Tumor Fusion Burden as a hallmark of immune infiltration in prostate cancer

Prostate cancer is the second leading cause of cancer-associated death in men. Despite having a relatively lower tumor mutational burden than most tumor types, multiple gene fusions such as TMPRSS2:ERG have been characterized and linked to more aggressive disease. Individual tumor samples have been found to contain multiple fusions and it remains unknown whether these fusions increase tumor immunogenicity. Here we investigate the role of fusion burden on the prevalence and expression of key molecular and immune effectors in prostate cancer tissue specimens representing different stages of disease progression and androgen sensitivity including hormone sensitive and castration resistant prostate cancer. We found that tumor fusion burden was inversely correlated with tumor mutational burden and not associated with disease stage. High fusion burden correlated with high immune infiltration, PD-L1 expression on immune cells and immune signatures representing activation of T cells and M1 macrophages while inversely correlating with immune suppressive signatures. Our findings suggest that high tumor fusion burden may be a more appropriate biomarker than tumor mutational burden in prostate cancer as it more closely associates with immunogenicity and suggests that tumors with high fusion burden could be potential candidates for immunotherapeutic agents.