A mouse model of hereditary coproporphyria identified in an ENU mutagenesis screen

Noncoding function of super enhancer derived mRNA in modulating neighboring gene expression and TAD interaction

Super enhancers are important regulators of gene expression that often overlap with protein-coding genes. However, it is unclear whether the overlapping protein-coding genes and the mRNA derived from them contribute to enhancer activity. Using an erythroid-specific super enhancer that overlaps the Cpox gene as a model, we found that Cpox mRNA has a non-coding function in regulating neighboring protein-coding genes, eRNA expression and TAD interactions. Depletion of Cpox mRNA leads to accumulation of H3K27me3 and release of p300 from the Cpox locus, activating an intra-TAD enhancer and gene expression. Additionally, we identified a head-to-tail interaction between the TAD boundary genes Cpox and Dcbld2 that is facilitated by a novel type of repressive loop anchored by p300 and PRC2/H3K27me3. Our results uncover a regulatory role for mRNA transcribed within a super enhancer context and provide insight into head-to-tail inter-gene interaction in the regulation of gene expression and oncogene activation.

A critical role for heme synthesis and succinate in the regulation of pluripotent states transitions

Using embryonic stem cells (ESCs) in regenerative medicine or in disease modeling requires a complete understanding of these cells. Two main distinct developmental states of ESCs have been stabilized in vitro, a naïve pre-implantation stage and a primed post-implantation stage. Based on two recently published CRISPR-Cas9 knockout functional screens, we show here that the exit of the naïve state is impaired upon heme biosynthesis pathway blockade, linked in mESCs to the incapacity to activate MAPK- and TGFβ-dependent signaling pathways after succinate accumulation. In addition, heme synthesis inhibition promotes the acquisition of 2 cell-like cells in a heme-independent manner caused by a mitochondrial succinate accumulation and leakage out of the cell. We further demonstrate that extracellular succinate acts as a paracrine/autocrine signal, able to trigger the 2C-like reprogramming through the activation of its plasma membrane receptor, SUCNR1. Overall, this study unveils a new mechanism underlying the maintenance of pluripotency under the control of heme synthesis.

A novel ENU-induced Cpox mutation causes microcytic hypochromic anemia in mice

Mouse models of red blood cell abnormalities are important for understanding the underlying molecular mechanisms of human erythrocytic diseases. DBA.B6-Mha (Microcytic hypochromic anemia) congenic mice were generated from the cross between N-ethyl-N-nitrosourea (ENU)-mutagenized male C57BL/6J and female DBA/2J mice as part of the RIKEN large-scale ENU mutagenesis project. The mice were established by backcrossing with DBA/2J mice for more than 20 generations. These mice showed autosomal-dominant microcytic hypochromic anemia with decreased mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) levels and increased red blood cell distribution width (RDW) and plasma ferritin levels. Linkage analysis indicated that the Mha locus was located within an interval of approximately 1.95-Mb between D16Nut1 (58.35 Mb) and D16Mit185 (60.30 Mb) on mouse chromosome 16. Mutation analysis revealed that DBA.B6-Mha mice had a point mutation (c.921-2A>G) at the acceptor site of intron 4 in the coproporphyrinogen oxidase (Cpox) gene, a heme-synthesizing gene. RT-PCR revealed that the Cpox mRNA in DBA.B6-Mha mice caused splicing errors. Our results suggest that microcytic hypochromic anemia in DBA.B6-Mha mice is owing to impaired heme synthesis caused by splice mutations in Cpox. Therefore, the DBA.B6-Mha mice may be used to elucidate the molecular mechanisms underlying microcytic hypochromic anemia caused by mutations in Cpox. Although low MCV levels are known to confer malarial resistance to the host, there were no marked changes in the susceptibility of DBA.B6-Mha mice to rodent malarial (Plasmodium yoelii 17XL) infection.

Murine models of the human porphyrias: Contributions toward understanding disease pathogenesis and the development of new therapies

Mouse models of the human porphyrias have proven useful for investigations of disease pathogenesis and to facilitate the development of new therapeutic approaches. To date, mouse models have been generated for all major porphyrias, with the exception of X-linked protoporphyria (XLP) and the ultra rare 5-aminolevulinic acid dehydratase deficient porphyria (ADP). Mouse models have been generated for the three autosomal dominant acute hepatic porphyrias, acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), and variegate porphyria (VP). The AIP mice, in particular, provide a useful investigative model as they have been shown to have acute biochemical attacks when induced with the prototypic porphyrinogenic drug, phenobarbital. In addition to providing important insights into the disease pathogenesis of the neurological impairment in AIP, these mice have been valuable for preclinical evaluation of liver-targeted gene therapy and RNAi-mediated approaches. Mice with severe HMBS deficiency, which clinically and biochemically mimic the early-onset homozygous dominant AIP (HD-AIP) patients, have been generated and were used to elucidate the striking phenotypic differences between AIP and HD-AIP. Mice modeling the hepatocutaneous porphyria, porphyria cutanea tarda (PCT), made possible the identification of the iron-dependent inhibitory mechanism of uroporphyrinogen decarboxylase (UROD) that leads to symptomatic PCT. Mouse models for the two autosomal recessive erythropoietic porphyrias, congenital erythropoietic porphyria (CEP) and erythropoeitic protoporphyria (EPP), recapitulate many of the clinical and biochemical features of the severe human diseases and have been particularly useful for evaluation of bone marrow transplantation and hematopoietic stem cell (HSC)-based gene therapy approaches. The EPP mice have also provided valuable insights into the underlying pathogenesis of EPP-induced liver damage and anemia.

Porphyrin-Induced Protein Oxidation and Aggregation as a Mechanism of Porphyria-Associated Cell Injury

Genetic porphyrias comprise eight diseases caused by defects in the heme biosynthetic pathway that lead to accumulation of heme precursors. Consequences of porphyria include photosensitivity, liver damage and increased risk of hepatocellular carcinoma, and neurovisceral involvement, including seizures. Fluorescent porphyrins that include protoporphyrin-IX, uroporphyrin and coproporphyrin, are photo-reactive; they absorb light energy and are excited to high-energy singlet and triplet states. Decay of the porphyrin excited to ground state releases energy and generates singlet oxygen. Porphyrin-induced oxidative stress is thought to be the major mechanism of porphyrin-mediated tissue damage. Although this explains the acute photosensitivity in most porphyrias, light-induced porphyrin-mediated oxidative stress does not account for the effect of porphyrins on internal organs. Recent findings demonstrate the unique role of fluorescent porphyrins in causing subcellular compartment-selective protein aggregation. Porphyrin-mediated protein aggregation associates with nuclear deformation, cytoplasmic vacuole formation and endoplasmic reticulum dilation. Porphyrin-triggered proteotoxicity is compounded by inhibition of the proteasome due to aggregation of some of its subunits. The ensuing disruption in proteostasis also manifests in cell cycle arrest coupled with aggregation of cell proliferation-related proteins, including PCNA, cdk4 and cyclin B1. Porphyrins bind to native proteins and, in presence of light and oxygen, oxidize several amino acids, particularly methionine. Noncovalent interaction of oxidized proteins with porphyrins leads to formation of protein aggregates. In internal organs, particularly the liver, light-independent porphyrin-mediated protein aggregation occurs after secondary triggers of oxidative stress. Thus, porphyrin-induced protein aggregation provides a novel mechanism for external and internal tissue damage in porphyrias that involve fluorescent porphyrin accumulation.

Bone marrow transplantation corrects haemolytic anaemia in a novel ENU mutagenesis mouse model of TPI deficiency

In this study, we performed a genome-wide N-ethyl-N-nitrosourea (ENU) mutagenesis screen in mice to identify novel genes or alleles that regulate erythropoiesis. Here, we describe a recessive mouse strain, called RBC19, harbouring a point mutation within the housekeeping gene, Tpi1, which encodes the glycolysis enzyme, triosephosphate isomerase (TPI). A serine in place of a phenylalanine at amino acid 57 severely diminishes enzyme activity in red blood cells and other tissues, resulting in a macrocytic haemolytic phenotype in homozygous mice, which closely resembles human TPI deficiency. A rescue study was performed using bone marrow transplantation of wild-type donor cells, which restored all haematological parameters and increased red blood cell enzyme function to wild-type levels after 7 weeks. This is the first study performed in a mammalian model of TPI deficiency, demonstrating that the haematological phenotype can be rescued.

Summary: In a novel ENU mutagenesis mouse model of TPI deficiency, bone marrow transplantation was conducted to demonstrate that haemolytic and red blood cell glycolytic defects can be effectively rescued.