The frequency and distribution of isolabelling in Chinese hamster chromosomes after exposure to x-rays

Factors that influence formation of sister chromatid exchanges in human blood lymphocytes

Sister chromatid exchange (SCE) reflects an interchange of DNA sequences between helices in a replicating chromosome. This was initially accomplished by Taylor and colleagues (1957) using tritiated thymidine incorporation followed by autoradiography. The development of an elegant technique for differential staining of sister chromatids by incorporating a thymidine analog, 5-bromodeoxyuridine (BrdU) has greatly simplified the detection of SCEs in metaphase chromosomes. In recent years, the analysis of SCE has been considered to be a highly sensitive and additional (i.e., with chromosome aberrations) end point for measuring mutagenic/carcinogenic potential of various environmental agents and is increasingly being used to detect and differentiate among chromosome fragility human diseases that predispose to neoplasia. Attention has been focused to see if the induction of SCEs in lymphocyte cultures can be used as a reliable "biological dosimeter" for genetic risk assessment and to monitor the exposed populations. Several physical or preparatory as well as biological factors that modify the response and formation of SCEs make the monitoring difficult. The purpose of this article is to review and analyze these factors to facilitate an effective development of a standard protocol for SCE testing and for appropriate evaluation of test results. This may also provide clues to understand the yet unknown molecular mechanism(s) and biological significance of SCE formation.

Effect of temperature variation on sister-chromatid exchange and cell-cycle duration in cultured human lymphocytes

The relation of temperature variation and cell-cycle duration and their influence on SCE frequencies in human peripheral blood lymphocytes was studied. The results indicate that the frequency of SCEs is affected by variation in temperature. Either increase or decrease in the incubation temperature of cells leads to a higher frequency of SCEs than in control cultures grown at 37 degrees C. With a decrease in temperature to 35 degrees C the increase in SCE is accompanied by prolonged cell cycle. No significant difference in cell cycle was observed with the increase of temperature to 39 degrees C.

Relationship of DNA repair to chromosome aberrations, sister-chromatid exchanges and survival during liquid-holding recovery in X-irradiated mammalian cells

The repair of X-ray-induced DNA single strand breaks and DNA--protein cross-links was investigated in stationary phase, contact-inhibited mouse cells by the alkaline-elution technique. Approx. 90% of X-ray-induced single strand breaks were rejoined during the first hour of repair, whereas most of the remaining breaks were rejoined more slowly during the next 5 h. At early repair times, the number of residual non-rejoined single strand breaks was approx. proportional to the X-ray dose. DNA--protein cross-links were removed at a slower rate (T1/2 approx. 10--12 h). Cells were held in stationary growth for various periods of time after irradiation before subculture at low density to score for colony survival (potentially lethal damage repair), chromosome aberrations in the first mitosis, and sister-chromatid exchanges in the second mitosis. Both cell killing and the frequency of chromosome aberrations decreased during the first several hours of recovery, reaching a minimum level by 6 h; this decrease correlated temporally with the repair of the slowly rejoining DNA-strand breaks. Relatively few sister-chromatid exchanges were observed when the cells were subcultured immediately after X-ray. The exchange frequency rose to maximum levels after a 4-h recovery interval, and returned to control levels after 12 h of recovery. The possible relationship of DNA repair to these changes in survival, chromosome aberrations, and sister-chromatid exchanges during liquid-holding recovery is discussed.

Chromosome aberration formation and sister chromatid exchange in relation to DNA repair in human cells

Apparent association between the ability to induce chromosome aberrations and sister chromatid exchanges and mutagenic-carcinogenic potential found in a variety of physical and chemical agents has led us to speculate that these cytogenetic changes might be reflection of DNA damage and repair and might provide induces of mutagenic changes. However, the mechanisms of their formation and their relation to DNA repair as well as the mechanism of their linking to mutation are by no means well understood. Studies in some human genetic mutant cells defective in their ability to repair DNA damage indicate, as a testable proposition, that sister chromatid exchanges and chromosome aberrations are cytological manifestations of replication-mediated dual-step repair pathways that are in operation to tolerate DNA damage when damage-bearing DNA enters and passes through semiconservative replication. The observations are also in line with idea that the majority of sister chromatid exchanges can arise when damage DNA attempts replication possibly by a process relating with the replicative bypass repair mechanisms such as those proposed by Fujiwara and Tatsumi [34] and Higgins et al. [54], while chromosome aberration formation and some fraction of sister chromatid exchanges are related with the post-replication repair processes which attempt to rescue damaged template post-replicationally by de novo synthesis or recombination type repair systems. The former sister chromatid exchange-relating process seems to link mutation to less extent, if any, than the latter process, which is caffeine sensitive and likely to be error-prone.

Sister chromatid exchange in lymphocytes from patients with malignant lymphoma

Sister chromatid exchange (SCE) frequencies were studied in differentially stained lymphocytes from 47 patients with malignant lymphoma. Thirteen patients were untreated when studied. The mean SCE frequency [+/- standard error (SE)] for these patients was 12.7 +/- 0.9 per mitosis. The mean score for 40 controls was 6.1 +/- 0.3. SCE mean scores were significantly higher in the untreated patients than in the controls (P less than 0.001). Seven patients were treated with radiotherapy alone. They demonstrated a mean SCE frequency (8.8 +/- 0.8) significantly lower (P less than 0.01) than that found in untreated patients. Eleven patients received cyclophosphamide within 4 weeks prior to study. They demonstrated a mean SCE frequency (14.3 +/- 1.3) significantly higher (P less than 0.05) than that found in patients who had received regimens that did not contain cyclophosphamide in the prior 4 weeks (11.1 +/- 1.3) or who had been off drugs for at least 8 weeks (10.1 +/- 0.8). Our data suggest that untreated patients with malignant lymphoma have elevated SCE frequencies, which may be further increased by certain chemotherapeutic agents.

Replication bypass model of sister chromatid exchanges and implications for Bloom's syndrome and Fanconi's anemia

A model of the sister chromatid exchange (SCE) process is outlined as a replication mechanism to bypass DNA crosslinks. The model suggests that when normal bidirectional replication advances from both sides towards a crosslink along the two opposite parental strands, the complementary parental strand segments can be temporarily displaced at each contralateral 5' side from the crosslink. The free ends produced in this first step will be terminally aligned but will have opposite polarity. The second step of the bypass can, however, be completed by either of two rejoining processes--terminal ligation of the free ends via nascent Okazaki pieces or aberrant complementation by overlapping the free ends. This bypass mechanism (1) allows replication to continue past a crosslink leaving it intact but (2) results in the switching of parental strands and their attached incomplete nascent strands above and below the crosslink site producing an exchange between sister chromatids. This model is compatible with the findings of current SCE studies using the new BUDR/stain techniques as well as with previous autoradiographic studies. It also suggests that the chromatid breaks and deletions in Fanconi's Anemia represent a defect in step two of the replication bypass mechanism and that the high frequency of SCE's and quadriradials in Bloom's Syndrome represent the SCE overload effects of a defect in crosslink repair.

Induction of sister chromatid exchanges in xeroderma pigmentosum cells after exposure to ultraviolet light

The role of DNA repair mechanisms in the induction of sister chromatid exchanges (SCE) after exposure to ultraviolet radiation was investigated in xeroderma pigmentosum cells. Cells from different excision-deficient XP strains, representing the 5 complementation groups in XP, A, B, C, D and E, and from excision-proficient XP variant strains were irradiated with low doses of UVR (0-3.5 J/m2). The number of SCE was counted after two cycles in the presence of BUdR. In cells of the complementation groups A, B, C and D the number of SCE was significantly higher than in UV-exposed control cells. The frequencies of SCE in group E cells and in XP varient cells were not different from those in control cells. Treatment with caffeine (0-200 microgram/ml) did not result in a different response of variant cells compared with normal cells. A simple correlation between SCE frequency and residual excision-repair activity was not observed. The response of the excision-repair deficient cells suggest that unrepaired damage, produced by UVR is involved in the production of SCE.

Mechanisms for sister chromatid exchanges and their relation to the production of chromosomal aberrations

By taking advantage of the fact that fluorescent light (FL) induces strand breaks only in bromodeoxyuridine(BrdUy-substituted DNA, and that those breaks eventually lead to the formation of sister chromatid exchanges (SCEs), the response of SCEs to FL was studied carefully in Chinese hamster chromosomes in which, out of four DNA strands, BrdU-substitution had occurred either in one or three strands. The FL-induced SCE frequency did not differ greatly between these two types of chromosomes. However, when they were submitted to caffeine treatment, a drastic increase in the frequency was detected in the trifilarly-substituted chromosomes while a significant decrease occurred in the unifilarly-substituted chromosomes. Based on these results, a working hypothesis was developed that the SCE can arise by at least two different mechanisms, one operating at replicating points probably utilizing the machinery of DNA replication, and the other acting only in the post-replicational DNA portion, probably in a similar fashion as assumed in a general model of crossing over in the eukaryote. These dual mechansims may account for the discrepancy encountered in the explanations of the induction of SCEs by various exogenous agents as well as spontaneous SCEs. The present study also showed that some, but clearly not all, of chromatid deletions are the outcome of the failure to complete SCEs arising through these mechanisms.

The human leukocyte test system. VI. The use of sister chromatid exchanges as possible indicators for mutagenic activities

The trifunctional alkylating chemical mutagen trenimon increases the frequency of sister chromatid exchanges in human leukocyte chromosomes in vitro, as revealed by a BUdR-Giemsa method. Treatment with lead acetate exhibited negative results in this respect. The use of sister chromatid exchanges as possible indicators for mutagenic activities is discussed.