Expression data from bone marrow and fetal liver of Col18-wt and Col18-KO mice

Perturbations in the mechanisms controlling hematopoietic stem cells (HSCs) quiescence are believed to be at the roots of leukemic transformation. Through its ability to seize growth factors, provide docking to HSCs and modulate cell-to-cell signaling, the extracellular matrix (ECM) has been variously involved in HSCs quiescence and malignant transformation. In this context, different collagens (including Collagen VI and X) have been demonstrated to regulate HSCs growth and differentiation. Different reports suggest that Collagen XVIII (Col18), a member of the multiplexins collagen family, could play similar roles in maintaining HSCs and, potentially, in leukemic transformation, though any knowledge about its actual involvement in hematopoiesis is still lacking. Here we show that Col18 is needed to keep HSCs quiescent, and that its lack increases the risk of hematological neoplasms. We found that mice lacking Col18, in fact, have increased frequency of HSCs in both fetal liver and adult bone marrow, with a significant accumulation of myeloid precursors in the hematopoietic niches and of mature and immature myeloid cells in the blood. Along with hematopoietic deregulation, mice lacking Col18 showed a significant increase in the development of spontaneous lymphomas, a genetic predisposition to leukemic transformation, and lack of cell-to-ECM adhesion. We also confirmed that, in leukemic patients, Col18 is downregulated through genetic and epigenetic mechanisms and that such a downregulation decreases survival and contributes to a poorer prognosis. Our results demonstrate that Col18 is necessary to keep HSCs quiescent and to prevent excessive myeloid growth, and that alterations in its expression within the hematopoietic niche are likely to facilitate leukemic transformation and affect life expectancy of patients. Total RNA was isolated from E14.5 fetal liver (FL) or adult (3-months-old) bone marrow (BM) of mice lacking Col18a1 (Col18-wt) or wild-type (Col18-wt) littermates. Samples from Col18-wt and Col18-KO FL and BM were pooled in pairs (2 littermates/sample) before RNA extraction, and two independent replicates per genotype were prepared for each tissue before hybridization to Affymetrix Mouse Genome 430.2 array.