Early T stage colorectal cancers (CRC) that invade lymph nodes (Stage IIIA) are rare and greatly under-represented in large-scale genomic mapping projects. We retrieved 10 Stage IIIA CRC cases, matched these to 16 Stage 1 CRC cases (T1 depth without lymph node metastasis) and carried out deep sequencing of 409 genes using the IonTorrent system. Tumour mutational burdens (TMB) ranged from 2.4 to 77.2/Mb sequenced. No stage-related mutational differences were observed, consistent with reanalysis of The Cancer Genome Atlas (TCGA) Stage I and IIIA datasets. We next examined mutational burdens and observed that the top five cancers were microsatellite stable (MSS) genotypes (mean TMB 49.3/Mb), while the other 16 MSS cancers had a mean TMB of 5.9/Mb. To facilitate comparison with TCGA hypermutator CRC, we included four microsatellite instability-high (MSI-H) samples with the high mutation burden MSS cases to form a TMB-High group. Comparison of TMB-High with TMB-Low groups revealed differences in mutational frequency of ATM, ALK, NSD1, UBR5, BCL9, CARD11, KDM5C, MN1, PTPRT and PIK3CA, with ATM and UBR5 validated in reanalysis of TCGA hypermutator Stages I and IIIA samples. Variants in ATM were restricted to the TMB-High group, and in four of five MSS specimens, we observed the co-occurrence of mutations in homologous recombination repair (HRR) genes in either two of ATM, CDK12, PTEN or ATR, with at least one of these being a likely pathogenic truncating mutation. No MSI-H specimens carried nonsense mutations in HRR genes. These findings add to our knowledge of early T stage CRC and highlight a potential therapeutic vulnerability in the HRR pathway of TMB-H MSS CRC.
Keywords: ATM; colorectal cancer; homologous recombination repair; tumour mutation burden.