Development of Notch-dependent T-cell leukemia by deregulated Rap1 signaling.

SPA-1 (signal-induced proliferation associated gene-1) functions as a suppressor of myeloid leukemia by negatively regulating Rap1 signaling in hematopoietic progenitor cells (HPCs). Herein, we showed that transplantation of HPCs expressing farnesylated C3G (C3G-F), a Rap1 guanine nucleotide exchange factor, resulted in a marked expansion of thymocytes bearing unique phenotypes (CD4/CD8 double positive [DP] CD3(-) TCRbeta(-)) in irradiated recipients. SPA-1(-/-) HPCs expressing C3G-F caused a more extensive expansion of DP thymocytes, resulting in lethal T-cell acute lymphoblastic leukemia (T-ALL) with massive invasion of clonal T-cell blasts into vital organs.

Generation of a conditional knockout allele for mammalian Spen protein Mint/SHARP.

The Spen protein family is found in worms, flies, and mammals, and is implicated in diverse biological processes from embryogenesis to aging. Spen proteins have three N-terminal RNA recognition motifs and a C-terminal SPOC domain. The mammalian Spen proteins Mint and its human ortholog SHARP interact with the Notch-signaling mediator RBP-J as well as Msx2 and several unliganded nuclear hormone receptors, and impart transcription-repressing activity to these molecules by recruiting corepressors through the SPOC domain.

Role of SPA-1 in phenotypes of chronic myelogenous leukemia induced by BCR-ABL-expressing hematopoietic progenitors in a mouse model.

SPA-1 is a negative regulator of Rap1 signal in hematopoietic cells, and SPA-1-deficient mice develop myeloproliferative disorders (MPD) of long latency. In the present study, we showed that the MPDs in SPA-1(-/-) mice were associated with the increased hematopoietic stem cells expressing LFA-1 in bone marrow and their premature mobilization to spleen with extensive extramedullary hematopoiesis, resembling human chronic myelogenous leukemia (CML). We further showed that human BCR-ABL oncogene caused a partial down-regulation of endogenous SPA-1 gene expression in mouse hematopoietic progenitor cells (HPC) and immature hematopoietic cell lines.