A case report of spontaneous mutation (C33 > U) in the iron-responsive element of l-ferritin causing hyperferritinemia-cataract syndrome

Abstract

The hereditary hyperferritinemia cataract syndrome (HHCS) is an autosomal dominant disorder characterized by juvenile-onset cataracts and elevated serum ferritin levels. It is caused by mutation in the iron response element (IRE) within the 5′UTR of l-ferritin gene. The mutation results in a loss of post-transcriptional negative feedback exerted by the interaction between iron regulatory proteins 1, 2 (IRP1 and IRP2) and IRE, which leads to uncontrolled expression of l-ferritin. In this paper, we describe the molecular pathogenesis of non-hereditary hyperferritinemia cataract syndrome (non-H-HCS) in a patient with typical HHCS ocular lens morphology and high ferritin levels without obvious family history. Initial sequencing of the full-length l-ferritin cloned from genomic DNA demonstrated a mutation (C33 > T) in the IRE of the affected patient but not in her unaffected family members. The mutation (C/T heterozygote) was also detected in cDNA derived from her blood mononuclear cells. Structure-prediction-modeling indicates that this mutation would significantly alter the secondary structure of the IRE, resulting in a loss of the interaction between IRP and IRE. By using IRP1/IRP2-human IgG1 Fc fusion proteins, we established a novel in vitro report system (modified ELISA) to verify impaired IRE/IRP binding. Both the C33 > U and A40G mutations (the first identified mutation for HHCS) showed a dramatically decreased binding to IRP1/IRP2 protein, compared to the normal IRE RNA. Surprisingly, a decrease in l-ferritin mRNA levels was observed in the affected patient compared to controls suggesting a mechanism of transcriptional negative feedback by high intracellular l-ferritin protein levels not described heretofore. Taken together, spontaneous mutation in the IRE of l-ferritin may cause non-H-HCS by the same mechanism as HHCS. In addition, under abnormal circumstances, the protein level of l-ferritin may be principally controlled by post-transcriptional regulation rather than the transcriptional regulation. The successful establishment of an ELISA report system provides an alternative method to evaluate precisely the interaction between protein and RNA.