Targeted mutagenesis of the murine IRP1 and IRP2 genes reveals context-dependent RNA processing differences in vivo

Ablation of Iron Regulatory Protein 2 produces a neurological disorder characterized by motor, somatosensory, and executive dysfunction in mice

Iron is an important cofactor for many proteins and is used to create Fe-S clusters and heme prosthetic groups that enzymes use to catalyze enzymatic reactions. Proteins involved in the import, export, and sequestration of iron are regulated by Iron Regulatory Proteins (IRPs). Recently, a patient with bi-allelic loss of function mutations in IREB2 leading to the absence of IRP2 protein was discovered. The patient failed to achieve developmental milestones and was diagnosed with dystonic cerebral palsy, epilepsy, microcytic hypochromic anemia, and frontal lobe atrophy. Several more IREB2 deficient patients subsequently identified manifested similar neurological problems. To better understand the manifestations of this novel neurological disease, we subjected an Irp2-null mouse model to extensive behavioral testing. Irp2-null mice had a significant motor deficit demonstrated by reduced performance on rotarod and hanging wire tests. Somatosensory function was also compromised in hot and cold plate assays. Their spatial search strategy was impaired in the Barnes maze and they exhibited a difficulty in flexibly adapting their response in the operant touchscreen reversal learning task. The latter is a cognitive behavior known to require an intact prefrontal cortex. These results suggest that loss of Irp2 in mice causes motor and behavioral deficits that faithfully reflect the IREB2 patient's neurodegenerative disorder.

Diurnal control of iron responsive element containing mRNAs through iron regulatory proteins IRP1 and IRP2 is mediated by feeding rhythms

BACKGROUND

Cellular iron homeostasis is regulated by iron regulatory proteins (IRP1 and IRP2) that sense iron levels (and other metabolic cues) and modulate mRNA translation or stability via interaction with iron regulatory elements (IREs). IRP2 is viewed as the primary regulator in the liver, yet our previous datasets showing diurnal rhythms for certain IRE-containing mRNAs suggest a nuanced temporal control mechanism. The purpose of this study is to gain insights into the daily regulatory dynamics across IRE-bearing mRNAs, specific IRP involvement, and underlying systemic and cellular rhythmicity cues in mouse liver.

RESULTS

We uncover high-amplitude diurnal oscillations in the regulation of key IRE-containing transcripts in the liver, compatible with maximal IRP activity at the onset of the dark phase. Although IRP2 protein levels also exhibit some diurnal variations and peak at the light-dark transition, ribosome profiling in IRP2-deficient mice reveals that maximal repression of target mRNAs at this timepoint still occurs. We further find that diurnal regulation of IRE-containing mRNAs can continue in the absence of a functional circadian clock as long as feeding is rhythmic.

CONCLUSIONS

Our findings suggest temporally controlled redundancy in IRP activities, with IRP2 mediating regulation of IRE-containing transcripts in the light phase and redundancy, conceivably with IRP1, at dark onset. Moreover, we highlight the significance of feeding-associated signals in driving rhythmicity. Our work highlights the dynamic nature and regulatory complexity in a metabolic pathway that had previously been considered well-understood.

Cardiac iron metabolism during aging - Role of inflammation and proteolysis

Iron is the most abundant trace element in the human body. Since iron can switch between its 2-valent and 3-valent form it is essential in various physiological processes such as energy production, proliferation or DNA synthesis. Especially high metabolic organs such as the heart rely on iron-associated iron-sulfur and heme proteins. However, due to switches in iron oxidation state, iron overload exhibits high toxicity through formation of reactive oxygen species, underlining the importance of balanced iron levels. Growing evidence demonstrates disturbance of this balance during aging. While age-associated cardiovascular diseases are often related to iron deficiency, in physiological aging cardiac iron accumulates. To understand these changes, we focused on inflammation and proteolysis, two hallmarks of aging, and their role in iron metabolism. Via the IL-6-hepcidin axis, inflammation and iron status are strongly connected often resulting in anemia accompanied by infiltration of macrophages. This tight connection between anemia and inflammation highlights the importance of the macrophage iron metabolism during inflammation. Age-related decrease in proteolytic activity additionally affects iron balance due to impaired degradation of iron metabolism proteins. Therefore, this review accentuates alterations in iron metabolism during aging with regards to inflammation and proteolysis to draw attention to their implications and associations.

Iron Homeostasis in the CNS: An Overview of the Pathological Consequences of Iron Metabolism Disruption

Iron homeostasis disruption has increasingly been implicated in various neurological disorders. In this review, we present an overview of our current understanding of iron metabolism in the central nervous system. We examine the consequences of both iron accumulation and deficiency in various disease contexts including neurodegenerative, neurodevelopmental, and neuropsychological disorders. The history of animal models of iron metabolism misregulation is also discussed followed by a comparison of three patients with a newly discovered neurodegenerative disorder caused by mutations in iron regulatory protein 2.

Dysregulation of the sensory and regulatory pathways controlling cellular iron metabolism in unilateral obstructive nephropathy

Chronic kidney disease involves disturbances in iron metabolism including anemia caused by insufficient erythropoietin (EPO) production. However, underlying mechanisms responsible for the dysregulation of cellular iron metabolism are incompletely defined. Using the unilateral ureteral obstruction (UUO) model in Irp1+/+ and Irp1-/- mice, we asked if iron regulatory proteins (IRPs), the central regulators of cellular iron metabolism and suppressors of EPO production, contribute to the etiology of anemia in kidney failure. We identified a significant reduction in IRP protein level and RNA binding activity that associates with a loss of the iron uptake protein transferrin receptor 1 (TfR1), increased expression of the iron storage protein subunits H- and L-ferritin, and a low but overall variable level of stainable iron in the obstructed kidney. This reduction in IRP RNA binding activity and ferritin RNA levels suggests the concomitant rise in ferritin expression and iron content in kidney failure is IRP dependent. In contrast, the reduction in the Epo mRNA level in the obstructed kidney was not rescued by genetic ablation of IRP1, suggesting disruption of normal hypoxia-inducible factor (HIF)-2α regulation. Furthermore, reduced expression of some HIF-α target genes in UUO occurred in the face of increased expression of HIF-α proteins and prolyl hydroxylases 2 and 1, the latter of which is not known to be HIF-α mediated. Our results suggest that the IRP system drives changes in cellular iron metabolism that are associated with kidney failure in UUO but that the impact of IRPs on EPO production is overridden by disrupted hypoxia signaling.NEW & NOTEWORTHY This study demonstrates that iron metabolism and hypoxia signaling are dysregulated in unilateral obstructive nephropathy. Expression of iron regulatory proteins (IRPs), central regulators of cellular iron metabolism, and the iron uptake (transferrin receptor 1) and storage (ferritins) proteins they target is strongly altered. This suggests a role of IRPs in previously observed changes in iron metabolism in progressive renal disease. Hypoxia signaling is disrupted and appeared to dominate the action of IRP1 in controlling erythropoietin expression.

Exploiting the passenger ACO1-deficiency arising from 9p21 deletions to kill T-cell lymphoblastic neoplasia cells

Precursor T-cell lymphoblastic neoplasms are aggressive malignancies in need for more effective and specific therapeutic treatments. A significant fraction of these neoplasms harbor deletions on the locus 9p21, targeting the tumor suppressor CDKN2A but also deleting the aconitase 1 (ACO1) gene, a neighboring housekeeping gene involved in cytoplasm and mitochondrial metabolism. Here we show that reducing the aconitase activity with fluorocitrate decreases the viability of T-cell lymphoblastic neoplasia cells in correlation to the differential aconitase expression. The consequences of the treatment were evidenced in vitro using T-cell lymphoblastic neoplasia cell lines exhibiting 9p21 deletions and variable levels of ACO1 expression or activity. Similar results were observed in melanoma cell lines, suggesting a true potential for fluorocitrate in different cancer types. Notably, ectopic expression of ACO1 alleviated the susceptibility of cell lines to fluorocitrate and, conversely, knockdown experiments increased susceptibility of resistant cell lines. These findings were confirmed in vivo on athymic nude mice by using tumor xenografts derived from two T-cell lines with different levels of ACO1. Taken together, our results indicate that the non-targeted ACO1 deficiency induced by common deletions exerts a collateral cellular lethality that can be used as a novel therapeutic strategy in the treatment of several types of cancer.

Cellular iron sensing and regulation: Nuclear IRP1 extends a classic paradigm

The classic view is that iron regulatory proteins operate at the post-transcriptional level. Iron Regulatory Protein 1 (IRP1) shifts between an apo-form that binds mRNAs and a holo-form that harbors a [4Fe4S] cluster. The latter form is not considered relevant to iron regulation, but rather thought to act as a non-essential cytosolic aconitase. Recent work in Drosophila, however, shows that holo-IRP1 can also translocate to the nucleus, where it appears to downregulate iron metabolism genes, preparing the cell for a decline in iron uptake. The shifting of IRP1 between states requires a functional mitoNEET pathway that includes a glycogen branching enzyme for the repair or disassembly of IRP1's oxidatively damaged [3Fe4S] cluster. The new findings add to the notion that glucose metabolism is modulated by iron metabolism. Furthermore, we propose that ferritin ferroxidase activity participates in the repair of the IRP1 [3Fe4S] cluster leading to the hypothesis that cytosolic ferritin directly contributes to cellular iron sensing.

Glycogen branching enzyme controls cellular iron homeostasis via Iron Regulatory Protein 1 and mitoNEET

Iron Regulatory Protein 1 (IRP1) is a bifunctional cytosolic iron sensor. When iron levels are normal, IRP1 harbours an iron-sulphur cluster (holo-IRP1), an enzyme with aconitase activity. When iron levels fall, IRP1 loses the cluster (apo-IRP1) and binds to iron-responsive elements (IREs) in messenger RNAs (mRNAs) encoding proteins involved in cellular iron uptake, distribution, and storage. Here we show that mutations in the Drosophila 1,4-Alpha-Glucan Branching Enzyme (AGBE) gene cause porphyria. AGBE was hitherto only linked to glycogen metabolism and a fatal human disorder known as glycogen storage disease type IV. AGBE binds specifically to holo-IRP1 and to mitoNEET, a protein capable of repairing IRP1 iron-sulphur clusters. This interaction ensures nuclear translocation of holo-IRP1 and downregulation of iron-dependent processes, demonstrating that holo-IRP1 functions not just as an aconitase, but throttles target gene expression in anticipation of declining iron requirements.

Higher organisms regulate cellular iron concentrations through Iron Regulatory Proteins (IRPs), which regulate specific messenger RNAs. Here Huynh et al. show that IRP1 requires a Glycogen Branching Enzyme for proper function, and that IRP1 has additional regulatory roles in cell nuclei.

FBXO7 sensitivity of phenotypic traits elucidated by a hypomorphic allele

FBXO7 encodes an F box containing protein that interacts with multiple partners to facilitate numerous cellular processes and has a canonical role as part of an SCF E3 ubiquitin ligase complex. Mutation of FBXO7 is responsible for an early onset Parkinsonian pyramidal syndrome and genome-wide association studies have linked variants in FBXO7 to erythroid traits. A putative orthologue in Drosophila, nutcracker, has been shown to regulate the proteasome, and deficiency of nutcracker results in male infertility. Therefore, we reasoned that modulating Fbxo7 levels in a murine model could provide insights into the role of this protein in mammals. We used a targeted gene trap model which retained 4-16% residual gene expression and assessed the sensitivity of phenotypic traits to gene dosage. Fbxo7 hypomorphs showed regenerative anaemia associated with a shorter erythrocyte half-life, and male mice were infertile. Alterations to T cell phenotypes were also observed, which intriguingly were both T cell intrinsic and extrinsic. Hypomorphic mice were also sensitive to infection with Salmonella, succumbing to a normally sublethal challenge. Despite these phenotypes, Fbxo7 hypomorphs were produced at a normal Mendelian ratio with a normal lifespan and no evidence of neurological symptoms. These data suggest that erythrocyte survival, T cell development and spermatogenesis are particularly sensitive to Fbxo7 gene dosage.

Brain Iron Metabolism and Regulation

With the development of research, more and more evidences suggested that mutations in the genes associated with brain iron metabolism induced diseases in the brain. Brain iron metabolism disorders might be one cause of neurodegenerative diseases. This review mainly summarizes the normal process of iron entry into the brain across the blood-brain barrier, and the distribution and transportation of iron among neurons and glial cells, as well as the underlying regulation mechanisms. To understand the mechanisms of iron metabolism in the brain will provide theoretical basis to prevent and cure brain diseases related to iron metabolism disorders.

Disruption of the Hepcidin/Ferroportin Regulatory System Causes Pulmonary Iron Overload and Restrictive Lung Disease

Emerging evidence suggests that pulmonary iron accumulation is implicated in a spectrum of chronic lung diseases. However, the mechanism(s) involved in pulmonary iron deposition and its role in the in vivo pathogenesis of lung diseases remains unknown. Here we show that a point mutation in the murine ferroportin gene, which causes hereditary hemochromatosis type 4 (Slc40a1^C326S), increases iron levels in alveolar macrophages, epithelial cells lining the conducting airways and lung parenchyma, and in vascular smooth muscle cells. Pulmonary iron overload is associated with oxidative stress, restrictive lung disease with decreased total lung capacity and reduced blood oxygen saturation in homozygous Slc40a1^C326S/C326S mice compared to wild-type controls. These findings implicate iron in lung pathology, which is so far not considered a classical iron-related disorder.

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Ferroportin resistance to hepcidin binding leads to pulmonary iron overload.

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Lung iron accumulation is restricted to specific cell types.

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Iron overload causes restrictive lung disease and decreased blood oxygen saturation.

Pulmonary iron accumulation is associated with a wide spectrum of lung diseases, such as chronic obstructive pulmonary disease and cystic fibrosis. Impaired lung function was further reported in patients with thalassemia major, a disease hallmarked by transfusional iron overload. So far, the mechanism(s) leading to pulmonary iron deposition and its role in disease onset and progression are still unknown. Our study shows that in a murine disease model, in which the control of systemic iron homeostasis is disrupted, iron accumulates in the lung and correlates with oxidative stress, restrictive lung disease and decreased blood oxygen saturation. These findings implicate iron overload in lung pathology, which is not considered a classical iron-related disorder.

Sumoylation in Craniofacial Disorders

Craniofacial development requires a complex series of coordinated and finely tuned events to take place, during a relatively short time frame. These events are set in motion by switching on and off transcriptional cascades that involve the use of numerous signalling pathways and a multitude of factors that act at the site of gene transcription. It is now well known that amidst the subtlety of this process lies the intricate world of protein modification, and the posttranslational addition of the small ubiquitin -like modifier, SUMO, is an example that has been implicated in this process. Many proteins that are required for formation of various structures in the embryonic head and face adapt specific functions with SUMO modification. Interestingly, the main clinical phenotype reported for a disruption of the SUMO1 locus is the common birth defect cleft lip and palate. In this chapter therefore, we discuss the role of SUMO1 in craniofacial development, with emphasis on orofacial clefts. We suggest that these defects can be a sensitive indication of down regulated SUMO modification at a critical stage during embryogenesis. As well as specific mutations affecting the ability of particular proteins to be sumoylated, non-genetic events may have the effect of down-regulating the SUMO pathway to give the same result. Enzymes regulating the SUMO pathway may become important therapeutic targets in the preventative and treatment therapies for craniofacial defects in the future.

Transcriptome Analysis of Targeted Mouse Mutations Reveals the Topography of Local Changes in Gene Expression

The unintended consequences of gene targeting in mouse models have not been thoroughly studied and a more systematic analysis is needed to understand the frequency and characteristics of off-target effects. Using RNA-seq, we evaluated targeted and neighboring gene expression in tissues from 44 homozygous mutants compared with C57BL/6N control mice. Two allele types were evaluated: 15 targeted trap mutations (TRAP); and 29 deletion alleles (DEL), usually a deletion between the translational start and the 3' UTR. Both targeting strategies insert a bacterial beta-galactosidase reporter (LacZ) and a neomycin resistance selection cassette. Evaluating transcription of genes in +/- 500 kb of flanking DNA around the targeted gene, we found up-regulated genes more frequently around DEL compared with TRAP alleles, however the frequency of alleles with local down-regulated genes flanking DEL and TRAP targets was similar. Down-regulated genes around both DEL and TRAP targets were found at a higher frequency than expected from a genome-wide survey. However, only around DEL targets were up-regulated genes found with a significantly higher frequency compared with genome-wide sampling. Transcriptome analysis confirms targeting in 97% of DEL alleles, but in only 47% of TRAP alleles probably due to non-functional splice variants, and some splicing around the gene trap. Local effects on gene expression are likely due to a number of factors including compensatory regulation, loss or disruption of intragenic regulatory elements, the exogenous promoter in the neo selection cassette, removal of insulating DNA in the DEL mutants, and local silencing due to disruption of normal chromatin organization or presence of exogenous DNA. An understanding of local position effects is important for understanding and interpreting any phenotype attributed to targeted gene mutations, or to spontaneous indels.

Reporter Gene Silencing in Targeted Mouse Mutants Is Associated with Promoter CpG Island Methylation

Targeted mutations in mouse disrupt local chromatin structure and may lead to unanticipated local effects. We evaluated targeted gene promoter silencing in a group of six mutants carrying the tm1a Knockout Mouse Project allele containing both a LacZ reporter gene driven by the native promoter and a neo selection cassette. Messenger RNA levels of the reporter gene and targeted gene were assessed by qRT-PCR, and methylation of the promoter CpG islands and LacZ coding sequence were evaluated by sequencing of bisulfite-treated DNA. Mutants were stratified by LacZ staining into presumed Silenced and Expressed reporter genes. Silenced mutants had reduced relative quantities LacZ mRNA and greater CpG Island methylation compared with the Expressed mutant group. Within the silenced group, LacZ coding sequence methylation was significantly and positively correlated with CpG Island methylation, while promoter CpG methylation was only weakly correlated with LacZ gene mRNA. The results support the conclusion that there is promoter silencing in a subset of mutants carrying the tm1a allele. The features of targeted genes which promote local silencing when targeted remain unknown.

Targeted disruption of the SUCNR1 metabolic receptor leads to dichotomous effects on obesity

A number of metabolites have signaling properties by acting through G-protein-coupled receptors. Succinate, a Krebs cycle intermediate, increases after dysregulated energy metabolism and can bind to its cognate receptor succinate receptor 1 (Sucnr1, or GPR91) to activate downstream signaling pathways. We show that Sucnr1 is highly expressed in the white adipose tissue (WAT) compartment of mice and regulates adipose mass and glucose homeostasis. Sucnr1(-/-) mice were generated, and weight gain was monitored under basal and nutritional stress (high-fat diet [HFD]) conditions. On chow diet, Sucnr1(-/-) mice had increased energy expenditure, were lean with a smaller WAT compartment, and had improved glucose buffering. Lipolysis measurements revealed that Sucnr1(-/-) mice were released from succinate-induced inhibition of lipolysis, demonstrating a function of Sucnr1 in adipose tissue. Sucnr1 deletion also protected mice from obesity on HFD, but only during the initial period; at later stages, body weight of HFD-fed Sucnr1(-/-) mice was almost comparable with wild-type (WT) mice, but WAT content was greater. Also, these mice became progressively hyperglycemic and failed to secrete insulin, although pancreas architecture was similar to WT mice. These findings suggest that Sucnr1 is a sensor for dietary energy and raise the interesting possibility that protocols to modulate Sucnr1 might have therapeutic utility in the setting of obesity.

Iron misregulation and neurodegenerative disease in mouse models that lack iron regulatory proteins

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are two cytosolic proteins that maintain cellular iron homeostasis by binding to RNA stem loops known as iron responsive elements (IREs) that are found in the untranslated regions of target mRNAs that encode proteins involved in iron metabolism. IRPs modify the expression of iron metabolism genes, and global and tissue-specific knockout mice have been made to evaluate the physiological significance of these iron regulatory proteins (Irps). Here, we will discuss the results of the studies that have been performed with mice engineered to lack the expression of one or both Irps and made in different strains using different methodologies. Both Irp1 and Irp2 knockout mice are viable, but the double knockout (Irp1(-/-)Irp2(-/-)) mice die before birth, indicating that these Irps play a crucial role in maintaining iron homeostasis. Irp1(-/-) mice develop polycythemia and pulmonary hypertension, and when these mice are challenged with a low iron diet, they die early of abdominal hemorrhages, suggesting that Irp1 plays an essential role in erythropoiesis and in the pulmonary and cardiovascular systems. Irp2(-/-) mice develop microcytic anemia, erythropoietic protoporphyria and a progressive neurological disorder, indicating that Irp2 has important functions in the nervous system and erythropoietic homeostasis. Several excellent review articles have recently been published on Irp knockout mice that mainly focus on Irp1(-/-) mice (referenced in the introduction). In this review, we will briefly describe the phenotypes and physiological implications of Irp1(-/-) mice and discuss the phenotypes observed for Irp2(-/-) mice in detail with a particular emphasis on the neurological problems of these mice.

WDR72 models of structure and function: a stage-specific regulator of enamel mineralization

Amelogenesis Imperfecta (AI) is a clinical diagnosis that encompasses a group of genetic mutations, each affecting processes involved in tooth enamel formation and thus, result in various enamel defects. The hypomaturation enamel phenotype has been described for mutations involved in the later stage of enamel formation, including Klk4, Mmp20, C4orf26, and Wdr72. Using a candidate gene approach we discovered a novel Wdr72 human mutation in association with AI to be a 5-base pair deletion (c.806_810delGGCAG; p.G255VfsX294). To gain insight into the function of WDR72, we used computer modeling of the full-length human WDR72 protein structure and found that the predicted N-terminal sequence forms two beta-propeller folds with an alpha-solenoid tail at the C-terminus. This domain iteration is characteristic of vesicle coat proteins, such as beta'-COP, suggesting a role for WDR72 in the formation of membrane deformation complexes to regulate intracellular trafficking. Our Wdr72 knockout mouse model (Wdr72(-/-)), containing a LacZ reporter knock-in, exhibited hypomineralized enamel similar to the AI phenotype observed in humans with Wdr72 mutations. MicroCT scans of Wdr72(-/-) mandibles affirmed the hypomineralized enamel phenotype occurring at the onset of the maturation stage. H&E staining revealed a shortened height phenotype in the Wdr72(-/-) ameloblasts with retained proteins in the enamel matrix during maturation stage. H(+)/Cl(-) exchange transporter 5 (CLC5), an early endosome acidifier, was co-localized with WDR72 in maturation-stage ameloblasts and decreased in Wdr72(-/-) maturation-stage ameloblasts. There were no obvious differences in RAB4A and LAMP1 immunostaining of Wdr72(-/-) mice as compared to wildtype controls. Moreover, Wdr72(-/-) ameloblasts had reduced amelogenin immunoreactivity, suggesting defects in amelogenin fragment resorption from the matrix. These data demonstrate that WDR72 has a major role in enamel mineralization, most notably during the maturation stage, and suggest a function involving endocytic vesicle trafficking, possibly in the removal of amelogenin proteins.

Resistance of ferroportin to hepcidin binding causes exocrine pancreatic failure and fatal iron overload

The regulatory axis between the iron hormone hepcidin and its receptor, the iron exporter ferroportin (FPN), is central to iron homeostasis. Mutations preventing hepcidin-mediated degradation of FPN cause systemic iron overload. We have introduced a point mutation (C326S) into the murine Fpn locus, resembling human hereditary hemochromatosis type 4, including elevated plasma iron and ferritin levels, high transferrin saturation, hepatic iron overload, and iron depletion of duodenal enterocytes and reticuloendothelial macrophages. Unlike other mouse models of iron overload, homozygous C326S mice die between 7 and 14 months of age. Pancreatic acinar cells display marked iron accumulation, oxidative damage and degeneration, associated with failure of the exocrine pancreas and severe body weight loss. Rescue experiments reveal iron overload and exocrine pancreatic failure as leading causes of death. This work uncovers the critical importance of the hepcidin-ferroportin regulatory axis for life and unveils the sensitivity of the exocrine pancreas to iron overload.

Lysosomal storage disorders and iron

Lysosomal storage disorders are a group of about 50 rare metabolic diseases that result from defects in lysosomal function. The majority is recessively inherited and caused by mutations in genes encoding lysosomal proteins as the basis for its pathobiology. The lysosome plays a pivotal role in a cell's ability to recycle and degrade unwanted material. One of its functions relates to regulating iron levels throughout the body. Iron is a double-edged sword: It is absolutely required for an organism's survival, but high levels of iron quickly lead to cell death. In addition, recent results have put the lysosome on the map of pathways leading to common neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. It is plausible that the mechanisms through which the lysosome acts in these diseases also involve iron and this would have significant implications in our understanding of the molecular etiology of these disorders.

The IRP/IRE system in vivo: insights from mouse models

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) post-transcriptionally control the expression of several mRNAs encoding proteins of iron, oxygen and energy metabolism. The mechanism involves their binding to iron responsive elements (IREs) in the untranslated regions of target mRNAs, thereby controlling mRNA translation or stability. Whereas IRP2 functions solely as an RNA-binding protein, IRP1 operates as either an RNA-binding protein or a cytosolic aconitase. Early experiments in cultured cells established a crucial role of IRPs in regulation of cellular iron metabolism. More recently, studies in mouse models with global or localized Irp1 and/or Irp2 deficiencies uncovered new physiological functions of IRPs in the context of systemic iron homeostasis. Thus, IRP1 emerged as a key regulator of erythropoiesis and iron absorption by controlling hypoxia inducible factor 2α (HIF2α) mRNA translation, while IRP2 appears to dominate the control of iron uptake and heme biosynthesis in erythroid progenitor cells by regulating the expression of transferrin receptor 1 (TfR1) and 5-aminolevulinic acid synthase 2 (ALAS2) mRNAs, respectively. Targeted disruption of either Irp1 or Irp2 in mice is associated with distinct phenotypic abnormalities. Thus, Irp1(-/-) mice develop polycythemia and pulmonary hypertension, while Irp2(-/-) mice present with microcytic anemia, iron overload in the intestine and the liver, and neurologic defects. Combined disruption of both Irp1 and Irp2 is incombatible with life and leads to early embryonic lethality. Mice with intestinal- or liver-specific disruption of both Irps are viable at birth but die later on due to malabsorption or liver failure, respectively. Adult mice lacking both Irps in the intestine exhibit a profound defect in dietary iron absorption due to a "mucosal block" that is caused by the de-repression of ferritin mRNA translation. Herein, we discuss the physiological function of the IRE/IRP regulatory system.

Impact of genetic background on neonatal lethality of Gga2 gene-trap mice

The functional redundancy of the three mammalian Golgi-localized, γ-ear-containing, ADP-ribosylation factor-binding proteins (GGAs) was addressed in a previous study. Using insertional mutagenesis, we found that Gga1 or Gga3 homozygous knockout mice were for the most part normal, whereas mice homozygous for two different Gga2 gene-trap alleles exhibited either embryonic or neonatal lethality in the C57BL/6 background, depending on the source of the vector utilized (Byg vs. Tigm, respectively). We now show that the Byg strain harbors a disrupted Gga2 allele that is hypomorphic, indicating that the Byg lethality is attributable to a mechanism independent of GGA2. This is in contrast to the Tigm Gga2 allele, which is a true knockout and establishes a role for GGA2 during the neonatal period. Placement of the Tigm Gga2 allele into the C57BL6/Ola129Sv mixed background results in a lower incidence of neonatal lethality, showing the importance of genetic background in determining the requirement for GGA2 during this period. The Gga2(-/-) mice that survive have reduced body weight at birth and this runted phenotype is maintained through adulthood.

Iron regulatory protein-1 protects against mitoferrin-1-deficient porphyria

Mitochondrial iron is essential for the biosynthesis of heme and iron-sulfur ([Fe-S]) clusters in mammalian cells. In developing erythrocytes, iron is imported into the mitochondria by MFRN1 (mitoferrin-1, SLC25A37). Although loss of MFRN1 in zebrafish and mice leads to profound anemia, mutant animals showed no overt signs of porphyria, suggesting that mitochondrial iron deficiency does not result in an accumulation of protoporphyrins. Here, we developed a gene trap model to provide in vitro and in vivo evidence that iron regulatory protein-1 (IRP1) inhibits protoporphyrin accumulation. Mfrn1(+/gt);Irp1(-/-) erythroid cells exhibit a significant increase in protoporphyrin levels. IRP1 attenuates protoporphyrin biosynthesis by binding to the 5'-iron response element (IRE) of alas2 mRNA, inhibiting its translation. Ectopic expression of alas2 harboring a mutant IRE, preventing IRP1 binding, in Mfrn1(gt/gt) cells mimics Irp1 deficiency. Together, our data support a model whereby impaired mitochondrial [Fe-S] cluster biogenesis in Mfrn1(gt/gt) cells results in elevated IRP1 RNA-binding that attenuates ALAS2 mRNA translation and protoporphyrin accumulation.

Abnormal body iron distribution and erythropoiesis in a novel mouse model with inducible gain of iron regulatory protein (IRP)-1 function

Disorders of iron metabolism account for some of the most common human diseases. Cellular iron homeostasis is maintained by iron regulatory proteins (IRP)-1 and 2 through their binding to cis-regulatory iron-responsive elements (IREs) in target mRNAs. Mouse models with IRP deficiency have yielded valuable insights into iron biology, but the physiological consequences of gain of IRP function in mammalian organisms have remained unexplored. Here, we report the generation of a mouse line allowing conditional expression of a constitutively active IRP1 mutant (IRP1*) using Cre/Lox technology. Systemic activation of the IRP1* transgene from the Rosa26 locus yields viable animals with gain of IRE-binding activity in all the organs analyzed. IRP1* activation alters the expression of IRP target genes and is accompanied by iron loading in the same organs. Furthermore, mice display macrocytic erythropenia with decreased hematocrit and hemoglobin levels as well as impaired erythroid differentiation. Thus, inappropriately high IRP1 activity causes disturbed body iron distribution and erythropoiesis. This new mouse model further highlights the importance of appropriate IRP regulation in central organs of iron metabolism. Moreover, it opens novel avenues to study diseases associated with abnormally high IRP1 activity, such as Parkinson’s disease or Friedreich’s ataxia.

F-box and leucine-rich repeat protein 5 (FBXL5) is required for maintenance of cellular and systemic iron homeostasis

Maintenance of cellular iron homeostasis requires post-transcriptional regulation of iron metabolism genes by iron regulatory protein 2 (IRP2). The hemerythrin-like domain of F-box and leucine-rich repeat protein 5 (FBXL5), an E3 ubiquitin ligase subunit, senses iron and oxygen availability and facilitates IRP2 degradation in iron replete cells. Disruption of the ubiquitously expressed murine Fbxl5 gene results in a failure to sense increased cellular iron availability, accompanied by constitutive IRP2 accumulation and misexpression of IRP2 target genes. FBXL5-null mice die during embryogenesis, although viability is restored by simultaneous deletion of the IRP2, but not IRP1, gene. Mice containing a single functional Fbxl5 allele behave like their wild type littermates when fed an iron-sufficient diet. However, unlike wild type mice that manifest decreased hematocrit and hemoglobin levels when fed a low-iron diet, Fbxl5 heterozygotes maintain normal hematologic values due to increased iron absorption. The responsiveness of IRP2 to low iron is specifically enhanced in the duodena of the heterozygotes and is accompanied by increased expression of the divalent metal transporter-1. These results confirm the role of FBXL5 in the in vivo maintenance of cellular and systemic iron homeostasis and reveal a privileged role for the intestine in their regulation by virtue of its unique FBXL5 iron sensitivity.

A precious metal: Iron, an essential nutrient for all cells

Iron is an important cofactor required for a number of essential cell functions and hence is a vital nutrient. However, iron can also be dangerous as a catalyst of free radical reactions. Accordingly, intracellular iron homeostasis and body iron balance are tightly regulated. In this review, we presented an overview of the remarkable new insights that over the last years have been gained into the multifaceted and sophisticated molecular mechanisms controlling iron acquisition, storage and release. We also reviewed the data about nutrition-related abnormalities of iron metabolism, such as iron overload and deficiency. Finally, we discussed how pathogenic microorganisms and host cells compete for iron, a battle whose outcome has a relevant role in infectious disease.

Iron and neurodegeneration: from cellular homeostasis to disease

Accumulation of iron (Fe) is often detected in the brains of people suffering from neurodegenerative diseases. High Fe concentrations have been consistently observed in Parkinson's, Alzheimer's, and Huntington's diseases; however, it is not clear whether this Fe contributes to the progression of these diseases. Other conditions, such as Friedreich's ataxia or neuroferritinopathy are associated with genetic factors that cause Fe misregulation. Consequently, excessive intracellular Fe increases oxidative stress, which leads to neuronal dysfunction and death. The characterization of the mechanisms involved in the misregulation of Fe in the brain is crucial to understand the pathology of the neurodegenerative disorders and develop new therapeutic strategies. Saccharomyces cerevisiae, as the best understood eukaryotic organism, has already begun to play a role in the neurological disorders; thus it could perhaps become a valuable tool also to study the metalloneurobiology.

Analysis of Gga null mice demonstrates a non-redundant role for mammalian GGA2 during development

Numerous studies using cultured mammalian cells have shown that the three GGAs (Golgi-localized, gamma-ear containing, ADP-ribosylation factor- binding proteins) function in the transport of cargo proteins between the trans- Golgi network and endosomes. However, the in vivo role(s) of these adaptor proteins and their possible functional redundancy has not been analyzed. In this study, the genes encoding GGAs1-3 were disrupted in mice by insertional mutagenesis. Loss of GGA1 or GGA3 alone was well tolerated whereas the absence of GGA2 resulted in embryonic or neonatal lethality, depending on the genetic background of the mice. Thus, GGA2 mediates a vital function that cannot be compensated for by GGA1and/or GGA3. The combined loss of GGA1 and GGA3 also resulted in a high incidence of neonatal mortality but in this case the expression level of GGA2 may be inadequate to compensate for the loss of the other two GGAs. We conclude that the three mammalian GGAs are essential proteins that are not fully redundant.

Multiple mRNA decapping enzymes in mammalian cells

Regulation of RNA degradation plays an important role in the control of gene expression. One mechanism of eukaryotic mRNA decay proceeds through an initial deadenylation followed by 5' end decapping and exonucleolytic decay. Dcp2 is currently believed to be the only cytoplasmic decapping enzyme responsible for decapping of all mRNAs. Here we report that Dcp2 protein modestly contributes to bulk mRNA decay and surprisingly is not detectable in a subset of mouse and human tissues. Consistent with these findings, a hypomorphic knockout of Dcp2 had no adverse consequences in mice. In contrast, the previously reported Xenopus nucleolar decapping enzyme, Nudt16, is an ubiquitous cytoplasmic decapping enzyme in mammalian cells. Like Dcp2, Nudt16 also regulates the stability of a subset of mRNAs including a member of the motin family of proteins involved in angiogenesis, Angiomotin-like 2. These data demonstrate mammalian cells possess multiple mRNA decapping enzymes, including Nudt16 to regulate mRNA turnover.

Ferritin nanoparticles as magnetic resonance reporter gene

Dynamic imaging of gene expression in live animals is among the exciting challenges of molecular imaging. To achieve that, one of the approaches is to use reporter genes that encode for the synthesis of easily detectable products. Such reporter genes can be designed to be expressed under the control of the regulatory elements included in a promoter region of a gene of interest, thus allowing the use of the same reporter gene for the detection of multiple genes. The most commonly used reporter genes include the firefly light-generating enzyme luciferase and the green fluorescent protein detectable by bioluminescence and fluorescence optical imaging, respectively. Over the last years a number of studies demonstrated the ability to use the iron-binding protein ferritin as a reporter gene that allows the detection of gene expression by magnetic resonance imaging (MRI). MRI provides high spatial resolution and soft tissue contrast for deep tissues along with a large arsenal of functional and anatomical contrast mechanisms that can be correlated with gene expression, and can potentially be translated into clinical use.

Cell-autonomous and systemic context-dependent functions of iron regulatory protein 2 in mammalian iron metabolism

Mice with total and constitutive iron regulatory protein 2 (IRP2) deficiency exhibit microcytosis and altered body iron distribution with duodenal and hepatic iron loading and decreased iron levels in splenic macrophages. To explore cell-autonomous and systemic context-dependent functions of IRP2 and to assess the systemic consequences of local IRP2 deficiency, we applied Cre/Lox technology to specifically ablate IRP2 in enterocytes, hepatocytes, or macrophages, respectively. This study reveals that the hepatic and duodenal manifestations of systemic IRP2 deficiency are largely explained by cell-autonomous functions of IRP2. By contrast, IRP2-deficient macrophages from otherwise IRP2-sufficient mice do not display the abnormalities of macrophages from systemically IRP2-deficient animals, suggesting that these result from IRP2 disruption in other cell type(s). Mice with enterocyte-, hepatocyte-, or macrophage-specific IRP2 deficiency display normal red blood cell and plasma iron parameters, supporting the notion that the microcytosis in IRP2-deficient mice likely reflects an intrinsic defect in hematopoiesis. This work defines the respective roles of IRP2 in the determination of critical body iron parameters such as organ iron loading and erythropoiesis.

Regulation of iron acquisition and storage: consequences for iron-linked disorders

Mammalian iron homeostasis must be meticulously regulated so that this essential element is available for use, but at the same time prevented from promoting the formation of toxic radicals. Controlling the entry of iron into blood plasma is the main mechanism by which iron stores in the body are physiologically manipulated and regulated. Defects in iron acquisition at the cellular and systemic levels lead to human disorders, which involve either iron overload or iron deficiency. Discoveries of iron transporters and insights into their regulation have provided important information about iron metabolism and genetic iron disorders.

Iron regulatory proteins are essential for intestinal function and control key iron absorption molecules in the duodenum

Iron regulatory proteins (IRPs) orchestrate the posttranscriptional regulation of critical iron metabolism proteins at the cellular level. Redundancy between IRP1 and IRP2 associated with embryonic lethality of doubly IRP-deficient mice has precluded the study of IRP function in vivo. Here we use Cre/Lox technology to generate viable organisms lacking IRP expression in a single tissue, the intestine. Mice lacking intestinal IRP expression develop intestinal malabsorption and dehydration postnatally and die within 4 weeks of birth. We demonstrate that IRPs control the expression of divalent metal transporter 1 (DMT1) mRNA and protein, a limiting intestinal iron importer. IRPs are also shown to be critically important to secure physiological levels of the basolateral iron exporter ferroportin. IRPs are thus essential for intestinal function and organismal survival and coordinate the synthesis of key iron metabolism proteins in the duodenum.

Identification of target mRNAs of regulatory RNA-binding proteins using mRNP immunopurification and microarrays

RNA-binding proteins (RBPs) frequently regulate the post-transcriptional fate of target mRNAs. To identify novel target mRNAs of RBPs, we incubate total RNA with recombinant RBP and immunoselect the messenger-ribonucleoproteins using a specific anti-RBP antibody. The mRNA composition of the supernatant and/or immunoprecipitated fraction is analyzed using dual-color microarrays in comparison with control reaction. From start to finish, the protocol takes approximately 6 d.

Iron-regulatory proteins: molecular biology and pathophysiological implications

Iron is required for key cellular functions, and there is a strong link between iron metabolism and important metabolic processes, such as cell growth, apoptosis and inflammation. Diseases that are directly or indirectly related to iron metabolism represent major health problems. Iron-regulatory proteins (IRPs) 1 and 2 are key controllers of vertebrate iron metabolism and post-transcriptionally regulate expression of the major iron homeostasis genes. Here we discuss how dysregulation of the IRP system can result from both iron-related and unrelated effectors and explain how this can have important pathological consequences in several human disorders.

An introduction to the metabolic determinants of anthracycline cardiotoxicity

Antitumor therapy with doxorubicin and other anthracyclines is limited by the possible development of cardiomyopathy upon chronic administration. Several lines of evidence suggest that a close link exists between cardiotoxicity and the amount of anthracycline that accumulates in the heart and then undergoes one- or two- electron reduction to toxic metabolites or by-products. Alternative metabolic pathways lead to an oxidative degradation of anthracyclines, possibly counteracting anthracycline accumulation and reductive bioactivation; unfortunately, however, the actual role of anthracycline oxidation is only partially characterized. Here, we briefly review the biochemical foundations of reductive versus oxidative anthracycline metabolism. We show that multiple links exist between one pathway of toxic biactivation and another, limiting the search and clinical development of "better anthracyclines" that retain antitumor activity but induce less cardiotoxicity than the available analogues.

Metabolic derangement of methionine and folate metabolism in mice deficient in methionine synthase reductase

Hyperhomocyst(e)inemia is a metabolic derangement that is linked to the distribution of folate pools, which provide one-carbon units for biosynthesis of purines and thymidylate and for remethylation of homocysteine to form methionine. In humans, methionine synthase deficiency results in the accumulation of methyltetrahydrofolate at the expense of folate derivatives required for purine and thymidylate biosynthesis. Complete ablation of methionine synthase activity in mice results in embryonic lethality. Other mouse models for hyperhomocyst(e)inemia have normal or reduced levels of methyltetrahydrofolate and are not embryonic lethal, although they have decreased ratios of AdoMet/AdoHcy and impaired methylation. We have constructed a mouse model with a gene trap insertion in the Mtrr gene specifying methionine synthase reductase, an enzyme essential for the activity of methionine synthase. This model is a hypomorph, with reduced methionine synthase reductase activity, thus avoiding the lethality associated with the absence of methionine synthase activity. Mtrr(gt/gt) mice have increased plasma homocyst(e)ine, decreased plasma methionine, and increased tissue methyltetrahydrofolate. Unexpectedly, Mtrr(gt/gt) mice do not show decreases in the AdoMet/AdoHcy ratio in most tissues. The different metabolite profiles in the various genetic mouse models for hyperhomocyst(e)inemia may be useful in understanding biological effects of elevated homocyst(e)ine.

IRP1-independent alterations of cardiac iron metabolism in doxorubicin-treated mice

Iron aggravates the cardiotoxicity of doxorubicin (DOX), a widely used anticancer anthracycline. The amount of iron in the cell is regulated by the iron regulatory proteins (IRPs)-1 and -2 that control the posttranscriptional expression of key iron metabolism genes. In vitro and cell culture studies revealed the ability of DOX to modulate the activity of both IRPs. However, conflicting data were obtained from different cell types and experimental conditions. To investigate the connection between acute DOX cardiotoxicity and the IRPs in a mammalian organism, we analyzed IRP activity and the expression of IRP target genes in the heart of mice subjected to DOX treatment. DOX exposure elicits a differential modulation of the two IRPs with reduced IRP2 activity and unchanged IRP1 activity. IRP2 downmodulation is associated with the upregulation of the ferritin L and H genes and decreased expression of the transferrin receptor 1 (TfR1). To directly test the role of IRP1 in DOX cardiotoxicity, the DOX response was analyzed in mice lacking IRP1. DOX-mediated IRP2 downmodulation and regulation of ferritin and TfR1 expression is identical in Irp1 (-/-) mice compared to wild type, as is the degree of oxidative damage of the heart assessed by thioredoxin and thiobarbituric acid reactive substance levels and by brain natriuretic peptide mRNA expression. These data demonstrate that the alterations of cardiac iron homeostasis related to acute anthracycline cardiotoxicity occur independently of IRP1. The observed IRP2 downmodulation could serve as a means to counteract DOX cardiotoxicity by reducing the "free" cellular iron pool.

Complete loss of iron regulatory proteins 1 and 2 prevents viability of murine zygotes beyond the blastocyst stage of embryonic development

Iron regulatory proteins 1 and 2 (IRPs) are homologous mammalian cytosolic proteins that sense intracellular iron levels and post-transcriptionally regulate expression of ferritin, transferrin receptor, and other iron metabolism proteins. Adult mice with homozygous targeted deletion of IRP2 develop microcytic anemia, elevated red cell protoporphyrin IX levels, high serum ferritin, and adult-onset neurodegeneration. Mice with homozygous deletion of IRP1 develop no overt abnormalities, but mice that lack both copies of IRP2 and one copy of IRP1 develop a more severe anemia and neurodegeneration than mice with deletion of IRP2 alone. Here, we have demonstrated that IRP1-/- IRP2-/- embryos do not survive gestation, and that although IRP1-/- IRP2-/blastocysts can be genotyped and harvested, implanted embryos with the IRP1-/- IRP2-/genotype are undetectable at embryonic day 6.5 and beyond. Blastocysts derived from a cross in which 25% of the fertilized embryos were expected to have the IRP1-/- IRP2-/genotype often showed brown discoloration and abnormal morphology. These abnormal blastocysts likely have the IRP1-/- IRP2-/- genotype, and the brown discoloration may be attributable to ferritin overexpression and sequestration of ferric iron in ferritin, whereas abnormal morphology may be due to concomitant functional iron deficiency. These results demonstrate that IRPs are indispensable for regulation of mammalian iron homeostasis at the post-implantation stage of murine embryonic development.

The mitochondrial ATP-binding cassette transporter Abcb7 is essential in mice and participates in cytosolic iron–sulfur cluster biogenesis

Proteins with iron-sulfur (Fe-S) clusters participate in multiple metabolic pathways throughout the cell. The mitochondrial ABC half-transporter Abcb7, which is mutated in X-linked sideroblastic anemia with ataxia in humans, is a functional ortholog of yeast Atm1p and is predicted to export a mitochondrially derived metabolite required for cytosolic Fe-S cluster assembly. Using an inducible Cre/loxP system to delete exons 9 and 10 of the Abcb7 gene, we examined the phenotype of mice deficient in Abcb7. We found that Abcb7 was essential in extra-embryonic tissues early in gestation and that the mutant allele exhibits an X-linked parent-of-origin lethality effect. Furthermore, using X-chromosome inactivation assays and tissue-specific deletions, Abcb7 was found to be essential for the development and function of numerous other cell types and tissues. A notable exception to this was liver, where loss of Abcb7 impaired cytosolic Fe-S cluster assembly but was not lethal. In this situation, control of iron regulatory protein 1, a key cytosolic modulator of iron metabolism, which is responsive to the availability of cytosolic Fe-S clusters, was impaired and contributed to the dysregulation of hepatocyte iron metabolism. Altogether, these studies demonstrate the essential nature of Abcb7 in mammals and further substantiate a central role for mitochondria in the biogenesis of cytosolic Fe-S proteins.

Iron-responsive degradation of iron-regulatory protein 1 does not require the Fe-S cluster

The generally accepted role of iron-regulatory protein 1 (IRP1) in orchestrating the fate of iron-regulated mRNAs depends on the interconversion of its cytosolic aconitase and RNA-binding forms through assembly/disassembly of its Fe-S cluster, without altering protein abundance. Here, we show that IRP1 protein abundance can be iron-regulated. Modulation of IRP1 abundance by iron did not require assembly of the Fe-S cluster, since a mutant with all cluster-ligating cysteines mutated to serine underwent iron-induced protein degradation. Phosphorylation of IRP1 at S138 favored the RNA-binding form and promoted iron-dependent degradation. However, phosphorylation at S138 was not required for degradation. Further, degradation of an S138 phosphomimetic mutant was not blocked by mutation of cluster-ligating cysteines. These findings were confirmed in mouse models with genetic defects in cytosolic Fe-S cluster assembly/disassembly. IRP1 RNA-binding activity was primarily regulated by IRP1 degradation in these animals. Our results reveal a mechanism for regulating IRP1 action relevant to the control of iron homeostasis during cell proliferation, inflammation, and in response to diseases altering cytosolic Fe-S cluster assembly or disassembly.

Altered body iron distribution and microcytosis in mice deficient in iron regulatory protein 2 (IRP2)

Iron regulatory protein 2 (IRP2)-deficient mice have been reported to suffer from late-onset neurodegeneration by an unknown mechanism. We report that young adult Irp2-/- mice display signs of iron mismanagement within the central iron recycling pathway in the mammalian body, the liver-bone marrow-spleen axis, with altered body iron distribution and compromised hematopoiesis. In comparison with wild-type littermates, Irp2-/- mice are mildly microcytic with reduced serum hemoglobin levels and hematocrit. Serum iron and transferrin saturation are unchanged, and hence microcytosis is not due to an overt decrease in systemic iron availability. The liver and duodenum are iron loaded, while the spleen is iron deficient, associated with a reduced expression of the iron exporter ferroportin. A reduction in transferrin receptor 1 (TfR1) mRNA levels in the bone marrow of Irp2-/- mice can plausibly explain the microcytosis by an intrinsic defect in erythropoiesis due to a failure to adequately protect TfR1 mRNA against degradation. This study links a classic regulator of cellular iron metabolism to systemic iron homeostasis and erythropoietic TfR1 expression. Furthermore, this work uncovers aspects of mammalian iron metabolism that can or cannot be compensated for by the expression of IRP1. (Blood. 2005;106: 2580-2589)

Generation of conditional alleles of the murineiron regulatory protein (IRP)-1 and -2 genes

Central aspects of cellular iron metabolism are controlled by IRP1 and IRP2, which are ubiquitously expressed in mouse organs and cells. Total and constitutive deficiency of both IRPs causes embryonic lethality in the mouse. To bypass the early lethality and to study organ-specific and/or temporal functions of IRP1 and/or IRP2 we generated Irp1 and Irp2 conditional alleles. We used mouse lines where a betaGeo gene trap construct was inserted into the second intron of the Irp1 and the Irp2 gene, generating hypomorphic alleles by interrupting the corresponding open reading frame near the amino-termini. The gene trap cassettes are flanked by Frt sites and were co-inserted with LoxP sites flanking exon 3. Flp-mediated removal of the gene trap construct generates floxed alleles with wildtype functions. For both Irp genes, Cre-assisted deletion of exon 3 generates complete null alleles that, in the case of IRP2, are associated with altered body iron distribution and compromised hematopoiesis. If not removed, the gene trap construct causes partially penetrant embryonic lethality unrelated to IRP deficiency when inserted within the Irp1 but not the Irp2 locus. We discuss the implications for functional genomics in the mouse.