Connective Tissue Growth Factor Induces Anti-Apoptotic Factors in Chronic Myeloid Leukemia Stem Cells

Chronic Myeloid Leukemia (CML) is a progressive hematopoietic malignancy where expression of the oncogenic fusion protein BCR-ABL1 in leukemia stem cells (LSCs) prevents the proper differentiation of myeloid progenitor populations, leading to accumulation of undifferentiated blasts. Current treatments target BCR-ABL1 with tyrosine kinase inhibitors (TKIs). Though effective if administered continuously, TKIs generally fail to eradicate the bone marrow niche-residing LSCs responsible for patient relapse or progression of CML to its terminal stage, Blast Crisis (BC), as evidenced by the high molecular relapse rate following TKI discontinuation. Previous studies performed by ourselves and others show that BC progenitors (CD34⁺CD38⁺Lin^-) exhibit stem-like behaviors, such as quiescence, self-renewal, and induction of pro-survival gene expression through alternative splicing of BCL2 family members, and thus behave like LSCs. Notably, BC CML LSCs co-cultured on LSC (SL/M2) supportive stroma are resistant to TKIs compared to culturing the cells alone, indicating a role of the extracellular matrix (ECM) in promoting LSC survival. We performed RNA-seq and qRT-PCR of CD34⁺CD38⁺Lin^- progenitor cells in CML patient samples and found a significant increase in CTGF (Connective Tissue Growth Factor) expression in BC CML versus chronic phase (CP). Interestingly, CTGF is an ECM protein that enhances cell adhesion and has been shown to predict therapeutic resistance in cancers, such as acute lymphoblastic leukemia (ALL). Lentiviral overexpression of CTGF in a CML cell line K562 and CD34⁺ CP CML patient samples caused proliferation and a decrease in apoptosis markers (cleaved caspase-3), as measured by FACS analysis. Moreover, qRT-PCR analysis of mRNA indicated an increase in pro-survival BCL2 family gene expression. These changes were not observed in normal CD34⁺ cord blood cells. Currently, lentiviral CTGF transduction of CP CML followed by transplantation into RAG2^-/-gc^-/- and NSG-S mice will be used to determine the effects of CTGF on LSC maintenance in vivo. In conclusion, CTGF promotes CML LSC survival in vitro and thus could be a key factor in BC transformation and TKI resistance.