ENU-induced phenovariance in mice: inferences from 587 mutations

Background

We present a compendium of N-ethyl-N-nitrosourea (ENU)-induced mouse mutations, identified in our laboratory over a period of 10 years either on the basis of phenotype or whole genome and/or whole exome sequencing, and archived in the Mutagenetix database. Our purpose is threefold: 1) to formally describe many point mutations, including those that were not previously disclosed in peer-reviewed publications; 2) to assess the characteristics of these mutations; and 3) to estimate the likelihood that a missense mutation induced by ENU will create a detectable phenotype.

Findings

In the context of an ENU mutagenesis program for C57BL/6J mice, a total of 185 phenotypes were tracked to mutations in 129 genes. In addition, 402 incidental mutations were identified and predicted to affect 390 genes. As previously reported, ENU shows strand asymmetry in its induction of mutations, particularly favoring T to A rather than A to T in the sense strand of coding regions and splice junctions. Some amino acid substitutions are far more likely to be damaging than others, and some are far more likely to be observed. Indeed, from among a total of 494 non-synonymous coding mutations, ENU was observed to create only 114 of the 182 possible amino acid substitutions that single base changes can achieve. Based on differences in overt null allele frequencies observed in phenotypic vs. non-phenotypic mutation sets, we infer that ENU-induced missense mutations create detectable phenotype only about 1 in 4.7 times. While the remaining mutations may not be functionally neutral, they are, on average, beneath the limits of detection of the phenotypic assays we applied.

Conclusions

Collectively, these mutations add to our understanding of the chemical specificity of ENU, the types of amino acid substitutions it creates, and its efficiency in causing phenovariance. Our data support the validity of computational algorithms for the prediction of damage caused by amino acid substitutions, and may lead to refined predictions as to whether specific amino acid changes are responsible for observed phenotypes. These data form the basis for closer in silico estimations of the number of genes mutated to a state of phenovariance by ENU within a population of G3 mice.

The Tpl2 mutation Sluggish impairs type I IFN production and increases susceptibility to group B streptococcal disease

Sluggish was identified in a population of third generation mice descended from N-ethyl-N-nitrosourea-mutagenized sires. Macrophages from homozygotes exhibited impaired TNF-alpha production in response to all TLR ligands tested and displayed impaired type I IFN production in response to TLR7 and TLR9 stimulations. The phenotype was confined to a critical region on mouse chromosome 18 and then ascribed to a T to A transversion in the acceptor splice site of intron 4 at position 13346 of the Map3k8 gene, resulting in defective splicing. The Map3k8(Sluggish) mutation does not result in susceptibility to viral infections, but Sluggish mice displayed high susceptibility to group B streptococcus infection, with impaired TNF-alpha and type I IFN production in infected macrophages. Our data demonstrate that the encoded protein kinase Tpl2 plays an essential role in cell signaling in the immune response to certain pathogens.

Mice with mutations of Dock7 have generalized hypopigmentation and white-spotting but show normal neurological function

The classical recessive coat color mutation misty (m) arose spontaneously on the DBA/J background and causes generalized hypopigmentation and localized white-spotting in mice, with a lack of pigment on the belly, tail tip, and paws. Here we describe moonlight (mnlt), a second hypopigmentation and white-spotting mutation identified on the C57BL/6J background, which yields a phenotypic copy of m/m coat color traits. We demonstrate that the 2 mutations are allelic. m/m and mnlt/mnlt phenotypes both result from mutations that truncate the dedicator of cytokinesis 7 protein (DOCK7), a widely expressed Rho family guanine nucleotide exchange factor. Although Dock7 is transcribed at high levels in the developing brain and has been implicated in both axon development and myelination by in vitro studies, we find no requirement for DOCK7 in neurobehavioral function in vivo. However, DOCK7 has non-redundant role(s) related to the distribution and function of dermal and follicular melanocytes.

Pancreatic endoderm derived from human embryonic stem cells generates glucose-responsive insulin-secreting cells in vivo

Development of a cell therapy for diabetes would be greatly aided by a renewable supply of human beta-cells. Here we show that pancreatic endoderm derived from human embryonic stem (hES) cells efficiently generates glucose-responsive endocrine cells after implantation into mice. Upon glucose stimulation of the implanted mice, human insulin and C-peptide are detected in sera at levels similar to those of mice transplanted with approximately 3,000 human islets. Moreover, the insulin-expressing cells generated after engraftment exhibit many properties of functional beta-cells, including expression of critical beta-cell transcription factors, appropriate processing of proinsulin and the presence of mature endocrine secretory granules. Finally, in a test of therapeutic potential, we demonstrate that implantation of hES cell-derived pancreatic endoderm protects against streptozotocin-induced hyperglycemia. Together, these data provide definitive evidence that hES cells are competent to generate glucose-responsive, insulin-secreting cells.

Production of pancreatic hormone-expressing endocrine cells from human embryonic stem cells

Of paramount importance for the development of cell therapies to treat diabetes is the production of sufficient numbers of pancreatic endocrine cells that function similarly to primary islets. We have developed a differentiation process that converts human embryonic stem (hES) cells to endocrine cells capable of synthesizing the pancreatic hormones insulin, glucagon, somatostatin, pancreatic polypeptide and ghrelin. This process mimics in vivo pancreatic organogenesis by directing cells through stages resembling definitive endoderm, gut-tube endoderm, pancreatic endoderm and endocrine precursor--en route to cells that express endocrine hormones. The hES cell-derived insulin-expressing cells have an insulin content approaching that of adult islets. Similar to fetal beta-cells, they release C-peptide in response to multiple secretory stimuli, but only minimally to glucose. Production of these hES cell-derived endocrine cells may represent a critical step in the development of a renewable source of cells for diabetes cell therapy.

Conditional expression of Smad7 in pancreatic beta cells disrupts TGF-beta signaling and induces reversible diabetes mellitus

Identification of signaling pathways that maintain and promote adult pancreatic islet functions will accelerate our understanding of organogenesis and improve strategies for treating diseases like diabetes mellitus. Previous work has implicated transforming growth factor-β (TGF-β) signaling as an important regulator of pancreatic islet development, but has not established whether this signaling pathway is required for essential islet functions in the adult pancreas. Here we describe a conditional system for expressing Smad7, a potent inhibitor of TGF-β signaling, to identify distinct roles for this pathway in adult and embryonic β cells. Smad7 expression in Pdx1⁺ embryonic pancreas cells resulted in striking embryonic β cell hypoplasia and neonatal lethality. Conditional expression of Smad7 in adult Pdx1⁺ cells reduced detectable β cell expression of MafA, menin, and other factors that regulate β cell function. Reduced pancreatic insulin content and hypoinsulinemia produced overt diabetes that was fully reversed upon resumption of islet TGF-β signaling. Thus, our studies reveal that TGF-β signaling is crucial for establishing and maintaining defining features of mature pancreatic β cells.

TGF-β signaling is known to regulate the development of pancreatic β cells; here the authors show that TGF-β is also required for the maintenance of β cell identity in the adult.

GDF11 modulates NGN3+ islet progenitor cell number and promotes beta-cell differentiation in pancreas development

Identification of endogenous signals that regulate expansion and maturation of organ-specific progenitor cells is a major goal in studies of organ development. Here we provide evidence that growth differentiation factor 11 (GDF11), a member of the TGF-beta ligand family, governs the number and maturation of islet progenitor cells in mouse pancreas development. Gdf11 is expressed in embryonic pancreatic epithelium during formation of islet progenitor cells that express neurogenin 3. Mice deficient for Gdf11 harbor increased numbers of NGN3+ cells, revealing that GDF11 negatively regulates production of islet progenitor cells. Despite a marked expansion of these NGN3+ islet progenitors, mice lacking Gdf11 have reduced beta-cell numbers and evidence of arrested beta-cell development, indicating that GDF11 is also required for beta-cell maturation. Similar precursor and islet cell phenotypes are observed in mice deficient for SMAD2, an intracellular signaling factor activated by TGF-beta signals. Our data suggest that Gdf11 and Smad2 regulate islet cell differentiation in parallel to the Notch pathway, which previously has been shown to control development of NGN3+ cells. Thus, our studies reveal mechanisms by which GDF11 regulates the production and maturation of islet progenitor cells in pancreas development.

Dissolution of uranium dioxide in supercritical fluid carbon dioxide

Uranium dioxide can be dissolved in supercritical CO2 with a CO2-philic TBP-HNO3 complexant to form a highly soluble UO2(NO3)(2).2TBP complex; this new method of dissolving UO2 that requires no water or organic solvent may have important applications for reprocessing of spent nuclear fuels and for treatment of nuclear wastes.

Evaluation of an amethocaine gel preparation for percutaneous analgesia before venous cannulation in children

We have evaluated the efficacy and safety of a preparation of 4% amethocaine gel in alleviating the pain of venous cannulation in children. In an initial open study of 148 children, clinically acceptable anaesthesia was achieved in 92% of cases. The preparation was then compared with 5% EMLA cream in a single-blind study in 94 patients using an application time of 40 min. We found clinically acceptable conditions in 85% of patients receiving amethocaine gel compared with 66% in the EMLA group. There were no significant adverse effects noted in each group, although 37% of those children treated with amethocaine gel showed localized erythema at the application site. The results suggest that amethocaine gel has greater efficacy and a faster onset time than EMLA cream when used for this purpose in children.

The functional deadspace of needle-free injection ports

This study examined six needle-free injection ports comprising three taps (Sorenson Intralock, Top type L, Viggo Connecta), a one-way valve (Braun RV 1000) and two cannulae with integral injection ports (Vasofix, Venflon 2) with regard to deadspace, protection against blood spillage, and potential difficulties in use. A conventional bung (Baxter), requiring needle use for drug administration, was included for comparison. Where necessary, ports were combined with cannulae in order to simulate clinical practice and allow fair comparison with the Vasofix and Venflon cannulae. Total deadspace was then calculated by filling with normal saline and measuring weight increase. Deadspace (0.10 to 0.43 ml) varied with injection port type and cannula size. Such volumes can contain significant amounts of drug and may be hazardous if not flushed after injection. Only the Vasofix and Venflon Cannulae and the Braun RV1000 protected adequately against blood spillage. Costs varied widely (A$0.19 to A$2.20 excluding cannula). The Vasofix and Venflon 2 cannulae were found to be nearest the ideal.

The effect of clonidine on the response to stimulation of non-adrenergic non-cholinergic nerves in the guinea-pig urinary bladder in-vitro

The effects of clonidine on the response of the guinea-pig urinary bladder detrusor muscle to stimulation of non-adrenergic non-cholinergic (NANC) nerves were investigated in-vitro. In tissues from both treated and chemically sympathectomized animals, in the presence of atropine (10(-5) M) to inhibit cholinergic responses, clonidine (10(-10)-10(-6) M) invariably enhanced the contractile response to NANC nerve stimulation at 2, 5, 10 and 20 Hz. The enhancement was not inhibited by yohimbine (10(-5) M), phentolamine (10(-5) M), or the histamine H1- or H2-receptor antagonists mepyramine (10(-6) M) or cimetidine (10(-5) M) respectively. Phentolamine (10(-5) M), like clonidine, enhanced the response to stimulation of NANC nerves at 2, 5, 10 and 20 Hz. Xylazine, another alpha 2-agonist, which, unlike both clonidine and phentolamine, is not a substituted 2-imidazoline compound, failed to enhance the response in the bladder to NANC nerve stimulation. The results suggest that clonidine enhances the response to NANC nerve stimulation, independently of its effects on alpha-adrenoceptors, by increasing the amount of NANC transmitter available.