Summary

Background: Tight regulatory loops orchestrate commitment to B cell fate within bone marrow. Genetic lesions in this gene regulatory network underlie the emergence of the most common childhood cancer, acute lymphoblastic leukemia (ALL). The initial genetic hits, including the common translocation that fuses ETV6 and RUNX1 genes, lead to arrested cell differentiation. Here, we aimed to characterize transcription factor activities along the B-lineage differentiation trajectory as a reference to characterize the aberrant cell states present in leukemic bone marrow, and to identify those transcription factors that maintain cancer-specific cell states for more precise therapeutic intervention. Methods: We compared normal B-lineage differentiation and in vivo leukemic cell states using single cell RNA-sequencing (scRNA-seq) and several complementary genomics profiles. Based on statistical tools for scRNA-seq, we benchmarked a workflow to resolve transcription factor activities and gene expression distribution changes in healthy bone marrow lymphoid cell states. We compared these to ALL bone marrow at diagnosis and in vivo during chemotherapy, focusing on leukemias carrying the ETV6-RUNX1 fusion. Results: We show that lymphoid cell transcription factor activities uncovered from bone marrow scRNA-seq have high correspondence with independent ATAC- and ChIP-seq data. Using this comprehensive reference for regulatory factors coordinating B-lineage differentiation, our analysis of ETV6-RUNX1-positive ALL cases revealed elevated activity of multiple ETS-transcription factors in leukemic cells states, including the leukemia genome-wide association study hit ELK3. The accompanying gene expression changes associated with natural killer cell inactivation and depletion in the leukemic immune microenvironment. Moreover, our results suggest that the abundance of G1 cell cycle state at diagnosis and lack of differentiation-associated regulatory network changes during induction chemotherapy represent features of chemoresistance. To target the leukemic regulatory program and thereby overcome treatment resistance, we show that inhibition of ETS-transcription factors reduced cell viability and resolved pathways contributing to this using scRNA-seq. Conclusions: Our data provide a detailed picture of the transcription factor activities characterizing both normal B-lineage differentiation and those acquired in leukemic bone marrow and provide a rational basis for new treatment strategies targeting the immune microenvironment and the active regulatory network in leukemia.

Overall design

Primary patient samples from diagnosis (N=6 BM samples, subtype ETV6-RUNX1, 4000-8000 cells per donor) with day 15 post-treatment sample collected from two cases. Cell line experiment in REH comparing two drugs targeting ETS-factors (TK216 ERG/FLI1 and XRP44X ELK3) at 72 h (N=1, 900-2500 cells per treatment).

Contributors

Juha Mehtonen 1, Susanna Teppo 2, Mari Lahnalampi 1, Aleksi Kokko 1, Riina Kaukonen 3, Laura Oksa 2, Maria Bouvy-Liivrand 1, Alena Malyukova 4, Artturi Mäkinen 2, Saara Laukkanen 2, Petri I Mäkinen 5, Samuli Rounioja 6, Pekka Ruusuvuori 2, Olle Sangfelt 4, Riikka Lund 3, Tapio Lönnberg 3, Olli Lohi 2 7, Merja Heinäniemi 8

Contact

merja.heinaniemi@uef.fi.(Merja Heinäniemi)