UROS Gene Research Articles

Mutation-Specific Guide RNA for Compound Heterozygous Porphyria On-target Scarless Correction by CRISPR/Cas9 in Stem Cells.

SummaryCRISPR/Cas9 is a promising technology for gene correction. However, the edition is often biallelic, and uncontrolled small insertions and deletions (indels) concomitant to precise correction are created. Mutation-specific guide RNAs were recently tested to correct dominant inherited diseases, sparing the wild-type allele. We tested an original approach to correct compound heterozygous recessive mutations. We compared editing efficiency and genotoxicity by biallelic guide RNA versus mutant allele-specific guide RNA in iPSCs derived from a congenital erythropoietic porphyria patient carrying compound heterozygous mutations resulting in UROS gene invalidation. We obtained UROS function rescue and metabolic correction with both guides with the potential of use for porphyria clinical intervention. However, unlike the biallelic one, the mutant allele-specific guide was free of on-target collateral damage. We recommend this design to avoid genotoxicity and to obtain on-target scarless gene correction for recessive disease with frequent cases of compound heterozygous mutations.

Genetic background influences hepcidin response to iron imbalance in a mouse model of hemolytic anemia (Congenital erythropoietic porphyria)

Clinical severity is heterogeneous among patients suffering from congenital erythropoietic porphyria (CEP) suggesting a modulation of the disease (UROS deficiency) by environmental factors and modifier genes. A KI model of CEP due to a missense mutation of UROS gene present in human has been developed on 3 congenic mouse strains (BALB/c, C57BL/6, and 129/Sv) in order to study the impact of genetic background on disease severity. To detect putative modifiers of disease expression in congenic mice, hematologic data, iron parameters, porphyrin content and tissue samples were collected.Regenerative hemolytic anemia, a consequence of porphyrin excess in RBCs, had various expressions: 129/Sv mice were more hemolytic, BALB/c had more regenerative response to anemia, C57BL/6 were less affected. Iron status and hemolysis level were directly related: C57BL/6 and BALB/c had moderate hemolysis and active erythropoiesis able to reduce iron overload in the liver, while, 129/Sv showed an imbalance between iron release due to hemolysis and erythroid use.The negative control of hepcidin on the ferroportin iron exporter appeared strain specific in the CEP mice models tested. Full repression of hepcidin was observed in BALB/c and 129/Sv mice, favoring parenchymal iron overload in the liver. Unchanged hepcidin levels in C57BL/6 resulted in retention of iron predominantly in reticuloendothelial tissues. These findings open the field for potential therapeutic applications in the human disease, of hepcidin agonists and iron depletion in chronic hemolytic anemia.

Tuning intracellular homeostasis of human uroporphyrinogen III synthase by enzyme engineering at a single hotspot of congenital erythropoietic porphyria.

Congenital erythropoietic porphyria (CEP) results from a deficiency in uroporphyrinogen III synthase enzyme (UROIIIS) activity that ultimately stems from deleterious mutations in the uroS gene. C73 is a hotspot for these mutations and a C73R substitution, which drastically reduces the enzyme activity and stability, is found in almost one-third of all reported CEP cases. Here, we have studied the structural basis, by which mutations in this hotspot lead to UROIIIS destabilization. First, a strong interdependency is observed between the volume of the side chain at position 73 and the folded protein. Moreover, there is a correlation between the in vitro half-life of the mutated proteins and their expression levels in eukaryotic cell lines. Molecular modelling was used to rationalize the results, showing that the mutation site is coupled to the hinge region separating the two domains. Namely, mutations at position 73 modulate the inter-domain closure and ultimately affect protein stability. By incorporating residues capable of interacting with R73 to stabilize the hinge region, catalytic activity was fully restored and a moderate increase in the kinetic stability of the enzyme was observed. These results provide an unprecedented rationale for a destabilizing missense mutation and pave the way for the effective design of molecular chaperones as a therapy against CEP.

Congenital Erythropoietic Porphyria Due to Co-Inheritance of GATA1 and UROS Gene Mutations

AbstractAbstract 3208The Congenital Erythropoietic Porphyria (CEP) is a rare form of Chronic Porphyria characterized by severe photosensivity. Secondary infections of cutaneous lesions may lead to scarring, deformities and disfigurement of the light-exposed parts of the body such as hands, ears, nose, and eyelids. Erythrodontia, osteodystrophia, combining osteolysis and osteoporosis, hypercellular bone marrow, hemolytic anemia, hypersplenism, splenomegaly and red-coloured urine are present in almost all patients. Generally, CEP is an autosomic recessive disease caused by mutations in homozygosis or compound heterozygosis in UROS gene coding for the fourth enzyme of the heme biosynthesis pathway. At present a new form of CEP with an X-linked inheritance associated with R216W mutation in the GATA 1 gene has been described only in one French-English family by Philips JD et al. GATA1 is a key zinc finger transcription factor that coordinates hematopoietic cell differentiation. We described a Turkish family where a severe CEP phenotype segregates with thalassemia and thrombocytopenia. The propositus is an 4 year old boy who presented photosensitive bullous dermatosis, splenomegaly, short stature and pallor. His mother was chronically anemic (Hb 10.9 g/dL) and thrombocytopenic (96×109/L) known as chronic immune thrombocytopenic purpura, whereas the father and the brother were asymptomatic. At birth, the boy had hypochromic, microcytic anemia, thrombocytopenia and thrombasthenia. Reticulocyte counts slightly increased. At 2 months he was transfused, splenomegaly and heart murmur were detected on physical examination. At this point his Hb level was 6,9 g/dl, WBC 7,400×109/L, platelets 64–106×109/L, MCV 70 flL, RDW 28%. The patient's peripheral-blood smear revealed microcytic, hypochromic red cells, target cells, basophilic stippling, and rare nucleated red cells, findings compatible with thalassemia but DNA analysis for beta and alpha thalassemia was normal. A bone marrow aspirate revealed a hypercellular marrow and erythroid hyperplasia. Dyserythropoiesis was noted with nuclear budding, nuclear bridging, and occasional multinucleation. Megakaryocytes were decreased in number, Hb electrophoresis revealed increased levels of fetal hemoglobin (HbA2 2%; Hb F 68%). At 3 years liver MRI detected iron overload. Ferritin level was 329ng/dL. Because of some skin lesions on his body he was then tested with urine and plasma analysis and the results were consistent with the diagnosis of congenital erythropoietic porphyria. Urine total porphyrins were 5414 nmol/mmol creatinin (N<35) with an excess of isomer I of both Uroporphyrins and Coproporhyrins. Total plasma porhyrins were 312 nmol/L (N<10). We analyzed the UROS gene and we identified a new mutation c.338A>T (D113V) in heterozygosis in the propositus, in his father and in his brother. On the basis of hematological status we decided to analyze also GATA1 gene and the mutation c.646C>T (R216W) has been detected in the child and on one allele of his mother who presents a balanced pattern of X-inactivation. The patient is similar to the case of association of CEP with an hematologic phenotype of beta-thalassemia intermedia. Markedly elevated Hb F levels, and thrombocytopenia result from mutations in the transacting factor GATA-1 presented by Philips JD et al. This patient additionally has a severe liver iron overload. A newborn sibling died at birth because of sepsis and neonatal hepatitis. Liver iron overload was also detected in the sibling's autopsy. All components of family were heterozygous for the H63D mutation in HFE gene but no other HAMP, HJV and SLC40A1 mutations were found. The patient started a regular transfusion program and Deferasirox was given for hemosiderosis after Desferroxamine uncompliance. This is the first report of the association of GATA1 and UROS gene mutations that could explain the very severe hematological phenotype. Disclosures:No relevant conflicts of interest to declare.

Suppression of the Barleyuroporphyrinogen III synthaseGene by aDsActivation Tagging Element Generates Developmental Photosensitivity

Chlorophyll production involves the synthesis of photoreactive intermediates that, when in excess, are toxic due to the production of reactive oxygen species (ROS). A novel, activation-tagged barley (Hordeum vulgare) mutant is described that results from antisense suppression of a uroporphyrinogen III synthase (Uros) gene, the product of which catalyzes the sixth step in the synthesis of chlorophyll and heme. In homozygous mutant plants, uroporphyrin(ogen) I accumulates by spontaneous cyclization of hydroxyl methylbilane, the substrate of Uros. Accumulation of this tetrapyrrole intermediate results in photosensitive cell death due to the production of ROS. The efficiency of Uros gene suppression is developmentally regulated, being most effective in mature seedling leaves compared with newly emergent leaves. Reduced transcript accumulation of a number of nuclear-encoded photosynthesis genes occurs in the mutant, even under 3% light conditions, consistent with a retrograde plastid-nuclear signaling mechanism arising from Uros gene suppression. A similar set of nuclear genes was repressed in wild-type barley following treatment with a singlet oxygen-generating herbicide, but not by a superoxide generating herbicide, suggesting that the retrograde signaling apparent in the mutant is specific to singlet oxygen.

Identification and characterization of theArabidopsisgene encoding the tetrapyrrole biosynthesis enzyme uroporphyrinogen III synthase

UROS (uroporphyrinogen III synthase; EC 4.2.1.75) is the enzyme responsible for the formation of uroporphyrinogen III, the precursor of all cellular tetrapyrroles including haem, chlorophyll and bilins. Although UROS genes have been cloned from many organisms, the level of sequence conservation between them is low, making sequence similarity searches difficult. As an alternative approach to identify the UROS gene from plants, we used functional complementation, since this does not require conservation of primary sequence. A mutant of Saccharomyces cerevisiae was constructed in which the HEM4 gene encoding UROS was deleted. This mutant was transformed with an Arabidopsis thaliana cDNA library in a yeast expression vector and two colonies were obtained that could grow in the absence of haem. The rescuing plasmids encoded an ORF (open reading frame) of 321 amino acids which, when subcloned into an Escherichia coli expression vector, was able to complement an E. coli hemD mutant defective in UROS. Final proof that the ORF encoded UROS came from the fact that the recombinant protein expressed with an N-terminal histidine-tag was found to have UROS activity. Comparison of the sequence of AtUROS (A. thaliana UROS) with the human enzyme found that the seven invariant residues previously identified were conserved, including three shown to be important for enzyme activity. Furthermore, a structure-based homology search of the protein database with AtUROS identified the human crystal structure. AtUROS has an N-terminal extension compared with orthologues from other organisms, suggesting that this might act as a targeting sequence. The precursor protein of 34 kDa translated in vitro was imported into isolated chloroplasts and processed to the mature size of 29 kDa. Confocal microscopy of plant cells transiently expressing a fusion protein of AtUROS with GFP (green fluorescent protein) confirmed that AtUROS was targeted exclusively to chloroplasts in vivo.

A knock-in mouse model of congenital erythropoietic porphyria

Congenital erythropoietic porphyria (CEP) is a recessive autosomal disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. The severity of the disease, the lack of specific treatment except for allogeneic bone marrow transplantation, and the knowledge of the molecular lesions are strong arguments for gene therapy. An animal model of CEP has been designed to evaluate the feasibility of retroviral gene transfer in hematopoietic stem cells. We have previously demonstrated that the knockout of the Uros gene is lethal in mice ( Uros del model). This work describes the achievement of a knock-in model, which reproduces a mutation of the UROS gene responsible for a severe UROS deficiency in humans (P248Q missense mutant). Homozygous mice display erythrodontia, moderate photosensitivity, hepatosplenomegaly, and hemolytic anemia. Uroporphyrin (99% type I isomer) accumulates in urine. Total porphyrins are increased in erythrocytes and feces, while Uros enzymatic activity is below 1% of the normal level in the different tissues analyzed. These pathological findings closely mimic the CEP disease in humans and demonstrate that the Uros mut248 mouse represents a suitable model of the human disease for pathophysiological, pharmaceutical, and therapeutic purposes.