FBXW7β isoform drives transcriptional activation of a proinflammatory TNF cluster in normal and malignant pro-B cells

Abstract

Non-canonical exon usage plays many important roles in cellular phenotypes, but its contribution to human B-cell development remains sketchily understood. To fill this gap, we collected various B-cell fractions from bone marrow and tonsil donors, performed RNA-seq, and examined transcript variants. We identified 150 genes that harbor local splicing variations in all pairwise comparisons. One of them encodes FBXW7, an E3 ubiquitin ligase implicated as a cancer driver in several blood cancers. Surprisingly, we discovered that in normal human pro-B cells, the predominant transcript utilized an alternative first exon to produce the poorly characterized FBXW7{beta} isoform, previously thought to be restricted to neural tissues. The FBXW7{beta} transcript was also abundant in cell lines and primary samples of pediatric B-cell acute lymphoblastic leukemia (B-ALL), which originates in the bone marrow. When overexpressed in a heterologous cell system, this transcript yielded the expected protein product, as judged by anti-FLAG immunoblotting and mass spectrometry. Furthermore, in REH B-ALL cells, FBXW7{beta} mRNA was the only FBXW7 isoform enriched in the polyribosome fraction. To shed light on possible functions of FBXW7{beta}, we utilized gain- and loss-of-function approaches and identified an FBXW7{beta}-dependent inflammatory gene signature, apparent in a subset of B-ALL with high FBXW7{beta} expression. This signature contained several members of the TNF superfamily, including those comprising the HLA Class III cluster (LTB, LST1, NCR3, LTA, and NFKBIL1). Our findings suggest that FBXW7{beta} expression drives proinflammatory responses, which could contribute to normal B-cell development, leukemogenesis and responses to anti-cancer therapies. Key pointsO_LIPreviously thought to be restricted to neural tissues, FBXW7{beta} is the predominant FBXW7 isoform in normal and malignant human pro-B cells. C_LIO_LIFBXW7{beta} promotes transcriptional activation of a proinflammatory gene cluster that contains TNF superfamily members. C_LI