Jim Drysdale

A Human Mitochondrial Ferritin Encoded by an Intronless Gene

Ferritin is a ubiquitous protein that plays a critical role in regulating intracellular iron homoeostasis by storing iron inside its multimeric shell. It also plays an important role in detoxifying potentially harmful free ferrous iron to the less soluble ferric iron by virtue of the ferroxidase activity of the H subunit. Although excess iron is stored primarily in cytoplasm, most of the metabolically active iron in cells is processed in mitochondria. Little is yet known of how these organelles regulate iron homeostasis and toxicity. Here we report an unusual intronless gene on chromosome 5q23.1 that encodes a 242-amino acid precursor of a ferritin H-like protein. This 30-kDa protein is targeted to mitochondria and processed to a 22-kDa subunit that assembles into typical ferritin shells and has ferroxidase activity. Immunohistochemical analysis showed that it accumulates in high amounts in iron-loaded mitochondria of erythroblasts of subjects with impaired heme synthesis. This new ferritin may play an important role in the regulation of mitochondrial iron homeostasis and heme synthesis.

Characterization of Serum Ferritin in Iron Overload: Possible Identity to Natural Apoferritin

Serum ferritins from a patient with haemochromatosis and from a patient with transfusional siderosis were compared with tissue isoferritins on the basis of their iron content, isoferritin spectrum and immunological properties. Both serum ferritins had a low iron content and corresponded to only the most basic isoferritins in liver. The serum ferritins were very similar to the natural apoferritin from liver in all respects.

Multiple subunits in human ferritins: Evidence for hybrid molecules☆

Abstract The subunit composition of human heart and liver ferritins was examined by both sodium dodecyl sulfate gel electrophoresis and acetic acidurea gel electrophoresis. These analyses indicated that both tissues contained two subunit types of similar size but different surface charge. One subunit was common to both tissues. The implications of these findings in relation to the known heterogeneity of isoferritins are discussed, and a new model of ferritin structure is proposed.

Genes for the H subunit of human ferritin are present on a number of human chromosomes

SummaryDNa has been extracted from hamster-human and mouse-human hybrid cell lines, restricted with EcoRI, and hybridised to a probe for the H subunit of human ferritin, pDBR2. Sequences highly homologous to this probe have been found on at least eight human chromosomes: 1, 2, 3, 6p21→6cen, 11, 14, 20, and Xq23–25→Xqter. Only the gene on chromosome 11 appears to be expressed in these hybrids Southern blotting of DNA from somatic cell hybrids containing different subsets of human chromosomes. The study shows that H subunit sequences are found on at least nine different chromosomes.

Identification of two human ferritin H genes on the short arm of chromosome 6.

We have found by analyses of human-hamster hybrid cells that two human ferritin H genes lie near the locus of the iron storage disease idiopathic hemochromatosis on chromosome 6p. One of these genes was isolated and shown to be a processed pseudogene. Comparison of its sequence with those of other ferritin H pseudogenes indicates that they may be derived from a functional H gene other than that on chromosome 11.

Human ferritin light chain gene sequences mapped to several sorted chromosomes

SummaryThe iron storage ferritin light-chain gene exhibits multiple restriction enzyme fragments which have been mapped by analyzing sorted human chromosomes. A dual laser chromosome sorter was used to construct spot-blot filter panels representing 22 chromosome fractions. Hybridization of radiolabeled human ferritin-L gene probe to spot-blot panels revealed the ferritin-L gene on more than one chromosome. Miniaturized restriction enzyme analysis was used to map each of the ferritin-L restriction fragments uniquely to one of three chromosomes. This combination of sorted chromosome analyses provides a rapid method to map homologous DNA sequences located on more than one chromosome.

Evidence for distinct mRNAs for ferritin subunits

Abstract Poly A enriched RNA from iron loaded HeLa cells and rat liver were translated separately and together in wheat germ lysates to investigate the origins of the H and L subunits of ferritin. Most of the ferritin translated from the HeLa RNA was of the H type, while that from the liver RNA was mostly L type. Mixtures of these RNAs gave H L ratios which correlated with the relative amounts of added HeLa and rat RNAs. These results indicate that the H and L subunits of ferritin are not derived by post-translational modification but from distinct mRNA species.

Studies on heterogeneity in ferritin subunits

Subunits prepared by dissociating rat and human ferritins by acid/urea or SDS can be resolved by isoelectric focusing in urea/Triton gels into many discrete forms. Most of these are not true isosubunits but aggregation artefacts formed during electrofocusing. The distribution of H and L subunit classes in these aggregates indicates that HeLa and heart ferritins contain similar classes of H and L subunits but that one or both of these classes is different in liver and kidney ferritins. To avoid aggregation artefacts, we examined subunits synthesised in vitro from exogenous mRNA. Our results indicate that HeLa and rat liver cells synthesise only one class of L subunit but two classes of H subunit.

Ferritin H gene polymorphism in idiopathic hemochromatosis

SummaryThe authors studied the H ferritin restriction polymorphism in 83 hemochromatosis patients and 84 controls as well as in 19 nuclear families. No significant difference was found with the ten restriction enzymes used (HindIII, EcoRI, EcoRV, PvuII, BamHI, PstI, BglI, BglII, HincII, and TaqI). Hence, the genomic abnormality responsible for idiopathic hemochromatosis is not a major deletion of an H ferritin gene. A higher frequency of one HindIII fragment, although nonsignificant when the number of comparisons made is taken into account, was observed in the patients. This HindIII fragment hybridizes with the H ferritin probe and with a 28 S ribosomal probe, and its segregation with HLA haplotypes (hence its assignment to chromosome 6) is uncertain. Its possible meaning in the expression of the disease is discussed.

Conserved mutations in human ferritin H pseudogenes: a second functional sequence or an evolutionary quirk?

This paper describes a search for a second functional human ferritin H gene in a collection of genomic clones. Nine new H-like sequences have been mapped to chromosomes 1p22-31, 1q32-42, 2q32-33, 3q21-23, 13q12, 14, 17p11-pter and X. These were examined for evidence of possible functionality by sequencing and by searching for possible introns. All except an uncharacterized sequence on chromosome 13 appear to be processed pseudogenes. However, nearly all share several conserved differences with the known functional sequence. These differences occur at regions of unusual structure. It is not known whether these sequences are derived from a second functional gene or from site-specific mutations in the generation of pseudogenes from the known functional gene. We also show that several hominoids contain H gene families with similar complexities to humans and that most of the human genes have counterparts in chimpanzees and gorillas.