At the level of mammalian somatic cells in renewal tissues, events or conditions that may tilt this balance towards instability have aroused special interest in relation to carcinogenesis. Mutations affecting DNA (and its subsequent repair) would, of course, be an important consideration here. These can occur spontaneously through endogenous cellular processes or as a result of exposure to mutagenic environmental agents. It is in this context that we discuss the rather unique destabilizing effects of ionizing radiation (IR) in terms of its ability to cause large-scale structural rearrangements in the genome.
We present arguments that support the conclusion that these and other important effects of IR originate largely from microscopically visible chromosomal aberrations. All exogenous agents capable of producing chromosomal aberrations (CA), that is,. For that reason, cytogenetic damage has long been a favored substitute endpoint for evaluating carcinogenic and mutagenic potential. As described below, a very important conclusion from ionizing radiation (IR) mutagenesis studies in cells of higher eukaryotic organisms has been that large-scale genomic structural changes generally dominate the spectrum of new mutations, compared to point mutations or other changes small intragenic.
We will summarize here several lines of evidence that support this conclusion. Of course, the spectrum of mutations can differ greatly, depending on the mutagenic agent, but here our emphasis will be on mutagenic events resulting from large-scale structural changes in the mammalian genome caused by IR. These include deletions, insertions, inversions and translocations, any of which can alter genes, alter the control of gene expression, or even result in the expression of new fusión sequences. IR is virtually unique in its effectiveness in producing rapid double-stranded breaks (DSB) of DNA randomly throughout the genome, which is the injury necessary for the development of these structural rearrangements.
To the extent that chromosomal aberrations underlie the important biological effects of IR, it follows that studies related to its formation and detection are essential to understand the mechanisms of action of radiation. Here, special attention is paid to advances in methodologies that have enabled the detection of CA previously considered “cryptic” and discuss some far-reaching implications with respect to our ability to fully explain them. Similarly, internal changes that occur within a given chromosome can be symmetrical or asymmetrical (Fig. Symmetrical, commonly called inversions, include interarm types in which the centromere is within the inverted region (pericentric inversions) and intra-arm types that do not include the centromere (paracentric inversions).
Asymmetric interchanges include inter-arm types, where a centric ring and an associated compound acentric fragment are formed, and intra-arm types that result in the formation of an acentric ring fragment and a (shortened) chromosome that no longer contains the deleted region previously occupied by the ring fragment. expelled. Small acentric rings of this type are commonly referred to as interstitial (ID) deletions. Being primarily ring-type structures, they should not be confused with terminal removals, which are discussed below.
There are several reasons why the study of investments deserves special attention. Being symmetrical exchanges, they retain their attachment to the spindle apparatus, allowing their orderly segregation to daughter cells during cell division. Therefore, they are not lethal, unless the breakpoints themselves alter vital gene sequences or important regulatory elements. The highly transmissible nature of inversions makes them potential sources of inheritable IR-induced mutations, including driver mutations associated with carcinogenesis.
We will return to this point in later sections. The following is an analysis of how the use of the methods described above has impacted, or is likely to do so in the near future, our basic understanding of the effects of radiation on mammalian cells. Before leaving the topic of complex exchanges, it is important to note that their unexpectedly high frequency does not argue against the proposition that dose rate and dose fractionation effects still involve a fundamental time-dependent factor based on meeting and erroneous reincorporation between breaks. Chromosó.
For independently produced breaks, a minimum of 2 (and often more) breaks are needed to produce an exchange by improper joining of the broken ends. However, the erroneous reunion or reinstatement of breakups still occurs evenly. And since initial radiogenic ruptures can come back together and disappear over time, there will be fewer potentially interactive groups of 2 or more of these. Consequently, fewer exchanges of any degree of complexity per total unit dose can occur when the dose is extended over time, such as with time-fractionated or reduced dose rate exposures.
The general consensus of these studies is that most of the recovered mutants harbored deletions of some (and in some cases all) of the 9 exons. From the previous discussion, this should come as no surprise. The question we are now raising has to do with the molecular nature of those 6-TG resistant mutants whose PCR profile is normal. The default assumption has always been that they are small base pair changes that escape PCR detection, but is that really the case? Thinking in terms of exchange breakpoints, a translocation of part of the gene to another chromosome or another part of the X chromosome would most likely destroy the gene product, but would not affect the multiplex PCR amplification signal.
An exon-targeting PCR test would not rule out this, and the following “envelope back” calculation can be made in support of this proposal. For those reluctant to abandon the idea that IR mutagenesis was largely the result of point mutations, it must have seemed difficult to admit the obvious. There was nothing “wrong” with the experimental systems they had been using. They were OK for what they were designed for, measuring small base pair changes.
Unfortunately, their use led to a serious underestimation of somatic cell mutagenesis caused by large-scale rearrangements. By the 1980s, writing was on the wall, and the idea that there was a simple equivalence between X-ray mutagenesis in microbial and mammalian cells was quietly abandoned, at least by those who had been following the controversy. Some may claim that the elegant biophysical approach has begun to reap diminishing returns, and that the wise path to follow is to accelerate cytogenetics in the molecular realm. While we generally agree on this point, we would add the following warning note.
All techniques, new and old, tend to focus on the problems they are most adept at tackling, not necessarily those of fundamental importance. The adage “For those who have only a hammer, the whole word is a nail” applies. Since its inception, cytogenetics has been involved in a series of controversial and key arguments related to the fundamental actions of IR on living cells, which in many cases serve to identify and frame those same arguments. It is logical that new ideas and theories that arise from the implementation of modern molecular strategies should, as a main impetus, seek harmony with the substantially rich cytogenetic experience.
The fact that chromosomal aberrations are shown to be the basis of the most important biological effects of IR underscores this conclusion. Co-founders and members of the Scientific Advisory Board of KromatiD, Inc. All authors analyzed data, exchanged ideas and edited the manuscript. PREVIOUS NEXT Article Your MyKarger account has been created.
A password reset link has been sent to your email address. Follow the instructions and try to log in again. Chromosomes are made up of long, thin DNA molecules. When cells are exposed to radiation or carcinogens, DNA sometimes breaks and broken ends can rejoin in patterns different from their original arrangement.
The abnormalities that occur are called “chromosomal aberrations” and can be visualized in mitosis when cells divide. This assay detects chromosomal damage in vitro using human cell lines or lymphocytes treated with the test substance both in the presence and absence of metabolic activation (Figure 27,. Chromosomal damage is evaluated as an increase in the number of micronucleated cells in the first interphase cell after exposure (Krishna et al. This cell population is identified as binucleate cells (2 nuclei) when cytochalasin B is used during testing.
Cytochalasin B is a compound that has been shown to block cytokinesis (cytoplasmic division) without affecting nuclear division. Relative changes in the proportion of binucleate cells in treated cultures compared to control cultures can be used as an indicator of xenobiotic-induced cytotoxicity. Micronuclei (small nuclei) are formed from acentric fragments of chromosomes (clastogenic mechanism) or whole chromosomes (aneugenic mechanism) that are not included in the main nucleus of the interphase cell (Krishna et al. Variations of this methodology have also been used for the mechanisms induced by the test article (Krishna et al.
An immunostaining technique is used to differentiate micronuclei containing kinetochore (s), thus evaluating the aneugenic potential of a test substance (shown in Figure 27,. Chromosomal breakdown syndromes are a group of genetic disorders that are generally transmitted in an autosomal recessive mode of inheritance. In culture, cells from affected individuals show high rates of chromosomal disruption or instability, leading to chromosomal rearrangements. Disorders are characterized by a defect in DNA repair mechanisms or genomic stability, and patients with these disorders show a greater predisposition to cancer.