The Translocation t(7;12)(q36;p13) Induces Myeloid Leukemia in Immuno-Compromised but Not Immunocompetent Mice

Introduction:

Non-random cytogenetic aberrations are often involved in the development of AML in children and several aberrations can serve as diagnostic markers, prognosis predictors and impact choice of therapy. In infant AML, a chromosomal translocation t(7;12)(q36;p13) has been found in up to 20-30 % of the cases, making it the second most common genetic aberration in this age group after KMT2A (MLL) rearrangements. Previous studies indicate that this patient group has a dismal prognosis with virtually no event-free survival. Limiting the chances to improve this is the lack of understanding how the t(7;12)(q36;p13) is involved in leukemia development. The translocation leads to a gene fusion MNX1-ETV6 but also to increased MNX1 gene expression. Although both ETV6 and MNX1 are expressed in normal hematopoietic tissues, the role of the fusion protein MNX1-ETV6in the development of AML is not established. Also unclear is whether the driver of leukemogenesis is the fusion itself or the simultaneous overexpression of MNX1. The aim of this study was to assess the transformation capacity and the molecular mechanism of the MNX1-ETV6 fusion and the overexpressed MNX1in vitro and in vivo using murine transplantation models.

Material and methods:

In a liquid culture system, we introduced the MNX1-ETV6 fusion, MNX1 overexpression, or empty vector into primary murine (C57BL/6) hematopoietic progenitor cells with retroviral transfection. Cells were isolated from either adult bone marrow after 5-FU stimulation, or from fetal liver at E14.5. After enrichment by fluorescence activated cell sorting based on vector co-expressed green/yellow fluorescence protein, transfected cells were used for in vitro experiments and for transplantation into lethally irradiated immunocompetent C57BL/6 mice or sub-lethally irradiated immunocompromised NSGW41 mice. In vitro, cells were assessed with RNA sequencing for gene expression, gamma H2AX assay for DNA double strand brakes, flow cytometry for lineage marker expression, apoptosis and proliferation, and with colony forming unit assay.

Results:

Upon transplantation, only fetal liver cells transduced with MNX1 or with MNX1-ETV6 fusion were able to induce leukemia in immunocompromised (NSGW41) mice. When MNX1 or MNX1-ETV6 transduced cells were transplanted into immunocompetent mice (C57BL/6) mice, no leukemia development was seen, when either fetal liver or adult bone marrow cells were used for transduction. However, when immunocompromised mice were transplanted with MNX1 or MNX1-ETV6 fusion expressing cells they typically developed signs of disease after 1-2 months and exhibited leukocytosis and elevated blast cells in blood and bone marrow, severe anemia, and enlarged spleens with infiltration of leukemic cells. The cells showed expression of predominantly myeloid markers. In vitro, cells with overexpression of MNX1 or MNX1-ETV6 fusion expression also showed altered lineage differentiation in favor of myeloid differentiation. In addition, MNX1 overexpressing cells, but not MNX1-ETV6 expressing cells, exhibited increased proliferation and colony formation capacity. Both MNX1 overexpressing and MNX1-ETV6 fusion expressing cells showed increased DNA damage as evident from an increased gamma-phosphorylated H2AX in fetal liver and adult bone marrow transduced cells respectively, accompanied with G1 arrest, compared to cells transduced with empty vectors. Both MNX1 and MNX1-ETV6 also led to increased apoptosis in adult bone marrow (3-fold) and to a lesser extent in fetal liver cells (1.5-fold). Results from transcriptome sequencing showed enrichment for specific pathways in G2/M transition of cell cycle in cells transduced by either MNX1or the MNX1-ETV6 fusion. Further investigations to elucidate the molecular mechanisms and pathways through which MNX1 and/or MNX1-ETV6 induce leukemia is ongoing.

Conclusions:

MNX1 overexpression and MNX1-ETV6 fusion, both characteristics of infant AML with t(7;12)(q36;p13), induced leukemogenic effects in both fetal liver cells and adult bone marrow cells, but could cause a myeloid leukemia only under immunocompromised conditions. This may be of importance for the exclusive prevalence of this AML subtype in young children, with the highest peak during the first six months of life when the immune system is less developed.