LAUSR

2634 Background: Immune checkpoint inhibitors (ICI) block proteins which enable cancer cells to evade the immune system. Recent studies have shown that the higher the tumor mutation burden (TMB) the greater the likelihood of response to ICI therapy. Analysis of TMB has focused on WES of paired tumor and normal samples. This study tests the feasibility of measuring TMB from a SNP-based resequencing assay (myChoice HRD Plus). Methods: WES and myChoice HRD Plus were performed on matched tumor and normal DNA from 44 breast and 12 colon tumors. myChoice HRD Plus combines homologous recombination deficiency analysis with resequencing of 108 genes and microsatellite instability analysis. WES-based TMB was calculated by identifying all variants in paired samples, and subtracting germline variants. Two SNP-based TMB (SbTMB) methods were utilized to calculate TMB. The first used germline subtraction similar to the WES-based method. The second utilized an algorithm which removed background germline variants. Median sequence length to calculate TMB was 9.7 Mb for WES, 4.6 Mb for SbTMB (germline subtraction), and 1.9 Mb for SbTMB (algorithm). Results: Correlation coefficients for WES vs. SbTMB (germline subtraction) and SbTMB (algorithm) were 0.895 and 0.908, respectively. The two SbTMB methods had a correlation coefficient of 0.834. SbTMB measures of TMB were generally higher than WES-based TMB with a mean increase in score of 1.6 variants/Mb for SbTMB (germline subtraction; p = 4.6x10-6) and 1.5 variants/MB for SbTMB (algorithm; p = 1.2x10-5). No significant difference in magnitude of TMB score between the SbTMB measures was observed (0.04 variants/Mb; p = 0.88). Conclusions: SNP-based methods for calculating TMB produced highly concordant scores compared to WES-based methods. SbTMB assays produced elevated TMB scores, consistent with selective pressure against mutations in coding regions of genes, necessitating a higher score threshold for when using a SbTMB assay. This SbTMB analysis expands the utility of myChoice HRD Plus, and provides a method for calculation of TMB without sequencing a germline comparator.