The genetics of juvenile polyposis syndrome

Germline mutations in the SMAD4 gene were described in juvenile polyposis patients just prior to the commencement of this project. Assessment of the true contribution of SMAD4 mutations in the large JPS cohort available was undertaken using a variety of techniques, with immunohistochemistry found to be a reliable indicator of the presence of a germline SMAD4 mutation. Morphological analysis of juvenile polyps uncovered subtle differences that may aid the segregation of juvenile polyps according to their SMAD4 mutation status. The contribution of SMAD4 inactivation to colorectal cancer was investigated and found to be at a higher frequency (39% in microsatellite stable tumours) and to probably occur earlier in tumorigenesis than otherwise reported, with mutations occurring after divergence of MSI+ and MSI- pathways, but before aneuploidy/polyploidy. Juvenile polyps were shown to be clonal lesions, by detection of loss of heterozygosity at the SMAD4 locus in polyps from individuals with a germline SMAD4 mutation. Furthermore, the 'landscaper' model for tumorigenesis was disproved in relation to JPS as the cells targeted for deletion were shown to be the epithelial cells rather than the suspected stromal cells. Several candidate genes, mainly those belonging to the same gene family as SMAD4, were screened for germline mutations in JPS patients without SMAD4 mutations. No pathogenic changes were identified. A subsequent genome wide linkage and comparative genomic hybridisation search did not reveal any area of the genome with convincing evidence of a new JPS gene. The reason for this failure is almost certainly considerable remaining genetic heterogeneity in JPS. This was evidenced by the finding of germline mutations in BMPR1A/ALK3 in 32% of JPS patients. Another genome screen was performed for the related tumour-development disorder hereditary mixed polyposis syndrome (HMPS) and linkage found to 15ql3-14. Identification of these genes is likely to rely on the ascertainment of large JPS families and/or candidate gene screening, combined with LOH analysis.