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PubMed2004

"Holo"er than thou: chromosome segregation and kinetochore function in C. elegans.

Kinetochores are proteinaceous organelles that assemble on centromeric DNA to direct chromosome segregation in all eukaryotes. While many aspects of kinetochore function are conserved, the nature of the chromosomal domain upon which kinetochores assemble varies dramatically between different species. In monocentric eukaryotes, kinetochores assemble on a localized region of each chromosome. In contrast, holocentric species such as the nematode Caenorhabditis elegans have diffuse kinetochores that form along the entire length of their chromosomes. Here, we discuss the nature of chromosome segregation in C. elegans. In addition to reviewing what is known about kinetochore function, chromosome structure, and chromosome movement, we consider the consequences of the specialized holocentric architecture on chromosome segregation.

Animals↗
PubMed2004

Telomere erosion and chromosomal instability in cells expressing the HPV oncogene 16E6.

Progression to advanced-stage cervical carcinomas is characterized by a recurrent pattern of chromosomal rearrangements. Structural chromosome rearrangements are generated through the fusion of broken chromosome ends. These chromosome breaks may be induced by mutagenic agents such as ionizing radiation, or chromosome ends may be exposed through extensive telomere shortening. The human papilloma virus oncogene 16E6 induces telomerase activity in human keratinocytes, a model system for cervical tumor formation. The present study explores the relationship between 16E6 expression, telomerase activity, and chromosomal instability. We show that the frequency of anaphase bridges is dependent on the level of telomerase activity in 16E6/E7-expressing clones, and is the result of telomere shortening. High frequencies of anaphase bridges, associated with low telomerase activity, correlate with increased chromosome instability. Anaphase bridge formation is also associated with the presence of micronuclei, which are shown to contain unstable chromosomes frequently involved in rearrangements. As anaphase bridges are observed in both high and low telomerase 16E6/E7 clones, but not in hTERT-expressing control clones, expression of 16E6 in these immortalized clones is not sufficient to stabilize shortened telomeres completely. We suggest a model in which HPV-induced tumorigenesis may be dependent on persistent bridge-breakage-fusion cycles that allow for continued genomic rearrangements.

Anaphase↗
PubMed1992

The presence of interstitial telomeric sequences in constitutional chromosome abnormalities.

We describe a novel chromosome structure in which telomeric sequences are present interstitially, at the apparent breakpoint junctions of structurally abnormal chromosomes. In the linear chromosomes with interstitial telomeric sequences, there were three sites of hybridization of the telomere consensus sequence within each derived chromosome: one at each terminus and one at the breakpoint junction. Telomeric sequences also were observed within a ring chromosome. The rearrangements examined were constitutional chromosome abnormalities with a breakpoint assigned to a terminal band. In each case (with the exception of the ring chromosome), an acentric segment of one chromosome was joined to the terminus of an apparently intact recipient chromosome. One case exhibited apparent instability of the chromosome rearrangement, resulting in somatic mosaicism. The rearrangements described here differ from the telomeric associations observed in certain tumors, which appear to represent end-to-end fusion of two or more intact chromosomes. The observed interstitial telomeric sequences appear to represent nonfunctional chromosomal elements, analogous to the inactivated centromeres observed in dicentric chromosomes.

Abnormalities, Multiple↗
PubMed1982

[Chromosome disorders in retinoblastoma cells].

The karyotype of 7 unilateral retinoblastomas was studied. Loss or a gain of chromosomes and structural chromosomal changes were observed in all tumors. Modal chromosome number was pseudodiploid in four and near-diploid in three tumors. There were no sex chromosomes (X or Y) in 3 tumors. Four tumors were characterized by the presence of an identical marker chromosome--i(6p). Two other markers--lp+ and 17q+ (probably, not quite identical)--were present in 4 and 3 tumors, respectively. The cells of two retinoblastomas contained both i (6p) and 17q+ markers.

Child, Preschool↗
PubMed1993

What is a chromosome like in Leishmania?

Hereby, we present evidences of the presence of centromeric proteins in Leishmania mexicana. These proteins were partially purified and used to bind Leishmania DNA. The protein-DNA complex was immunoprecipitated and the DNA extracted, like in the human sample. Two major fragments of 1.2 and 2.2 Kbp, respectively, were resolved in agarose gel electrophoresis. A model of chromosomal structure and chromosomal segregation in Leishmania is presented.

Animals↗
PubMed2001

The structural maintenance of chromosomes (SMC) family of proteins in mammals.

Structural maintenance of chromosomes (SMC) family proteins play critical roles in chromosome structural changes. SMC proteins are known to be involved in two major chromosome structural organization events required for mitotic segregation of chromosomes: mitotic chromosome condensation and sister chromatid cohesion. In eukaryotes, two separate sets of SMC heterodimers form the cores of two distinct multiprotein complexes termed 'condensin' and 'cohesin', each specialized for condensation or cohesion, respectively. It is clear that both condensin and cohesin are conserved in mammals, including humans. The mammalian complexes demonstrate dynamic changes in intracellular distribution in a cell cycle-dependent manner. At any point in the cell cycle, the intracellular localization of the majority of mammalian cohesin and condensin appears to be complementary. Cohesin is associated with chromatin in interphase, while condensin is largely cytoplasmic. Similarly, in mitosis, cohesin is mostly excluded from chromosomes while condensin is distinctly bound to them. Cell cycle-dependent targeting of the two complexes appears to play a major role in regulating their cell cycle-specific activities, and how this redistribution is controlled is an area of active research. Finally, there is evidence that SMC proteins may be involved in DNA recombination and repair. This review focuses on what we have learned about SMC family proteins in humans and other mammalian species in comparison to those in lower eukaryotes. The authors present their own views with regard to some of the major outstanding questions surrounding the nature and functions of the SMC family of proteins.

Animals↗
PubMed1998

Structure and chromosomal localization of the mouse bombesin receptor subtype 3 gene.

Bombesin (BN)-like peptides/neurotransmitters mediate a broad range of physiological funtions in the gastrointestinal tract and the central nervous system through binding to their specific, high-affinity mammalian bombesin receptors. This family of heptahelical, G-protein coupled receptors includes the gastrin-releasing peptide receptor (GRP-R, or bb2), neuromedin B receptor (NMB-R, or bb1), and the bombesin receptor subtype 3 (BRS-3, or bb3). The tissue distribution of BRS-3 is quite dissimilar compared to the other two BN receptors, GRP-R and NMB-R, and a natural ligand for BRS-3 is currently unknown. Nothing is known about mechanisms regulating BRS-3 gene expression and possible association with disease. To gain insight into the underlying structure and chromosomal localization of the BRS-3 genes, bacteriophage P1 genomic clones, harboring the genes for the human and mouse BRS-3, respectively, were isolated and their structure and chromosomal localizations determined. The protein-coding region of both genes is divided into three exons and spans approximately 5kb. The loci of the BRS-3 genes were mapped to a syntenic region of the human (Xq25) and mouse (XA7.1-7.2) X-chromosome, respectively. The structural data of the BRS-3 genes derived from this study will permit future investigations of the mechanisms regulating their expression.

Amino Acid Sequence↗
PubMed2002

Abnormal chromosome migration and chromosome aberrations in mouse oocytes during meiosis II in the presence of topoisomerase II inhibitor ICRF-193.

Using a mouse parthenogenetic system, effects of ICRF-193, a noncleavable complex-forming topoisomerase II inhibitor, on female meiosis II chromosomes and pronuclear chromosomes were studied. Eggs were exposed to the inhibitor (10 microM) at various times after parthenogenetic stimulation, and chromosomes of them were analyzed at the first cleavage metaphase. When eggs were exposed to the inhibitor during the period from metaphase II to anaphase II, a significant increase in incidences of structural chromosome aberrations (51.1% versus 1.3% in the control) and aneuploidy (30.3% versus 0.7% in the control) was found. Structural chromosome aberrations were observed in 10-20% of eggs following treatments during telophase II, but there was no increased incidence of aneuploidy in treatments during this meiotic stage. When pronuclear eggs at S phase were targeted by the inhibitor, no significant increase in chromosome aberrations was found.Interestingly, when chromatids moved to each pole during anaphase II in the presence of ICRF-193, most of them oriented their centromeres toward the spindle equator as if moving backwards. Moreover, lagging chromatids with the centromeres present were observed in more than 50% of treated eggs. However, chromosomal bridges that resulted from chromosome stickiness did not appear in any egg.These findings indicate that ICRF-193 can induce structural chromosome aberrations and aneuploidy in mouse secondary oocytes in meiotic stage-dependent manner. The induction of aneuploidy is due to disruption of the separation of sister centromeres at anaphase II. There appears to be mechanism(s) other than cleavable complex formation or chromosome stickiness behind the induction of structural chromosome aberrations by ICRF-193.

Animals↗
PubMed1991

Test of a semiselective screen for induced aneuploidy in germ cells of Drosophila melanogaster females with structurally normal chromosomes.

A new semiselective screen (only female progeny survive) for induced aneuploidy in germ cells of Drosophila melanogaster (referred to as 20/Q56 for the X-chromosome mutation markers in the parental females) has been validated by recovering cold, colchicine and N,N-dimethylnitrosamine (DMN) induced chromosome gain and loss events in females that contain structurally normal chromosomes. In addition, the spontaneous and induced results from the 20/Q56 assay, which identifies gain events at division I and loss events at divisions I and II of meiosis, were compared with a nonselective (all progeny survive) modified mating scheme that identifies gains and losses at both divisions of oogenesis. Females with the same genotypes are treated in the two mating schemes and are then mated with males that contain different marked Y chromosomes. The spontaneous rates of chromosome gains and losses were not significantly different in the two mating schemes (these rates ranged from 0.008 to 0.022%), supporting previous reports that spontaneous aneuploidy occurs at a higher frequency at division I of meiosis in females of D. melanogaster than at division II. Both the 20/Q56 and modified screens were able to identify significant increases in aneuploidy after adult treatments with cold shock (10 degrees C and 5 degrees C), colchicine (5 ppm and 10 ppm), and DMN (100 ppm). Brood analysis (five 2-day or five 3-day broods) showed that the largest increases in aneuploidy after cold treatment occurred in the first brood, which contains a high proportion of stage 14 oocytes, whereas colchicine induced the highest frequencies in the latter broods and DMN was effective in all but the last brood. Although the 20/Q56 mating scheme identifies gain events only in division I of meiosis whereas the modified mating scheme identifies gains in both divisions, the 20/Q56 scheme is just as effective in identifying induced aneuploidy as is the modified scheme. There were no significant differences in the frequencies of induced gains or losses in the two schemes. These results also suggest that the 3 treatments induced chromosome gain events mainly at division I of oogenesis. Taken together, the results from this study suggest that the 20/Q56 mating scheme in D. melanogaster, which is semiselective and therefore less expensive and time-consuming to perform, is an appropriate test system to screen for chemical induced aneuploidy in germ cells of a higher organism.

Aneuploidy↗
PubMed1997

The HRX proto-oncogene product is widely expressed in human tissues and localizes to nuclear structures.

Chromosomal rearrangement of the HRX (MLL, ALL-1, Htrx) gene situated at chromosome band 11q23 is one of the most frequent genetic changes in infant leukemias of myeloid and lymphoid lineage and in treatment-induced secondary leukemias. The HRX gene codes for a predicted 431-kD protein that shows significant homology to the Drosophila trithorax protein, an Hox epigenetic regulator. Typically, the region encoding the HRX gene is rearranged, mostly in reciprocal translocations with a number of partners, resulting in a range of fusion genes. However, this is not the only abnormality affecting HRX because partial duplication of the gene, as well as interstitial deletions, can occur. Despite extensive studies of HRX at the genetic level, the protein products of the HRX gene and their patterns of expression in normal and leukemic cells remain uncharacterized. In this study we analyzed the distribution and localization of HRX proteins in cell lines and human tissues, using both polyclonal and monoclonal antibodies. The specificity of these reagents was confirmed using cells transfected with the HRX-ENL fusion gene. Western blot analyses of protein extracts from cells carrying the t(11;19) and t(4;11) translocations showed HRX chimeric proteins whose migrations corresponded to the sizes predicted from analyses of translocation-induced fusion mRNAs expressed by the derivative 11 chromosomes. Immunocytochemical analysis showed a punctate distribution of wild-type and chimeric HRX proteins within cell nuclei, suggesting that HRX localizes to nuclear structures in cells with and without 11q23 translocations. Nuclear staining was found in the majority of tissues studied with the strongest reactivity in cerebral cortex, kidney, thyroid, and lymphoid tissues. Thus, HRX is widely expressed in most cell types including hematopoietic cells, a finding that precludes an immunocytochemical approach for diagnosis of leukemias bearing 11q23 structural abnormalities.

Animals↗
PubMed1976

Subunit structure of chromosomes in mitotic nuclei of physarum polycephalum.

We have investigated the subunit structure of mitotic chromosomes of the acellular slime mould Physarum polycephalum, using the nuclease susceptibility of isolated mitotic nuclei as a probe. A characteristic pattern of DNA digestion products is obtained, containing approximately integral multiples of a basic 140 base pair DNA segment that resembles very closely the pattern in G2 phase nuclei of Physarum and of calf lymphocyte nuclei. These results demonstrate that during the process of chromosome condensation there is no alteration at the primary level of chromatin structure that is responsible for the characteristic DNA digestion pattern.

Chromatin↗
PubMed1978

Relationship of R6K replicating forms to the folded chromosome of Escherichia coli.

An examination of the relationship of both nonreplicating and replicating forms of R6K plasmid DNA to the Escherichia coli folded chromosome showed that both forms cosediment with the chromosome in neutral sucrose gradients. Approximately 20% of the nonreplicatin molecules was found as freely sedimenting forms when the folded-configuration of the chromosomes was preserved. However, under the same conditions negligible amounts of the replicating forms were found as freely sedimenting molecules. Thus, it is concluded that the replicating forms, when compared with nonreplicating molecules, are preferentially associated with the folded chromosomal structure. Exposure of the folded chromosomal structure to RNase resulted in an unfolding of the chromosome and a concomitant increase in the amount of freely sedimenting replicating and nonreplicating forms of R6K DNA. Analyses of the single-stranded length of RNase-released nascent molecules suggest that they replicate in continuous association with the folded chromsome complex. Nonenzymatic unfolding of the chromosomes by progressively lowering the sodium ion concentration during lysis resulted in a progressive increase in the release of nonreplicating molecules. Replicating molecules wer not released by unfolding the chromosome in this fashion.

Centrifugation, Density Gradient↗
PubMed1990

[Hyperdiploidy (greater than 50 chromosomes) has the most favorable prognosis among the major karyotypic subgroups of childhood acute lymphoblastic leukemia].

Thirty-four children, including nine relapsed cases with acute lymphoblastic leukemia (ALL) having hyperdiploidy (greater than 50 chromosomes) were studied on clinical and cytogenetic characteristics. The majority of children initially with hyperdiploidy (greater than 50 chromosomes), who showed favorable prognostic features such as lower leukocyte counts, lower serum lactic dehydrogenase levels, ages between 2 and 10 years, or the presence of common ALL antigen, had the most favorable outcome among childhood ALL (5-year survival rate was 100%). Even nine children, who showed poor prognostic features such as ages over 10 years, leukocyte counts over 2 X 10(4)/mm3 or lymphomatous signs, had also the same favorable outcome. There were no differences in clinical features between 6 patients with additional chromosomal structural abnormalities and 19 patients without them. Duplication of the long arm of chromosome 1 was frequently observed as additional chromosomal structural abnormalities. Patients with hyperdiploidy (greater than 50 chromosomes) observed at relapse, who had the same favorable clinical features as those at diagnosis, had a poorer prognosis. These findings show that initial hyperdiploidy (greater than 50 chromosomes) is an independent favorable prognostic sign in childhood ALL and additional chromosomal structural abnormalities may not indicate a poor prognosis among childhood ALL with hyperdiploidy (greater than 50 chromosomes). On the other hand, relapsed children with hyperdiploidy (greater than 50 chromosomes) have not a favorable outcome after the onset of relapse.

Child↗
PubMed1994

The structure of human S-phase chromosome fibres.

Recent in situ hybridization studies suggested that within the range of 0.1-1.0 Mb, human interphase chromosomes follow a random walk model (i.e. they behave as flexible polymers without major constraints). However, chromosome structure may differ in the G1, S, and G2 phases, and phase-specific constraints may be masked if the chromosome analysis does not discriminate between the phases. Therefore, using confocal microscopy, we examined the structure of S-phase chromosomes labelled with 5-iododeoxyuridine after prolonged treatment with 5-fluorodeoxyuridine. In the S-phase, labelled 0.32 mu chromosome fibres mostly appear as semi-circles with an average diameter of 0.83 +/- 0.03 mu. These semi-circles are joined together to form different 3D structures, and two semicircles frequently adopt s- or omega-like conformations involving about 2.5 mu of the chromosome contour length (L). Morphometric analysis of the S-phase fibres suggests that our data fit both the random flexible polymer model and also a model in which two constrained semi-circles are attached to each other by a flexible joint, thus eliminating constraints at long distances (L more than 2 mu).

Breast Neoplasms↗
PubMed2001

Meiotic stage-dependent induction of chromosome aberrations in Chinese hamster primary oocytes exposed to topoisomerase II inhibitor etoposide.

To investigate the chromosomal effects of topoisomerase II (topo-II)-interactive drugs on mammalian primary oocytes, female Chinese hamsters were treated with etoposide (VP-16) at various intervals pre- and post-human chorionic gonadotropin (hCG) injections. Chromosome analysis of oocytes at metaphase II (M II) showed that treatment with VP-16 at 50h pre-hCG had no effect, but the treatments between 24h pre-hCG and 2h post-hCG often caused structural chromosome aberrations. Although treatment at 4h post-hCG had no effect, subsequent treatments at 6 and 8h post-hCG produced a significant increase in structural chromosome aberrations. No effect was found following treatment at 10h post-hCG. The incidence of aneuploidy following exposure to VP-16 was also dependent on the time of hCG injection. Taking the time course of meiotic progression in primary oocytes following hCG injection and pharmacokinetics of VP-16 into consideration, it is likely that meiotic stages from late dictyate to diakinesis are highly sensitive to VP-16, while stages at dictyate and from metaphase I (M I) to telophase I (telo I) are relatively insensitive to the drug. Moreover, the effect of VP-16 on structural chromosome aberrations and aneuploidy was dose-dependent. Chromosome analysis at M I detected a frequent occurrence of structural chromosome aberrations in treated oocytes. This suggests that structural aberrations may be caused by disruption of cleavable complexes during chromosome condensation. Detection of chromosome bridges during anaphase I/telophase I (ana I/telo I) may support the hypothesis that induction of aneuploidy by VP-16 is due to failure in decatenation of recombinant homologous chromosomes.

Aneuploidy↗
PubMed1997

Occurrence of deletions, associated with genetic instability in Streptomyces ambofaciens, is independent of the linearity of the chromosomal DNA.

The chromosomal structures of mutant strains of Streptomyces ambofaciens which have arisen from genetic instability were investigated by using pulsed-field gel electrophoresis and probing with sequences cloned from the unstable region which maps near the ends of the linear chromosomal DNA. The chromosomal structures of seven mutant strains harboring large deletions were classified into three types. (i) Deletions internal to one chromosomal arm were characterized in two of the mutant strains. In these strains, a linear chromosomal structure was retained, as were parts of the terminal inverted repeats sequences (TIRs) and the proteins bound to them. (ii) Four of the mutants presented a deletion including all sequences from the TIRs. A junction fragment homologous to sequences originating from internal region of both arms was characterized. Consequently, the chromosomal DNA of these strains was deduced to be circularized. Furthermore, chromosomal stability was assessed in the progeny of these circular DNA mutants. Additional deletion events were detected in 11 mutants among the 13 strains isolated, demonstrating that circular chromosomes do not correspond to a stabilization of the chromosome structure and that the occurrence of deletion could be independent of the presence of chromosomal ends. (iii) A mutant with DNA amplification was shown to have a linear chromosome with a deletion of all sequences between the amplified region and the end of the chromosome. The other chromosomal arm remained unaffected by deletion and associated with protein.

Bacterial Proteins↗
PubMed1981

Evaluation of chemically induced cytogenetic lesions in rabbit oocytes. I. The test system and the effects of streptonigrin.

An appropriate method for evaluating transmissible genetic damage in female germ cells is presented. Analysis of karyotype preparations from preimplantation (6- day) rabbit embryos for consistent structural chromosome abnormalities is described as a sensitive way in which these determinations can be made. A table prepared from summarized research reports shows that less than 1 in 2000 rabbit blastocysts karyotyped thus far has a consistent structural chromosome abnormality. In the new data presented in this study, none of the 90 control blastocysts karyotyped had consistent structural chromosome abnormalities while 13 of 278 from female rabbits treated with streptonigrin (NSC-45383) before conception did. These findings demonstrate that karyotype analysis for consistent chromosome lesions in preimplantation rabbit embryos from females exposed to chemical clastogens can serve as a sensitive and quantitative means of estimating effects on oocytes. The results of the study also show that the consistent structural chromosome lesions in the blastocyst karyotypes are what would be predicted from the mode of action of streptonigrin and the segregation of chromosomes during meiosis. The relative efficiency of this system based on the rabbit as compared to another recently described system bases on the hamster is also discussed.

Animals↗