Lama4 Deficiency Leads to Delayed Onset of Chronic Myeloid Leukemia Possibly By Providing an Unfavorable Niche
Authors: Mansson Alma , Qian Hong 
Published: December 2024

Introduction
Although tyrosine kinase inhibitors (TKIs) have dramatically improved the survival rate of patients with chronic myeloid leukemia (CML), it has been challenging to achieve treatment-free mission in most of CML patients since TKIs often fail to eradicate CML-initiating stem cells (LSCs), leading to CML persistence or relapse post-TKI treatment discontinuation. Evidence suggests that CML LSC persistence is due to the protection of bone marrow (BM) niche. However, the molecular mechanisms remain unclear, limiting the opportunity to identify new therapeutic targets that could deprive the protective signals. We have recently shown that expression of Lama4, a functional chain for several laminin isoforms and exclusively expressed in non-hematopoietic cells, in BM niche impacts acute myeloid leukemia (AML) progression and chemoresistance in mice (Cai and Kondo et al., Blood, 2022). Furthermore, LAMA4 mRNA was downregulated in BM mesenchymal stem cells of patients with CML at diagnosis (Dolinska, Cai, Månsson et al., Blood, 2023). However, the functional impact of LAMA4 expression on CML progression remains to be investigated.
Method
To explore the impact of Lama4 on CML development, we have here utilized the inducible Scl-tTA×TRE-BCR::ABL1 (BCR::ABL1) transgenic mouse crossed with Lama4-/- and Lama4+/+ mice. The CML progression was monitored by symptomatic CML onset, survival rate and blood lineage analysis using hematology analyzer and multi-color fluorescent-activated cell sorting (FACS) after BCR::ABL1 activation. The specific effect of Lama4 on the fitness of BCR::ABL1+ cells was determined by droplet digital PCR of FACS-sorted hematopoietic cell subsets from blood and BM of Lama4+/+ and Lama4-/- BCR::ABL1 mice.
Result
In contrast to the impact of Lama4 on AML, deletion of Lama4 delayed CML onset (p&gt;0.009) thus prolonged the survival (p&gt;0.04) of BCR::ABL1×Lama4-/- mice compared to BCR::ABL1×Lama4+/+ mice after CML induction by tetracycline withdrawal. This is accompanied by an overall reduced frequency of BCR::ABL1+ cells in the blood of BCR::ABL1×Lama4-/- mice while the BCR::ABL1+ fraction within hematopoietic stem cells (HSCs) remained comparable to that in Lama4+/+ mice. Concomitantly, an increase of immature myelomonocytes (CD11B+GR1low) in blood (p=0.007 at 2 weeks and p=0.02 at 4 weeks) and reduced granulocyte-macrophage progenitors in BM (p=0.02) at 6 weeks post-tetracycline withdrawal was detected by FACS in the BCR::ABL1×Lama4-/- mice. Furthermore, while the HSCs (LSKCD150+) in BCR::ABL1x Lama4-/- BM remain similar to that in BCR::ABL1xLama4+/+ mice, the extramedullary HSC in Lama4-/- mouse spleen seemed to be reduced at 4 weeks post tetracycline withdrawal (p=0.02). This together with smaller spleens (p=0.02) and reduced liver infiltration of white blood cells (0/4 in BCR::ABL1×Lama4-/- vs 3/4 in BCR::ABL1xLama4+/+ mice at week 4) indicated slower extramedullary hematopoiesis in BCR::ABL1×Lama4-/- mice than that in BCR::ABL1xLama4+/+ mice.
Conclusion
Taken together, our data point to that loss of Lama4 delayed CML onset and progression, associated with reduced frequency of BCR::ABL1+ blood mononuclear cells. Contrary to the promoting effect of Lama4 deficient niche on AML progression, our data suggest Lama4-/- niche suppresses CML development possibly by providing an unfavorable niche for BCR::ABL1+ HSC differentiation. These results provide another evidence for differential niche regulation of AML and CML. The underlying mechanisms and therapeutic effects of LAMA4 peptides are currently under investigation.