Haploinsufficiency of the Insulin Receptor in the Presence of a Splice-Site Mutation in Ppp2r2a Results in a Novel Digenic Mouse Model of Type 2 Diabetes.

Insulin resistance in mice typically does not manifest as diabetes due to multiple compensatory mechanisms. Here we present a novel digenic model of type 2 diabetes in mice heterozygous for a null allele of the Insulin Receptor and an N -ethyl- N -nitrosourea induced alternative splice mutation in the regulatory protein phosphatase 2A (PP2A) subunit PPP2R2A. Inheritance of either allele independently results in insulin resistance but not overt diabetes. Doubly heterozygous mice exhibit progressive hyperglycaemia, hyperinsulinaemia and impaired glucose tolerance from 12 weeks of age without significant increase in bodyweight. Alternative splicing of Ppp2r2a decreased PPP2R2A protein levels. This reduction in PPP2R2A containing PP2A phosphatase holoenzyme was associated with decreased serine/threonine protein kinase AKT protein levels. Ultimately, reduced insulin stimulated phosphorylated AKT levels were observed, a result that was confirmed in Hepa1-6, C2C12 and differentiated 3T3-L1 cells knocked down using Ppp2r2a siRNAs. Altered AKT signaling and expression of gluconeogenic genes in the fed state contributed to an insulin resistance and hyperglycaemia phenotype. This model demonstrates how genetic changes with individually small phenotypic effects can interact to cause diabetes and how expression difference hypomorphic alleles of PPP2R2A and potentially other regulatory proteins have deleterious effects and may therefore be relevant in determining diabetes risk.