Spatial tumour heterogeneity and branched evolution in DIPG
From our deep sequencing and immunohistochemical studies of DIPG, we know that H3F3A and TP53 mutations are heterozygous and are present in all tumour cells. However, other genes are mutated with lower allelic frequency, suggesting that they may represent later genetic hits present in a clone of the original tumour and pointing to the possibility of tumour genetic heterogeneity. In addition to these indications of genetic heterogeneity, our histologic studies demonstrate morphologic and proteomic heterogeneity. Detailed histopathological studies by our group have revealed that DIPGs represent a diverse histologic spectrum, including multiple cases in which different tumour regions are observed to contain a histological grade unique to that of the primary site. Further, we have shown that approximately one third of DIPG patients have leptomeningeal spread of their tumour and that diffuse invasion of the brainstem, spinal cord and thalamus is common, with some cases showing distant spread as far as the frontal lobes. Although DIPGs are histologically heterogeneous, the degree of genetic heterogeneity between the main mass in the pons and the infiltrative and metastatic compartments is not well studied. We are interested in understanding how DIPGs evolve over time, both genetically as well as epigenetically and transcriptionally. This will be critical for clinical decision making and design of future therapeutics.