LIM Domain Protein 1 (Ldb1) Is Required for Lmo2 Oncogene-Induced Thymocyte Self-Renewal and T-Cell Leukemia in a Mouse Model of Human T-ALL

LIM domain Only-2 (LMO2) is one of the most frequently deregulated oncogenes in T-cell acute lymphoblastic leukemia (T-ALL) and is generally expressed in the clinically aggressive Early Thymocyte Precursor ALL. LMO2 encodes a small protein with 2 LIM domains that is part of a large multiprotein complex in hematopoietic stem and progenitor cells (HSPC), where it is required for HSC specification and maintenance. Many of LMO2's protein partners in HSPCs are expressed in T-ALL implying that protein complexes like those scaffolded by LMO2 in HSPCs also play a role in leukemia. LDB1 is concordantly expressed with LMO2 in human T-ALL although its expression is more widespread than LMO2. In this study, we analyzed a critical component of the Lmo2 associated complex, LIM domain binding 1 (Ldb1), in the CD2-Lmo2 transgenic mouse model of human T-ALL. To further define Ldb1's role in leukemia, we induced its conditional knockout in CD2-Lmo2 transgenic mice with the use of Lck-Cre, Rag1-Cre, and Il7r-Cre transgenic mice. CD2-Lmo2 transgenic mice develop T-ALL with high penetrance and closely model the human disease.

We discovered that the penetrance and latency of Lmo2-induced T-ALL were markedly attenuated in the Lck-Cre model and T-ALL onset was completely abrogated in the Rag1-Cre and Il7r-Cre models. The latter two models induced more efficient deletions of Ldb1, earlier in the T-cell differentiation program compared to Lck-Cre. Interestingly, Lck-Cre deletion was efficient in thymocytes without the Lmo2 transgene. In striking contrast, Ldb1 deletion could not be induced in CD2-Lmo2 transgenic T-cell progenitors. Consistent with this finding, T-ALLs that developed in CD2-Lmo2/floxed-Ldb1/Lck-Cre mice had incomplete deletion of Ldb1. These results imply that Ldb1 is a required factor for Lmo2 to induce T-ALL. To further probe the pathogenesis of Lmo2-induced T-ALL, we analyzed preleukemic phenotypes in the Rag1-Cre (or Il7r-Cre) conditional knockout models. Our results showed that Ldb1 is required for the induction of thymocyte self-renewal and radioresistance. Ldb1 was also required for the acquisition of the pre-leukemic ETP gene expression signature observed in immature CD2-Lmo2 transgenic thymocytes. Detailed biochemical experiments show that LMO2 protein is directly stabilized by LDB1 in leukemia cells perhaps on chromatin.

In conclusion, these results support a model where Lmo2-induced T-ALL is caused by a failure to downregulate Ldb1/Lmo2 nucleated transcription complexes that normally function to enforce self-renewal in bone marrow hematopoietic progenitor cells.