Association of foreign DNA sequence with male sterility and translocation in a line of transgenic mice

Evaluation of eGFP expression in the ChAT-eGFP transgenic mouse brain

BACKGROUND

A historically definitive marker for cholinergic neurons is choline acetyltransferase (ChAT), a synthesizing enzyme for acetylcholine, (ACh), which can be found in high concentrations in cholinergic neurons, both in the central and peripheral nervous systems. ChAT, is produced in the body of the neuron, transported to the nerve terminal (where its concentration is highest), and catalyzes the transfer of an acetyl group from the coenzyme acetyl-CoA to choline, yielding ACh. The creation of bacterial artificial chromosome (BAC) transgenic mice that express promoter-specific fluorescent reporter proteins (green fluorescent protein-[GFP]) provided an enormous advantage for neuroscience. Both in vivo and in vitro experimental methods benefited from the transgenic visualization of cholinergic neurons. Mice were created by adding a BAC clone into the ChAT locus, in which enhanced GFP (eGFP) is inserted into exon 3 at the ChAT initiation codon, robustly and supposedly selectively expressing eGFP in all cholinergic neurons and fibers in the central and peripheral nervous systems as well as in non-neuronal cells.

METHODS

This project systematically compared the exact distribution of the ChAT-eGFP expressing neurons in the brain with the expression of ChAT by immunohistochemistry using mapping and also made comparisons with in situ hybridization (ISH).

RESULTS

We qualitatively described the distribution of ChAT-eGFP neurons in the mouse brain by comparing it with the distribution of immunoreactive neurons and ISH data, paying special attention to areas where the expression did not overlap, such as the cortex, striatum, thalamus and hypothalamus. We found a complete overlap between the transgenic expression of eGFP and the immunohistochemical staining in the areas of the cholinergic basal forebrain. However, in the cortex and hippocampus, we found small neurons that were only labeled with the antibody and not expressed eGFP or vice versa. Most importantly, we found no transgenic expression of eGFP in the lateral dorsal, ventral and dorsomedial tegmental nuclei cholinergic cells.

CONCLUSION

While the majority of the forebrain ChAT expression was aligned in the transgenic animals with immunohistochemistry, other areas of interest, such as the brainstem should be considered before choosing this particular transgenic mouse line.

Emergence of complex rearrangements at translocation breakpoints in a transgenic mouse; implications for mechanisms involved in the formation of chromosome rearrangements

Cryptic complex rearrangements as a result of a reciprocal chromosome translocation have been characterised in a transgenic mouse strain. Analysis of the breakpoint junctions in our previous studies showed that the ada transgene was integrated at the breakpoint forming a fusion gene with Golga3 (Mea2). In this study, further detailed analysis around the translocation junctions revealed that the surrounding regions were composed of 13 fragments of defined transgenic chromosome origins over approximately 1.9-Mb areas. Exactly the same cluster structure of these 13 breakpoint fragments already existed in the second generation of the transgenic mice. Our results show that this highly complex rearrangement has been conserved as the incipient form without any additional changes for 18 years up to the present generation, suggesting simultaneous occurrence of multiple events in the founder mouse.

High Toxicity, Low Receptor Density, and Low Integration Frequency Severely Impede the Use of Adenovirus Vectors for Production of Transgenic Mice

Although many methods are available for introducing genes into the mammalian germ line, none is ideal for genetic manipulation of livestock or primates. These organisms produce relatively few offspring in each reproductive cycle and they have long generation times. For these reasons, a recent report that adenovirus vectors can efficiently insert genes into the mouse germ line by embryo infection is of considerable interest. Adenovirus vectors have a high cloning capacity, can be produced in high titers, and can infect a wide variety of cell types. We have investigated in more detail the potential for such vectors to infect embryos and integrate their DNA into the genome. We exposed mouse embryos to adenovirus vectors that express bacterial beta-galactosidase (LacZ), and studied expression in the preimplantation period, toxicity of the vectors, and the frequency with which fetuses and pups integrate vector DNA. Our findings indicate that fully functional adenovirus receptor does not appear until the two-cell stage of development. Successful infection is associated with high toxicity, such that viral titers must be balanced to achieve high infection with tolerable levels of toxicity. Screening of 94 animals after embryo infection revealed a single positive polymerase chain reaction signal, which is indicative of the presence of the lacZ gene. This finding could not be confirmed by Southern blotting, which indicates that the founder animal was a genetic mosaic for the exogenous DNA. We conclude from these experiments that adenovirus gene transfer vectors are not readily usable for germ line gene insertion.

Germline alteration by gene therapy: assessing and reducing the risks

Developments in gene therapy are certain to lead to the treatment of an increasing variety of diseases, some of which will affect patients who might wish to have children following their gene therapy treatment. These circumstances raise the concern that germline integration of gene therapy vector DNA could occur. Although our current understanding of reproductive biology and of the biodistribution of gene therapy vectors administered to extragonadal sites indicate that this risk is low, animal experiments and clinical studies designed specifically to address this question are warranted; because of this risk, every gene therapy vector should be tested for its potential to integrate into germ cells and preimplantation embryos.

A new mouse mutation causing male sterility and histoincompatibility

Male sterility and histoincompatibility, mshi, is an autosomal recessive mutation in BALB/cBy mice that causes reduced testis size and sterility in homozygous males. The testes of homozygous mutants are highly disorganized and appear to have a block in the regulation of male germ cell proliferation. No heterozygous effect is detectable. Reproduction is unaffected in females carrying the mutation. The mutation also affects histocompatibility; most homozygous males and females reject sex-matched skin grafts from BALB/cBy mice. We used an intercross between BALB/cBy and CAST/Ei to map the mshi mutation to the proximal end of Chromosome (Chr) 10. The most likely gene order places the mutation between D10Mit80 and D10Mit16, near the interferon gamma receptor locus, Ifgr, which may be a candidate gene for this mutation.

Mini-mouse: phenotypic characterization of a transgenic insertional mutant allelic to pygmy

A phenotypic analysis was performed on two alleles at the pygmy locus which arose by insertional mutagenesis in transgenic mice. Similar to the spontaneous mutant pygmy, the adult insertional transgenic mutants are 40% of the size of wild-type litter-mates whereas adult heterozygotes are 80% of wild-type litter-mates. An analysis of the various organs revealed that, in general, there was a reduction in weight of each organ commensurate with the overall reduction in body size. However, two organs did not follow this pattern, the brain being disproportionately larger and the adrenals disproportionately smaller in the mutant mice. In addition, mini-mice have less adipose tissue than their wild-type or heterozygous litter-mates. A developmental analysis determined that mutants could first be identified on the basis of reduced body weight at day 15.5 of gestation. The small size is not due to a growth hormone deficiency so these mice differ from other known dwarf mouse mutants. Therefore they should provide insight into the growth hormone-resistant human dwarfisms and help in furthering our knowledge of mammalian growth and development.

Induced reciprocal translocation in transgenic mice near sites of transgene integration

Transgenic mice (JCP0 #18), heterozygous for an insertion of approximately 50 copies of the rat peripheral myelin (P0) protein cDNA, displayed a pattern of reduced litter size that suggested a chromosome rearrangement. Chromosome banding studies of fetal cells disclosed the presence of an apparently balanced translocation between a Chromosome (Chr) 1 and 14 with breakpoints at bands 1H3 and 14C3. In situ hybridization of biotin-labeled P0 rat cDNA probe to chromosome spreads and detection of specific signal with fluorescein isothiocyanate-conjugated avidin revealed a strong signal on the 1(14) translocation chromosome at the site of the breakpoint. A weaker signal was present near the breakpoint on the 14(1) derivative chromosome. These results suggest an etiologic relationship between the insertion of the transgene and the origin of the translocation. To further elucidate possible mechanisms, we first mapped the endogenous P0 gene (gene symbol Mpp). As previously reported (You et al., Genomics 9: 751, 1991), we found that Mpp is located on Chr 1 in the region of the translocation breakpoint in JCP0 mice. Subsequently, we have carried out pulsed-field gel and standard Southern analyses with P0 gene probes, but found no evidence for a direct involvement of the endogenous P0 gene in the process that generated the balanced reciprocal translocation. Thus, we favor the hypothesis that, during repair of DNA strand breakage--possibly induced by the microinjection procedure--the transgene copies were ligated to broken ends of Chrs 1 and 14. According to convention, this translocation is designated T(1;14)1Po. Homozygotes are phenotypically normal and breed well; they will be useful for genetic and physical mapping of Chrs 1 and 14.

Germ-cell deficient (gcd), an insertional mutation manifested as infertility in transgenic mice

A genetic analysis is necessary to gain a greater understanding of the complex developmental processes in mammals. Toward this end, an insertional transgenic mouse mutant has been isolated that results in abnormal germ-cell development. This recessive mutation manifests as infertility in both males and females and is specific for the reproductive organs, since all other tissues examined were histologically normal. A developmental analysis of the gonadal tissues demonstrated that the germ cells were specifically depleted as early as day 11.5 of embryonic development, while the various somatic cells were apparently unaffected. Therefore, the mutated locus must play a critical role in the migration/proliferation of primordial germ cells to the genital ridges of developing embryos. In addition, females homozygous for the mutation could potentially be a valuable animal model of a human syndrome, premature ovarian failure. This mutation has been named germ-cell deficient, gcd.

Symplastic spermatids (sys): a recessive insertional mutation in mice causing a defect in spermatogenesis

A line of transgenic mice that carries an insertional mutation in a gene essential for spermatogenesis is described. Males homozygous for the transgenic insert are sterile, while female homozygotes and both male and female heterozygotes exhibit normal fertility. Developing spermatids in homozygous males form prominent abnormal multinucleated syncytia (symplasts) and do not complete maturation. In addition, abnormal cytoplasmic vacuolation is commonly seen in Sertoli cells. One flank of the transgenic integration site within the genome has been cloned and used to show linkage between homozygosity for the transgene and the mutant phenotype. The flank maps to mouse chromosome 14 approximately 4 centimorgans proximal to the gene encoding esterase-10 (Es-10). As no other gene that is known to be essential for spermatogenesis has been mapped to this region of the genome and as the mutant phenotype is unique, the transgenic insert appears to affect a previously unidentified gene. We have named the mutation "symplastic spermatids" (sys).