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on known targets including MYC and MYB or by other mechanisms. The finding of reduced numbers of B cells at the pre-B-cell level while pro-B cells are similar upon JQ1 treatment suggests there is a differentiation block, but this needs further clarification.
Although JQ1 induced numerical expansion of megakaryocytes in the BM, other myeloid cells, platelets and RBC remained constant upon treatment. Thus, JQ1 treatment induces profound cell type-specific changes in hematopoietic cell populations displaying high mRNA expression of BET proteins. As JQ1 targets multiple pro- teins of the BET family, the individual contribution of spe- cific molecular targets to the observed phenotype has to be identified in future studies.
JQ1 induces LT-HSC proliferation, thereby leading to an expansion of LT-HSC, ST-HSC and MPP in BM and spleen. Our data further indicate that JQ1 induces HSC mobilization. At the functional level, JQ1 treatment increased the quantity of HSC leading to better engraft- ment and faster recovery of the hematopoietic system without inducing HSC exhaustion. It is worthy of note that we did not challenge the secondary recipients of JQ1- or control-treated BM with myelosuppressive stimuli, but only measured their lifespan as a read-out for HSC
exhaustion. It might, therefore, be conceivable that tran- sient myeloablation of secondary recipients would reveal potential differences in hematopoietic recovery between both groups. Such experiments are especially important as we found marginal induction of apoptosis in HSC upon JQ1 treatment. Therefore, this aspect should be analyzed in future studies.
Interestingly, recent reports have shown that MYB plays an important role in hematopoiesis by recruiting BRD4, which is functionally suppressed by BET inhibitors such as JQ1.24 In line with this, the MYB polymorphism M303V disrupts the interaction of MYB with its transcriptional co- activator p300, thereby inducing megakaryocytosis, lym- phopenia, as well as a pronounced increase in HSC.25 The striking phenotypic similarities of reduced MYB transacti- vation and JQ1 treatment suggest that the results reported in this study could result from JQ1-mediated suppression of MYB. Furthermore, and in line with our observations, impaired function of MYB was reported to increase cycling of LT-HSC.25 In any case, additional studies are warranted to verify whether JQ1 induces its effects on hematopoiesis via MYB.
With respect to BRD4 inhibition, a previous study demonstrated that short-term reconstitution of LSK cells
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Figure 5. JQ1 treatment accelerates blood recovery after myeloab- lation. Animals were injected daily with 50 mg/kg JQ1 intraperi- toneal (i.p.) for 21 days before receiving sublethal whole body irra- diation with 5 Gy. (A and B) Flowcytometric quantification of hematopoietic stem cells (HSC) (A) and MPP (B) over the course of six weeks after myelosuppression (n=3-5; *P<0.05). (C-H) Analysis of the recovery of leukocytes (C), B cells (D), T cells (E), monocytes (F), thrombocytes (G) and granulocytes (H) after myelosuppression over the course of six weeks (n=3-5; *P<0.05).
haematologica | 2018; 103(6)