Iron Regulatory Protein/Iron Responsive Element (IRP/IRE) system: associated diseases and new target mRNAs (PPP1R1B)

Abstract

Iron is a biometal involved in many physiological processes essential for life. Regulation of both systemic and cellular iron homeostasis is crucial for health. The IRP/IRE post-transcriptional regulatory system is in charge to control iron uptake, utilization, storage and export at cellular level. There are two iron regulatory proteins (IRP1 and IRP2) which recognize and bind conserved mRNA motifs named iron responsive elements (IREs). These IREs are located in the mRNA untranslated regions (UTR) of genes involved in iron metabolism. IRP/IRE binding is produced under cellular iron depletion conditions, modifying the expression of several proteins related with cellular iron acquisition, mobilization and storage. Depending on the location of the IREs in the target mRNA, the binding of the IRPs regulates differentially its expression. When IREs are present in the 3' UTR their binding to the IRPs stabilize the mRNA, protecting it front degradation and increasing its translation. On the other hand, the translation is inhibited when IRPs bind IREs located in the 5’ UTR. IRP/IRE system missregulation leads to human diseases denoting the importance of this post-transcriptional gene regulation. As part of my thesis (objective 1) we describe new cases of patients with Hereditary Hyperferritinemia Cataract Syndrome (HHCS) with mutations in the FTL IRE (Paris1 c.-160A>G and Madrid/Philadelphia c.-167G>T mutations) and we did an exhaustive revision of all literature reported cases. A genome-wide study made by our group, in order to characterize all the mRNAs that interact with the IRPs, identified 263 mRNAs that are potential IRP-target genes (44 mRNAs bind both IRPs, 102 bind specifically IRP1 and 117 bind specifically IRP2). The objective 3 of my thesis is focused in the characterization of one of these novel IRP-target mRNAs: PPP1R1B (DARPP-32 protein). DARPP-32 is a dopaminergic neurotransmission integrator which its main function is to inhibit protein phosphatase 1 (PP1). We identified a functional 5’IRE in mouse Ppp1r1b mRNA that binds in vitro with IRP1. Further, DARPP-32 protein expression is downregulated in two human cell lines under iron deprivation by an iron chelator and upregulated with an iron source, in agreement with an IRP/IRE regulation by a 5’IRE. Finally, in my thesis objective 2, we also contributed to the characterization of a Slc11a2 IRE knock-in mouse model to dissect the Dmt1 3’ IRE function in iron homeostasis.