Date Published: October 11, 2018
Publisher: Public Library of Science
Author(s): Stefan Nagel, Roderick A. F. MacLeod, Corinna Meyer, Maren Kaufmann, Hans G. Drexler, Aamir Ahmad.
Homeobox genes encode transcription factors which regulate basic processes in development and cell differentiation. Several members of the NKL subclass are deregulated in T-cell progenitors and support leukemogenesis. We have recently described particular expression patterns of nine NKL homeobox genes in early hematopoiesis and T-cell development. Here, we screened NKL homeobox gene activities in normal B-cell development and extended the NKL-code to include this lymphoid lineage. Analysis of public expression profiling datasets revealed that HHEX and NKX6-3 were the only members differentially active in naïve B-cells, germinal center B-cells, plasma cells and memory B-cells. Subsequent examination of different types of B-cell malignancies showed both aberrant overexpression of NKL-code members and ectopic activation of subclass members physiologically silent in lymphopoiesis including BARX2, DLX1, EMX2, NKX2-1, NKX2-2 and NKX3-2. Based on these findings we performed detailed studies of the B-cell specific NKL homeobox gene NKX6-3 which showed enhanced activity in patient subsets of follicular lymphoma, mantle cell lymphoma and diffuse large B-cell lymphoma (DLBCL), and in three DLBCL cell lines to serve as in vitro models. While excluding genomic and chromosomal rearrangements at the locus of NKX6-3 (8p11) promoter studies demonstrated that B-cell factors MYB and PAX5 activated NKX6-3 transcription. Furthermore, aberrant BMP7/SMAD1-signalling and deregulated expression of chromatin complex components AUTS2 and PCGF5 promoted NKX6-3 activation. Finally, NKL homeobox genes HHEX, HLX, MSX1 and NKX6-3 were expressed in B-cell progenitors and generated a regulatory gene network in cell lines which we propose may provide physiological support for NKL-code formation in early B-cell development. Together, we identified an NKL-code in B-cell development whose violation may deregulate differentiation and promote malignant transformation.
Hematopoietic stem cells (HSCs) generate common myeloid and lymphoid progenitor (CMP/CLP) cells which, respectively, initiate the differentiation into myeloid and lymphoid cell lineages. The latter produces all types of lymphocytes comprising B-cells, T-cells, and NK-cells. Early B-cell development begins with the B-cell progenitor (BCP) and takes place in the bone marrow. The final differentiation steps to memory B-cells and plasma cells via naïve and germinal center (GC) B-cells occur in lymph nodes and in the spleen [1–3]. Lymphopoiesis including B-cell development is regulated mainly at the transcriptional level [3,4]. Accordingly, several transcription factors (TFs) including PAX5, MYB, BCL6 and PRDM1/BLIMP1 generate a B-cell specific regulatory network controlling fundamental differentiation processes [5,6]. Deregulation of these TFs by chromosomal rearrangement or gene mutation contributes to the generation of B-cell malignancies [7,8].
In this study we analyzed physiological activities of NKL homeobox genes in B-cell development. Considered together with previously published data, this examination detected expression patterns which complete the NKL-code for early hematopoiesis and lymphopoiesis (Fig 1). Additional results of our work are summarized in Fig 8, which shows a gene regulatory network surrounding NKL homeobox gene NKX6-3. TFs MYB and PAX5, BMP7/SMAD1-signalling, and PRC1.5-mediator AUTS2 all activate expression of NKX6-3.