Search PubMed⌕ Search

Biomedical subjects

D A Fox

Publications and source records attributed to D A Fox.

At least 55 records · Page 3Linked to original sources
PubMed1995

A method for monoclonal antibody isotype switching: anti-CD60 VH expression in a heavy chain-deficient hybridoma variant.

Alteration of monoclonal antibody isotype is desirable for a variety of purposes, including obtaining an improved reagent for investigative or therapeutic use. A variety of approaches for isotype switching, particularly from IgM to various IgG subclasses, have been described. Antibodies that recognize carbohydrate determinants on glycoproteins, glycolipids, or polysaccharides are generally of the IgM isotype. This includes all available antibodies to the human CD60 antigen, a determinant with cell coactivating properties described on a subset of T lymphocytes and on other cell types. In this report a new method for monoclonal antibody isotype switching is presented. A plasmid containing the VH regions of anti-CD60 linked to C gamma 1 was transfected into a spontaneously arising variant of the CD60 hybridoma that produced kappa light chain but no heavy chain. This transfected hybridoma line maintains stable production of useful quantities of IgG1 monoclonal anti-CD60 in vitro and in vivo.

Animals↗
PubMed1995

Trichloroacetic acid: investigation into the mechanism of chromosomal damage in the in vitro human lymphocyte cytogenetic assay and the mouse bone marrow micronucleus test.

Trichloroacetic acid (TCA) was tested for its ability to induce chromosomal damage in cultured human peripheral blood lymphocytes and in bone marrow cells of male and female C57BL/6JfBL10/Alpk mice. Two in vitro cytogenetic assays were conducted with TCA. In the first TCA, as free acid, was added to whole blood cultures at final concentrations of 500, 2000 and 3500 micrograms/ml in the presence and absence of an auxiliary metabolic activation system (rat liver S9-mix). Statistically significant increases in the percentage of aberrant cells compared with solvent control values were observed in cultures treated with TCA at 2000 and 5000 mu/ml. Investigation into the effects of TCA on the pH of the culture medium revealed significant reductions in pH at both these TCA concentrations. Neutralized TCA was then tested at concentrations of 500, 2,000 and 5000 micrograms/ml, also in the presence and absence of S9-mix. No statistically or biologically significant increases in the percentage of aberrant cells were observed in any of these cultures. In the mouse micronucleus test, neutralized TCA was administered in two equal intraperitoneal doses 24 h apart to C57BL/6JfBL10/Alpk mice (337, 675 and 1080 mg/kg in males; 405, 810 and 1300mg/kg in females). These dose levels represent 25%, 50% and 80% of the median lethal dose (MLD) in this strain of mouse. Bone marrow samples were taken 6 and 24 h after the second dose and the chromosomal damage assessed by analysis of the bone marrow for micronuclei. No statistically or biologically significant increases in the incidence of micronucleated polychromatic erythrocytes compared with the solvent control dosed animals were observed in either sex at the 6 h sampling time or in the females at the 24 h sampling time. A small but statistically significant increase in micronucleated polychromatic erythrocytes was observed in male mice 24 h after a dose of 675 mg/kg (50% MLD). Since no increases were noted at the 25 or 80% MLD, and the levels recorded are within the range of the concurrent solvent control values, the small increase observed in the males at the 50% MLD is considered not to be biologically significant. Flow cytometric studies on suspensions of isolated liver cell nuclei revealed that changes in FITC binding (indicating altered chromatin conformation) were induced by pH changes alone and were not caused by neutralized TCA.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Expression of CD60 on multiple cell lineages in inflammatory synovitis.

BACKGROUND: CD60 is a recently described T cell subset marker that is expressed on the surface of most T lymphocytes in synovial tissue and fluid and on a smaller proportion of peripheral T cells. Activation of T lymphocytes can be triggered through CD60. CD60 is also expressed by neuroectodermally derived cells in thymic epithelium and in skin. EXPERIMENTAL DESIGN: Immunohistologic analysis of CD60 expression in synovium and thymus was performed using formalin-fixed tissue samples. Nonlymphoid cell lines grown from similar tissues were analyzed by flow cytometry. RESULTS: CD60 was readily identified in formalin-fixed, paraffin-embedded tissues. Simultaneous examination of CD60 distribution and cell morphology demonstrated that, in addition to its presence on T cells, CD60 was also expressed by a variety of nonlymphoid cells in synovium, including synovial lining cells, vascular endothelium, and dendritic-appearing cells deep within synovial tissue. Synovial tissue expression of CD60 was similar in rheumatoid arthritis and in other forms of inflammatory arthritis. In addition, it was strongly expressed by giant cells in pigmented villonodular synovitis. Surface expression of CD60 was detected by flow cytometry on cultured synoviocytes and on other CD60+ nonlymphoid cells, thus excluding adsorption of CD60 shed by T cells as a sufficient explanation of the immunohistologic findings. CONCLUSIONS: These results define the T cell-activating CD60 determinant as a broadly distributed Ag within synovial tissue, with a possible functional role in the activation of a variety of cellular populations. CD60 may also be a marker for previously undescribed cell subsets in the synovial compartment, possibly including a cell population of neuroectodermal origin.

Adult↗
PubMed1994

Transcriptional regulation of CD6 expression on human T lymphocytes by phorbol ester.

Activation of protein kinase C (PKC) has emerged as a major common signal-transducing mechanism for T cell activation and regulation of expression of T cell surface glycoproteins. Surface expression of the CD6 Ag is known to increase with T cell activation and CD6 itself may be involved in regulation of T cell activation. Therefore, we performed experiments to determine whether activation of PKC by phorbol ester induced an increase in CD6 expression and to investigate the mechanisms of such an effect. CD6 surface expression was up-regulated substantially in response to PMA on both mature and immature T cells, but only negligibly on B cell lines. This increase was blocked by PKC inhibitors. The PMA-induced increase in CD6 surface expression was accompanied by an increase in total CD6 protein, as detected by using Western blot analysis of whole cell lysates. After PMA stimulation, Northern blot analysis showed that steady state levels of CD6 mRNA increased in response to PMA treatment. This increase was blocked by cycloheximide, demonstrating that it was dependent on new protein synthesis. Nuclear run-on analysis showed that the rate of CD6 mRNA transcription increased by approximately two to threefold after PMA stimulation of Jurkat cells. Experiments in which we used actinomycin D showed that PMA had no significant effect on the t1/2 of CD6 mRNA. The data suggest that the effect of PMA on CD6 expression is mediated primarily by an increase in CD6 mRNA transcription after PKC activation. mAbs were used to determine whether augmented CD6 expression could be induced by perturbation of specific T cell surface molecules. Up-regulation of CD6 expression occurred when thymocytes were cultured with anti-CD2 Abs, but not with Abs to other functional T cell surface structures, and not when mature T cells were cultured with the anti-CD2 mAbs. Up-regulation of CD6 expression by activation of PKC, triggered in thymocytes by ligation of CD2, could allow CD6 to provide additional regulatory signals required for events in later stages of T cell activation and differentiation.

Antigens, CD↗
PubMed1994

The human 4F2 antigen: evidence for cryptic and noncryptic epitopes and for a role of 4F2 in human T lymphocyte activation.

T lymphocyte activation can be triggered through multiple distinct, but functionally related, pathways. Murine monoclonal antibodies (mAbs) have been used to characterize the surface components of several of these pathways, as well as structures whose function is not yet known. One such cell surface structure is the heterodimeric 4F2 antigen, which is expressed on activated and proliferating cells. Two new mAbs that recognize the heavy chain of the 4F2 antigen have been produced in our laboratory. One antibody, UM7F8, is comitogenic with soluble anti-CD2 and immobilized (but not soluble) anti-CD3 mAbs. The second antibody, termed UM2G12, appears to recognize a cryptic epitope on the 4F2 heavy chain and is not comitogenic for T cells. In view of the functional effects seen with UM7F8, and the highly regulated expression of the 4F2 antigen, it seems likely that 4F2 has a specific role in T cell development and activation.

Antibodies, Monoclonal↗
PubMed1994

Immediate hypersensitivity reaction to cyclophosphamide.

We report a case of an acute anaphylactic reaction to intravenous cyclophosphamide in a patient with systemic lupus erythematosus. Skin testing for reactivity to cyclophosphamide and one of its metabolites, 4-hydroperoxycyclophosphamide, was performed on the patient and on 6 controls. The patient exhibited positive skin test responses; all controls had negative responses. The results suggest an acute hypersensitivity reaction related to sensitivity to cyclophosphamide and to 4-hydroperoxycyclophosphamide, through a shared antigenic determinant or dual sensitization.

Adult↗
PubMed1994

Substrate-dependent effects of calcium on rat retinal mitochondrial respiration: physiological and toxicological studies.

Cytosolic Ca2+ overload may play a key role in the process of lead-induced retinal injury and degeneration. We report that retinal calcium content was elevated following developmental and in vitro lead exposure. To determine the concentration-dependent effects of Ca2+ (5-1000 nM) on retinal mitochondrial bioenergetics an isolation procedure was developed. Isolated mitochondria were efficiently coupled; had good respiratory control ratios with the NAD-linked substrates, glutamate or pyruvate plus malate (G/M or P/M), and the FAD-linked substrate, succinate plus rotenone (S/R); and possessed a Na+/Ca2+ exchanger. The major finding was that at equimolar [Ca2+] > or = 35 nM, mitochondria were more sensitive to and exhibited a greater degree of inhibition of coupled and uncoupled respiration with NAD-linked substrates compared to S/R. At all [Ca2+], decreases in State 3 and uncoupled respiration were similar, thereby eliminating the ATP synthase and ADP/ATP translocase as sites of inhibition and suggesting that opening the mitochondrial permeability transition pore (MTP) did not contribute to the inhibition. The effects of toxicological [Ca2+] were: (1) blocked by ruthenium red, (2) blocked by dibucaine only in the presence of NAD-linked substrates, and (3) partially reversed by NAD+ with G/M after opening the MTP. Results with G/M suggest that Ca2+ acts on the inner membrane phospholipase A2 to decrease NADH CoQ reductase activity and/or produce a NAD+ leak, whereas with S/R, Ca2+ may inhibit succinate dehydrogenase. In conclusion, Ca2+ inhibits retinal mitochondrial ATP production, which may contribute to the retinal cell injury and death observed in developmentally lead-exposed rats.

Adenosine Triphosphate↗
PubMed1994

Lead-induced alterations in rod-mediated visual functions and cGMP metabolism: new insights.

Long-term scotopic (rod-mediated) visual deficits following developmental lead exposure occur in monkeys and hooded rats. This report describes and summarizes previous ERG and biochemical findings, presents new biochemical data aimed at determining the mechanism of inhibition of lead on rod cGMP-PDE, presents an integratory framework for understanding the ERG and cGMP results and speculates on the implications of the present data. A- and b-wave voltage-log intensity and latency-log intensity functions, generated from single-flash ERGs in fully dark-adapted rats, revealed that low and moderate level lead exposure caused decreases in absolute sensitivity and amplitude, and increases in latency. Rod- and cone-mediated flicker fusion frequency measures revealed selective rod deficits in temporal resolution. In addition, the slope of the increment threshold function was decreased, but only at scotopic adapting backgrounds, and dark adaptation was delayed. Prior exposure to lead produced a dose-response inhibition of retinal cGMP-phosphodiesterase (PDE) resulting in an increase in cGMP in dark-adapted and light-adapted states and an increase in the calcium content of rods. In vitro experiments with adult rat retinas incubated with 10(-9) to 10(-4) M Pb2+ revealed a concentration-dependent inhibition of cGMP-PDE which suggested that Pb2+ directly inhibited the rod cGMP-PDE. This was confirmed in experiments conducted with isolated, purified, trypsin-activated bovine rod cGMP-specific PDE exposed to 5 x 10(-8) to 10(-4) M Pb2+. The cGMP data are entirely consistent with the observed ERG changes. The ERG data is relevant to low level pediatric lead poisoning since rat rods are similar to human rods. Finally, since a lesion in the gene that codes for a cAMP-PDE leads to defective learning and memory in the Drosophila dunce flies, it is possible that lead-induced alterations in cyclic nucleotide phosphodiesterases contribute to the long-term CNS deficits produced by developmental lead exposure.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1994

Immunosuppressive drug therapy of systemic lupus erythematosus.

This article reviews the biologic effects of cyclophosphamide and azathioprine relevant to the treatment of systemic lupus erythematosus (SLE). Other agents used less commonly for SLE, such as methotrexate, chlorambucil, and cyclosporin-A, receive more limited attention. Clinical studies of efficacy and toxicity of these agents in their treatment of SLE are then described in detail.

Azathioprine↗
PubMed1993

An anti-CD2 monoclonal antibody that both inhibits and stimulates T cell activation recognizes a subregion of CD2 distinct from known ligand-binding sites.

The T lymphocyte glycoprotein CD2 appears to have an important role in human T cell development and activation. A novel anti-CD2 monoclonal antibody, designed UMCD2, was shown to block E rosetting, and therefore was defined as recognizing the Tll1 ligand-binding epitope. Binding of UMCD2 to T cells and thymocytes was blocked by several, but not all, anti-Tll1 antibodies, suggesting that the Tll1 epitope consists of more than one subepitope. In functional studies, the combination of UMCD2 plus anti-Tll3 was mitogenic for T cells; in some individuals, the level of activation was as high as that seen for the combination of anti-Tll2 plus anti-Tll3. However, when UMCD2 was added to other stimuli mitogenic for T lymphocytes, such as IL-2 or anti-CD3-Sepharose, it inhibited T cell responses. Although the combination of UMCD2 and anti-Tll3 induced an increase in cytoplasmic free calcium, the inhibitory activities of UMCD2 were not accompanied by effects on calcium fluxes. A panel of previously characterized CD2 mutants was then analyzed for binding of UMCD2 and other anti-CD2 monoclonals. Surprisingly, UMCD2 bound to all mutants tested, although the other anti-CD2 antibodies with specificity for the ligand-binding region of CD2 each failed to bind to one or more mutants. These data suggest that binding of antibody to a particular CD2 epitope can have opposite effects on the state of T cell activation, depending on the costimulus. Moreover, inhibitory effects mediated through CD2 may use a signaling mechanism distinct from that used in CD2 pathway activation. Of particular interest, the portion of the CD2 ligand-binding region recognized by UMCD2 is distinct from areas of the CD2 molecule that have previously been studied.

Amino Acid Sequence↗
PubMed1993

Activation of human T cells through CD6: functional effects of a novel anti-CD6 monoclonal antibody and definition of four epitopes of the CD6 glycoprotein.

The CD6 glycoprotein is expressed primarily on lymphocytes and conveys co-activating signals to T cells, but its exact function and ligand(s) are unknown. A novel mAb, termed UMCD6, was demonstrated to recognize CD6 by immunoprecipitation, Western blotting, and reactivity with COS cells transfected with CD6 cDNA. UMCD6 was mitogenic for T cells and was strongly synergistic with phorbol ester in inducing T cell activation. UMCD6 enhanced the autologous mixed lymphocyte reaction as previously observed with another anti-CD6 mAb, anti-T12. The activating effects of UMCD6 were more striking than those of other anti-CD6 mAbs and encompassed all of the diverse stimulatory properties previously reported for other anti-CD6 reagents. However, neither UMCD6 nor other anti-CD6 antibodies alone or in combination with phorbol ester or IL-2 were able to induce thymocytes to proliferate. Stimulation by UMCD6 is dependent on accessory cell function in a manner not accounted for simply by antibody cross-linking. UMCD6 did not induce an increase in cytoplasmic free Ca2+, but the CD6 activation pathway appears to involve protein kinase C. UMCD6 and a panel of seven other anti-CD6 mAbs were used in a series of experiments to define four discrete epitopes of CD6 using the criteria of antibody cross-blocking, reactivity on reduced Western blots, and resistance to controlled V8 protease digestion. The functional mAbs UMCD6, 2H1, and anti-T12 each recognized a different epitope. Taken together, the results of these studies strongly reinforce the hypothesis that CD6 plays a significant and distinct role in T cell activation, and suggest that multiple regions of CD6 may be functionally active.

Antibodies, Monoclonal↗
PubMed1993

Leukemic T cells from patients with cutaneous T-cell lymphoma demonstrate enhanced activation through CDw60, CD2, and CD28 relative to activation through the T-cell antigen receptor complex.

Antigen-dependent activation of T cells occurs through the T-cell antigen-receptor complex (TCR/CD3). Antigen-independent T-cell activation may occur through the surface molecules CDw60, CD2, and CD28. We wished to determine whether these antigen-independent T-cell-activation pathways could be involved in proliferation of leukemic T cells from patients with cutaneous T-cell lymphoma (CTCL). Whereas CDw60 was only expressed on 28% +/- 7% (mean +/- SEM) of blood T cells obtained from healthy control subjects (n = 4), CDw60 was expressed on 94% +/- 3% of blood T cells obtained from patients with CTCL (n = 4). Dual color immunofluorescence microscopy of the T-cell infiltrate in involved skin of these patients demonstrated that almost 100% of the T cells expressed CDw60. Not only did T cells in the patients with CTCL express CDw60, but triggering of the T cells with anti-CDw60 resulted in enhanced proliferation relative to anti-TCR/CD3 and mitogenic lectins. Other antigen-independent pathways also appeared highly active in the T cells from patients with CTCL because enhanced proliferation relative to anti-TCR/CD3 or mitogenic lectins was found when anti-CD2 or anti-CD28 plus phorbol ester was used as stimulant. Despite the brisk proliferation induced by anti-CDw60, anti-CD2, or anti-CD28, T cells from the patients did not produce detectable amounts of gamma-interferon. The inability to produce gamma-interferon correlates with our finding of absent (n = 3) or weak (n = 1) intercellular adhesion molecule-1 expression in the lesional keratinocytes in these patients. In conclusion, T cells of patients with CTCL demonstrate elevated expression of a T-cell-independent signaling molecule CDw60 and respond to antigen-independent activating signals.

Aged↗
PubMed1993

Monocyte-endothelial adhesion in chronic rheumatoid arthritis. In situ detection of selectin and integrin-dependent interactions.

Blood monocytes are the principal reservoir for tissue macrophages in rheumatoid synovitis. Receptor-mediated adhesive interactions between circulating cells and the synovial venules initiate recruitment. These interactions have been studied primarily in cultured endothelial cells. Thus the functional activities of specific adhesion receptors, such as the endothelial selectins and the leukocytic integrins, have not been evaluated directly in diseased tissues. We therefore examined monocyte-microvascular interactions in rheumatoid synovitis by modifying the Stamper-Woodruff frozen section binding assay initially developed to study lymphocyte homing. Specific binding of monocytes to venules lined by low or high endothelium occurred at concentrations as low as 5 x 10(5) cells/ml. mAbs specific for P-selectin (CD62, GMP-140/PADGEM) blocked adhesion by > 90% in all synovitis specimens examined. In contrast, P-selectin-mediated adhesion to the microvasculature was either lower or absent in frozen sections of normal foreskin and placenta. mAbs specific for E-selectin (ELAM-1) blocked 20-50% of monocyte attachment in several RA synovial specimens but had no effect in others. mAbs specific for LFA-1, Mo1/Mac 1, the integrin beta 2-chain, and L-selectin individually inhibited 30-40% of adhesion. An mAb specific for the integrin beta 1-chain inhibited the attachment of elutriated monocytes up to 20%. We conclude that P-selectin associated with the synovial microvasculature initiates shear-resistant adhesion of monocytes in the Stamper-Woodruff assay and stabilizes bonds formed by other selectins and the integrins. Thus the frozen section binding assay permits direct evaluation of leukocyte-microvascular adhesive interactions in inflamed tissues and suggests a prominent role for P-selectin in monocyte recruitment in vivo.

Arthritis, Rheumatoid↗
PubMed1993

Alterations in retinal Na+, K(+)-ATPase in diabetes: streptozotocin-induced and Zucker diabetic fatty rats.

The temporal pattern of changes in the specific activities of retinal Na+, K(+)-ATPase (Na, K-ATPase) and Mg(2+)-ATPase (Mg-ATPase) were determined at several time intervals following the onset of diabetes in streptozotocin-induced diabetic (STZ: at 1, 2, 4 and 6 months) Long-Evans hooded rats, spontaneously diabetic Zucker diabetic fatty (ZDF: at 1, 2 and 4 months) rats and their age-matched controls. These animals were utilized as models for insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM), respectively. Na, K-ATPase specific activity, using 10(3) M ouabain, was decreased (-6% to -14%) at all time points after the appearance of hyperglycemia in the ZDF rat, but was reduced only after 4 and 6 months in the STZ rat (-8% and -14%, respectively). In contrast, Mg-ATPase activity was significantly increased (13%) after 4 months in the ZDF rat and after 6 months in the STZ rat (8%). The concentration-dependent inhibitory effects of ouabain (10(-9) to 10(-3) M) on the activity of Na, K-ATPase in diabetic rats and age-matched controls was used to assess the time-dependent effects of diabetes on the alpha 3-high ouabain affinity or the alpha 1-low ouabain affinity retinal Na, K-ATPase isozymes. The retinal Na, K-ATPase activity for the alpha 3 isozyme was significantly lower at all times examined for the ZDF (-5% to -26%) and STZ-induced diabetic rats (-8% to -14%). This was reflected in the markedly decreased half-maximal inhibitory concentrations (IC50) of ouabain for the alpha 3 isozyme. For example, after four months of diabetes, the mean +/- SEM IC50 values were 12 +/- 3 nM in the STZ rats and 48 +/- 6 nM in the age-matched controls and 19 +/- 3 nM in the ZDF rats and 30 +/- 4 nM in the age-matched controls. In contrast, the activity of the alpha 1 isozyme was slightly, but significantly, decreased at 2 and 4 months in the ZDF rats (-4% to -7%) and after 4 and 6 months in the STZ-induced diabetic rats (-3% to -9%) while the IC50 values were unchanged. Moreover, the Hill coefficient for the alpha 3 isozyme was decreased in both diabetic groups while it was unchanged for the alpha 1 isozyme.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Activation of resting peripheral blood lymphocytes through the T cell receptor induces rapid phosphorylation of Op18.

Op18 is a highly conserved major cytosolic phosphoprotein that has been implicated in signal transduction in a wide variety of cell types. Freshly isolated peripheral blood lymphocytes (PBL) constitutively express low levels of mostly unphosphorylated Op18. After mitogenic stimulation of PBL, Op18 synthesis is induced at a time when cells are entering S-phase. In this study, we have examined the phosphorylation of Op18 in freshly isolated PBL after activation of the T cell receptor by OKT3. Quantitative analysis of Op18 phosphorylation was undertaken by metabolic labeling with 32Pi and PhosphorImager analysis of two-dimensional gels. After 10 or 15 min of activation by OKT3, one of the three major phosphorylated forms of Op18, designated Op18c, increased approximately 10-fold, which represented a most pronounced change among a large number of phosphoproteins analyzed. In time course experiments, increased Op18 phosphorylation to yield Op18c was observed as early as 2 min. Continued OKT3-induced activation for 20 to 72 h resulted in a further increase in phosphorylated Op18 forms, which paralleled new Op18 synthesis and occurred at a time when cells were entering S-phase, as determined by [3H]-thymidine incorporation. Inhibitors of lymphoid proliferation, cyclosporin A and RPM, had no effect on early (less than 15 min) phosphorylation. Addition of calphostin C, a specific inhibitor of protein kinase C, 1 min prior to stimulation of resting T cells with OKT3 completely inhibited further phosphorylation of Op18. Incubation of PBL with calphostin C for 75 min decreased constitutive levels of phosphorylated Op18. In contrast, inhibition of cyclic nucleotide-dependent protein kinases with HA1004 had no effect on Op18 phosphorylation. Activation of cAMP-dependent protein kinase with Forskolin or 8Br-cAMP did not increase Op18 phosphorylation. Our results suggest that Op18 phosphorylation is mediated by protein kinase C activation as an early event in T cell activation through the T cell receptor.

Cells, Cultured↗
PubMed1992

Similarity of spontaneous germinal and in vitro somatic cell mutation rates in humans: implications for carcinogenesis and for the role of exogenous factors in "spontaneous" germinal mutagenesis.

The rate of spontaneous mutation resulting in electrophoretic variants per cell generation in a human lymphoblastoid cell line, on the basis of experiments described in this paper, is found to be 7.2 x 10(-8) per locus. A review of similar data on electrophoretic variants resulting from spontaneous mutation in the human germ line leads to an estimate of 3.3 x 10(-8) per locus per cell generation. It is argued that the similarity of these two estimates, despite an average cell generation time of 18.5 hr for the cultured somatic cells but about 26 days in the germ line, suggests that spontaneous mutation involving nucleotide substitutions is much more dependent on cell generation than on time. This finding permits the inference that environmental (exogenous) variables make a relatively small contribution to the rate of this type of human germinal spontaneous mutation. While in vitro somatic-cell mutation rates, such as derived in this study, provide a basis for modeling the contribution of nucleotide substitutions in multihit/clonal theories of carcinogenesis, it is also argued that the complex of events involved in carcinogenesis, including chromosomal rearrangements and mitotic recombination, could have very different individual probabilities. Estimates for the rates of these other types of mutation are needed to provide a better understanding of the manner in which multiple mutations accumulate in malignant cells.

Alleles↗