Distinct genomic and immune microenvironment between thymomas and thymic carcinomas.

e13529 Background: Thymomas and thymic carcinomas which uniformly known as thymic epithelial tumors (TETs) are rare intrathoracic malignancies and a limited studies have been reported addressing the molecular biology and immune discrepancy. The main purpose of this study was to depict the genomic and transcriptomic landscape of thymomas and thymic carcinomas, as well as elucidate the differentiated immune microenvironment. Methods: Totally 15 thymomas and 7 thymic carcinomas patients were enrolled from January 2014 to July 2018. Treatment-naïve tissue samples were collected, and we also obtained matched peripheral blood mononucleocytes as negative control. DNA and RNA were co-extracted and performed with whole exon and transcriptome sequencing. The immune cell infiltration scores were estimated using ssGSEA algorithm. Results: Exome sequencing revealed that GTF2I mutation occurred in all of type A thymomas but was absent in the aggressive subtypes. The median tumor mutation burden of thymomas was 0.12/Mb, significantly lower than thymic carcinomas (median: 1.02/Mb, p = 0.001). Copy number variation was more common in thymic carcinomas than thymomas (83.3% vs 9.1%, p = 0.005). Top mutational signatures enriched in both thymomas and thymic carcinomas included age and Aristolochic acid exposure, while the APOBEC signature was more common in thymomas than thymic carcinomas (81.8% vs 16.7%, p = 0.03). As a confirmed immune escape event, loss of heterozygosity of human leukocyte antigen was identified in 9.1% of thymomas and 50% of thymic carcinomas. Via unsupervised clustering of immune infiltration, all tissue samples were classified into high- and low-infiltration subgroups. Remarkably, up to 71.4% of samples from thymic carcinomas and only 6.7% of samples from thymomas were defined as low immune cell infiltration. In consideration of specific immune cell types, macrophage ( p = 0.01) and neutrophil ( p = 0.02) were enriched in thymic carcinomas while CD56+ NK cell ( p = 0.005) was enriched in thymomas, indicating the evidential discrepancy about immune cell infiltration between two subtypes of TETs. Conclusions: This study elucidated the molecular and immune microenvironment discrepancy between two subtypes of TETs. From molecular perspective, thymomas and thymic carcinomas are entirely different diseases with different etiology and characterized by distinct immune infiltration, and thus should be managed with disparate therapeutic strategies. Findings in this study may also be useful in future targets development and exploration of immunotherapies in TETs.