ADVANCED SERRATED LESIONS OF THE COLON AND RECTUM
Serrated colorectal polyps are defined histologically by a sawtooth-like appearance of the epithelium, and comprise a spectrum of lesions, including hyperplastic polyps, SSA/Ps with or without dysplasia and TSAs. Opinion regarding the clinical significance of serrated polyps has changed from considering them safe and irrelevant to cancer to seeing them as having a role as important precursors of right-sided colon cancer with a methylator phenotype (108). SSA/Ps without dysplasia seem to be particularly dangerous lesions because they are more common than TSAs and have been more often incriminated as immediate precursors of cancer (75). However, TSAs are different from SSA/Ps in that, by definition, they all contain adenomatous epithelium. SSA/Ps with dysplasia also contain adenomatous epithelium, so these three subtypes of serrated polyps can be regarded as “advanced”.
Little is known about the natural history of the different subtypes of serrated colorectal lesions. Hyperplastic polyps are the most numerous, and are generally small and left-sided. Development of carcinoma directly from hyperplastic polyps is uncommon, and the studies describing their natural history are confounded by past difficulties in separating serrated lesions histologically. SSA/Ps without dysplasia are more direct precursors of CpG island methylator phenotype (CIMP) cancer (109). These are found in 2%–6% of patients undergoing screening colonoscopy (76, 110) and have a characteristic sessile or flat mucus-covered appearance. TSAs are less common, accounting for less than 1% of polyps in these same series. Because TSAs are much less common than SSA/Ps, there are fewer data on their molecular profile. TSAs may have KRAS or BRAF mutations, and either low or high levels of CIMP (22, 111). They do not show hypermethylation of MLH1 or develop microsatellite instability, but they do commonly have hypermethylation of the DNA repair gene MGMT (O6-methylguanine-DNA methyltransferase) (111). MGMT promoter methylation has been associated with both CIMP-low and CIMP-high colorectal cancer (112). TSAs are not usually diagnosed endoscopically, but rather are thought to be adenomas. It is only when the pathology report is read that the diagnosis is appreciated. A third category of advanced serrated colorectal polyp is the SSA/Ps with dysplasia. Here, adenomatous epithelium coexists with serrated epithelium in a way that is different to that observed with TSA. Current opinion is that SSA/Ps with dysplasia represent adenomatous dysplasia arising in an SSA/P (75), but I have made them a separate group here to identify whether there are any qualities that make them unique lesions. In the absence of systematic data describing the natural history of serrated lesions, the SSA/Ps with or without dysplasia and TSAs in Cleveland Clinic, Ohio of one surgeon were investigated.
This was a retrospective review of data collected prospectively from 2004 (the year when pathology reporting of serrated polyps became acceptably reproducible at Cleveland Clinic, Ohio) to 2010, and stored in a single-surgeon colonoscopy database. The institutional review board at the Cleveland Clinic approved this database and a waiver of consent was granted for this study.
All clinically significant lesions seen during colonoscopy were removed or biopsied, and tissue was routinely submitted for pathology. When there were multiple, small, pale polyps in the rectum and sigmoid, not all were removed or biopsied, but a random representative sample of one or two lesions was excised or biopsied.
All patients with a histological diagnosis of SSA/P with or without dysplasia or TSA were identified. Medical records, histological slides and endoscopic reports were reviewed. Demographic data, indication for colonoscopy, polyp location, and treatment were extracted.
Continuous data were described as the mean and standard deviation (for normally distributed data) and the median and range (for non-parametric data). Categorical data were given as numbers and percentages. The significance of differences between groups was assessed using the Student’s t-test for parametric continuous data and the Chi-squared or Fisher’s Exact test for categorical data.
From 2004 to 2010, 154 patients were identified as having 211 histologically advanced serrated polyps. There were 157 SSA/Ps without dysplasia in 111 patients (72.1%), 33 TSAs in 24 patients (15.6%), and 21 SSA/Ps with dysplasia in 19 patients (12.3%). The patient demographics are shown in Table 3.1. Patients with TSA were younger than the others (54.0±7.1 years versus 63.8±10.6 years for SSA/P and 66.5±7.8 years for SSA/Ps with dysplasia; p<0.05). Women comprised 55.9%, 50.0% and 63.2% of patients with SSA/Ps, TSAs and SSA/Ps with dysplasia, respectively (p=0.682). A family history of CRC was significantly more common in the SSA/P group (35.1% versus 8.4% in the TSA group versus 5.3% in the SSA/Ps with dysplasia group; p=0.001).
The diagnosis of SSA/P without dysplasia is based solely on histological criteria, which were uniformly applied in Cleveland Clinic from 2004 onwards. Therefore, it is important to define the histological characteristics of these advanced serrated polyps. In SSA/Ps without dysplasia, the basal half of the crypts are often dilated and extend laterally to grow parallel to the muscularis mucosae, forming a crypt shaped like an inverted « T » or an « L » (113). The proliferation zone extends from the crypt bases, and cells often have vesicular nuclei with slight chromatin irregularities (8, 82, 93).
CHAPTER I – INTRODUCTION 22
CHAPTER II – SERRATED POLYP PATHWAY 25
2.1 Definition and classification of serrated lesions
2.2 Hyperplastic polyps
2.3 Sessile serrated adenoma/polyps without dysplasia
2.4 Sessile serrated adenoma/polyps with dysplasia
2.5 Traditional serrated adenoma
2.6 Rationale for clinical research
CHAPTER III – ADVANCED SERRATED LESIONS OF THE COLON AND RECTUM
CHAPTER IV – SERRATED POLYP DETECTION RATE VERSUS ADENOMA DETECTION RATE DURING COLONOSCOPY
CHAPTER V – SNARING LARGE SERRATED POLYPS
CHAPTER VI – SERRATED PATHWAY COLORECTAL CANCERS
6.1 Historical evidence
6.2 Recent evidence
6.3 Unpublished date from the Cleveland Clinic laboratory
6.4 Guidelines for post-polypectomy surveillance
CHAPTER VII – MOLECULAR MECHANISMS UNDERLYING COLORECTAL CANCERS
7.1 Key molecular pathways
7.2 Molecular alterations in serrated pathway lesions
7.3 miRNAs and colorectal cancer
7.4 Epigenetic changes in serrated pathway
7.5 Rationale for laboratory research
CHAPTER VIII – IDENTIFICATION OF miRNAs RELEVANT IN THE
SERRATED COLORECTAL CANCER PATHWAY
CHAPTER IX – TUMOUR-SUPPRESSIVE FUNCTIONS OF miR-1247 AND
ITS TARGET PATHWAY
9.2 Methods and materials
9.3 Statistical analysis
CHAPTER X – SUMMARY AND DISCUSSION
10.1 Summary of clinical research
10.2 Summary of experimental findings
10.3 Clinical implications
10.4 Future perspective and conclusion
CHAPTER XI– APPENDIX- LABORATORY MATERIALS AND METHODS
11.1 Human tissue samples
11.2 Colorectal cancer cell lines
11.3 DNA and RNA purification
11.4. Techniques for studying DNA methylation
11.5 DNA polyacrylamide gel electrophoresis
11.6 Real-time quantitative PCR
11.7 Transient transfection of cell lines
11.8 Cell proliferation assays
11.9 Annexin V apoptosis assay
11.10 Cell migration assay
11.11 Western blot
11.14 Lentiviral infection
11.15 Subcutaneous xenografts
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CLINICAL SIGNIFICANCE AND MOLECULAR FEATURES OF THE SERRATED COLORECTAL CANCER PATHWAY