Rag1 Deficiency Does Not Affect Myelodysplasia but Leads to More Rapid Leukemic Transformation in a Mouse Model of MDS.

Abstract 2784

Poster Board II-760

MDS comprises a premalignant heterogeneous group of clonal stem cell disorders that also show bone marrow dysplasia and which often evolve to acute myeloid leukemia (AML). Aplastic anemia (AA) patients also share the bone marrow failure, anemia and resulting peripheral blood cytopenias of MDS. AA is thought to be caused by an oligoclonal expansion of cytotoxic T-cells that target haematopoietic stem and progenitor cells. The severe anemia and leucopenia characteristic of both diseases is relieved in AA patients and some MDS patients by immunosuppressive therapy, supporting the role of cytotoxic T-cells in the etiology of AA. However, the role of the lymphocytes in progressive MDS remains unclear. MDS has been associated with a number of genetic aberrations, including chromosomal translocations involving the NUP98 gene. Using mice that express a NUP98-HOXD13 (NHD13) transgene, previously shown to manifest the same clinical symptoms as those of MDS patients, we have followed a cohort of NHD13/Rag1^−/− mice to determine if the absence of lymphocytes, especially T cells, might 1) diminish the severity of the MDS, or 2) effect transformation and/or survival in the NHD13 mice, as would be predicted by an “immune surveillance” hypothesis of malignant transformation. Serial CBCs at two month time intervals were used to evaluate the extent of anemia and leucopenia in NHD13⁺ /Rag1^+/+ and NHD13/Rag1^−/−, as well as WT/Rag1^+/− and WT/Rag1^−/− control groups over a 15 month period. NHD13/Rag1^−/− mice were generated by crossing the NHD13⁺ (C57BL/6) with the B6;129S7-Rag1^tm1Mom/J mouse, and housed in a Specific Pathogen-Free (SPF) environment. Mice were euthanized and analyzed when CBCs indicated severe anemia/leucopenia or leukemic transformation, or when determined to be unwell (hunched, immobile, dyspnea) by observation. Flow cytometry, histology and genomic analyses further determined leukemia subtype, extent of infiltration and leukemia clonality. NHD13⁺ /Rag1^+/+ and NHD13/Rag1^−/− mice showed no significant differences at any two month time-point in hemoglobin (Hg), mean corpuscular volume (MCV), or platelet levels, and progressive MDS occurred in both groups. Consistent with previous studies, and excluding cases that showed evident transformation to acute leukemia, NHD13⁺ /Rag1^+/+ mice showed low WBC, neutrophil and lymphocyte numbers, which were not significantly different from the NHD13/Rag1^−/− mice. NHD13/Rag1^−/− mice did however show a significantly reduced survival when compared with the NHD13⁺ /Rag1^+/+ mice (Log-rank test, p = 0.0135), and survival medians of 11 and 13 months, respectively. Incidence of leukemic transformation was increased in the NHD13/Rag1^−/− compared with the NHD13⁺ /Rag1^+/+ mice (p=0.0079). A range of leukemia subtypes was observed in both the NHD13⁺ /Rag1^+/+ and NHD13/Rag1^−/− mice, including myeloid, B-cell, T-cell, and erythroid leukemias. In the SPF environment provided, the WT/Rag1^+/− and WT/Rag1^−/− control groups showed no significant difference in survival rates. Serial CBC data indicated that there was no significant difference in the timing or degree of peripheral blood cytopenias between the NHD13⁺ /Rag1^+/+ and NHD13/Rag1^−/− mice, supporting the conclusion that absence of lymphocytes does not lead to improvement in the peripheral blood cytopenias caused by the NHD13 transgene. This observation suggests that the NHD13 transgene does not produce MDS caused by an autoimmune phenomenon. The poorer survival and increased frequency of leukemic transformation in the NHD13/Rag1^−/− mice suggests that lymphocytes might play a role in the evolution of MDS to AML in the NHD13 mouse model, and supports the ‘immune surveillance' hypothesis.