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Biomedical subjects

D L Mitchell

Publications and source records attributed to D L Mitchell.

At least 91 records · Page 5Linked to original sources
PubMed1990

The regulation of DNA repair during development.

DNA repair is important in such phenomena as carcinogenesis and aging. While much is known about DNA repair in single-cell systems such as bacteria, yeast, and cultured mammalian cells, it is necessary to examine DNA repair in a developmental context in order to completely understand its processes in complex metazoa such as man. We present data to support the notion that proliferating cells from organ systems, tumors, and embryos have a greater DNA repair capacity than terminally differentiated, nonproliferating cells. Differential expression of repair genes and accessibility of chromatin to repair enzymes are considered as determinants in the developmental regulation of DNA repair.

Animals↗
PubMed1990

Relationship between pyrimidine dimers, 6-4 photoproducts, repair synthesis and cell survival: studies using cells from patients with trichothiodystrophy.

Trichothiodystrophy is a genetic disease which in the majority of cases studied is associated with a deficiency in the ability to repair UV damage in cellular DNA. Three categories of UV response have been identified. In type 1 the response is completely normal, whereas type 2 cells are deficient in excision-repair, with properties indistinguishable from those of XP complementation group D. Type 3 cells have normal survival following UV-irradiation and normal rates of removal of cyclobutane pyrimidine dimer sites. Nevertheless repair synthesis is reduced by 50% in these cell strains and this is associated with a marked reduction in the repair of 6-4 photoproducts from cellular DNA. The present results show that 50% or more of repair synthesis at early times after irradiation of normal primary human fibroblasts is attributable to repair of 6-4 products. They also suggest that repair of cyclobutane dimers is crucial for cell survival.

Cell Survival↗
PubMed1990

Repair of pyrimidine(6-4)pyrimidone photoproducts in mouse skin.

The induction and repair of cyclobutane pyrimidine dimers and pyrimidine(6-4)pyrimidone photoproducts in the epidermal DNA of ultraviolet-irradiated hairless mice were determined by radioimmunoassay. Few cyclobutane dimers were excised by 48 h after ultraviolet (UV) irradiation, whereas 50% of the (6-4) photoproducts were removed by 6 h, correlating with previously determined rates of unscheduled DNA synthesis in mouse skin. After this initial rapid phase of (6-4) photoproduct excision, a slower phase was observed between 6 and 48 h. These repair kinetics contrast with those for fibroblast cell cultures derived from mouse tissues irradiated with UV light yielding similar levels of damage. Although the initial rate of (6-4) photoproduct repair in cultured fibroblasts and epidermal cells was similar, the extent of repair in cultured cells was significantly greater, with most of the damage removed by 24 h. The kinetics for (6-4) photoproduct repair in mouse epidermal cells suggest that a significant population, such as terminally differentiated keratinocytes, may have a reduced repair capacity and that the culture process may select for more rapidly proliferating, repair-proficient stem cells.

Animals↗
PubMed1990

Replication of chromosomal and episomal DNA in X-ray-damaged human cells: a cis- or trans-acting mechanism?

Episomal plasmids and viruses in mammalian cells present small targets for X-ray-induced DNA damage. At doses up to 100 Gy, DNA strand breaks or endonuclease III-sensitive sites were not discernible in 10.3-kb Epstein-Barr virus-based plasmid DNA or in 4.9-kb defective simian virus 40 DNA. DNA replication in these small molecules, however, was inhibited strongly by X-ray doses of greater than or equal to 20 Gy, decreasing to only 20 to 40% of control values. Inhibition was relieved slightly by growth in caffeine but was increased by growth in 3-aminobenzamide. Inhibition of DNA replication in episomal DNA molecules that are too small to sustain significant damage directly to their DNA may be due to either (a) a trans-acting diffusible factor that transfers the consequences of DNA breakage to episomes and to other replicating molecules, (b) a cis-acting mechanism in which episomes are structurally linked to genomic chromatin, and replication of both episomal and chromosomal replicons is under common control, or (c) radiation damage on other cellular structures unrelated to DNA. The resolution of these cellular mechanisms may shed light on the X-ray-resistant replication in ataxia-telangiectasia and may suggest strategies for molecular characterization of potential trans- or cis-acting factors.

Benzamides↗
PubMed1990

Tissue reaction involving an intraoral skin graft and CP titanium abutments: a clinical report.

Focal areas of gingival hyperplasia surrounding the transmucosal portions of titanium implants have been attributed to poor hygiene, lack of attached gingival tissues, and titanium allergy. Following mandibular vestibuloplasty and placement of a split-thickness skin graft, two of five patients developed persistent proliferation of the epithelial tissue surrounding endosseous CP titanium dental implants. In both circumstances, traditional gingivectomy procedures, chemotherapeutic agents, and aggressive oral hygiene measures failed to adequately control the hyperplastic response. Following replacement of the titanium abutments with custom-fabricated gold abutments, the epithelial condition appeared to return to normal.

Adult↗
PubMed1990

Immunoprecipitation of pyrimidine(6-4)pyrimidone photoproducts and cyclobutane pyrimidine dimers in uv-irradiated DNA.

Biological studies suggest that a significant proportion of the cytotoxicity observed in mammalian cells after uv irradiation may be due to damage other than cyclobutane dimers in DNA. Although pyrimidine-pyrimidone (6-4) photoproducts have been implicated as major contributors to cell lethality, their induction has been measured at considerably less than cyclobutane pyrimidine dimers when measured by chromatographic techniques. Because the yield of (6-4) photoproducts may be reduced by their lability to extreme heat and pH, we have advised an alternative, immunological quantification which does not require DNA hydrolysis. Affinity-purified rabbit antisera were used to precipitate low molecular weight 32P-labeled PM2 DNA irradiated with increasing fluences of uv light. DNA of known molecular weight was used to determine rates of induction for antibody-binding sites associated with (6-4) photoproducts and cyclobutane dimers. These rates were calculated to be 0.6 (6-4) photoproducts and 1.2 cyclobutane dimers/10(8) Da/J/m2. At low uv fluences (6-4) photoproducts were induced at one-half the rate of cyclobutane dimers, whereas at higher fluences (6-4) photoproducts predominated.

Animals↗
PubMed1990

Photoreactivation of cyclobutane dimers and (6-4) photoproducts in the epidermis of the marsupial, Monodelphis domestica.

Radioimmunoassays were used to investigate the repair of cyclobutane pyrimidine dimers and pyrimidine (6-4)pyrimidone photoproducts ((6-4] photoproducts) in the epidermis of the South American opossum, Monodelphis domestica. In the absence of photoreactivating light, both types of photodamage were excised with similar kinetics, 50% of the damage remaining 8 h after UV irradiation in vivo. Exposure of UV-irradiated skin to photoreactivating light resulted in removal of most of the cyclobutane dimers and an enhanced rate of (6-4) photoproduct repair. Photoenhanced excision repair of non-dimer damage increases the range of biologically effective lesions removed by in vivo photoreactivation.

Animals↗
PubMed1989

The orbital shell: an aid in positioning the ocular component and verifying margin contours for oculofacial prostheses.

The correct location of the orientation (conversational gaze) of the ocular component is vital to esthetically pleasing oculofacial prostheses. This location should be verified before time and energy are spent on sculpturing the oculofacial prosthesis. A technique is described that uses a clear acrylic resin shell as a matrix and Play-doh material as a support medium to position and support the ocular component to verify the orientation. In addition, the thin acrylic resin shell can be used to refine border inaccuracies that may exist clinically.

Acrylic Resins↗
PubMed1989

A photoelastic study of stress induced by framework design in a maxillary resection.

Support of a framework and obturator in a patient with an acquired maxillary defect is diminished by the removal of a portion of the palate and alveolar bone. The intent of this study was to investigate, by three-dimensional photoelastic analysis, the stress transmission that occurs with four commonly used retentive systems. The individual designs were facial cast circumferential retention with palatal plating, swing-lock design with palatal plating, facial cast circumferential retention and palatal cast circumferential clasp reciprocation, and facial cast circumferential reciprocation with palatal I-bar retention. Stresses were observed and compared on the palate and around the teeth of the four models.

Chromium Alloys↗
PubMed1989

Inhibition of transient gene expression in Chinese hamster ovary cells by cyclobutane dimers and (6-4) photoproducts in transfected ultraviolet-irradiated plasmid DNA.

Using a transient gene expression assay to measure host cell reactivation, the effects of cyclobutane dimer and noncyclobutane dimer uv photoproducts on expression of a reporter gene were examined in normal and repair-deficient Chinese hamster ovary (CHO) cell lines. Ultraviolet damage in plasmid pRSV beta gal DNA, containing the Escherichia coli beta-galactosidase gene, resulted in reduced reporter gene expression in both uv-hypersensitive mutant CHO cell lines UV5 and UV61 relative to wild-type, parental AA8 cells. However, the effects of uv irradiation of transfected plasmid DNA on gene activity were reduced in UV61, a mutant with normal (6-4) photoproduct repair, compared to UV5, which is deficient in (6-4) photoproduct repair; this reduction correlated with the intermediate uv-hypersensitivity of UV61. Selective removal of cyclobutane dimers by in vitro photoreactivation of uv-irradiated plasmid DNA prior to transfection substantially increased reporter gene activity in both uv-hypersensitive mutant cell lines. This increase was significantly greater in UV61 than in UV5, consistent with UV5 being deficient in repair of both (6-4) photoproducts and cyclobutane dimers. These results suggest that unrepaired (6-4) photoproducts in transfected pRSV beta gal plasmid DNA are responsible for a significant fraction of the reduction in transient gene expression observed in recipient uv-hypersensitive CHO cell mutants.

Animals↗
PubMed1989

Intermediate (6-4) photoproduct repair in Chinese hamster V79 mutant V-H1 correlates with intermediate levels of DNA incision and repair replication.

A DNA-repair mutant isolated from Chinese hamster V79 cells, V-H1, has been characterized as having only slightly reduced unscheduled DNA synthesis (UDS) and intermediate levels of DNA incision and repair replication after UV exposure. This observation was unexpected, since V-H1 has been shown by genetic complementation analysis to belong to the UV5 complementation class (i.e., class 2), exhibiting equivalent UV hypersensitivity and hypermutability as UV5 cells, which are defective in incision, UDS and repair replication. We have examined the repair of cyclobutane dimers and (6-4) photoproducts in V-H1 and V79 cells and shown that V-H1 cells are deficient in cyclobutane dimer repair, but exhibit intermediate (6-4) photoproduct repair, unlike UV5 cells which are completely deficient in (6-4) photoproduct repair. Our results confirm observations made in other UV-hypersensitive Chinese hamster cell mutants in CHO complementation class 2, and suggest that the gene affected in these mutants (ERCC2) may be involved in at least two distinct repair pathways in hamster cells.

Animals↗
PubMed1989

UV mutagenesis, cytotoxicity and split-dose recovery in a human-CHO cell hybrid having intermediate (6-4) photoproduct repair.

Somatic cell hybrids constructed between UV-hypersensitive Chinese hamster ovary cell line UV20 and human lymphocytes were used to examine the influence of a human DNA repair gene, ERCC1, on UV photoproduct repair, mutability at several drug-resistance loci, UV cytotoxicity and UV split-dose recovery. In hybrid cell line 20HL21-4, which contains human chromosome 19, UV-induced mutagenesis at the APRT, HPRT and Na+/K+-ATPase loci was comparable to that in repair-proficient CHO AA8 cells, whereas cell line 20HL21-7, a reduced human-CHO hybrid not containing human chromosome 19, exhibited a hypermutable phenotype at all 3 loci indistinguishable from that of UV20 cells. The response of 20HL21-4 cells to UV cytotoxicity reflected substantial but incomplete restoration of wild-type UV cytotoxic response, whereas responses of UV20 and 20HL21-7 cell lines to UV cytotoxicity were essentially the same, reflecting several-fold UV hypersensitivity. Repair of UV-induced (5-6) cyclobutane dimers and (6-4) photoproducts was examined by radioimmunoassay; (6-4) photoproduct repair was deficient in UV20 and 20HL21-7 cell lines, and intermediate in 20HL21-4 cells relative to wild-type CHO AA8 cells. UV split-dose recovery in 20HL21-4 cells was also intermediate relative to AA8 cells. These results show that the human ERCC1 gene on chromosome 19 is responsible for substantial restoration of UV survival and mutation responses in repair-deficient UV20 cells, but only partially restores (6-4) UV photoproduct repair and UV split-dose recovery.

Adenine Phosphoribosyltransferase↗
PubMed1989

Diabetes-specific social learning variables and self-care behaviors among persons with type II diabetes.

This study assessed the relationship between diabetes-specific social learning factors and diabetes self-care. Predictor variables were collected within the categories of knowledge (e.g., behavioral demonstrations, pencil-and-paper tests), beliefs/expectations (e.g., self-efficacy, self-motivation), skills (e.g., problem-solving and refusal skills), and environmental support (e.g., barriers to adherence, family support). Different results emerged across the three areas of the regimen assessed: diet, exercise, and glucose testing. Multiple-regression analyses revealed that the social learning variables consistently improved the prediction of self-care beyond that attributable to demographic variables but that the categories of social learning variables most closely related to self-care varied across regimen areas. These findings suggest that programs to enhance diabetes self-care could beneficially focus on life-style behaviors and employ strategies to increase regimen-related expectations and diabetes-specific social and problem-solving skills.

Adult↗
PubMed1989

Excision repair of UV radiation-induced DNA damage in Caenorhabditis elegans.

Radioimmunoassays were used to monitor the removal of antibody-binding sites associated with the two major UV radiation-induced DNA photoproducts [cyclobutane dimers and (6-4) photoproducts]. Unlike with cultured human cells, where (6-4) photoproducts are removed more rapidly than cyclobutane dimers, the kinetics of repair were similar for both lesions. Repair capacity in wild type diminished throughout development. The radioimmunoassays were also employed to confirm the absence of photoreactivation in C. elegans. In addition, three radiation-sensitive mutants (rad-1, rad-2, rad-7) displayed normal repair capacities. An excision defect was much more pronounced in larvae than embryos in the fourth mutant tested (rad-3). This correlates with the hypersensitivity pattern of this mutant and suggests that DNA repair may be developmentally regulated in C. elegans. The mechanism of DNA repair in C. elegans as well as the relationship between the repair of specific photoproducts and UV radiation sensitivity during development are discussed.

Animals↗
PubMed1989

CHO mutant UV61 removes (6-4) photoproducts but not cyclobutane dimers.

The CHO mutant UV61 was previously assigned to complementation group 6 of UV-sensitive rodent cell mutants. UV61 is less sensitive to killing by UV radiation than mutants such as UV5, which is highly defective in the incision process that acts on UV-induced lesions. The D37 for cell survival is approximately 4 J/m2 for UV61, compared with 10 J/m2 for the parental AA8 line and approximately 2 J/m2 for UV5. Similarly, mutation induction at the hprt and aprt loci shows an intermediate response to UV61. In a post-replication recovery assay, the kinetics of maturation of pulse-labelled nascent DNA were normal after UV irradiation in UV61. Data from alkaline elution and alkaline unwinding assays showed that the rates of break accumulation and resealing, measured 0-120 min after irradiation, were also normal in the mutant. This repair incision correlated with the rapid, normal removal of pyrimidine(6-4)pyrimidone photoproducts in UV61 measured using a radioimmunoassay that is specific for this class of damage. In contrast, after exposure to 10 or 15 J/m2, no detectable removal of cyclobutane dimers from DNA was found in UV61 while AA8 cells removed 32% by 24 h. We suggest that the mutation in UV61 specifically lowers the affinity of a repair protein for cyclobutane dimers, which are also inefficiently removed from the bulk DNA of normal CHO cells. The resistance of UV61 to killing by the direct acting chemical 7-bromomethylbenz[a]anthracene was only slightly greater than that of UV5, indicating defective repair of bulky chemical adducts in addition to cyclobutane dimers.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Inhibition of transient gene expression in Chinese hamster ovary cells by triplet-sensitized UV-B irradiation of transfected DNA.

The biological effectiveness of thymine-thymine cyclobutane dimers specifically induced by photosensitized ultraviolet-B irradiation was analyzed by host-cell reactivation of triplet-sensitized, UV-B irradiated plasmid pRSV beta gal DNA transfected into normal and repair-deficient Chinese hamster ovary cells. For comparison, pRSV beta gal DNA was also UV-C irradiated and transfected into the same cell lines. Ultraviolet endonuclease-sensitive site induction was determined after UV-C irradiation or acetophenone-sensitized UV-B irradiation of plasmid pRSV beta gal DNA. These data were used to calculate the number of cyclobutane pyrimidine dimers required to inactivate expression of the lacZ reporter gene in each irradiation condition. Transfection with UV-C-irradiated plasmid DNA resulted in a significantly greater reduction of reporter gene expression than did transfection with acetophenone-sensitized UV-B-irradiated pRSV beta gal DNA at equivalent induction of enzyme-sensitive sites. Since only a fraction of the inhibition could be accounted for by noncyclobutane dimer photoproducts, these results suggest that cytosine-containing pyrimidine cyclobutane dimers may be more effective than thymine-thymine dimers in inhibiting transient gene expression as measured in such host-cell reactivation experiments in mammalian cells.

Animals↗
PubMed1989

The biology of the (6-4) photoproduct.

The (6-4) photoproduct is an important determinant of the lethal and mutagenic effects of UV irradiation of biological systems. The removal of this lesion appears to correlate closely with the early DNA repair responses of mammalian cells, including DNA incision events, repair synthesis and removal of replication blocks. The processing of (6-4) photoproducts and cyclobutane dimers appears to be enzymatically coupled in bacteria and most mammalian cell lines examined (i.e. a mutation affecting the repair of one lesion also often affects the other), although exceptions exist in which repair capacity may be evident for one photoproduct and not the other (e.g. UV61 and the XP revertant cell line). These differences in the processing of the two photoproducts in some cell lines of human and rodent origin suggest that in mammalian cells, different pathways for the repair of (6-4) photoproducts and cyclobutane dimers may be used. This observation is further supported by pleiotropic repair phenotypes such as those observed in CHO complementation class 2 mutants (e.g., UV5, UVL-1, UVL-13, and V-H1). Indirect data, from HCR of UV irradiated reported genes and the cytotoxic responses of UV61, suggest that the (6-4) photoproduct is cytotoxic in mammalian cells and may account for 20 to 30% of the cell killing after UV irradiation of rodent cells. Cytotoxicity of the (6-4) photoproduct may be important in the etiology of sunlight-induced carcinogenesis, affecting mutagenesis as well as tumorigenesis. The intricate photochemistry of the (6-4) photoproduct, its formation and photoisomerization, is in itself extremely interesting and may also be relevant to sunlight carcinogenesis. The data reviewed in this article support the notion that the (6-4) photoproduct and its Dewar photoisomer are important cytotoxic determinants of UV light. The idea that the (6-4) photoproduct is an important component in the spectrum of UV-induced cytotoxic damage may help clarify our understanding of why rodent cells survive the effects of UV irradiation as well as human cells, without apparent cyclobutane dimer repair in the bulk of their DNA. The preferential repair of cyclobutane dimers in essential genes has been proposed to account for this observation (Bohr et al., 1985, 1986; Mellon et al., 1986). The data reviewed here suggest that understanding the repair of a prominent type of noncyclobutane dimer damage, the (6-4) photoproduct, may also be important in resolving this paradox.

Animals↗