Samd9L-Deficient Mice Develop Myeloid Malignancies Mimicking Human Diseases with Monosomy 7

Abstract 869

Monosomy 7 is a common chromosomal abnormality found frequently in MDS and AML. We previously identified a common microdeletion cluster in subband 7q21.3 in JMML patients (Asou et al., 2009). This cluster contains three poorly characterized genes: sterile alpha motif (SAM) domain-9 (Samd9) which is absent in mice, Samd9-like (Samd9L) and Miki (LOC253012). In this study we established and characterized Samd9L-deficient mice in detail and found that these mice develop myeloid malignancies that mimic human diseases with −7/7q-.

Samd9L^−/− (n=10), Samd9L^+/− (n=11) and Samd9L^+/+ (n=13) littermates were observed for signs of illness for 2 years. All but one Samd9L^+/+ mice maintained consistent WBC counts and hemoglobin levels throughout the observation period. Meanwhile, seven among 21 Samd9L^+/− and Samd9L^−/− mice developed neutropenia and/or anemia and 2 mice showed an apparent WBC increase after the age of 18 months. Based on Bethesda proposals for classification, the types and frequencies of myeloid diseases demonstrated by mice included myeloid dysplasia (5/10 Samd9L^−/−; 2/11 Samd9L^+/−), myeloid leukemia (1/10 Samd9L^−/−; 1/11 Samd9L^+/−) and myeloproliferative disease (1/11 Samd9L^+/−).

To determine whether loss of the Samd9L gene predisposes mice to myeloid diseases, newborn mice were injected with MOL4070A retrovirus which selectively induces AML or MDS in mice with propensity to myeloid malignancies. There was a significant effect, in that almost all Samd9L^+/− (12/14) and Samd9L^−/− (10/12) mice died from myeloid neoplasms, roughly one year earlier than mice that developed spontaneous myeloid malignancies without retroviral infection. In contrast, only 2 of 30 MOL4070A-infected Samd9L^+/+ mice developed myeloid diseases. Unlike uninfected Samd9L^+/− and Samd9L^−/− mice, which preferentially developed MDS (7/21), 16 of 26 virus-infected mice showed myeloid leukemias of various subtypes including undifferentiated, myelomonocytic, or monocytic leukemia that expressed combinations of surface markers for the granulocytic, monocytic, erythroid or megakaryocytic lineage. By applying inverse PCR method, two common retrovirus integration sites, Evi1 and Fbxl10 genes, were identified in leukemic samples specifically from Samd9L^−/− and Samd9L^+/− mice. The enhanced incidence of leukemia in mice with Samd9L-deficiency by Evi1 or Fbxl10 overexpression was further analyzed by transplanting Samd9L^−/− or Samd9L^+/+ bone marrow cells transduced with retrovirus expressing Evi1 or Fbxl10. All mice (11/11) receiving transferred Samd9L^−/−Evi1 bone marrow cells died from hematopoietic malignancies, particularly myeloid disorders (10/11). While not a statistically significant result, we also found that three of 13 mice receiving transferred Samd9L ^−/−Fbxl10 bone marrow cells developed myeloid malignancies.

The development of MDS in Samd9L^+/− and Samd9L^−/− mice of advancing age mimics the typical clinical association between −7/7q- and sporadic MDS in elderly humans, while development of wide-variety of myeloid leukemia subtypes in retrovirus-infected Samd9L^+/− and Samd9L^−/− mice indicates that Samd9L gene-deficiency can promote diverse leukemogenic pathways, mimicking another clinical feature of −7/7q-: i.e., deep involvement in therapy-related AML/MDS as well as AML/MDS among patients with a propensity for myeloid diseases.

As we reported last year, Samd9L protein localizes to the early endosome. Cells expressing Samd9L at low levels internalize ligand-bound cytokine receptors normally, but there is a delay in homotypic fusion of endosomes that results in prolonged cellular activation to cytokine signals. Moreover, colony replating assay revealed that Samd9L^+/− and Samd9L^−/− bone marrow cells maintain colony forming ability beyond 7th plating. Although detailed mechanisms remain to be elucidated, we hypothesize that excess cytokine-receptor signaling due to Samd9/Samd9L insufficiency induces self-renewal of hematopoietic stem cells and/or delays in differentiation of early progenitors, resulting in the development of myeloid neoplasms in cooperation with genetic and age-related epigenetic alterations.