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PubMed1999

[S-phase arrest associated apoptosis leads to diversities of drug resistance and mutation rate in human breast cancer cells].

To find out different abilities of drug resistance and mutation rate and its fundamental mechanism among the human breast cancer cells, two variants, L-2 and Br-1, derived from a common parent cell line of MDA-MB-435, were assayed for sensitivity to PALA and determined for their mutation rate to drug resistance by clone formation and fluctuation analysis respectively. Further, by means of cell growth rate test, flow cytometry, measurement of DNA fragmentation and western blot, the relationship between drug resistance and mutation rate and cell cycle control and apoptosis was explored. The results showed that the mechanism of S-phase arrest associated apoptosis created such a different ability of drug resistance and stability of genome in two cell lines with identical p53 mutation.

Antineoplastic Agents↗
PubMed2005

Incidence and mutation rates of Huntington's disease in Spain: experience of 9 years of direct genetic testing.

BACKGROUND: Prior to the discovery of the Huntington's disease (HD) mutation, the prevalence, incidence, and new mutation rates for this disease were based on the presence of progressive choreic movements and a positive family history. OBJECTIVE: To evaluate the uptake of the HD genetic analysis in Spain, and to provide additional information on the epidemiology of this disease from the experience of 9 years of direct genetic testing. METHODS: From 1994 to 2002, CAG repeat length was determined in 317 patients with symptoms compatible with HD. In all cases, demographic, clinical, and family data were carefully reviewed. RESULTS: HD diagnosis (CAG repeat length >/=36) was confirmed in 166 (52%) symptomatic cases. Of these, 76 (45.8%) reported a positive family history and in 21 cases (12.7%) family history was negative. New mutation events were genetically proven in three families and highly suspected in another, estimating that the minimum new mutation rate for HD in our population is >4%, with a potential mutation rate of 8%. More than 16% of all HD cases had late onset (>59 years) of symptoms, and in three quarters of these the family history was negative. The incidence rate for the autonomous communities of Navarra and the Basque country, based on the number of newly diagnosed cases by genetic testing, was 4.7 per million per year. CONCLUSIONS: Direct HD genetic testing shows that the incidence and mutation rates of the disease are 2-3 times higher than previously reported. We also demonstrated the relevance of CAG repeat length assessment in diagnosing patients with late onset of symptoms and negative family history for HD.

Adolescent↗
PubMed1980

No sex difference in mutations rates of Duchenne muscular dystrophy.

Segregation analysis was performed to evaluate possible differences in mutation rates in man. It was based on 514 males with Duchenne type muscular dystrophy (DMD) from five of nineteen hospitals for muscular disease in Japan. The estimated proportion of sporadic cases (new mutations) was 0.29 +/- 0.046, which is in excellent agreement with the expected 0.333 if there is no sex difference in mutation rates. The rate of mutation was estimated to be 6.3 X 10(-5) per generation. The incidence and prevalence rates among males were estimated to be 217 X 10(-6) and 49.9 X 10(-6), respectively.

Humans↗
PubMed1989

Lack of correlation between deoxyribonucleotide pool sizes, spontaneous mutation rates and malignant potential in Chinese hamster ovary cells.

To examine the relationship between altered spontaneous mutation rates and malignant characteristics of cells, two hydroxyurea-resistant Chinese hamster ovary cell lines, with alterations in ribonucleotide reductase, were examined for their rates of spontaneous mutation to 6-thioguanine and ouabain resistance, tumor growth rates and their ability to form experimental lung metastases. The most resistant cell line, HR-R2T, showed no changes in the rate of spontaneous mutation to 6-thioguanine or ouabain resistance compared to the parental wild-type cell line; however, the mutant line formed lung metastases in experimental metastasis assays with BALB/c nu/nu mice, and exhibited metastatic abilities significantly different from the wild-type population. Furthermore, the HR-R2T population did not show imbalances in any of the deoxyribonucleoside triphosphate pool sizes, which are frequently observed in cells altered in ribonucleotide reductase activity. The second hydroxyurea-resistant line, HNR-AT, had gross alterations in dCTP and dGTP pools and although the rate of spontaneous mutation to 6-thioguanione resistance was unaltered, it showed a moderate decrease in the rate of spontaneous mutation to ouabain resistance when compared to the parental wild-type population. Interestingly, the HNR-AT cell line did not form any lung metastases in the experimental metastasis assay. Both mutant cell lines, HR-R2T, and HNR-AT, had increased tumor growth rates in C57 BALB/c "beige" nude (nu/nu) mice as compared to the parental wild-type population. In total, the results obtained with the two mutant cell lines question the association of altered mutation rates with increased metastatic potential. Although several explanations are possible for the altered malignant properties exhibited by HR-R2T and HNR-AT cells, it is interesting to note that the results are consistent with earlier suggestions that changes in ribonucleotide reductase may accompany modifications in the malignant characteristics of cells.

Animals↗
PubMed1997

The effect of the interval between dose applications on the observed specific-locus mutation rate in the mouse following fractionated treatments of spermatogonia with ethylnitrosourea.

Our earlier analyses have suggested an apparent threshold dose-response for ethylnitrosourea-induced specific-locus mutations in treated spermatogonia of the mouse to be due to a saturable repair process. In the current study a series of fractionated-treatment experiments was carried out in which male (102 x C3H)F1 mice were exposed to 4 x 10, 2 x 40. 4 x 20 or 4 x 40 mg ethylnitrosourea per kg body weight with 24 h between applications; 4 x 40 mg ethylnitrosourea per kg body weight with 72 h between dose applications; and 2 x 40, 4 x 20 and 4 x 40 mg ethylnitrosourea per kg body weight with 168 h between dose applications. For all experiments with 24-h intervals between dose applications, there was no effect due to dose fractionation on the observed mutation rates, indicating the time interval between dose applications to be shorter than the recovery time of the repair processes acting on ethylnitrosourea-induced DNA adducts. In contrast, a fractionation interval of 168 h was associated with a significant reduction in the observed mutation rate due to recovery of the repair process. However, although reduced, the observed mutation rates for fractionation intervals of 168 h were higher than the spontaneous specific-locus mutation rate. These observations contradict the expectation for a true threshold dose response. We interpret this discrepancy to be due to the differences in the predictions of a mathematical abstraction of experimental data and the complexities of the biological system being studied. Biologically plausible explanations of the discrepancy are presented.

Animals↗
PubMed1997

Mutation rate and excess African heterozygosity.

Global studies of within-group genetic variation have revealed a tendency for some traits, but not all, to show higher heterozygosity in sub-Saharan African populations. Although excess African diversity has been interpreted as reflecting a greater "age" of sub-Saharan African populations, more recent research has shown that this excess is more likely a consequence of a larger African long-term effective population size. The observation that certain traits, particularly classic genetic markers and RFLPs, do not show this pattern has been interpreted as ascertainment bias. Here, I examine another possible factor: that excess African heterozygosity is in part a function of mutation rate. Simple equilibrium and nonequilibrium models of absolute excess heterozygosity are examined. The results indicate that there is little excess African heterozygosity for traits with low mutation rates and greater excess heterozygosity for traits with moderate to high aggregate mutation rates. Observed data are consistent with these models. Also, depending on population size and time depth, traits with high levels of mutation might show less excess heterozygosity than those with moderate to high mutation rates. Another measure of diversity, mean sequence divergence, shows an increase in excess diversity for traits with high mutation rates.

Africa South of the Sahara↗
PubMed2003

Evolution of evolvability via adaptation of mutation rates.

We examine a simple form of the evolution of evolvability-the evolution of mutation rates-in a simple model system. The system is composed of many agents moving, reproducing, and dying in a two-dimensional resource-limited world. We first examine various macroscopic quantities (three types of genetic diversity, a measure of population fitness, and a measure of evolutionary activity) as a function of fixed mutation rates. The results suggest that (i) mutation rate is a control parameter that governs a transition between two qualitatively different phases of evolution, an ordered phase characterized by punctuated equilibria of diversity, and a disordered phase of characterized by noisy fluctuations around an equilibrium diversity, and (ii) the ability of evolution to create adaptive structure is maximized when the mutation rate is just below the transition between these two phases of evolution. We hypothesize that this transition occurs when the demands for evolutionary memory and evolutionary novelty are typically balanced. We next allow the mutation rate itself to evolve, and we observe that evolving mutation rates adapt to values at this transition. Furthermore, the mutation rates adapt up (or down) as the evolutionary demands for novelty (or memory) increase, thus supporting the balance hypothesis.

Adaptation, Physiological↗
PubMed1955

Mutation rates of bacteria in steady state populations.

The breeder and the chemostat have been used to measure mutation rates for two mutations under a variety of steady state growth conditions. These rates have been found to be higher in complex medium than in minimal (F) medium. The effects of changes in nutritional conditions on these high rates have been described. In addition, the mutation rates at short generation times, in complex medium, have been shown to decrease with increasing generation time.

Bacteria↗
PubMedDate not supplied

Highly polymorphic minisatellite sequences: allele frequencies and mutation rates for five locus-specific probes in a Caucasian population.

Six human minisatellite sequences (MS1, MS8, MS29, MS31, MS43, g3) have been subcloned into a stable host/vector system. Allele frequencies at the hypervariable loci detected by five of these probes were determined in a Caucasian population (200 individuals). Mendelian inheritance has been demonstrated in 5 large multi-generation pedigrees. The mutation rate has been determined in 59 families. The highest mutation rate was observed with MS1, as might be predicted from the observed high heterozygosity and in agreement with previous direct measurement of germ line mutation rates. The data presented on allele frequencies and mutation rates provide preliminary data supporting the use of these probes in paternity analysis and forensic investigations.

Alleles↗
PubMed1989

How much difference does chromosome banding make? Adjustments in prevalence and mutation rates of human structural cytogenetic abnormalities.

A collaborative analysis was undertaken of 226 karyotypes with structural chromosome abnormalities diagnosed primarily with low level banding resolution, about 300 to 400 bands per karyotype. We estimate that in this series, use of low level banding was required to detect about 78% of pericentric inversions, about 51% of reciprocal translocations, about 47% of all balanced translocations, about 35% of unbalanced rearrangements other than rings, Robertsonian translocations and extra structurally abnormal chromosomes, about 11% of all unbalanced rearrangements, and about 35% of all structural abnormalities. Adjustment factors derived from these figures were applied to prevalence and mutation rates of structural mutation rates derived from published large scale studies of livebirths. Had low level banding been used in these earlier studies we estimate that the rate of all structural abnormalities would have been about 60% higher than those reported (3.8 per 1000 vs. 2.3 per 1000 in the original studies). The increase is much higher for balanced abnormalities, 75% (3.4 per 1000 vs. 1.9 per 1000), than for unbalanced abnormalities, 5% (0.42 per 1000 vs. 0.405 per 1000). The increase in mutation rates for de novo cytogenetic abnormalities was similarly, considerably higher after such adjustment: the rates per 100,000 gametes increased from 18.0 to 35.0 for balanced rearrangements, from 8.2 to 10.1 for unbalanced abnormalities and from 26.2 to 45.1 for all abnormalities. These estimates illustrate the difference even low level banding makes to detection of structural cytogenetic abnormalities and why contemporary studies using such methods cannot be compared with earlier large scale population studies or livebirths without some type of adjustment such as those suggested here.(ABSTRACT TRUNCATED AT 250 WORDS)

Birth Rate↗
PubMed1982

De novo mutations producing unstable Hbs or Hbs M. II. Direct estimates of minimum nucleotide mutation rates in man.

Cases of unstable hemoglobin and hemoglobin M disease that have appeared as de novo mutants over a span of approximately 50 years were used to deriving minimal, direct estimates of mutation rates per nucleotide per generation in man. The estimates are based upon analysis of data related to 40 cases of unstable Hbs and 15 of Hbs M that arose in 13 countries. The estimated rate calculated using all de novo beta-gene mutants is 7.4 X 10(-9) per nucleotide per generation; that derived using de novo alpha-gene mutants is 10.0 X 10(-9). Subsequent calculations of mutation rates per alpha- and beta-chain gene and extrapolation of these rates to a hypothetical gene of 1000 nucleotides yield an estimated mutation rate of 8.6 X 10(-6) per 1000 nucleotides per generation. Even though some instances of false paternity may have biased these estimates in an upward direction, under-reporting of Hb M cases, and particularly of unstable hemoglobins, makes it likely that the cited values are minimal estimates of mutation rates at the molecular level.

Anemia, Hemolytic↗
PubMed1994

Minisatellite mutation rate variation associated with a flanking DNA sequence polymorphism.

Human minisatellite mutation in the male germline frequently involves complex interallelic gene conversion events restricted to one end of the tandem repeat array. Some alleles at minisatellite MS32 show reduced variability in human populations and are associated with a G to C transversion upstream of the array. Analysis of single sperm demonstrated a frequently profound reduction in mutation rate at alleles carrying the C variant. This mutation suppression acts in cis, but does not affect the ability of an allele to act as sequence donor during gene conversion. This mutation rate polymorphism provides strong evidence for elements near the minisatellite that regulate tandem repeat instability.

Africa↗
PubMed1998

Mutation rate in human microsatellites: influence of the structure and length of the tandem repeat.

In 10,844 parent/child allelic transfers at nine short-tandem-repeat (STR) loci, 23 isolated STR mismatches were observed. The parenthood in each of these cases was highly validated (probability >99.97%). The event was always repeat related, owing to either a single-step mutation (n=22) or a double-step mutation (n=1). The mutation rate was between 0 and 7 x 10(-3) per locus per gamete per generation. No mutations were observed in three of the nine loci. Mutation events in the male germ line were five to six times more frequent than in the female germ line. A positive exponential correlation between the geometric mean of the number of uninterrupted repeats and the mutation rate was observed. Our data demonstrate that mutation rates of different loci can differ by several orders of magnitude and that different alleles at one locus exhibit different mutation rates.

Adult↗
PubMed2003

Retroviral mutation rates and reverse transcriptase fidelity.

Genetic variation in retroviral populations provides a mechanism for retroviruses to escape host immune responses and develop resistance to all known antiretroviral drugs. Retroviruses, like all RNA viruses, exhibit a high mutation rate. Polymerization errors during DNA synthesis by reverse transcriptase, which lacks a proofreading activity, is a major mechanism for generating genetic variation within retroviral populations. In this review, we summarize our current understanding of the processes that contribute to the generation of mutations in retroviruses. An overview of in vivo and in vitro studies of retroviral mutation rates determined by various fidelity assays is provided. Extensive mutational analyses of RTs are beginning to elucidate the relationship between structural determinants of RTs and fidelity of DNA synthesis. Recently, it was observed that the Y586F mutation in MLV RT results in a dramatic increase in the mutation rate in the vicinity of adenine-thymie tracts (AAAA, TTTT, and AATT), which are associated with bends in DNA. These results indicate that the template-primer duplex is a component of the polymerase active site and its structure can influence nucleotide selectivity and the mutation rate. Additionally, the results also suggest that the Y586 residue and the RNase H primer grip are structural determinants of RT that have evolved to attenuate the effects of unusual conformations of the template-primer duplex, such as bends in DNA, on fidelity of DNA synthesis.

Animals↗
PubMed1996

Maximum likelihood estimation of spontaneous mutation rates from large initial populations.

When estimating a spontaneous mutation rate from either a single culture (C=1) or from the C parallel cultures (C>1) of a fluctuation experiment, the use of a large initial population size N0 to seed each culture will permit a gaussian approximation for the probability distribution of the number M of mutants at the time when the culture(s) has (have) grown to size N=N02g, i.e., experienced g doublings. Using this gaussian approximation we find that the maximum likelihood estimate mu of the expected number mu of mutants present in a culture in generation g is (exactly) (equation: see text) where r = 2g / g and M 2 is the average of the squares of the C mutant counts. The maximum likelihood estimate p of the unknown mutation rate p is p = 2 mu / gN assuming an 'ideal' experiment and that there were no mutants in the initial population. A well-behaved maximum likelihood estimate is known to be efficient in large samples and we illustrate by Monte Carlo simulation that indeed p is better (has smaller mean squared error) than our previous (Rossman et al., 1995) estimator (equation: see text) (M is the average mutant count) provided N0 is of the order 1/p or larger. This advantage exists even without a fluctuation experiment, i.e., for C = 1.

Animals↗
PubMed2003

Genetic hits and mutation rate in colorectal tumorigenesis: versatility of Knudson's theory and implications for cancer prevention.

The multistep model of carcinogenesis is now widely accepted for colorectal cancer and other common epithelial tumors of the adult. The challenges ahead are to define the number of genetic hits necessary for conversion of a normal cell into a cancer cell and to determine whether the observed increase in the mutation rate (mutator phenotype) is required. The beauty of Knudson's two-hit theory is its ability to explain diverse experimental situations and guide specific predictions, including some directly relevant to cancer prevention.

Adenoma↗
PubMedDate not supplied

Factors affecting electromorph mutation rates in man: an analysis of data from Australian Aborigines.

The constraints of molecular size and structure on the relative magnitudes of electromorph mutation rates as calculated indirectly have been studies using data for Australian Aborigines. The role of sample size in detecting rare electromorphs is important. In addition, subunit size shows a positive and subunit number a negative correlation with mutation rate. The differences in mutation rates were 2--9-fold when calculated for different categories of the data. The importance of physicochemical constraints are discussed.

Alleles↗
PubMed2002

Deterministic mutation rate variation in the human genome.

Several studies of substitution rate variation have indicated that the local mutation rate varies over the mammalian genome. In the present study, we show significant variation in substitution rates within the noncoding part of the human genome using 4.7 Mb of human-chimpanzee pairwise comparisons. Moreover, we find a significant positive covariation of lineage-specific chimpanzee and human local substitution rates, and very similar mean substitution rates down the two lineages. The substitution rate variation is probably not caused by selection or biased gene conversion, and so we conclude that mutation rates vary deterministically across the noncoding nonrepetitive regions of the human genome. We also show that noncoding substitution rates are significantly affected by G+C base composition, partly because the base composition is not at equilibrium.

Amino Acid Substitution↗