Functional Analysis of Aberrantly Spliced Caspase8 Variants in Adult T-Cell Leukemia Cells

The genetic and molecular alterations responsible for leukemogenesis and progression of HTLV-infected adult T-cell leukemia (ATL) have not been fully clarified. Previously, we reported that various genes are not only overexpressed but also abnormally spliced in ATL cells. Here, we identified various CASP8 transcript variants in PBMCs from a smoldering-type ATL patient, which encode aberrant truncated caspase 8 (Casp8) isoforms. Among those, we focus on the three transcript variants, CASP8L (including the first 136 bp of the intron 8 between exon 8 and exon 9), CASP8-ΔE4 (without the exon 4), and CASP8-ΔE7 (without the exon 7), because they encode isoforms, Casp8L, Casp8-ΔE4, and Casp8-ΔE7, respectively, without the C-terminal catalytic domains. In this study, we conducted in vitro characterization and functional analysis of those mutant Casp8 isoforms to clarify their changed functions compared with the wild-type (WT)-Casp8. We demonstrated that these abnormal Casp8 isoforms showed lower ability to induce apoptosis than WT-Casp8 due to their dominant-negative interactions with WT-Casp8, which impair WT-Casp8 homodimerization that is essential for induction of apoptosis. Moreover, Casp8L and Casp8-ΔE7, which have only two death-effector domains, significantly activated NFκB by forming filament-like structures, which probably function as scaffolds for the IKK complex formation. In view of increasing levels of these abnormal CASP8 transcripts in primary PBMCs from HTLV-1 carriers and patients with ATL, we propose a possibility that overexpression of those Casp8 mutants, with lower proapoptotic activities and higher NFκB-activating functions than WT-Casp8, may be one of the molecular abnormalities causing malignant transformation and growth of ATL cells. IMPLICATIONS: We describe naturally occurring CASP8 transcription variants in PBMCs from patients with ATL, which encode truncated Casp8-mutant isoforms with lower proapoptotic activities and higher NFκB-activating functions compared with WT-Casp8.