Reflections and meditations upon complex chromosomal exchanges

Twenty Years of Reflections in Mutation Research

Reflections is a component of Mutation Research Reviews devoted to historical and philosophical themes pertaining to the subject of mutation. Reflections was initiated in 1999 and has included a broad array of topics centered on mutation research, but overlapping other scientific fields and touching upon history, sociology, politics, philosophy and ethics. This commentary offers an editor's reflections on the 44 papers in the Reflections series, including the people who contributed to the series and the topics that they discussed.

Understanding cancer development processes after HZE-particle exposure: roles of ROS, DNA damage repair and inflammation

During space travel astronauts are exposed to a variety of radiations, including galactic cosmic rays composed of high-energy protons and high-energy charged (HZE) nuclei, and solar particle events containing low- to medium-energy protons. Risks from these exposures include carcinogenesis, central nervous system damage and degenerative tissue effects. Currently, career radiation limits are based on estimates of fatal cancer risks calculated using a model that incorporates human epidemiological data from exposed populations, estimates of relative biological effectiveness and dose-response data from relevant mammalian experimental models. A major goal of space radiation risk assessment is to link mechanistic data from biological studies at NASA Space Radiation Laboratory and other particle accelerators with risk models. Early phenotypes of HZE exposure, such as the induction of reactive oxygen species, DNA damage signaling and inflammation, are sensitive to HZE damage complexity. This review summarizes our current understanding of critical areas within the DNA damage and oxidative stress arena and provides insight into their mechanistic interdependence and their usefulness in accurately modeling cancer and other risks in astronauts exposed to space radiation. Our ultimate goals are to examine potential links and crosstalk between early response modules activated by charged particle exposure, to identify critical areas that require further research and to use these data to reduced uncertainties in modeling cancer risk for astronauts. A clearer understanding of the links between early mechanistic aspects of high-LET response and later surrogate cancer end points could reveal key nodes that can be therapeutically targeted to mitigate the health effects from charged particle exposures.

Subdiffusion supports joining of correct ends during repair of DNA double-strand breaks

The mobility of damaged chromatin regions in the nucleus may affect the probability of mis-repair. In this work, live-cell observation and distance tracking of GFP-tagged DNA damage response protein MDC1 was used to study the random-walk behaviour of chromatin domains containing radiation-induced DNA double-strand breaks (DSB). Our measurements indicate a subdiffusion-type random walk process with similar time dependence for isolated and clustered DSBs that were induced by 20 MeV proton or 43 MeV carbon ion micro-irradiation. As compared to normal diffusion, subdiffusion enhances the probability that both ends of a DSB meet, thus promoting high efficiency DNA repair. It also limits their probability of long-range movements and thus lowers the probability of mis-rejoining and chromosome aberrations.

Nuclear dynamics of radiation-induced foci in euchromatin and heterochromatin

Repair of double strand breaks (DSBs) is essential for cell survival and genome integrity. While much is known about the molecular mechanisms involved in DSB repair and checkpoint activation, the roles of nuclear dynamics of radiation-induced foci (RIF) in DNA repair are just beginning to emerge. Here, we summarize results from recent studies that point to distinct features of these dynamics in two different chromatin environments: heterochromatin and euchromatin. We also discuss how nuclear architecture and chromatin components might control these dynamics, and the need of novel quantification methods for a better description and interpretation of these phenomena. These studies are expected to provide new biomarkers for radiation risk and new strategies for cancer detection and treatment.

Mechanisms and impacts of chromosomal translocations in cancers

Chromosomal aberrations have been associated with cancer development since their discovery more than a hundred years ago. Chromosomal translocations, a type of particular structural changes involving heterologous chromosomes, have made a critical impact on diagnosis, prognosis and treatment of cancers. For example, the discovery of translocation between chromosomes 9 and 22 and the subsequent success of targeting the fusion product BCR-ABL transformed the therapy for chronic myelogenous leukemia. In the past few decades, tremendous progress has been achieved towards elucidating the mechanism causing chromosomal translocations. This review focuses on the basic mechanisms underlying the generation of chromosomal translocations. In particular, the contribution of frequency of DNA double strand breaks and spatial proximity of translocating loci is discussed.

Evidence for formation of DNA repair centers and dose-response nonlinearity in human cells

The concept of DNA "repair centers" and the meaning of radiation-induced foci (RIF) in human cells have remained controversial. RIFs are characterized by the local recruitment of DNA damage sensing proteins such as p53 binding protein (53BP1). Here, we provide strong evidence for the existence of repair centers. We used live imaging and mathematical fitting of RIF kinetics to show that RIF induction rate increases with increasing radiation dose, whereas the rate at which RIFs disappear decreases. We show that multiple DNA double-strand breaks (DSBs) 1 to 2 μm apart can rapidly cluster into repair centers. Correcting mathematically for the dose dependence of induction/resolution rates, we observe an absolute RIF yield that is surprisingly much smaller at higher doses: 15 RIF/Gy after 2 Gy exposure compared to approximately 64 RIF/Gy after 0.1 Gy. Cumulative RIF counts from time lapse of 53BP1-GFP in human breast cells confirmed these results. The standard model currently in use applies a linear scale, extrapolating cancer risk from high doses to low doses of ionizing radiation. However, our discovery of DSB clustering over such large distances casts considerable doubts on the general assumption that risk to ionizing radiation is proportional to dose, and instead provides a mechanism that could more accurately address risk dose dependency of ionizing radiation.

Complex exchanges are responsible for the increased effectiveness of C-ions compared to X-rays at the first post-irradiation mitosis

The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.

A patient with 22q11.2 deletion and Opitz syndrome-like phenotype has the same deletion as velocardiofacial patients

Five patients were previously described with the Opitz (GBBB) syndrome (OMIM 145410) phenotype and 22q11.2 deletion determined by FISH but the precise limits of their deletions have not been determined. Since one locus for Opitz syndrome maps to 22q11.2 and chromosomal arrangements are frequently complex and could inactivate such a locus, we performed high-resolution array-based comparative genomic hybridization (CGH) on a new Opitz syndrome-like phenotype patient with a 22q11.2 deletion. He shares the same deletion as patients with velocardiofacial and DiGeorge syndrome.

Image-based modeling reveals dynamic redistribution of DNA damage into nuclear sub-domains

Several proteins involved in the response to DNA double strand breaks (DSB) form microscopically visible nuclear domains, or foci, after exposure to ionizing radiation. Radiation-induced foci (RIF) are believed to be located where DNA damage occurs. To test this assumption, we analyzed the spatial distribution of 53BP1, phosphorylated ATM, and γH2AX RIF in cells irradiated with high linear energy transfer (LET) radiation and low LET. Since energy is randomly deposited along high-LET particle paths, RIF along these paths should also be randomly distributed. The probability to induce DSB can be derived from DNA fragment data measured experimentally by pulsed-field gel electrophoresis. We used this probability in Monte Carlo simulations to predict DSB locations in synthetic nuclei geometrically described by a complete set of human chromosomes, taking into account microscope optics from real experiments. As expected, simulations produced DNA-weighted random (Poisson) distributions. In contrast, the distributions of RIF obtained as early as 5 min after exposure to high LET (1 GeV/amu Fe) were non-random. This deviation from the expected DNA-weighted random pattern can be further characterized by “relative DNA image measurements.” This novel imaging approach shows that RIF were located preferentially at the interface between high and low DNA density regions, and were more frequent than predicted in regions with lower DNA density. The same preferential nuclear location was also measured for RIF induced by 1 Gy of low-LET radiation. This deviation from random behavior was evident only 5 min after irradiation for phosphorylated ATM RIF, while γH2AX and 53BP1 RIF showed pronounced deviations up to 30 min after exposure. These data suggest that DNA damage–induced foci are restricted to certain regions of the nucleus of human epithelial cells. It is possible that DNA lesions are collected in these nuclear sub-domains for more efficient repair.

DNA damages are daily cellular events. If such events are left unchecked in an organism, they can lead to DNA mutations and possibly cancer over a long period of time. Consequently, cells have very efficient DNA repair machinery. Many studies have focused on the different molecular factors involved in the repair machinery, neglecting to consider the spatial context where damage occurs. Therefore, little is known about the role the nuclear architecture might have in the DNA damage response. In this study, we introduce computer modeling and image analysis tools in order to relate the position of DNA damage markers to morphologically distinct regions of the nucleus. Using these tools, we show that radiation-induced damages locate preferentially in non-condensed DNA regions or at the boundary of regions with condensed DNA. These results contradict the current dogma that the molecular response to randomly generated DNA damages is independent of their nuclear locations. Instead, this suggests the existence of repair centers in the nucleus. Overall, our approach shows that nuclear architecture plays a role in the DNA damage response, reminding us that the nucleus is not simply a soup of DNA and proteins.

Recurrent locus-specific mutation resulting from a cryptic ectopic insertion in Neurospora

New mutations are found among approximately 20% of progeny when one or both parents carry eas allele UCLA191 (eas(UCLA), easily wettable, hydrophobin-deficient, linkage group II). The mutations inactivate the wild-type allele of cya-8 (cytochrome aa3 deficient, linkage group VII), resulting in thin, "transparent" mycelial growth. Other eas alleles fail to produce cya-8 mutant progeny. The recurrent cya-8 mutations are attributed to repeat-induced point mutation (RIP) resulting from a duplicated copy of cya-8+ that was inserted ectopically at eas when the UCLA191 mutation occurred. As expected for RIP, eas(UCLA)-induced cya-8 mutations occur during nuclear proliferation prior to karyogamy. When only one parent is eas(UCLA), the new mutations arise exclusively in eas(UCLA) nuclei. Mutation of cya-8 is suppressed when a long unlinked duplication is present. Stable cya-8 mutations are effectively eliminated in crosses homozygous for rid, a recessive suppressor of RIP. The eas(UCLA) allele is associated with a long paracentric inversion. A discontinuity is present in eas(UCLA) DNA. The eas promoter is methylated in cya-8 progeny of eas(UCLA), presumably by the spreading of methylation beyond the adjoining RIP-inactivated duplication. These findings support a model in which an ectopic insertion that created a mutation at the target site acts as a locus-specific mutator via RIP.

Radiation-induced bystander effects and the DNA paradigm: an "out of field" perspective

Over the past 20 years there has been increasing evidence that cells and the progeny of cells surviving a very low dose of ionizing radiation [micro-mGy] can exhibit a wide range of non-monotonic effects such as adaptive responses, low dose hypersensitivity and other delayed effects. These effects are inconsistent with the expected dose-response, when based on extrapolation of high dose data and cast doubt on the reliability of extrapolating from high dose data to predict low dose effects. Recently the cause of many of these effects has been tentatively ascribed to so-called "bystander effects". These are effects that occur in cells not directly hit by an ionizing track but which are influenced by signals from irradiated cells and are thus highly relevant in situations where the dose is very low. Not all bystander effects may be deleterious although most endpoints measured involve cell damage or death. In this commentary, we consider how these effects impact the historical central dogma of radiobiology and radiation protection, which is that DNA double strand breaks are the primary radiation-induced lesion which can be quantifiably related to received dose and which determine the probability that a cancer will result from a radiation exposure. We explore the low dose issues and the evidence and conclude that in the very low dose region, the primary determinant of radiation exposure outcome is the genetic and epigenetic background of the individual and not solely the dose. What this does is to dissociate dose from effect as a quantitative relationship, but it does not necessarily mean that the effect is ultimately unrelated to DNA damage. The fundamental thesis we present is that at low doses fundamentally different mechanisms underlie radiation action and that at these doses, effect is not quantitatively related to dose.

Actions of radiation on living cells in the "post-bystander" era

Over the past 20 years there has been increasing evidence that cells and the progeny of cells surviving a dose of ionizing radiation can exhibit a wide range of effects inconsistent with the level of dose received. Recently, the cause of these delayed effects has been ascribed to so-called bystander effects, occurring in cells not directly hit by an ionizing track, but which are influenced by signals from irradiated cells. These effects are not necessarily deleterious, although most of the literature deals with adverse delayed effects. What is important to consider is what, if anything, these effects mean for what is still the central dogma of radiobiology and radiation protection, i.e., that DNA double-strand breaks are the primary radiation-induced lesion that can be quantifiably related to received dose, and which determine the probability that a cancer will result from a radiation exposure. In this chapter we review the history of radiation biology which led to the DNA paradigm. We explore the issues and the evidence which are now challenging the view that dose deposition in DNA is all important. We conclude that in the low-dose region, the primary determinant of radiation exposure outcome is the genetic and epigenetic background of the individual and not the dose. This effectively dissociates dose from effect as a quantitative relationship, but it does not necessarily mean that the effect is unrelated to DNA damage somewhere in the system.

Chromosomes Lacking Telomeres are Present in the Progeny of Human Lymphocytes Exposed to Heavy Ions

High-charge and energy (HZE) nuclei represent one of the main health risks for human space exploration, yet little is known about the mechanisms responsible for the high biological effectiveness of these particles. We have used in situ hybridization probes for cross-species multicolor banding (RxFISH) in combination with telomere detection to compare yields of different types of chromosomal aberrations in the progeny of human peripheral blood lymphocytes exposed to either high-energy iron ions or gamma rays. Terminal deletions showed the greatest relative variation, with many more of these types of aberrations induced after exposure to accelerated iron ions (energy 1 GeV/nucleon) compared with the same dose of gamma rays. We found that truncated chromosomes without telomeres could be transmitted for at least three cell cycles after exposure and represented about 10% of all aberrations observed in the progeny of cells exposed to iron ions. On the other hand, the fraction of cells carrying stable, transmissible chromosomal aberrations was similar in the progeny of cells exposed to the same dose of densely or sparsely ionizing radiation. The results demonstrate that unrejoined chromosome breaks are an important component of aberration spectra produced by the exposure to HZE nuclei. This finding may well be related to the ability of such energetic particles to produce untoward late effects in irradiated organisms.

Increased complexity of radiation-induced chromosome aberrations consistent with a mechanism of sequential formation

Complex chromosome aberrations (any exchange involving three or more breaks in two or more chromosomes) are effectively induced in peripheral blood lymphocytes (PBL) after exposure to low doses (mostly single particles) of densely ionising high-linear energy transfer (LET) alpha-particle radiation. The complexity, when observed by multiplex fluorescence in situ hybridisation (m-FISH), shows that commonly four but up to eight different chromosomes can be involved in each rearrangement. Given the territorial organisation of chromosomes in interphase and that only a very small fraction of the nucleus is irradiated by each alpha-particle traversal, the aim of this study is to address how aberrations of such complexity can be formed. To do this, we applied theoretical "cycle" analyses using m-FISH paint detail of PBL in their first cell division after exposure to high-LET alpha-particles. In brief, "cycle" analysis deconstructs the aberration "observed" by m-FISH to make predictions as to how it could have been formed in interphase. We propose from this that individual high-LET alpha-particle-induced complex aberrations may be formed by the misrepair of damaged chromatin in single physical "sites" within the nucleus, where each "site" is consistent with an "area" corresponding to the interface of two to three different chromosome territories. Limited migration of damaged chromatin is "allowed" within this "area". Complex aberrations of increased size, reflecting the path of alpha-particle nuclear intersection, are formed through the sequential linking of these individual sites by the involvement of common chromosomes.

The involvement of key DNA repair pathways in the formation of chromosome rearrangements in embryonic stem cells

It is vital that embryonic stem (ES) cells, which give rise to the diverse tissues of the mature organism, maintain genetic stability. To understand mechanisms for the prevention and causation of chromosomal instability, we have used spectral karyotyping (SKY) to analyse ES cells from wild-type and repair-gene knockout mice. We chose cells deficient in Ku70 (DNA end joining), Xrcc2 (gene conversion), Ercc1 (single-strand annealing) and Csb (transcription-coupled repair) to represent potentially-important DNA repair pathways, plus an Xpc-deficient line to examine loss of global nucleotide excision repair (NER). Spontaneous and radiation (X-ray or alpha-particle)-induced chromosome changes were assessed to measure the influence of different levels of damage severity on response. We show that most repair pathways (except for global NER) protect against chromosome changes induced by ionizing radiations, while only homology-dependent pathways protect against spontaneous chromosomal change in ES cells. However, for a given level of damage, the prevalence of different types of changes alters in the different repair-deficient lines. Thus, loss of Ercc1, Csb or Ku70 leads to increased fragment formation, but loss of Xrcc2 promotes exchanges between chromosomes. Strikingly, we found that loss of the Csb gene function specifically protects ES cells from complex exchanges, suggesting a role for transcription-associated events in complex exchange formation.

Radiation biology: concepts for radiation protection

The opportunity to write a historical review of the field of radiation biology allows for the viewing of the development and maturity of a field of study, thereby being able to provide the appropriate context for the earlier years of research and its findings. The pioneering work of Muller, Sax, and McClintock, and many others, has stood the test of time. The idea that x-rays could damage the genetic material and result in interactions that could lead to gene mutations and a range of chromosomal alterations is now interpretable in terms of induced DNA damage and errors of DNA repair. The expanded idea that such genetic alterations can be induced by DNA damage that is produced by one or two tracks of ionizing radiation remains the mainstay of radiation biology. The impact of the more recent molecular approaches to unraveling the mechanism behind this simple concept has confirmed this fundamental observation. The remarkable advances have allowed for a fairly complete understanding of the specific types of DNA damage induced by ionizing radiations and the pivotal role played by the errors of repair of double-strand breaks. Given our considerably enhanced knowledge of the details of the DNA repair processes involved, misrepair is a very unlikely event. The role of potential confounders of the concept of dose-response (e.g., bystander effects, genomic instability, and adaptive responses) is taking on a growing importance to the field. The evolving need is to begin to consider mechanistically-based dose-response models for cancer risk such that any potential impact of confounders on the response at low, environmental doses can be assessed. Thus, radiation biology research has always had a focus on how best to protect human health from radiation exposures and will continue to do so.

Impact of transforming viruses on cellular mutagenesis, genome stability, and cellular transformation

It is estimated that 15% of all cancers are etiologically linked to viral infection. Specific cancers including adult T-cell leukemia, hepatocellular carcinoma, and uterine cervical cancer are associated with infection by human T-cell leukemia virus type I, hepatitis B virus, and high-risk human papilloma virus, respectively. In these cancers, genomic instability, a hallmark of multistep cancers, has been explicitly linked to the expression of oncoproteins encoded by these viruses. This review discusses mechanisms utilized by these viral oncoproteins, Tax, HBx, and E6/E7, to mediate genomic instability and cellular transformation.

mFISH Analysis Reveals Complexity of Chromosome Aberrations in Individuals Occupationally Exposed to Internal Plutonium: A Pilot Study to Assess the Relevance of Complex Aberrations as Biomarkers of Exposure to High-LET α Particles

We recently demonstrated that a significant proportion of apparently stable insertions induced after exposure to a mean of one alpha particle/cell, detected using three-color FISH, were part of larger unstable complexes when visualized by 24-color FISH. Interestingly, regardless of the long-term persistence capability of the cell, the complexity of each alpha-particle-induced complex appeared to be specific to the nuclear traversal of a single alpha particle. To assess whether aberrations of a similar complexity are observed in vivo and also to examine the usefulness of detecting such aberrations as a biomarker of chronic exposure to alpha particles, we have carried out a limited pilot study of Russian workers with large body burdens of alpha-particle-emitting plutonium. We found unstable cells containing non-transmissible complex aberrations in all of the plutonium-exposed subjects analyzed by mFISH. In addition, all of the complexes seen were consistent with those previously observed in vitro. Non-transmissible complex aberrations were more common than transmissible-type complexes, consistent with ongoing/chronic exposure, and insertions were dominant features of both types of complex. Accordingly, this preliminary study supports the proposal that aberration complexity and non-transmissibility are the major cytogenetic features of alpha-particle exposure that could potentially be exploited as a specific indicator of chronic exposures to high-LET alpha particles.

Distribution of breakpoints and fragment sizes in human chromosome 5 after heavy-ion bombardment

PURPOSE

To measure the location of heavy ion-induced residual breakpoints in human chromosome 5 and the size distribution of chromosome fragments involved in inter- or intrachromosomal exchanges.

MATERIALS AND METHODS

Human peripheral blood lymphocytes were exposed to 4 Gy accelerated 56Fe (iron) ions (500 MeV per nucleon (MeV n(-1)), linear energy transfer=200 keV microm(-1)). Cells were then stimulated to grow in vitro for 48 h, and chromosomes were prematurely condensed by calyculin A. Chromosome 5 was painted using high-resolution multicolour banding. The location of the observed residual breakpoints and the size of all chromosome 5 fragments involved in structural aberrations were measured using dedicated image analysis software.

RESULTS

Mapping of 283 breakpoints revealed a slight deviation from randomness, with an excess of breakpoints clustered in two small bands and an under representation of breaks at the telomeric end in the q-arm. Breakpoints per unit length were similar in p- and q-arms. The distribution of chromosome fragments has a maximum for very small fragments (< 10% of the chromosome size), indicating a severe fragmentation of chromosome 5 after heavy-ion bombardment. Only fragments < 40% of the chromosome size were involved in intrachromosomal exchanges (interstitial deletions or inversions), whereas fragments up to 75% of the whole chromosome 5 were found in interchromosomal exchanges.

CONCLUSIONS

Residual breakpoints after exposure to high-energy iron ions were not distributed randomly along chromosome 5, although the p- and q-arms displayed similar radiosensitivity. Large fragments are either restituted or misrejoined to other chromosome ends, whereas small intrachromosomal fragments can produce either inter- or intrachromosomal exchanges.

Comparing DNA Damage-Processing Pathways by Computer Analysis of Chromosome Painting Data

Chromosome aberrations are large-scale illegitimate rearrangements of the genome. They are indicative of DNA damage and informative about damage processing pathways. Despite extensive investigations over many years, the mechanisms underlying aberration formation remain controversial. New experimental assays such as multiplex fluorescent in situ hybridyzation (mFISH) allow combinatorial "painting" of chromosomes and are promising for elucidating aberration formation mechanisms. Recently observed mFISH aberration patterns are so complex that computer and graph-theoretical methods are needed for their full analysis. An important part of the analysis is decomposing a chromosome rearrangement process into "cycles." A cycle of order n, characterized formally by the cyclic graph with 2n vertices, indicates that n chromatin breaks take part in a single irreducible reaction. We here describe algorithms for computing cycle structures from experimentally observed or computer-simulated mFISH aberration patterns. We show that analyzing cycles quantitatively can distinguish between different aberration formation mechanisms. In particular, we show that homology-based mechanisms do not generate the large number of complex aberrations, involving higher-order cycles, observed in irradiated human lymphocytes.

Radiation biology: concepts for radiation protection

The opportunity to write a historical review of the field of radiation biology allows for the viewing of the development and maturity of a field of study, thereby being able to provide the appropriate context for the earlier years of research and its findings. The pioneering work of Muller, Sax, and McClintock, and many others, has stood the test of time. The idea that x-rays could damage the genetic material and result in interactions that could lead to gene mutations and a range of chromosomal alterations is now interpretable in terms of induced DNA damage and errors of DNA repair. The expanded idea that such genetic alterations can be induced by DNA damage that is produced by one or two tracks of ionizing radiation remains the mainstay of radiation biology. The impact of the more recent molecular approaches to unraveling the mechanism behind this simple concept has confirmed this fundamental observation. The remarkable advances have allowed for a fairly complete understanding of the specific types of DNA damage induced by ionizing radiations and the pivotal role played by the errors of repair of double-strand breaks. Given our considerably enhanced knowledge of the details of the DNA repair processes involved, misrepair is a very unlikely event. The role of potential confounders of the concept of dose-response (e.g., bystander effects, genomic instability, and adaptive responses) is taking on a growing importance to the field. The evolving need is to begin to consider mechanistically-based dose-response models for cancer risk such that any potential impact of confounders on the response at low, environmental doses can be assessed. Thus, radiation biology research has always had a focus on how best to protect human health from radiation exposures and will continue to do so.

Does chromosome 22 have anything to do with sex determination: further studies on a 46,XX,22q11.2 del male

Several years ago, we presented a patient with true hermaphroditism and partial duplication of chromosome 22 and no evidence of SRY (Aleck et al. [1999: Am J Med Genet 85:2-4]). Recently a 46,XX male with velocardiofacial syndrome and a deletion of 22q11.2 and no evidence of Y chromosomal loci in blood DNA was reported (Phelan et al. [2003: Am J Med Genet 116A:77-79]). We have restudied this patient as he enters puberty. Because chromosomal deletions sometimes involve micro rearrangements of nearby material, we have extensively studied this individual's chromosome 22 looking for evidence of any gene duplication. We studied a number of variable number tandem repeat (VNTR) loci along chromosome 22 in the patient and both parents. Normal Mendelian inheritance of the VNTRs was found. We then used quantitative multiplex PCR of short fluorescent fragments (QMPSF) to delineate the 22q11.2 deletion in this patient (Jacquet et al. [2002: Hum Molec Genet 11:2243-2249]) and found a pattern of deletion typical of the velocardiofacial DiGeorge syndrome. Finally, the patient's DNA has been analyzed using a full coverage human chromosome 22 genomic microarray (array comparative genomic hybridization [CGH]) for evidence of rearrangements outside the classical velocardiofacial DiGeorge associated deletion (Buckley et al. [2002: Hum Molec Genet 11:3221-3229]). The array-CGH profile of this patient confirms the deletion encompassing the typically deleted region associated with the velocardiofacial DiGeorge syndrome and provides no support for additional gene copy number aberrations on 22q. Thus, there is no evidence of any chromosome 22 trisomic material. In this case, the rare events of sex reversal and 22q11.2 deletion may have occurred together by chance.