Lymphoid cell growth and transformation are suppressed by a key regulatory element of the gene encoding PU.1

Abstract

Tight regulation of transcription factors, such as PU.1, is crucial for generation of all hematopoietic lineages. We previously reported that mice with a deletion of an upstream regulatory element (URE) of the gene encoding PU.1 (Sfpi1) developed acute myeloid leukemia. Here we show that the URE has an essential role in orchestrating the dynamic PU.1 expression pattern required for lymphoid development and tumor suppression. URE deletion ablated B2 cells but stimulated growth of B1 cells in mice. The URE was a PU.1 enhancer in B cells but a repressor in T cell precursors. TCF transcription factors coordinated this repressor function and linked PU.1 to Wnt signaling. Failure of appropriate PU.1 repression in T cell progenitors with URE deletion disrupted differentiation and induced thymic transformation. Genome-wide DNA methylation assessment showed that epigenetic silencing of selective tumor suppressor genes completed PU.1-initiated transformation of lymphoid progenitors with URE deletion. These results elucidate how a single transcription factor, PU.1, through the cell context-specific activity of a key cis-regulatory element, affects the development of multiple cell lineages and can induce cancer.

Spi-C Has Opposing Effects to PU.1 on Gene Expression in Progenitor B Cells

“…Recent data has shown that deletion of PU.1 in B cells can induce a B-2 to B-1 cell switch, accompanied by loss of B220 expression (27). In addition, deletion of a regulatory element upstream of the PU.1 gene eliminates B-2 cells, but enhances the B-1 cell population (58). Our data indicate that pro-B cells ectopically expressing Spi-C have dramatically reduced levels of B220 on their surface, which could suggest that these cells may be switching to a B-1 cell phenotype.…”

Bcl11b and combinatorial resolution of cell fate in the T-cell gene regulatory network

“…This balance is tipped during commitment, with the silencing of the phase 1 regulatory genes and down-regulation of Kit, but only limited aspects of this process are understood. GATA-3, Runx1, and TCF-1 (or its relative LEF-1) eventually play roles in silencing expression of phase 1 regulatory genes encoding PU.1 and Bcl11a during commitment, as demonstrated by gain-and loss-offunction data (27,(36)(37)(38)(39) (Fig. 1B).…”

The role of PU.1 and GATA-1 transcription factors during normal and leukemogenic hematopoiesis

“…36,38,39 Conditional deletion of the PU.1 gene in adult bone marrow progenitors showed its requirement for both monocyte and lymphoid progeny whereas granulocyticlike cells were increased indicating that PU.1 may inhibit adult in vivo granulopoiesis at latter stages of development. 34,40 Similarly, PU.1 has only a minor function in advanced differentiation states of B cells 25,34,35,[41][42][43] and is not required in mature T cells. [44][45][46] Collectively the above-mentioned facts reveal an important and crucial role of PU.1 as a primary transcriptional determinant of hematopoietic cell fate with current evidence showing its substantial role in the cell fate control of HSC and progenitor populations rather than the maturation of terminally differentiated cells.…”

“…24,25,80,81,82 Targeted disruption of the URE in mice significantly reduces PU.1 expression and results in the development of acute myeloid leukemia with frequent cytogenetic aberrations. 24 The URE structure is dynamically regulated by chromatin loops associated with the chromatin remodeling regulator SATB1 bound directly to the URE.…”

“…Deletion of URE in the mouse has a detrimental effect on the PU.1 protein level reducing it to 20% of normal levels. 24,25 Various transcription factors can bind to the URE including PU.1 itself, Elf1, FLI-1, Runx-1 and C/EBP. 11,23,26,27 The URE regulates transcription of both sense as well as antisense non-coding transcripts and this provides an additional layer of modifying PU.1 activity via the regulation of protein translation.…”