Molecular and functional mapping of the piebald deletion complex on mouse chromosome 14

Spry2 regulates signalling dynamics and terminal bud branching behaviour during lung development

Development of mammalian lung involves reiterative outgrowth and branching of an epithelial tube into the surrounding mesenchymal bed. Each coordinated growth and branching cycle is driven by reciprocal signalling between epithelial and adjacent mesenchymal cells. This signalling network includes FGF, SHH, BMP4 and other pathways. We have characterized lung defects in 36Pub mice carrying a deletion that removes an antagonist of FGF signalling, Spry2. Spry2 deficient mice show an enlarged cystic structure located in the terminus of each lobes. Our study shows that Spry2 deficient lungs have reduced lung branching and the cystic structure forms in the early lung development stage. Furthermore, mice carrying a targeted disruption of Spry2 fail to complement the lung phenotype characterized in 36Pub mice. A Spry2-BAC transgene rescues the defect. Interestingly, cystic structure growth is accompanied by the reduced and spatially disorganized expression of Fgf10 and elevated expression of Shh and Bmp4. Altered signalling balance due to the loss of Spry2 causes a delayed branch cycle and cystic growth. Our data underscores the importance of restricting cellular responsiveness to signalling and highlights the interplay between morphogenesis events and spatial localization of gene expression.

The effect of dose rate on the frequency of specific-locus mutations induced in mouse spermatogonia is restricted to larger lesions; a retrospective analysis of historical data

A series of 19 large-scale germ-cell mutagenesis experiments conducted several decades ago led to the conclusion that low-LET radiation delivered to mouse spermatogonia at dose rates of 0.8 R/min and below induced only about one-third as many specific-locus mutations as did single, acute exposures at 24 R/min and above. A two-hit origin of the mutations was deemed unlikely in view of the then prevailing evidence for the small size of genetic lesions in spermatogonia. Instead, the dose-rate effect was hypothesized to be the result of a repair system that exists in spermatogonia, but not in more mature male reproductive cells. More recent genetic and molecular studies on the marker genes have identified the phenotypes associated with specific states of the mutant chromosomes, and it is now possible retrospectively to classify individual past mutations as "large lesions" or "other lesions". The mutation-frequency difference between high and low dose rates is restricted to the large lesion mutations, for which the dose-curve slopes differ by a factor exceeding 3.4. For other lesion mutations, there is essentially no difference between the slopes for protracted and acute irradiations; induced other lesions frequencies per unit dose remain similar for dose rates ranging over more than 7 orders of magnitude. For large lesions, these values rise sharply at dose rates >0.8 R/min, though they remain similar within the whole range of protracted doses, failing to provide evidence for a threshold dose rate. The downward bend at high doses that had been noted for X-ray-induced specific-locus mutations as a whole and ascribed to a positive correlation between spermatogonial death and mutation load is now found to be restricted to large lesion mutations. There is a marked difference between the mutation spectra (distributions among the seven loci) for large lesions and other lesions. Within each class, however, the spectra are similar for acute and protracted irradiation.

Creating a "hopeful monster": mouse forward genetic screens

One of the most straightforward approaches to making novel biological discoveries is the forward genetic screen. The time is ripe for forward genetic screens in the mouse since the mouse genome is sequenced, but the function of many of the genes remains unknown. Today, with careful planning, such screens are within the reach of even small individual labs. In this chapter we first discuss the types of screens in existence, as well as how to design a screen to recover mutations that are relevant to the interests of a lab. We then describe how to create mutations using the chemical N-ethyl-N-nitrosourea (ENU), including a detailed injection protocol. Next, we outline breeding schemes to establish mutant lines for each type of screen. Finally, we explain how to map mutations using recombination and how to ensure that a particular mutation causes a phenotype. Our goal is to make forward genetics in the mouse accessible to any lab with the desire to do it.

Sprouty2 controls proliferation of palate mesenchymal cells via fibroblast growth factor signaling

Cleft palate is one of the most common craniofacial deformities. The fibroblast growth factor (FGF) plays a central role in reciprocal interactions between adjacent tissues during palatal development, and the FGF signaling pathway has been shown to be inhibited by members of the Sprouty protein family. In this study, we report the incidence of cleft palate, possibly caused by failure of palatal shelf elevation, in Sprouty2-deficient (KO) mice. Sprouty2-deficient palates fused completely in palatal organ culture. However, palate mesenchymal cell proliferation estimated by Ki-67 staining was increased in Sprouty2 KO mice compared with WT mice. Sprouty2-null palates expressed higher levels of FGF target genes, such as Msx1, Etv5, and Ptx1 than WT controls. Furthermore, proliferation and the extracellular signal-regulated kinase (Erk) activation in response to FGF was enhanced in palate mesenchymal cells transfected with Sprouty2 small interfering RNA. These results suggest that Sprouty2 regulates palate mesenchymal cell proliferation via FGF signaling and is involved in palatal shelf elevation.

An essential gene mutagenesis screen across the highly conserved piebald deletion region of mouse chromosome 14

The piebald deletion complex is a set of overlapping chromosomal deficiencies on distal mouse chromosome 14. We surveyed the functional genetic content of the piebald deletion region in an essential gene mutagenesis screen of 952 genomes to recover seven lethal mutants. The ENU-induced mutations were mapped to define genetic intervals using the piebald deletion panel. Lethal mutations included loci required for establishment of the left-right embryonic axis and a loss-of-function allele of Phr1 resulting in respiratory distress at birth. A functional map of the piebald region integrates experimental genetic data from the deletion panel, mutagenesis screen, and the targeted disruption of specific genes. A comparison of several genomic intervals targeted in regional mutagenesis screens suggests that the piebald region is characterized by a low gene density and high essential gene density with a distinct genomic content and organization that supports complex regulatory interactions and promotes evolutionary stability.

The requirement for Phr1 in CNS axon tract formation reveals the corticostriatal boundary as a choice point for cortical axons

Phr1 is the single well-conserved murine ortholog of the invertebrate ubiquitin ligase genes highwire (in Drosophila) and rpm-1 (in Caenorhabditis elegans). The function and mechanism of action of highwire and rpm-1 are similar--both cell-autonomously regulate synaptogenesis by down-regulating the ortholog of the mitogen-activated protein kinase kinase kinase dual leucine zipper kinase (MAPKKK DLK). Here, using a targeted conditional mutant, we demonstrate that Phr1 also plays essential roles in mammalian neural development. As in invertebrates, Phr1 functions cell-autonomously to sculpt motor nerve terminals. In addition, Phr1 plays essential roles in the formation of major CNS axon tracts including those of the internal capsule, in part via cell-nonautonomous mechanisms, and these results reveal a choice point for cortical axons at the corticostriatal boundary. Furthermore, whereas the neurite morphology phenotypes of highwire and rpm-1 are suppressed by loss of DLK in flies and worms, Phr1-dependent CNS defects persist in Phr1, DLK double mutants. Thus, in the mammalian nervous system Phr1 is required for formation of major CNS axon tracts via a mechanism that is both cell-nonautonomous and independent of DLK.

A dosage-dependent role for Spry2 in growth and patterning during palate development

The formation of the palate involves the coordinated outgrowth, elevation and midline fusion of bilateral shelves leading to the separation of the oral and nasal cavities. Reciprocal signaling between adjacent fields of epithelial and mesenchymal cells directs palatal shelf growth and morphogenesis. Loss of function mutations in genes encoding FGF ligands and receptors have demonstrated a critical role for FGF signaling in mediating these epithelial-mesenchymal interactions. The Sprouty family of genes encode modulators of FGF signaling. We have established that mice carrying a deletion that removes the FGF signaling antagonist Spry2 have cleft palate. We show that excessive cell proliferation in the Spry2-deficient palate is accompanied by the abnormal progression of shape changes and movements required for medially directed shelf outgrowth and midline contact. Expression of the FGF responsive transcription factors Etv5, Msx1, and Barx1, as well as the morphogen Shh, is restricted to specific regions of the developing palate. We detected elevated and ectopic expression of these transcription factors and disorganized Shh expression in the Spry2-deficient palate. Mice carrying a targeted disruption of Spry2 fail to complement the craniofacial phenotype characterized in Spry2 deletion mice. Furthermore, a Spry2-BAC transgene rescues the palate defect. However, the BAC transgenic mouse lines express reduced levels of Spry2. The resulting hypomorphic phenotype demonstrates that palate development is Spry2 dosage sensitive. Our results demonstrate the importance of proper FGF signaling thresholds in regulation of epithelial-mesenchymal interactions and cellular responses necessary for coordinated morphogenesis of the face and palate.

Comparison of the genetic effects of equimolar doses of ENU and MNU: while the chemicals differ dramatically in their mutagenicity in stem-cell spermatogonia, both elicit very high mutation rates in differentiating spermatogonia

Mutagenic, reproductive, and toxicity effects of two closely related chemicals, ethylnitrosourea (ENU) and methylnitrosourea (MNU), were compared at equimolar and near-equimolar doses in the mouse specific-locus test in a screen of all stages of spermatogenesis and spermiogenesis. In stem-cell spermatogonia (SG), ENU is more than an order of magnitude more mutagenic than MNU. During post-SG stages, both chemicals exhibit high peaks in mutation yield when differentiating spermatogonia (DG) and preleptotene spermatocytes are exposed. The mutation frequency induced by 75mgMNU/kg during this peak interval is, to date, the highest induced by any single-exposure mutagenic treatment - chemical or radiation - that allows survival of the exposed animal and its germ cells, producing an estimated 10 new mutations per genome. There is thus a vast difference between stem cell and differentiating spermatogonia in their sensitivity to MNU, but little difference between these stages in their sensitivity to ENU. During stages following meiotic metaphase, the highest mutation yield is obtained from exposed spermatids, but for both chemicals, that yield is less than one-quarter that obtained from the peak interval. Large-lesion (LL) mutations were induced only in spermatids. Although only a few of the remaining mutations were analyzed molecularly, there is considerable evidence from recent molecular characterizations of the marker genes and their flanking chromosomal regions that most, if not all, mutations induced during the peak-sensitive period did not involve lesions outside the marked loci. Both ENU and MNU treatments of post-SG stages yielded significant numbers of mutants that were recovered as mosaics, with the proportion being higher for ENU than for MNU. Comparing the chemicals for the endpoints studied and additional ones (e.g., chromosome aberrations, toxicity to germ cells and to animals, teratogenicity) revealed that while MNU is generally more effective, the opposite is true when the target cells are SG.

Interactions between Sox10, Edn3 and Ednrb during enteric nervous system and melanocyte development

The requirement for SOX10 and endothelin-3/EDNRB signalling pathway during enteric nervous system (ENS) and melanocyte development, as well as their alterations in Waardenburg-Hirschsprung disease (hypopigmentation, deafness and absence of enteric ganglia) are well established. Here, we analysed the genetic interactions between these genes during ENS and melanocyte development. Through phenotype analysis of Sox10;Ednrb and Sox10;Edn3 double mutants, we show that a coordinate and balanced interaction between these molecules is required for normal ENS and melanocyte development. Indeed, double mutants present with a severe increase in white spotting, absence of melanocytes within the inner ear, and in the stria vascularis in particular, and more severe ENS defects. Moreover, we show that partial loss of Ednrb in Sox10 heterozygous mice impairs colonisation of the gut by enteric crest cells at all stages observed. However, compared to single mutants, we detected no apoptosis, cell proliferation or overall neuronal or glial differentiation defects in neural crest cells within the stomach of double mutants, but apoptosis was increased in vagal neural crest cells outside of the gut. These data will contribute to the understanding of the molecular basis of ENS, pigmentation and hearing defects observed in mouse mutants and patients carrying SOX10, EDN3 and EDNRB mutations.

X-ray-induced deletion complexes in embryonic stem cells on mouse chromosome 15

Chromosomal deletions have long been used as genetic tools in dissecting the functions of complex genomes, and new methodologies are still being developed to achieve the maximum coverage. In the mouse, where the chromosomal deletion coverage is far less extensive than that in Drosophila, substantial coverage of the genome with deletions is strongly desirable. This article reports the generation of three deletion complexes in the distal part of mouse Chromosome (Chr) 15. Chromosomal deletions were efficiently induced by X rays in embryonic stem (ES) cells around the Otoconin 90 (Oc 90), SRY-box-containing gene 10 (Sox 10), and carnitine palmitoyltransferase 1b (Cpt 1 b) loci. Deletions encompassing the Oc 90 and Sox 10 loci were transmitted to the offspring of the chimeric mice that were generated from deletion-bearing ES cells. Whereas deletion complexes encompassing the Sox 10 and the Cpt 1 b loci overlap each other, no overlap of the Oc 90 complex with the Sox 10 complex was found, possibly indicating the existence of a haploinsufficient gene located between Oc 90 and Sox 10. Deletion frequency and size induced by X rays depend on the selective locus, possibly reflecting the existence of haplolethal genes in the vicinity of these loci that yield fewer and smaller deletions. Deletions induced in ES cells by X rays vary in size and location of breakpoints, which makes them desirable for mapping and for functional genomics studies.

Effects of Male Germ-Cell Stage on the Frequency, Nature, and Spectrum of Induced Specific-Locus Mutations in the Mouse

By means of the mouse specific-locus test (SLT) with visible markers, which is capable of detecting intragenic mutations as well as larger lesions, about 20 mutagens have been studied comparatively across arrays of male germ-cell stages. In addition, a very large historical control, accumulated over decades, provides data on spontaneous mutations in males. Each mutagen has a characteristic germ-cell-stage sensitivity pattern. Although most chemicals yield their maximum numbers of mutations following exposure of spermatozoa and late spermatids, mutagens have now been identified that peak in each of the major stages of spermatogenesis and spermiogenesis, including those in which effects on recombination can also be induced. Stem-cell spermatogonia have yielded positive results with only five of 15 mutagenic chemicals. In postspermatogonial stages, all chemicals, as well as radiations, induce primarily large lesions (LL). By contrast, in spermatogonia (either stem-cell or differentiating) all chemicals except one (bleomycin) produce very few such lesions. The spectrum of relative mutation frequencies at the seven loci of the SLT is characteristic for treated germ-cell stage and mutagen. Treatments that induce primarily LL are characterized by a great preponderance of s (Ednrb)-locus mutations (possibly due to a paucity of haplo-insufficient genes in the surrounding region); and those that induce very few, if any, LL by a great preponderance of p-locus mutations. Spontaneous locus-spectra differ from both types of treatment-induced spectra; moreover, there are two distinct types of spontaneous spectra, depending on whether mutations occurred in mitotic cells or during the perigametic interval.

Evidence for a conserved function in synapse formation reveals Phr1 as a candidate gene for respiratory failure in newborn mice

Genetic studies using a set of overlapping deletions centered at the piebald locus on distal mouse chromosome 14 have defined a genomic region associated with respiratory distress and lethality at birth. We have isolated and characterized the candidate gene Phr1 that is located within the respiratory distress critical genomic interval. Phr1 is the ortholog of the human Protein Associated with Myc as well as Drosophila highwire and Caenorhabditis elegans regulator of presynaptic morphology 1. Phr1 is expressed in the embryonic and postnatal nervous system. In mice lacking Phr1, the phrenic nerve failed to completely innervate the diaphragm. In addition, nerve terminal morphology was severely disrupted, comparable with the synaptic defects seen in the Drosophila hiw and C. elegans rpm-1 mutants. Although intercostal muscles were completely innervated, they also showed dysmorphic nerve terminals. In addition, sensory neuron terminals in the diaphragm were abnormal. The neuromuscular junctions showed excessive sprouting of nerve terminals, consistent with inadequate presynaptic stimulation of the muscle. On the basis of the abnormal neuronal morphology seen in mice, Drosophila, and C. elegans, we propose that Phr1 plays a conserved role in synaptic development and is a candidate gene for respiratory distress and ventilatory disorders that arise from defective neuronal control of breathing.

Development of an enhanced GFP-based dual-color reporter to facilitate genetic screens for the recovery of mutations in mice

Mutagenesis screens to isolate a variety of alleles leading to null and non-null phenotypes represent an important approach for the characterization of gene function. Genetic schemes that use visible markers permit the efficient recovery of chemically induced mutations. We have developed a universal reporter system to visibly mark chromosomes for genetic screens in the mouse. The dual-color reporter is based on a single vector that drives the ubiquitous coexpression of the enhanced GFP (EGFP) spectral variants yellow and cyan. We show that widespread expression of the dual-color reporter is readily detected in embryonic stem cells, mice, and throughout developmental stages. CRE-loxP- and FLPe-FRT-mediated deletion of each color cassette demonstrates the modular design of the marker system. Random integration followed by plasmid rescue and sequence-based mapping was used to introduce the marker to a defined genomic location. Thus, single-step placement will simplify the construction of a genomewide bank of marked chromosomes. The dual-color nature of the marker permits complete identification of genetic classes of progeny as embryos or mice in classic regionally directed screens. The design also allows for more efficient and novel schemes, such as marked suppressor screens, in the mouse. The result is a versatile reporter that can be used independently or in combination with the growing sets of deletion and inversion resources to enhance the design and application of a wide variety of genetic schemes for the functional dissection of the mammalian genome.

Overlapping deletions define novel embryonic lethal loci in the mouse t complex

SUMMARY

The t complex region of mouse chromosome 17 contains genetic information critical for embryonic development. To identify and map loci required for normal embryogenesis, a set of overlapping deletions (D17Aus9(df10J), D17Aus9(df12J), and D17Aus9(df13J)) surrounding the D17Aus9 locus and one encompassing the T locus, Del(17)T(7J), were bred in various combinations and the consequences of nullizygosity in overlapping regions were examined. The results indicated that there are at least two functional units within 1 cM of D17Aus9. l17J1 is a peri-implantation lethal mutation within the region deleted in D17Aus9(df13J), whereas l17J2 is a later-acting lethal defined by the region of overlap between Del(17)T(7J) and D17Aus9(df12J). Del(17)T(7J)/D17Aus9(df12J) embryos die around 10.5 dpc. The development of the mutant embryos is characterized by lack of axial rotation, an abnormal notochord structure, and a ballooning pericardium. These studies demonstrate the value of overlapping deletion complexes, as opposed to individual deletion complexes, for the identification, mapping, and analysis of genes required for embryonic development.

Functional and comparative genomic analysis of the piebald deletion region of mouse chromosome 14

Several developmentally important genomic regions map within the piebald deletion complex on distal mouse chromosome 14. We have combined computational gene prediction and comparative sequence analysis to characterize an approximately 4.3-Mb segment of the piebald region to identify candidate genes for the phenotypes presented by homozygous deletion mice. As a result we have ordered 13 deletion breakpoints, integrated the sequence with markers from a bacterial artificial chromosome (BAC) physical map, and identified 16 known or predicted genes and >1500 conserved sequence elements (CSEs) across the region. The candidate genes identified include Phr1 (formerly Pam) and Spry2, which are mouse homologs of genes required for development in Drosophila melanogaster. Gene content, order, and position are highly conserved between mouse chromosome 14 and the orthologous region of human chromosome 13. Our studies combining computational gene prediction with genetic and comparative genomic analyses provide insight regarding the functional composition and organization of this defined chromosomal region.