—  SYMPOSIUM #48  —

Molecular Pathogenesis of Gastrointestinal Neoplasia
Moderators: Dr. Wataru Yasui and Dr. Jeremy Jass

Section 4 - Alternative Pathways of Colorectal Tumorigenesis

Michael J O'Brien and Shi Yang
Boston University School of Medicine


The Serrated Polyp Neoplasia Pathway
The serrated polyp neoplasia pathway comprises a morphologically distinct group of colorectal neoplasms and represents, as proposed by Jass et al. [1], an alternative molecular pathway to colorectal cancer. The sequence begins in a hyperplastic polyp (or precursor aberrant crypt focus(ACF) and progresses via an atypical hyperplastic polyp variant to a serrated adenoma and ultimately to carcinoma. The predominant carcinomas of this pathway, accounting for up to 10 – 15% of all colorectal carcinomas(CRC), are adenocarcinomas that show defective DNA mismatch repair(MMR) resulting in microsatellite instability(MSI-High), but they also include some proportion of carcinomas that are microsatellite stable(MSS) or MSI-Low(MSI-L).

Serrated Carcinoma Characteristics
Most if not all sporadic MSI carcinomas have a serrated histogenesis. [2] They tend to occur in the proximal colon, have an epidemiological association with cigarette smoking but are, nonetheless, typically encountered in elderly females. [3] MSI carcinomas of the serrated pathway show a polyclonal or polymorphous histological pattern [4] that includes well differentiated, often cystic and papillary glands, areas of mucinous differentiation and poorly differentiated, insular, trabecular or undifferentiated (medullary) architectural patterns. A lymphocytic response is apt to be prominent but less so than in HNPCC. [4] Nuclei of these tumors tend to be round and vesicular with nucleoli contrasting with the irregularity and hyperchromaticity of those of conventional CRC. Significantly, when a residual adenoma is present it is likely to be a serrated adenoma differing also in that respect from CRC in HNPCC which develops in a traditional adenoma.

Molecular Genetic Profiles
Adenomas (or aberrant crypt foci) of the traditional adenoma-carcinoma sequence are instigated by mutations of the APC or b-catenin gene that lead to disruption of the Wnt signaling pathway; the end-point carcinomas are characterized by chromosomal instability as distinct from microsatellite instability. [5] Inactivation of suppressor genes that govern the progress to malignancy in these neoplasms is due, primarily, to chromosomal deletions (loss of heterozygosity). [6] The initiating or significant early event of the serrated polyp neoplasia pathway, by contrast, appears to be activation of the RAS-RAF-MAP-Kinase signaling pathway, most frequently by mutations of the BRAF oncogene. [7] CpG-island methylation of promoter regions of key suppressor and mutator genes rather than chromosomal deletion is the main engine of progression in the serrated pathway. [8] In the case of BRAF(V600E) mutated precursors, in particular, it can ultimately silence both alleles of the hMLH1 gene resulting in MSI. [9] An activating mutation of KRAS2 appears to play an occasional role in both pathways, [2] but is more frequently associated with the traditional adenoma-carcinoma sequence where such mutations often appear in the intermediate stages of adenoma progression. [6]

Serrated Adenoma
The penultimate stage in the progression to carcinoma in this pathway is likely to be a serrated adenoma, [7] although some have proposed that the atypical hyperplastic polyp variant serrated polyp with abnormal proliferation (syn.s: sessile serrated adenoma; sessile serrated polyp) may proceed directly to carcinoma. [10, 11] Serrated adenomas, which represent less than 1% of all adenomas, were first categorized as a distinct histological entity by Longacre and Fenoglio-Preiser in 1990. [12] They have a serrated crypt architecture resembling that of a hyperplastic polyp but exhibit dysplasia throughout, as in traditional adenomas. On one end of the morphologic spectrum, these lesions may be difficult to distinguish from the HP variant SPAP (syn: SSA; SSP), while on the opposite end of the spectrum SAs may resemble traditional adenomas. Some have emphasized the presence of a predominant eosinophilic columnar cell phenotype in addition to uniformly distributed dysplasia to distinguish SA from SPAP. [11] Varying proportions of SAs may show a phenotype that closely resembles traditional adenoma or show a predominant goblet cell rather than an eosinophilic columnar cell phenotype. A traditional adenoma-like pattern seen contiguous to early MSI-H adenocarcinomas may represent a phenotypic variant of high grade dysplasia in SAs and SPAPs. Goldstein [10] has suggested that this resemblance to traditional adenomas is spurious because a careful examination of nuclear detail can reveal a basilar and cuboidal and finely granular appearance that differs from the pencillate pseudostratified nuclei with coarsely granular chromatin of traditional (APC mutated) adenomas.

Hyperplastic Polyps
Torlakovic and Snover, [13] in a study of a series of 6 patients with the hyperplastic polyposis syndrome, including 4 with associated adenocarcinoma, first drew attention to an HP variant that was prone to occur proximally in the colon and to show patterns of disordered growth very similar to that of SA, although it lacked definite adenomatous or dysplastic changes. These polyps tended to be sessile and the authors proposed the term sessile serrated adenoma(SSA) for this form of HP. Taking a different approach, Goldstein et al. [14] also drew attention to the prevalence of HPs of this type in the proximal colon in patients who later developed sporadic MSI cancer. Informed by their earlier study of HPs in hyperplastic polyposis, Torlakovic et al. [15] applied a statistical analysis method called cluster analysis, generally used for the analysis of microchip gene expression patterns, to a panel of 24 different morphologic parameters in 289 sporadic HPs. The analysis identified 3 major histologic sub-types of HPs, or serrated polyps: Goblet Cell Serrated Polyp (GCSP), Serrated Polyp with Microvesicular Mucin (MVSP) and Serrated Polyp with Abnormal Proliferation (SPAP; syns: SSA;SPP). Briefly, GCSPs are characterized by surface papillation and prominence of goblet cells; MVSPs are distinguished by the predominance of microvesicular cells over goblet cells and serration extending to mid crypt; SPAPs (syn.s: SSAs;SSPs) are identified by the abnormal architecture of the crypt bases with branching, dilatation, papillary infolding, irregular shapes and long axes parallel to the bowel lumen; other features include dystrophic goblet cells and mature cells with gastric foveolar phenotype in the crypt base (inverted crypts).

Since then, molecular studies have validated this classification by demonstrating associations between the 3 main subtypes and specific molecular genetic profiles. [16] Such studies also demonstrate that HPs are in fact neoplasms, in that they represent clonal proliferations of genetically altered epithelial cells of the colorectal mucosa. Furthermore, as Jass has suggested, their potential importance may be that they represent "sanctuary neoplasms," that is, lesions that permit the accumulation of genetic changes that in a normal crypt cell would activate apoptosis. [1] Although the vast majority of HPs probably succumb to apoptosis, this protection may in some instances result in the cumulative acquisition of mutations or epigenetic alterations of the genome that can result in further neoplastic proliferation and ultimately invasive adenocarcinoma. Some of the genetic factors that govern such outcomes may yield to studies of an autosomal dom inantly inherited predisposition to serrated neoplasia, distinct from hyperplastic polyposis, recently described by Young et al. [17] The MVSP variant in particular, which is the prototypical HP, is appropriately labeled an early neoplasm in that it appears to have the potential, particularly if located proximally, to transition to a more advanced lesion, namely SPAP(syn: SSA;SSP) and SA. The GCSP variant on the other hand, representing the common HP of the distal rectosigmoid appears to be self limited and has not been shown to have a capacity to progress further. [2]

Separate Pathways to Distinct Colorectal Carcinoma Endpoints
The presentation will include data from a recently completed study of BRAF and KRAS, CIMP and MSI status in multiple cases of each of the histological categories, including end-point carcinomas with residual adenoma, of the serrated polyp neoplasia pathway and the traditional adenoma-carcinoma sequence. [2] The findings of the study indicate that a BRAF(V600E) mutation is a specific marker for a serrated polyp pathway that has its origin in a hyperplastic polyp (MVSP) and a potential endpoint as MSI carcinoma. CIMP-H develops early in this sequence and MSI-H develops late. The data provided a less complete picture of a second serrated pathway, identified by a KRAS2 mutation in serrated adenomas, but show that the progressive stages of both iterations of the serrated neoplasia pathway are separate and distinct from those of the traditional adenoma-carcinoma sequence.

Future Directions
The serrated polyp neoplasia pathway is an important alternative model of carcinogenesis in the colon that invites numerous basic and clinical research questions. Among the former are the specific molecular pathways that favor HP persistence over apoptosis, that account for the key transitions in the serrated pathway such from MVSP to SPAP or from SPAP to SA, that clarify the role of KRAS or the nature of the specificity of BRAF(V600E) for MSI endpoints. This model will also provide a framework for studies of the phenomenon of CPG island methylation and the role of genetic predisposition. Clinical studies are needed that can elucidate a time-frame of the progress of serrated neoplasia and provide evidence-based guidance for developing recommendations for risk assessment and surveillance of individuals with the precursor lesions of the serrated pathway.

Selected References
  1. Jass JR, Whitehall VL, Young J et al. Emerging concepts in colorectal neoplasia. Gastroenterology 2002;123:862-76.

  2. O'Brien MJ, Yang S, Mack C et al. Comparison of Microsatellite Instability, CpG island methylation phenotype, BRAF and KRAS status in serrated polyps indicates separate pathways to distinct colorectal endpoints. Am J Surg Pathol (In press)

  3. Huang CS, O'Brien MJ, Yang S et al. Hyperplastic polyps, serrated adenomas, and the serrated polyp neoplasia pathway. Am J Gastroenterol 2004;99:2242-55.

  4. Young J, Simms LA, Biden KG et al. Features of colorectal cancers with high-level microsatellite instability occurring in familial and sporadic settings: parallel pathways of tumorigenesis. Am J Pathol 2001;159:2107-16.

  5. Tighe A, Johnson VL, Taylor SS. Truncating APC mutations have dominant effects on proliferation, spindle checkpoint control, survival and chromosome stability. J Cell Sci 2004;117:6339-53.

  6. Vogelstein B, Fearon ER, Hamilton SR et al. Genetic alterations during colorectal-tumor development. N Engl J Med 1988;319:525-32.

  7. Yang S, Farraye FA, Mack C et al. BRAF and KRAS Mutations in hyperplastic polyps and serrated adenomas of the colorectum: relationship to histology and CpG island methylation status. Am J Surg Pathol 2004;28:1452-9.

  8. Issa JP, Shen L, Toyota M. CIMP, at last. Gastroenterology 2005;129:1121-4.

  9. Wang L, Cunningham JM, Winters JL et al. BRAF mutations in colon cancer are not likely attributable to defective DNA mismatch repair. Cancer Res 2003;63:5209-12.

  10. Goldstein N. Small Colonic Microsatellite Unstable Adenocarcinomas and High-Grade Epithelial Dysplasias in Sessile Serrated Adenoma Polypectomy Specimens. A Study of Eight Cases. Am J Clin Pathol 2006; 125[1], 132-45.

  11. Snover D, Jass JR, Fenoglio-Preiser, Batts KP. Serrated Polyps of the Large Intestine. A Morphological and Molecular Review of an Evolving Concept. Am J Clin Pathol 2005;124:380-391.

  12. Longacre TA, Fenoglio-Preiser CM. Mixed hyperplastic adenomatous polyps/serrated adenomas. A distinct form of colorectal neoplasia. Am J Surg Pathol 1990;14:524-37.

  13. Torlakovic E, Snover DC. Serrated adenomatous polyposis in humans. Gastroenterology 1996;110:748-55.

  14. Goldstein NS, Bhanot P, Odish E, Hunter S. Hyperplastic-like colon polyps that preceded microsatellite-unstable adenocarcinomas. Am J Clin Pathol 2003;119:778-96.

  15. Torlakovic E, Skovlund E, Snover DC et al. Morphologic reappraisal of serrated colorectal polyps. Am J Surg Pathol 2003;27:65-81.

  16. O'Brien MJ, Yang S, Clebanoff JL et al. Hyperplastic (serrated) polyps of the colorectum: relationship of CpG island methylator phenotype and K-ras mutation to location and histologic subtype. Am J Surg Pathol 2004;28:423-34.

  17. Young J, Jass J. The case for a genetic predisposition to serrated neoplasia in the colorectum: Hypothesis and review of the literature. Cancer Epidemiol Biomarkers Prev 2006;(in press).