Abstract P2-08-08: Concurrent predictors of an immune responsive tumor microenvironment within tumor mutational burden-high breast cancer
Authors: Sammons Sarah , Elliott Andrew , Force Jeremy , Chumsri Saranya , Anders Carey , Tan Antoinette R , Magee Daniel , Zeng Jia , Korn W. Michael , Kahsraw Mustafa , Torres Evanthia T Roussos 
Published: March 2022

Abstract
Background: Data supporting high tumor mutational burden (TMB-H) as a lone biomarker for an immune responsive tumor microenvironment (TME) or of response to immune checkpoint inhibition (ICI) in metastatic breast cancer (MBC) is weak; yet the tumor agnostic approval in TMB-H advanced tumors makes ICI an option in the clinic. PDL-1 positivity (PDL-1+) is the only consistent biomarker for ICI benefit in metastatic triple-negative BC (mTNBC). We sought to further evaluate concurrent predictors of an immune responsive or non-responsive TME within TMB-H MBC. Methods: Tumor samples (N=5621) obtained from patients with MBC were analyzed by next-generation sequencing (NGS) of DNA (592-gene panel or whole exome sequencing) and RNA (whole transcriptome sequencing) at Caris Life Sciences (Phoenix, AZ). Friends of Cancer Research TMB Harmonization Project (Merino et al., 2020) recommendations, with the TMB-H threshold set to ≥ 10 muts/Mb, were used. PDL-1+ (≥1% immune cells stained) assessed by IHC (Ventana SP142). Deficient-mismatch repair (dMMR) and high microsatellite instability (MSI-H) were determined by IHC and NGS, respectively. Gene expression profiling assessed 21 immune-related signatures and TMEs using MCP-counter (Betch et al., 2016) and quanTIseq (Finotello et al., 2019). Immune cell types were classified as immune-responsive (CD8+ and CD4+ T cells, B cells, myeloid dendritic cells, M1 macrophages, NK cells, and neutrophils) or non-responsive (Tregs, M2 macrophages, and monocytes). P-values adjusted by the Benjamini-Hochberg procedure. Exploratory p-values reported for sub-analyses of TMB-H responsive vs nonresponsive TME and PDL1+ vs PDL1- subgroups. Results: TMB-H was identified in 461/5621 (8.2%) MBC samples. TMB-H tumors exhibit significant enrichment of dMMR/MSI-H (7 vs 0%, p&lt;0.0001) and IHC-PDL1+ (36 vs 28%, p&lt;0.05) compared to TMB-L. TMB-H was not associated with increased abundance or proportion of immune responsive cell types or immune response gene signatures (e.g. antigen presentation), yet positive trends were observed. To identify predictive biomarkers of immune responsive (hot) or non-responsive (cold) TME profiles within TMB-H MBC, we generated a composite TME score representing the overall proportion of hot vs cold immune cell types. APOBEC mutagenesis was common in TMB-H tumors (72%) and was more frequently associated with an immune cold TME (77 vs 66% in hot TME subgroup, p=0.068). Within TMB-H tumors, cold TMEs were enriched with mutations (muts) in PIK3CA (61 vs 47%), CHEK2 (5 vs 0%), CBFB (4 vs 0%), amplifications (amp) of CCND1 (13 vs 4%), FGF19 (16 vs 5%), FGF4 (11 vs 4%), and MMLT6 (6 vs 0%), and expression of IHC-AR (75 vs 58%) compared to hot TMEs (all p&lt;0.05). Hot TMEs had increased frequency of MSI-H (12 vs 5%) and KMT2D muts (12 vs 3%) compared to cold TMEs (all p&lt;0.05). Amongst TMB-H and TMB-L tumors, PDL1+ was significantly associated with increased proportion of immune responsive cell types (ie CD8+) compared to PDL1- tumors (0.14 vs -0.08 TME score, p&lt;0.0001). Comparison of TMB-H tumors by PDL1 status revealed enrichment of TP53 (67 vs 53%) and B2M (8 vs 0%) muts in PDL1+ tumors, while IHC-ER positivity (68 vs 54%) and CDH1 muts (32 vs 16%) were increased in PDL1- tumors (all p&lt;0.05). Conclusions: High TMB alone does not strongly correlate with immune infiltrate or immune response gene signatures in MBC. Within TMB-H MBC, concurrent mutations in MSI-H and KMT2D are associated with an immune responsive TME while mutations in PIK3CA, CHEK2, CBFB, amplifications of CCND1, FGF19, FGF4, and MMLT6, and expression of IHC-AR are associated with an immune non-responsive TME. Co-occurring biomarkers within TMB-H breast cancer warrant evaluation in prospective cohorts for response or resistance to ICI to help develop composite biomarkers in breast cancer.
Citation Format: Sarah Sammons, Andrew Elliott, Jeremy Force, Saranya Chumsri, Carey Anders, Antoinette R Tan, Daniel Magee, Jia Zeng, W. Michael Korn, Mustafa Kahsraw, Evanthia T Roussos Torres. Concurrent predictors of an immune responsive tumor microenvironment within tumor mutational burden-high breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-08-08.