Role of Lama4 Expression in Bone Marrow Niche for the Progression and Treatment Response of Chronic Myeloid Leukemia

Although tyrosine kinase inhibitors (TKIs) have dramatically improved the treatment outcomes of chronic myeloid leukemia (CML), TKIs cannot eliminate CML-initiating stem cells (LSCs), leading to CML persistence and relapse post TKI-discontinuation. Thus, it is imperative to develop more effective therapeutic options to overcome the treatment challenges. Targeting the bone marrow (BM) niche has become one of the research focuses in drug discovery since increasing evidence suggests that BM niches protect CML LSCs. Laminin alpha4 chain (Lama4), a functional chain for several laminin isoforms, is widely expressed in bone marrow (BM). We have recently shown the critical impact of LAMA4 expression in the microenvironment for hematopoiesis regeneration including megakaryocyte maturation and acute myeloid leukemia progression in mice (Cai et al., Blood, 2022). Furthermore, we have by RNA sequencing shown downregulation of LAMA4 mRNA in freshly isolated BM mesenchymal stem cells (MSCs) from newly diagnosed patients with CML (Dolinska, Cai, Mansson, et al., Blood, 2023). However, the functional impact of the LAMA4 expression on CML LSCs remains to be investigated.

We have here explored the role of LAMA4 in CML development and drug response by using both mouse models and primary patient BM samples. To first assess the in vivo impact of LAMA4 expression in BM niches on CML development, we transplanted BM cells (CD45.2) from Scl-tTA×TRE-BCR-ABL1 (BCR-ABL1) mice prior or following tetracycline withdrawal into irradiated Lama4^+/+ and Lama4^-/- CD45.1 mice and analyzed CML progression and BCR-ABL1 LSC activity by flow cytometry and droplet digital PCR (ddPCR). Following transplantation of unfractionated BM cells from the BCR-ABL1 mice with CML induced by tetracycline withdrawal, Lama4^-/- recipients showed significantly increased platelet counts in blood compared to Lama4^+/+ recipients at 2-8 weeks post-transplantation (P=0.005 at week 2, P= 0.008, n=7-8, at week 8), which is in contrast to the impaired platelet recovery in non-transplanted Lama4^-/- mice following sublethal irradiation. This suggests that Lama4 deficient microenvironment is favorable for platelet production from BCR-ABL1 CML cells. Consistent with this, megakaryocyte-erythrocyte progenitors (MEPs) derived from BCR-ABL1⁺ cells were significantly increased in Lama4^-/- BM at the endpoint (8 weeks, p=0.02, n=3). Megakaryocyte progenitors (MkPs) seemed to be increased in Lama4^-/- recipient BM (P=0.05, n=3). Intriguingly, when LIN^-SCA1⁺KIT⁺ CD150⁺ hematopoietic stem cells from BCR-ABL1 mice without prior tetracycline withdrawal were transplanted, the donor-derived MEPs were significantly reduced in Lama4^-/- BM (P=0.02, n=4-6), which is accompanied with a trend of reduction in MkP but no changes in platelets. ddPCR analysis indicated that the ratio of BCR-ABL1⁺ cell fraction to total CD45.2⁺ cells (detected by FACS) seemed to be lower in Lama4^-/- recipients (33.6%) than that in Lama4^+/+ recipients (41.5%), raising a possibility that Lama4^-/- niches is less suitable for maintenance of BCR-ABL1⁺ cells. Together, these data point to a possible differential regulation of CML stem and progenitor cells by the Lama4 expression. More work will be done to dissect the regulatory mechanisms through in vitro co-cultures, transplantation and confocal imaging. In parallel, the functional impact of LAMA4 on human CML LSC survival was evaluated by a revised long-term culture-initiating cell assay using CML patient derived MSCs. The preliminary result indicated that recombinant LAMA4 peptides appeared to inhibit CML patient BM-derived CD34⁺CD38^- cell growth.

In conclusion, LAMA4 is downregulated in BM MSCs and endothelial cells of CML patients at diagnosis. Lama4^-/- microenvironment might act differentially in regulating BCR-ABL1⁺ hematopoietic stem and progenitor cell activity post-transplantation. However, more work is required to validate the finding and the effect of LAMA4 peptides on the megakaryocyte differentiation and maturation.