Abstract 1506: Characterization of fibronectin type III domain containing 3B (FNDC3B) as a novel fusion partner of retinoic acid receptor alpha in granulocytic differentiation

Acute promyelocytic leukemia (APL) is characterized by the promyelocytic leukemia-retinoic acid receptor alpha (PML-RARA) fusion. Recently, we identified fibronectin type III domain containing 3B (FNDC3B) as a novel RARA fusion partner resulting from the t(3;17)(q26;q21) translocation in a variant APL patient. Unlike other RARA fusion partners, FNDC3B appears as the only partner involved in granulocytic differentiation but the molecular mechanisms underlying its actions remain to be characterized. FNDC3B mRNA expression in normal and malignant hematopoietic cells was determined by quantitative RT-PCR. Transcriptional control of FNDC3B was studied by reporter gene assays, site-directed mutagenesis and chromatin immunoprecipitation (ChIP) assays. FNDC3B-regulated pathways were identified by transcriptome analyses. Using BloodSpot, we observed that FNDC3B expression was progressively up-regulated as human hematopoietic stem cells (HSCs) differentiate along the granulocytic lineage, with the highest level in terminally differentiated polymorphonuclear cells. Concordantly, we demonstrated lower expression of FNDC3B in human cord blood CD34 + HSCs than HL-60 promyelocytes. The specific involvement of FNDC3B in granulocytic differentiation was further suggested by higher FNDC3B expression and promoter activity in human promyelocytic than monocytic cell lines. Mutational analyses indicated that two putative CCAAT-enhancer binding protein epsilon (CEBPE) and one Sp1 binding sites were critical for FNDC3B promoter activity in HL-60 cells. The specific association of CEBPE and Sp1 with the FNDC3B promoter could be confirmed by ChIP assays. Knockdown of CEBPE in HL60 cells by small interfering RNA (siRNA) reduced FNDC3B expression and promoter activity. Furthermore, we observed a strong positive correlation between CEBPE and FNDC3B expression in the Cancer Genome Atlas (TCGA)-acute myeloid leukemia (AML) dataset (n=173, r=0.66, P Citation Format: Zhe Wand, Chi Keung Cheng, Terry Wong, Kin Mang Lau, Thomas S.k. Wan, Kam Tong Leung, Margaret Heung-Ling Ng. Characterization of fibronectin type III domain containing 3B (FNDC3B) as a novel fusion partner of retinoic acid receptor alpha in granulocytic differentiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1506.