Functional characterization of a novel non-coding mutation \u201cGhent +49A\u2009>\u2009G\u201d in the iron-responsive element of L-ferritin causing hereditary hyperferritinaemia-cataract syndrome

Stijn Van De Sompele,Elfride De Baere,Jorge Couso,Audrey Meunier,Lucie Pécheux,Mayka Sanchez

doi:10.1038/s41598-017-18326-6

Abstract

Hereditary hyperferritinaemia-cataract syndrome (HHCS) is a rare disorder usually caused by heterozygous mutations in the iron-responsive element (IRE) in the 5′ untranslated region (5′UTR) of the L-ferritin gene (FTL), disturbing the binding of iron regulatory proteins (IRPs) and the post-transcriptional regulation of ferritin expression. Here, the proband of a consanguineous family displayed moderate bilateral cataracts and elevated serum ferritin in the absence of iron overload. The parents and siblings showed variable degrees of mild bilateral cataracts combined with elevated levels of circulating ferritin. Sequencing of FTL identified a novel 5′UTR mutation c.-151A > G, also named “Ghent +49A > G”. The zygosity of the mutation, occurring in homozygous and heterozygous state in the proband and other affected family members respectively, correlated well with severity of ophthalmological and hematological manifestations. The substitution is expected to impair the secondary structure of the upper IRE stem. Functional characterization of +49A > G by electrophoretic mobility shift assays demonstrated a reduced binding affinity for IRP1 compared to the wild-type IRE of FTL. Overall, we have expanded the repertoire of deleterious biallelic FTL IRE mutations in HHCS with this novel +49A > G mutation, the zygosity of which correlated well with the disease expression.

Highlights

Hereditary hyperferritinaemia-cataract syndrome (HHCS) (OMIM 600886) is a rare autosomal dominant disease, characterized by the combination of elevated serum ferritin in the absence of iron overload or other hematological abnormalities and progressive cataracts of a highly distinctive morphology[1]
The expression of the ferritin subunits is modulated by intracellular iron levels through a tightly regulated negative feedback system based on the interaction between cytoplasmic iron-sensing mRNA-binding proteins, the iron regulatory proteins (IRP1 and IRP2), and a non-coding cis-acting stem-loop structure located in the 5′ untranslated region (5′UTR) of both L- and H-ferritin mRNA, the iron-responsive elements (IREs)[6]
When intracellular iron levels increase the IRP/IRE binding affinity decreases due to conformational changes of IRP1 and the proteasomal degradation of IRP2, resulting in the initiation of ferritin mRNA translation and the consequent upregulation of ferritin synthesis[8,9]

Summary

Introduction

Hereditary hyperferritinaemia-cataract syndrome (HHCS) (OMIM 600886) is a rare autosomal dominant disease (estimated prevalence of 1 in 200,000), characterized by the combination of elevated serum ferritin in the absence of iron overload or other hematological abnormalities and progressive cataracts of a highly distinctive morphology[1]. The expression of the ferritin subunits is modulated by intracellular iron levels through a tightly regulated negative feedback system based on the interaction between cytoplasmic iron-sensing mRNA-binding proteins, the iron regulatory proteins (IRP1 and IRP2), and a non-coding cis-acting stem-loop structure located in the 5′ untranslated region (5′UTR) of both L- and H-ferritin mRNA, the iron-responsive elements (IREs)[6]. These IREs comprise four important structural parts: the hexanucleotide apical loop, the upper and lower stem consisting of palindromic sequences and the cytosine bulge, an unpaired cytosine residue (C-bulge) located in between the upper and lower stem[7]. Our aim was to molecularly explain the variable expressivity of hyperferritinaemia and bilateral cataracts in different members of a consanguineous family with HHCS, and to functionally characterize a novel mutation in the 5′UTR of FTL

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Conclusion