Autoregulation of the Transcription Factor PU.1 Via Its Evolutionarily Conserved Upstream Regulatory Element Is Critical in Adult Mouse Hematopoiesis.
Authors: Staber Philipp B. , Zhang Pu , Ye Min , Huang Gang , Bartholdy Boris , DiRuscio Annalisa , Ebralidze Alexander K. , Tenen Daniel G. 
Published: October 2019

Abstract
Abstract 1468
Poster Board I-491

Background:
Levels of the Ets transcription factor PU.1 control normal hematopoietic differentiation and even modest alterations can lead to leukemia and lymphoma. Regulation of PU.1 levels at different stages of hematopoiesis requires multiple interactions between several regulatory elements and transcription factors. Our previous studies identified a potential autoregulatory mechanism of the PU.1 gene through the combined activity of the proximal promoter and an evolutionarily conserved upstream regulatory element (URE), located at –14 kb relative to the transcription start site in mice. PU.1 binds to a conserved PU.1 site in the PU.1 URE both in vitro and in vivo.

Approach:
To ask at which stages of hematopoietic differentiation autoregulation of PU.1 via binding to its URE might play a role, we developed a mouse model with targeted disruption of the PU.1 binding site in the PU.1 URE.

Results:
Targeted mutation of the PU.1 autoregulatory site in PU.1 URE abolished PU.1 binding as verified by Chromatin Immuno-precipitation (ChIP). PU.1 URE activity was manifestly reduced resulting in a variety of lineage-specific abnormalities. As shown here in adult mice, the absence of the autoregulatory PU.1 site affected PU.1 expression in a lineage dependent manner. PU.1 expression was markedly decreased in phenotypic long term hematopoietic stem cells (LT-HSC: CD150+/CD48−/ c-kit+/sca-1+/lin−) and short term HSCs (ST-HSCs: CD150−/CD48+/ c-kit+/sca-1+/lin−) and, to a lesser extent, in Common Myeloid Progenitors (CMPs: lin−/c-kit+/Sca-1−/CD34+/FcrRlow), and Megakaryocyte/Erythrocyte Progenitors (MEPs: lin−/c-kit+/Sca-1−/CD34−/FcrRhigh). Within the lymphoid linage, PU.1 levels were unchanged in Common Lymphoid Progenitors (CLPs: lin−/c-kitlow/Sca-1low /IL-7Ra+/Thy1.1−) and pre-B-cells (B220+/ CD43−), up in pro-B-cells (B220+/CD43+), and down in mature B cells. Myeloid cells appeared to be unaffected. Interestingly, while PU.1 levels were decreased in LT- and ST-HSC populations, only phenotypic LT-HSCs were reduced in number. To further analyze HSC function of PU.1 site mutated mice we performed limiting dilution transplantation assays and measured the frequency of competitive repopulation units (CRU) using the congenic Ly5.1/Ly5.2 system. Our preliminary data indicated a decrease of LT-HSC function in PU.1 site mutated mice, although their homing and engraftment functions were not affected. This was also observed in mice with targeted disruption of all three AML-1 sites that are in close proximity of the PU.1 site at the PU.1 URE. While AML-1 itself appeared not to influence LT-HSC function (M. Ichikawa, T. Asai et al. Nature Medicine, 2004), we found that the conformational changes of the URE present in mice with disrupted AML-1 binding sites, as measured by Quantitative Chromosome Conformation Capture, impede PU.1 binding to its autoregulatory site.

Conclusion:
PU.1 indeed autoregulates its expression via binding to the -14kb URE in a lineage specific manner in vivo. Our data point to a critical role of PU.1 autoregulation especially for long-term hematopoietic stem cell function.

Disclosures:
No relevant conflicts of interest to declare.