Germline variants affecting the exonuclease domains of POLE and POLD1 predispose to multiple colorectal adenomas and/or colorectal cancer (CRC). The aim of this study was to estimate the prevalence of previously described heterozygous germline variants POLE c.1270C4G, p.(Leu424Val) and POLD1 c.1433G4A, p.(Ser478Asn) in a Dutch series of unexplained familial, early onset CRC and polyposis index cases. We examined 1188 familial CRC and polyposis index patients for POLE p.(Leu424Val) and POLD1 p.(Ser478Asn) variants using competitive allele-specific PCR. In addition, protein expression of the POLE and DNA mismatch repair genes was studied by immunohistochemistry in tumours from POLE carriers. Somatic mutations were screened using semiconductor sequencing. We detected three index patients (0.25%) with a POLE p.(Leu424Val) variant. In one patient, the variant was found to be de-novo. Tumours from three patients from two families were microsatellite instable, and immunohistochemistry showed MSH6/MSH2 deficiency suggestive of Lynch syndrome. Somatic mutations but no germline MSH6 and MSH2 variants were subsequently found, and one tumour displayed a hypermutator phenotype. None of the 1188 patients carried the POLD1 p.(Ser478Asn) variant. POLE germline variant carriers are also associated with a microsatellite instable CRC. POLE DNA analysis now seems warranted in microsatellite instable CRC, especially in the absence of a causative DNA mismatch repair gene germline variant.
INTRODUCTIONFaithful DNA replication and the repair of errors are both essential for the maintenance of genomic stability and suppression of carcinogenesis. 1 Duplication of genomes with high accuracy is achieved through three mechanisms: the high selectivity of DNA polymerases; exonucleolytic proofreading; and post replication mismatch repair. 2 The DNA polymerases ε (POLε) and δ (POLδ) are required for the efficient genome replication in the eukaryotic replication fork. 3 Their major component proteins, encoded by POLE and POLD1, respectively, possess an intrinsic 3′-5′ proofreading domain that removes incorrectly inserted nucleotides during DNA synthesis. [4][5][6][7][8][9] Studies in the yeast have shown that mutations in the proofreading domains of POLε or POLδ increase spontaneous mutation rates. 8,9 In addition, somatic mutations in the proofreading domains of POLD1 and POLE have been identified in microsatellite instable (MSI) and hypermutated subgroups of colorectal cancers (CRCs). [10][11][12] Recently, Palles et al reported that heterozygous germline variants in the proofreading domain of the DNA polymerases POLE and POLD1 predispose, with a high penetrance, to multiple colorectal adenomas, early onset CRC (OMIM #114500) and endometrial cancer (OMIM #608089). These variants were found by whole-genome sequencing and linkage analysis in three large families with a dominant pattern of CRC and multiple adenomas. 13 Subsequent screening of 3805 CRC patients revealed that these variants are relatively rare: POLE