(W0930-03-18) Aberrant Splicing of PI3Kδ Promotes Drug Resistance in Hematological Malignancies

Purpose: The PI3K/Akt/mTOR pathway is frequently hyperactivated in cancers and is associated with poor prognosis and therapeutic resistance¹. PI3Kδ-S, a novel splice isoform of PI3Kδ (a PI3K isoform highly associated with leukocytes) has been linked to tumor aggressiveness and drug resistance in African American prostate cancer^2–4. However, the role of PI3Kδ-S in blood cancers has not been explored. Lymphoma and leukemia account for 10% of annual cancer diagnoses, and 40% of childhood and adolescent cases in the United States⁵. This study intends to explore the expression and impact of PI3Kδ-S in lymphoma and leukemia. The hypothesis of this study is that increased PI3Kδ-S expression correlates to drug resistance.

Methods: Expression of PI3Kδ-S and full-length PI3Kδ-L was quantified in lymphoma and leukemia cell lines (JeKo-1, Ramos (Ra1), SU-DHL-5, and Hs 505.T) using RT-PCR and immunofluorescence. Drug sensitivity was assessed with MTT and apoptosis assays. Western blotting was used to evaluate the activation of PI3K/Akt/mTOR signaling. Patient-derived lymphoma and leukemia tissue samples were analyzed using RNAscope. To model acquired resistance, JeKo-1, SU-DHL-5, and Ramos (Ra1) were exposed to idelalisib for an extended period to generate cell lines resistant to idelalisib. The resistant cell lines were then characterized for their changes in PI3Kδ-S expression levels and pathway activation.

Results: PI3Kδ-S expression was significantly higher in Hs 505.T and Ramos (Ra1) compared to JeKo-1 and SU-DHL-5, which correlates with idelalisib resistance and PI3K/Akt/mTOR activation. Immunofluorescence illustrated nuclear localization of PI3Kδ-S in cell lines highly expressing PIK3CD-S.  The RNAscope state indicated that PI3Kδ-S was highly elevated in lymphoma/leukemia patient samples (n=127) compared to normal tissue (n=22) (p < 0.01). Compared to the parental wild-type cell lines, our RT-PCR results further revealed that a higher PIK3CD-S/PIK3CD-L ratio was achieved in the idelalisib resistant cells upon longer treatment of idelalisib.

Conclusion: PI3Kδ-S is enriched in drug-resistant lymphoma and leukemia cell lines, and malignant patient samples. A higher PIK3CD-S/PIK3CD-L expression ratio is associated with enhanced PI3K/Akt/mTOR signaling and resistance to idelalisib (summarized in Fig. 1 below). The findings support PI3Kδ-S as a potential biomarker for drug resistance and as a novel therapeutic target in hematological malignancies.

References: 1. Fruman, D. A. & Rommel, C. PI3K and cancer: lessons, challenges and opportunities. Nat Rev Drug Discov 13, 140–156 (2014).
2. Ha, S., Gujrati, H. & Wang, B. D. Aberrant PI3Kδ splice isoform as a potential biomarker and novel therapeutic target for endocrine cancers. Front Endocrinol (Lausanne) 14, (2023).
3. Wang, B. D. et al. Alternative splicing promotes tumour aggressiveness and drug resistance in African American prostate cancer. Nature Communications 2017 8:1 8, 1–15 (2017).
4. Ha, S. & Wang, B. D. Molecular Insight into Drug Resistance Mechanism Conferred by Aberrant PIK3CD Splice Variant in African American Prostate Cancer. Cancers (Basel) 15, 1337 (2023).
5. Siegel Mph, R. L., Giaquinto, A. N., Ahmedin, |, Dvm, J. & Siegel, R. L. Cancer statistics, 2024. CA Cancer J Clin 74, 12–49 (2024).

Acknowledgements: This work was supported by NIH/NIGMS grant 5SC1GM127256 (to B.-D.W.).


Fig.1 Molecular model of drug resistance mechanism driven by overexpression of aberrant PI3Kδ-S splice isoform.