A central problem in human biology remains the discovery of causal molecular links between mutations identified in genome-wide association studies (GWAS) and their corresponding disease traits. This challenge is magnified for variants residing in non-coding regions of the genome. Single-nucleotide polymorphisms (SNPs) in the 5ʹ untranslated region (5ʹ-UTR) of the ferritin light chain (FTL) gene that cause hyperferritinemia are reported to disrupt translation repression by altering iron regulatory protein (IRP) interactions with the FTL mRNA 5ʹ-UTR. Here, we show that human eukaryotic translation initiation factor 3 (eIF3) acts as a distinct repressor of FTL mRNA translation, and eIF3-mediated FTL repression is disrupted by a subset of SNPs in FTL that cause hyperferritinemia. These results identify a direct role for eIF3-mediated translational control in a specific human disease.

Polypyrimidine tract-binding proteins (PTBPs) are RNA binding proteins that regulate a number of posttranscriptional events. Human PTBP1 transits between the nucleus and cytoplasm and is thought to regulate RNA processes in both. However, information about PTBP1 mRNA isoforms and regulation of PTPB1 expression remains incomplete. Here we mapped the major PTBP1 mRNA isoforms in HEK293T cells and identified alternative 5 ′ ′ ′ ′ ′ and 3 ′ ′ ′ ′ ′ untranslated regions (5 ′ ′ ′ ′ ′-UTRs, 3 ′ ′ ′ ′ ′-UTRs), as well as alternative splicing patterns in the protein coding region. We also assessed how the observed PTBP1 mRNA isoforms contribute to PTBP1 expression in different phases of the cell cycle. Previously, PTBP1 mRNAs were shown to crosslink to eukaryotic translation initiation factor 3 (eIF3). We find that eIF3 binds differently to each PTBP1 mRNA isoform in a cell cycle dependent manner. We also observe a strong correlation between eIF3 binding to PTBP1 mRNAs and repression of PTBP1 levels during the S phase of the cell cycle. Our results provide evidence of translational regulation of PTBP1 protein levels during the cell cycle, which may affect downstream regulation of alternative splicing and translation mediated by PTBP1 protein isoforms.

16Iron is essential to life, but excess iron is detrimental as it catalyzes reactive hydroxyl radical 17 production. To prevent iron toxicity, intracellular iron homeostasis is regulated by ferritin, a 18 protein complex composed of a mixture of two subunits: ferritin light chain (FTL) and ferritin 19 heavy chain (FTH). Ferritin expression is regulated post-transcriptionally by the iron response 20 proteins (IRPs), which bind an RNA hairpin -the iron responsive element (IRE) -located in 21 the FTL and FTH mRNA 5ʹ-untranslated region (5ʹ-UTR). Here, we show FTL translation is also 22 regulated in an IRP-independent manner by eukaryotic translation initiation factor 3 (eIF3). We 23 find that eIF3 represses FTL translation by interacting with a region of the 5ʹ-UTR immediately 24 downstream of the IRE. Furthermore, we find that loss of eIF3-mediated regulation of FTL 25 translation likely contributes to a subset of clinically-observed hyperferritinemia cases. 26 Introduction 27Iron is essential for a spectrum of metabolic pathways and cellular growth. However, if 28 not properly managed, excess iron catalyzes the production of harmful radicals that lead to lipid 29 peroxidation, protein and DNA damage, and ultimately cell death (Palmer, Roberts, and Bero 30 1994). To safeguard against these damaging effects, cells ubiquitously express ferritin, an iron-31 sequestering protein complex. Ferritin is composed of a dynamic mixture of 24 individual 32 subunits of two types, the ferritin heavy chain (FTH) and the ferritin light chain (FTL), which 33 assemble into a hollow shell that encapsulates and stores up to 4,500 Fe 3+ atoms (Harrison and 34 Arosio 1996) (Anderson and McLaren 2012). Though the two subunits that make up this 35 complex are structurally similar, they are not functionally interchangeable (Harrison and Arosio 36 1996). The FTH subunit catalyzes the oxidation of Fe 2+ to Fe 3+ , while the FTL subunit lacks 37 catalytic activity and instead serves as a nucleation site for mineralization, increases complex 38All rights reserved. No reuse allowed without permission.was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. ribosomal subunit recruitment and consequently inhibits translation. This inhibition prevents the 50 presence of excess ferritin and ensures sufficient levels of accessible iron. In response to an 51 increase in the labile iron pool (LIP) or under high iron conditions, IRP1 is loaded with an iron 52 sulfur cluster produced by mitochondria that prevents IRE binding, and IRP2 is targeted for 53 proteasomal degradation (Fig. 1A,B) (Wilkinson and Pantopoulos 2014). These processes 54 relieve the transcript from IRP/IRE generated steric hindrances, allowing for translation to 55 proceed and for the excess iron to be deposited into newly formed ferritin complexes. IRP-IRE 56 interactions are known to be disrupted in clinically relevant cases of hyperferritinemia, in which 57 single nucleotide mutations throughout the IRE ...

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