Clonal expansion sets the stage for cancer genesis by allowing for the accumulation of molecular alterations. While genetic mutations that induce clonal expansion and malignancy, such as Tet2, have been identified, these mutations are also frequently found in healthy individuals. Here, we tracked preleukemic clonal expansion using genetic barcoding in an inducible Tet2 knockout mouse model and found that only a small fraction of hematopoietic stem cells (HSCs) expanded excessively upon Tet2 knockout. These overexpanded HSCs expressed significantly lower levels of genes associated with leukemia and RNA splicing compared to non-overexpanded Tet2 knockout HSCs. Knocking down Rbm25, an identified RNA splicing factor, accelerated the expansion of Tet2-knockout hematopoietic cells in vitro and in vivo. Our data suggest that mutations of an epigenetic factor Tet2 induces variability in the expression of an RNA splicing factor Rbm25, which subsequently drives heterogeneous preleukemic clonal expansion. This heterogeneous clonal expansion could contribute to the variable disease risks across individuals.

Cellular heterogeneity is a major cause of treatment resistance in cancer. Despite recent advances in single-cell genomic and transcriptomic sequencing, it remains difficult to relate measured molecular profiles to the cellular activities underlying cancer. Here, we present an integrated experimental system that connects single cell gene expression to heterogeneous cancer cell growth, metastasis, and treatment response. Our system integrates single cell transcriptome profiling with DNA barcode based clonal tracking in patient-derived xenograft models. We show that leukemia cells exhibiting unique gene expression signatures respond to different chemotherapies in distinct but consistent manners across multiple mice. In addition, we uncover an unexpected yet common form of leukemia expansion that is spatially confined to the bone marrow of single anatomical sites and driven by cells with distinct gene expression signatures. Our integrated system directly and effectively interrogates the molecular and cellular basis of the intratumoral heterogeneity underlying disease progression and treatment resistance.