Recurrent PPP2R1A Mutations in Uterine Cancer Act through a Dominant-Negative Mechanism to Promote Malignant Cell Growth

Abstract

Somatic missense mutations in the Ser/Thr protein phosphatase 2A (PP2A) Aα scaffold subunit gene PPP2R1A are among the few genomic alterations that occur frequently in serous endometrial carcinoma (EC) and carcinosarcoma, two clinically aggressive subtypes of uterine cancer with few therapeutic options. Previous studies reported that cancer-associated Aα mutants exhibit defects in binding to other PP2A subunits and contribute to cancer development by a mechanism of haploinsufficiency. Here we report on the functional significance of the most recurrent PPP2R1A mutations in human EC, which cluster in Aα HEAT repeats 5 and 7. Beyond predicted loss-of-function effects on the formation of a subset of PP2A holoenzymes, we discovered that Aα mutants behave in a dominant-negative manner due to gain-of-function interactions with the PP2A inhibitor TIPRL1. Dominant-negative Aα mutants retain binding to specific subunits of the B56/B' family and form substrate trapping complexes with impaired phosphatase activity via increased recruitment of TIPRL1. Accordingly, overexpression of the Aα mutants in EC cells harboring wild-type PPP2R1A increased anchorage-independent growth and tumor formation, and triggered hyperphosphorylation of oncogenic PP2A-B56/B' substrates in the GSK3β, Akt, and mTOR/p70S6K signaling pathways. TIPRL1 silencing restored GSK3β phosphorylation and rescued the EC cell growth advantage. Our results reveal how PPP2R1A mutations affect PP2A function and oncogenic signaling, illuminating the genetic basis for serous EC development and its potential control by rationally targeted therapies. Cancer Res; 76(19); 5719-31. ©2016 AACR.

STRIPAK directs PP2A activity toward MAP4K4 to promote oncogenic transformation

“…S3A). In addition, it was recently shown that disruption of interactions between PP2A core enzyme and canonical B subunits by mutations in PP2A Aα (P179R, R18G) promotes PP2A interactions with members of the STRIPAK complex (Haesen et al 2016), reinforcing the notion that ST phenocopies the effect of cancer-associated PP2A mutations. More generally, these observations suggest that striatins act as key regulators of PP2A that impart substrate specificity.…”

Cancer driver mutations in endometriosis: Variations on the major theme of fibrogenesis

“…PPP2R1B is reported to be downregulated in cultured dermal fibroblasts from patients with systemic sclerosis, a fibrotic disorder . The reported PPP2R1A activating mutation (p.S256F) is shown recently in endometrial cancer cells to behave in a dominant‐negative manner due to gain‐of‐function interactions with the PP2A inhibitor TIPRL1, resulting in hyperphosphorylation of AKT, GSK3β, and mTOR signaling pathways . That is, the activating mutation leads to reduced PP2A activity and consequent increased AKT activity.…”

Targeted Therapies in Type II Endometrial Cancers: Too Little, but Not Too Late

“…Remarkably, based on structural studies, the same residues or neighbouring residues were predicted to be important for interaction with the B subunits [133,148]. However, experimental evidence for this hypothesis revealed a much more sophisticated image, in that mutations at these positions indeed resulted in binding deficiencies of Aα mutants with several regulatory B subunit types, but specifically preserved binding to others, most notably to B56δ and B56γ [149,150]. Some mutations, e.g., p.(P179R), also diminished binding to the catalytic C subunit [149].…”

“…However, experimental evidence for this hypothesis revealed a much more sophisticated image, in that mutations at these positions indeed resulted in binding deficiencies of Aα mutants with several regulatory B subunit types, but specifically preserved binding to others, most notably to B56δ and B56γ [149,150]. Some mutations, e.g., p.(P179R), also diminished binding to the catalytic C subunit [149]. Moreover, Haesen et al suggested a dominant negative mechanism for these mutants, as the PP2A trimers that could still be formed proved catalytically impaired through the increased recruitment of a cellular PP2A inhibitor, TIPRL1 [149].…”

“…Some mutations, e.g., p.(P179R), also diminished binding to the catalytic C subunit [149]. Moreover, Haesen et al suggested a dominant negative mechanism for these mutants, as the PP2A trimers that could still be formed proved catalytically impaired through the increased recruitment of a cellular PP2A inhibitor, TIPRL1 [149]. Accordingly, ectopic expression of the Aα mutants increased anchorage-independent cell growth in vitro, xenografted tumour growth in vivo, and resulted in hyperactivation of, again, the PI3K/Akt/mTOR pathway.…”