Morphological, immunophenotypical and molecular features of hypermutation in colorectal carcinomas with mutations in DNA polymerase ε (POLE)

Colorectal carcinomas (CRC) with mismatch repair (MMR) deficiency have increased tumour mutation burden and respond to immune check-point inhibitor therapy. The Cancer Genome Atlas identified hypermutated CRCs with somatic mutations in DNA polymerase ε (POLE) with mutation burdens exceeding that of MMR-deficient CRCs.To identify the morphological, immunophenotypical and molecular features of POLE-mutated CRCs, 63 consecutive MMR-intact CRCs were evaluated by Sanger sequencing for POLE exonuclease domain mutations in exons 9, 11, 13 and 14 and confirmed by next-generation sequencing. Tumour immune microenvironment and IMMUNOSCORE®1 were assessed in POLE-mutated CRCs using immunohistochemistry to detect CD3+ /CD8+ tumour-infiltrating lymphocytes and compared to 59 non-POLE mutated MMR-intact CRC, 10 non-POLE mutated MMR-deficient CRCs and 223 normal colonic mucosa.A total of 4.8% CRC (four MMR-intact primary and one MMR-intact metastasis) harboured POLE mutations in amino acid 286 in exon 9 (p.P286R) or exon 13 (p.V411L). POLE-mutated CRCs arose in the transverse colon and rectum, were male-predominant, younger and showed increased tumour-infiltrating lymphocytes and immune cells at the tumour-stromal interface. The patient with metastatic POLE-mutated CRC was placed on PD-1 inhibitor treatment with marked and sustained response. These data indicate that POLE-mutated CRCs have hypermutated phenotypes despite MMR-intact status, with mutation burdens higher than that in microsatellite-unstable CRCs. Given the recent approval for treatment of microsatellite-unstable cancer with immune check-point inhibitors, assessment of POLE status may help to guide therapeutic decisions for hypermutated tumours with intact MMR that would otherwise be missed by routine testing.