junctions, lymphocyte trafficking, macrophage, dendritic cell and natural killer cell function and T cell activation, differentiation and function support the role of the immune system in the development of IBD. Variants in genes involved in each of these processes have been discovered in IBD patients supporting the role for a genetic predisposition to this immune dysfunction. The era of genome wide association studies is slowly being replaced by the era of meaningful genotype-phenotype associations. Several IBD phenotypes are relative to surgery. Determining genetic correlates with these phenotypes may assist in surgical decision and lead to a more personalised approach to several aspects of the treatment of the disease, including surgical intervention. Aims: The aim of this thesis project was to determine genetic correlates of surgically relevant phenotypes in IBD, namely 1) disease location in Crohn¿s disease (CD), 2) age at diagnosis of IBD and 3) septic anal disease in CD. This work sparked an interest in another inflammatory colonic pathology with a likely genetic basis which was used as a control in earlier studies using our Biobanks; diverticulitis. We then aimed to find a genetic correlate with diverticulitis requiring surgery. Methods: The Hershey Medical Center Division of Colon and Rectal Surgery¿s IBD Biobank was used to obtain patient details and tissue samples. A custom designed IBD-associated single nucleotide polymorphism (SNP) chip (Illumina, San Diego, CA) containing over 300 SNPs was used for all IBD projects. Polymerase chain reaction and Taqman (Applied Biosystems, Foster City, CA) genotyping was used for the diverticulitis experiments. Real time PCR and immunohistochemistry were performed to determine the downstream tissue effects of the polymorphisms discovered in 2 of the correlations; anal disease and surgical diverticulitis. Results: 1) Disease location project: SNP rs16967637 in the STAT5 gene was significantly associated with small bowel sparing CD when the enteritis group was compared to either a combined colitis/ileocolic group or to those with only ileocolic disease. 2) Age at diagnosis project: The NOD2 SNP rs2076756 was associated with younger age at diagnosis in CD when studying age as a continuous variable. Depending on age categories compared, SNPs in POU5F1, TNFSF15 and HLA DRB1*501 were associated with age of CD diagnosis. The LAMB1 SNP rs886774 was found to be associated with UC that was diagnosed at ¿16 vs. >17 years of age. 3) Septic anal disease project: The TAGAP SNP rs212388 was associated with septic anal disease in CD. Increased TAGAP expression was demonstrated in moderate or severely diseased tissue versus tissue with no or mild disease. TAGAP expression was increased in more distal tissue with a statistically significant difference seen when comparing transverse versus sigmoid colon with moderate or severe disease. 4) Diverticulitis studies: SNP rs7848647 associated with the TNFSF15 gene correlated with diverticulitis requiring surgery vs healthy controls. An approximately 7 fold upregulation of TNFSF15 mRNA was demonstrated in all diverticulitis tissue when compared to Controls. In the 11 paired samples of diverticulitis affected and unaffected tissue from the same patients, no significant difference in TNFSF15 expression was seen. Conclusions: These SNP associations were derived from a carefully characterised cohort of surgically treated IBD and diverticulitis patients. Some findings confirmed previously known data (ie NOD2 and age at diagnosis) but others, interestingly, were unique, novel associations (ie TNFSF15 and surgical diverticulitis, LAMB1 and age at UC diagnosis). Due to the increasing availability and decreasing cost of genome sequencing it will become commonplace in the near future for large amounts of genetic data to be available for each individual. Genetic associations such as those described in this thesis may assist in predicting disease behaviour, determining prognosis and predicting response to medical and surgical therapy in both IBD and diverticulitis patients.
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