Abstract

Homeobox genes code for transcription factors with essential regulatory impact on cellular processes during embryogenesis and in the adult. Increasingly, members of the circa 200 gene strong family are emerging as major oncogenic players, prompting our investigation into possible homeobox gene dysregulation in Hodgkin lymphoma (HL) in which no recurrent oncogene involvement has been known. Accordingly, we screened 6 well characterized HL cell lines (HDLM-2, KM-H2, L-1236, L-428, L-540, SUP-HD1) and 3 non-Hodgkin lymphoma (NHL) cell lines (RC-K8, RI-1, SC-1) for homeobox gene expression using Affymetrix U133-2.0 whole-genome oligonucleotide microarrays. Of 15 candidate genes thus shown to reveal HL-specific expression patterns, 5 homeobox genes were shortlisted as potentially key dysregulatory targets in HL after additional RT-PCR expression analysis relative to controls. While 3/5 homeobox genes were upregulated in HL (HOXB9, HOXC8, HLXB9), 2/5 were downregulated (BOB1, PAX5). Furthermore, cloning and sequencing RT-PCR products obtained with degenerate primers recognizing conserved homeobox motifs confirmed the predominant expression of HOXB9 in HL cells. However, fluorescence in situ hybridization (FISH) analysis of the HOXB locus (at 17q21) revealed no cytogenetic aberrations, indicating that its activation is conducted non-chromosomally in HL cells. Surprisingly, known target genes of HOXB9 and HOXC8 remained unperturbed, implying novel downstream effector pathways in HL cells. Antisense oligos directed against HOXB9 and forced expression experiments using cloned full length HOXB9 cDNA indicated its involvement in both proliferation and apoptosis. Cell cycle regulators BTG1, BTG2 and GEMININ have been described to interact with HOXB9 and may represent potential targets deserving investigation. We recently showed that HLXB9 promotes IL6 expression in HL cells in response to a constitutively active PI3K signalling pathway therein (

Nagel et al.,
Leukemia
19
,
841
–6,
2005
). Our most recent data indicate that HLXB9 is also expressed in various NHL cell lines including anaplastic, diffuse and mediastinal large cell as well as follicular B-cell lymphomas while expression is notably absent from Burkitt, mantle cell and natural killer T-cell lymphomas reflecting their pathologic classification. Intriguingly, our data highlight unexpected similarities between HL and prostate cancer cells which together uniquely overexpress HOXB9, HOXC8 and HLXB9 (or its close homolog GBX2). Additional genes expressed in prostate carcinoma (HOXB13, PRAC1, PRAC2) were detected in two HL cell lines (KM-H2 and L-428) suggesting further parallels may be revealed. Detection of downregulated B-cell differentiation factors BOB1 and PAX5 in our panel of HL cell lines validated this approach. Both factors were previously implicated in oncogenesis of HL lacking IGH rearrangements and other key B-cell characteristics. In summary, we identified a unique homeobox gene expression pattern involving HOXB9, HOXB13, HOXC8 and HLXB9 in HL cell lines resembling that of prostate carcinoma cells. Overexpressed HOXB9 contributes to proliferation and protects against apoptosis in HL cells potentially via interacting with cell cycle regulators BTG1/2 and/or GEMININ.

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