The genetics of inherited predispositions to colorectal polyps: a quick guide for clinicians

Sporadic gastric juvenile polyposis with a novel SMAD4 nonsense mutation in a mosaic pattern

A 50-year-old female was diagnosed with gastric hyperplastic polyps 7 years before and was followed up at another hospital. She was referred to our hospital because of the growth of gastric polyps and progression of anemia. She had no family history of polyposis. The polyps were observed only in the stomach, increased in size and number, and the erythematous edema got worse. Endoscopic mucosal resection (EMR) of the gastric polyp was performed. Pathologically, the gastric polyp was hamartomatous polyp, and the intervening mucosa between polyps showed no atypical structure without inflammation. Given that gastric juvenile polyposis (GJP) was clinically suspected, a genetic test using peripheral blood was performed. Target resequencing and Sanger sequencing analysis revealed a nonsense mutation in the SMAD4 gene at codon 169. The mutation was detected at a low frequency of 11%, and considered a mosaic mutation. Therefore, she was diagnosed with a sporadic GJP, and total gastrectomy was performed. Immunostaining of SMAD4 for the resected specimen showed a mixture of stained and unstained area in the epithelium of the polyp, indicating partial loss of SMAD4 expression. To our knowledge, this is the first reported case of GJP with a nonsense SMAD4 mutation at codon 169 in a mosaic pattern.

Synonymous mutation adenomatous polyposis coliΔ486s affects exon splicing and may predispose patients to adenomatous polyposis coli/mutY DNA glycosylase mutation‑negative familial adenomatous polyposis

Familial adenomatous polyposis (FAP) is an autosomal dominant-inherited colorectal cancer. Recent advances in genetics have indicated that the majority of patients with FAP carry germline mutations of the adenomatous polyposis coli (APC) and mutY DNA glycosylase (MUTYH) genes. However, a large subset of families with a history of FAP have undetectable pathogenic alterations, termed APC/MUTYH mutation-negative FAP. To investigate the germline mutations in the APC and MUTYH genes in Chinese patients with FAP, 13 unrelated patients were enrolled. Through genetic sequencing, four known pathogenic alterations (Lys1061LysfsTer2, Glu1309AspfsTer4, Arg283Ter and Ser1196Ter) of APC and two novel disease-associated pathogenic mutations (Tyr152Ter and Ter522Gly) in MUTYH were identified in six individuals. For samples that did not present with pathogenic alterations, the functional effects of missense, synonymous and intronic mutations were analyzed using bioinformatics tools and databases. Bioinformatics prediction suggested that the synonymous mutation Tyr486Tyr in APC (APC^∆486s) was likely a disease-causing polymorphism and may have induced the exon skipping of APC. A hybrid mini-gene assay was performed, which confirmed that the synonymous single nucleotide polymorphism APC^∆486s induced major splicing defects with skipping of exon 12 in APC. The data of the present study suggested that the synonymous polymorphism APC^∆486s was a potential pathogenic alteration that predisposed APC/MUTYH mutation-negative patients to FAP.

Polyp Genetics

The genetic events involved in the transformation of normal colonic epithelium to neoplastic polyps to invasive carcinoma, as initially proposed by Fearon and Vogelstein, form the foundation of our understanding of colorectal cancer. The identification of the polyp as the precursor lesion to colorectal cancer is the basis of many of our current practices for screening, surveillance, and prevention. The last three decades have seen a veritable explosion in our understanding of the molecular events involved in the pathogenesis of colorectal cancer. It is now clear that there are multiple genetic pathways in the polyp to carcinoma sequence. Some polyps previously thought to be nonneoplastic have now been shown to have malignant potential. Finally, increased understanding of the sequence of genetic events has led to the development of targeted therapeutics. The clinical translation of these scientific advances has made a significant impact on the management of patients with colorectal cancer. Accordingly, it is imperative that all clinicians caring for these patients have an understanding of the genetics of colorectal polyps and cancer. In this article, we review the etiology and genetic pathways to carcinoma associated with a range of polyps of the colon and rectum.

Familial Colorectal Cancer: Understanding the Alphabet Soup

While most colorectal cancers (CRCs) originate from nonhereditary spontaneous mutations, one-third of cases are familial or hereditary. Hereditary CRCs, which account for < 5% of all CRCs, have identifiable germline mutations and phenotypes, such as Lynch syndrome and familial adenomatous polyposis (FAP). Familial CRCs, which account for up to 30% of CRCs, have no identifiable germline mutation or specific pattern of inheritance, but higher-than-expected incidence within a family. Since the discovery that certain genotypes can lead to development of CRC, thousands of mutations have now been implicated in CRC. These new findings have enhanced our ability to identify at-risk patients, initiate better surveillance, and take preventative measures. Given the large number of genes now associated with hereditary and familial CRCs, clinicians should be familiar with the alphabet soup of genes to provide the highest quality of care for patients and families.

Magnifying chromoendoscopic and endocytoscopic findings of juvenile polyps in the colon and rectum

A precise endoscopic diagnosis is necessary for endoscopic therapy for neoplastic and non-neoplastic lesions, including juvenile polyps (JPs). Therefore, the present study aimed to clarify the characteristic endoscopic findings of JPs. A total of 154 JPs were evaluated by magnifying chromoendoscopy, 20 of which were also assessed by endocytoscopy using an ultra-high magnification endoscope. Endoscopic images were evaluated in terms of gross appearance, color, pit pattern, surface inflammatory changes and vascularity of polyps. Endocytoscopic images were evaluated with regard to the morphology of glandular cavities, nuclei of glandular cells and interstitial features. Reddish surfaces (98.1%), surface erosion (92.2%), open pits (90.3%) and low pit density (90.3%) were observed in the majority of JPs by chromoendoscopy. In addition, dilated ductal openings surrounded by normal glandular cells (100%), greater distances between gland basal layers (100%) and interstitial infiltration by inflammatory cells (100%) were observed in all JPs examined by endocytoscopy. These findings indicate that there is a tetralogy of magnifying chromoendoscopic findings characteristic of JPs: Reddish surfaces, surface erosion, open pits and low pit density. There is also a triad of endocytoscopic findings characteristic of JPs, namely dilated ductal openings surrounded by normal glandular cells, greater distances between gland basal layers, and interstitial infiltration by inflammatory cells. The aforementioned magnifying chromoendoscopic and endocytoscopic characteristics of JPs may be useful factors for diagnosing JPs.