Increased IRP1 activity in Friedreich ataxia.

In low-iron conditions, the cytosolic iron-regulatory protein IRP1 binds to iron-responsive elements (IREs) in mRNAs encoding iron-regulated proteins. In high-iron conditions, IRP1 incorporates an iron-sulfur cluster (ISC), which interferes with IRE binding and prevents intracellular iron accumulation. Here we demonstrate an incomplete shift of IRP1 to its ISC form in Friedreich ataxia (FRDA) fibroblasts, associated with decreased activities of ISC respiratory complexes. Our data suggest an impaired adaptive response to iron accumulation in FRDA cells.

Transferrin receptor hyperexpression in primary erythroblasts is lost on transformation by avian erythroblastosis virus.

In primary chicken erythroblasts (stem cell factor [SCF] erythroblasts), transferrin receptor (TfR) messenger RNA (mRNA) and protein were hyperexpressed as compared to nonerythroid chicken cell types. This erythroid-specific hyperexpression was abolished in transformed erythroblasts (HD3E22 cells) expressing the v-ErbA and v-ErbB oncogenes of avian erythroblastosis virus. TfR expression in HD3E22 cells could be modulated by changes in exogenous iron supply, whereas expression in SCF erythroblasts was not subject to iron regulation. Measurements of TfR mRNA half-life indicated that hyperexpression in SCF erythroblasts was due to a massive stabilization of transcripts even in the presence of high iron levels. Changes in mRNA binding activity of iron regulatory protein 1 (IRP1), the primary regulator of TfR mRNA stability in these cells, correlated well with TfR mRNA expression; IRP1 activity in HD3E22 cells and other nonerythroid cell types tested was iron dependent, whereas IRP1 activity in primary SCF erythroblasts could not be modulated by iron administration. Analysis of avian erythroblasts expressing v-ErbA alone indicated that v-ErbA was responsible for these transformation-specific alterations in the regulation of iron metabolism. In SCF erythroblasts high amounts of TfR were detected on the plasma membrane, but a large fraction was also located in early and late endosomal compartments, potentially concealing temporary iron stores from the IRP regulatory system. In contrast, TfR was almost exclusively located to the plasma membrane in HD3E22 cells. In summary, stabilization of TfR mRNA and redistribution of Fe-Tf/TfR complexes to late endosomal compartments may contribute to TfR hyperexpression in primary erythroblasts, effects that are lost on leukemic transformation.

Ferritin crystal cataracts in hereditary hyperferritinemia cataract syndrome.

PURPOSE: Hereditary hyperferritinemia cataract syndrome (HHCS) is a genetic disease defined by cataracts, hyperferritinemia, and ferritin light-chain (L-ferritin) gene mutations. HHCS was diagnosed in this study in one of the first families known to be affected in the United States, and the basis of lens opacities in HHCS was determined. METHODS: DNA amplification and sequencing of the human L-ferritin gene was used for mutation detection. RNA electrophoretic mobility shift analysis was performed to demonstrate functional consequences of a new mutation. Opacities were characterized by immunohistochemical and electron microscopic analyses of human HHCS lens aspirate. RESULTS: HHCS was diagnosed in five members of one family who had all three hallmark features: hyperferritinemia, a prominent cataract or history, and the finding of a novel mutation in the L-ferritin gene (C33T). This mutation interferes with function of the L-ferritin transcript in an RNA gel shift assay. Light-diffracting crystalline deposits were present in cataractous lenses from two affected family members but not in control lenses. Immunohistochemical analysis showed strong anti-L-ferritin reactivity in the crystalline deposits. Analysis of these deposits by transmission electron microscopy with fast Fourier transformation demonstrated macromolecular crystalline structure of the deposits. The data were consistent with a face-centered cubic crystal having a unit crystal cell size of 17 nm, both findings characteristic of ferritin crystals grown in vitro. CONCLUSIONS: HHCS cataract is due to numerous small opacities, predominantly in the lens cortex, that are light-diffracting ferritin crystals. Patients with HHCS may be recognized by a family history of cataracts and hyperferritinemia without increased serum iron.