PF151 RAS/MAPK ACTIVATION COOPERATES WITH GAIN OF CHROMOSOME 21 IN B CELL LEUKEMIA AND IS AN ATTRACTIVE TARGET TO IMPROVE THE OUTCOME OF DS CHILDREN WITH B‐ALL
Authors: A.P. Laurent, A. Siret, C. Ignacimouttou, M. Diop, Y.‐C. Tsai, D. Roos‐Weil, D. Plassard, S. Lagarde, N. Prade, H. Aid, G. Meurice, O. Bernard, C. Barin Bonnigal, E. MacIntyre, L. Cheung, R. Kotecha, B. Bornhauser, J.‐P. Bourquin, P. Ballerini, E. Delabesse, T. Mercher, S. Malinge
Journal: HemaSphere (2019)

Background: B cell precursors acute lymphoblastic leukemia (B‐ALL) is the most commonly diagnosed childhood cancer. Major advances in treatments protocols, allowing for the 5‐year overall survival rate to reach 90%. However, many children still undergo intensive therapy and will develop relapse after a complete remission. Children with Down syndrome (DS) have a 27‐fold increased risk of developing B‐ALL and face a dismal prognosis due to treatment‐related mortality, and an increased risk of infections and relapse rate. Aims: The goal of the study was to better understand the molecular bases of DS‐ALL and develop specific therapies to improve outcomes. Methods: Here, we comprehensively characterized DS‐ALL samples, extended our study to other groups of childhood B‐ALL carrying gains of chromosome 21 (B‐ALL+21) and developed Patient Derived Xenograft (PDX) models suitable for preclinical studies. We performed exome and RNA sequencing to characterize the genetic and transcriptomic landscape of DS‐ALL (n = 8), iAMP21‐ALL (n = 7), HeH‐ALL (n = 14) compared to other pediatric B‐ALL (n = 12). Results: Somatic mutations leading to RAS/MAPK pathway activation (affecting the N/KRAS , JAK2 , FLT3 and CBL genes) were present in 75% (22/29) of B‐ALL +21 samples, confirming recent observations by other group, and strengthening the link between RAS/MAPK pathway activation and +21. To gain insights into the molecular bases of this cooperation, we ectopically expressed the constitutively active mutant KRAS G12D in murine bone marrow progenitors. We observed that KRAS G12D expression in trisomic progenitors led to an increased number of CFU‐preB colonies that presented increased self‐renewal capacities compared to wild‐type, trisomy 21 or KRAS G12D alone. RNA sequencing showed that 261 genes were synergistically deregulated by KRAS G12D +Trisomie 21 in murine B cell progenitors, including 24 genes that were transcriptionally deregulated in B‐ALL+21 with RAS/MAPK mutations patient samples as compared to healthy B‐cell progenitors. Together, tshese results indicated that constitutive RAS activation and +21 functionally cooperates at the cellular and molecular level to impose an altered differentiation of B‐cell progenitors. From this cohort, we also developed 18 preclinical PDX models (13 B‐ALL+21 and 5 Others) to test the efficacy of inhibitors of the RAS/MAPK pathway. We showed that the most potent compound, Trametinib (GSK1120212), significantly decreased viability of B‐ALL+21 blasts presenting a constitutive RAS/MAPK activation in vitro (IC50 = 0,1 ± 0,25 μM compared to 3,95 ± 4,7 μM for B‐ALL samples without RAS/MAPK activation). In vivo treatments with Trametinib significantly decreased leukemia burden in the bone marrow and spleen compared to vehicle controls, and increased survival of DS‐ALL, iAMP21‐ALL and HeH‐ALL PDX models. Finally, we assessed the efficacy of Trametinib in combination with conventional chemotherapy agents. We observed that Trametinib synergised with Vincristine in vitro , and delayed leukemia progression in vivo to enhance survival of DS‐ALL PDXs. Summary/Conclusion: Altogether, we built a comprehensive cohort of preclinical models of DS‐ALL and B‐ALL+21, and suggested that inhibiting RAS/MAPK pathway activation in combination with current chemotherapy may represent a promising strategy to improve the outcome of Down syndrome children with B‐ALL.