Hypothesis: The length-dependent vulnerability of short exons to splicing dysregulation in cancer isn't random—short exons tend to encode phospho-regulatory protein domains. My proposal is that short exons in PP2A regulatory subunit genes (especially PPP2R5A/B/C and PPP2R2A family members) carry disproportionately high densities of serine/threonine residues targeted by oncogenic kinases. When spliceosome stress hits—whether from mutations or drugs—these short exons get misspliced more often, generating dominant-negative isoforms that disrupt PP2A holoenzyme assembly and push the phosphorylation balance toward oncogenic signaling.

Mechanistic basis: SF3B1 mutations clearly promote decay of PPP2R5A transcripts, leading to MYC S62 and BCL2 S70 phosphorylation—this establishes PP2A as a key tumor-suppressive phosphatase in spliceosome-driven malignancy [1]. But the mechanism probably goes beyond simple transcript decay. The observation that aberrant splicing shows length-dependent vulnerability—short exons are disproportionately affected across cancer types [2]—points to a structural explanation. Short exons evolutionarily constrain protein interaction interfaces and regulatory motifs. PP2A regulatory subunits, which determine substrate specificity, are enriched for short alternatively spliced exons encoding B56-binding motifs and phosphorylation sites. The mutual exclusivity of SF3B1, SRSF2, and U2AF1 mutations in myeloid malignancies [3] likely reflects convergent targeting of this vulnerable node: different spliceosome perturbations produce similar depletion of functional PP2A regulatory isoforms through distinct molecular mechanisms—enhanced decay, altered splicing efficiency, or shifted splice site selection.

Testable predictions:

1. Short exons in PP2A regulatory genes will show significantly higher phospho-serine/threonine residue density compared to short exons in the broader transcriptome
2. SF3B1, SRSF2, or U2AF1 mutant cells will exhibit coordinated missplicing of multiple PP2A regulatory subunit genes, not just PPP2R5A
3. Exon-skipping events affecting PP2A regulatory gene short exons will produce dominant-negative protein isoforms that impair PP2A holoenzyme catalytic activity
4. Restoring PP2A activity through small-molecule activators will preferentially suppress tumor growth in spliceosome-mutant cells exhibiting short exon missplicing, compared to cells with wild-type spliceosomes

This hypothesis ties together the length-dependent vulnerability observation [2] with the PP2A-SF3B1 oncogenic axis [1] into a unified mechanistic framework with therapeutic implications.

Falsifiability: If short exons in PP2A regulatory genes don't show elevated phospho-residue density, or if missplicing of these exons doesn't impair PP2A function in functional assays, the hypothesis would fall apart.

References: [1] https://aacrjournals.org/cancerdiscovery/article/10/6/806/2746/Mutations-in-the-RNA-Splicing-Factor-SF3B1-Promote [2] https://www.science.org/doi/10.1126/sciadv.abn9232 [3] https://pubmed.ncbi.nlm.nih.gov/23335386/