B-Cell precursor acute lymphoblastic leukemia (B-ALL) is the most common malignancy in children. Recently, we and others described a new subtype of the disease, affecting 60% of children with Down Syndrome (DS) and about 10% of patients with sporadic ALL, in which chromosomal rearrangements result in over-expression of the cytokine receptor-like factor 2 (CRLF2) receptor1-4. This over-expression is often accompanied by mutations in additional proteins in the CRLF2 pathway, such as JAK2, a downstream effector in the pathway5-9, and IL7RA, the second subunit in the TSLP receptor10. Based on mutation analyses, aberrant CRLF2 expression was thought to play a causal role in the development of B-ALL. While some data obtained in mouse systems support this assertion2,3,6, no studies have been performed in human cells to ascertain whether or not CRLF2 contributes to B-ALL pathogenesis. Due to the prominent difference between mouse and human B lymphoid development, particularly in the TSLP/IL7 pathways, it is important to study the contribution of activation of the TSLP pathway to the development of B-ALL in human cells. In the research described here, I hypothesized that aberrant expression of CRLF2 in cooperation with secondary mutations in the TSLP pathways contributes to B-ALL initiation.
This hypothesis was tested primarily by utilizing cord-blood (CB) hematopoietic-progenitors transduced with a set of lentiviral vectors carrying CRLF2 alone or in combination with JAK2 or IL7RA mutations. Outcome of forced TSLP pathway activation was cell context specific.
Expression of CRLF2 in CB hematopoietic-progenitors from a ubiquitous promoter resulted in skewed differentiation towards the myeloid lineage while transcription of the same genes from a B-cell-specific promoter accelerated B-lymphoid differentiation in vitro, underscoring the importance of expressing the genes of interest in the right cellular context for B-ALL pathogenesis.
Transduced CB cells were transplanted in NOD/LtSz-scid IL2Rγnull (NSG) mice, which are known to support human B-Cell differentiation. Transplanted cells expressing CRLF2 with mutant IL7RA exhibited population expansion, enhanced B-cell differentiation, and a significant block in differentiation at the pro-pre B-cell stage, resembling the stage of differentiation of leukemic blast cells.