Classes Of Aberrations Research Articles

Multisite Evaluation and Validation of Optical Genome Mapping for Prenatal Genetic Testing

Prenatal diagnostic testing of amniotic fluid, chorionic villi, or more rarely, fetal cord blood is recommended following a positive or unreportable noninvasive cell-free fetal DNA test, abnormal maternal biochemical serum screen, abnormal ultrasound, or increased genetic risk for a cytogenomic abnormality based on family history. Although chromosomal microarray is recommended as the first-tier prenatal diagnostic test, in practice, multiple assays are often assessed in concert to achieve a final diagnostic result. The use of multiple methodologies is costly, time consuming, and labor intensive. Optical genome mapping (OGM) is an emerging technique with application for prenatal diagnosis because of its ability to detect and resolve, in a single assay, all classes of pathogenic cytogenomic aberrations. In an effort to characterize the potential of OGM as a novel alternative to traditional standard of care (SOC) testing of prenatal samples, OGM was performed on a total of 200 samples representing 123 unique cases, which were previously tested with SOC methods (92/123=74.7% cases tested with at least two SOCs). OGM demonstrated an overall accuracy of 99.6% when compared with SOC methods, a positive predictive value of 100%, and 100% reproducibility between sites, operators, and instruments. The standardized workflow, cost-effectiveness, and high-resolution cytogenomic analysis demonstrate the potential of OGM to serve as a first-tier test for prenatal diagnosis.

Fast mutual exclusivity algorithm nominates potential synthetic lethal gene pairs through brute force matrix product computations

Mutual Exclusivity analysis of genomic aberrations contributes to the exploration of potential synthetic lethal (SL) relationships thus guiding the nomination of specific cancer cells vulnerabilities. When multiple classes of genomic aberrations and large cohorts of patients are interrogated, exhaustive genome-wide analyses are not computationally feasible with commonly used approaches. Here we present Fast Mutual Exclusivity (FaME), an algorithm based on matrix multiplication that employs a logarithm-based implementation of the Fisher’s exact test to achieve fast computation of genome-wide mutual exclusivity tests; we show that brute force testing for mutual exclusivity of hundreds of millions of aberrations combinations can be performed in few minutes. We applied FaME to allele-specific data from whole exome experiments of 27 TCGA studies cohorts, detecting both mutual exclusivity of point mutations, as well as allele-specific copy number signals that span sets of contiguous cytobands. We next focused on a case study involving the loss of tumor suppressors and druggable genes while exploiting an integrated analysis of both public cell lines loss of function screens data and patients’ transcriptomic profiles. FaME algorithm implementation as well as allele-specific analysis output are publicly available at https://github.com/demichelislab/FaME.

Cytogenetic analysis of 130 renal oncocytomas identify three distinct and mutually exclusive diagnostic classes of chromosome aberrations.

The cytogenetic alterations in renal oncocytoma (RO) are poorly understood. We analyzed 130 consecutive RO for karyotypic alterations. Clonal chromosome abnormalities were identified in 63 (49%) cases, which could be categorized into three classes of mutually exclusive cytogenetic categories. Class 1 (N = 20) RO had diploid karyotypes with characteristic 11q13 rearrangement in balanced translocations with 10 or more different chromosome partners in all cases. We identified recurrent translocation partners at 5q35, 6p21, 9p24, 11p13-14, and 11q23, and confirmed that CCND1 gene rearrangement at 11q13 utilizing fluorescence in situ hybridization (FISH). Class 2 RO (N = 25) exhibited hypodiploid karyotypes with loss of chromosome 1 and/or losses of Y in males and X in females in all cases. The class 3 tumors comprising of 18 cases showed diverse types of abnormalities with the involvement of two or more chromosomes exclusive of abnormalities seen in classes 1 and 2 tumors. Furthermore, karyotypically uninformative cases were subjected to FISH analysis to identify classes 1 and 2 abnormalities. In this group, we found similar frequencies of CCND1 rearrangement, loss of chromosome 1 or Y as with karyotypically abnormal cases. We validated our results against 91 tumors from the Mitelman database. Correlation of clinical data with all the three classes of ROs showed no clear evidence of overall patient survival. Our findings support the hypothesis that RO exhibit three principal cytogenetic categories, which may have different roles in initiation and/or progression. These cytogenetic markers provide a key tool in the diagnostic evaluation of RO.

Abstract 3415: Cytogenetics of renal oncocytomas identify three distinct and mutually exclusive diagnostic classes of chromosome aberrations

Abstract Christopher B. Anderson1, Michael Lipsky1, Subhadra V. Nandula2,3, Christopher E. Freeman2, Thomas Matthews2, Catlin Walsh2, Gen Li1, Matthias Szabolcs2, Mahesh M. Mansukhani 2, James M. McKiernan1, Vundavalli V. Murty 2 1Departments of Urology, 2Pathology and Cell Biology, Columbia University Medical Center, New York, New York; 3Cancer Genetics Inc, Rutherford, New Jersey The cytogenetic alterations in renal oncocytoma (RO) are poorly understood. We analyzed 130 consecutive RO for karyotypic alterations. Clonal chromosome abnormalities were identified in 63 (48.5%) cases, which could be categorized into 3 classes of mutually exclusive cytogenetic categories. Class 1 RO had diploid karyotypes with 11q13 rearrangement in balanced translocations with ten or more different chromosome partners in all 20 (31.7%) cases. We identified recurrent translocation partners at 5q35, 6p21, 9p24, 11p13-14 and 11q23, and confirmed that CCND1 gene rearrangement at 11q13 utilizing fluorescence in situ hybridization (FISH). Class 2 RO exhibited hypodiploid karyotypes with loss of chromosome 1 and concurrent losses of Y in males and X in females in 25 (39.7%) cases. The class 3 tumors comprising of 18 (30.2%) cases showed diverse types of abnormalities with the involvement of 2 or more chromosomes exclusive of abnormalities seen in Class 1 and 2 tumors. Furthermore, karyotypically uninformative cases were subjected to FISH analysis to identify class 1 and 2 abnormalities. In this group we found similar frequencies of CCND1 rearrangement, loss of 1 or Y as with karyotypically abnormal cases. We validated our results against 91 tumors from the Mittleman database. Correlation of clinical data with all the 3 classes of RO showed no clear evidence of overall patient survival. Our findings support the hypothesis that RO exhibit 3 principal cytogenetic categories, which may have different roles in initiation and/or progression. These cytogenetic markers provide a key tool in the diagnostic evaluation of RO. Citation Format: Christopher B. Anderson, Michael Lipsky, Subhadra V. Nandula, Freeman E. Christopher, Matthews Thomas, Caitlin E. Walsh, Gen Li, Matthias Szabolcs, Mahesh M. Mansukhani, James M. McKiernan, Murty V. Vundavalli. Cytogenetics of renal oncocytomas identify three distinct and mutually exclusive diagnostic classes of chromosome aberrations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3415.

The effects of silver ions and silver nanoparticles on cell division and expression of cdc2 gene in Allium cepa root tips

The effects of silver nanoparticles (AgNPs), silver ions (Ag+), and polyvinylpyrrolidone (PVP) on mitosis and expression of a gene encoding cyclin-dependent kinase 2 (cdc2) in onion roots were compared. Three concentrations (5, 10, and 15 mg dm-3) were employed in combination with three incubation times (3, 6, and 9 h). PVP enhanced mitotic index and cdc2 expression. Both silver forms decreased mitotic index and cdc2 expression. Genotoxicity of both silver forms were indicated by three major distinguishable classes of chromosome aberrations: spindle disturbances, clastogenic aberrations, and chromosome stickiness. Concerning Ag+ treatments, significant enhancements in occurrence of any chromosome aberration type was associated with significant decrease in mitotic index. On the other hand, disturbed spindle in AgNPs treatments was observed even in absence of significant reduction in mitotic index suggesting that AgNPs inhibit cellular events occurring during mitosis to proceed normally rather than starting of cell division.

Identification of large rearrangements in cancer genomes with barcode linked reads.

Large genomic rearrangements involve inversions, deletions and other structural changes that span Megabase segments of the human genome. This category of genetic aberration is the cause of many hereditary genetic disorders and contributes to pathogenesis of diseases like cancer. We developed a new algorithm called ZoomX for analysing barcode-linked sequence reads—these sequences can be traced to individual high molecular weight DNA molecules (>50 kb). To generate barcode linked sequence reads, we employ a library preparation technology (10X Genomics) that uses droplets to partition and barcode DNA molecules. Using linked read data from whole genome sequencing, we identify large genomic rearrangements, typically greater than 200kb, even when they are only present in low allelic fractions. Our algorithm uses a Poisson scan statistic to identify genomic rearrangement junctions, determine counts of junction-spanning molecules and calculate a Fisher's exact test for determining statistical significance for somatic aberrations. Utilizing a well-characterized human genome, we benchmarked this approach to accurately identify large rearrangement. Subsequently, we demonstrated that our algorithm identifies somatic rearrangements when present in lower allelic fractions as occurs in tumors. We characterized a set of complex cancer rearrangements with multiple classes of structural aberrations and with possible roles in oncogenesis.

Oncologists’ use of tumor sequencing findings and patient derived benefit.

e13016 Background: Despite increasing use of tumor genomic sequencing in clinical oncology, there is currently limited understanding of how oncologists interpret and apply this information to a patient’s clinical management. Methods: We surveyed oncologists who referred patients with refractory cancers to the Michigan Oncology Sequencing (MI-ONCOSEQ) program which uses integrative clinical sequencing to detect multiple classes of somatic and germline molecular aberrations including point mutations, amplifications, insertions/deletions, gene fusions, and outlier gene expression from June 2014-February 2015. Patients were also surveyed approximately 2 weeks after notification of completion of sequencing. In addition, we conducted chart reviews to ascertain how many patients’ clinical management was changed or derived clinical benefit as a result of sequencing. Results: Forty-three oncologists (37% female, 98% academic medical center practice), referring 112 patients ( M = 2, range: 1-9) completed the survey (response rate = 93%). Oncologists reported an intention to change 22% of patients’ treatment plans on the basis of sequencing results, with referral to a clinical trial (N = 12; 50%) as the most frequently endorsed intended change. However, few patients (N = 9, 38%) had an actual change in clinical management. Barriers identified included patient factors, ineligibility for identified treatment, and loss to follow-up/deceased. Of the 57 patients who completed post-sequencing surveys, 34 (60%) reported that results of sequencing had not been disclosed. However, patient records had documentation that results disclosure had indeed taken place in 41% of cases. Conclusions: Although expectation of enrollment in clinical trials is frequently proposed as the indication for genomic sequencing, practice barriers remain. These include lack of access to studies, severity of patient illness and challenges to effective communication of results.

Transcriptome meta-analysis of lung cancer reveals recurrent aberrations in NRG1 and Hippo pathway genes.

Lung cancer is emerging as a paradigm for disease molecular subtyping, facilitating targeted therapy based on driving somatic alterations. Here, we perform transcriptome analysis of 153 samples representing lung adenocarcinomas, squamous cell carcinomas, large cell lung cancer, adenoid cystic carcinomas and cell lines. By integrating our data with The Cancer Genome Atlas and published sources, we analyze 753 lung cancer samples for gene fusions and other transcriptomic alterations. We show that higher numbers of gene fusions is an independent prognostic factor for poor survival in lung cancer. Our analysis confirms the recently reported CD74-NRG1 fusion and suggests that NRG1, NF1 and Hippo pathway fusions may play important roles in tumors without known driver mutations. In addition, we observe exon skipping events in c-MET, which are attributable to splice site mutations. These classes of genetic aberrations may play a significant role in the genesis of lung cancers lacking known driver mutations.

Analysis of bleomycin-induced chromosomal aberrations in Chinese hamster primary embryonic cells by FISH using arm-specific painting probes.

Chinese hamster primary embryonic cells (at G1 phase) were treated with 1.0 or 3.0 microg/ml bleomycin and chromosomal aberrations in first division metaphases were analysed by fluorescence in situ hybridization (FISH) using arm-specific painting probes for chromosomes 3, 4, 8 and 9. We observed that bleomycin induced all classes of chromosome-type aberrations very efficiently. The interesting findings were: (i) the frequency of induced interstitial translocations (i.e. insertions) was approximately equal to that of reciprocal translocations; (ii) the frequency of induced pericentric inversions was higher than that of centric rings. In our earlier studies, we found that X-rays induced a low frequency of interstitial translocations in comparison with reciprocal translocations and equal frequencies of centric rings and pericentric inversions. These data suggest that bleomycin differs from X-rays with respect to the induction of some specific types of aberrations. The results of a chi2 test examining the hypothesis that formed aberrations among the chromosomes or chromosome arms are randomly distributed on the basis of their relative lengths revealed a differential involvement of these chromosomes in the aberrations following exposure to bleomycin. In general, chromosome 8 was found to be more involved in induced aberrations than expected, chromosome 4 was randomly involved, whereas chromosomes 3 and 9 were less involved. This study demonstrates the utility of arm-specific painting probes for efficient detection of a large variety of chromosomal aberrations induced by bleomycin.

Chromosomal breakage correlates with delayed lethality in normal and ataxia telangiectasia cell lines treated with bleomycin

Cytogenetic damage and cytotoxicity produced by a range of acute treatments with bleomycin (BLM) were investigated in three B-lymphoblastoid cell lines: EM-OC, PA-AT and CP-NC: established from heterozygous, and homozygous, ataxia telangiectasia (AT) and normal donors, respectively. The following endpoints were studied: (1) the maximum (initial) yields of different classes of chromosomal aberrations (CA) produced in G 2-phase; (2) the percentage distributions of cells within bins with different numbers of CA; (3) short-term (0–48 h) viability and proliferation; and (4) (delayed) lethality, as measured by a modified limiting-dilution assay. While only slight losses of short-term viability and proliferation were detectable after standard BLM pulse-treatment at 25 or 100 μg/ml (results for latter dose given in parentheses), both regimes subsequently led to intense lethality as follows: CP-NC, 79.3% (95.1%); EM-OC, 90.4% (96.7%) and PA-AT, 98.4% (99.9%). Relative lethality after the respective BLM treatments increased 6.2 × (48.3 × ) among PA-AT cells more than in controls —resembling the corresponding ratios for chromosome breaks (csb) of 7.0 × (30.9 × ), more than those for chromatid breaks (ctb) of 3.1 × (2.7 × ). EM-OC exhibited slightly increased relative lethality after the respective BLM treatments at 2.15 × (1.45), and increased aberration sensitivities for csb of 3.0 × (14.0), while the corresponding ratios for ctb were actually lower at 0.66 × (0.50 × ). Significant correlations were observed between lethality and both, the yields of most types of chromosomal aberrations (excepting unambiguous exchanges) and also, the percentages of cells bearing CA.

Current cytogenetic methods for detecting exposure and effects of mutagens and carcinogens.

Most mutagens and genotoxic carcinogens are efficient inducers of chromosomal alterations in exposed cells. Two important classes of aberrations, namely structural and numerical, are recognized and both types of aberrations are associated with congenital abnormalities and neoplasia in humans. These alterations can be easily detected and quantified in human peripheral blood lymphocytes. Conventional staining techniques can be used to detect these aberrations; this technique was used to estimate absorbed dose in the case of a radiation accident in Goiania, Brazil. A recently introduced fluorescent in situ hybridization technique (FISH) using DNA probes has increased the sensitivity and ease of detecting chromosome aberrations, especially stable chromosome aberrations. This technique allows, to some extent, the estimation of absorbed radiation dose from past exposures. Numerical aberrations can be directly estimated in metaphases by counting the number of FISH-painted chromosomes. Micronuclei are formed by lagging chromosome fragments or whole chromosomes during the anaphase stage of cell division. The nature of micronuclei as to whether they possess a centromere can be determined either by CREST staining (calcinosis, Raynoud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) or FISH with centromere-specific DNA probes. In several carcinogen-exposed populations, such as heavy smokers or people exposed to arsenic, aneuploidy appears to be more common than structural aberrations. In victims of radiation accidents, aneuploidy (hyperploidy) has been found to be common in addition to structural aberrations.

Report from the working group on the in vivo mammalian bone marrow chromosomal aberration test

The following summary represents a consensus of the working group, except where noted. The goal of this working group was to identify the minimal requirements needed to conduct a scientifically valid and practical in vivo chromosomal aberration assay. For easy reference, the items discussed are listed in the order in which they appear in OECD guideline 475. Specific disagreement with the current and/or proposed OECD guideline is presented in the text. Introduction, purpose, scope, relevance, application, and limits of test: This test would not be appropriate in situations where there was sufficient evidence to indicate that the test article or reactive metabolites could not reach the bone marrow. Test substances: Solid and liquid test substances should be dissolved, if possible, in water or isotonic saline. If insoluble in water/saline, the test substance should be dissolved or homogeneously suspended in an appropriate vehicle (e.g., vegetable oil). A suspension was not considered suitable for an intravenous injection. The use of dimethyl sulfoxide as an organic solvent was not recommended. The use of any uncommonly used solvent/vehicle should be justified. Freshly prepared solutions or suspensions of the test substance should be employed unless stability data demonstrate the acceptability of storage. Selection of species: Any commonly used rodent species was deemed acceptable but rats or mice were preferred, with no strain preference. Number and sex: A consensus could not be reached as to the requirement for both sexes versus one sex in this assay. It was suggested that a single sex should be used unless pharmacokinetic and/or toxicity data indicated a difference in metabolism and/or sensitivity between males and females. The size of the experiment (i.e., number of cells per animal, number of animals per treatment group) should be based on statistical considerations. Lacking a formal analysis, it was agreed that at least 100 metaphase cells should be scored per animal while at least five animals of any one sex should be evaluated per treatment group. Recently, a formal analysis of the numbers of cells to score per animal and numbers of animals to score per treatment group was conducted at a workshop on statistics for in vivo mutagenicity tests (Adler et al., 1994). The conclusion of this workshop was that, based on a type I error of 0.05 and a power of 80% to detect at least a doubling in the control frequency, the minimal number of cells to score per animal was 200 and the minimal number of animals to score per sex per treatment group was four.(ABSTRACT TRUNCATED AT 400 WORDS)

Chromosomal aberrations in native small mammals (peromyscus leucopus) at a petrochemical waste disposal site: II. Cryptic and inherited aberrations detected by G-band analysis

Wild rodents (Peromyscus leucopus) collected from a hazardous waste site polluted with petrochemicals and known to have significantly increased frequencies of somatic chromosome aberrations in a standard metaphase chromosome aberration (SMCA) assay were examined by G-banding, a differential staining technique. Several classes of aberrations not detectable with SMCA were identified, including Robertsonian fusions, pericentric and paracentric inversions, and interstitial deletions. Four classes of these more cryptic aberrations were significantly higher in animals from the hazardous waste site compared to animals from reference sites. Several animals also carried apparently inherited aberrations in chromosomes 5, 10, and 21, suggesting that aberrations induced in a parent may be transmitted to offspring. When all classes of aberrations were converted to numbers of lesions per cell, animals from the hazardous waste site did not show elevated frequencies of lesions or aberrant cells, compared to animals from reference sites, unless inherited aberrations were removed from the data set. Then animals collected from the hazardous waste site showed a significantly increased number of aberrations per cell. Two chromosomes (5 and 21) sustained more aberrations than the other members of the complement, suggesting that some chromosomes may be more susceptible to induced damage than others. The method of G-banding itself may obscure much of the damage attributable to single chromatid breaks in SMCA but reveals types of damage not detectable in SMCA, indicating that the application of any single genetic assay technique may seriously underestimate actual levels of induced damage in exposed organisms.

Chromosome instability in lymphocytes, fibroblasts, and colon epithelial-like cells from patients with familial polyposis coli

Detailed chromosome studies on early-passage skin fibroblast cultures from 17 patients with familial polyposis coli (FPC) showed an increase in all classes of chromosome aberrations compared with controls. The most striking difference was in the number of cytogenetically abnormal clones, some of which reached 80–100% in early cultures, suggesting either presence in vivo or considerable proliferative advantage. The occasional occurrence of very high levels of tetraploidy in these cultures was thought to be a manifestation of clone formation. The same type of chromosome instability was found to extend to colon fibroblasts and lymphocytes but without evidence of clone formation, apart from high tetraploidy in the former. Chromosomes in colon epithelial-like cultures were remarkably stable initially, despite the time span of several months required to establish them. However, from the outset one of these cultures was composed of three abnormal clones. After approximately 18 months in culture, stable and unstable chromosome rearrangements clearly increased in all tested cultures, although the cells were not senescent, suggesting a possible shift toward malignancy.

Dose-Yield Relationships for Individual Types of Chromosome Aberrations in Cells Irradiated in Early S

The dose-yield relationships for individual classes of chromosome aberrations were measured for cells irradiated in early S after release from thymidine block. It was found that the yield for asymmetrical chromosome exchanges as a function of dose was quadratic, whereas that for sister unions was linear. These data cannot be reconciled with the Chadwick-Leenhouts hypothesis, which predicts that the initial lesion leading to aberration formation is always of a mixed one-hit, two-hit type, resulting in DNA double-stranded breaks. The data support the classical theory of aberration formation that initial lesions are of a one-hit type and that exchanges are of the two-hit type, involving two initial lesions, each of the one-hit type.

Alteration in Frequencies of Different Classes of Chromosome Aberrations Throughout the First Cell Cycle Following X Irradiation

The frequencies of different classes of aberrations induced by 250 rad of X rays following pulse-labeling with tritiated thymidine were scored in successive time samples of metaphase cells over one cell cycle in a cell line with a very simple karyotype. The experimental conditions precluded the possibility that cells were in their second division following irradiation. It was found that chromosome-type aberrations were common in cells apparently in early S at the time of irradiation, that is, in labeled metaphases found at later collection times, and gradually declined in frequency at earlier collection times. The most frequent class of aberrations in early S cells was sister unions. Sister unions were also found in cells irradiated in G/sub 1/. The data suggest that those classes of aberrations usually described as of the isochromatid type are more probably of the chromosome type, or are chromosome-chromatid interchanges.

Chromosomal abnormalities and the morphology of mouse sperm heads.

In the mouse, numerous mutagens, teratogens and carcinogens have been shown to induce marked elevations in the fraction of sperm with head shape abnormalities. Since carcinogens and teratogens may act by causing genetic damage, a likely explanation of these results is that the sperm abnormalities are also caused by genetic damage. There are two more or less distinct classes of genetic damage, chromosomal aberrations and point mutations. In this paper, we provide evidence, that in general, chromosomal aberrations are not responsible for causing abnormally shaped sperm. Chromosomal aberrations could have caused abnormal sperm morphology in a number of ways. One possibility was that the mere presence of a translocated chromosome within the germ cell led to the malformation of the sperm head. A second possibility was that chromosomal imbalance, i.e., aneuploidy, duplications or deficiencies, within the spermatid or haploid cells caused abnormalities in shape. We tested these hypotheses by measuring the level of abnormally shaped sperm in mice homozygous and heterozygous for 24 various reciprocal and Robertsonian translocations. The diploid cells of these mice are known to be chromosomally balanced, containing translocated chromosomes. A predictable proportion of their gametes are, however, chromosomally unbalanced and carry translocated chromosomes. It was found that the levels of sperm abnormalities in these mice were convincingly unrelated to the levels predicted by any of the above hypotheses. Based on these results it seems that sperm abnormalities in mice are not due to the mere presence of translocated chromosomes in germ cells and also not due to chromosomal aneuploidy or duplication-deficiencies of chromosomal segments in the spermatid during development of the sperm.

The effect of structural aberrations of the chromosomes on reproductive fitness in man. I. Methodology.

Methods are presented to estimate relative fitness of carriers as a function of fertility, survival, are generation time in pedigrees under incomplete ascertainment. A large sample of diverse chromosomal aberrations reveals significant effects on all three parameters, giving a relative fitness of .769 +/- .039. There is no significant shift in segregation frequency. Implications of these results for population dynamics of structural rearrangements are discussed. The following paper applies these methods to specific classes of aberrations.

A model for the production of chromosome damage by mitomycin C.

A model is presented, which is based on the idea that the chromosome damage induced by Mitomycin C results directly from repair or misrepair of DNA molecules responsible for the linear continuity of the chromosomes. Testing the model with human cells confirms the prediction that exchanges with complete joining occur between chromosome regions containing homologous, repetitive DNA. Most probably incomplete exchanges involve homologous, but unique DNA sequences. — Prerequisites determining the MC-induced aberration patterns are the distribution of the chemical due to compartmentalization, the somatic pairing of chromosomes, and the occurrence of repeated or unique DNA sequences. — The scoring of different classes of MC-induced chromatid aberrations (attenuation, constriction, gap, break) in alcohol/acetic acid-fixed chromosome has a limited value.

On the Mechanisms of the Synergy of 5-Fluorouracil Deoxyriboside (FUDR) and Ionizing Radiations

SUMMARYIn order to elucidate the mechanisms by which 5 deoxyfluoro-uridine (FUDR) provokes synergistic effect with ionizing radiations in Vicia faba L., three concentrations of the compound were tested after seed irradiations with γ-rays (Co60) and fast neutrons at three different presoaking times.In all cases synergistic effect was obtained but lower for neutrons than for γ-rays. It was found to be conspicuous after presoaking the seeds as regards to both chromosome shattering and other aberrations. The classes of aberrations that enhanced with γ-rays and FUDR included mainly chromosome breaks, chromosome exchanges and microfragments. From these experiments it could be suggested that the mechanisms by which FUDR exerts its action at the chromosome level may be manifold and cannot only be imputed to the inhibition of just one enzymatic system.