Hoxb4-deficient mice undergo normal hematopoietic development but exhibit a mild proliferation defect in hematopoietic stem cells

Abstract

Enforced expression of IntroductionHematopoiesis relies on the unique abilities of relatively few hematopoietic stem cells to self-renew and generate progenitors that will differentiate into the mature cells forming the blood system. This dynamic process is tightly regulated by a complex of internal and external signals, such as transcription factors, growth factors, and cell cycle regulators (for reviews, see Orkin 1 and Verfaillie 2 ). Many transcription factors, including homeobox (Hox) transcription factors, have been shown to be key players in the proliferation and differentiation of early progenitor cells. 3,[4][5][6] Specific expression patterns of multiple Hox genes have been detected in normal and leukemic hematopoiesis. 7,8 Enforced expression of Hox genes has been shown to affect the ability of progenitors and stem cells to proliferate and differentiate. [9][10][11][12][13][14][15][16][17] One of these genes, Hoxb4, has been implicated in the regulation of hematopoietic stem cell regeneration, 8 and retrovirally engineered overexpression in murine bone marrow cells dramatically increases the stem cell pool ex vivo and in vivo, resulting in faster, more complete recovery of the stem cells in transplantation studies with no adverse effect on differentiation or lineage distribution. 14,[18][19][20][21] This is in contrast to the overexpression of other Hox genes, which can perturb the proliferation and lineage commitment of primitive progenitors and can give rise to hematopoietic malignancies. 10,11,13,15,16,[22][23][24] However, recent studies have suggested that the effect of Hoxb4 is concentration dependent and is not necessarily restricted to proliferation. [25][26][27] Thus, the level of Hoxb4 expression has to be within a specific range for Hoxb4 to increase stem cell proliferation without adverse effects on differentiation. Although enforced expression of Hoxb4 in hematopoietic cells has been studied in detail, its physiologic role in hematopoiesis is poorly understood. Recently, we described a mouse model deficient in Hoxb3 and Hoxb4, showing reduced proliferative capacity of the stem cell pool without otherwise perturbing hematopoiesis. 28 Here we report a novel mouse model in which the Hoxb4 gene alone has been completely removed through the Cre/loxP technique. Hoxb4-deficient mice have a phenotype similar to that of double Hoxb3/Hoxb4 knockout (KO) mice, although the effects are milder in the Hoxb4 Ϫ/Ϫ mice. The phenotype observed seems mainly confined to the stem cell pool, resulting in reduced proliferative capacity of bone marrow and fetal liver hematopoietic stem cells (HSCs) without affecting differentiation or lineage choice. Deficiency of Hoxb4 or Hoxb3 and Hoxb4 affects the expression of other Hox genes and the expression of cell cycle regulators, indicating a complex regulatory role of these Hox genes. Collectively, these findings indicate that Hoxb4 improves proliferative recruitment of HSCs in settings demanding high proliferation, such as transplantation, but that it has a less pro...

Near-maximal expansions of hematopoietic stem cells in culture using NUP98-HOX fusions

“…To achieve comparable levels of reconstitution, 10-fold higher transplant doses of NUP98-HOXB4-transduced cells were required and ~100-fold and greater than 1000-fold higher transplant NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript doses of HOXB4-or GFP-transduced cells were needed. As shown in Figure 3B, in vitroexpanded HSC transduced with either NUP98-HOXB4 or NUP98-HOXA10 made substantial contributions to all major peripheral blood lineages, as well as matching high-level contributions to the red blood cell compartment, although the proportion of myeloid cells in some recipients was modestly increased and the proportions of B and T cells correspondingly decreased, a phenomenon previously observed for recipients of HOXB4-transduced cells [33][34][35]. However, no recipients followed over the 6-to 8-month time frame of the experiments showed any evidence of a myeloproliferative disorder or leukemia consistent with a previously observed lack of leukemogenic activity for NUP98-HOXB4 and long disease latency in recipients of large doses of NUP98-HOXA10-transduced cells [28].…”

Hox genes in hematopoiesis and leukemogenesis

“…Gene expression profiling of c-kit þ Hoxb1-Hoxb9 À/À fetal liver cells revealed a downregulation of most Hoxa genes, and upregulation of three Hoxc genes including the Hoxb4 paralog, Hoxc4. The apparent discrepancy in hematopoietic defects reported by Brun et al (2004) and Bijl et al (2006) has been explained in the latter study to be the result of the different gene targeting strategies used, a phenomenon for which a precedent exists. Nevertheless, both studies provide strong support for gene redundancy and compensatory mechanisms, as well as cross-regulatory interactions, among Hox genes.…”

“…These mice did, however, exhibit subtle reductions in HSC and progenitor numbers and a modest impairment of competitive repopulating ability (Brun et al, 2004). Compound Hoxb3/b4 À/À knockout mice exhibited qualitatively similar but quantitatively more pronounced hematopoietic differences (Bjornsson et al, 2003).…”

“…Hox regulation of hematopoietic and leukemic stem cells B Argiropoulos and RK Humphries example, definitive hematopoiesis was not disrupted in Hoxb4-deficient mice (Brun et al, 2004;Bijl et al, 2006). These mice did, however, exhibit subtle reductions in HSC and progenitor numbers and a modest impairment of competitive repopulating ability (Brun et al, 2004).…”

Hematopoietic stem cells: generation and self-renewal

“…56 Interestingly, HoxB4 knockout mice exhibit normal hematopoiesis, with only a mild defect in the proliferative potential of HSC. 57 This discrepancy is likely due to the redundant function of other Hox family members. Growth factor independence 1 (Gfi1) is a zincfinger-containing transcriptional repressor.…”