Identification of an origin of bidirectional DNA replication in the ubiquitously expressed mammalian CAD gene.

Most DNA replication origins in eukaryotes localize to nontranscribed regions, and there are no reports of origins within constitutively expressed genes. This observation has led to the proposal that there may be an incompatibility between origin function and location within a ubiquitously expressed gene. The biochemical and functional evidence presented here demonstrates that an origin of bidirectional replication (OBR) resides within the constitutively expressed housekeeping gene CAD, which encodes the first three reactions of de novo uridine biosynthesis (carbamoyl-phosphate synthetase, aspartate carbamoyltransferase, and dihydroorotase). The OBR was localized to a 5-kb region near the center of the Syrian hamster CAD transcriptional unit. DNA replication initiates within this region in the single-copy CAD gene in Syrian baby hamster kidney cells and in the large chromosomal amplicons that were generated after selection with N-phosphonacetyl-L-aspartate, a specific inhibitor of CAD. DNA synthesis also initiates within this OBR in autonomously replicating extrachromosomal amplicons (CAD episomes) located in an N-phosphonacetyl-L-aspartate-resistant clone (5P20) of CHOK1 cells. CAD episomes consist entirely of a multimer of Syrian hamster CAD cosmid sequences (cCAD1). These data limit the functional unit of replication initiation and timing control to the 42 kb of Syrian hamster sequences contained in cCAD1. In addition, the data indicate that the origin recognition machinery is conserved across species, since the same OBR region functions in both Syrian and Chinese hamster cells. Importantly, while cCAD1 exhibits characteristics of a complete replicon, we have not detected autonomous replication directly following transfection. Since the CAD episome was generated after excision of chromosomally integrated transfected cCAD1 sequences, we propose that prior localization within a chromosome may be necessary to "license" some biochemically defined OBRs to render them functional.

Localization of a bidirectional DNA replication origin in the native locus and in episomally amplified murine adenosine deaminase loci.

Gene amplification is frequently mediated by the initial production of acentric, autonomously replicating extrachromosomal elements. The 4,000 extrachromosomal copies of the mouse adenosine deaminase (ADA) amplicon in B-1/50 cells initiate their replication remarkably synchronously in early S phase and at approximately the same time as the single-copy chromosomal locus from which they were derived. The abundance of ADA sequences and favorable replication timing characteristics in this system led us to determine whether DNA replication initiates in ADA episomes within a preferred region and whether this region is the same as that used at the corresponding chromosomal locus prior to amplification. This study reports the detection and localization of a discrete set of DNA fragments in the ADA amplicon which label soon after release of synchronized B-1/50 cells into S phase. A switch in template strand complementarity of Okazaki fragments, indicative of the initiation of bidirectional DNA replication, was found to lie within the same region. This putative replication origin is located approximately 28.5 kbp upstream of the 5' end of the ADA gene. The same region initiated DNA replication in the single-copy ADA locus of the parental cells. These analyses provide the first evidence that the replication of episomal intermediates involved in gene amplification initiates within a preferred region and that the same region is used to initiate DNA synthesis within the native locus.

Double minute chromosomes can be produced from precursors derived from a chromosomal deletion.

Recent experiments have shown that gene amplification can be mediated by submicroscopic, autonomously replicating, circular extrachromosomal molecules. We refer to those molecules as episomes (S. Carroll, P. Gaudray, M. L. DeRose, J. F. Emery, J. L. Meinkoth, E. Nakkim, M. Subler, D. D. Von Hoff, and G. M. Wahl, Mol. Cell. Biol. 7:1740-1750, 1987). The experiments reported in this paper explore the way episomes are formed and their fate in the cell over time. The data reveal that in our system the episomes are initially 250 kilobases, but gradually enlarge until they become double minute chromosomes. In addition, we show that episomes or double minute chromosomes can integrate into chromosomes. Our results also suggest that episomes can be produced by deletion of the corresponding sequences from the chromosome.

FGF-18, a novel member of the fibroblast growth factor family, stimulates hepatic and intestinal proliferation.

The fibroblast growth factors (FGFs) play key roles in controlling tissue growth, morphogenesis, and repair in animals. We have cloned a novel member of the FGF family, designated FGF-18, that is expressed primarily in the lungs and kidneys and at lower levels in the heart, testes, spleen, skeletal muscle, and brain. Sequence comparison indicates that FGF-18 is highly conserved between humans and mice and is most homologous to FGF-8 among the FGF family members. FGF-18 has a typical signal sequence and was glycosylated and secreted when it was transfected into 293-EBNA cells. Recombinant murine FGF-18 protein (rMuFGF-18) stimulated proliferation in the fibroblast cell line NIH 3T3 in vitro in a heparan sulfate-dependent manner. To examine its biological activity in vivo, rMuFGF-18 was injected into normal mice and ectopically overexpressed in transgenic mice by using a liver-specific promoter. Injection of rMuFGF-18 induced proliferation in a wide variety of tissues, including tissues of both epithelial and mesenchymal origin. The two tissues which appeared to be the primary targets of FGF-18 were the liver and small intestine, both of which exhibited histologic evidence of proliferation and showed significant gains in organ weight following 7 (sometimes 3) days of FGF-18 treatment. Transgenic mice that overexpressed FGF-18 in the liver also exhibited an increase in liver weight and hepatocellular proliferation. These results suggest that FGF-18 is a pleiotropic growth factor that stimulates proliferation in a number of tissues, most notably the liver and small intestine.

Expression of keratinocyte growth factor in embryonic liver of transgenic mice causes changes in epithelial growth and differentiation resulting in polycystic kidneys and other organ malformations.

Expression of human keratinocyte growth factor (KGF/FGF-7) was directed to hepatocytes during the later period of mouse gestation using a human apolipoprotein E (ApoE) gene promoter and its associated liver-specific enhancer. Human KGF was detectable in liver extracts and serum prepared from e17.5-e19.5 embryos, concomitant with the appearance of morphological abnormalities in several organs which express KGF receptor. The most striking phenotypic aberration in the ApoE-hKGF transgenic embryos was marked hyperplasia and cystic dilation of the cortical and medullary kidney collecting duct system, a phenotype resembling infantile polycystic kidney disease in humans. Transgenic embryos had enlarged livers, with prominent biliary epithelial hyperplasia, and also exhibited enhanced bronchiolar epithelial and type II pneumocyte proliferation. There was variable hyperplasia of intestinal epithelia, and urothelium of the urinary bladder and ureters. When compared to age-matched littermate controls, marked epidermal papillomatous acanthosis and hyperkeratosis in the skin, with a notable decrease in the number of developing hair follicles was seen in transgenic embryos. The pancreas exhibited significant ductal hyperplasia, with an increase in the number of ductal epithelial cells staining positive for insulin expression. High systemic levels of KGF during the latter stages of embryogenesis causes abnormalities in epithelial growth and differentiation within multiple organ systems and results in perinatal lethality. Correct temporal and spatial expression of KGF during the latter stages of organ development is likely to play a critical role in mesenchymal-epithelial signaling required for normal embryonic growth and development.

The evolution of trade-offs: effects of inbreeding on fecundity relationships in the cricket Gryllus firmus.

The evolution of traits is modulated by their interrelationships with each other, particularly when those relationships result in a fitness trade-off. In this paper we explore the consequences of genetic architecture on functional relationships between traits. Specifically, we address the consequences of inbreeding on these relationships. We show that the linear regression between two traits will not be affected if there is no dominance genetic variance in either trait, whereas the intercept but not the slope of the regression will change if there is dominance genetic variance in one trait only. We test the latter hypothesis using fecundity relationships in the cricket Gryllus firmus. Data from pedigree analysis and an inbreeding experiment show that there is significant dominance genetic variance in fecundity, but not head width (an index of body size) or dorsal longitudinal muscle (DLM) mass. Fecundity increases with head width, but decreases with DLM mass. As predicted, the intercepts of the regressions of fecundity on these two morphological traits decrease with inbreeding, but there is little or no change in slope. Gryllus firmus is wing dimorphic, with the macropterous (LW) morph having a lower fecundity than the micropterous (SW) morph. We hypothesize that the difference in fecundity arises primarily because of a competition for resources in the LW females between DLM maintenance (i.e., mass) and egg production. As a consequence, we predict that the fecundity within each morph should decline linearly with the inbreeding coefficient at the same rate in both morphs. The result of this will be a change in the relative fitness of the two morphs, that of the SW morph increasing with inbreeding. This prediction is supported. These results indicate that trade-offs will evolve and such changes will affect evolutionary trajectories by altering the pattern of relationships among fitness components.

Can alphabet recall be part of a visuo-spatial task?

Fifty male and 50 female undergraduates were subjects in an experiment where they were required to recall the alphabet and to count either the number of letters containing the sound 'ee' or the number of letters with curves in the upper-case form. The results fail to replicate those of Coltheart et al. (1975), but are consistent with current models of hemispheric specialization. Males were found to complete the visuo-spatial task slower than the females and it is argued that this is due to their greater degree of hemispheric specialization which necessitates transfer of information between hemispheres.

Molecular dissection of an extrachromosomal amplicon reveals a circular structure consisting of an imperfect inverted duplication.

A mouse fibroblast line, B-1/50, with a 4300-fold amplification of the adenosine deaminase gene locus (Yeung et al., 1983, J. Biol. Chem. 258: 8338-8345), was shown by in situ hybridization to harbor the amplified sequences on variously sized extrachromosomal elements. We show here that the smallest circle is approximately 500 kb. We describe a facile screening technique for identifying cosmid and yeast artificial chromosome (YAC) clones derived from the amplicon. A closed molecular map was generated by arranging the cosmids and YACs into a contig spanning over 250 kb of the adenosine deaminase gene locus. YACs from the two ends of this contig were shown to delimit a 250-kb inverted duplication. Long-range mapping of a SalI partial digest of B-1/50 DNA is also consistent with the interpretation that the 500-kb adenosine deaminase amplicon in B-1/50 cells is an inverted duplication. The finding that this amplicon is the only or predominant structure containing amplified sequences in the B-1/50 cell line suggests that such structures are not inherently prone to high frequency rearrangement, even when present at such high copy number. This study provides the first molecular description of the structure of an episome involved in mammalian gene amplification. The implications of this finding for models of gene amplification and episome formation are discussed.

Effect of chromosomal position on amplification of transfected genes in animal cells.

A single protein (CAD) contains the first three enzymatic activities of de novo uridine biosynthesis. The chromosomal location of CAD genes introduced into Chinese hamster ovary cells significantly affects the frequency and cytogenetic result of their amplification. The amplification frequency in one transformant is 100-fold that of the others; in another, amplification of donated genes inserted near a centromere results in chromosome instability and rearrangements.

Fgf-10 is required for both limb and lung development and exhibits striking functional similarity to Drosophila branchless.

Fgf-10-deficient mice (Fgf-10(-/-)) were generated to determine the role(s) of Fgf-10 in vertebrate development. Limb bud initiation was abolished in Fgf-10(-/-) mice. Strikingly, Fgf-10(-/-) fetuses continued to develop until birth, despite the complete absence of both fore- and hindlimbs. Fgf-10 is necessary for apical ectodermal ridge (AER) formation and acts epistatically upstream of Fgf-8, the earliest known AER marker in mice. Fgf-10(-/-) mice exhibited perinatal lethality associated with complete absence of lungs. Although tracheal development was normal, main-stem bronchial formation, as well as all subsequent pulmonary branching morphogenesis, was completely disrupted. The pulmonary phenotype of Fgf-10(-/-) mice is strikingly similar to that of the Drosophila mutant branchless, an Fgf homolog.

Pulmonary malformation in transgenic mice expressing human keratinocyte growth factor in the lung.

Expression of human keratinocyte growth factor (KGF/FGF-7) was directed to epithelial cells of the developing embryonic lung of transgenic mice disrupting normal pulmonary morphogenesis during the pseudoglandular stage of development. By embryonic day 15.5(E15.5), lungs of transgenic surfactant protein C (SP-C)-KGF mice resembled those of humans with pulmonary cystadenoma. Lungs were cystic, filling the thoracic cavity, and were composed of numerous dilated saccules lined with glycogen-containing columnar epithelial cells. The normal distribution of SP-C proprotein in the distal regions of respiratory tubules was disrupted. Columnar epithelial cells lining the papillary structures stained variably and weakly for this distal respiratory cell marker. Mesenchymal components were preserved in the transgenic mouse lungs, yet the architectural relationship of the epithelium to the mesenchyme was altered. SP-C-KGF transgenic mice failed to survive gestation to term, dying before E17.5. Culturing mouse fetal lung explants in the presence of recombinant human KGF also disrupted branching morphogenesis and resulted in similar cystic malformation of the lung. Thus, it appears that precise temporal and spatial expression of KGF is likely to play a crucial role in the control of branching morphogenesis during fetal lung development.

Osteoprotegerin: a novel secreted protein involved in the regulation of bone density.

A novel secreted glycoprotein that regulates bone resorption has been identified. The protein, termed Osteoprotegerin (OPG), is a novel member of the TNF receptor superfamily. In vivo, hepatic expression of OPG in transgenic mice results in a profound yet nonlethal osteopetrosis, coincident with a decrease in later stages of osteoclast differentiation. These same effects are observed upon administration of recombinant OPG into normal mice. In vitro, osteoclast differentiation from precursor cells is blocked in a dose-dependent manner by recombinant OPG. Furthermore, OPG blocks ovariectomy-associated bone loss in rats. These data show that OPG can act as a soluble factor in the regulation of bone mass and imply a utility for OPG in the treatment of osteoporosis associated with increased osteoclast activity.