Date of Award

2013

Document type

Thesis

Degree Name

MD (Medical Doctor)

First Supervisor

Walter A. Koltun

Second Supervisor

Professor Arnold Hill

Funder/Sponsor

The American Society of Colon and Rectal Surgeons (ASCRS) Research Foundation "International Research Fellowship Award"

Keywords

Inflammatory Bowel Diseases, Genetics, Surgery

Abstract

BACKGROUND: Severe pouchitis (SP) and Crohn's disease (CD)-like complications confound the success o f the ileal pouch anal anastomosis (IPAA) in patients with ulcerative colitis. Furthermore, there are no clear criteria for judging the severity of disease in patients with CD. Yet classification of patients into low- and high-risk severity groups would benefit both medical and surgical management. To date, approximately 83 single nucleotide polymorphisms (SNPs) within 55 genes have been associated with IBD.

OBJECTIVE: (1) To determine whether the NOD2 gene correlated with post operative IPAA complications (CD-like/pouchitis) (2) To identify SNPs that correlate with complications after IPAA that could be utilized in a gene signature fashion to predict IPAA postoperative complications aid in preoperative surgical decision making. (3) To identify genetic determinants (SNPs) that could be markers of CD severity by the use of frequency of ileocolic surgery as a surrogate for disease severity.

DESIGN: IPAA patients were retrospectively sub-classified into the following groups:1) IPAA with CD-like complications (perianal fistula, pouch inlet stricture/upstream small-bowel disease, or biopsies showing granulomata) occurring at least 6 months after ileostomy closure; 2) IPAA with mild pouchitis (≥3 episodes/y for 2 consecutive years); 3) IPAA with SP (≥4 episodes/y for 2 consecutive years or need for continuous antibiotics); 4) IPAA without complications or pouchitis; The severity of CD was quantified by dividing the total number of ileocolectomy procedures by the time between IBD diagnosis and the patient's last clinic visit, the rationale being that more severe disease would be associated with a more frequent need for surgery. The 3 NOD2 SNPs (rs2066844, rs2066845, and rs2066847) were genotyped using polymerase chain reaction. Genotyping for 83 IBD associated SNPs was performed on a customized Illumina Veracode genotyping platform.

RESULTS: NOD2 mutations were significantly higher in the SP group (67%) compared with both asymptomatic (5.4%, P < .001) and CD-like pouch groups (14.3%, P = .008) groups. The top 2 SNPs for CD-like complications were in the 10q21 locus and the gene for PTGER4 (p = 0.006 and 0.007), whereas for SP it was NOD2 and TNFSF15 (p = 0.003 and 0.011). Probability equations suggested that the risk of these 2 complications greatly increased with increasing number of risk alleles, going as high as 92% for SP and 65% for CD-like complications. The average number of ileocolectomies per patient was 1.7 (range, 1-5) with an average duration of disease o f 14.7 years. SNP rs4958847 in the IRGM gene was the most significant SNP as being associated with ileocolectomy. Patients carrying the "at-risk" allele for this SNP (n = 20) had an average of 1 surgery every 6.87 ± 1.33 years in comparison with patients carrying the wild-type genotype (n =46) who averaged 1 surgery in 11.43 ± 1.21 years (p = 0.007).

CONCLUSIONS: In this IPAA patient cohort, asymptomatic IPAA patients have a low incidence of NOD2 mutations not significantly different from patients with mild pouchitis or healthy controls. Patients with SP had the highest incidence of NOD2 mutations vs. CD-like, suggesting a different genetic makeup in these 2 patient groups. Preoperative genetic analysis and use of gene signatures hold promise for improved preoperative surgical patient selection to minimize these IPAA complications. SNP rs4958847 in the IRGM gene correlated very significantly with frequency of surgery in patients with ileocolonic CD. This SNP may be a marker for disease severity and/or early recurrence after ileocolectomy and may assist in surgical and medical decision making.

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Comments

A thesis submitted to the Royal College of Surgeons in Ireland for the degree of Doctor of Medicine from the National University of Ireland in 2013.

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