BET-Inhibition By JQ1 Promotes Proliferation and Self-Renewal Capacity of Hematopoietic Stem Cells

Background/significance: Bromodomain and extra terminal domain (BET) proteins have been shown to be critical for maintaining the function of leukemia stem cells. Although BET-inhibitors are in clinical development and show promising activity in different hematologic malignancies, a systematic analysis of the consequences of pharmacological BET-inhibition on healthy hematopoietic (stem) cells is outstanding.

Methods: C57BL/6J (WT) or B6.SJL-Ptprca Pepcb/BoyJ mice, which carry the congenic pan-leukocyte marker CD45.1 were treated for 3 weeks with the BET-Inhibitor JQ1 or a placebo. For competitive repopulation transplantations, bone marrow from treated CD45.1 mice was transplanted together with WT competitor cells (CD45.2) into lethally irradiated WT mice (9 Gy) in different ratios. Engraftment of all major hematopoietic lineages was monitored in peripheral blood (PB) and BM using differential blood counting and flow cytometry. For the determination of HSC frequency in vivo, extreme limiting dilution analysis (ELDA) was carried out by transplanting 2x10³, 2x10⁴ or 2x10⁵ JQ1- or placebo-treated bone marrow cells from WT mice together with 2x10⁵ bone marrow cells from CD45.2 mice. Engraftment was assumed when CD45.1⁺ myeloid cells were present in the peripheral blood in frequencies > 1 %.

Colony formation assays were used to measure the abundance of colony forming progenitor cells (M3434) as well as megakaryocytic progenitors (MegaCult-C). For analysis of recovery after myelosuppression, JQ1- or placebo-treated WT mice received sublethal irradiation (5 Gy) and hematopoietic repopulation was monitored. For analysis of HSC proliferation, 1 mg BrdU was injected 24 h, 12 h, and 6 h before sacrifice.

Results: As a first step, we analyzed the mRNA expression patterns of Brd2-4 in all major hematopoietic subpopulations, because BET-family members represent the main molecular targets of JQ1. Expression of Brd4 mRNA is most abundant in megakaryocytes (MK), followed by B cells, T cells and hematopoietic stem cells (HSC) and expression levels of Brd2+3 show similar distribution.

JQ1-treatment decreases the numbers of pre-, immature and mature B cells while numbers of early pro-B cells remain constant (pre-B cells: 125 ± 7 vs. 52 ± 5 and immature B cells: 121 ± 9 vs. 26 ± 5 x10⁴ cells/tibia; n=4; *p<0.05). Furthermore, it increases apoptosis in all T cell subsets, all together leading to reduced cellularity in thymus, BM and spleen. Intriguingly, JQ1 induces a 3-fold increase in the number of hematopoietic stem and progenitor cells (25 ± 2 vs. 70 ± 5 x10³ LSK/femur; n=5; *p<0.05) and mobilizes HSC into the peripheral blood (4 ± 1 vs. 28 ± 2 x10⁶ CFU/ml blood; n=5; *p<0.05). Consistently, extreme limiting dilution analysis (ELDA) showed a 3-fold increase in stem cell frequencies in the bone marrow upon JQ1-treatment (1/19480 vs. 1/6284; n=8-12; *p<0.05).

We found that the elevated HSC numbers arise from increased cycling of LT-HSC, as these cells incorporate more BrdU upon JQ1-treatment (29 ± 2 vs. 41 ± 3 % BrdU⁺ LT-HSC in bone marrow; n=5; *p<0.05). Moreover and in contrast to placebo, JQ1-treated bone marrow cells preserve their colony-forming properties during several rounds of replating in vitro, indicating sustained stemness (18 ± 4 vs. 77 ± 10 CFU/1x10⁴ bone marrow cells after third replating; n=3-8; *p<0.05). Due to increased numbers of HSC, JQ1-treated bone marrow engrafts better after competitive stem cell transplantation and repopulates the hematopoietic system significantly faster after sublethal myeloablation. Six weeks after irradiation, total leukocyte counts of JQ1-treated mice fully recover to levels before irradiation (94 ± 7 % recovery; n=5; *p<0.05), whereas recovery is slower in the control group (47 ± 5 % recovery; n=5; *p<0.05).

Conclusions: JQ1 induces proliferation and mobilization of HSC, thereby increasing their number in bone marrow and peripheral blood. Therefore, BET-inhibition might benefit patients after myelosuppression or stem cell transplantation.