Comprehensive Analysis of Homeobox Genes in Hodgkin Lymphoma Cell Lines Identified Dysregulated Expression of HOXB9 Mediated by Constitutive Active ERK5 Signalling Pathway and BMI1.

Within the human genome nearly 200 homeobox genes have been identified. These are functionally involved in distinct developmental processes during embryogenesis and in the adult. Many have been implicated in cancer mostly following ectopic expression. In this study, we analyzed homeobox gene expression in Hodgkin lymphoma (HL) cell lines using degenerate primers for RT-PCR and Affymetrix U133A expression arrays followed by extended RT-PCR analysis of candidate homeobox genes in additional hematopoietic cell lines. We identified upregulated HOXB9 expression in HL, as well as in anaplastic large cell lymphoma and mediastinal B-cell lymphoma cell lines. Analysis of HOXB9 regulation in HL cells revealed activation by E2F3A, and repression by BMI1, respectively. Constitutive ERK5 pathway activation was identified in all HL cell lines analyzed together with primary HL lymph nodes biopsies. Our data show that ERK5 regulates HOXB9 expression, probably mediated via BMI1 repression. Expression analysis of microRNA gene mir-196a1 which is located upstream of HOXB9 indicated coregulation together with HOXB9. Functional analysis of HOXB9 by knockdown or overexpression assays revealed its influence in both proliferation and apoptosis in HL cells. In summary we identified upregulated expression of HOXB9 in HL, and its regulation mediated by constitutively active ERK5 signalling, thus representing novel therapeutic targets in both HL and NHL subtypes.