Contribution of New Adenomatous Polyposis Predisposition Genes in an Unexplained Attenuated Spanish Cohort by Multigene Panel Testing

Abstract

Attenuated adenomatous polyposis (AAP) is a heterogeneous syndrome in terms of clinical manifestations, heritability and etiology of the disease. Genetic heterogeneity and low penetrance alleles are probably the best explanation for this variability. Certainly, it is known that APC and MUTYH are high penetrance predisposition genes for adenomatous polyposis, but they only account for 5-10% of AAP. Other new predisposition genes, such as POLE, POLD1, NTHL1, AXIN2 or MSH3, have been recently described and have been associated with AAP, but their relative contribution is still not well defined. In order to evaluate the genetic predisposition to AAP in a hospital based population, germline DNAs from 158 AAP subjects were screened for genetic variants in the coding regions and intron-exon boundaries of seven associated genes through a next-generation sequencing (NGS) custom gene panel. Splicing, segregation studies, somatic mutational screening and RNA quantitative expression assays were conducted for selected variants. In four of the probands the adenoma susceptibility could be explained by actionable mutations in APC or MUTYH, and one other patient was a double carrier of two truncating variants in both POLE and NTHL1. Furthermore, 16 additional patients harbored uncertain significance variants in the remaining tested genes. This report gives information about the contribution of the newly described adenomatous polyposis predisposition genes in a Spanish attenuated polyposis cohort. Our results highly support the convenience of NGS multigene panels for attenuated polyposis genetic screening and reveals POLE frameshift variants as a plausible susceptibility mechanism for AAP.

Figure 1

Pedigrees of the explained polyposis cases. (a) MUTYH biallelic mutations carrier families. (b) APC mutations carrier families. ^†polyposis diagnosis after family mutation identification. Black square = cancer; black circle = >10 adenomas; ⁺⁼variant carrier; ⁻non carrier; ^+/+biallelic carrier; GC = gastric cancer; CRC = colorectal cancer; BC = breast cancer; y = years; A = adenomas.

Figure 2

Identification of POLE and NTHL1 double heterozygote. (a) POLE and NTHL1 protein domains and location of truncating variants detected in patient 83. (b) Pedigree chart; POLE+ = POLE variant carrier; POLE− = POLE wild type genotype; NTHL1+ = NTHL1variant carrier; NTHL1− = NTHL1wild type genotype; LC = lung cancer; A = adenoma. (c) POLE wild type expression in blood and tissue cDNA samples. (d) NTHL1 wild type allele expression in blood and tissue cDNA samples. COLON pool sample has been used as a reference sample in both NTHL1 and POLE assays. Significant ρ-values are indicated with *(ρ < 0.05) and **(ρ < 0.01). Each sample was analyzed in triplicates. AC = adenocarcinoma; AD = adenoma; CRC = colorectal cancer.