Genetic and Immunological Characterization of Brain Metastases from Solid Cancers

Brain metastasis, a leading cause of cancer death, is a clinical challenge. Recently, genetic characterization of brain metastatic lesions based on next generation sequencing-based advanced technologies, such as single-cell RNA sequencing, has been performed to develop novel efficient therapies. The present study aimed to investigate brain-metastasis-specific biomarkers as well as relevant prognostic factors.

The genetic profiles and expression levels of immune response-associated genes and 820 cancer-associated genes were compared between primary cancer lesions and metastatic cancer lesions obtained from nine cancer patients at the Shizuoka Cancer Center. Cytokine and chemokine marker genes were analyzed via quantitative PCR. T-cell receptor (TCR) repertoire profiling was performed for the same patients. For survival analysis, survival data of 52 cancer patients with brain metastases were utilized.

Comparison of driver mutation profiling between primary and metastatic lesions revealed shared core mutations in both lesions and a few new mutations in metastatic lesions. A high tumor mutation burden (TMB) was detected in metastatic lesions. Volcano plot analysis revealed specific features of the metastatic tumor microenvironment, such as cancer signaling promotion and immune suppression due to decreased immune cell infiltration. Survival analysis revealed that three genes, the TREML2 gene, the BTLA gene on activated microglia and the CERS2 gene on metastatic tumor, were potent prognostic factors.

High TMB in metastatic lesions indicates potential benefit from immune checkpoint inhibitor usage for brain metastasis and TREML2 and BTLA are factors associated with poor prognosis. Activated microglia may be novel targets for the treatment of brain metastasis.