A novel type of congenital hypochromic anemia associated with a nonsense mutation in the STEAP3/TSAP6 gene.

STEAP3/TSAP6 encodes a ferrireductase that is involved in the acquisition of iron by developing erythroblasts and steap3/tsap6 null-mice display severe microcytic anemia. We report a family in which 3 siblings born to healthy parents display transfusion-dependent hypochromic anemia. A nonsense STEAP3/TSAP6 was identified in the siblings at the heterozygous state. This mutation was inherited from their father while no mutation was found in their mother. A large variability of expression was found between normal alleles in a control population, confirming a previous report that STEAP3/TSAPS6 is an expressed quantitative trait locus (e-QTL). Determination of the relative allele expression showed that the "normal" allele was expressed at a significantly higher level in the father than in the affected siblings relative to the shared mutated allele. The blood level of STEAP3/TSAP6 mRNA was severely reduced in the siblings, while both parents were in the lower range of normal controls. The STEAP3/TSAP6 protein was also reduced in lymphocytic cell lines from the patients. Collectively, our data support the hypothesis that STEAP3/TSAP6 deficiency leads to severe anemia in the affected siblings and results from the combination of a mutated allele inherited from their father and a weakly expressed allele inherited from their mother.

Missense SLC25A38 variations play an important role in autosomal recessive inherited sideroblastic anemia.

BACKGROUND: Congenital sideroblastic anemias are rare disorders with several genetic causes; they are characterized by erythroblast mitochondrial iron overload, differ greatly in severity and some occur within a syndrome. The most common cause of non-syndromic, microcytic sideroblastic anemia is a defect in the X-linked 5-aminolevulinate synthase 2 gene but this is not always present. Recently, variations in the gene for the mitochondrial carrier SLC25A38 were reported to cause a non-syndromic, severe type of autosomal-recessive sideroblastic anemia. Further evaluation of the importance of this gene was required to estimate the proportion of patients affected and to gain further insight into the range and types of variations involved. DESIGN AND METHODS: In three European diagnostic laboratories sequence analysis of SLC25A38 was performed on DNA from patients affected by congenital sideroblastic anemia of a non-syndromic nature not caused by variations in the 5-aminolevulinate synthase 2 gene. RESULTS: Eleven patients whose ancestral origins spread across several continents were homozygous or compound heterozygous for ten different SLC25A38 variations causing premature termination of translation (p.Arg117X, p.Tyr109LeufsX43), predicted splicing alteration (c.625G>C; p.Asp209His) or missense substitution (p.Gln56Lys, p.Arg134Cys, p.Ile147Asn, p.Arg187Gln, p.Pro190Arg, p.Gly228Val, p.Arg278Gly). Only three of these variations have been described previously (p.Arg117X, p.Tyr109LeufsX43 and p.Asp209His). All new variants reported here are missense and affect conserved amino acids. Structure modeling suggests that these variants may influence different aspects of transport as described for mutations in other mitochondrial carrier disorders. CONCLUSIONS: Mutations in the SLC25A38 gene cause severe, non-syndromic, microcytic/hypochromic sideroblastic anemia in many populations. Missense mutations are shown to be of importance as are mutations that affect protein production. Further investigation of these mutations should shed light on structure-function relationships in this protein.

A new missense mutation in the L ferritin coding sequence associated with elevated levels of glycosylated ferritin in serum and absence of iron overload.

BACKGROUND: Elevated serum ferritin levels are frequently encountered in clinical situations and once iron overload or inflammation has been ruled out, many cases remain unexplained. Genetic causes of hyperferritinemia associated to early cataract include mutations in the iron responsive element in the 5' untranslated region of the L ferritin mRNA, responsible for the hereditary hyperferritinemia cataract syndrome. DESIGN AND METHODS: We studied 91 probands with hyperferritinemia comprising 25 family cases belonging to families with at least two cases of unexplained hyperferritinemia, and 66 isolated cases. In the families, we also analyzed 30 relatives. Hyperferritinemia was considered as unexplained when transferrin saturation was below 45% and/or serum iron below 25 mumol/L and/or no tissue iron excess was detected, when inflammation had been ruled out and when iron responsive element mutation was absent. We carried out sequencing analysis of the FTL gene coding the L ferritin. RESULTS: A novel heterozygous p.Thr30Ile mutation in the NH2 terminus of L ferritin subunit was identified in 17 probands out of the cohort. The mutation was shown to cosegregate with hyperferritinemia in all the 10 families studied. No obvious clinical symptom was found associated with the presence of the mutation. This unique mutation is associated with an unusually high percentage of ferritin glycosylation. CONCLUSIONS: This missense mutation of FTL represents a new cause of genetic hyperferritinemia without iron overload. We hypothesized that the mutation increases the efficacy of L ferritin secretion by increasing the hydrophobicity of the N terminal "A" alpha helix.

Wild-type and mutant ferroportins do not form oligomers in transfected cells.

Ferroportin [FPN; Slc40a1 (solute carrier family 40, member 1)] is a transmembrane iron export protein expressed in macrophages and duodenal enterocytes. Heterozygous mutations in the FPN gene result in an autosomal dominant form of iron overload disorder, type-4 haemochromatosis. FPN mutants either have a normal iron export activity but have lost their ability to bind hepcidin, or are defective in their iron export function. The mutant protein has been suggested to act as a dominant negative over the wt (wild-type) protein by multimer formation. Using transiently transfected human epithelial cell lines expressing mouse FPN modified by the addition of a haemagglutinin or c-Myc epitope at the C-terminus, we show that the wtFPN is found at the plasma membrane and in Rab5-containing endosomes, as are the D157G and Q182H mutants. However, the delV162 mutant is mostly intracellular in HK2 cells (human kidney-2 cells) and partially addressed at the cell surface in HEK-293 cells (human embryonic kidney 293 cells). In both cell types, it is partially associated with the endoplasmic reticulum and with Rab5-positive vesicles. However, this mutant is complex-glycosylated like the wt protein. D157G and G323V mutants have a defective iron export capacity as judged by their inability to deplete the intracellular ferritin content, whereas Q182H and delV162 have normal iron export function and probably have lost their capacity to bind hepcidin. In co-transfection experiments, the delV162 mutant does not co-localize with the wtFPN, does not prevent its normal targeting to the plasma membrane and cannot be immunoprecipitated in the same complex, arguing against the formation of FPN hetero-oligomers.

Transferrin receptor 1 mRNA is downregulated in placenta of hepcidin transgenic embryos.

We have previously shown that hepcidin transgenic embryos are severely anemic and die around birth. Here, we report that embryonic hepcidin transgene expression decreases transferrin receptor 1 (TfR1) mRNA level in placenta, as shown by cDNA microarray analysis and quantitative RT-PCR, by a mechanism which is independent of placenta iron content and iron responsive element/iron regulatory protein (IRE/IRP) activity. On the contrary, iron injections into pregnant mothers result in increased placenta iron and ferritin content, and reduced IRE binding activity of IRP1 leading to decreased TfR1 mRNA level. Taken together, these results suggest that hepcidin action on placenta is mostly through transcriptional downregulation of the iron uptake machinery.

Association study between iron-related genes polymorphisms and Parkinson's disease.

We have conducted a case-control study in order to test for an association between 8 intragenic polymorphisms of 5 iron-related genes (transferrin, transferrin receptor1, HFE, frataxin and lactoferrin) and Parkinson disease. Comparison of genotypes and allele frequencies did not differ significantly between cases and controls for all studied polymorphisms except the G258S transferrin polymorphism, for which a higher frequency of the G allele was found among cases (p=0.033), particularly among cases with onset older than 60 (p=0.0017) and with negative family history (p=0.022). This finding suggests that genetic variations in the control of iron metabolism may contribute to the pathogenesis of the disease.

Genetic variation at KIT locus may predispose to melanoma.

As loss of KIT frequently occurs in melanoma progression, we hypothesized that KIT is implicated in predisposition to melanoma (MM). Thus, we sequenced the KIT coding region in 112 familial MM cases and 143 matched controls and genotyped tag single-nucleotide polymorphisms (SNPs) in two cohorts of melanoma patients and matched controls. Five rare KIT substitutions, all predicted possibly or probably deleterious, were identified in five patients, but none in controls [RR = 2.26 (1.26-2.26)]. Expressed in melanocyte lines, three substitutions inhibited KIT signaling. Comparison with exomes database (7020 alleles) confirmed a significant excess of rare deleterious KIT substitutions in patients. Additionally, a common SNP, rs2237028, was associated with MM risk, and 6 KIT variants were associated with nevus count. Our data strongly suggest that rare KIT substitutions predispose to melanoma and that common variants at KIT locus may also impact nevus count and melanoma risk.

Two new human DMT1 gene mutations in a patient with microcytic anemia, low ferritinemia, and liver iron overload.

DMT1 mediates the pH-dependent uptake of Fe(2+) from the diet in duodenal enterocytes and in most other cells. It transfers iron from the endosomes to the cytosol following the uptake of the transferrin-transferrin receptor complex. DMT1 mutations are responsible for severe hypochromic microcytic anemia in rodents and in 2 human patients described recently. We report a compound heterozygote for 2 new DMT1 mutations, associated with microcytic anemia from birth and progressive liver iron overload. The first mutation is a GTG deletion in exon 5, leading to the V114 in-frame deletion in transmembrane domain 2, and the second is a G --> T substitution in exon 8 leading to the G212V replacement in transmembrane domain 5. Together with the 2 previously reported cases, this patient defines a new syndrome of congenital microcytic hypochromic anemia, poorly responsive to oral iron treatment, with liver iron overload associated paradoxically with normal to moderately elevated serum ferritin levels.

A physiological model to study iron recycling in macrophages.

Following erythrophagocytosis (EP) of senescent red blood cells (RBCs), heme iron is recycled to the plasma by tissue macrophages. This process is critical for mammalian iron homeostasis but remains elusive. We characterized a cellular model using artificially-aged murine RBCs and murine bone marrow-derived macrophages (BMDMs) and study mRNA and protein expression of HO-1, ferroportin and ferritin after EP. In vitro ageing of RBCs was obtained by raising intracellular calcium concentration. These RBCs exhibit several features of erythrocyte senescence including externalization of phosphatidyl-serine, specific binding and phagocytosis by BMDMs. During the first hours of EP, we observed a rapid increase of HO-1 and ferroportin mRNAs and proteins, whereas ferritin protein expression was progressively induced with no major changes in RNA levels. At later stages after EP, a different pattern of expression was observed with a net decrease of ferroportin, a sustained high level of HO-1, and a strong increase in ferritins. Taken together, these results suggest that after EP, iron is rapidly extracted from heme and exported by ferroportin. Surprisingly, the gene expression profile at late stages after EP, which is indicative of iron storage, is reminiscent of what is observed in inflammation. However, phagocytosis of artificially-aged red blood cells seems to repress the proinflammatory response of macrophages.

Novel haptoglobin insertion/deletion polymorphism is associated with the lipid profile and C-reactive protein (CRP) concentration.

We discovered a hitherto undescribed insertion/deletion (I/D) polymorphism of 7 base pairs in intron 4 of the HP1 allele and intron 4 and 6 of the HP2 allele. Genotyping was performed in 311 Belgian subjects. The association between serum haptoglobin (Hp) and lipid concentrations and the Hp I/D genotypes was investigated. Genotype distribution in 103 Hp 1-1 phenotypes (50.5% DD, 39.8% DI, 9.7% II) and D allele frequency (0.70) were in close agreement with the Hardy-Weinberg equilibrium. No association between Hp concentrations and the Hp I/D genotypes could be found. Apolipoprotein (apo)A2, apoB and cholesterol concentrations were slightly lower in Hp II compared to DI and DD. Low-density lipoprotein (LDL)-cholesterol and ultrasensitive C-reactive protein (CRP) concentrations and the apoA1/apoA2 ratios differed significantly between Hp D/I genotypes. We added a further component to the molecular heterogeneity of Hp by the detection of an I/D polymorphism. Studies on the Hp I/D polymorphism in various populations open perspectives for further investigation of the distribution pattern of the human Hp gene. The association of the Hp I/D polymorphism with various lipid parameters might add a further component to the complex and multifaceted lipid metabolism.

Molecular analyses of patients with hyperferritinemia and normal serum iron values reveal both L ferritin IRE and 3 new ferroportin (slc11A3) mutations.

Unexplained hyperferritinemia is a common clinical finding, even in asymptomatic persons. When early onset bilateral cataracts are also present, the hereditary hyperferritinemia-cataract syndrome (HHCS), because of heterozygous point mutation in the L ferritin iron-responsive element (IRE) sequence, can be suspected. We sequenced the L ferritin exon 1 in 52 DNA samples from patients referred to us for molecular diagnosis of HHCS. We identified 24 samples with a point mutation/deletion in the IRE. For the 28 samples in which no IRE mutation was present, we also genotyped HFE mutations and sequenced both H ferritin and ferroportin genes. We found an increased frequency of His63Asp heterozygotes (12 of 28) but no H ferritin mutations. We identified 3 new ferroportin mutations, producing, respectively, Asp157Gly, Gln182His, and Gly323Val amino acid replacements, suggesting that these patients have dominant type 4 hemochromatosis. This study demonstrates that both L ferritin IRE and ferroportin mutations can account for isolated hyperferritinemia. The presence of cataract does not permit the unambiguous identification of patients with HHCS, although the existence of a family history of cataract was only encountered in these patients. This raises the intriguing possibility that lens ferritin accumulation might be a factor contributing to age-related cataract in the general population. Additional causes of isolated hyperferritinemia remain to be identified.

Molecular analysis of nonrandom 8q12 deletions in acute lymphoblastic leukemia: identification of two candidate genes.

Acute lymphoblastic leukemia is the most common malignancy in childhood. High-resolution allelotyping performed in our laboratory showed new chromosomal sites of nonrandom deletions. We have focused our work on 8q12 deletions, which we have found in about 4% of patients (eight of 205 informative cases). These deletions were of small size (less than 1 Mb) in all but one patient, and the deleted region common to all patients was delineated between two microsatellite markers (D8S1113 and D8S1763). This region was sequenced entirely from two overlapping bacterial artificial chromosomes. The common deleted region (120 kb) had a low GC content (37%), was composed more than 50% of LINE sequences, and contained only two candidate genes. The centromeric deletion borders were clustered within an interval of 33 kb between two microsatellite markers. This interval contains the first exon of an HMG-1-related gene (KIAA0808) and a putative gene, DL8q12, predicted to encode a protein with 231 amino acid residues with no homolog in protein databases. Analysis of the available mRNA from lymphoblastic cells of two patients with 8q12 deletions using common polymorphisms in the 3' UTR of KIAA0808 showed monoallelic expression of this gene. Identification of a biallelic polymorphism in the first exon of DL8q12 showed that this gene was deleted in two of four informative cases. Sequencing of the exons of both genes from all patients with 8q12 deletions did not show any mutation, which suggests that neither of these genes behaves as a classic tumor suppressor gene.