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Biomedical subjects

K A Mahon

Publications and source records attributed to K A Mahon.

35 records · Page 2Linked to original sources
PubMed1992

Development shows some backbone. HFSP Workshop on Genetic Control of Vertebrate Development cosponsored by the Human Frontier Science Program, European Science Foundation, and European Molecular Biology Organization, Les Diablerets, Switzerland, May 26-30, 1991.

This meeting aptly illustrated the power of a combined analysis of development in a range of vertebrate systems. Each system has its own inherent strengths: the mouse has gene transfer technology and targeted mutagenesis, the frog and chick have experimental embryology, and the zebrafish has genetics. It is the synergistic effect of considering all of these systems in combination that is without measure. In the past, the study of vertebrate development has been relegated to a largely descriptive phase. Initially, this was through analysis of morphological changes taking place during development. More recently, this has taken the form of cataloging the expression patterns of genes transcribed in development. It is clear that we are now entering an era when a functional analysis of development can get underway.

Animals↗
PubMed1991

Inactivation of a sperm motility gene by insertion of an epidermal growth factor receptor transgene whose product is overexpressed and compartmentalized during spermatogenesis.

Transgenic mice were generated with a human epidermal growth factor (EGF) receptor cDNA driven by the chicken beta-actin gene promoter. One line (AE24) that exhibited a unique expression pattern in which dramatically elevated levels of EGF receptor RNA were found only in the testis was established, suggesting that the beta-actin promoter was being influenced by an adjacent testis-specific enhancer. EGF receptor RNA was detected in primary spermatocytes, whereas the synthesis of receptor protein was restricted to elongate spermatids, indicating that transgene expression was under translational control. At spermiation, the EGF receptor was sequestered in residual bodies and excluded from mature sperm by a compartmentalization mechanism. About half of AE24 homozygous males were sterile because of sperm paralysis, whereas heterozygous males and females of either genotype were completely fertile. Electron microscopic analysis of sperm flagella from sterile AE24 homozygotes revealed an aberrant axonemal structure in which outer doublet microtubules were missing from the middle piece, resembling changes observed in the sperm of some infertile humans. Flagellar axonemal disassembly was observed in the vas deferens and epididymis but not in the testis, suggesting that outer doublets were assembled in a grossly normal manner but possessed a latent instability. These results demonstrate that in the AE24 mouse line the EGF receptor transgene was integrated into and inactivated an endogenous autosomal gene, causing sperm flagellar axonemal disruption and male sterility.

Actins↗
PubMed1991

Ghox 4.7: a chick homeobox gene expressed primarily in limb buds with limb-type differences in expression.

Homeobox genes play a key role in specifying the segmented body plan of Drosophila, and recent work suggests that at least several homeobox genes may play a regulatory role during vertebrate limb morphogenesis. We have used degenerate oligonucleotide primers from highly conserved domains in the homeobox motif to amplify homeobox gene segments from chick embryo limb bud cDNAs using the polymerase chain reaction. Expression of a large number of homeobox genes (at least 17) is detected using this approach. One of these genes contains a novel homeobox loosely related to the Drosophila Abdominal B class, and was further analyzed by determining its complete coding sequence and evaluating its expression during embryogenesis by in situ hybridization. Based on sequence and expression patterns, we have designated this gene as Ghox 4.7 and believe that it is the chick homologue of the murine Hox 4.7 gene (formerly Hox 5.6). Ghox 4.7 is expressed primarily in limb buds during development and shows a striking spatial restriction to the posterior zone of the limb bud, suggesting a role in specifying anterior-posterior pattern formation. In chick, this gene also displays differences in expression between wing and leg buds, raising the possibility that it may participate in specifying limb-type identity.

Animals↗
PubMed1991

Spatially restricted expression of a member of a new family of murine Distal-less homeobox genes in the developing forebrain.

The homeobox genes of Drosophila perform key functions in embryonic pattern formation, and their vertebrate counterparts may play similar developmental roles. Using polymerase chain reaction technology, we have identified four murine homologs of the Drosophila Distal-less homeobox gene that are expressed in midgestation embryos. The homeodomains encoded by these genes vary considerably from other known homeodomain sequences and represent a new family of vertebrate homeobox genes. We isolated a cDNA for one of these genes (Dlx-2) and studied its expression by in situ hybridization from 8.5 days postcoitum (pc) until postnatal day 1. Dlx-2 shows a restricted pattern of expression in the ventral forebrain, extending from the olfactory bulb to the ventral diencephalon. This domain of expression may delineate an ontogenetically defined subdivision within the forebrain. The murine Distal-less genes are the first homeobox genes described whose expression in the central nervous system is exclusively restricted to the forebrain. Thus, the Distal-less genes may contribute missing positional cues not provided by the previously identified vertebrate homeobox genes.

Amino Acid Sequence↗
PubMed1989

Differentiation and oncogenesis: phenotypically distinct lens tumors in transgenic mice.

We report on two lines of transgenic mice that express a murine alpha A-crystallin/SV40 tumor antigen fusion gene in the eye lens. The alpha T1 line develops fast growing, poorly differentiated lens tumors, whereas the alpha T2 line produces lens tumors that are slow growing and well differentiated. There is a striking difference between these two lines in the temporal and spatial patterns of tumor antigen expression during initial lens development. In the alpha T1 line, the transgene is expressed very early in development in most lens cells, and no primary fiber differentiation takes place. In the alpha T2 line, transgene expression occurs after primary fiber formation has been initiated, and is restricted to differentiating fiber cells. The anterior epithelium from both alpha T lines undergoes normal development and remains morphologically normal until after birth, although in alpha T1 mice, these anterior cells produce considerable amounts of SV40 tumor antigens. This suggests that the state of differentiation of the lens cell plays an important role in its response to oncogene products.

Animals↗
PubMed1988

Expression and amplification in transgenic mice of a polyoma virus mutant regulatory region.

Two hybrid gene constructs consisting of wild-type and mutant polyoma regulatory regions fused to a bacterial reporter gene were inserted in the mouse germline. Both transgenes were expressed in a large number of different organs. However, marker gene expression controlled by the polyoma wild-type regulatory region was not detectable in the early embryo and remained low throughout the life of the animal while expression controlled by the polyoma F9-1 mutation was detectable in blastocysts and was significantly higher at later stages of development. The F9-1 hybrid gene was also amplifiable when large T-antigen was supplied in trans to mice or to kidney cells derived from these transgenic mice. Amplification resulted in the appearance of several hundred copies of episomal transgenes and a marked increase of marker gene RNA and protein. Our results suggest that the F9-1 mutation does not alter the target spectrum of gene expression in vivo but does create a more efficient enhancer element in the polyoma early control region. Transgene amplification based upon use of the polyoma regulatory elements may be a means of increasing expression of genes in transgenic mice.

Age Factors↗
PubMed1988

Prenatal lethality in a transgenic mouse line is the result of a chromosomal translocation.

We have produced a line of transgenic mice that is characterized by prenatal lethality. These mice bear a chimeric plasmid containing the long terminal repeat of the Rous sarcoma virus linked to the coding region of the chloramphenicol acetyltransferase gene (pRSV-CAT). Mice heterozygous for the pRSV-CAT integration site are semisterile, producing litters approximately equal to 40% of the average size when crossed to normal mice. Approximately 50% of the progeny from such a cross bear the pRSV-CAT sequences and also produce litters of smaller size. An analysis of embryogenesis revealed that normal numbers of embryos implanted, but 60% failed to develop past day 7. Eight other independent transgenic lines containing RSV-CAT show no evidence of embryonic lethality; thus, it is unlikely that the defect observed is due to the direct effects of RSV-CAT expression. We have found that carrier mice bear a reciprocal translocation between chromosomes 6 and 17, T(6A2-6A3;17D-17E1), that can explain the apparent dominant embryonic lethality seen in this line. The site of integration has been localized by in situ hybridization at or near the translocation breakpoint in one of the translocated chromosomes (6(17)). Because the foreign DNA is present in one of the translocated chromosomes, we propose that this rearrangement was elicited by the introduction of foreign DNA.

Animals↗
PubMed1988

Expression of homeobox gene Hox 1.1 during mouse embryogenesis.

Many of the genes controlling segmentation and pattern formation in Drosophila contain a conserved 183 bp sequence known as the homeobox. Homeobox sequences have been found in a range of metazoan species, including the vertebrates mouse and man. This striking conservation suggests that homeobox genes may play a fundamental role in developmental processes. If this is the case then it might be expected that vertebrate homeobox genes will be differentially expressed during embryogenesis and that the timing of their expression will coincide with major morphogenetic events. Here the spatial and temporal patterns of expression of murine homeobox genes will be explored, concentrating on the Hox 1.1 gene as an example. Using in situ hybridization to localize RNA transcripts, it has been found that Hox 1.1 is expressed in a region-specific manner during the formation and differentiation of the embryonic anteroposterior axis. Although striking patterns of expression of Hox 1.1 and other homeobox genes are seen in overtly segmented structures of the embryo (i.e. somites, prevertebral elements, neural tube and dorsal spinal ganglia) expression is also seen in tissues with no obvious segmental origin. The results suggest that homeobox genes probably do not play an exclusive role in segmentation in vertebrates, but are consistent with a role in the assignment of positional identity along the axis of the embryo.

Animals↗
PubMed1987

Oncogenesis of the lens in transgenic mice.

Neoplastic tumors of the ocular lens of vertebrates do not naturally occur. Transgenic mice carrying a hybrid gene comprising the murine alpha A-crystallin promoter (-366 to +46) fused to the coding sequence of the SV40 T antigens developed lens tumors, which obliterated the eye cavity and even invaded neighboring tissue, thus establishing that the lens is not refractive to oncogenesis. Large-T antigen was detected early in lens development; it elicited morphological changes and specifically interfered with differentiation of lens fiber cells. Both alpha- and beta-crystallins persisted in many of the lens tumor cells, while gamma-crystallin was selectively reduced. Accessibility, characteristic morphology, and defined protein markers make this transparent epithelial eye tissue a potentially useful system for testing tumorigenicity of oncogenes and for studying malignant transformation from its inception until death of the animal.

Animals↗
PubMed1987

Crystallin genes: lens specificity of the murine alpha A-crystallin gene.

The abundant soluble proteins of the eye lens, the crystallins, are encoded by several gene families which are developmentally regulated in the embryonic lens. We have studied the expression of the murine alpha A-crystallin gene. Transfection experiments using the pSVO-CAT vector and explanted lens epithelia from embryonic chickens demonstrated proximal (-88 to -60) and distal (-111 to -85) regulatory sequences which interact when the alpha A-crystallin promoter is activated in the lens cells. Transgenic mouse experiments showed that the sequence between positions -366 to +46 of the alpha A-crystallin promoter can drive foreign genes selectively in the lens. A fusion gene consisting of this alpha A-crystallin promoter sequence and the T-antigen gene of SV40 produced a lens tumor in transgenic mice. Thus, crystallin promoters provide a useful model for tissue-specific gene expression and permit targeting the expression of foreign genes to a highly differentiated tissue during development.

Animals↗
PubMed1986

Transcription of a satellite DNA in the newt.

Satellite 2 is an abundant, 330-bp tandemly repeated sequence in the genome of the newt, Notophthalmus viridescens. This sequence is distributed throughout the genome on each of the 11 chromosomes. Both strands of satellite 2 are transcribed on the lampbrush chromosomes during oogenesis, probably as a result of readthrough from upstream structural gene promoters. In addition to these heterogeneous nuclear transcripts, satellite 2 is homologous to stable, strand-specific cytoplasmic transcripts in a variety of different tissues. The majority of these transcripts correspond in size to the entire satellite 2 repeat unit, or to whole multiples of the repeat. The transcripts present in the ovary have been sequenced by primer extension and were found to be more homogeneous than eight independently cloned satellite 2 DNA repeats. We propose that the stable cytoplasmic transcripts are encoded by a small subset of genomic satellite 2 sequences.

Animals↗
PubMed1984

Histone RNA in amphibian oocytes visualized by in situ hybridization to methacrylate-embedded tissue sections.

We present an in situ hybridization method for detecting cellular RNAs in tissue sections using methacrylate as the embedding medium. The technique offers the advantage of superior morphological preservation compared with previously published procedures. Since sections can be cut 1 micron or less in thickness, full advantage is taken of the short path length of 3H electrons. Applying this procedure to developing amphibian oocytes, we investigated the accumulation and localization of RNA complementary to the histone genes and their adjacent spacers. Histone RNA begins to accumulate in the cytoplasm of late pachytene-early diplotene oocytes, rapidly reaching a maximum concentration during Dumont stage 1. After this stage the concentration of histone RNA declines. RNA transcribed from histone coding regions is located almost exclusively in the cytoplasm of oocytes. Transcripts of the spacer regions, which are known to be synthesized on oocyte lampbrush chromosomes, do not accumulate in the oocytes. [3H]RNA complementary to U2 small nuclear RNA, used in these experiments as a control, hybridized predominantly to the nucleus of the oocytes.

Animals↗
PubMed1984

Research with the mentally retarded and mentally ill: rights and duties versus compelling state interest.

This paper examines the assumptions underlying the ethical reasoning involved when the natural right to informed consent has been violated. Ramifications of the compelling need of the state to protect society by overriding the rights of individual freedom receive special attention. Specifically these issues are addressed: natural rights versus state's rights to preserve rights, state's rights versus parental autonomy, rights and duties versus paternalism, self interest and self survival versus the common good, coercion versus progress in research, and benefit versus cost. Building on these ethical considerations, the authors present a theoretical framework for ethical decision making with research involving the mentally retarded and the mentally ill.

Civil Rights↗
PubMed1981

Transcripts from both strands of a satellite DNA occur on lampbrush chromosome loops of the newt Notophthalmus.

We have isolated genomic DNA clones from the newt Notophthalmus which contain one or more copies of a 222 bp tandemly repeated sequence (satellite 1). Satellite 1 occurs in the pericentromeric heterochromatin of all chromosomes and at the sphere loci of chromosomes 2 and 6. Lampbrush chromosome loops associated with the spheres contain transcripts of satellite 1, as shown by in situ hybridization. The matrix on these loops consists of multiple thin--thick transcription units, most but not all of which have the same polarity within a given loop. By in situ hybridization with single-stranded probes we show that transcription units of opposite morphological polarity contain transcripts of opposite strands of satellite 1. Satellite 1 occurs as the spacer between clusters of histone genes. We postulate that transcription initiates at a histone gene promoter, fails to terminate at the end of the gene and continues without interruption into the adjacent satellite DNA. In this way, long transcripts are produced which contain both histone and satellite 1 sequences. A general failure of termination signals might explain the unusually long transcripts of lampbrush chromosomes.

Animals↗