Sequence Specific Single Stranded DNA Binding Protein 2 (SSBP2) Is a Regulator of Hematopoietic Stem Cell Activity

Abstract 1384

Interstitial deletions of chromosome 5q are common, non-random anomalies in myelodysplastic syndromes (MDS) and acute myelogenous leukemia (AML). While most deletions are large, spanning 5q13–33, a subset of patients deletes only the 5q11–14 region. We isolated SSBP2, a candidate gene from a critical region of loss at 5q14. SSBP2 expression is undetectable in AML cell lines, primary AML and MDS samples with chromosome 5q deletions. Decreased Ssbp2 expression due to viral integration has been reported in a case of primate AML resulting from HOXB4 retroviral transduction¹ . We have established SSBP2, to be a member of Lim Domain Binding Protein (LDB1)-Lim Only 2(LMO2)- TAL1/LYL1containing transcriptional complexes. SSBP2-LDB1, interactions promote gene expression by protecting LDB1 from proteosomal degradation^2,3 . LDB1-LMO2-bHLH complexes are also critical regulators of hematopoietic stem cells (HSC), a target of transformation in MDS and AML. The transcriptional cofactor LDB1 is required for hematopoiesis as part of an autoregulatory loop, which maintains the expression of TAL1, LMO2, E2A and GATA1⁴ .

Since Ssbp2 is a candidate gene from 5q and an in vivo regulator of LDB1 stability³ , we sought to determine its role in hematopoietic stem cell homeostasis. In normal marrow, Ssbp2 expression is highest in Lin⁻, Sca1⁺, c-Kit⁺ (LSK) cells and decreases during lineage commitment. Ssbp2^−/− mice are apparently normal upto six months of age although multiple hematopoitic tissues exhibit modest hypoplasia. However, hematopoietic recovery from 5-Fluoro uracil treatment is considerably delayed in these mice. Furthermore, competitive repopulation assays reveal significantly reduced short and long-term engraftment with Ssbp2^−/− bone marrow. Enforced expression of Ssbp2 rescues the competitive repopulating activity. Increased expression of E2A and its' transcriptional target CDKN1A in Ssbp2^−/− LSK cells appears to underlie the impaired stem cell activity. Our results identify SSBP2 to be a novel regulator of HSC function by maintaining the LDB1 mediated regulatory loop. Furthermore, these findings suggest defective stem cell homeostasis in the absence of SSBP2, when combined with mitogenic signals might contribute to the evolution of MDS/AML in a subset of patients.