Abundant and equipotent founder cells establish and maintain acute lymphoblastic leukaemia

High frequencies of blasts in primary acute lymphoblastic leukaemia (ALL) samples have the potential to induce leukaemia and to engraft mice. However, it is unclear how individual ALL cells each contribute to drive leukaemic development in a bulk transplant and the extent to which these blasts vary functionally. We used cellular barcoding as a fate mapping tool to track primograft ALL blasts in vivo. Our results show that high numbers of ALL founder cells contribute at similar frequencies to leukaemic propagation over serial transplants, without any clear evidence of clonal succession. These founder cells also exhibit equal capacity to home and engraft to different organs, although stochastic processes may alter the composition in restrictive niches. Our findings enhance the stochastic stem cell model of ALL by demonstrating equal functional abilities of singular ALL blasts and show that successful treatment strategies must eradicate the entire leukaemic cell population. Overall design: Four different ALL primograft samples were transduced with a lentiviral vector barcode library with an estimated complexity in excess of 100,000 barcodes and transplanted intrafemorally into NSG mice. Samples were collected from spleen, bone marrow and meninges, certain samples were transplanted into secondary and tertiary recipients. Barcode complexity was assessed using Illumina MiSeq.