Identification of Germline Mutations in Genes Involved in Classic FAP in Patients from Northern Brazil

Abstract

Background: Colorectal cancer is a common cancer worldwide, with 5-10% of cases being hereditary. Familial adenomatous polyposis syndrome (FAP) is caused by germline mutations in the APC gene or rarely in the MUTYH gene.

Patients and methods: This work did not identify germline mutations in the MUTYH, NTHL1, POLD1 and POLE genes in 15 individuals belonging to five families with classic FAP, who had the mutation in the APC gene confirmed in a previous study. Our results support mutations in the APC gene as the main genetic contribution of classical FAP with severe phenotype. In the family that had the most aggressive form of the disease, we performed an array-based Comparative Genomic Hybridization analysis and identified the germinal loss of an allele of the NOTCH2 and BMPR2 genes in the mother (proband) and daughter. In order to validate the involvement of these genes in the other four families of this study, we analyzed the DNA copy number variation in the peripheral blood of the 15 participants.

Results: FAP is a syndrome with considerable genetic and phenotypic heterogeneity and this phenomenon may explain the presence of secondary genetic alterations, such as the allelic loss of NOTCH2 and BMPR2 genes, found only in one family in this study. The CNV analysis confirmed that only the two members of the FAP2 family (patient 02H and 02F) had a deletion of these two genes, as the aCGH methodology had found. The other study participants did not show allelic loss for these two genes.

Conclusion: Validation in a larger number of families could confirm the presence of these new genetic alterations in classic FAP and improve understanding of the different types of aggressiveness of the disease.

Keywords: CNV; Familial adenomatous polyposis; aCGH; carcinogenesis; germline mutations.

Figure 1. Pedigrees of families with FAP. (A): FAP1 - Family with eight affected members, presence of mutations in the APC gene, low expression of mRNAs and deletion of an allele of the BMPR2 and NOTCH2 genes; (B): FAP2 - family with four affected members and presence of APC mutations; (C): FAP3 - a family with five members affected with APC mutations and three members without onset of disease, but with presence of APC mutations. The numbers under the symbols represent age at diagnosis. Solid symbols represent affected limbs with confirmed diagnoses of adenomatous polyposis. Left upper arrows indicate the probands. In all affected limbs with mutations in the APC gene, mutations in the MUTYH, NTHL1, POLD1 and POLE genes were not detected (adapted from Moreira-Nunes et al., 2015) (11).