Mutagenesis of diploid mammalian genes by gene entrapment

Application of an inducible transposon with anther culture in generation of di-haploid homologous mutants

BACKGROUND

Insertional mutagenesis represents one of the most effective ways to acquire information about a plant gene's function. However, it is hindered by the autosomal genome being diploid and therefore, most mutations being recessive. The problem is addressed by inducing the transposition during anther culture so that selected mutations can be transmitted and then regenerated to a homozygous state.

RESULTS

To this end, we treated transgenic rice floral tissues containing the inducible transposon with an inducer, salicylic acid. Excision events were detected in regenerated calli and subsequent plantlets. DNA blot and PCR assay were used to determine the homogeneity of knockout mutants. About 5% of the mutants containing transposition events were homozygous. Furthermore, the inducible transposon was active during calli regeneration.

CONCLUSIONS

This strategy could be applicable to improve transposition efficiency in microspore development stages to create stable di-haploid mutants in plants.

Cutting edge: the "death" adaptor CRADD/RAIDD targets BCL10 and suppresses agonist-induced cytokine expression in T lymphocytes

The expression of proinflammatory cytokines and chemokines in response to TCR agonists is regulated by the caspase-recruitment domain membrane-associated guanylate kinase 1 (CARMA1) signalosome through the coordinated assembly of complexes containing the BCL10 adaptor protein. We describe a novel mechanism to negatively regulate the CARMA1 signalosome by the "death" adaptor protein caspase and receptor interacting protein adaptor with death domain (CRADD)/receptor interacting protein-associated ICH-1/CED-3 homologous protein with a death domain. We show that CRADD interacts with BCL10 through its caspase recruitment domain and suppresses interactions between BCL10 and CARMA1. TCR agonist-induced interaction between CRADD and BCL10 coincides with reduction of its complex formation with CARMA1 in wild-type, as compared with Cradd-deficient, primary cells. Finally, Cradd-deficient spleen cells, CD4(+) T cells, and mice respond to T cell agonists with strikingly higher production of proinflammatory mediators, including IFN-γ, IL-2, TNF-α, and IL-17. These results define a novel role for CRADD as a negative regulator of the CARMA1 signalosome and suppressor of Th1- and Th17-mediated inflammatory responses.

Gene trap mutagenesis in the mouse

Gene trapping in mouse embryonic stem (ES) cells is an efficient method for the mutagenesis of the mammalian genome. Insertion of a gene trap vector disrupts gene function, reports gene expression, and provides a convenient tag for the identification of the insertion site. The trap vector can be delivered to ES cells by electroporation of a plasmid, by retroviral infection, or by transposon-mediated insertion. Recent developments in trapping technology involve the utilization of site-specific recombination sites, which allow the induced modification of trap alleles in vitro and in vivo. Gene trapping strategies have also been successfully developed to screen for genes that are acting in specific biological pathways. In this chapter, we review different applications of gene trapping, and we provide detailed experimental protocols for gene trapping in ES cells by retroviral and transposon gene trap vectors.

A wider context for gene trap mutagenesis

Gene trapping is a technology originally developed for the simultaneous identification and mutation of genes by random integration in embryonic stem (ES) cells. While gene trapping was developed before efficient and high-throughput gene targeting, a significant proportion of the publically available mutant ES cell lines and mice were generated through a number of large-scale gene trapping initiatives. Moreover, elements of gene trap vectors continue to be incorporated into gene targeting vectors as a means to increase the efficiency of homologous recombination. Here, we review the current state of gene trapping technology and the applications of specific types of gene trap vector. As a component of this analysis, we consider the behavior of specific vector types both from the perspective of their application and how they can inform our annotation of the mammalian transcriptome. We consider the utility of gene trap vectors as tools for cell-based expression analysis, targeted screening in embryonic differentiation, and for use in cell lines derived from different lineages.

Expression-independent gene trap vectors for random and targeted mutagenesis in embryonic stem cells

Promoterless gene trap vectors have been widely used for high-efficiency gene targeting and random mutagenesis in embryonic stem (ES) cells. Unfortunately, such vectors are only effective for genes expressed in ES cells and this has prompted the development of expression-independent vectors. These polyadenylation (poly A) trap vectors employ a splice donor to capture an endogenous gene's polyadenylation sequence and provide transcript stability. However, the spectrum of mutations generated by these vectors appears largely restricted to the last intron of target loci due to nonsense-mediated mRNA decay (NMD) making them unsuitable for gene targeting applications. Here, we present novel poly A trap vectors that overcome the effect of NMD and also employ RNA instability sequences to improve splicing efficiency. The set of random insertions generated with these vectors show a significantly reduced insertional bias and the vectors can be targeted directly to a 5' intron. We also show that this relative positional independence is linked to the human beta-actin promoter and is most likely a result of its transcriptional activity in ES cells. Taken together our data indicate that these vectors are an effective tool for insertional mutagenesis that can be used for either gene trapping or gene targeting.

Dyggve-Melchior-Clausen syndrome: chondrodysplasia resulting from defects in intracellular vesicle traffic

Dyggve-Melchior-Clausen syndrome and Smith-McCort dysplasia are recessive spondyloepimetaphyseal dysplasias caused by loss-of-function mutations in dymeclin (Dym), a gene with previously unknown function. Here we report that Dym-deficient mice display defects in endochondral bone formation similar to that of Dyggve-Melchior-Clausen syndrome and Smith-McCort dysplasia, demonstrating functional conservation between the two species. Dym-mutant cells display multiple defects in vesicle traffic, as evidenced by enhanced dispersal of Golgi markers in interphase cells, delayed Golgi reassembly after brefeldin A treatment, delayed retrograde traffic of an endoplasmic reticulum-targeted Shiga toxin B subunit, and altered furin trafficking; and the Dym protein associates with multiple cellular proteins involved in vesicular traffic. These results establish dymeclin as a novel protein involved in Golgi organization and intracellular vesicle traffic and clarify the molecular basis for chondrodysplasia in mice and men.

Subunit 1 of the prefoldin chaperone complex is required for lymphocyte development and function

Prefoldin is a hexameric chaperone that facilitates posttranslational folding of actins and other cytoskeletal proteins by the Tcp1-containing ring complex chaperonin, TriC. The present study characterized mice with a null mutation in Pfdn1, which encodes the first subunit of the Prefoldin complex. Pfdn1-deficient mice displayed phenotypes characteristic of defects in cytoskeletal function, including manifestations of ciliary dyskinesia, neuronal loss, and defects in B and T cell development and function. B and T cell maturation was markedly impaired at relatively early stages, namely at the transitions from pre-pro-B to pre-B cells in the bone marrow and from CD4-CD8- double-negative to CD4+CD8+ double-positive T cells in the thymus. In addition, mature B and T lymphocytes displayed cell activation defects upon Ag receptor cross-linking accompanied by impaired Ag receptor capping in B cells. These phenotypes illustrate the importance of cytoskeletal function in immune cell development and activation.

Developmental and adult phenotyping directly from mutant embryonic stem cells

Tetraploid embryo complementation assay has shown that mouse ES cells alone are capable of supporting embryonic development and adult life of mice. Newly established F(1) hybrid ES cells allow the production of ES cell-derived animals at a high enough efficiency to directly make ES cell-based genetics feasible. Here we report the establishment and characterization of 12 new F(1) hybrid ES cell lines and the use of one of the best (G4) in a gain- and loss-of-function genetic study, where the in vivo phenotypes were assessed directly from ES cell-derived embryos. We found the generation of G4 ES cell-derived animals to be very efficient. Furthermore, even after two consecutive rounds of genetic modifications, the majority of transgenic lines retained the original potential of the parental lines; with 10-40% of chimeras producing ES cell-derived animals/embryos. Using these genetically altered ES cells, this success rate, in most cases, permitted the derivation of a sufficient number of mutants for initial phenotypic analyses only a few weeks after the establishment of the cell lines. Although the experimental design has to take into account a moderate level of uncontrolled damage on ES cell lines, our proof-of-principle experiment provides useful data to assist future designs harnessing the power of this technology to accelerate our understanding of gene function.