Development of the First Transgenic Animal Model for IRF4-Induced Lymphoid Malignancy
Authors: Amanda Stella , Wong Regina , Tan Tze King , Ali Muhammad Zulfaqar , Li Yan , Gong Zhi Yuan , Iida Shinsuke , Sanda Takaomi 
Published: November 2019

Introduction
The transcription factor IRF4, a member of Interferon Regulatory Factor (IRF) family, is a critical regulator for the production of functional B and T lymphocytes. The dysregulation of IRF4 expression has been frequently implicated in various mature B- and T-lymphoid malignancies such as multiple myeloma and peripheral T-cell lymphomas in which IRF4 acts as an oncogene. However, in vivo model systems to investigate the oncogenic property of IRF4 has not been established. One of common downstream targets of IRF4 across different cancers is the oncogene, MYC, which promotes cell proliferation. Other downstream targets or collaborating factors of IRF4 in these malignancies have not been fully elucidated. Here, we established the first transgenic animal model for IRF4-induced lymphoma, which displays multiple spectrum of tumors providing an in vivo platform to study IRF4 oncogenicity.
Methods and Results
To identify the specific lineages and stages of the hematopoietic cells that can be transformed by IRF4, we utilized the zebrafish lck promoter, which is active both in T- and B-lymphocytes. We overexpressed human IRF4 gene in zebrafish under this promoter along with a fluorescent marker gene. We confirmed successful expression of both IRF4 and marker genes in the lymphocyte population. Strikingly, multiple F0 transgenic zebrafish showed an expansion of fluorescent signals arising from the thymus by 5 months, while control fish showed thymic involution after 4 months. Approximately 20% of F1 animals developed lymphoma within 5 months and progressed to leukemia by 8 months. Histopathological examination revealed massive infiltration of abnormal lymphocytes into skin, spinal cord, muscles, and liver, which resembles clinical and pathological features observed in several types of human mature T-cell neoplasms. Interestingly, the tumor onset and progression were significantly accelerated when crossed with p53e7/e7-mutant fish, which possesses transactivation-dead p53 variant. This reveals the synergy between IRF4 expression and p53 loss of function, which have also been observed in several types of human lymphomas. Notably, analysis of gene expression profiles using the single-cell RNA-sequencing platform revealed a simultaneous development of both B- and T-cell tumors, which consist of multiple clones at the early stage of tumorigenesis. T-cell-derived tumors became dominant at the late stage of tumorigenesis. Importantly, the expressions of mycb, the zebrafish orthologue of human MYC, as well as of other lymphoid transcription factors were highly upregulated in those tumors in parallel with IRF4 expression.
Conclusion
Our study demonstrated that IRF4 serves as a driver oncogene in the development of T- and B-cell malignancies. IRF4 accelerates the tumor progression by taking advantage of the impaired function of p53, as demonstrated by massive infiltration into distal organs recapitulating human lymphomas. Taken together, our zebrafish IRF4 model provides a very powerful platform to investigate the plausible mechanisms and pathways through which IRF4 exert its oncogenic property in lymphomagenesis.

Disclosures
Iida: Teijin Pharma: Research Funding; Astellas: Research Funding; Gilead: Research Funding; Sanofi: Research Funding; MSD: Research Funding; Abbvie: Research Funding; Kyowa Kirin: Research Funding; Chugai: Research Funding; Novartis: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding; Daichi Sankyo: Honoraria, Research Funding; Takeda: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Janssen: Honoraria, Research Funding.