Abstract S1-07: Immune sculpting of the triple negative breast cancer genome
Authors: Karn T , Jiang T , Hatzis C , Sänger N , El-Balat A , Holtrich U , Becker S , Bianchini G , Pusztai L 
Published: March 2017

Background: Tumors with infiltrating lymphocytes (TIL) demonstrate a better prognosis particularly in TNBC and HER2 positive breast cancer. Two competing hypothesis predict contrasting relationships of TILs and genomic heterogeneity. On one hand, a strong immune response may lead to “pruning” of intratumor heterogeneity by eliminating immunogenic clones resulting in a near equilibrium, hence better prognosis, while cancers that escape the surveillance may evolve towards greater clonal heterogeneity and genomic complexity. In some cancers, the predicted neoantigens are less frequent than expected by chance also suggesting immune mediated elimination of neoplastic clones (Rooney et al. 2015). Studies also showed an inverse association between immune cell infiltration and intratumor clonal heterogeneity (Morris et al. 2016). On the other hand, cancers with greater genomic instability and mutational burden will have larger clonal heterogeneity and therefore more neoantigens and greater immune infiltration. Indeed, a positive correlation between overall mutation load and immune activity in the tumor microenvironment was observed in pooled data across a broad range of cancer types (Brown et al. 2014, Rooney et al. 2015, Schumacher and Schreiber 2015).

Methods: We assessed these two competing hypothesis and examined the relationship between genomic complexity and immune gene expression in different breast cancer subtypes. We used previously described immune metagene expression (DNA microarray n=655) as measures of immune infiltration in the TCGA data set (RNA-Seq n=1215). We compared somatic mutations, mutation count, neoantigen load, clonal heterogeneity metrics and the distribution of mutations in 119 canonical cancer genes and 12 cancer pathways between good and poor prognosis TNBC (n=208) corresponding to high and low immune infiltration.

Results:A positive but weak correlation between mutation count and immune metagene expression was observed when all breast cancer subtypes were analyzed together (P=0.08). This was driven by the generally higher mutation count and immune infiltration in TNBC. When TNBC was analyzed separately, good prognosis TNBC with high immune infiltration had lower total mutation count (P=0.021) and predicted neo-antigen count (P=0.035). Clonal heterogeneity was also lower in good prognosis TNBC (P=0.001). There was a strong inverse relationship of dispersion in mutation variant allele frequencies and immune metagene expression. CASP8 was the top enriched mutation in TNBC with high immune infiltration (P=0.007 with no adjustment for multiple testing).

Conclusions:High immune infiltration is associated with reduced intratumor heterogeneity in TNBC suggesting immune sculpting of the tumor and a near equilibrium between the cancer and immune surveillance. Surgical resection of the primary tumor may tilt the balance towards the immune system resulting in the better prognosis of high-TIL TNBC. TNBC with low immune infiltration has greater clonal heterogeneity and mutation load and may represent the consequence of escape from immune surveillance. Mutation of CASP8 may be one way to evade tumor cell killing in high-TIL TNBC as previously noted.

Citation Format: Karn T, Jiang T, Hatzis C, Sänger N, El-Balat A, Holtrich U, Becker S, Bianchini G, Pusztai L. Immune sculpting of the triple negative breast cancer genome [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr S1-07.