LIQUID‐BIOPSY BASED GENOTYPING OF PRIMARY CENTRAL NERVOUS SYSTEM LYMPHOMA (PCNSL) (2021)

Introduction: In Primary Central Nervous System Lymphoma (PCNSL) tissue samples are mainly obtained through stereotactic or open surgical biopsies, bearing significant risks. Furthermore, the genomic landscape of PCNSL is less well described than in diffuse large B-cell lymphoma (DLBCL). Consequently, sequencing of circulating tumor DNA (ctDNA) could be a helpful tool to minimally invasively genotype PCNSL at diagnosis and even more importantly, at relapse, when repeat biopsies are not routinely performed due to associated risks. Response assessment is currently carried out by magnetic resonance imaging (MRI) but lacks accuracy in distinguishing remaining scar tissue from active lymphoma. Thus, a high unmet medical need for improved prognostic assessment including genotyping and highly sensitive response monitoring aided by ctDNA sequencing exists. Methods: Cell-free DNA (cfDNA) was extracted from plasma samples of 17 patients suffering from previously untreated PCNSL between 8/2018 and 4/2020. CfDNA sequencing targeted a newly designed panel of ∼350 genes. Sequencing and bioinformatic analysis was performed as previously published (Sobesky et al., bioRxiv 2021). Results: We successfully extracted sufficient cfDNA in 17/17 (100%) of patients from 4 ml plasma. The median amount of cfDNA was 16,7 ng/ml plasma (range 1,4-119,9 ng/ml). Overall, 120 different genes displayed somatic variants (SNVs or InDels). Somatic variants were detectable in all patients with a median of 10 mutations per patient (range 1-43). Overall, we identified 29 significantly mutated genes by MutSig2CV analysis (Figure 1). Among the most recurrently mutated genes were KMT2D (7 patients), NFKBIA (4 patients) and EP300 (4 patients). The mutated allele frequency was generally low (median 0.95%; range 0.14%-16.00%). KMT2D and KMT2B mutations had high allele frequencies, suggesting they constitute events early in PCNSL oncogenesis. Surviving patients had significantly lower ctDNA burden measured by average mutated allele frequency than patients who died (1,61% vs. 4,13%; p = 0,011). At the meeting, a full analysis of our cohort including copy number variations and further genotype-phenotype links will be presented. Conclusions: Our results show that detection of ctDNA in peripheral blood of patients suffering from PCNSL is feasible. Furthermore, we were able to detect several recurrent genetic aberrations that were previously shown to be present in DLBCL as well as PCNSL in tissue genotyping. Despite the fact that the mutated allele frequency was rather low compared to previous reports from DLBCL, our pipeline allowed us to detect genetic aberrations in 100% of cases in our series. Liquid biopsy-based genotyping of PCNSL might offer tremendous advantages for diagnostic approaches, discovery of potential therapeutic targets and assessment of response or early detection of relapse. Somatic variants (SNVs or InDels) limited to significant genes (p < 0.05) following MutSig2CV analysis The research was funded by: Else-Kröner-Fresenius-Stiftung Keywords: Liquid biopsy, Extranodal non-Hodgkin lymphoma, Pathology and Classification of Lymphomas No conflicts of interest pertinent to the abstract.