Deletion of a Key PU.1 Gene Regulatory Element Induces T-Cell Lymphoma.

The transcription factor PU.1 is indispensable for the generation of myeloid and B-lymphoid cells. PU.1 expression is highly regulated, and enforced expression of PU.1 in early erythroid or T-cell progenitors leads to erythroleukemia or an early block in T-cell differentiation, respectively. While PU.1 function in hematopoietic differentiation is well characterized, the regulation of the PU.1 gene is still poorly understood. We previously identified a -14 kb upstream regulatory element (URE) in the PU.1 gene. Replacement of this element with a neomycin cassette led to mice with hypomorphic PU.1 alleles, and uncovered a dosage effect of this lineage specific transcription factor in acute myeloid leukemia. However, since insertion of a neomycin gene might interfere with PU.1 expression, it was excised from the genome to eliminate any effects other than deletion of the URE. We found that UREΔ/Δ animals exhibited markedly decreased PU.1 expression in multiple lineages, resulting in a profound reduction in macrophage and B-cell numbers in bone marrow and spleen. Thus, normal PU.1 levels are required for development of both lineages in vivo. Furthermore, UREΔ/Δ animals exhibited decreased thymocyte numbers due to a partial block in double negative T-cell differentiation at the transition from DN1/2 to DN3/4 cells. Within a period of 3 to 8 months, many animals developed aggressive, clonal, lymphoblastic T-cell lymphomas, which were dominant over the myeloid leukemia in this model. Gene profiling analysis of early thymocyte populations sorted from preleukemic mice revealed several key genes known to be involved in normal T-cell development and transformation. The role of these novel PU.1 downstream targets is currently being analyzed and the data will be presented. Together, this murine model demonstrates (1) a crucial function of the URE for endogenous PU.1 expression, and (2) a PU.1 dosage effect in the induction of fatal T-cell malignancies. These results show that deletion of a key regulatory element can be a sufficient event for malignant transformation, thus highlighting the need to increase the search for such elements in tumor suppressor genes and proto-oncogenes and to validate their contribution to human disease.