The distribution of radiation-induced breaks in the chromosomes of irradiated subjects

Distribution of radiation-induced breakpoints in chromosomes and its bands in persons recovered from acute radiation sickness and personnel from Chernobyl NPP were investigated using G-banding staining. The frequency of damaged bands and breakpoints in groups exposed to radiation was significantly higher as compared with the control group. It was shown that in exposed to radiation persons damage depends on its length. Most frequently damaged bands in the observed groups were determined. The G-negative bands and telomeres of chromosomes were more sensitive to radiation.     

The frequency and distribution of sister chromatid exchanges in human chromosomes

Summary A detailed procedure is described for a rapid detection of phosphoglucomutase-2 (=phosphopentomutase; PGM-2) on Cellogel following electrophoresis of extracts of human red blood cells and other tissues, including cultured fibroblasts and various types of primate-rodent somatic hybrid cells.The present study indicated that there is only one locus for phosphopentomutase in man. The data from a selected panel of 20 independent clones of man-mouse somatic cell hybrids, investigated for the presence of human chromosomes and for the presence or absence of human PGM-2 favored the assignment of the human PGM-2 locus to chromosome 4.     

The inter- and intrachromosomal effects of the bar-stone translocation in Drosophila melanogaster

Summary Females homozygous and heterozygous for the B ^S translocation were tested to determine the extent of the intra- and interchromosomal effects caused by the rearrangement. The heterozygous translocation produces an increase in crossing over at the tip of the X and in the centromere region of chromosome 2. The homozygous translocation has no effect on crossing over in these regions, but an unexpected increase is observed near the centromere region of the B ^S segment. This result is not predicted by the time-delay model for interchromsomal effects.Supported by USPHS Training Grant 5T1 GM 1145-05 and NSF GB 18786.     

High yields of isochromatid breaks and successive formation of chromosome exchanges may lead to reproductive cell death following high-LET irradiation

To clarify the relationship between cell death and chromosomal aberrations following exposure to heavy-charged ion particles beams, exponentially growing Human Salivary Gland Tumor cells (HSG cells) were irradiated with various kinds of high energy heavy ions; 13 keV/μm carbon ions as a low-LET charged particle radiation source, 120 keV/μm carbon ions and 440 keV/μm iron ions as high-LET charged particle radiation sources. X-rays (200 kVp) were used as a reference. Reproductive cell death was evaluated by clonogenic assays, and the chromatid aberrations in G2/M phase and their repairing kinetics were analyzed by the calyculin A induced premature chromosome condensation (PCC) method. High-LET heavy-ion beams introduced much more severe and un-repairable chromatid breaks and isochromatid breaks in HSG cells than low-LET irradiation. In addition, the continuous increase of exchange aberrations after irradiation occurred in the high-LET irradiated cells. The cell death, initial production of isochromatid breaks and subsequent formation of chromosome exchange seemed to be depend similarly on LET with a maximum RBE peak around 100–200 keV/μm of LET value. Conversely, un-rejoined isochromatid breaks or chromatid breaks/gaps seemed to be less effective in reproductive cell death. These results suggest that the continuous yield of chromosome exchange aberrations induced by high-LET ionizing particles is a possible reason for the high RBE for cell death following high-LET irradiation, alongside other chromosomal aberrations additively or synergistically.     

Distribution of the points of breaks in radiation-induced chromosome rearrangements along the polytene chromosomes of Anopheles messeae

Distribution of 431 rearrangement breaks induced by X-ray treatment in polytene chromosomes of Anopheles messeae was studied. No differences were revealed in the distribution pattern of inversion and translocation breaks. The frequency both of inversion and translocation breaks is much greater in the distal parts of autosomes than in the proximal ones. Within autosomes, breaks are grouped in a number of subdivisions. The points of breaks are Non-randomly distributed. The causes for this non-randomness are discussed.     

Dog chromosome-specific paints reveal evolutionary inter- and intrachromosomal rearrangements in the American mink and human

Forty chromosome-specific paint probes of the domestic dog (Canis familiaris, 2n = 78) were used to delineate conserved segments on metaphase chromosomes of the American mink (Mustela vison, 2n = 30) by fluorescence in situ hybridisation. Half of the 38 canine autosomal probes each painted one pair of homologous segments in a diploid mink metaphase, whereas the other 19 dog probes each painted from two to five pairs of discrete segments. In total, 38 canine autosomal paints highlighted 71 pairs of conserved segments in the mink. These painting results allow us to establish a complete comparative chromosome map between the American mink and domestic dog. This map demonstrates that extensive chromosome rearrangements differentiate the karyotypes of the dog and American mink. The 38 dog autosomes could be reconstructed from the 14 autosomes of the American mink through at least 47 fissions, 25 chromosome fusions, and six inversions. Furthermore, comparison of the current dog/mink map with the published human/dog map discloses 23 cryptic intrachromosomal rearrangements in 10 regions of conserved synteny in the human and American mink genomes and thus further refined the human/mink comparative genome map.     

Imbalanced segregation of recombinant haplotypes in hybrid populations reveals inter- and intrachromosomal Dobzhansky-Muller incompatibilities

Dobzhansky-Muller incompatibilities (DMIs) are a major component of reproductive isolation between species. DMIs imply negative epistasis and are exposed when two diverged populations hybridize. Mapping the locations of DMIs has largely relied on classical genetic mapping. Approaches to date are hampered by low power and the challenge of identifying DMI loci on the same chromosome, because strong initial linkage of parental haplotypes weakens statistical tests. Here, we propose new statistics to infer negative epistasis from haplotype frequencies in hybrid populations. When two divergent populations hybridize, the variance in heterozygosity at two loci decreases faster with time at DMI loci than at random pairs of loci. When two populations hybridize at near-even admixture proportions, the deviation of the observed variance from its expectation becomes negative for the DMI pair. This negative deviation enables us to detect intermediate to strong negative epistasis both within and between chromosomes. In practice, the detection window in hybrid populations depends on the demographic scenario, the recombination rate, and the strength of epistasis. When the initial proportion of the two parental populations is uneven, only strong DMIs can be detected with our method unless migration prevents parental haplotypes from being lost. We use the new statistics to infer candidate DMIs from three hybrid populations of swordtail fish. We identify numerous new DMI candidates, some of which are inferred to interact with several loci within and between chromosomes. Moreover, we discuss our results in the context of an expected enrichment in intrachromosomal over interchromosomal DMIs.     

Positional control of the distribution of spontaneous sister chromatid exchanges along mouse chromosomes]

The use of a new method having combined C-band staining and differential staining of sister chromatids allowed to determine a pattern of distribution of spontaneous sister chromatid exchanges (SCE) along cytologically marked chromosomes 1, 2 and 6 of house mouse. All chromosomes displayed the same pattern of SCE distribution: SCEs are most frequent in the middle part of the chromosome arm and rather rare near the centromere and the telomere. It has been suggested that this pattern of distribution is positional, rather chromatin-specific. The chromosome 1 carrying paracentric inversion with breakpoints in the middle part of the arm and just near the telomere has the same pattern of SCE distribution as normal chromosome 1. Double insertion of homogeneously staining regions in the middle part of the chromosome 1 produces increase in the SCE number per chromosome proportional to the physical length of the insertion. In contrast to meiotic recombination, interference between SCEs is not detected. No evidence for existence of the hot-spots of SCE on the junctions between C-positive and C-negative regions, as well as between G-bands and R-bands, has been produced.     

Factors limiting the number of radiation-induced chromosome exchanges. I. Distance: evidence from non-interaction of x-ray- and neutron-induced breaks

Soaked seeds of Vicia faba were exposed to fractionated doses of x-rays or x-rays and fast neutrons. When the two-hit (exchange) chromosome aberrations were scored at the first mitosis of the root tip, it was observed that with short fractionation times the radiation-induced breaks from the two x-ray doses could rejoin with one another to form exchanges in proportion to the square of the total dose. If, however, one dose was x-rays and the second neutrons, then no quantitatively determinable interaction occurred between the breaks induced by each of the doses, and the aberration yield was simply the sum of that induced by each fraction. The phenomenon of non-interaction as observed by these dose fractionation studies and also by the linear dose response curve for two-break aberrations induced by neutrons has led to calculations of the distance over which two breaks can rejoin. The distance is evidently much smaller than the previously accepted value of 1 micro.     

Modeling the repair of radiation-induced DNA double-strand breaks

A problem often overlooked in the study of the repair of radiation-induced DNA double-strand breads (DSBs) is the question of what the status of a regular site is in the DNA duplex immediately after a radiation treatment. Here, we suggest a mixed repair mechanism which consists of a gradual process and an instantaneous process. A comparison of the present kinetic model with those which have appeared in the literature shows that the former is a generalization of the latter. We have shown that different repair mechanisms may lead to equivalent mathematical representations. Therefore, care must be taken in interpreting the repair mechanism on the basis of the experimentally observed transient number of DSBs.     

mBAND analysis of chromosomal aberrations in human epithelial cells exposed to low- and high-LET radiation

Energetic heavy ions pose a potential health risk to astronauts who have participated in extended space missions. High-LET radiation is much more effective than low-LET radiation in the induction of biological effects, including cell inactivation, genetic mutations, cataracts and cancer. Most of these biological end points are closely correlated with chromosomal damage, which can be used as a biomarker for radiation damage. Multicolor banding in situ hybridization (mBAND) has proven to be highly useful for the study of intrachromosomal aberrations, which have been suggested as a biomarker of exposure to high-LET radiation. To investigate biological signatures of radiation quality and the complexity of intrachromosomal aberrations, we exposed human epithelial cells in vitro to (137)Cs gamma rays or iron ions (600 MeV/nucleon) and collected chromosomes using a premature chromosome condensation technique. Aberrations in chromosome 3 were analyzed using mBAND probes. The results of our study confirmed the observation of a higher incidence of inversions for high-LET radiation. However, detailed analysis of the inversion type revealed that both iron ions and gamma rays induced a low incidence of simple inversions. Half of the inversions observed in the low-LET-irradiated samples were accompanied by other types of intrachromosome aberrations, but few inversions were accompanied by interchromosome aberrations. In contrast, iron ions induced a significant fraction of inversions that involved complex rearrangements of both inter- and intrachromosome exchanges.     

Spatial distribution of radiation-induced double-strand breaks in plasmid DNA as resolved by atomic force microscopy

Atomic force microscopy (AFM) has been used to directly visualize, size and compare the DNA fragments resulting from exposure to low- and high-LET radiation. Double-stranded pUC-19 plasmid ("naked") DNA samples were irradiated by electron-beam or reactor neutron fluxes with doses ranging from 0.9 to 10 kGy. AFM scanning in the tapping mode was used to image and measure the DNA fragment lengths (ranging from a few bp up to 2864 bp long). Double-strand break (DSB) distributions resulting from high-LET neutron and lower-LET electron irradiation revealed a distinct difference between the effects of these two types of radiation: Low-LET radiation-induced DSBs are distributed more uniformly along the DNA, whereas a much larger proportion of neutron-induced DSBs are distributed locally and densely. Furthermore, comparisons with predictions of a random DSB model of radiation damage show that neutron-induced DSBs deviate more from the model than do electron-induced DSBs. In summary, our high-resolution AFM measurements of radiation-induced DNA fragment-length distributions reveal an increased number of very short fragments and hence clustering of DSBs induced by the high-LET neutron radiation compared with low-LET electron radiation and a random DSB model prediction.