PO-338 Recurrent glioblastoma: a complex scenario dominated by loss of MMR proteins

Introduction Glioblastoma (GBM) is the most common primary brain tumour in adults and the Stupp protocol represents the standard of care. However, the tumour invariably relapses suggesting marked intra-tumour genetic heterogeneity enabling rapid adaptation to therapy. In-depth characterisation of recurrent GBM (rGBM) might contribute to better understand mechanisms behind tumour progression and enable rGBM treatment with targeted drugs. Material and methods Matched GBM samples have been collected at diagnosis and recurrence from adult patients (n=57) treated with the Stupp protocol. Expression of mismatch repair (MMR) proteins (MLH1, PMS2, MSH2, MSH6) was evaluated by IHC, followed by exome sequencing of 3 pairs showing loss of MSH6 reactivity as well as of 3 MSH6 positive pairs. In addition, established genetic and epigenetic markers of GBM were investigated along with their correlation with loss of MMR proteins and patients’ survival. Results and discussions According to IHC results, 13 out of 52 rGBM samples (25%) lacked expression of MMR proteins. In particular, 11 among the 13 samples (85%) showed partial or total reduction of MSH6 expression. Conversely, almost all GBM samples at diagnosis (96.4%) stained positive for the 4 MMR markers. Consistent with IHC data, exome sequencing disclosed lack of variants in MMR genes in primary samples whereas rGBM samples lacking MSH6 expression were mutated in the abovementioned genes and shared a c.3438+1G>A* splicing variant in MSH6 with a potential loss of function effect. Moreover, MSH6 negative relapsed specimens were characterised by 30 to 100-fold more variants compared to the matched primary ones and lacked microsatellite instability. Notably, MMR deficiency was associated with significant telomere shortening. Conversely, the tumour pairs expressing MMR proteins showed an almost comparable number of mutations in primary versus relapsed samples and absence of variants in MMR genes both in the initial tumours and in their recurrent counterpart. Conclusion Our study shows that IHC staining is a valuable tool to identify a subset of rGBM patients with alterations in MMR genes linked to high mutational burden and, hence, potentially eligible for drugs targeting immune checkpoint inhibitors.