Deletion Of Tifab, a Novel Candidate Gene On Chromosome 5q, Results In Hematopoietic Defects By Changing The Dynamic Range Of Innate Immune Pathway Activation

Myelodysplastic syndromes (MDS) are defined by blood cytopenias due to ineffective hematopoiesis, and predisposition to acute myeloid leukemia (AML) or bone marrow failure (BMF). The most common cytogenetic alteration in MDS is deletion of chromosome 5q (del(5q)). A search of annotated genes within or near the commonly-deleted regions (CDR) in del(5q) revealed a novel uncharacterized gene, TRAF-interacting protein with forkhead-associated domain B (TIFAB). TIFAB resides within the proximal CDR on band 5q31.1, and consistent with haploinsufficiency, expression of TIFAB mRNA is at least 50% lower in del(5q) MDS as compared to diploid 5q MDS and age-matched control marrow cells. Restoring TIFAB in human AML cell lines with low endogenous levels of TIFAB results in increased apoptosis, diminished proliferation, and impaired leukemic progenitor function, suggesting that it functions as a tumor suppressor. To investigate whether loss of TIFAB is important to the pathophysiology of del(5q) MDS/AML, we characterized a novel germline TIFAB knockout (KO) mouse for hematopoietic stem/progenitor cell (HSPC) function. Whole-body TIFAB KO mice do not exhibit changes in steady-state hematopoiesis even beyond 1 year of age. However, the consequences of 5-fluorouracil (5-FU) treatment on TIFAB KO mice were more severe as compared to wild type (WT)-treated mice, suggesting that HSPC function of TIFAB KO mice is affected following hematopoietic stress. To further investigate the consequences of TIFAB loss on HSPC function, TIFAB KO marrow cells were transplanted into WT syngeneic recipient mice. For 3 months post-BM transplant, WT and TIFAB KO-transplanted mice displayed similar blood and BM proportions. However, beyond 6 months post-BM transplantation, TIFAB KO mice displayed progressive hematopoietic defects, including skewed HSPC proportions, altered myeloid differentiation, and pancytopenia. Importantly, approximately 40% of mice transplanted with TIFAB KO BM cells developed a BMF-like disease associated with BM dysplasia and pancytopenia. Cell-intrinsic HSC defects in TIFAB KO mice were confirmed by performing competitive BM transplantations. In support of a HSC defect, TIFAB KO HSC were out-competed by co-transplanted WT HSC. To uncover the molecular consequences that explain the HSPC defects in TIFAB KO mice, we isolated Lin-cKit+Sca1+ (LSK) and performed a microarray analysis. TIFAB KO LSK exhibited an increase in HSC-, IFN-, and p53-related gene signatures and downregulation of LPS-stimulated gene signatures as compared to WT LSK. Despite a repression of an LPS-stimulated gene signature, TIFAB KO BM cells are hyper-sensitive to LPS stimulation, suggesting that loss of TIFAB alters the innate immune pathway. As a potential explanation for altered LPS sensitivity, TIFAB loss induces an increase in TRAF6 protein, a key modulator of the innate immune pathway. Taken together, our results provide evidence that TIFAB exhibits tumor suppressor-like functions and that deletion of TIFAB contributes to an MDS-like phenotype in mice by changing the dynamic range of the innate immune pathway reactivity in HSPC cells.