Biochemische en functionele karakterisatie van ziekte-geassocieerde veranderingen in Proteïne Fosfatase 2A

We report inherited dysregulation of protein phosphatase 2A activity as a novel cause of intellectual disability (ID). De novo missense mutations in 2 PP2A subunit genes (PPP2R1A and PPP2R5D) were identified in 16 individuals with ID of unknown etiology. Mutant B’δ was A and C binding–deficient, while mutant Aα subunits bound B’δ well but were unable to bind C or bound a catalytically impaired C, suggesting a dominant-negative effect where mutant subunits hinder dephosphorylation of B’δ-anchored substrates. The same dominant-negative mechanism caused increased cancer cell growth in high grade uterine cancers bearing somatic PPP2R1A missense mutations, some of which were identical to all the Aα mutations found in ID. PPP2R1A mutation represents one of only a few genomic alterations occurring with high frequency in serous endometrial carcinoma and carcinosarcoma, typically aggressive uterine cancers. This thesis reports on the biochemical and functional consequences of eleven recurrent mutations clustering into Aα HEAT-repeats 5 and 7. Besides the predicted loss-of-function effects on formation of some PP2A holoenzymes, we confirm the dominant-negative effects of these mutations in the cancer context and this on specific, tumor suppressive PP2A complexes. Dominant-negative Aα mutants retain binding to specific B-type subunits, but form substrate-trapping complexes with impaired phosphatase activity due to increased incorporation of a cellular PP2A inhibitor, TIPRL1. In addition, we reveal gain-of-binding of the Aα mutants to the B”’/striatin subunits of PP2A, which we suspect also contributes to the dominant behavior of the mutants. Accordingly, ectopic expression of these mutants in endometrial carcinoma cells increases anchorage-independent growth and tumor formation in vivo, and causes hyperphosphorylation of several oncogenic PP2A substrates in the Akt and mTOR signaling pathways. Although identical Aα mutations are found in both pathologies, a major cancer risk for the ID patients seems unlikely. Together, our data not only reveal how mutations affect PP2A assembly and function, oncogenic signaling and ID disease pathogenesis, they also provide a basis for improved design of targeted therapies.nrpages: 177status: publishe