3394

IRF8

H-ICSBP|ICSBP|ICSBP1|IMD32A|IMD32B|IRF-8;interferon regulatory factor 8;IRF8;interferon regulatory factor 8

interferon consensus sequence binding protein 1

16q24.1

protein-coding

INTRODUCTION: The NUP98-TOP1 fusion gene is one of 18 distinct translocations identified in acute myeloid leukemia involving the N-terminal portion of the nucleoporin NUP98. We previously reported that expression of NUP98-TOP in murine bone marrow induces a lethal, transplantable leukemia. However, the long latency suggests the in vivo acquisition of additional mutations and/or time required for clonal outgrowth of rare transformed cells arising from the collaboration of NUP98-TOP1 and a cooperating event. The aim of this study was to test whether retroviral insertional mutagenesis contributes to disease onset and whether integration site analysis can identify collaborating genes. METHODS: The genomic sites of retroviral integration in NUP98-TOP1-induced leukemic mice were analyzed. This screen identified a proviral integration that disrupts expression of the Interferon consensus sequence binding protein (ICSBP) tumor suppressor gene. Intriguingly, an ICSBP deficiency induces a chronic myeloid leukemia-like disease in mice and its reduced expression has been observed in several human leukemias. To ascertain whether an ISCBP deficiency collaborates with NUP98-TOP1 in leukemogenesis, we expressed NUP98-TOP1 in ICSBP(-/-) bone marrow. RESULTS: The in vivo myeloproliferation induced by NUP98-TOP1 was markedly exaggerated with the ICSBP(-/-) deficiency. Moreover, NUP98-TOP1/ICSBP(-/-) mice had a reduced survival compared with NUP98-TOP1/ICSBP(+/+) mice. CONCLUSION: These results reveal the novel finding of collaboration between the ICSBP tumor suppressor gene and NUP98-TOP1 in leukemogenesis. Moreover they further illustrate the power of retroviral integration site analysis for identifying novel cooperating tumor suppressor genes.

16q24 is frequently deleted in multiple tumors including cancers of nasopharynx, esophagus, breast, prostate and liver. By array comparative genomic hybridization (aCGH), we refined a 16q24 hemizygous deletion in nasopharyngeal carcinoma (NPC) cell lines. Semi-quantitative RT-PCR analysis revealed interferon regulatory factor 8 (IRF8) as the only downregulated gene within this deletion. IRF8 belongs to a family of interferon (IFN) regulatory factors that modulate various important physiologic processes including host defense, cell growth and differentiation and immune regulation. In contrast to the broad expression of IRF8 in normal adult and fetal tissues, transcriptional silencing and promoter methylation of IRF8 were frequently detected in multiple carcinoma (except for hepatocellular) cell lines (100% in NPC, 88% in esophageal and 18-78% in other carcinoma cell lines) and in a large collection of primary carcinomas (78% in NPC, 36-71% in other carcinomas). Methylation of the IRF8 promoter led to the disruption of its response to IFN-gamma stimulation. Pharmacological and genetic demethylation could restore IRF8 expression, indicating a direct epigenetic mechanism. Ectopic expression of IRF8 in tumor cells lacking its expression strongly inhibited their clonogenicity, confirming its tumor suppressor function. Thus, IRF8 was identified as a functional tumor suppressor, which is frequently silenced by epigenetic mechanism in multiple carcinomas.