Search PubMed⌕ Search

Biomedical subjects

Ohtsura Niwa

Publications and source records attributed to Ohtsura Niwa.

At least 19 recordsLinked to original sources
PubMed2006

Indirect mechanisms of genomic instability and the biological significance of mutations at tandem repeat loci.

Radiation induction of genomic instability has two features: induction of untargeted mutation and delayed mutation. These phenomena have been studied mostly in tissue culture cells, but analyses have also been conducted in whole body systems. The study of response in whole body systems frequently applies repeat sequences as markers to detect mutations. These studies have generated conflicting findings. In addition, lack of knowledge of the mechanisms involved in repeat mutation confounds the interpretation of the biological significance of increased rates of repeat mutation. In this review, some of the existing controversies of genomic instability are discussed in relation to the mechanism of repeat mutation. Analyses of published and unpublished studies indicate a mechanistic similarity between radiation-induced genomic instability at repeat loci and dynamic mutations of triplet repeats. Because of their repetitive nature, repeat sequences frequently block progression of replication forks and are consequently resolved by slippage and/or recombination. Irradiation of cells induces S checkpoints and promotes slippage/recombination mediated repeat mutations. Thus, genomic instability at repeat loci might be viewed as a consequence of cellular attempts to restore the stability of replication in the face of the stalled replication fork; this process can occur both spontaneously as well as after exposure to radiation.

Animals↗
PubMed2006

Radiation carcinogenesis in mouse thymic lymphomas.

Ionizing radiation is a well-known carcinogen for various human tissues and a complete carcinogen that is able to initiate and promote neoplastic progression. Studies of radiation-induced mouse thymic lymphomas, one of the classic models in radiation carcinogenesis, demonstrated that even the unirradiated thymus is capable of developing into full malignancy when transplanted into the kidney capsule or subcutaneous tissue of irradiated mice. This suggests that radiation targets tissues other than thymocytes to allow expansion of cells with tumorigenic potential in the thymus. The idea is regarded as the 'indirect mechanism' for tumor development. This paper reviews the indirect mechanism and genes affecting the development of thymic lymphomas that we have analyzed. One is the Bcl11b/Rit1 tumor suppressor gene and the other is Mtf-1 gene affecting tumor susceptibility.

Animals↗
PubMed2006

Radiation induced dynamic mutations and transgenerational effects.

Many studies have confirmed that radiation can induce genomic instability in whole body systems. Although the molecular mechanisms underlying induced genomic instability are not known at present, this interesting phenomenon could be the manifestation of a cellular fail-safe system in which fidelity of repair and replication is down-regulated to tolerate DNA damage. Two features of genomic instability namely, delayed mutation and untargeted mutation, require two mechanisms of ;damage memory' and ;damage sensing, signal transduction and execution' to induce mutations at a non damaged-site. In this report, the phenomenon of transgenerational genomic instability and possible mechanisms are discussed using mouse data collected in our laboratory as the main bases.

Animals↗
PubMed2005

Transcription-independent suppression of DNA synthesis by p53 in sperm-irradiated mouse zygotes.

Cell cycle arrest in response to DNA damage is important for the maintenance of genomic integrity in higher eukaryotes. We have previously reported the novel p53-dependent S-phase checkpoint operating in mouse zygotes fertilized with irradiated sperm. In the present study, we analysed the detail of the p53 function required for this S-phase checkpoint in mouse zygotes. The results indicate that ATM kinase is likely to be indispensable for the p53-dependent S-phase checkpoint since the suppression was abrogated by inhibitors such as caffeine and wortmannin. However, ATM phosphorylation site mutant proteins were still capable of suppressing DNA synthesis when microinjected into sperm-irradiated zygotes lacking the functional p53, suggesting that the target of the phosphorylation is not p53. In addition, the suppression was not affected by alpha-amanitin, and p53 protein mutated at the transcriptional activation domain was also functional in the suppression of DNA synthesis. However, p53 proteins mutated at the DNA-binding domain were devoid of the suppressing activity. Taken together, the transcription-independent function of p53 associated with the DNA-binding domain is involved in the S-phase checkpoint in collaboration with yet another unidentified target protein(s).

Alleles↗
PubMed2005

Predisposition to mouse thymic lymphomas in response to ionizing radiation depends on variant alleles encoding metal-responsive transcription factor-1 (Mtf-1).

Genetic predisposition to cancers is significant to public health because a high proportion of cancers probably arise in a susceptible human subpopulation. Using a mouse model of gamma-ray-induced thymic lymphomas, we performed linkage analysis and haplotype mapping that suggested Mtf-1, metal-responsive transcription factor-1 (Mtf-1), as a candidate lymphoma susceptibility gene. Sequence analysis revealed a polymorphism of Mtf-1 that alters the corresponding amino acid at position 424 in the proline-rich domain from a serine in susceptibility strains to proline in resistant strains. The transcriptional activity of Mtf-1 encoding serine and proline was compared by transfecting the DNA to Mtf-1-null cells, and the change to proline conferred a higher metal responsiveness in transfections. Furthermore, the resistant congenic strains possessing the Mtf-1 allele of proline type exhibited higher radiation inducibility of target genes than susceptible background strains having the Mtf-1 allele of serine type. Since products of the targets such as metallothionein are able to suppress cellular stresses generated by irradiation, these results suggest that highly inducible strains having Mtf-1 of proline type are refractory to radiation effects and hence are resistant to lymphoma development.

Amino Acid Sequence↗
PubMed2005

Comparison of properties of spontaneous and radiation-induced mouse thymic lymphomas: role of Trp53 and radiation.

Mouse thymic lymphomas are readily induced by radiation and also arise without irradiation when the mice are null in Trp53 functions. In the present study, spontaneous thymic lymphomas in Trp53-/- mice were compared to those arising in irradiated Trp53+/- mice, revealing three features characteristic of the spontaneous lymphomas. (1) Mp53D2, a Trp53 modifier that affects the latent period of radiogenic thymic lymphomas in Trp53+/- mice, had no effect on the development of spontaneous lymphomas. (2) A sex difference in the latency was found. (3) A marked difference was noted in the frequency of allelic loss at the Ikaros gene on chromosome 11, encoding a transcription factor required for normal lymphocyte development and differentiation; 2% in the lymphomas of Trp53-/- mice and 78% in the radiogenic lymphomas of Trp53+/- mice, suggesting that loss of Trp53 may reduce the requirement for the loss of Ikaros for lymphomagenesis. Furthermore, allelic loss analysis on chromosome 19 localized a region that may harbor an unknown tumor suppressor gene. These results suggest intricate steps of lymphomagenesis influenced by the presence or absence of Trp53.

Animals↗
PubMed2004

Generation of pluripotent stem cells from neonatal mouse testis.

Although germline cells can form multipotential embryonic stem (ES)/embryonic germ (EG) cells, these cells can be derived only from embryonic tissues, and such multipotent cells have not been available from neonatal gonads. Here we report the successful establishment of ES-like cells from neonatal mouse testis. These ES-like cells were phenotypically similar to ES/EG cells except in their genomic imprinting pattern. They differentiated into various types of somatic cells in vitro under conditions used to induce the differentiation of ES cells and produced teratomas after inoculation into mice. Furthermore, these ES-like cells formed germline chimeras when injected into blastocysts. Thus, the capacity to form multipotent cells persists in neonatal testis. The ability to derive multipotential stem cells from the neonatal testis has important implications for germ cell biology and opens the possibility of using these cells for biotechnology and medicine.

Animals↗
PubMed2004

Involvement of V(D)J recombinase in the generation of intragenic deletions in the Rit1/Bcl11b tumor suppressor gene in gamma-ray-induced thymic lymphomas and in normal thymus of the mouse.

Mouse thymic lymphomas induced by gamma-irradiation exhibited homozygous deletions of the Rit1/Bcl11b tumor suppressor gene on chromosome 12 at high frequencies. Internal deletions of one allele were frequently accompanied by loss of the other allele. In order to elucidate the mechanism of these internal deletions, the sites of breakage and rejoining were examined by PCR mapping and sequencing. The 5' site of the deletions clustered within an approximately 5 kb region of intron 1 and the 3' site was confined to a site in intron 3. These sites contained P and/or N nucleotides and cryptic sequences recognizable by the RAG1/2 recombinase in the vicinity. This suggests that the Rit1 intragenic deletions were generated by endogenous illegitimate V(D)J recombinase activity and such aberrant recombination was also detected by nested PCR of DNA from the thymus of unirradiated mice but not of RAG2-deficient mice. A rough estimate indicated that there reside as many as 10(3)-10(4) thymocytes having Rit1 deletions, assuming the presence of 10(8) thymocytes in the thymus of unirradiated mice. Moreover, the recombination frequency was not affected by gamma-irradiation. These results show no effect of radiation on Rit1 mutations and suggest an indirect mechanism for its role in lymphomagenesis.

Animals↗
PubMed2003

Induced genomic instability in irradiated germ cells and in the offspring; reconciling discrepancies among the human and animal studies.

Many studies confirmed that radiation induces genomic instability in whole-body systems. However, the results of the studies are not always consistent with each other. Attempts are made in the present review to resolve the discrepancies. Many of the studies in human and experimental animals utilize the length change mutation of minisatellite sequences as a marker of genomic instability. Minisatellite sequences frequently change their length, and the data obtained by conventional Southern blotting give rather qualitative information, which is sometimes difficult to scrutinize quantitatively. This is the problem inevitably associated with the study of minisatellite mutations and the source of some conflicts among studies in humans and mice. Radiation induction of genomic instability has also been assessed in whole-body experimental systems, using other markers such as the mouse pink-eyed unstable allele and the specific pigmentation loci of medaka fish (Oryzias latipes). Even though there are some contradictions, all these studies have demonstrated that genomic instability is induced in the germ cells of irradiated parents, especially of males, and in offspring born to them. Among these, transmission of genomic instability to the second generation of irradiated parents is limited to the mouse minisatellite system, and awaits further clarification in other experimental systems.

Animals↗
PubMed2003

The D5Mit7 locus on mouse chromosome 5 provides resistance to gamma-ray-induced but not N-methyl-N-nitrosourea-induced thymic lymphomas.

Susceptibility to gamma-ray induction of thymic lymphomas in mouse strains is controlled by low-penetrance genetic variant alleles. Our previous genome-wide scan of a mouse backcross between BALB/c and MSM strains suggested the existence of a BALB/c resistance locus near D5Mit5 on chromosome 5. To confirm this resistance, we produced congenic mice carrying a 28.4 cM region between D5Mit4 and D5Mit315 from the MSM parental strain on the BALB/c background. Lymphomas were induced in their progeny by gamma-ray irradiation or administration of N-methyl-N-nitrosourea (MNU), an alkylating agent. The incidence of radiogenic lymphomas was 87.5% in mice of the M/M genotype at D5Mit7, significantly higher than the 46% incidence in mice of the C/M genotype, indicating highly significant linkage between the locus and the resistance (P = 0.000054). In contrast, the frequencies of MNU-induced thymic lymphomas were similar between the two genotypes (P = 0.35 in chi2 test). These results confirm the presence of a resistance allele for gamma-ray induction of thymic lymphomas near the D5Mit7 locus and strongly suggest that this locus modifies carcinogenic risk from exposure to radiation but not to alkylating agents.

Animals↗
PubMed2003

Bcl11b is required for differentiation and survival of alphabeta T lymphocytes.

The gene Bcl11b, which encodes zinc finger proteins, and its paralog, Bcl11a, are associated with immune-system malignancies. We have generated Bcl11b-deficient mice that show a block at the CD4-CD8- double-negative stage of thymocyte development without any impairment in cells of B- or gammadelta T cell lineages. The Bcl11b-/- thymocytes showed unsuccessful recombination of V(beta) to D(beta) and lacked the pre-T cell receptor (TCR) complex on the cell surface, owing to the absence of Tcrb mRNA expression. In addition, we saw profound apoptosis in the thymus of neonatal Bcl11b-/- mice. These results suggest that Bcl11b is a key regulator of both differentiation and survival during thymocyte development.

Animals↗
PubMed2003

Mapping of genetic modifiers of thymic lymphoma development in p53-knockout mice.

The strain dependency of the spectrum and latency of tumors has been reported in p53-deficient (KO) mice, suggesting the presence of modifiers for the outcome of the p53 deficiency. The modifiers provide clues to the oncogenic pathway in cells lacking p53, the most frequently mutated gene in a wide variety of human cancers. To search the modifiers, we induced 160 lymphomas and 69 skin tumors by gamma-irradiation of p53(KO/+) backcross mice between BALB/c and MSM strains and performed genome scan. BALB/c-derived alleles at three loci on chromosome 19, Mp53D1 (modifier of p53-deficiency) at D19Mit5, Mp53D2 at D19Mit90 and Mp53D3 at D19Mit123, extended the latency of thymic lymphoma development (P values in Mantel-Cox test were 0.0007, 0.0007 and 0.0003, respectively). Mp53D3 also increased the latency of skin tumors (P value, 0.0008). The linkage of Mp53D2 was confirmed by the experiment using 94 p53-KO mice consomic for chromosome 19, providing a significant linkage. However, the linkage was not confirmed for Mp53D1 or Mp53D3, suggesting epistasis of genes involved in the tumorigenesis.

Alleles↗
PubMed2003

Homozygous deletions and point mutations of the Rit1/Bcl11b gene in gamma-ray induced mouse thymic lymphomas.

Allelic loss (LOH) mapping and sequence analysis were conducted for gamma-ray induced mouse thymic lymphomas and a novel tumor suppressor gene, Rit1/Bcl11b, on chromosome 12 was isolated. Bi-allelic changes were found in 17 of the 66 p53-proficient lymphomas with Rit1 LOH but in only 2 of the 54 p53-deficient lymphomas. This suggests an association between the presence of functional p53 and inactivation of the Rit1 gene in the lymphoma development. Introduction of Rit1 into HeLa cells lacking Rit1 expression suppressed cell growth. These results indicate that loss-of-function mutations of Rit1 contribute to mouse lymphomagenesis and possibly to human cancer development.

Alleles↗
PubMed2003

The D4Mit12 locus on mouse chromosome 4 provides susceptibility to both gamma-ray-induced and N-methyl-N-nitrosourea-induced thymic lymphomas.

Low-penetrance genes control different susceptibilities to gamma-ray-induced thymic lymphomas in mouse strains. Our previous genetic analyses with backcross mice between BALB/c and MSM strains and congenic lines localized one such gene near the D4Mit12 locus on chromosome 4. N-Methyl-N-nitrosourea (MNU) is a guanine base-alkylating agent and differs from gamma-radiation in its mechanism of mutagenic action. Accordingly, in this study, we examined whether or not the locus also provides susceptibility to MNU-induced thymic lymphomas using 84 offsprings derived from congenic mice for D4Mit12. Association analysis provided a suggestive linkage at D4Mit12 (P = 0.0075) and the linkage was sustained by the peak of likelihood ratio statistical values being at the same position as that for the gamma-ray-induced lymphomas. The results strongly suggest that the BALB/c allele near D4Mit12 is associated with susceptibility to lymphomas induced by two carcinogenic agents having different mechanisms of mutagenic action.

Animals↗
PubMed2002

p53-dependent S-phase damage checkpoint and pronuclear cross talk in mouse zygotes with X-irradiated sperm.

One difficulty in analyzing the damage response is that the effect of damage itself and that of cellular response are hard to distinguish in irradiated cells. In mouse zygotes, damage can be introduced by irradiated sperm, while damage response can be studied in the unirradiated maternal pronucleus. We have analyzed the p53-dependent damage responses in irradiated-sperm mouse zygotes and found that a p53-responsive reporter was efficiently activated in the female pronucleus. [(3)H]thymidine labeling experiments indicated that irradiated-sperm zygotes were devoid of G(1)/S arrest, but pronuclear DNA synthesis was suppressed equally in male and female pronuclei. p53(-/-) zygotes lacked this suppression, which was corrected by microinjection of glutathione S-transferase-p53 fusion protein. In contrast, p21(-/-) zygotes exhibited the same level of suppression upon fertilization by irradiated sperm. About a half of the 6-Gy-irradiated-sperm zygotes managed to synthesize a full DNA content by prolonging S phase, while the other half failed to do so. Regardless of the DNA content, all the zygotes cleaved to become two-cell-stage embryos. These results revealed the presence of p53-dependent pronuclear cross talk and a novel function of p53 in the S-phase DNA damage checkpoint of mouse zygotes.

Animals↗
PubMed2002

Persistent induction of somatic reversions of the pink-eyed unstable mutation in F1 mice born to fathers irradiated at the spermatozoa stage.

Untargeted mutation and delayed mutation are features of radiation-induced genomic instability and have been studied extensively in tissue culture cells. The mouse pink-eyed unstable (p(un)) mutation is due to an intragenic duplication of the pink-eyed dilution locus and frequently reverts back to the wild type in germ cells as well as in somatic cells. The reversion event can be detected in the retinal pigment epithelium as a cluster of pigmented cells (eye spot). We have investigated the reversion p(um) in F1 mice born to irradiated males. Spermatogonia-stage irradiation did not affect the frequency of the reversion in F1 mice. However, 6 Gy irradiation at the spermatozoa stage resulted in an approximately twofold increase in the number of eye spots in the retinal pigment epithelium of F1 mice. Somatic reversion occurred for the paternally derived p(un) alleles. In addition, the reversion also occurred for the maternally derived, unirradiated p(un) alleles at a frequency equal to that for the paternally derived allele. Detailed analyses of the number of pigmented cells per eye spot indicated that the frequency of reversion was persistently elevated during the proliferation cycle of the cells in the retinal pigment epithelium when the male parents were irradiated at the spermatozoa stage. The present study demonstrates the presence of a long-lasting memory of DNA damage and the persistent up-regulation of recombinogenic activity in the retinal pigment epithelium of the developing fetus.

Animals↗