Abstract
Brain tumours comprising of high-grade tumours that arise in the brain (glioblastoma) or those that metastasise to the brain have a poor patient prognosis. Large research efforts try to identify glioblastoma subgroups that are relevant to patient prognosis, survival and treatment response. Activation of a telomere maintenance mechanism is a hallmark of cancer and the type of maintenance mechanism can be used to divide glioblastoma into three different subgroups: telomerase, alternative lengthening of telomeres (ALT), and a group where the telomere maintenance mechanism is non-determined (NDTMM).
The current study aimed to better understand the composition of the NDTMM subgroup using RNA-Seq data. Currently the methods used to identify the telomere maintenance based subgroups are not applicable to a diagnostic setting. The second aim was to identify markers that could distinguish different subgroups on paraffin embedded tissue sections.
Markers of interest selected from RNA-Seq data were validated in up to 136 glioblastomas using immunohistochemistry. RNA-Seq data revealed an increased tumour associated macrophage (TAM) immune phenotype in NDTMM compared to telomerase and ALT tumours. An increased content of TAMs was confirmed using immunohistochemistry for the macrophage/activated microglia marker CD163 (79% in NDTMM, 2% in telomerase and 14% in ALT. P < 0.0001). From these results and the results from the validation of potential telomere maintenance mechanism markers three subgroups emerged from the NDTMM subgroup. Telomerase and ALT tumours with a high content of macrophages were called non-ALTM and ALTM and a true NDTMM group called MGMT. The new subgroups along with the original ALT and telomerase groups showed significantly different responses to treatment, extent of surgical resection, and patient survival times. The ALTM and non-ALTM subgroups had a worse patient survival compared to ALT and non-ALT tumours respectively, suggesting the increased macrophage content produces a more aggressive tumour (ALTM compared to ALT P = 0.0004, non-ALTM compared to non-ALT P = 0.013). Consistent with this, no individuals that survived greater than 24 months had ALTM or non-ALTM tumours.
The results from this study suggest an increased content of TAMS in glioblastoma interferes with identifying the telomere maintenance mechanism. This study is of clinical relevance as it supports the need for more targeted personalised treatment to treat subgroups that do not respond to current treatments.
A smaller aspect of this study investigated brain tumour metastases to try and identify molecular markers that were present in both brain tumour metastases and glioblastoma to try and confirm a hypothesis that tumours that metastasise to the brain adopt brain markers in order to survive in the brain. A high frequency of brain tumour metastases were positive for paired-box transcription factor 8, consistent with the hypothesis. Melanoma brain metastasis showed PAX8 positivity in 81.8% of samples. However, more samples are needed in the metastasis cohort in order to draw any definitive conclusions and so this research is ongoing.