High cumulative risk of colorectal cancers and desmoid tumours and fibromatosis in South Asian APC mutation carriers

Management of familial adenomatous polyposis (FAP) is guided by the cumulative risk of colorectal cancer (CRC) and aggressive fibromatosis/desmoid (AF/D). The first non-Caucasian FAP cohort with cumulative risk estimates for CRC and AF/D shows distinct differences with the Caucasian and other Asian cohorts. The strong correlation between the adenomatous polyposis coli (APC) mutation location with the FAP phenotype and the geoethnic differences in APC mutation spectrum, genetic constitution, lifestyle and sporadic CRC rates, mandates the use of population-specific cumulative risk estimates for CRC and desmoid for counselling and risk management. On genotype-phenotype correlation in 83 individuals with classical FAP and a confirmed pathogenic/likely Pathogenic (P/LP) APC variant (n=76) or obligate carrier of the family variant (n=7), we observed a high cumulative CRC risk of 40% and 85% by 40 and 60 years, respectively. The observed 30% cumulative risk by 50 years for desmoids was higher than previous European and Asian cohorts and was significantly associated with prophylactic surgery (OR: 4.58, 95% CI 1.06 to 19.78) and APC mutation 3' of codon 1309 (OR: 13.07, 95% CI 3.58 to 47.56) and also 3' of codon 1444 (OR: 8.0, 95% CI 1.83 to 34.94). Global cooperation is required to establish FAP genotype-phenotype associations and population-specific risk estimates to guide genetic counselling and risk management.