Lenalidomide promotes p53 degradation by inhibiting MDM2 auto-ubiquitination in myelodysplastic syndrome with chromosome 5q deletion

Allelic deletion of the RPS14 gene is a key effector of the hypoplastic anemia in patients with myelodysplastic syndrome (MDS) and chromosome 5q deletion (del(5q)). Disruption of ribosome integrity liberates free ribosomal proteins to bind to and trigger degradation of mouse double minute 2 protein (MDM2), with consequent p53 transactivation. Herein we show that p53 is overexpressed in erythroid precursors of primary bone marrow del(5q) MDS specimens accompanied by reduced cellular MDM2. More importantly, we show that lenalidonnide (Len) acts to stabilize MDM2, thereby accelerating p53 degradation. Biochemical and molecular analyses showed that Len inhibits the haplodeficient protein phosphatase 2A catalytic domain alpha (PP2Ac alpha) phosphatase resulting in hyperphosphorylation of inhibitory serine-166 and serine-186 residues on MDM2, and displaces binding of RPS14 to suppress MDM2 autoubiquitination whereas PP2Ac alpha overexpression promotes drug resistance. Bone marrow specimens from del(5q) MDS patients resistant to Len overexpressed PP2Ac alpha accompanied by restored accumulation of p53 in erythroid precursors. Our findings indicate that Len restores MDM2 functionality in the 5q-syndrome to overcome p53 activation in response to nucleolar stress, and therefore may warrant investigation in other disorders of ribosomal biogenesis. Oncogene (2013) 32, 1110-1120; doi:10.1038/onc.2012.139; published online 23 April 2012