A transposon surveillance mechanism that safeguards plant male fertility during stress

Although plants are able to withstand a range of environmental conditions, spikes in ambient temperature can impact plant fertility causing reductions in seed yield and notable economic losses^1,2. Therefore, understanding the precise molecular mechanisms that underpin plant fertility under environmental constraints is critical to safeguarding future food production³. Here, we identified two Argonaute-like proteins whose activities are required to sustain male fertility in maize plants under high temperatures. We found that MALE-ASSOCIATED ARGONAUTE-1 and -2 associate with temperature-induced phased secondary small RNAs in pre-meiotic anthers and are essential to controlling the activity of retrotransposons in male meiocyte initials. Biochemical and structural analyses revealed how male-associated Argonaute activity and its interaction with retrotransposon RNA targets is modulated through the dynamic phosphorylation of a set of highly conserved, surface-located serine residues. Our results demonstrate that an Argonaute-dependent, RNA-guided surveillance mechanism is critical in plants to sustain male fertility under environmentally constrained conditions, by controlling the mutagenic activity of transposons in male germ cells.

Meiotic Studies on Combinations of Chromosomes With Different Sized Centromeres in Maize

Multiple centromere misdivision derivatives of a translocation between the supernumerary B chromosome and the short arm of chromosome 9 (TB-9Sb) permit investigation of how centromeres of different sizes behave in meiosis in opposition or in competition with each other. In the first analysis, heterozygotes were produced between the normal TB-9Sb and derivatives of it that resulted from centromere misdivision that reduced the amounts of centromeric DNA. These heterozygotes could test whether these drastic differences would result in meiotic drive of the larger chromosome in female meiosis. Cytological determinations of the segregation of large and small centromeres among thousands of progeny of four combinations were made. The recovery of the larger centromere was at a few percent higher frequency in two of four combinations. However, examination of phosphorylated histone H2A-Thr133, a characteristic of active centromeres, showed a lack of correlation with the size of the centromeric DNA, suggesting an expansion of the basal protein features of the kinetochore in two of the three cases despite the reduction in the size of the underlying DNA. In the second analysis, plants containing different sizes of the B chromosome centromere were crossed to plants with TB-9Sb with a foldback duplication of 9S (TB-9Sb-Dp9). In the progeny, plants containing large and small versions of the B chromosome centromere were selected by FISH. A meiotic "tug of war" occurred in hybrid combinations by recombination between the normal 9S and the foldback duplication in those cases in which pairing occurred. Such pairing and recombination produce anaphase I bridges but in some cases the large and small centromeres progressed to the same pole. In one combination, new dicentric chromosomes were found in the progeny. Collectively, the results indicate that the size of the underlying DNA of a centromere does not dramatically affect its segregation properties or its ability to progress to the poles in meiosis potentially because the biochemical features of centromeres adjust to the cellular conditions.

Weight-of-the-evidence evaluation of 2,4-D potential for interactions with the estrogen, androgen and thyroid pathways and steroidogenesis

A comprehensive weight-of-the-evidence evaluation of 2,4-dichlorophenoxyacetic acid (2,4-D) was conducted for potential interactions with the estrogen, androgen and thyroid pathways and with steroidogenesis. This assessment was based on an extensive database of high quality in vitro, in vivo ecotoxicological and in vivo mammalian toxicological studies. Epidemiological studies were also considered. Toxicokinetic data provided the basis for determining rational cutoffs above which exposures were considered irrelevant to humans based on exceeding thresholds for saturation of renal clearance (TSRC); extensive human exposure and biomonitoring data support that these boundaries far exceed human exposures and provide ample margins of exposure. 2,4-D showed no evidence of interacting with the estrogen or androgen pathways. 2,4-D interacts with the thyroid axis in rats through displacement of thyroxine from plasma binding sites only at high doses exceeding the TSRC in mammals. 2,4-D effects on steroidogenesis parameters are likely related to high-dose specific systemic toxicity at doses exceeding the TSRC and are not likely to be endocrine mediated. No studies, including high quality studies in the published literature, predict significant endocrine-related toxicity or functional decrements in any species at environmentally relevant concentrations, or, in mammals, at doses below the TSRC that are relevant for human hazard and risk assessment. Overall, there is no basis for concern regarding potential interactions of 2,4-D with endocrine pathways or axes (estrogen, androgen, steroidogenesis or thyroid), and thus 2,4-D is unlikely to pose a threat from endocrine disruption to wildlife or humans under conditions of real-world exposures.

Plant centromeres

Plant centromeres are generally composed of tandem arrays of simple repeats that are typical of a particular species, but that evolve rapidly. Centromere specific retroelements are also present. These arrays associate with a centromere specific variant of histone H3 that anchors the site of the kinetochore. Although such DNA arrays are typical of the centromere, the specification of centromere activity has an epigenetic component as shown by the fact that centromeres are formed in the absence of such repeats and that centromeres in dicentric chromosomes regularly undergo inactivation.

Labeling meiotic chromosomes in maize with fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) can be used to visualize chromosomal features using repetitive or single gene probes above a minimum target size. When applied to meiosis, each chromosome of the karyotypic complement can be identified, which can facilitate an understanding of the interrelationship of different chromosomes during this process. On the other hand, the pachytene stage of early meiosis is characterized by slightly but not strongly condensed chromosomes that permit more detailed analyses of adjacent features than can be achieved with somatic metaphase chromosomes.

Parameters affecting telomere-mediated chromosomal truncation in Arabidopsis

Conversion of a double-strand break into a telomere is a dangerous, potentially lethal event. However, little is known about the mechanism and control of de novo telomere formation (DNTF). DNTF can be instigated by the insertion of a telomere repeat array (TRA) into the host genome, which seeds the formation of a new telomere, resulting in chromosome truncation. Such events are rare and concentrated at chromosome ends. Here, we introduce tetraploid Arabidopsis thaliana as a robust genetic model for DNTF. Transformation of a 2.6-kb TRA into tetraploid plants resulted in a DNTF efficiency of 56%, fivefold higher than in diploid plants and 50-fold higher than in human cells. DNTF events were recovered across the entire genome, indicating that genetic redundancy facilitates recovery of DNTF events. Although TRAs as short as 100 bp seeded new telomeres, these tracts were unstable unless they were extended above a 1-kb size threshold. Unexpectedly, DNTF efficiency increased in plants lacking telomerase, and DNTF rates were lower in plants null for Ku70 or Lig4, components of the nonhomologous end-joining repair pathway. We conclude that multiple competing pathways modulate DNTF, and that tetraploid Arabidopsis will be a powerful model for elucidating the molecular details of these processes.

Chromosome painting for plant biotechnology

Fluorescence in situ hybridization (FISH) is an invaluable tool for chromosome analysis and engineering. The ability to visually localize endogenous genes, transposable elements, transgenes, naturally occurring organellar DNA insertions - essentially any unique sequence larger than 2 kb - greatly facilitates progress. This chapter details the labeling procedures and chromosome preparation techniques used to produce high-quality FISH signals on somatic metaphase and meiotic pachytene spreads.

Conserved telomere maintenance component 1 interacts with STN1 and maintains chromosome ends in higher eukaryotes

Orthologs of the yeast telomere protein Stn1 are present in plants, but other components of the Cdc13/Stn1/Ten1 (CST) complex have only been found in fungi. Here we report the identification of conserved telomere maintenance component 1 (CTC1) in plants and vertebrates. CTC1 encodes an approximately 140 kDa telomere-associated protein predicted to contain multiple OB-fold domains. Arabidopsis mutants null for CTC1 display a severe telomere deprotection phenotype accompanied by a rapid onset of developmental defects and sterility. Telomeric and subtelomeric tracts are dramatically eroded, and chromosome ends exhibit increased G overhangs, recombination, and end-to-end fusions. AtCTC1 both physically and genetically interacts with AtSTN1. Depletion of human CTC1 by RNAi triggers a DNA damage response, chromatin bridges, increased G overhangs, and sporadic telomere loss. These data indicate that CTC1 participates in telomere maintenance in diverse species and that a CST-like complex is required for telomere integrity in multicellular organisms.

Age-specific reference ranges for polychlorinated biphenyls (PCB) based on the NHANES 2001-2002 survey

The Centers for Disease Control and Prevention (CDC) conducted analyses for 34 polychlorinated biphenyl (PCB) congeners in blood samples collected from a statistically representative sample of the U.S. population during the National Health and Nutrition Examination Survey (NHANES) and reported overall population percentiles. Because the serum concentrations of many persistent organochlorine compounds are strongly age dependent, data were analyzed from the NHANES 2001-2002 sampling cycle to identify age-specific reference ranges for the measured congeners on a lipid-adjusted serum basis. In addition, reference ranges were estimated for the sum of the 34 measured PCB congeners. Because many congeners were frequently nondetectable, estimates for summed PCB levels are dependent upon the assumption used to replace nondetectable concentrations in the calculation. The effect of nondetects on the summed congeners totals is particularly strong for younger ages. The NHANES 2001-2002 PCB serum data demonstrate strong age-related trends, with older individuals displaying higher concentrations of most congeners and of summed PCB congeners. These age-specific reference ranges for PCB concentrations are critical for accurate interpretation of measured serum concentrations of PCB congeners in individuals.

Distinct chromosomal distributions of highly repetitive sequences in maize

The majority of genomic DNA in most plant species is made up of repetitive elements including satellites and retrotransposons. The maize genome is intermediate in size and abundance of repetitive elements between small genomes such as Arabidopsis and rice and larger genomes such as wheat. Although repetitive elements are present throughout the maize genome, individual families are non-randomly distributed along chromosomes. In this work we use fluorescence in-situ hybridization (FISH) to examine the distribution of abundant LTR retroelement families and satellites contained in heterochromatic blocks called knobs. Different retroelement families have distinct patterns of hybridization. Prem1 and Tekay, two very closely related elements, both hybridize along the length of all chromosomes but do so with greater intensity near the centromeres, although subtle differences are detectable between the hybridization patterns. Opie, Prem2/Ji, and Huck are enriched away from the centromeres and Grande is distributed uniformly along the chromosomes. Double labeling with proximally and distally enriched elements on pachytene chromosomes produces alternating blocks of element enrichment. The maize elements hybridized in the same general patterns to chromosomes of maize relatives including Zea diploperennis and Tripsacum dactyloides. Additionally, abundant Tripsacum LTR retroelements are enriched in similar chromosomal regions among the different species. The 180 bp knob satellite is present in large blocks at interstitial locations on chromosome arms. With long exposures, smaller sites of hybridization are detected at the ends of chromosomes, adjacent to the telomere tract. This distal position for knob satellites is conserved among Zea and Tripsacum species.

Localization and transcription of a retrotransposon-derived element on the maize B chromosome

Dispensable chromosomes in addition to the normal complement in diverse taxa are called B chromosomes. The maize B chromosome is discernible in mitotic chromosome spreads as a small compact chromosome composed mainly of heterochromatin. Although much of this chromosome consists of repetitive elements common to the A chromosomes, several sequences specific to the B chromosome have been identified. In the work described here we used the sequence from a B-specific RAPD (random amplification of polymorphic DNA) marker, pBGBM18.2, to isolate another DNA element, StarkB, present in many copies on the B chromosome. StarkB was mapped to the third and fourth blocks of distal heterochromatin using translocation breakpoints and fluorescent in-situ hybridization (FISH). Sequence analysis revealed that StarkB is composed of fragments from the A genome as well as B-specific sequences. The StarkB element is much larger than the other B-specific elements and is not present in large tandem arrays. Different copies of StarkB varied by small insertions, deletions, and duplications as well as single-nucleotide polymorphisms. Reverse transcriptase PCR showed that portions of the StarkB element are expressed. Using the LTR divergence of retroelements interrupting the B-specific sequences, the minimum age of the StarkB repeat array and, by inference, of the B chromosome, was estimated to be 2 million years.

Plant chromosomes from end to end: telomeres, heterochromatin and centromeres

Recent evidence indicates that heterochromatin in plants is composed of heterogeneous sequences, which are usually composed of transposable elements or tandem repeat arrays. These arrays are associated with chromatin modifications that produce a closed configuration that limits transcription. Centromere sequences in plants are usually composed of tandem repeat arrays that are homogenized across the genome. Analysis of such arrays in closely related taxa suggests a rapid turnover of the repeat unit that is typical of a particular species. In addition, two lines of evidence for an epigenetic component of centromere specification have been reported, namely an example of a neocentromere formed over sequences without the typical repeat array and examples of centromere inactivation. Although the telomere repeat unit is quite prevalent in the plant kingdom, unusual repeats have been found in some families. Recently, it was demonstrated that the introduction of telomere sequences into plants cells causes truncation of the chromosomes, and that this technique can be used to produce artificial chromosome platforms.

Recent proliferation and translocation of pollen group 1 allergen genes in the maize genome

The dominant allergenic components of grass pollen are known by immunologists as group 1 allergens. These constitute a set of closely related proteins from the beta-expansin family and have been shown to have cell wall-loosening activity. Group 1 allergens may facilitate the penetration of pollen tubes through the grass stigma and style. In maize (Zea mays), group 1 allergens are divided into two classes, A and B. We have identified 15 genes encoding group 1 allergens in maize, 11 genes in class A and four genes in class B, as well as seven pseudogenes. The genes in class A can be divided by sequence relatedness into two complexes, whereas the genes in class B constitute a single complex. Most of the genes identified are represented in pollen-specific expressed sequence tag libraries and are under purifying selection, despite the presence of multiple copies that are nearly identical. Group 1 allergen genes are clustered in at least six different genomic locations. The single class B location and one of the class A locations show synteny with the rice (Oryza sativa) regions where orthologous genes are found. Both classes are expressed at high levels in mature pollen but at low levels in immature flowers. The set of genes encoding maize group 1 allergens is more complex than originally anticipated. If this situation is common in grasses, it may account for the large number of protein variants, or group 1 isoallergens, identified previously in turf grass pollen by immunologists.

Centromere function and nondisjunction are independent components of the maize B chromosome accumulation mechanism

Supernumerary or B chromosomes are selfish entities that maintain themselves in populations by accumulation mechanisms. The accumulation mechanism of the B chromosome of maize (Zea mays) involves nondisjunction at the second pollen mitosis, placing two copies of the B chromosome into one of the two sperm. The B chromosome long arm must be present in the same nucleus for the centromere to undergo nondisjunction. A centromere, containing all of the normal DNA elements, translocated from the B chromosome to the short arm of chromosome 9 was recently found to be epigenetically silenced for centromeric function. When intact B chromosomes were added to this genotype, thus supplying the long arm, the inactive centromere regained the property of nondisjunction causing the translocation chromosome 9 to be differentially distributed to the two sperm or resulted in chromosome breaks in 9S, occasionally producing new translocations. Translocation of the inactive B centromere to chromosome 7 transferred the nondisjunction property to this chromosome. The results provide insight into the molecular and evolutionary basis of this B chromosome accumulation mechanism by demonstrating that nondisjunction is caused by a process that does not depend on normal centromere function but that the region of the chromosome required for nondisjunction resides in the centromeric region.

A hemicentric inversion in the maize line knobless Tama flint created two sites of centromeric elements and moved the kinetochore-forming region

A maize line, knobless Tama flint (KTF), was found to contain a version of chromosome 8 with two spatially distinct regions of centromeric elements, one at the original genetic position and the other at a novel location on the long arm. The new site of centromeric elements functions as the kinetochore-forming region resulting in a change of arm length ratio. Examination of fluorescence in situ hybridization markers on chromosome 8 revealed an inversion between the two centromere sites relative to standard maize lines, indicating that this chromosome 8 resulted from a hemicentric inversion with one breakpoint approximately 20 centi-McClintocks (cMc) on the long arm (20% of the total arm length from the centromere) and the other in the original cluster of centromere repeats. This inversion moved the kinetochore-forming region but left the remainder of the centromere repeats. In a hybrid between a standard line (Mo17) and KTF, both chromosome 8 homologues were completely synapsed at pachytene despite the inversion. Although the homologous centromeres were not paired, they were always correctly oriented at anaphase and migrated to opposite poles. Additionally, recombination on 8L was severely repressed in the hybrid.

Single-gene detection and karyotyping using small-target fluorescence in situ hybridization on maize somatic chromosomes

Combined with a system for identifying each of the chromosomes in a genome, visualizing the location of individual genetic loci by fluorescence in situ hybridization (FISH) would aid in assembling physical and genetic maps. Previously, large genomic clones have been successfully used as FISH probes onto somatic chromosomes but this approach is complicated in species with abundant repetitive elements. In this study, repeat-free portions of sequences that were anchored to particular chromosomes including genes, gene clusters, large cDNAs, and portions of BACs obtained from public databases were used to label the corresponding physical location using FISH. A collection of probes that includes at least one marker on each chromosome in the maize complement was assembled, allowing a small-target karyotyping system to be developed. This set provides the foundation onto which additional loci could be added to strengthen further the ability to perform chromosomal identification in maize and its relatives. The probes were demonstrated to produce signals in several wild relatives of maize, including Zea luxurians, Z. diploperennis, and Tripsacum dactyloides.

Telomere-mediated chromosomal truncation in maize

Direct repeats of Arabidopsis telomeric sequence were constructed to test telomere-mediated chromosomal truncation in maize. Two constructs with 2.6 kb of telomeric sequence were used to transform maize immature embryos by Agrobacterium-mediated transformation. One hundred seventy-six transgenic lines were recovered in which 231 transgene loci were revealed by a FISH analysis. To analyze chromosomal truncations that result in transgenes located near chromosomal termini, Southern hybridization analyses were performed. A pattern of smear in truncated lines was seen as compared with discrete bands for internal integrations, because telomeres in different cells are elongated differently by telomerase. When multiple restriction enzymes were used to map the transgene positions, the size of the smears shifted in accordance with the locations of restriction sites on the construct. This result demonstrated that the transgene was present at the end of the chromosome immediately before the integrated telomere sequence. Direct evidence for chromosomal truncation came from the results of FISH karyotyping, which revealed broken chromosomes with transgene signals at the ends. These results demonstrate that telomere-mediated chromosomal truncation operates in plant species. This technology will be useful for chromosomal engineering in maize as well as other plant species.

Cytological visualization of DNA transposons and their transposition pattern in somatic cells of maize

Global genomic analysis of transposable element distributions of both natural (En/Spm, Ac-Ds, and MuDR/Mu) and modified (RescueMu) types was performed by fluorescence in situ hybridization (FISH) on somatic chromosomes coupled with karyotyping of each chromosome. In lines without an active transposable element, the locations of silent En/Spm, Ac-Ds, and MuDR/Mu elements were visualized, revealing variation in copy number and position among lines but no apparent locational bias. The ability to detect single elements was validated by using previously mapped active Ac elements. Somatic transpositions were documented in plants containing an engineered Mutator element, RescueMu, via use of the karyotyping system. By analyzing the RescueMu lines, we found that transposition of RescueMu in root-tip cells follows the cut-and-paste type of transposition. This work demonstrates the utility of FISH and karyotyping in the study of transposon activity and its consequences.

Retroelement genome painting: cytological visualization of retroelement expansions in the genera Zea and Tripsacum

Divergence of abundant genomic elements among the Zea and Tripsacum genera was examined cytologically and a tool kit established for subsequent studies. The LTR regions from the CRM, Huck, Grande, Prem1, Prem2/Ji, Opie, Cinful-1, and Tekay retroelement families were used as FISH probes on mitotic chromosome spreads from a "trispecies" hybrid containing chromosomes from each of three species: Zea mays (2n = 20), Z. diploperennis (2n = 20), and Tripsacum dactyloides (2n = 36). Except for Tekay, which painted both Zea and Tripsacum chromosomes with nearly equal intensity, the retroelement probes hybridized strongly to the Zea chromosomes, allowing them to be distinguished from those of Tripsacum. Huck and Grande hybridized more intensely to maize than to Z. diploperennis chromosomes. Tripsacum genomic clones containing retroelement sequences were isolated that specifically paint Tripsacum chromosomes. The retroelement paints proved effective for distinguishing different genomes in interspecific hybrids and visualizing alien chromatin from T. dactyloides introgressed into maize lines. Other FISH probes (180-bp knob, TR-1, 5S, NOR, Cent4, CentC, rp1, rp3, and alpha-ZeinA) could be simultaneously visualized with the retroelement probes, emphasizing the value of the retroelement probes for cytogenetic studies of Zea and Tripsacum.

High frequency of centromere inactivation resulting in stable dicentric chromosomes of maize

Somatic chromosome spreads from maize (Zea mays L.) plants containing B-A translocation chromosomes undergoing the chromosome type breakage-fusion-bridge cycle were examined by FISH. The size and type of extra chromosomes varied among cells of the same individual. A collection of minichromosomes derived from the chromosome type breakage-fusion-bridge cycle was examined for the presence of stable dicentric chromosomes. Six of 23 chromosomes in the collection contained two regions with DNA sequences typical of centromeres. Functional analysis and immunolabeling of CENH3, the centromere-specific histone H3 variant, revealed only one functional centromere per chromosome, despite the duplicate centromere sequences. One plant was found with an inactive B centromere that had been translocated to the short arm of chromosome 9. The translocated centromere region appeared identical to that of a normal B chromosome. The inactivation of the centromeres was stable for at least four generations. By using dicentrics from dispensable chromosomes, centromere inactivation was found to be quite common under these circumstances.

Minichromosomes derived from the B chromosome of maize

Fourteen minichromosomes derived from the B chromosome of maize are described. The centromeric region of the B chromosome contains a specific repetitive DNA element called the B repeat. This sequence was used to determine the transmission frequency of the different types of minichromosomes over several generations via Southern blot analysis at each generation. In general, the minichromosomes have transmission rates below the theoretical 50% frequency of a univalent chromosome. The gross structure of each minichromosome was determined using fluorescence in situ hybridization (FISH) on root tip chromosome spreads. The presence of the B centromeric repeat and of the adjacent heterochromatic knob sequences was determined for each minichromosome. In two cases, the amount of the centromeric knob repeat is increased relative to the progenitor chromosome. Other isolates have reduced or undetectable levels of the knob sequence. Potential uses of the minichromosomes are discussed.

Molecular and functional dissection of the maize B chromosome centromere

The centromere of the maize (Zea mays) B chromosome contains several megabases of a B-specific repeat (ZmBs), a 156-bp satellite repeat (CentC), and centromere-specific retrotransposons (CRM elements). Here, we demonstrate that only a small fraction of the ZmBs repeats interacts with CENH3, the histone H3 variant specific to centromeres. CentC, which marks the CENH3-associated chromatin in maize A centromeres, is restricted to an approximately 700-kb domain within the larger context of the ZmBs repeats. The breakpoints of five B centromere misdivision derivatives are mapped within this domain. In addition, the fraction of this domain remaining after misdivision correlates well with the quantity of CENH3 on the centromere. Thus, the functional boundaries of the B centromere are mapped to a relatively small CentC- and CRM-rich region that is embedded within multimegabase arrays of the ZmBs repeat. Our results demonstrate that the amount of CENH3 at the B centromere can be varied, but with decreasing amounts, the function of the centromere becomes impaired.

Advances in plant chromosome identification and cytogenetic techniques

Recent developments that improve our ability to distinguish slightly diverged genomes from each other, as well as to distinguish each of the nonhomologous chromosomes within a genome, add a new dimension to the study of plant genomics. Differences in repetitive sequences among different species have been used to develop multicolor fluorescent in situ hybridization techniques that can define the components of allopolyploids in detail and reveal introgression between species. Bacterial artificial chromosome probes and repetitive sequence arrays have been used to distinguish each of the nonhomologous somatic chromosomes within a species. Such karyotype analysis opens new avenues for the study of chromosomal variation and behavior, as well as for the localization of individual genes and transgenes to genomic position.

Sequences associated with A chromosome centromeres are present throughout the maize B chromosome

Maize chromosome spreads containing the supernumerary B chromosome were hybridized with probes from various repetitive elements including CentC, CRM, and CentA, which have been localized to centromeric regions on the A chromosomes. Repetitive elements that are enriched or found exclusively near the centromeres of A chromosomes hybridized to many sites distinct from the centromere on the B chromosome. To examine whether these elements recruit kinetochore proteins at locations other than the canonical B centromere, cells were labeled with antibodies against CENH3, a key kinetochore protein. No labeling was detected outside the normal centromere and no evidence of B chromosome holocentromeric activity was observed. This finding suggests that, as in other higher eukaryotes, DNA sequence alone is insufficient to dictate kinetochore location in plants. Additionally, examination of the B centromere region in pachytene chromosomes revealed that the B-specific element ZmBs hybridizes to a much larger region than the site of hybridization of CentC, CRM, and CentA and the labeling by anti-CENH3 antibodies.

Chromosome painting using repetitive DNA sequences as probes for somatic chromosome identification in maize

Study of the maize (Zea mays L.) somatic chromosomes (2n = 20) has been difficult because of a lack of distinguishing characteristics. To identify all maize chromosomes, a multicolor fluorescence in situ hybridization procedure was developed. The procedure uses tandemly repeated DNA sequences to generate a distinctive banding pattern for each of the 10 chromosomes. Fluorescence in situ hybridization screening trials of nonsubtracted or subtracted PCR libraries resulted in the isolation of microsatellite 1-26-2, subtelomeric 4-12-1, and 5S rRNA 2-3-3 clones. These three probes, plus centromeric satellite 4 (Cent4), centromeric satellite C (CentC), knob, nucleolus-organizing region (NOR), pMTY9ER telomere-associated sequence, and tandemly repeated DNA sequence 1 (TR-1) were used as a mixture for hybridization to root-tip chromosomes. All 10 chromosomes were identified by the banding and color patterns in the 14 examined lines. There was significant quantitative variation among lines for the knob, microsatellite, TR-1, and CentC signals. The same probe mixture identifies meiotic pachytene, late prophase I, and metaphase I chromosomes. The procedure could facilitate the study of chromosomal structure and behavior and be adapted for other plant species.

What's in a centromere?

The complete sequence of rice centromere 8 reveals a small amount of centromere-specific satellite sequence in blocks interrupted by retrotransposons and other repetitive DNA, in an arrangement that is similar in size and content to other centromeres of multicellular eukaryotes.

The complete sequence of rice centromere 8 reveals a small amount of centromere-specific satellite sequence in blocks interrupted by retrotransposons and other repetitive DNA, in an arrangement that is strikingly similar in overall size and content to other centromeres of multicellular eukaryotes.

The role of DNA sequence in centromere formation

Centromeres are key to the correct segregation and inheritance of genetic information. Eukaryotic centromeres, which are located in large blocks of highly repetitive DNA, have been notoriously difficult to sequence. Several groups have recently succeeded in analyzing centromeric sequences in human, Drosophila and Arabidopsis, providing new insights into the importance of DNA sequence for centromere function.

Pan-trk inhibition decreases metastasis and enhances host survival in experimental models as a result of its selective induction of apoptosis of prostate cancer cells

During the progression of prostate cancer, molecular changes occur resulting in the autocrine production of a series of neurotrophins by the malignant cells. This is coupled with expression of high-affinity cognate receptors for these ligands, termed trk receptors, by these cancer cells. The binding of the neurotrophins to their trk receptors activates the receptor's latent tyrosine kinase activity inducing a series of signal transduction pathways within these prostate cancer cells. These molecular changes result in the acquisition by prostate cancer cells of a restricted requirement for these trk signaling pathways for optimal survival. CEP-701 is an indolocarbazole compound specifically designed as a potent inhibitor (IC(50), 4 nM) of the tyrosine kinase activity of the trk receptors required for initiation of these survival pathways. In the present studies, the consequences of CEP-701 inhibition of these trk signaling survival pathways were tested in vivo using both rat (R3327 AT 6.3 and H) and human (TSU-pr1 and CWR-22Rv1) prostatic cancer models. These in vivo studies demonstrated that treatment with CEP-701 inhibits the growth of both rodent and human prostate cancers, without being toxic to the normal tissue including the host prostate. Because of this selective effect, CEP-701 inhibits metastasis and growth of both primary and metastatic sites of prostate cancer. Based upon this profile, long-term survival studies were performed using the slow-growing Dunning H rat prostate cancer model. For these latter studies, the dosing regimen was 10 mg CEP-701/kg/dose twice a day via gavage 5 days a week. This regimen maintains CEP-701 tumor tissue concentrations of 25-50 nM. Such chronic dosing increased (P < 0.001) the median survival of rats bearing the slow growing H prostate cancers from 408 days (395-432 days, 95% confidence interval) for the vehicle group (n = 18) to 566 days (497-598 days, 95% confidence interval) for the CEP-701-treated group (n = 24).

A review of the developmental and reproductive toxicity of styrene

The reproductive and developmental toxicity of styrene has been studied in animals and humans. The animal studies on styrene have diverse study designs and conclusions. Developmental or reproductive toxicity studies have been conducted in rats, mice, rabbits, and hamsters. In most cases, high doses are required to elicit effects, and the effects are not unique to reproduction or development. In a number of the reports, either the experimental designs are limited or the descriptions of the designs and the endpoints measured are insufficient to draw conclusions about the toxicity of styrene. The more complete and better-reported studies show that styrene does not cause developmental toxicity at dose levels that are not maternally toxic. Some neurochemical or neurobehavioral effects have been reported at high exposures. Styrene does not affect fertility or reproductive function. Considerable animal toxicity data on styrene support the conclusion that styrene is neither an endocrine-active substance nor an endocrine disrupter. Human studies often suffer from either inadequate exposure data or exposure to a wide variety of materials, so that attribution of effects to styrene exposure is impossible. Furthermore, investigators often have failed to account for other exposures in the workplace or for other potentially confounding factors in their studies. Menstrual cycle irregularities and congenital abnormalities were initially reported; however, the better and more recent reports do not show that styrene causes developmental or reproductive effects in humans. Human studies also support the conclusion that styrene is not an endocrine disrupter. Although some study authors have concluded that styrene is either a human or an animal reproductive or developmental toxicant, careful review demonstrates that such conclusions are not justified.

Workshop to identify critical windows of exposure for children's health: reproductive health in children and adolescents work group summary

This work group report addresses the central question: What are the critical windows during development (preconception through puberty) when exposure to xenobiotics may have the greatest adverse impact on subsequent reproductive health? The reproductive system develops in stages, with sex-specific organogenesis occurring prenatally and further maturational events occurring in the perinatal period and at puberty. Complex endocrine signals as well as other regulatory factors (genetics, growth factors) are involved at all stages. Evidence from animal models and human studies indicates that many specific events can be perturbed by a variety of toxicants, with endocrine-mediated mechanisms being the more widely studied. Prioritized research needs include basic studies on the cellular-molecular and endocrine regulation of sexual differentiation and development; increased efforts regarding potential adverse effects on development in females, including breast development; expanded animal studies on different classes of chemicals, comparing responses during development (prenatal and postnatal) with responses in adults; and, more extensive explorations regarding the reproductive biology and toxicology of puberty in humans.

Reevaluation of the cancer potency factor of toxaphene: recommendations from a peer review panel

This reevaluation of the current U.S. EPA cancer potency factor for toxaphene is based upon a review of toxaphene carcinogenesis bioassays in mice conducted by Litton Bionetics (unpublished report, 1978) and the National Cancer Institute (NCI) (Technical Report Series No. 37, conducted by Gulf South Research Institute, 1979). The mechanistic data available for toxaphene, including consideration of the potential of the compound to induce genotoxicity, was examined with an emphasis on whether this information supports a change in the cancer potency factor. If a quantitative dose-response assessment for toxaphene is to be performed, the data from both the NCI and Litton cancer bioassays should be used. Additionally, liver tumor results from female mice, rather than male mice, should be used for estimating potential human cancer risk because the background rate of liver tumors in females is lower and less variable than that exhibited by males. An ED(10) was estimated as the point of departure. The mechanistic data were not sufficient to fully support a margin of exposure approach. Therefore, we believe that applying a linear extrapolation from the ED(10) to the origin is an appropriate means to estimate risk at low doses. This is a highly conservative approach and, when it is applied, we conclude that the current EPA cancer potency factor should be reduced from 1.1 (mg/kg/day)(-1) to 0.1 (mg/kg/day)(-1).

Chemical testing strategies for predicting health hazards to children

The United States Environmental Protection Agency has proposed the development of a Children's Health Test Program under the Toxic Substances Control Act. The Environmental Protection Agency's proposal for the children's health test battery has 12 different assays including general toxicity, genotoxicity, carcinogenicity, neurotoxicity, and developmental and reproductive toxicity. The current Environmental Protection Agency testing proposal is an "all or nothing" test battery. An alternative and preferable approach would be to use a science-based, tiered testing scheme. It is proposed that the Screening Information Dataset program, currently used by the Organization for Economic Co-operation and Development (OECD) for the Screening Information Dataset-High Production Volume test battery, or equivalent, be considered for the first step. Step 1 would include acute and repeat dose toxicity testing, developmental toxicity testing (first species OECD 414 or OECD 422), reproductive toxicity screening (OECD 415 or 422), and genetic toxicity testing. For this step, the rat would be the initial and only species tested unless the mouse was used for in vivo genetic toxicity. Step 2 of the proposed children's health test battery would include developmental testing (second species OECD 414) or special mode of action studies performed for those chemicals that proved to be developmental toxicants in Step 1. Those chemicals that tested positive as reproductive toxicants in Step 1 would be tested in a two-generation reproduction study (OECD 416) or a special mode of action study. Steps 1 and 2 provide information on whether oncogenicity or developmental neurotoxicity testing is useful. Step 3 would include chronic toxicity/oncogenicity testing for those chemicals that tested positive for genetic toxicity in Step 1, and positive for developmental concerns in Step 2. In this step, chemicals would also be tested for developmental neurotoxicity if they showed evidence of neuropathy, behavioral effects, or neurotoxic potential in earlier studies. This stepwise approach would conserve resources and answer scientific questions in a logical, orderly, timely, and cost-effective manner.

The potential health effects of phthalate esters in children's toys: a review and risk assessment

Diisononyl phthalate (DINP) is one of several dialkyl phthalate esters that are widely used as plasticizers to impart softness and flexibility to normally rigid polyvinyl chloride (PVC) products. During the past 2 years, concern has been voiced by public interest groups and regulatory agencies in Europe, Canada, and the United States regarding the potential adverse health effects of DINP migrating from children's toys during mouthing activities. Concern has focused on potential chronic effects on the kidney and liver. In chronic high-dose studies with rodents, DINP causes a dose-related decrease in body weight, an increase in liver weight, and changes in liver cell histopathology (hypertrophy). To a lesser extent, the rodent kidney is also a target for prolonged high-level exposures of DINP. Prolonged high-level exposure of rodents to DINP leads to an increased incidence of liver tumors (adenomas and carcinomas). The chronic cancer and noncancer effects of DINP on rodent liver are consistent with its known action as a peroxisome proliferator. Peroxisome proliferation is a threshold-based effect that is reversible on cessation of exposure to proliferators such as DINP. Because rodents are uniquely responsive and humans and nonhuman primates are particularly nonresponsive to peroxisome proliferators, rodents are very poor animal models for use in human risk assessment of adverse effects mediated through peroxisome proliferation. Because DINP exerts its effects on rodent liver through a known threshold-based mechanism of little, if any, relevance to humans, a highly conservative risk assessment can be conducted using a NOAEL uncertainty factor approach. Chronic rodent no-observed-effect levels (NOELs) based on end points such as increased liver weight and changes in liver pathology that are early indicators of peroxisome proliferation but should not be considered adverse range from about 100 to 400 mg/kg/day. Application of a 100-fold uncertainty factor yields acceptable daily intakes (ADIs) ranging from 1 to 4 mg/kg/day. Estimates of DINP migration from soft PVC materials have been obtained from a variety of in vitro methods (simulated saliva and controlled agitation) as well as in vivo methods (controlled chewing) that more closely resemble child chewing and mouthing activities. Recent estimates by the Consumer Product Safety Commission (CPSC) suggest that maximum exposures occur in infants 3-12 months of age. The geometric mean (50th percentile) exposure is 5.7 microg/kg/day and the 95th percentile is 94.3 microg/kg/day. These exposure values are 17,500-70,000 and 1100-4200 times, respectively, lower than the chronic rodent NOAEL for DINP and 175-700 and 11-42 times lower than the corresponding ADI of 1-4 mg/kg/day. It is concluded, with a high degree of confidence, that the use of DINP in soft PVC toys and other children's products does not present a significant risk to children. The scientific evidence supports the continued use of DINP as a plasticizer in children's products.

FIFRA Subdivision F testing Guidelines: are these tests adequate to detect potential hormonal activity for crop protection chemicals? Federal Insecticide, Fungicide, and Rodenticide Act

Recently, a major topic of discussion has been the impact of synthetic chemicals that possess the capacity to alter hormonal activity, the so-called "endocrine modulators," with potentially the capacity to alter the reproductive capability of humans. Particularly, various synthetic pesticides and industrial chemicals that persist in the environment and/or bioaccumulate have been implicated. Further, it has been alleged that the standard tests for pesticide registration as required by the U.S. Environmental Protection Agency (EPA) and other regulatory agencies may be inadequate to detect endocrine modulating effects. To address these shortcomings, it has been proposed that very specific tests for estrogen receptor binding, or in vitro cell response to chemicals, be used to identify potential endocrine modulators. However, such approaches have certain flaws that limit their application as screens. First, very specific tests, like receptor binding, evaluate only a single chemical event per test. Such tests do not measure toxicity or biological response. Isolated systems are very important for studying mechanisms of action or structure activity relationships, but can only provide a preliminary screen for a single mechanism of toxicity. Isolated systems can not be used to regulate a chemical without additional information. Second, they fail to test many other parts of the neuroendocrine control of the reproductive system. Testing for adverse effects in highly specific in vitro systems failed to replace whole-animal models in carcinogenesis and will also fail in reproductive toxicology because this system is too complicated for such as in vitro approach to be accurately predictive. Advanced tests, such as the EPA multigeneration study, are more effective, and reliable means for evaluation than any specific and narrowly focused screening tests. Experience has shown that a better approach to testing chemicals is to evaluate their effects on the whole animal. When one part of the system is adversely affected, various processes may be indirectly affected and can be detected in the animal model. For example, a modulation of testosterone synthesis could lead to (1) altered accessory sex organ morphology, size, and function; (2) decreased sperm counts; and (3) even decreased fertility. These and many other effects would be noted in toxicity studies that are already required for the registration of crop protection chemicals. The developmental and reproductive toxicity guidelines were recently reviewed in a hearing that included the representatives from the EPA, the public, and the Scientific Advisory Panel. The EPA kept the basic study design the same, but added a few new endpoints to further assess chemical-induced effects on reproductive development and function. The review presented herein concentrates on the required Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) testing for pesticides, and demonstrates how the massive arrays of sensitive endocrine endpoints that are delineated in FIFRA Subdivision F have been successfully used to detect both weak and potent hormonally modulating chemicals. For example, (1) diethyl-stilbestrol (DES), which is a potent synthetic therapeutic estrogen, (2) DDT, which is weakly estrogenic but persistent and bioaccumulating, and (3) dioxins, which have antiestrogenic properties, were all found as being hormonally active in tests similar or identical to FIFRA tests. All food-use pesticides have been evaluated using a comprehensive multigeneration reproduction test. Hence, the FIFRA testing procedures have been demonstrated to identify endocrine modulators of sufficient potency to represent a concern to human health.

Testicular and germ cell toxicity: in vitro approaches

Research on testicular toxicology has been advanced significantly by the introduction of in vitro testing systems. In vivo systems, however, are still essential parts of the risk assessment process, and they are unlikely to be eliminated by in vitro model systems. While in vivo systems are needed to study the integrated male reproductive system, in vitro systems are uniquely suited to investigate specific mechanisms of action in the testis. In vitro systems substantially improve the interpretation and use of in vivo systems. In vitro models can be used alone or in combination with each other to test hypotheses about testicular toxicity. Numerous systems are described in the literature, including Sertoli-germ cell cocultures, Sertoli cell-enriched cultures, germ cell-enriched cultures, Leydig cell cultures, and Leydig-Sertoli cell cocultures. These systems have been used to test relative toxicologic activity of selected chemicals in a class, to investigate the cellular response to certain toxicants, to study the metabolic capability of cells, and to describe the interaction of adjacent cell types.

The antitumor effects of the quinoline-3-carboxamide linomide on Dunning R-3327 rat prostatic cancers

Linomide (N-phenylmethyl-1,2-dihydro-4-hydroxyl-1-methyl-2-oxo-quinoline-3- carboxamide) is a quinoline 3-carboxamide which previously has been demonstrated to produce immunomodulator and antitumor effects when given in vivo. To test the possible antitumor effects of linomide against prostatic cancers, rats bearing five distinct Dunning R-3327 rat prostatic cancer sublines were treated daily with i.p. injections of linomide. These studies demonstrated that linomide has a reproducible antitumor effect against all of the prostatic cancers tested regardless of their growth rate, degree of morphologic differentiation, metastatic ability, or androgen responsiveness. This antitumor effect is observed only in vivo, not in vitro, and involves a cytotoxic response of the prostatic cancer cells. This cytotoxic response results in the retardation of the growth rate (i.e., increased tumor volume doubling time) of primary prostatic cancers and in metastatic lesions. Linomide's growth retardation is reversible, and thus continuous daily treatment with linomide is required for maximal antitumor response. Pretreatment of rats with linomide before tumor inoculation has no effect in addition to that produced by initiating linomide treatment at the time of tumor inoculation. No enhancement of either natural killer cell number or natural killer cell cytotoxic activity is induced by linomide treatment in the tumor-bearing rats. In addition, depletion of natural killer cell activity via injections of asialo-GM1 antiserum does not prevent the antitumor effects of linomide in vivo. Likewise, the antitumor effects of linomide are also produced in prostatic cancer-bearing athymic nude rats. These results suggest that the requirement for host involvement in the antitumor effects of linomide against rat prostatic cancers may involve both immune and nonimmune host mechanism(s) (e.g., antiangiogenesis).

Reproductive toxicity of triethylene glycol and its diacetate and dimethyl ether derivatives in a continuous breeding protocol in Swiss CD-1 mice

Triethylene glycol and two of its derivatives were evaluated for reproductive toxicity in a continuous breeding protocol with Swiss CD-1 mice. Triethylene glycol (TEG: 0, 0.3, 1.5, and 3%), triethylene glycol diacetate (TGD: 0, 0.75, 1.5, and 3%), and triethylene glycol dimethyl ether (TGDME: 0, 0.25, 0.5, and 1%) were administered in drinking water to breeding pairs (20 pairs per treatment group, 40 control pairs) during a 98-day cohabitation period. Reproductive function was assessed by the number of litters per pair, live pups per litter, proportion of pups born alive, and pup weight. There were no apparent effects on reproductive function in the animals receiving TEG or TGD at doses up to 3% in the drinking water (representing 6.78 or 5.45 g/kg, respectively). However, some developmental toxicity was demonstrated for both TEG and TGD. Continuous exposure of dams to 1.5 or 3% TEG significantly reduced live pup weight at birth compared to control and 0.3% TEG, while exposure to 3% TGD during lactation significantly (but reversibly) reduced pup body weights on Postnatal Days 14 and 21. In contrast, TGDME was toxic to the reproductive system as evidenced by decreases at the highest dose (1% TGDME; 1.47 g/kg) in the proportion of pairs that produced at least one litter, live pups per litter, and proportion of pups born alive, with dose-related trends seen in the latter two parameters. A crossover mating trial showed that TGDME was more toxic to the female than the male reproductive system. These data indicate that TGDME (1.47 g/kg) is a reproductive toxicant in Swiss mice while reproductive toxicity was not demonstrated in mice receiving TEG or TGD (at doses up to 6.78 or 5.45 g/kg, respectively).

Reproductive toxicity of sulfamethazine in Swiss CD-1 mice during continuous breeding

Sulfamethazine (SMZ) was evaluated for reproductive toxicity in Swiss CD-1 mice using a continuous breeding protocol. SMZ was administered in the diet at 0, 0.25, 0.5, or 1% (w/w), which represented an average daily intake of 0, 313, 625, or 1250 mg SMZ/kg/day, respectively. Exposure of F0 male and female mice to 1% SMZ for 126 days resulted in a significant decrease in the mean number of live pups per litter and the number of litters produced (task 2); the percentage pups born alive to 1% SMZ females showed a nonsignificant decrease versus control females. The effects on fertility were rapid to onset (1 to 4 weeks) and cumulative in nature. F0 male and female body weights were slightly depressed from 3 weeks to the end of the study. The crossover mating trial (task 3) revealed that the adverse effect on fertility involved both treated partners in that litter size decreased when either 1% SMZ males were bred to control females or 1% SMZ females were mated with control males. After approximately 155 days of exposure of F0 mice to 1% SMZ, the terminal body weight of 1% SMZ females was significantly decreased and that of 1% SMZ males showed a nonsignificant decrease. In addition, the liver weight to body weight ratio of the males was increased. Further, the prostate and seminal vesicle weight to body weight ratios were decreased in 1% SMZ males relative to control males. No treatment-related gross or histopathological lesions were noted for the pituitary or reproductive organs of either sex. Sperm assessment indicated no significant difference in the epididymal sperm concentration or percentage motile or abnormal sperm.(ABSTRACT TRUNCATED AT 250 WORDS)

Reproductive toxicity evaluation of acetaminophen in Swiss CD-1 mice using a continuous breeding protocol

Acetaminophen (APAP) was evaluated for reproductive toxicity in Swiss CD-1 mice using a continuous breeding protocol. APAP was administered in the diet at 0, 0.25, 0.5, and 1.0% (w/w), which represented average daily intakes of 0, 357, 715, and 1430 mg APAP/kg/day, respectively. Exposure of parental (P) breeding pairs to 1% APAP in the diet for 14 weeks during cohabitation significantly decreased the number of litters per pair, and reduced, although not significantly, the number of live pups per litter. Importantly, 6 of 19 high-dose P pairs failed to produce a fifth litter, and this fully accounted for the diminished number of litters in this group. In addition, the fifth litter that was produced by the 13 high-dose P pairs averaged only about 9 live pups per litter, which correspondingly reduced the overall group average for this parameter. In comparison, the control and two lower-dose P pairs produced 11 or 12 live pups per litter on average. Although the birth weights for F1 pups in the final litter were unaffected by prenatal APAP exposure, postnatal growth was adversely affected as evidenced by retarded weight gain as measured at 28 and 74 +/- 10 days of age for all three dietary levels. At 1% APAP this weight gain effect was more pronounced at Day 28 than at Day 74 +/- 10, suggesting that nursing pups may have been exposed to higher concentrations or may be more sensitive to APAP and/or an active metabolite than were the young adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostatic involution in rats induced by a novel 5 alpha-reductase inhibitor, SK&F 105657: role for testosterone in the androgenic response

Within the prostate, androgen stimulates glandular cell secretion and proliferation while inhibiting glandular cell death. Due to its predominant nuclear localization, higher affinity for the androgen receptor, and more than 10-fold higher intracellular concentration than testosterone, dihydrotestosterone (DHT), not testosterone, appears to be the active intracellular androgen within the prostate of intact male hosts. The issue has remained unanswered, however, whether testosterone itself, without irreversible conversion to DHT by the 5 alpha-reductase enzyme, is capable of androgenic effects in the prostate. To address this issue, a novel dead end (i.e. product) inhibitor of the 5 alpha-reductase enzyme, SK&F 105657, was administered to intact or castrated male rats treated with either exogeneous testosterone or DHT. When administered twice a day orally at 25 mg/kg.dose, SK&F 105657 reduced the prostatic DHT content of either intact or castrated rats maintained with exogeneous testosterone to the same low level as that produced by surgical castration. Unlike castration, however, such SK&F 105657 treatment increased the prostatic testosterone content by more than 5-fold. The decrease in prostatic DHT coupled with a raise in testosterone are specifically due to the in vivo inhibition of the 5 alpha-reductase activity, since they were not observed in castrated rats maintained with exogeneous DHT. Treatment of intact or castrated male rats with exogeneous testosterone and oral SK&F 105657 (25 mg/kg, twice daily) resulted in a substantial inhibition of prostatic secretion, an inhibition of prostatic glandular cell proliferation, and an increase in prostatic glandular cell death. The magnitude of the changes, however, was not as great as that observed after surgical castration. The results are, however, specific for 5 alpha-reductase inhibition, since they were not observed in castrated rats given exogeneous DHT. These results demonstrate that if the prostatic testosterone content is elevated to sufficient levels, androgenic effects are induced without a requirement for an elevation in prostatic DHT content. Thus, the conversion of testosterone to DHT appears to function as a means of amplifying androgenic stimulation in the prostate.

Response of rat and human prostatic cancers to the novel 5 alpha-reductase inhibitor, SK&F 105657

The response of two androgen-responsive rat prostatic cancers (i.e., Dunning R-3327 H and G sublines) and one androgen-responsive human prostatic cancer (i.e., PC-82) to the 5 alpha-reductase inhibitor, SK&F 105657, was tested in vivo. SK&F 105657 was administered orally twice a day at a dose of 25 or 50 mg/kg/dose. The rat R-3327 G tumor and the human PC-82 tumor have a low to undetectable level of tissue 5 alpha-reductase activity and both responded to SK&F 105657 treatment with a reproducible inhibition of tumor growth. Associated with this antitumor effect was a major decrease (i.e., greater than 70%) in tissue dihydrotestosterone (DHT) content in both tumors. By contrast, the rat R-3327 H prostatic cancer has a much higher level of tissue 5 alpha-reductase activity, and neither tumor DHT content nor growth of the tumor was inhibited by treatment with SK&F 105657. Drug treatment of rats bearing R-3227 H tumors resulted in a similar reduction in the DHT content, wet weight, and DNA content of the ventral prostate as that produced in R-3327 G tumor-bearing rats which experienced an antitumor response. These results suggest that SK&F 105657 can produce antitumor effects if a substantial reduction in tissue DHT is achieved. Such reduction in tissue DHT, secondary to inhibition of the tissue 5 alpha-reductase enzyme, appears to be more difficult to achieve in tumors than in the normal prostate. In order to achieve such a DHT reduction in tumor tissue, prostatic cancers with low 5 alpha-reductase activity could be treated with SK&F 105657 on a dose regimen that lowers serum DHT to surgical castration levels, while concomitantly inhibiting the already low tumor tissue 5 alpha-reductase activity.

Assessment of ethylene glycol monobutyl and monophenyl ether reproductive toxicity using a continuous breeding protocol in Swiss CD-1 mice

A continuous breeding reproduction study design was utilized to examine the reproductive toxicity of ethylene glycol monobutyl ether (EGBE) and ethylene glycol monophenyl ether (EGPE). Swiss CD-1 mice were administered EGBE in drinking water (0, 0.5, 1.0, and 2.0%, i.e., 0.7, 1.3, and 2.1 g/kg body wt/day) and EGPE was administered via the feed (0, 0.25, 1.25, and 2.5%, i.e., 0, 0.4, 2.0, and 4 g/kg body wt/day). Both male and female mice were dosed for 7 days prior to and during a 98-day cohabitation period. EGBE was toxic at the high (2%) and mid dose (1%) to adult F0 female mice: 13 out of 22 females at the high dose and 6 out of 20 at the mid dose died during the cohabitation period. Both the high- and mid-dose animals produced fewer litters/pair, fewer pups/litter, with decreased pup weight. These effects occurred in the presence of decreased body weight, decreased water consumption, and increased kidney weight. A crossover mating trial indicated that the reproductive effects could be attributed primarily to an effect on the female. This was substantiated at necropsy where testes and epididymis weights were normal as were sperm number and motility. Fertility of the offspring of the 0.5% group was normal in the presence of increased liver weights. With respect to EGPE, there was no change in the ability to produce five litters during the continuous breeding period. There was, however, a significant but small (10-15%) decrease in the number of pups/litter and in pup weight in the high-dose group. A crossover mating trial suggested a female component of the reproductive toxicity of EGPE. While fertility was only minimally compromised, severe neonatal toxicity was observed. By Day 21 there were only 8 out of 40 litters in the mid- and high-dose groups which had at least one male and female/litter. Second generation reproductive performance of the mid-dose group (1.25%) was unaffected except for a small decrease in live pup weight. In summary the reproductive toxicity of EGBE and EGPE was only evident in the female and occurred at doses which elicited general toxicity. EGBE was particularly toxic to adult female mice while EGPE was particularly toxic to immature mice of both sexes.

Reproductive effects of diethylene glycol and diethylene glycol monoethyl ether in Swiss CD-1 mice assessed by a continuous breeding protocol

Diethylene glycol (DEG) and diethylene glycol monoethyl ether (DEGEE) were evaluated for reproductive toxicity in CD-1 mice using a continuous breeding protocol. Compounds were administered in the drinking water at 0, 0.35, 1.75, and 3.5% w/v (DEG) or 0, 0.25 1.25, and 2.5% w/v (DEGEE). Exposure of the breeding pairs to 3.5% DEG for 14 weeks produced statistically significant decreases in the number of litters per pair, live pups per litter, proportion of pups born alive, and live pup weight. There was also a significant increase in the cumulative days to litter and a significant decrease in the number of pairs producing the third, fourth, and fifth litters for the 3.5% DEG-exposed mice. A crossover mating trial of the F0 mice to determine the affected sex was inconclusive, but suggested that offspring development was compromised in females exposed to 3.5% DEG. Slight maternal (F0) toxicity was noted for the 3.5 DEG group (7% decrease in body weight). The F1 generation, at 3.5% DEG, had decreased body weights at birth and exhibited poor postnatal survival. At the intermediate dose of DEG, body weights of both sexes were depressed at weaning, at onset of mating, and at necropsy. However, no adverse effects on reproduction were observed. DEGEE had no effect on reproduction in the F0 or F1 generation mice despite a 34% decrease in cauda epididymal sperm motility in the F1 males at 2.5% DEGEE. Other signs of toxicity observed in these F1 mice included increased relative liver weights. These data indicate that DEG is a reproductive toxicant in Swiss mice affecting fertility and reproductive performance, albeit at high doses (equivalent to 6.1 g/kg/day). However, its monoethyl derivative, DEGEE, is without adverse effects on fertility and reproductive performance.

Results and evaluations of 48 continuous breeding reproduction studies conducted in mice

Results of 48 continuous breeding reproduction (RACB) studies are summarized and control data from these studies are used to determine the statistical sensitivity of each endpoint from different parts of these studies. Results of testing individual chemicals compared well with results of multigeneration studies reported in the literature. Continuous breeding studies were able to discriminate reproductive toxicants from nontoxicants, and provided valuable structure-activity information. When mice in continuous cohabitation produce multiple litters, the statistical sensitivity of fertility endpoints is quite high and is comparable to that associated with other sensitive indicators of reproductive function, such as testis weight and sperm parameter measures. The principal advantages of the RACB protocol in comparison to multigeneration studies are: (1) the increased sensitivity and statistical power, (2) the ability to monitor progression of toxicity and to detect subfertility, (3) use of a battery of endpoints including sperm measures, (4) the ability to determine the affected sex(es), and (5) slightly reduced testing time.

Reproductive toxicity of 2,2-bis(bromomethyl)-1,3-propanediol in a continuous breeding protocol in Swiss (CD-1) mice

The effect of 2,2-bis(bromomethyl)-1,3-propanediol (BMP) on reproduction in Swiss CD-1 mice was evaluated by use of a continuous breeding protocol. BMP was administered in the feed at 0.1, 0.2, and 0.4% concentrations. Both male and female F0 mice (20 pairs per treatment group, 40 pairs of control animals) were dosed 7 days prior to and during a 98-day cohabitation period. Although the fertility index was unchanged in the high-dose group, BMP exposure significantly decreased the numbers of litters per pair, pups born alive per litter, and pup weight when adjusted for litter size. Crossover mating between treated and control F0 animals indicated a specific effect only on female reproductive capacity. At the highest dose, BMP caused a body weight decrease in the F0 animals of both sexes with no effect on relative organ weights. Sperm concentration, motility, morphology, and estrual cyclicity were unaffected by BMP exposure. Histopathology in the F0 animals revealed specific kidney lesions in both sexes; males were more sensitive than females. The last litter born in the 98-day breeding phase was reared to age 74 days and then mated to nonsiblings of the same treatment group. The effect of high-dose BMP exposure on F1 fertility, body and organ weights, sperm parameters, and estrual cyclicity was the same as that for the F0 animals, with the exception of the lack of renal lesions seen in the F1 females. These data show that BMP impaired fertility in female mice in both generations in the absence of an effect on reproductive organ weights and estrual cyclicity.

Reproductive toxicity of three phthalic acid esters in a continuous breeding protocol

A continuous breeding protocol was utilized to examine the reproductive toxicity of three phthalate esters. CD-1 mice were given diets with either di-n-propyl phthalate (DPrP: 0.0, 1.25, 2.5, or 5.0%), di-n-pentyl phthalate (DPP: 0.0, 0.5, 1.25, or 2.5%), or di-n-octyl phthalate (DOP: 0.0, 1.25, 2.5, or 5.0%). Both male and female mice (20 pairs per treatment group, 40 pairs of control animals) were dosed for 7 days prior to and during a 98-day cohabitation period. Reproductive function was evaluated during the cohabitation period by measuring number of litters per pair, live pups per litter, and pup weight. There was no apparent effect on reproductive function in the animals exposed to DOP at dose levels sufficient to cause a significant increase in liver weight. Both DPP and DPrP were toxic to the reproductive system as evidenced by a complete inhibition of fertility at 1.25 and 2.5% DPP or 5.0% DPrP, and reduced fertility (litters/pair and live pups/litter, 0.5% DPP; live pups/litter, 2.5% DPrP). Toxicity of DPP had a strong male component and female component, whereas DPrP was more toxic to the female than the male reproductive system. DPP and DPrP treatment was associated with decreased body weight, increased liver weight, decreased testis and epididymis weights, decreased epididymal sperm concentration, and elevated seminiferous tubule atrophy. A comparison of seven phthalate esters tested using this continuous breeding protocol indicates the relative order of reproductive toxicity as diethylhexyl, dihexyl, dipentyl, dibutyl, dipropyl; diethyl and dioctyl are nontoxic.

Evaluation of two in vitro assays to screen for potential developmental toxicants

To evaluate two in vitro assays for their ability to detect known developmental toxicants and nontoxicants, a series of 44 coded compounds were assayed by two independent laboratories using standardized protocols. The two test systems were (1) the human embryonic palatal mesenchymal cell growth inhibition assay and (2) the mouse ovarian tumor cell attachment inhibition assay. After all compounds were tested, they were decoded and ranked according to the minimum IC50 value (the millimolar concentration of compound which inhibits growth or attachment by 50% compared to the solvent control) from either test. The in vitro test result concordance with established in vivo animal and human test results was examined over a wide range of concentration levels (above which the in vitro results were called positive and below which they were considered negative). A positive response from either test was defined as a positive in vitro response. Concordance was defined as the number of correct responses divided by the number of chemicals tested. At the 1 mM level, the concordance of data from the combined in vitro assays with the in vivo data was 66% in one laboratory and 58% in the other. The maximum agreement between the combined in vitro and in vivo data was reached at the 20 mM level, where there was a 73 and 74% concordance of results in the two laboratories. At that level there was a 16 and 10% incidence of false negative results, and a 54 and 77% incidence of false positive results. A portion of these false negative compounds may require metabolic activation. The use of either assay alone was not as accurate as using a positive result from either test. Agreement of the in vitro data at the 10 mM level with available human data was 71 and 75% for each laboratory. The data indicate that the two assays are complimentary and as such the combination of these assays could be useful as a preliminary screen to establish priorities for in vivo developmental toxicity testing.

Subchronic toxicity of orally administered (gavage and dosed-feed) theophylline in Fischer 344 rats and B6C3F1 mice

Theophylline, a methylated xanthine closely resembling caffeine and theobromine, is a widely used pharmaceutical agent for the treatment of respiratory disorders and certain acute cardiovascular conditions. The National Toxicology Program has conducted 13-week subchronic toxicity studies in F344 rats and B6C3F1 mice (10 animals/group) following administration of theophylline via the diet or by gavage. Administration of theophylline in the feed (0, 1000, 2000, and 4000 ppm) resulted in no mortality or body weight effects in F344 rats, but did induce periarteritis of the arteries adjacent to mesenteric lymph nodes and the pancreas, particularly arterioles in the latter. Also observed in rats dosed with theophylline via the diet was an increased severity of chronic nephropathy in males, especially at the high dose. Administration of theophylline at the same concentrations in the feed to B6C3F1 mice resulted in no mortality, but terminal body weights were significantly decreased in all dosed groups. An increased incidence of hepatocellular glycogen depletion was observed in male and female mice, and this change is believed to represent a physiological alteration exacerbated by the administration of theophylline. Administration of theophylline by gavage to F344 rats (0, 37.5, 75, and 150 mg/kg) resulted in the early death of one high-dose male and female and significantly decreased or increased terminal body weights of high-dose males and females, respectively. Similar to the results of the dosed-feed study, male and female rats receiving theophylline by gavage demonstrated a dose-related increase in the incidence and severity of perivascular inflammation of mesenteric arteries. Gavage administration of theophylline to B6C3F1 mice (0, 75, 150, and 300 mg/kg) resulted in the early death of all high-dose females and 3/10 high-dose males and significant depression of terminal body weights in high- and mid-dose males and low-dose females. As in the dosed-feed study, the primary histopathologic change in the mouse subchronic gavage study was hepatocellular glycogen depletion, although in this case it was seen only in females. In summary, the major target organs for orally administered theophylline in 13-week subchronic toxicity studies appear to be the mesenteric arteries in F344 rats and the liver in B6C3F1 mice. On the basis of organ weight changes and/or minor histopathologic effects, many other tissues were also affected, particularly the kidneys in dosed-feed male rats and the uterus in gavage-dosed female rats.