RNA-seq Reveals Differences in Expressed Tumor Mutation Burden in Colorectal and Endometrial Cancer With and Without Defective DNA Mismatch Repair

Tumor mutation burden (TMB) is an evolving biomarker for predicting response to immune checkpoint inhibitors (ICI’s). The goal of this study was to evaluate the use of RNA-seq for determining TMB in patients with defective DNA MMR (dMMR) due to MMR mutations or MLH1 promoter hypermethylation (HM) and tumors without dMMR. We included tumors and paired normal tissue from 58 patients for TMB analysis using RNA-seq. Forty-six tumors were MSI-H (29 with a DNA MMR germline mutation and 17 with MLH1 promoter HM) and 12 were MSS. TMB was measured using the expressed somatic nucleotide variants (eTMB). We developed a method that leverages mutational signatures to remove FFPE derived artifact from total eTMB and thereby accurately measure the eTMB. There was a significant difference in the eTMB observed between MSI-H and MSS cases; MSI-H cases had a median of 27.3 mutations/Mb compared to 6.7 mutations/Mb for MSS cases (p=3.5e-9). Among tumors with dMMR the tumors with DNA MMR mutations had a significantly higher eTMB (p=0.037) than tumors with MLH1 promoter HM: a median of 28.1 mutations/Mb vs. a median of 17.5 mutations/Mb. Furthermore, through multivariate analysis we found that MSI status, tissue type (endometrial or colorectal) and patient ages are significantly associated with eTMB. These results demonstrate that RNA-seq analysis can be used to measure eTMB in FFPE tumor specimens. Further studies are needed to determine how eTMB as determined by RNA-seq compares to TMB as determined by DNA-based NGS assays