Alcam Mediated Interaction Regulates Normal Hematopoietic Stem Cell Function and Cbfβ-SMMHC Induced Leukemogenesis

Abstract 4091

The inv(16) acute myeloid leukemia (AML)-associated CBFβ-SMMHC fusion protein impairs hematopoietic differentiation and predisposes to leukemic transformation. Alcam, which encodes the activated leukocyte cell adhesion molecule (CD166), is a cell surface immunoglobulin superfamily member mediating homophilic adhesion as well as heterotypic interactions with CD6. We found that Alcam expression marks long-term repopulating HSCs (LT-HSC), multipotent progenitors (MPP), a subset of granulocyte-macrophage progenitors (GMP), and that Alcam expression is lost or reduced in subsets of pre-leukemic and leukemic progenitors expressing the Cbfβ-SMMHC fusion protein. We characterized the role of Alcam in HSC differentiation and self-renewal using an Alcam-null (Alcam^−/−) mouse model (Weiner et al. 2004 Mol Cell Neurosci 27:1, 59–69). We show that Alcam is highly expressed in LT-HSCs where its level progressively increases with age. Young adult Alcam^−/− mice had normal homeostatic hematopoiesis, and normal numbers of phenotypic HSCs. However, Alcam^−/− HSCs had reduced long-term replating capacity in vitro and reduced long-term engraftment potential upon transplantation. We show that Alcam^−/− BM contain a markedly lower frequency of long-term repopulating cells than wild type (WT). Further, the long-term repopulating potential and engraftment efficiency of Alcam^−/− LT-HSCs was greatly compromised despite a progressive increase in phenotypic LT-HSC numbers during long-term serial transplantation. In addition, an age-associated increase in phenotypic LT-HSC cellularity was observed in Alcam^−/− mice. This increase was predominately within the CD150^hi fraction, and was accompanied by significantly reduced leukocyte output. Moreover, Alcam^−/− LT-HSCs display premature elevation of Selp expression, a hallmark of HSC aging. To understand the role of Alcam in leukemic transformation, we generated conditional Cbfb-MYH11 knock-in (Cbfb^56M/+/Mx1-Cre), Alcam-deficient (Alcam^−/−) mice. Interestingly, we found that loss of Alcam drastically delayed or reduced leukemia incidence. Transplantation of Alcam^−/−/Cbfb^56M/+/Mx1-Cre pre-leukemic bone marrow cells into WT recipients also led to delayed and reduced incidence of leukemia development. These results suggest that Alcam contributes to leukemia transformation in a cell-intrinsic manner. Collectively, our study reveals that Alcam regulates the functional integrity and self-renewal of LT-HSCs, and contributes to leukemia initiation induced by CBFβ-SMMHC.