Oncogenic Splicing of the Apoptosis-Stimulating Protein of p53-2 (ASPP2) Is an Initiating Step in Leukemogenesis Facilitating Acquisition of Structural Genomic Mutations

ASPP2 is an independent haploinsufficient tumor suppressor which initiates induction of apoptosis after cellular damage in a TP53-dependent manner. We recently reported the identification of two C-terminally truncated splice variants of ASPP2 with high frequency in acute leukemias. Tantalizingly, these variants lack the functionally relevant TP53 binding sites, suggesting a dominant-negative, oncogenic function. The most prevalent isoform lacking exon 17 (ASPP2k) is detected in up to 60% of acute leukemias. A cell sort for the CD34+ stem cell fraction confirmed ASPP2k-expression in the leukemia-initiating clone. Immunoblotting revealed translation of ASPP2k into a genuine protein isoform.

Generation of isoform-specific antibodies allows to discriminate ASPP2k from the wildtype isoforms. Interestingly, ASPP2k is the sole isoform expressed in some native leukemia samples. Other samples reveal a heterozygous expression pattern suggesting a dominant-negative function of the C-terminally truncated ASPP2k isoform.

Forced expression of ASPP2k using a HisMax plasmid vector in murine IL3-dependent pro-B Ba/F3 cells induced a more aggressive phenotype with mitotic failure and perturbed cellular proliferation compared to the empty vector controls. Of note, IL3 was successfully weaned in the ASPP2k cells - indicating that autoactivating genomic alterations must have occurred upholding cellular integrity and viability. Fractionated gamma irradiation accelerated this process. In contrast, the empty vector strains did not survive IL3 weaning. Chromosomal analyses of IL3-independent Ba/F3 ASPP2kcells revealed structural alterations including monosomies and additional marker chromosomes.

Furthermore, we demonstrate that ASPP2k is stress-inducible. Changing culture conditions (e.g. temperature) of cell lines or ex vivo native AML samples resulted in a dramatic increase of the ASPP2k isoform. Consecutive treatment of cells with daunorubicin lead to relative resistance of pre-stressed cells compared to their counterparts cultured under standard conditions. Vice versa, specific knock-down of ASPP2k resulted in a significant increase in induction of apoptosis upon chemotherapy compared to empty vector controls.

Our data demonstrate that ASPP2k plays a distinctive role as an anti-apoptotic regulator of the TP53 checkpoint rendering cells to a more aggressive phenotype as evidenced by proliferation and apoptosis rates - and facilitates acquisition of genomic mutations, a first initiating step in leukemogenesis.