The gene encoding the splicing factor SF2/ASF is a proto-oncogene

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Abstract

Alternative splicing modulates the expression of many oncogene and tumor-suppressor isoforms. We have tested whether some alternative splicing factors are involved in cancer. We found that the splicing factor SF2/ASF is upregulated in various human tumors, in part due to amplification of its gene, SFRS1. Moreover, slight overexpression of SF2/ASF is sufficient to transform immortal rodent fibroblasts, which form sarcomas in nude mice. We further show that SF2/ASF controls alternative splicing of the tumor suppressor BIN1 and the kinases MNK2 and S6K1. The resulting BIN1 isoforms lack tumor-suppressor activity; an isoform of MNK2 promotes MAP kinase–independent eIF4E phosphorylation; and an unusual oncogenic isoform of S6K1 recapitulates the transforming activity of SF2/ASF. Knockdown of either SF2/ASF or isoform-2 of S6K1 is sufficient to reverse transformation caused by the overexpression of SF2/ASF in vitro and in vivo. Thus, SF2/ASF can act as an oncoprotein and is a potential target for cancer therapy.

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Figure 1: Upregulation and amplification of SF2/ASF in human tumors.
Figure 2: SF2/ASF transforms immortal cells and is tumorigenic in nude mice.
Figure 3: SF2/ASF overexpression protects E1A-sensitized MEF cells against apoptosis and enhances the proliferation of Ras-transformed cells.
Figure 4: Specific alternative splicing changes induced by overexpression of splicing factors or knockdown of SF2/ASF.
Figure 5: SF2/ASF promotes expression of an oncogenic isoform of S6K1 and induces eIF4E phosphorylation.
Figure 6: Knockdown of SF2/ASF reverses transformation of NCI-H460 cells and SF2/ASF-overexpressing NIH 3T3 cells.
Figure 7: Knockdown of S6K1 isoform-2 blocks SF2/ASF-mediated transformation.
Figure 8: A model for transformation by SF2/ASF.

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Acknowledgements

We thank G. Dreyfuss (University of Pennsylvania) for antibodies to hnRNP A1 and A2; D. Pant for advice on statistical tests; A. Rosenberg, M. Nolan and S. Muthuswamy (Cold Spring Harbor Laboratory) for the breast-cancer cell lines; and M. Hemann, L. Zender, R.-M. Xu, A. Neuwald, S. Powers and R. Lucito for helpful discussions. This work was supported by grant CA13106 from the US National Cancer Institute.

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Correspondence to Adrian R Krainer.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Fig. 1

Amplification and overexpression of SF2/ASF in cancer cell lines, and correlation with alternative splicing of its targets. (PDF 667 kb)

Supplementary Fig. 2

Analysis of transduced splicing factor levels in mouse, rat and human cell lines. (PDF 652 kb)

Supplementary Fig. 3

Alternative splicing of mouse Rps6kb1 and primary structure of the two S6K1 isoforms. (PDF 29 kb)

Supplementary Fig. 4

Correlation between SF2/ASF expression and S6K1 isoform ratios in 50 lung tumors. (PDF 84 kb)

Supplementary Fig. 5

Knockdown of isoform-2 of S6K1 reverses NCI-H460 cell transformation. (PDF 355 kb)

Supplementary Table 1

Alternative splicing changes induced by overexpression or knockdown of specific splicing factors. (PDF 116 kb)

Supplementary Table 2

Primers, probes and shRNA templates. (PDF 133 kb)

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Karni, R., de Stanchina, E., Lowe, S. et al. The gene encoding the splicing factor SF2/ASF is a proto-oncogene. Nat Struct Mol Biol 14, 185–193 (2007) doi:10.1038/nsmb1209

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