Clonal expansion in the human upper gastrointestinal tract.

Abstract

The high incidence of gastrointestinal cancers in the general population and the presence of premalignant dysplastic precursor lesions in the gastrointestinal tract make the gastrointestinal tract an ideal environment to study cancer clonality and clonal expansion. Background: Intestinal metaplastic (IM) glands in the human stomach are clonal, contain multiple stem cells and spread by fission. This mechanism of gland fission causes field cancerisation. We hypothesised that gastric adenocarcinoma (GA) progresses through a series of genetic events arising from a founder mutation. A process analogous to niche succession may also take place in the normal oesophagus. We hypothesise that oesophageal squamous cell cancer occurs by a process of field cancerisation of the oesophagus. RHBDF2 has been identified as the gene responsible for tylosis with oesophageal carcinoma (TOC). We hypothesise that RHBDF2 germline gain of function mutations might be lost during tumour progression in TOC and this might affect iRhom2 localisation in the cell. Methods and results: A cohort of 23 patients with dysplasia and a cohort of 51 GA patients were screened for genes accounting for 75% of all somatic mutations previously reported in GA. Only 13% of dysplastic patients and 31.4% of GA patients had mutations. Three dysplastic patients and six GA patients were analysed by microdissection. Small gastric cancer foci in a cohort of hereditary diffuse gastric cancer (HDGC) patients (n=5) were also screened by laser-capture microdissection (LCM) for mutations in TP53. A cohort of 30 patients was screened for common mutations in OSCC and for RHBDF2 mutations. 36.36% of the patients presented mutations. Three patients with mutations were randomly selected and areas of oesophageal squamous cell dysplasia and OSCC were analysed by LCM. Three TOC patients were also analysed by LCM and immunohistochemistry was performed for iRhom2 and ADAM17. Conclusions: The usual mutational events established for GA development during the metaplasiadysplasia- carcinoma sequence (MCS) do not fit the results from either of our two LCM mutation studies in the human stomach. Dysplasia was shown to be clonal and GA demonstrates genetic heterogeneity through clonal evolution. Field cancerisation could not be detected in HDGC using TP53 as a clonal marker. The low incidence of OSCC patients with mutations implies that other genes may be involved in the premalignant pathway leading to OSCC. Oesophageal squamous cell dysplasia and OSCC demonstrate clonal expansion through tumour progression. RHBDF2 mutations do not occur in sporadic OSCC but germline RHBDF2 mutations can be lost during tumour progression in TOC patients with LOH in 17q. Overall, the somatic mutation theory of carcinogenesis seems to hold true for both the progression to GA and OSCC, as both carcinomas seem to evolve from a single mutated stem cell and acquire genetic heterogeneity as the tumours evolve.