Prdm14 initiates lymphoblastic leukemia after expanding a population of cells resembling common lymphoid progenitors

Understanding the heterogeneous genetic mechanisms of tumor initiation in lymphoid leukemias (LL) will lead to improvements in prognostic classification and treatment regimens. In previous studies of mouse leukemias, we showed that retroviral insertion at the ecotropic viral insertion site 32 locus leads to increased expression of Prdm14, a pluripotency gene implicated in the self-renewal capacity of embryonic stem cells and the early stages of breast cancer. Here, we show that PRDM14 is also overexpressed in ∼25% of human lymphoid neoplasms, with increased frequencies in T-cell acute LL and hyperdiploid precursor B-cell acute LL. To test if Prdm14 overexpression could initiate leukemia, mice were transduced with bone marrow cells transfected with a Prdm14 expression vector. LLs developed in 96% of female mice and 42% of male mice. Before the onset of leukemia, differentiation of transduced cells was biased up to 1000-fold toward cells with features of common lymphoid progenitors (CLPs), and lymphoid differentiation showed a relative block at the pro-B stage. Microarray gene expression analysis of expanded CLP-like cells before the onset of leukemia demonstrated upregulation of genes involved in pluripotency, tumor initiation, early B-lineage commitment, Wnt/Ras signaling and the epithelial-to-mesenchymal transition. Among the dysregulated genes were imprinted genes and non-coding RNAs including Dlk1 and Meg3, which are also key pluripotency mediators. Heightened expression of the estrogen-dependent oncogene, Myb, in tumors suggests a basis for the increased frequency of cancer in female mice. These data provide the first direct evidence for the association of Prdm14 with cancer initiation in an in vivo mouse model and in human lymphoid malignancies, while suggesting mechanisms for Prdm14's mode of action.

Mice with alopecia, osteoporosis, and systemic amyloidosis due to mutation in Zdhhc13, a gene coding for palmitoyl acyltransferase

Protein palmitoylation has emerged as an important mechanism for regulating protein trafficking, stability, and protein-protein interactions; however, its relevance to disease processes is not clear. Using a genome-wide, phenotype driven N-ethyl-N-nitrosourea-mediated mutagenesis screen, we identified mice with failure to thrive, shortened life span, skin and hair abnormalities including alopecia, severe osteoporosis, and systemic amyloidosis (both AA and AL amyloids depositions). Whole-genome homozygosity mapping with 295 SNP markers and fine mapping with an additional 50 SNPs localized the disease gene to chromosome 7 between 53.9 and 56.3 Mb. A nonsense mutation (c.1273A>T) was located in exon 12 of the Zdhhc13 gene (Zinc finger, DHHC domain containing 13), a gene coding for palmitoyl transferase. The mutation predicted a truncated protein (R425X), and real-time PCR showed markedly reduced Zdhhc13 mRNA. A second gene trap allele of Zdhhc13 has the same phenotypes, suggesting that this is a loss of function allele. This is the first report that palmitoyl transferase deficiency causes a severe phenotype, and it establishes a direct link between protein palmitoylation and regulation of diverse physiologic functions where its absence can result in profound disease pathology. This mouse model can be used to investigate mechanisms where improper palmitoylation leads to disease processes and to understand molecular mechanisms underlying human alopecia, osteoporosis, and amyloidosis and many other neurodegenerative diseases caused by protein misfolding and amyloidosis.

Genome-wide identification of mouse congenital heart disease loci

Empirical evidence supporting a genetic basis for the etiology of congenital heart disease (CHD) is limited and few disease-causing mutations have been identified. To identify novel CHD genes, we performed a forward genetic screen to identify mutant mouse lines with heritable CHD. Lines with recessive N-ethyl-N-nitrsourea-induced CHD-causing mutations were identified using a three-generation backcross. A hierarchical screening protocol was used to test the hypothesis that the fetal-to-neonatal circulatory transition unmasks the specific structural heart defects observed in CHD. Mice with heart defects were efficiently ascertained by selecting for pups exhibiting perinatal lethality and characterizing their cardiac pathology. A marked increase of perinatal lethality was observed in the mutagen-treated cohort compared with an untreated backcross population. Cardiac pathology on perinatal lethals revealed cardiovascular defects in 79 pups from 47 of 321 mutagenized lines. All identified structural abnormalities were analogous to previously described forms of human CHD. Furthermore, the phenotypic recurrence and variance patterns across all lines were similar to human CHD prevalence and recurrence patterns. We mapped the locus responsible for heritable atrioventricular septal defects in six lines (avc1-6). Our screen demonstrated that 'sporadic' CHD may have major genetic component and established a practical, efficient approach for identifying CHD candidate genes.

Mutation in archain 1, a subunit of COPI coatomer complex, causes diluted coat color and Purkinje cell degeneration

Intracellular trafficking is critical for delivering molecules and organelles to their proper destinations to carry out normal cellular functions. Disruption of intracellular trafficking has been implicated in the pathogenesis of various neurodegenerative disorders. In addition, a number of genes involved in vesicle/organelle trafficking are also essential for pigmentation, and loss of those genes is often associated with mouse coat-color dilution and human hypopigmentary disorders. Hence, we postulated that screening for mouse mutants with both neurological defects and coat-color dilution will help identify additional factors associated with intracellular trafficking in neuronal cells. In this study, we characterized a mouse mutant with a unique N-ethyl-N-nitrosourea (ENU)-induced mutation, named nur17. nur17 mutant mice exhibit both coat-color dilution and ataxia due to Purkinje cell degeneration in the cerebellum. By positional cloning, we identified that the nur17 mouse carries a T-to-C missense mutation in archain 1 (Arcn1) gene which encodes the delta subunit of the coat protein I (COPI) complex required for intracellular trafficking. Consistent with this function, we found that intracellular trafficking is disrupted in nur17 melanocytes. Moreover, the nur17 mutation leads to common characteristics of neurodegenerative disorders such as abnormal protein accumulation, ER stress, and neurofibrillary tangles. Our study documents for the first time the physiological consequences of the impairment of the ARCN1 function in the whole animal and demonstrates a direct association between ARCN1 and neurodegeneration.

Mouse mutagenesis with the chemical supermutagen ENU

The generation and analysis of germline mutations in the mouse is one of the cornerstones of modern biological research. The chemical supermutagen N-ethyl-N-nitrosourea (ENU) is the most potent known mouse mutagen and can be used to generate point mutations throughout the mouse genome. The progeny of ENU-mutagenized males can be screened for autosomal dominant phenotypes, or they can be used to generate multigeneration pedigrees to screen for autosomal recessive traits. The introduction of balancer chromosomes into the breeding scheme can allow for the selective capture of mutations in a specific chromosomal region. More recent work has demonstrated that the use of animals that already have a mutation of interest can lead to the successful isolation of additional mutations that modify the original mutant phenotype. Further, modern molecular techniques ensure that mutations can be readily identified. We describe here the procedures for mutagenizing male mice with ENU and explain the various types of screens that can be performed for different kinds of induced mutations. The currently published research on ENU mutagenesis in the mouse has only scratched the surface of what is possible with this powerful technique, and further work is certain to deepen our knowledge of the role of the individual components of the mouse genome and the myriad relationships between them.

The role of quaking in mammalian embryonic development

Functional studies of the mouse quaking gene (Qk) have focused on its role in the postnatal central nervous system during myelination. However, the death of the majority of homozygous mouse quaking alleles revealed that quaking has a critical role in embryonic development prior to the start of myelination. Surprisingly, the lethal alleles revealed that quaking has a function in embryonic blood vessel formation and remodeling. Further studies ofthe extraembryonic yolk sac showed that Qk regulates visceral endoderm differentiated function at the cellular level, including the local synthesis of retinoic acid (RA), which then exerts paracrine control of endothelial cells within adjacent mesoderm. Endoderm-derived RA regulates proliferation of endothelial cells and extracellular matrix (ECM) production, which in a reciprocal manner, modulates visceral endoderm survival and function. Although exogenous RA can rescue endothelial cell growth control and ECM production in mutants carrying a lethal mutation, which lack functional Qk, neither visceral endoderm function nor vascular remodeling is restored. Thus, Qk also regulates cell autonomous functions of visceral endoderm that are critical for vascular remodeling. Interestingly, quaking is highly expressed during normal cardiac development, particularly in the outflow tract, suggesting potentially unique functions in the developing heart. Together, the work on Qk in mammalian embryos reveals an essential, yet under appreciated, role in cardiovascular development. This suggests that certain functions may remain conserved in the early embryo throughout the evolution of nonvertebrate and vertebrate organisms and that additional roles for quaking remain to be discovered.

Discovery of candidate disease genes in ENU-induced mouse mutants by large-scale sequencing, including a splice-site mutation in nucleoredoxin

An accurate and precisely annotated genome assembly is a fundamental requirement for functional genomic analysis. Here, the complete DNA sequence and gene annotation of mouse Chromosome 11 was used to test the efficacy of large-scale sequencing for mutation identification. We re-sequenced the 14,000 annotated exons and boundaries from over 900 genes in 41 recessive mutant mouse lines that were isolated in an N-ethyl-N-nitrosourea (ENU) mutation screen targeted to mouse Chromosome 11. Fifty-nine sequence variants were identified in 55 genes from 31 mutant lines. 39% of the lesions lie in coding sequences and create primarily missense mutations. The other 61% lie in noncoding regions, many of them in highly conserved sequences. A lesion in the perinatal lethal line l11Jus13 alters a consensus splice site of nucleoredoxin (Nxn), inserting 10 amino acids into the resulting protein. We conclude that point mutations can be accurately and sensitively recovered by large-scale sequencing, and that conserved noncoding regions should be included for disease mutation identification. Only seven of the candidate genes we report have been previously targeted by mutation in mice or rats, showing that despite ongoing efforts to functionally annotate genes in the mammalian genome, an enormous gap remains between phenotype and function. Our data show that the classical positional mapping approach of disease mutation identification can be extended to large target regions using high-throughput sequencing.

Removing the cloak of invisibility: phenotyping the mouse

If you study a human disease, it is likely that you have tried to generate a mouse model. Sometimes, these models are excellent; others are disappointing. Or, so we think. How often does our mouse mutant not model the human disease because of limitations in how we may look at it? In any living organism, many factors work together to produce the phenotype. Here, new phenotyping paradigms for assessing mouse biology and physiology are described and proposed. Advances in mouse phenotype assessments have paralleled human clinical diagnostics. The future brings a multitude of mouse strains that might be exposed to a variety of conditions. To assess health will require the ability to perform a broad-based phenotype assessment of every animal until we can understand how the perturbation of one system affects others.

A mouse chromosome 4 balancer ENU-mutagenesis screen isolates eleven lethal lines

BACKGROUND

ENU-mutagenesis is a powerful technique to identify genes regulating mammalian development. To functionally annotate the distal region of mouse chromosome 4, we performed an ENU-mutagenesis screen using a balancer chromosome targeted to this region of the genome.

RESULTS

We isolated 11 lethal lines that map to the region of chromosome 4 between D4Mit117 and D4Mit281. These lines form 10 complementation groups. The majority of lines die during embryonic development between E5.5 and E12.5 and display defects in gastrulation, cardiac development, and craniofacial development. One line displayed postnatal lethality and neurological defects, including ataxia and seizures.

CONCLUSION

These eleven mutants allow us to query gene function within the distal region of mouse chromosome 4 and demonstrate that new mouse models of mammalian developmental defects can easily and quickly be generated and mapped with the use of ENU-mutagenesis in combination with balancer chromosomes. The low number of mutations isolated in this screen compared with other balancer chromosome screens indicates that the functions of genes in different regions of the genome vary widely.

Genotype, phenotype, and karyotype correlation in the XO mouse model of Turner Syndrome

The murine model for Turner Syndrome is the XO mouse. Unlike their human counterparts, XO mice are typically fertile, and their lack of a second sex chromosome can be transmitted from one generation to the next as an X-linked dominant trait with male lethality. The introduction of an X-linked coat-color marker (tabby) has greatly facilitated the maintenance of this useful mouse strain. XO mice can be produced in large numbers, generation after generation, and rapidly identified on the basis of their sex and coat color. Although this breeding scheme appears to be effective at the phenotype level, its utility has never been conclusively proved at the molecular or cytogenetic levels. Here, we clone and sequence the tabby deletion break point and present a multiplex polymerase chain reaction-based assay for the tabby mutation. By combining the results of this assay with whole-chromosome painting data, we demonstrate that genotype, phenotype, and karyotype all show perfect correlation in the publicly available XO breeding stock. This work lays the foundation for the use of this strain to study Turner Syndrome in particular and the X chromosome in general.

Mouse Mutagenesis Using N-Ethyl-N-Nitrosourea (ENU)

INTRODUCTIONThis protocol describes chemical mutagenesis of male mice using N-ethyl-N-nitrosourea (ENU), which is the most efficient method for obtaining mouse mutations in phenotype-driven screens. A fractionated dose of ENU, an alkylating agent, can produce a mutation rate as high as 1.5 × 10(-3) in male mouse spermatogonial stem cells. Treatment with ENU produces point mutations that provide a unique mutant resource: They reflect the consequences of single gene changes independent of position effects, provide a fine structure dissection of protein function, display a range of mutant effects from complete or partial loss of function to exaggerated function, and discover gene functions in an unbiased manner. After treatment with ENU, mice are mated in genetic screens designed to uncover mutations of interest. Screens for dominant, recessive, and modifying mutations can be performed.

Overexpression of Eg5 causes genomic instability and tumor formation in mice

Proper chromosome segregation in eukaryotes is driven by a complex superstructure called the mitotic spindle. Assembly, maintenance, and function of the spindle depend on centrosome migration, organization of microtubule arrays, and force generation by microtubule motors. Spindle pole migration and elongation are controlled by the unique balance of forces generated by antagonistic molecular motors that act upon microtubules of the mitotic spindle. Defects in components of this complex structure have been shown to lead to chromosome missegregation and genomic instability. Here, we show that overexpression of Eg5, a member of the Bim-C class of kinesin-related proteins, leads to disruption of normal spindle development, as we observe both monopolar and multipolar spindles in Eg5 transgenic mice. Our findings show that perturbation of the mitotic spindle leads to chromosomal missegregation and the accumulation of tetraploid cells. Aging of these mice revealed a higher incidence of tumor formation with a mixed array of tumor types appearing in mice ages 3 to 30 months with the mean age of 20 months. Analysis of the tumors revealed widespread aneuploidy and genetic instability, both hallmarks of nearly all solid tumors. Together with previous findings, our results indicate that Eg5 overexpression disrupts the unique balance of forces associated with normal spindle assembly and function, and thereby leads to the development of spindle defects, genetic instability, and tumors.

Retroviral insertions in the VISION database identify molecular pathways in mouse lymphoid leukemia and lymphoma

AKXD recombinant inbred (RI) strains develop a variety of leukemias and lymphomas due to somatically acquired insertions of retroviral DNA into the genome of hematopoetic cells that can mutate cellular proto-oncogenes and tumor suppressor genes. We generated a new set of tumors from nine AKXD RI strains selected for their propensity to develop B-cell tumors, the most common type of human hematopoietic cancers. We employed a PCR technique called viral insertion site amplification (VISA) to rapidly isolate genomic sequence at the site of provirus insertion. Here we describe 550 VISA sequence tags (VSTs) that identify 74 common insertion sites (CISs), of which 21 have not been identified previously. Several suspected proto-oncogenes and tumor suppressor genes lie near CISs, providing supportive evidence for their roles in cancer. Furthermore, numerous previously uncharacterized genes lie near CISs, providing a pool of candidate disease genes for future research. Pathway analysis of candidate genes identified several signaling pathways as common and powerful routes to blood cancer, including Notch, E-protein, NFκB, and Ras signaling. Misregulation of several Notch signaling genes was confirmed by quantitative RT-PCR. Our data suggest that analyses of insertional mutagenesis on a single genetic background are biased toward the identification of cooperating mutations. This tumor collection represents the most comprehensive study of the genetics of B-cell leukemia and lymphoma development in mice. We have deposited the VST sequences, CISs in a genome viewer, histopathology, and molecular tumor typing data in a public web database called VISION (Viral Insertion Sites Identifying Oncogenes), which is located at http://www.mouse-genome.bcm.tmc.edu/vision.

Filamin B represses chondrocyte hypertrophy in a Runx2/Smad3-dependent manner

FILAMIN B, which encodes a cytoplasmic actin binding protein, is mutated in several skeletal dysplasias. To further investigate how an actin binding protein influences skeletogenesis, we generated mice lacking intact Filamin B. As observed in spondylocarpotarsal synostosis syndrome patients, Filamin B mutant mice display ectopic mineralization in many cartilaginous elements. This aberrant mineralization is due to ectopic chondrocyte hypertrophy similar to that seen in mice expressing Runx2 in chondrocytes. Accordingly, removing one copy of Runx2 rescues the Filamin B mutant phenotype, indicating that Filamin B is a regulator of Runx2 function during chondrocyte differentiation. Filamin B binds Smad3, which is known to interact with Runx2. Smad3 phosphorylation is increased in the mutant mice. Thus, Filamin B inhibits Runx2 activity, at least in part, through the Smad3 pathway. Our results uncover the involvement of actin binding proteins during chondrogenesis and provide a molecular basis to a human genetic disease.

Mutations in the cofilin partner Aip1/Wdr1 cause autoinflammatory disease and macrothrombocytopenia

A pivotal mediator of actin dynamics is the protein cofilin, which promotes filament severing and depolymerization, facilitating the breakdown of existing filaments, and the enhancement of filament growth from newly created barbed ends. It does so in concert with actin interacting protein 1 (Aip1), which serves to accelerate cofilin's activity. While progress has been made in understanding its biochemical functions, the physiologic processes the cofilin/Aip1 complex regulates, particularly in higher organisms, are yet to be determined. We have generated an allelic series for WD40 repeat protein 1 (Wdr1), the mammalian homolog of Aip1, and report that reductions in Wdr1 function produce a dramatic phenotype gradient. While severe loss of function at the Wdr1 locus causes embryonic lethality, macrothrombocytopenia and autoinflammatory disease develop in mice carrying hypomorphic alleles. Macrothrombocytopenia is the result of megakaryocyte maturation defects, which lead to a failure of normal platelet shedding. Autoinflammatory disease, which is bone marrow-derived yet nonlymphoid in origin, is characterized by a massive infiltration of neutrophils into inflammatory lesions. Cytoskeletal responses are impaired in Wdr1 mutant neutrophils. These studies establish an essential requirement for Wdr1 in megakaryocytes and neutrophils, indicating that cofilin-mediated actin dynamics are critically important to the development and function of both cell types.

The kinesin related motor protein, Eg5, is essential for maintenance of pre-implantation embryogenesis

Eg5 is a plus end directed kinesin related motor protein (KRP) previously shown to be involved in the assembly and maintenance of the mitotic spindle. KRPs are molecular motors capable of generating forces upon microtubules (MTs) in dividing cells and driving structural rearrangements necessary in the developing spindle. In vitro experiments demonstrate that loss of Eg5 results in cell cycle arrest and defective centrosome separation resulting in the development of monopolar spindles. Here we describe mice with a genetrap insertion in Eg5. Heterozygous mutant mice appear phenotypically normal. In contrast, embryos homozygous for the Eg5 null allele recovered at embryonic days 2.5-3.5 display signs of a proliferation defect as reduced cell numbers and failure of compaction and progression to the blastocyst stage was observed. These data, in conjunction with previous in vitro data, suggest that loss of Eg5 results in abnormal spindle structure, cell cycle arrest and thereby reduced cell proliferation of early cleavage pre-implantation embryos. These observations further support the conclusion that Eg5 is essential for cell division early in mouse development, and that maternal contribution may sustain the embryo through the maternal to zygotic transition at which point supplies of functional Eg5 are exhausted, preventing further cell cleavage.

Regional variation in the density of essential genes in mice

In most species, and particularly in vertebrates, the percentage of genes absolutely required for survival, the essential genes, has not been estimated. To obtain this estimation, we used the mouse as an experimental model to carry out high-efficiency N-ethyl-N-nitrosourea (ENU) mutagenesis screens in two balancer chromosome regions, and compared our results to a third previously published screen. The number of essential genes in each region was predicted based on allele frequencies. We determined that the density of essential genes differs by up to an order of magnitude among genomic regions. This indicates that extrapolating from regional estimates to genome-wide estimates of essential genes has a huge variance. A particularly high density of essential genes on mouse Chromosome 11 coincides with a high degree of regional linkage conservation, providing a possible causal explanation for the density variation. This is the first demonstration of regional variation in essential gene density in the mouse genome.

Craniofacial, skeletal, and cardiac defects associated with altered embryonic murine Zic3 expression following targeted insertion of a PGK-NEO cassette

Mutation in ZIC3 (OMIM #306955), a zinc finger transcription factor, causes heterotaxy (situs ambiguus) or isolated congenital heart defects in humans. Mice bearing a null mutation in Zic3 have left-right patterning defects with associated cardiovascular, vertebra/rib, and central nervous system malformations. Although XZic3 is thought to play a critical role in Xenopus neural crest development, no defects in tissues derived from neural crest are apparent in adult Zic3(null) mice. In this study we have characterized the effect of a PGK-neo cassette insertion 5' of the Zic3 locus. The Zic3 transcript in this new allele is up-regulated in ES cells and in E9.0 embryos, but no ectopic expression was detected. Unlike the Zic3(null) mutation in which only 20% of mutant animals survive to adulthood, there was no evidence of excess fetal death caused by the Zic3(neo) allele. Zic3(neo) mutant mice exhibited hemifacial microsomia, asymmetric low set ears, axial skeletal defects, kyphosis and scoliosis; a combination of defects which mimics Goldenhar Syndrome. Some Zic3(neo) mice had evidence of left-right axis patterning defects, but cardiac malformation was much less common than in the Zic3(null) mutants. A six-week old hemizygous mouse was found to have thoraco-cervical ectopia cordis, an extremely rare congenital malformation in humans and for which there is no precedent in a mouse model.

Point mutations in the melanocortin-4 receptor cause variable obesity in mice

Mutations in the melanocortin-4 receptor (MC4R) are associated with early-onset obesity in humans. Furthermore, a null Mc4r allele in mice leads to severe obesity due to hyperphagia and decreased energy expenditure. As part of independent N-ethyl- N-nitrosourea (ENU) mutagenesis screens, two obesity mutants, Fatboy and Southbeach, were isolated. Mapping revealed linkage to the melanocortin-4 receptor (Mc4r) and sequencing found single amino acid changes in Mc4r for each line. Expression of the mutant receptors in HEK 293 cells revealed defects in receptor signaling. The mutated Fatboy receptor (I194T) shows an increase in the effective concentration necessary for 50% of maximal signaling (EC(50)) when stimulated with alpha-MSH. Based on competitive binding, I194T is expressed on the cell surface at lower levels than the nonmutated receptor. In contrast, Southbeach (L300P) displays minimal receptor signaling when stimulated with the natural ligand alpha-MSH or the synthetic agonist NDP-alpha-MSH. Cell surface binding is absent, which usually indicates a lack of cell surface expression. However, antibody binding to Flag-tagged receptors by flow cytometry analysis and immunofluorescence demonstrates that L300P is translocated to the plasma membrane at a level comparable to the wild-type receptor. These results indicate a correlation with remaining receptor activity and the severity of the obesity in the mice homozygous for the mutations. Southbeach has less receptor activity and becomes more obese. These mutants will serve as good models for the variability in phenotype in humans carrying mutations in the MC4R gene.

Phenotypic characterization of Bbs4 null mice reveals age-dependent penetrance and variable expressivity

Bardet-Biedl syndrome (BBS) is a rare oligogenic disorder exhibiting both clinical and genetic heterogeneity. Although the BBS phenotype is variable both between and within families, the syndrome is characterized by the hallmarks of developmental and learning difficulties, post-axial polydactylia, obesity, hypogenitalism, renal abnormalities, retinal dystrophy, and several less frequently observed features. Eleven genes mutated in BBS patients have been identified, and more are expected to exist, since about 20-30% of all families cannot be explained by the known loci. To investigate the etiopathogenesis of BBS, we created a mouse null for one of the murine homologues, Bbs4, to assess the contribution of one gene to the pleiotropic murine Bbs phenotype. Bbs4 null mice, although initially runted compared to their littermates, ultimately become obese in a gender-dependent manner, females earlier and with more severity than males. Blood chemistry tests indicated abnormal lipid profiles, signs of liver dysfunction, and elevated insulin and leptin levels reminiscent of metabolic syndrome. As in patients with BBS, we found age-dependent retinal dystrophy. Behavioral assessment revealed that mutant mice displayed more anxiety-related responses and reduced social dominance. We noted the rare occurrence of birth defects, including neural tube defects and hydrometrocolpos, in the null mice. Evaluations of these null mice have uncovered phenotypic features with age-dependent penetrance and variable expressivity, partially recapitulating the human BBS phenotype.

Oncogenic function for the Dlg1 mammalian homolog of the Drosophila discs-large tumor suppressor

The fact that several different human virus oncoproteins, including adenovirus type 9 E4-ORF1, evolved to target the Dlg1 mammalian homolog of the membrane-associated Drosophila discs-large tumor suppressor has implicated this cellular factor in human cancer. Despite a general belief that such interactions function solely to inactivate this suspected human tumor suppressor protein, we demonstrate here that E4-ORF1 specifically requires endogenous Dlg1 to provoke oncogenic activation of phosphatidylinositol 3-kinase (PI3K) in cells. Based on our results, we propose a model wherein E4-ORF1 binding to Dlg1 triggers the resulting complex to translocate to the plasma membrane and, at this site, to promote Ras-mediated PI3K activation. These findings establish the first known function for Dlg1 in virus-mediated cellular transformation and also surprisingly expose a previously unrecognized oncogenic activity encoded by this suspected cellular tumor suppressor gene.

Defining the breakpoints of the quaking(viable) mouse mutation reveals a duplication from a Parkin intron

The quakingviable (qkv) mutant mouse shows a recessive neurological phenotype that includes central nervous system (CNS) dysmyelination, seizures, and tremor associated with voluntary movement. The molecular defect of qkv has been previously reported to be a spontaneous approximately 1 megabase (Mb) deletion in the proximal region of mouse chromosome 17 that occurred in the DBA mouse strain more than four decades ago. The mutation has recently been shown to affect three genes in the region: Quaking (qk), Parkin-coregulated gene (Pacrg), and Parkin. Here we determine the exact deletion breakpoints and demonstrate that the mutation is not just comprised of a approximately 1.1 Mb deletion, but also harbors a small 163 bp duplication fragment between the deletion breakpoints. Although the distal deletion breakpoint is within the fifth intron of the mouse Parkin gene, the duplicated sequence is derived from the sixth Parkin intron and shows positive transcriptional activity on a reporter gene in vitro. This complexity provides insight into a well-studied neurological mutant and may have a role in affecting the phenotype observed.

Novel lethal mouse mutants produced in balancer chromosome screens

Mutagenesis screens are a valuable method to identify genes that are required for normal development. Previous mouse mutagenesis screens for lethal mutations were targeted at specific time points or for developmental processes. Here we present the results of lethal mutant isolation from two mutagenesis screens that use balancer chromosomes. One screen was localized to mouse chromosome 4, between the STS markers D4Mit281 and D4Mit51. The second screen covered the region between Trp53 and Wnt3 on mouse chromosome 11. These screens identified all lethal mutations in the balancer regions, without bias towards any phenotype or stage of death. We have isolated 19 lethal lines on mouse chromosome 4, and 59 lethal lines on chromosome 11, many of which are distinct from previous mutants that map to these regions of the genome. We have characterized the mutant lines to determine the time of death, and performed a pair-wise complementation cross to determine if the mutations are allelic. Our data suggest that the majority of mouse lethal mutations die during mid-gestation, after uterine implantation, with a variety of defects in gastrulation, heart, neural tube, vascular, or placental development. This initial group of mutants provides a functional annotation of mouse chromosomes 4 and 11, and indicates that many novel developmental phenotypes can be quickly isolated in defined genomic intervals through balancer chromosome mutagenesis screens.

Visceral endoderm function is regulated byquaking and required for vascular development

The quaking (qkI) gene produces three major alternatively spliced variants (qkI-5,-6,-7) that encode for proteins that share the RNA binding, KH domain. Previous studies utilizing the qk(k2) allele, which contains an N-ethyl-N-nitrosourea (ENU)-induced point mutation in the KH domain, demonstrate that this functional region of qkI is required for embryonic vascular development. In the current studies we demonstrate that qk(l-1)/qk(l-1) mutants, which lack the QKI-5 splice variant, also died at midgestation due to vascular remodeling defects. In addition, although all three QKI isoforms were expressed in the visceral endoderm of wildtype yolk sacs, qkI-6 and qkI-7 transcript and protein expression were suppressed in qk(k2)/qk(k2) and qk(l-1)/qk(l-1) mutant yolk sacs, suggesting that the KH-domain of QKI-5 was required for qkI-6 and qkI-7 expression. Further studies revealed that the cellular role of qkI is to regulate visceral endoderm function, including the local synthesis of retinoic acid (RA) and the subsequent control of endothelial cell proliferation, matrix production, and visceral endoderm survival. Although these defects were rescued by exogenous RA, visceral endoderm function or vascular remodeling were not restored. Thus, we conclude that qkI regulates visceral endoderm function, which is critical for vascular remodeling.

A new ENU-induced allele of mouse quaking causes severe CNS dysmyelination

The mutant allelic series of the mouse quaking gene consists of the spontaneous quaking(viable) (qk(v)) allele, which is homozygous viable with a dysmyelination phenotype, and four ENU-induced alleles (qk(kt 1), qk(k2), qk(kt3/4), and qk(l-1)), which are homozygous embryonic lethal. Here we report the isolation of qk(e5), the first ENU-induced viable allele of quaking. Unlike qk(v)/qk(v), qk(e5)/qk(e5) animals have early-onset seizures, severe ataxia, and a dramatically reduced lifespan. Ultrastructural analysis of qk(e5)/qk(e5) brains reveals severe dysmyelination when compared with both wild-type and qk(v)/qk(v) brains. In addition, Calbindin detection in young adult qk(e5)/qk(e5) mice reveals Purkinje cell axonal swellings indicative of neurodegeneration , which is not seen in young adult qk(v)/qk(v) mice. Although the molecular defect in the qk(e5) allele is not evident by sequencing, protein expression studies show that qk(e5)/qk(e5) postnatal oligodendrocytes lack the QKI-6 and QKI-7 isoforms and have reduced QKI-5 levels. The oligodendrocyte developmental markers PDGF alpha R, NG 2, O4, CNP, and MBP are also present in the qk(e5)/qk(e5) postnatal brain although CNP and MBP levels are considerably reduced. Because the qk(v) allele is a large deletion that affects the expression of three genes, the new neurologic qk(e5) allele is an important addition to this allelic series.

Implementing large-scale ENU mutagenesis screens in North America

A step towards annotating the mouse genome is to use forward genetics in phenotype-driven screens to saturate the genome with mutations. The purpose of this article is to highlight the new projects in North America that are focused on isolating mouse mutations after ENU mutagenesis and phenotype screening.

Inactivation of Rai1 in mice recapitulates phenotypes observed in chromosome engineered mouse models for Smith–Magenis syndrome

Retinoic acid induced 1 (RAI1) is among the 20 genes identified in the critical region of Smith-Magenis syndrome (SMS), a genomic disorder with multiple congenital anomalies associated with a 3.7 Mb heterozygous deletion of 17p11.2. Heterozygous premature termination mutations in RAI1 have been identified recently in SMS patients without detectable deletions. To investigate Rai1 function, we generated a null allele in mice by gene targeting and simultaneously inserted a lacZ reporter gene into the Rai1 locus. X-gal staining of the Rai1(+/-) mice recapitulated the endogenous expression pattern of Rai1. The gene was predominantly expressed in the epithelial cells involved in organogenesis. Obesity and craniofacial abnormalities, which have been reported in SMS mouse models containing a heterozygous deletion of the syntenic SMS critical region, were observed in Rai1(+/-) mice. Thus, haploinsufficiency of Rai1 causes obesity and craniofacial abnormalities in mice. Interestingly, the penetrance of craniofacial anomalies is further reduced in Rai1(+/-) mice. Most homozygous mice died during gastrulation and organogenesis. The surviving Rai1(-/-) mice were growth retarded and displayed malformations in both the craniofacial and the axial skeleton. Using green fluorescence protein and GAL4 DNA binding domain fusions to Rai1, we showed that Rai1 is translocated to the nucleus and it has transactivation activity. Our data are consistent with Rai1 functioning as a transcriptional regulator, document that Rai1 haploinsufficiency is responsible for obesity and craniofacial phenotypes in mice with SMS deletions, and indicate Rai1 is important for embryonic and postnatal developments.

Mutation of l7Rn3 shows that Odz4 is required for mouse gastrulation

A mouse homolog of the Drosophila pair-rule gene Odd Oz (Odz4) maps to the critical region of the l7Rn3 locus on mouse chromosome 7. Here we show that Odz4 is an excellent candidate for this allelic series because (1) it spans the entire critical region, (2) the phenotypes correlate with embryonic expression, (3) the complex genetic inheritance of the alleles is consistent with complex transcriptional regulation, and (4) one allele has a mutation in a conserved amino acid. Odz4 uses five alternate promoters that encode both secreted and membrane-bound proteins. Intragenic complementation of the l7Rn3 alleles is consistent with these multiple-protein isoforms. Further, the allelic series shows that Odz4 is required to establish the anterior-posterior axis of the gastrulating mouse embryo and is necessary later for mesoderm-derived tissues such as somites, heart, and skeleton. Sequencing of RT-PCR products from five of the six alleles reveals a nonconservative amino acid change in the l7Rn3(m4) allele. This amino acid is important evolutionarily, as it is conserved to Drosophila. Together, our data indicate that Odz4 is mutated in the l7Rn3 allele series and performs roles in the mouse brain, heart, and embryonic patterning similar to those of its Drosophila counterpart.

A novel mouse model of hypogonadotrophic hypogonadism: N-ethyl-N-nitrosourea-induced gonadotropin-releasing hormone receptor gene mutation

An autosomal-recessive mutation that causes hypogonadotrophic hypogonadism was isolated during an N-ethyl-N-nitrosourea mutagenesis screen in mice. Affected males had micropenis and small, undescended testes with spermatogenesis arrested at the pachytene stage of meiosis, leading to sterility. Androgen-sensitive organs were small and immature. Affected females were externally normal but sterile with small ovaries due to an arrest at the secondary stage of folliculogenesis, and the uterus and oviducts were thin and immature. Circulating reproductive hormones were significantly decreased in affected males and females. There was also a dramatic reduction in the numbers of FSH- and LH-producing gonadotrophs. Meiotic mapping of the mutation and candidate gene sequencing determined that the N-ethyl-N-nitrosourea-induced lesion is in the third transmembrane domain of the GnRH receptor gene (Gnrhr). In vitro studies indicate that the mutant receptor is not coupled to the plasma membrane signal transduction system. Moreover, this mutant cannot be rescued with defined GnRH receptor pharmacoperones (pharmacological chaperones), an approach that rescues many other misfolded mutants. The mutant GnRH receptor was also shown to exert a dominant-negative effect on wild-type receptor function, indicating that the mutant receptor is unable to fold properly and likely misrouted within the cell, not reaching the plasma membrane. Surprisingly, Gnrhr mutant transcripts were significantly up-regulated in the pituitaries of Gnrhr mutants, revealing a previously unknown autoregulatory feedback loop. This is the first report of a mouse with a Gnrhr loss of function mutation. These GnRH-insensitive mice provide a novel animal model for the study of human idiopathic hypogonadotrophic hypogonadism.

Evi3, a zinc-finger protein related to EBFAZ, regulates EBF activity in B-cell leukemia

Retroviral insertions that activate proto-oncogenes are a primary cause of tumors in certain strains of mice. The AKXD recombinant inbred mice are predisposed to a variety of leukemias and lymphomas as a result of viral integration. One common insertion site, the ecotropic viral insertion site 3 (Evi3), has been implicated in most B-cell tumors in the AKXD-27 strain. The Evi3 gene encodes a zinc-finger protein with sequence similarity to the Early B-cell Factor-Associated Zinc-finger gene (EBFAZ). We show that the Evi3 gene is overexpressed in several tumors with viral insertions at Evi3, which results in the upregulation of Early B-cell Factor (EBF)-target gene expression, suggesting that Evi3 modulates EBF activity. Reconstitution of primary leukemia cells showed that these tumors express high densities of the B-cell surface proteins CD19 and CD38, which are EBF targets. Using a transactivation assay, we show that the terminal six zinc-fingers of Evi3 are required for modification of EBF activity. This is the first evidence that Evi3 expression in tumors alters the level of EBF target genes, and the first characterization of the Evi3 protein domains required for modulation of EBF activity. Further, these data imply that Evi3 misexpression initiates tumorigenesis by perturbing B-cell development via an interaction with EBF.

Comparative physical maps of the human and mouse Meckel syndrome critical regions

Meckel syndrome (MKS-OMIM 24900) is an autosomal recessive disease characterized by cystic kidneys, occipital encephalocele, polydactyly, and fibrotic changes of the liver, typically resulting in postnatal death. A Meckel syndrome critical region (MKS1) maps to human Chromosome (Chr) 17, in a region of homology to mouse Chr 11. Here we report the comparison of human Chr 17q23 with mouse Chr 11. We have generated physical maps of the human and mouse MKS1 critical regions. Additionally, we have created a transcript map of the MKS1 critical region in both species. By comparing these physical maps, we observe a high degree of similarity in gene order in the human and mouse Meckel syndrome critical regions. We have also examined the expression patterns of genes in the MKS1 region to assess their potential as MKS1 candidates. Finally, we have analyzed genes present in the other Meckel syndrome critical regions, MKS2 and MKS3, to determine whether any of the candidate genes for the three MKS loci have similar gene functions or are members of a common biological pathway.

Reduced penetrance of craniofacial anomalies as a function of deletion size and genetic background in a chromosome engineered partial mouse model for Smith–Magenis syndrome

Smith-Magenis syndrome (SMS) is a multiple congenital anomaly/mental retardation syndrome associated with del(17)(p11.2p11.2). The phenotype is variable even in patients with deletions of the same size. RAI1 has been recently suggested as a major gene for majority of the SMS phenotypes, but its role in the full spectrum of the phenotype remains unclear. Df(11)17/+ mice contain a heterozygous deletion in the mouse region syntenic to the SMS common deletion, and exhibit craniofacial abnormalities, seizures and marked obesity, partially reproducing the SMS phenotype. To further study the genetic basis for the phenotype, we constructed three lines of mice with smaller deletions [Df(11)17-1, Df(11)17-2 and Df(11)17-3] using retrovirus-mediated chromosome engineering to create nested deletions. Both craniofacial abnormalities and obesity have been observed, but the penetrance of the craniofacial phenotype was markedly reduced when compared with Df(11)17/+ mice. Overt seizures were not observed. Phenotypic variation has been observed in mice with the same deletion size in the same and in different genetic backgrounds, which may reflect the variation documented in the patients. These results indicate that the smaller deletions contain the gene(s), most likely Rai1, causing craniofacial abnormalities and obesity. However, genes or regulatory elements in the larger deletion, which are not located in the smaller deletions, as well as genes located elsewhere, also influence penetrance and expressivity of the phenotype. Our mouse models refined the genomic region important for a portion of the SMS phenotype and provided a basis for further molecular analysis of genes associated with SMS.

The extracellular matrix gene Frem1 is essential for the normal adhesion of the embryonic epidermis

Fraser syndrome is a rare recessive disorder characterized by cryptophthalmos, syndactyly, renal defects, and a range of other developmental abnormalities. Because of their extensive phenotypic overlap, the mouse blebbing mutants have been considered models of this disorder, and the recent isolation of mutations in Fras1 in both the blebbed mouse and human Fraser patients confirms this hypothesis. Here we report the identification of mutations in an extracellular matrix gene Fras1-related extracellular matrix gene 1 (Frem1) in both the classic head blebs mutant and in an N-ethyl-N-nitrosourea-induced allele. We show that inactivation of the gene results in the formation of in utero epidermal blisters beneath the lamina densa of the basement membrane and also in renal agenesis. Frem1 is expressed widely in the developing embryo in regions of epithelial/mesenchymal interaction and epidermal remodeling. Furthermore, Frem1 appears to act as a dermal mediator of basement membrane adhesion, apparently independently of the other known "blebs" proteins Fras1 and Grip1. Unlike both Fras1 and Grip1 mutants, collagen VI and Fras1 deposition in the basement membrane is normal, indicating that the protein plays an independent role in epidermal differentiation and is required for epidermal adhesion during embryonic development.

Checks and balancers: balancer chromosomes to facilitate genome annotation

Phenotype-driven mutagenesis screens are used to discover gene function in model organisms. Mutations that are induced by chemical mutagens can occur anywhere in the genome. However, the use of a balancer chromosome (where a phenotypically marked segment of a chromosome is inverted) in a mutagenesis screen enables mutations to be mapped in a defined region of the genome and maintained stably in a heterozygous state. Mouse balancer chromosomes can be engineered using Cre-loxP technology in selected regions of the genome. Balancer mutagenesis screens will provide a systematic functional analysis of the genes on mouse chromosomes, and consequently, will facilitate a functional annotation of the mammalian genome sequence.

Deletion of the Parkin coregulated gene causes male sterility in the quaking(viable) mouse mutant

Quaking(viable) (qk(v)) is a recessive neurological mouse mutation with severe dysmyelination of the CNS and spermiogenesis failure. The molecular lesion in the qk(v) mutant is a deletion of approximately 1 Mb on mouse chromosome 17 that alters the expression of the qk gene in oligodendrocytes. Complementation analysis between the qk(v) mutation and qk mutant alleles generated through chemical mutagenesis showed that the male sterility is a distinctive feature of the qk(v) allele. This observation suggested that the sperm differentiation defect in qk(v) is due to the deletion of a gene(s) distinct from qk. Here, we demonstrate that the deletion of Pacrg is the cause of male sterility in the qk(v) mutant. Pacrg is the mouse homologue of the human PARKIN-coregulated gene (PACRG), which encodes for a protein whose biochemical function remains unclear. We show that Pacrg is highly expressed in the testes in both mice and humans. In addition, the expression pattern of Pacrg during spermiogenesis suggests that it plays a role in sperm differentiation. In support of this hypothesis, we show that transgenic expression of Pacrg in testes restores spermiogenesis and fertility in qk(v) males. This finding provides the first in vivo evidence, to our knowledge, for the function of Pacrg in a model organism. Immunolocalization experiments on isolated spermatozoa show that the Pacrg protein is present in mature sperm. Remarkably, the mammalian Pacrg protein shares significant sequence similarities with gene products from flagellated protozoans, suggesting that Pacrg may be necessary for proper flagellar formation in many organisms.

Mutations in a Novel Locus on Mouse Chromosome 11 Resulting in Male Infertility Associated with Defects in Microtubule Assembly and Sperm Tail Function1

Traditional gene knock-out approaches using homologous recombination in embryonic stem cells are routinely used to provide functional information about genes involved in reproduction. In the present study, we examined a novel approach using N-ethyl-N-nitrosourea (ENU) together with a balancer chromosome mating strategy to identify new loci with functional roles in male fertility. Our genetic strategy is a forward-genetic approach; thus, our phenotypic investigation begins with the discovery of an abnormal phenotype without previous knowledge of the mutant locus. We isolated eight recessive mutations on chromosome 11 that resulted in male or female infertility from a screen of 184 founder pedigrees from ENU-treated males. After testing the six male infertile and two female infertile mutations for their ability to complement, we found that three independent recessive male infertile mutations failed to complement each other. The male infertility was associated with reduced epididymal sperm count, a block in late-spermatid differentiation, and increased apoptosis. Furthermore, the three male infertile mutants had severe defects in epididymal sperm morphology associated with incorrect microtubule assembly. Electron microscopy revealed unique defects in sperm head and tail morphology for each of the three alleles. One allele had an abnormal manchette assembly of the sperm head. The other two alleles had different abnormalities in the 9+2 patterning of the microtubules in the sperm tail axoneme, with one containing only five of the microtubule doublets and the other containing an extra doublet. The isolation of this allelic series identifies a new locus on mouse chromosome 11 that is required for spermiogenesis and male fertility.

A twist code determines the onset of osteoblast differentiation

Runx2 is necessary and sufficient for osteoblast differentiation, yet its expression precedes the appearance of osteoblasts by 4 days. Here we show that Twist proteins transiently inhibit Runx2 function during skeletogenesis. Twist-1 and -2 are expressed in Runx2-expressing cells throughout the skeleton early during development, and osteoblast-specific gene expression occurs only after their expression decreases. Double heterozygotes for Twist-1 and Runx2 deletion have none of the skull abnormalities observed in Runx2(+/-) mice, a Twist-2 null background rescues the clavicle phenotype of Runx2(+/-) mice, and Twist-1 or -2 deficiency leads to premature osteoblast differentiation. Furthermore, Twist-1 overexpression inhibits osteoblast differentiation without affecting Runx2 expression. Twist proteins' antiosteogenic function is mediated by a novel domain, the Twist box, which interacts with the Runx2 DNA binding domain to inhibit its function. In vivo mutagenesis confirms the antiosteogenic function of the Twist box. Thus, relief of inhibition by Twist proteins is a mandatory event precluding osteoblast differentiation.

The homeobox gene Hex induces T-cell-derived lymphomas when overexpressed in hematopoietic precursor cells

Proviral insertions at the viral insertion site Lvis1 occur frequently in B- and T-cell leukemias and lymphomas in AKXD mice and activate two nearby genes, the divergent homeobox gene Hex and the kinesin-related spindle protein gene Eg5. To determine whether Hex misexpression results in the altered differentiation or neoplastic transformation of hematopoietic lineages, we have transplanted mice with bone marrow cells transduced with retrovirus containing the Hex coding region. High levels of Hex expression in hematopoietic precursor cells inhibit contribution to mature blood cell lineages by these precursors. Hex bone marrow transplant recipient mice also develop hematologic neoplasms that appear to originate in the bone marrow. The tumors have clonal rearrangements of the TCR locus, are Thy1+, and are CD4+CD8+, CD4-CD8-, or mixed, indicating tumor origin from a precursor T-cell population. Tumors in transplant mice contain clonal and transcriptionally active Hex proviral insertions, demonstrating a causal role for Hex misexpression in the onset of these neoplasms. Our results demonstrate that Hex can act as a T lineage oncogene when misexpressed in hematopoietic precursor cells.

COP9 signalosome subunit 3 is essential for maintenance of cell proliferation in the mouse embryonic epiblast

Csn3 (Cops3) maps to the mouse chromosome 11 region syntenic to the common deletion interval for the Smith-Magenis syndrome, a contiguous gene deletion syndrome. It encodes the third subunit of an eight-subunit protein complex, the COP9 signalosome (CSN), which controls a wide variety of molecules of different functions. Mutants of this complex caused lethality at early development of both plants and Drosophila melanogaster. CSN function in vivo in mammals is unknown. We disrupted the murine Csn3 gene in three independent ways with insertional vectors, including constructing a approximately 3-Mb inversion chromosome. The heterozygous mice appeared normal, although the protein level was reduced. Csn3(-/-) embryos arrested after 5.5 days postcoitum (dpc) and resorbed by 8.5 dpc. Mutant embryos form an abnormal egg cylinder which does not gastrulate. They have reduced numbers of epiblast cells, mainly due to increased cell death. In the Csn3(-/-) mice, subunit 8 of the COP9 complex was not detected by immunohistochemical techniques, suggesting that the absence of Csn3 may disrupt the entire COP9 complex. Therefore, Csn3 is important for maintaining the integrity of the COP9 signalosome and is crucial to maintain the survival of epiblast cells and thus the development of the postimplantation embryo in mice.

Functional genetic analysis of mouse chromosome 11

Now that the mouse and human genome sequences are complete, biologists need systematic approaches to determine the function of each gene. A powerful way to discover gene function is to determine the consequence of mutations in living organisms. Large-scale production of mouse mutations with the point mutagen N-ethyl-N-nitrosourea (ENU) is a key strategy for analysing the human genome because mouse mutants will reveal functions unique to mammals, and many may model human diseases. To examine genes conserved between human and mouse, we performed a recessive ENU mutagenesis screen that uses a balancer chromosome, inversion chromosome 11 (refs 4, 5). Initially identified in the fruitfly, balancer chromosomes are valuable genetic tools that allow the easy isolation of mutations on selected chromosomes. Here we show the isolation of 230 new recessive mouse mutations, 88 of which are on chromosome 11. This genetic strategy efficiently generates and maps mutations on a single chromosome, even as mutations throughout the genome are discovered. The mutations reveal new defects in haematopoiesis, craniofacial and cardiovascular development, and fertility.

Tnfrsf13c (Baffr) is mis-expressed in tumors with murine leukemia virus insertions at Lvis22

In susceptible strains of mice, leukemia is caused by the somatic integration of murine leukemia retroviruses into the host genome. Integration sites that are common to several tumors are likely to affect genes that are important in oncogenesis. Here we present the analysis of a common site of retroviral integration on mouse chromosome 15, which includes the genomic structure of three genes near the integration site. One of the genes misexpressed at the insertion site has recently been characterized as a B-cell receptor, Tnfrsf13c (formerly Baffr), indicating that this approach is useful in defining genes that function in lymphocyte development and tumor progression. Current genome databases provide powerful resources for the rapid identification of genes at common proviral insertion sites. The characterization of these genes in tumor samples will allow a function to be assigned to many novel loci identified by the genome sequencing projects.

Evidence for translational regulation of the imprinted Snurf-Snrpn locus in mice

In studies of genomic imprinting in the Prader-Willi/Angelman domain, an agouti coat color cassette was inserted into the downstream open reading frame (ORF) of the imprinted bicistronic Snurf-Snrpn locus in the mouse. The fusion gene was maternally silenced, as is Snurf-Snrpn, and produced a tan abdomen only when inherited paternally in otherwise-black mice. A screen for dominant epigenetic or genetic events was performed with ENU mutagenesis, using a strategy whereby variation in abdominal color was scored at weaning. One mouse with maternal origin of the fusion gene had a tan abdomen and had an imprinting defect resulting in loss of both maternal methylation and silencing of the fusion gene. One mouse with paternal origin of the fusion gene was completely yellow and was found to have an ATG-to-AAG mutation in the initiation codon of the upstream ORF encoding SNURF. Northern blotting, immunoblotting, and transfection studies indicated that the ATG-to-AAG mutation causes a 15-fold or more increase in translation of the downstream ORF in two fusion constructs, and it is likely that similar translational control affects the normal Snurf-Snrpn transcript as well.

Expression, genomic structure and mapping of the thymus specific protease prss16: a candidate gene for insulin dependent diabetes mellitus susceptibility

PRSS16 is a serine protease specifically expressed by epithelial cells in the thymic cortex. The human gene is encoded on 6p21.3-p22 where recent linkage analysis has identified an association with insulin dependent diabetes mellitus (IDDM) susceptibility independent of HLA-DR3. To further investigate its potential role in autoimmunity, we characterized the mouse orthologue, Prss16. The genomic structure of Prss16 shows conservation with the human gene in size, number of exons and chromosomal location. Mapping of Prss16 places it on mouse chromosome 13 centromeric of thesatin locus. This region is comparable to the PRSS16 region on human chromosome 6 and has also been linked to quantitative trait locus for IDDM in the nonobese diabetic mouse. Similar to the human gene, Prss16 expression is highly specific in the mouse with expression limited to the cortical thymic epithelium. Notably, embryonic expression coincides with population of the thymic anlage with T-cell precursors and initiation of T-cell development. We also show that NOD and New Zealand Black mice, which have a disrupted thymic architecture and autoimmune phenotype, have lower levels of Prss16 expression compared to C57BL/6 mice. These findings support the role of Prss16 in T-cell development and susceptibility to autoimmunity in the mouse.

Quaking is essential for blood vessel development

For nearly 40 years functional studies of the mouse quaking gene (qkI) have focused on its role in the postnatal central nervous system during myelination. However, the homozygous lethality of a number of ENU-induced alleles reveals that quaking has a critical role in embryonic development prior to the start of myelination. In this article, we show that quaking has a previously unsuspected and essential role in blood vessel development. Interestingly, we found that quaking, a nonsecreted protein, is expressed in the yolk sac endoderm, adjacent to the mesodermal site of developing blood islands, where the differentiation of blood and endothelial cells first occurs. Antibodies against PE-CAM-1, TIE-2 and SM-alpha-actin reveal that embryos homozygous for the qk(k2) allele have defective yolk sac vascular remodeling and abnormal vessels in the embryo proper at midgestation, coinciding with the timing of embryonic death. However, these mutants exhibit normal expression of Nkx2.5 and alpha-sarcomeric actin, indicating that cardiac muscle differentiation was normal. Further, they had normal embryonic heart rates in culture, suggesting that cardiac function was not compromised at this stage of embryonic development. Together, these results suggest that quaking plays an essential role in vascular development and that the blood vessel defects are the cause of embryonic death.

Efficient generation and mapping of recessive developmental mutations using ENU mutagenesis

Treatment with N-ethyl-N-nitrosourea (ENU) efficiently generates single-nucleotide mutations in mice. Along with the renewed interest in this approach, much attention has been given recently to large screens with broad aims; however, more finely focused studies have proven very productive as well. Here we show how mutagenesis together with genetic mapping can facilitate the rapid characterization of recessive loci required for normal embryonic development. We screened third-generation progeny of mutagenized mice at embryonic day (E) 18.5 for abnormalities of organogenesis. We ascertained 15 monogenic mutations in the 54 families that were comprehensively analyzed. We carried out the experiment as an outcross, which facilitated the genetic mapping of the mutations by haplotype analysis. We mapped seven of the mutations and identified the affected locus in two lines. Using a hierarchical approach, it is possible to maximize the efficiency of this analysis so that it can be carried out easily with modest infrastructure and resources.

Dmbx1, a novel evolutionarily conserved paired-like homeobox gene expressed in the brain of mouse embryos

To identify novel homeobox genes expressed during mouse embryogenesis, we searched the databases and found a novel mouse paired-like homeobox gene, Dmbx1(diencephalon/mesencephalon-expressed brain homeobox gene 1), that is also conserved in zebrafish and human. Linkage analysis mapped mouse Dmbx1 to the mid-portion of chromosome 4 that is the homologous gene cluster region of human chromosome 1, where human DMBX1 is located. Both mouse and human Dmbx1/DMBX1 have four coding exons and their gene structures are conserved. Whole-mount in situ hybridization revealed that Dmbx1 expression is detected in 7.5-9.5 dpc mouse embryos. At 7.5 and 8.5 dpc, Dmbx1 is expressed in a sub-region of the anterior head folds. At 9.5 dpc, expression is observed in the caudal diencephalon as well as in the mesencephalon and is restricted to the neuroepithelium. Expression in adult tissues was detected in brain, stomach, and testis. Dmbx1 provides a unique marker of the developing anterior nervous system and should provide a useful molecular resource to elucidate the mechanisms that pattern the vertebrate brain.