Embryonic stem cells and somatic cells differ in mutation frequency and type

Association between Large-scale Genomic Homozygosity without Chromosomal Loss and Nonseminomatous Germ Cell Tumor Development

The genotype of a tumor determines its biology and clinical behavior. The genetic alterations associated with the unique embryonal morphology of nonseminomatous subtypes of testicular germ cell tumors remain to be established. Using single nucleotide polymorphism microarray analysis, we found in all of the 15 nonseminomas analyzed, large-scale chromosomal homozygosities, most of which were not associated with relative chromosome loss. This unusual genotype, distinguishing nonseminoma from seminomas and other human tumors, may be associated with the special embryonal development morphologic transition of this malignancy. Based on these genetic data, we hypothesized a new potential origin of nonseminomas through sperm fusion. Nonrandom involvement of certain chromosomes also suggests that genes on these chromosome regions may play an important role in nonseminoma development.

Tumor development: haploinsufficiency and local network assembly

According to the current models, tumor development is a continuous process of mutation accumulation, leading to several intermediate phenotypes and final phases of autonomy, unlimited growth and metastasis. One of the most important events in that process is the initial destabilization of cellular pathways that subsequently allow mutations to accumulate. The mechanisms involved in that stage are not clear. In principle, the estimated very low mutation frequency in human or mouse cells would suggest that accumulating the required number of mutations for tumor development should be a statistically unlikely event. However, this theory is contradicted by the high incidence of cancers. Here we discuss the role of protein haploinsufficiency as a contributor to the initial phases of tumor development, and suggest possible mechanisms that might be involved in that process.

Negative clonal selection in tumor evolution

Development of cancer requires the acquisition of multiple oncogenic mutations and selection of the malignant clone. Cancer evolves within a finite host lifetime and mechanisms of carcinogenesis that accelerate this process may be more likely to contribute to the development of clinical cancers. Mutator mutations are mutations that affect genome stability and accelerate the acquisition of oncogenic mutations. However, mutator mutations will also accelerate the accumulation of mutations that decrease cell proliferation, increase apoptosis, or affect other key fitness parameters. These "reduced-fitness" mutations may mediate "negative clonal selection," i.e., selective elimination of premalignant mutator clones. Target reduced-fitness loci may be "recessive" (both copies must be mutated to reduce fitness) or "dominant" (single-copy mutation reduces fitness). A direct mathematical analysis is applied to negative clonal selection, leading to the conclusion that negative clonal selection against mutator clones is unlikely to be a significant effect under realistic conditions. In addition, the relative importance of dominant and recessive reduced-fitness mutations is quantitatively defined. The relative predominance of mutator mutations in clinical cancers will depend on several variables, including the tolerance of the genome for reduced-fitness mutations, particularly the number and potency of dominant reduced-fitness loci.

Genetic and epigenetic properties of mouse male germline stem cells during long-term culture

Although stem cells are believed to divide infinitely by self-renewal division, there is little evidence that demonstrates their infinite replicative potential. Spermatogonial stem cells are the founder cell population for spermatogenesis. Recently, in vitro culture of spermatogonial stem cells was described. Spermatogonial stem cells can be expanded in vitro in the presence of glial cell line-derived neurotrophic factor (GDNF), maintaining the capacity to produce spermatogenesis after transplantation into testis. Here, we examined the stability and proliferative capacity of spermatogonial stem cells using cultured cells. Spermatogonial stem cells were cultured over 2 years and achieved approximately 10(85)-fold expansion. Unlike other germline cells that often acquire genetic and epigenetic changes in vitro, spermatogonial stem cells retained the euploid karyotype and androgenetic imprint during the 2-year experimental period, and produced normal spermatogenesis and fertile offspring. However, the telomeres in spermatogonial stem cells gradually shortened during culture, suggesting that they are not immortal. Nevertheless, the remarkable stability and proliferative potential of spermatogonial stem cells suggest that they have a unique machinery to prevent transmission of genetic and epigenetic damages to the offspring, and these characteristics make them an attractive target for germline modification.

DNA repair defects in stem cell function and aging

Cellular DNA is under constant challenge by exogenous and endogenous genotoxic stress, which results in both transient and accumulated DNA damage and genomic instability. All cells are equipped with DNA damage response pathways that trigger DNA repair, cell cycle arrest, and, if need be, apoptosis, to eliminate DNA damage or damaged cells. The consequences of these processes for stem cells can be profound: diminution in stem cell pools, or, because of altered gene expression, an increased chance for stem cell differentiation or malignant transformation. Furthermore, a number of DNA repair abnormalities are linked to premature aging syndromes, and these are associated with defects in the stem cell population. The specific DNA repair systems for which there are data regarding the impact of repair defects on stem cell function include O(6)-alkylguanine DNA alkyltransferase, nucleotide excision repair, base excision repair, mismatch repair, non-homologous DNA end-joining Fanconi's anemia protein complex, and homologous recombination. It has recently become clear that deficiencies of these processes are associated not only with cancer and/or aging but also with stem cell defects. This discovery raises the possibility of a link between aging and stem cell dysfunction. In this review, we provide evidence for a link between DNA repair systems and the maintenance and longevity of stem cells.

Spontaneous and mutagen-induced loss of DNA mismatch repair in Msh2-heterozygous mammalian cells

We have developed a simple procedure that enables the efficient selection of cells that are deficient for DNA mismatch repair (MMR). This selection procedure was used to investigate the frequency of fortuitous MMR-deficient cells in a mouse embryonic stem cell line, heterozygous for the MMR gene Msh2. We found a surprisingly high frequency (3 x 10(-4)) of Msh2-deficient cells. The wild type Msh2 allele was almost invariably lost by loss of heterozygosity. Single treatments with the genotoxic agents ethylnitrosourea, UVC light and mitomycin C resulted in a further increase of the number of Msh2-/- cells in the heterozygous cell line. This increase was not only due to induced loss of the wild type allele but also to a selective growth advantage of preexisting Msh2-/- cells to ethylnitrosourea and UVC. Mitomycin C, in contrast to ethylnitrosourea and UVC, uniquely induced loss of heterozygosity at Msh2. These mechanistically different ways of loss of the wild type Msh2 allele reflect the different repair pathways processing these damages. Heterozygous germ line defects in one of the MMR genes underlie the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome. Based on the results described here we hypothesize that mutagen-induced loss of MMR in the intestine of these patients contributes to the tissue specificity of carcinogenesis in HNPCC patients.

Bio-monitoring the genotoxicity of populations of Scots pine in the vicinity of a radioactive waste storage facility

Results of a long-term (1997-2002) study of the Scots pine populations growing in the vicinity of the radioactive waste storage facility ('Radon' LWPE) are presented. Cytogenetic disturbances in reproductive (seeds) and vegetative (needles) tissues sampled from Scots pine populations were studied to examine whether Scots pine trees have experienced environmental stress in areas with relatively low levels of pollution. The data clearly indicate the presence of mutagenic contaminants in the environment of the pine trees. An increased number of mitotic abnormalities, especially multipolar mitoses was found in the pine tree populations submitted to man-made exposure, which suggests that the cytogenetic damage is mainly caused by chemical contamination. A higher radioresistance of the Scots pine seeds from the impacted populations was shown by use of acute gamma-irradiation. During the observation period 1997-2002, pine trees exposed to anthropogenic pollution showed a steady increase of cytogenetic alterations in the root meristem cells.

Embryonic stem cells: prospects for developmental biology and cell therapy

Stem cells represent natural units of embryonic development and tissue regeneration. Embryonic stem (ES) cells, in particular, possess a nearly unlimited self-renewal capacity and developmental potential to differentiate into virtually any cell type of an organism. Mouse ES cells, which are established as permanent cell lines from early embryos, can be regarded as a versatile biological system that has led to major advances in cell and developmental biology. Human ES cell lines, which have recently been derived, may additionally serve as an unlimited source of cells for regenerative medicine. Before therapeutic applications can be realized, important problems must be resolved. Ethical issues surround the derivation of human ES cells from in vitro fertilized blastocysts. Current techniques for directed differentiation into somatic cell populations remain inefficient and yield heterogeneous cell populations. Transplanted ES cell progeny may not function normally in organs, might retain tumorigenic potential, and could be rejected immunologically. The number of human ES cell lines available for research may also be insufficient to adequately determine their therapeutic potential. Recent molecular and cellular advances with mouse ES cells, however, portend the successful use of these cells in therapeutics. This review therefore focuses both on mouse and human ES cells with respect to in vitro propagation and differentiation as well as their use in basic cell and developmental biology and toxicology and presents prospects for human ES cells in tissue regeneration and transplantation.

Mlh1 mediates tissue-specific regulation of mitotic recombination

Mitotic recombination (MR) between chromosome homologs in somatic cells is a major pathway to the loss of heterozygosity (LOH), which may cause cancer if tumor suppressor genes are involved. MR can be suppressed by DNA sequence heterology (homeology) in hybrid mice from matings between species or between subspecies. We now report that MR is relatively suppressed in F1 hybrids between inbred strains C57BL/6 and 129S2. The frequency of MR in fibroblasts is lower in F1 hybrid mice than in either of the two parental strains. However, MR in T cells is not affected by strain background. Thus, relatively small genetic differences are capable of restricting MR in a tissue-specific manner. Using Mlh1-deficient mice, we tested the role of mismatch repair in MR in two isogenic cell types. In fibroblasts of C57BL/6 x 129S2 F1 mice, the suppression of MR is alleviated in the absence of MLH1. In contrast, MR is not affected by Mlh1 status in T cells. The frequency of point mutations at the reporter gene loci Aprt and Hprt, on the other hand, is significantly increased in both T cells and fibroblasts of Mlh1(-/-) mice. Thus, different cell types respond differently to MLH1 deficiency, and the contribution of MR to tumorigenesis may be tissue-dependent in the absence of mismatch repair.

Quantification of random genomic mutations

Cancer cells contain numerous clonal mutations. It has been theorized that malignant cells sustain an elevated mutation rate and, as a consequence, harbor yet larger numbers of random point mutations. Testing this hypothesis has been precluded by lack of an assay to measure random mutations-that is, mutations that occur in only one or a few cells of a population. We have established a method that has permitted us to detect and identify rare random mutations in human cells, at a frequency of 1 per 10(8) base pairs. The assay is based on gene capture, by hybridization with a uracil-containing probe, followed by magnetic separation. Mutations that render the mutational target sequence non-cleavable by a restriction enzyme are quantified by dilution to single molecules and real-time quantitative PCR amplification. The assay can be extended to quantify mutation in any DNA-based organism, at different sites in the genome, in introns and exons, in unselected and selected genes, and in proliferating and quiescent cells.

zeta-/- Thalassemic mice are affected by two modifying loci and display unanticipated somatic recombination leading to inherited variation

Thalassemia is a disease caused by a variety of mutations affecting both the adult and embryonic alpha- and beta-globin loci. A mouse strain carrying an embryonic zeta-globin gene disrupted by the insertion of a PGK-Neo cassette displays an alpha-thalassemia-like syndrome. Embryonic survival of this zeta-null mouse is variable and strongly influenced by genetic background, the 129/SvEv mouse strain displaying a more severe phenotype than C57BL/6. We have identified two modifying loci on C57BL/6 chromosomes 2 and 5, which affect the penetrance of embryonic lethality in the 129/SvEv mouse. Through this work, we were able to observe an interesting effect on somatic recombination events in thalassemic embryos. We show that these events can occur on multiple chromosomes in very early embryonic cells, prior to their allocation to the germline. Our results demonstrate that somatic recombination events can be transmitted to subsequent generations.

Mutator phenotype in cancer: timing and perspectives

Normal human cells replicate their DNA with exceptional accuracy. During every division cycle, each daughter cell receives a full and accurate complement of genetic information. It has been estimated that approximately one error occurs during DNA replication for each 10(9) to 10(10) nucleotides polymerized. Stem cells, the cells that are progenitors of cancer, may replicate their genes even more accurately. In contrast, the malignant cells that constitute a tumor are markedly heterogeneous and exhibit multiple chromosomal abnormalities and alterations in the nucleotide sequence of DNA. To account for the disparity between the rarity of mutations in normal cells and the large numbers of mutations present in cancer, we initially hypothesized that during tumor progression, cancer cells must exhibit a mutator phenotype. In this perspective, we summarize the evidence supporting a mutator phenotype in human cancer, analyze recent measurements of mutations in human cancer, consider the timing for the expression of a mutator phenotype, and focus on the important consequences of large numbers of random mutations in human tumors.

Attempts towards derivation and establishment of bovine embryonic stem cell-like cultures

Current knowledge on the biology of mammalian embryonic stem cells (ESC) is stunningly sparse in light of their potential value in studies of development, functional genomics, generation of transgenic animals and human medicine. Despite many efforts to derive ESC from other mammalian species, ESC that retain their capacity for germ line transmission have only been verified in the mouse. However, the criterion of germ line transmission may not need to be fulfilled for exploitation of other abilities of these cells. Promising results with human ESC-like cells and adult stem cells have nourished great expectations for their potential use in regenerative medicine. However, such an application is far from reality and substantial research is required to elucidate aspects of the basic biology of pluripotent cells, as well as safety issues associated with the use of such cells in therapy. In this context, methods for the derivation, propagation and differentiation of ESC-like cultures from domestic animals would be highly desirable as biologically relevant models. Here, we review previously published efforts to establish bovine ESC-like cells and describe a procedure used in attempts to derive similar cells from bovine Day 12 embryos.

Environmental and chemical carcinogenesis

People are continuously exposed exogenously to varying amounts of chemicals that have been shown to have carcinogenic or mutagenic properties in experimental systems. Exposure can occur exogenously when these agents are present in food, air or water, and also endogenously when they are products of metabolism or pathophysiologic states such as inflammation. It has been estimated that exposure to environmental chemical carcinogens may contribute significantly to the causation of a sizable fraction, perhaps a majority, of human cancers, when exposures are related to "life-style" factors such as diet, tobacco use, etc. This chapter summarizes several aspects of environmental chemical carcinogenesis that have been extensively studied and illustrates the power of mechanistic investigation combined with molecular epidemiologic approaches in establishing causative linkages between environmental exposures and increased cancer risks. A causative relationship between exposure to aflatoxin, a strongly carcinogenic mold-produced contaminant of dietary staples in Asia and Africa, and elevated risk for primary liver cancer has been demonstrated through the application of well-validated biomarkers in molecular epidemiology. These studies have also identified a striking synergistic interaction between aflatoxin and hepatitis B virus infection in elevating liver cancer risk. Use of tobacco products provides a clear example of cancer causation by a life-style factor involving carcinogen exposure. Tobacco carcinogens and their DNA adducts are central to cancer induction by tobacco products, and the contribution of specific tobacco carcinogens (e.g. PAH and NNK) to tobacco-induced lung cancer, can be evaluated by a weight of evidence approach. Factors considered include presence in tobacco products, carcinogenicity in laboratory animals, human uptake, metabolism and adduct formation, possible role in causing molecular changes in oncogenes or suppressor genes, and other relevant data. This approach can be applied to evaluation of other environmental carcinogens, and the evaluations would be markedly facilitated by prospective epidemiologic studies incorporating phenotypic carcinogen-specific biomarkers. Heterocyclic amines represent an important class of carcinogens in foods. They are mutagens and carcinogens at numerous organ sites in experimental animals, are produced when meats are heated above 180 degrees C for long periods. Four of these compounds can consistently be identified in well-done meat products from the North American diet, and although a causal linkage has not been established, a majority of epidemiology studies have linked consumption of well-done meat products to cancer of the colon, breast and stomach. Studies employing molecular biomarkers suggest that individuals may differ in their susceptibility to these carcinogens, and genetic polymorphisms may contribute to this variability. Heterocyclic amines, like most other chemical carcinogens, are not carcinogenic per se but must be metabolized by a family of cytochrome P450 enzymes to chemically reactive electrophiles prior to reacting with DNA to initiate a carcinogenic response. These same cytochrome P450 enzymes--as well as enzymes that act on the metabolic products of the cytochromes P450 (e.g. glucuronyl transferase, glutathione S-transferase and others)--also metabolize chemicals by inactivation pathways, and the relative amounts of activation and detoxification will determine whether a chemical is carcinogenic. Because both genetic and environmental factors influence the levels of enzymes that metabolically activate and detoxify chemicals, they can also influence carcinogenic risk. Many of the phenotypes of cancer cells can be the result of mutations, i.e., changes in the nucleotide sequence of DNA that accumulate as tumors progress. These can arise as a result of DNA damage or by the incorporation of non-complementary nucleotides during DNA synthetic processes. Based upon the disparity between the infrequency of spontaneous mutations and the large numbers of mutations reported in human tumors, it has been postulated that cancers must exhibit a mutator phenotype, which would represent an early event in cancer progression. A mutator phenotype could be generated by mutations in genes that normally function to guarantee genetic stability. These mutations presumably arise via DNA damage by environmental or endogenous agents, but it remains to be determined whether the acquisition of a mutator phenotype is a necessary event during tumor progression.

Spontaneous multiple mutations show both proximal spacing consistent with chronocoordinate events and alterations with p53-deficiency

Analysis of spontaneous multiple mutations in normal and tumor cells may constrain hypotheses about the mechanisms responsible for multiple mutations and provide insight into the mutator phenotype. In a previous study, spontaneous doublets in Big Blue mice were dramatically more frequent than expected by chance and exhibited a mutation pattern similar to that observed for single mutations [Mutat. Res. 452 (2000) 219]. The spacing between mutations in doublets was generally closer than expected by chance and the distribution of mutation spacing fit an exponential, albeit with substantial scatter. We now analyze 2658 additional mutants and confirm that doublets are enhanced dramatically relative to chance expectation. The spacing, frequency and pattern of spontaneous doublets and multiplets (domuplets) are examined as a function of age, tissue type, p53-deficiency and neoplasia in the new and combined data. The new and combined data confirm that the distribution of the spacing between mutations in doublets is non-random with the mutations more closely spaced than expected by chance (P < 0.0005; combined data), consistent with temporally coordinate (chronocoordinate) events. An exponential provides an excellent fit to the distribution (R2 = 0.98) and estimates that half of doublets have mutations separated by 120 nucleotides or less (the "half-life of mutation spacing"). We make several novel observations: (i) singlets and doublets show similar overall increases in frequency with age (ii) doublet frequency may be lower in the male germline, consistent with the generally reduced mutation frequency in the male germline (iii) doublet frequencies are elevated in somatic tissues of p53-deficient mice (Li-Fraumini cancer syndrome model; P = 0.005) and (iv) doublets and singlets in tumors from p53-deficient mice have a different mutation pattern (P = 0.007). The observations are consistent with chronocoordinate occurrence of spontaneous doublets and multiplets due to a transient error-prone condition and do not suggest a major role for the recently discovered Y family of error-prone polymerases. The enhancement of doublets in p53-deficient mice may contribute to cancer risk.

Restoration of an absent G1 arrest and protection from apoptosis in embryonic stem cells after ionizing radiation

Response to DNA damage and cell-cycle regulation differ markedly between embryonic stem (ES) cells and somatic cells. ES cells require exquisitely sensitive mechanisms to maintain genomic integrity and do so, in part, by suppressing spontaneous mutation. Spontaneous mutation frequency in somatic cells is approximately 10(-4) compared with 10(-6) for ES cells. ES cells also lack a G(1) checkpoint and are hypersensitive to IR and other DNA-damaging agents. These characteristics facilitate apoptosis and the removal of cells with a mutational burden from the population, thereby keeping the population free of damaged cells. Here, we identify signaling pathways that are compromised and lead to a natural absence of aG(1) arrest in ES cells after DNA damage. The affected pathways are those mediated by p53 and p21 and by ATM, Chk2, Cdc25A, and Cdk2. In ES cells, Chk2 kinase is not intranuclear as in somatic cells but is sequestered at centrosomes and is unavailable to phosphorylate Cdc25A phosphatase and cause its degradation. Although ectopic expression of Chk2 does not rescue the p53/p21 pathway, its expression is sufficient to allow it to phosphorylate Cdc25A, activate downstream targets, restore a G(1) arrest, and protect the cell from apoptosis.

A LacZ-based transgenic mouse for detection of somatic gene repair events in vivo

Somatic gene repair of disease-causing chromosomal mutations is a novel approach for gene therapy. This method would ensure that the corrected gene is regulated by its endogenous promoter and expressed at physiological levels in the appropriate cell types. A reporter mouse, Gtrosa26(tm1Col), was generated by targeting a mutated LacZ gene to the Rosa26 locus in mouse embryonic stem (ES) cells. The LacZ gene contains a G to A point mutation, resulting in a Glu to Lys amino-acid substitution at position 461, which abrogates enzymatic activity. The gene is expressed in ES cells, primary embryonic fibroblasts, and in all tissues examined in the adult mouse, including the lung, liver, kidney, spleen, heart, brain and smooth muscle. This transgenic mouse will allow testing of gene repair strategies in vivo and identification of which cell types can be successfully targeted by chromosomal gene repair. Although low levels of gene repair were achieved in the ES cells used to generate the Gtrosa26(tm1Col) mouse, preliminary attempts at gene repair in vivo were unsuccessful, thus highlighting the difficulties that will have to be overcome to get this approach to work.

Unexpected embryonic stem (ES) cell mutations represent a concern in gene targeting: Lessons from ?fierce? mice

The exceptional value of gene targeting technology to generate mouse models of human disease exists under the shadow of potential genetic errors. We previously observed an unexpected brain-behavior phenotype that resulted from a gene-targeting experiment designed to delete the Zfa gene. Given that the transcription of Zfa is restricted to the germ cell lineage of adult testis, it was both a surprise and a concern when the resulting mice had a phenotype present in both sexes that included abnormal brains and violent behavior. We hypothesized that an unrelated mutation may have been responsible for the unexpected phenotype. Here we show that the single gene mutation, Nr2e1(frc) (fierce), which was responsible for the brain-behavior phenotype, existed in the embryonic stem (ES) cell even before the derivation of the Zfa knockout mice. Our work thus highlights a concern in gene targeting, namely, that ES cells can harbor unexpected mutations, which can lead to genotype-phenotype misattribution. Based on our findings, we caution the gene-targeting community to use low-passage ES cells, to characterize mice derived from more than one independently targeted ES cell clone, and to backcross mice to allow for segregation of distant but linked mutations.

Chromosome segregation defects contribute to aneuploidy in normal neural progenitor cells

Recent studies based predominantly on nucleotide hybridization techniques have identified aneuploid neurons and glia in the normal brain. To substantiate these findings and address how neural aneuploidy arises, we examined individual neural progenitor cells (NPCs) undergoing mitosis. Here we report the identification of chromosomal segregation defects in normal NPCs of the mouse cerebral cortex. Immunofluorescence in fixed tissue sections revealed the presence of supernumerary centrosomes and lagging chromosomes among mitotic NPCs. The extent of aneuploidy followed the prevalence of supernumerary centrosomes within distinct cell populations. Real-time imaging of live NPCs revealed lagging chromosomes and multipolar divisions. NPCs undergoing nondisjunction were also observed, along with interphase cells that harbored micronuclei or multiple nuclei, consistent with unbalanced nuclear division. These data independently confirm the presence of aneuploid NPCs and demonstrate the occurrence of mitotic segregation defects in normal cells that can mechanistically account for aneuploidy in the CNS.

Chromosome segregation defects contribute to aneuploidy in normal neural progenitor cells

Recent studies based predominantly on nucleotide hybridization techniques have identified aneuploid neurons and glia in the normal brain. To substantiate these findings and address how neural aneuploidy arises, we examined individual neural progenitor cells (NPCs) undergoing mitosis. Here we report the identification of chromosomal segregation defects in normal NPCs of the mouse cerebral cortex. Immunofluorescence in fixed tissue sections revealed the presence of supernumerary centrosomes and lagging chromosomes among mitotic NPCs. The extent of aneuploidy followed the prevalence of supernumerary centrosomes within distinct cell populations. Real-time imaging of live NPCs revealed lagging chromosomes and multipolar divisions. NPCs undergoing nondisjunction were also observed, along with interphase cells that harbored micronuclei or multiple nuclei, consistent with unbalanced nuclear division. These data independently confirm the presence of aneuploid NPCs and demonstrate the occurrence of mitotic segregation defects in normal cells that can mechanistically account for aneuploidy in the CNS.

Neurons derived from embryonic stem (ES) cells resemble normal neurons in their vulnerability to excitotoxic death

We determined whether embryonic stem (ES) cells could provide a model system for examining neuronal death mediated by glutamate receptors. Although limited evidence indicates that normal neurons can be derived from mouse ES cells, there have been no studies examining pathophysiological responses in mouse ES cell systems. Mouse ES cells, induced down a neural lineage by retinoic acid (RA), were found to have enhanced long-term survival when plated onto a layer of cultured mouse cortical glial cells. In these conditions, the ES cells differentiated into neural cells that appeared normal morphologically and displayed normal features of immunoreactivity when tested for neuron-specific elements. Varying the culture medium generated cultures of mixed neuronal/glial cells or enriched in oligodendrocytes. These cultures were viable for at least four weeks. Real-time PCR analysis of N-methyl-D-aspartate (NMDA) receptor subunits revealed an appropriate age-in-vitro dependent pattern of expression. Neurons derived from ES cells were vulnerable to death induced by a 24-h exposure to the selective glutamate receptor agonists NMDA, kainate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). This vulnerability to agonist-induced death increased with age in vitro, and related closely to expression of receptor subunits, as it does in cultured primary neurons. Experiments with selective receptor antagonists showed that glutamate receptors mediated the NMDA- and kainate-induced death. Neuronal differentiated ES cells therefore exhibited an excitotoxic response resembling that displayed by central nervous system (CNS) neurons. Thus, ES cells, which are very amenable to genetic manipulation, provide a valid system for studying glutamate receptor-mediated toxicity at the molecular level.

Radiation-induced versus endogenous DNA damage: commentary on Pollycove and Feinendegen

Quantitative comparison of ROS-induced endogenous DNA damage with DNA damage induced by ionizing radiation at environmental level, presented by Pollycove and Feinedegen, is timely and provides basic data for establishing a a sound rule for radiation protection. Main point added to their estimation is quantitative data on oxidative damage to DNA precursormolecules, such as oxo dGTP and their cellular decomposition efficiency. If the precursor damages are taken into account, the amount of initial endogenous DNA damage would be increased by a factor of 100 / 10 000

Somatic gene mutation and human disease other than cancer

While the focus of much mutation research is on germ-line mutation, somatic mutation is being found to be important in human disease. Neurofibromatosis 1 and McCune-Albright are disorders which are detected in the skin and other systems. The skin manifestations were essential for the demonstration of somatic mosaicism in neurofibromatosis 1, while analysis of blood DNA demonstrated somatic mutation in neurofibromatosis 2. Incontinentia pigmenti is also a disorder seen in skin and other tissues, but here it is the rare variant of the disorder in males, where it is usually lethal, that involves somatic mosaicism. Paroxysmal nocturnal hemoglobinuria is a disorder of the blood and cell separation of blood elements allows the demonstration of the somatic mosaicism. This review also discusses disorders in which somatic mosaicism, for mutations probably incompatible with life if the mutation had been germ-line, are likely to be involved. These include the Proteus syndrome, which involves both vascular tissues and bones, and two disorders which might be thought of as representing two subtypes of Proteus: Klippel-Trenaunay, which involves vascular tissues, and Maffuci, which involves bones. Embryonic mutations, which create mosaicism for both the soma and germ-line, are being increasingly found in a number of disorders and are discussed more briefly. Finally, reverse mutations involving the soma have been recently found in several disorders and such revertant mutations are also examined. While the review focuses on the clinical importance of somatic mutations, many of the mutations found to date are tabulated. It is too early to see if there is a different pattern of somatic mutation as compared to germ-line mutation. Although the parameters to allow careful quantitation are not yet available, it seems that the frequency of gene mutation in embryonic cells is not markedly different than that in the germ-line.

Extensive loss of heterozygosity is suppressed during homologous repair of chromosomal breaks

Loss of heterozygosity (LOH) is a common genetic alteration in tumors and often extends several megabases to encompass multiple genetic loci or even whole chromosome arms. Based on marker and karyotype analysis of tumor samples, a significant fraction of LOH events appears to arise from mitotic recombination between homologous chromosomes, reminiscent of recombination during meiosis. As DNA double-strand breaks (DSBs) initiate meiotic recombination, a potential mechanism leading to LOH in mitotically dividing cells is DSB repair involving homologous chromosomes. We therefore sought to characterize the extent of LOH arising from DSB-induced recombination between homologous chromosomes in mammalian cells. To this end, a recombination reporter was introduced into a mouse embryonic stem cell line that has nonisogenic maternal and paternal chromosomes, as is the case in human populations, and then a DSB was introduced into one of the chromosomes. Recombinants involving alleles on homologous chromosomes were readily obtained at a frequency of 4.6 x 10(-5); however, this frequency was substantially lower than that of DSB repair by nonhomologous end joining or the inferred frequency of homologous repair involving sister chromatids. Strikingly, the majority of recombinants had LOH restricted to the site of the DSB, with a minor class of recombinants having LOH that extended to markers 6 kb from the DSB. Furthermore, we found no evidence of LOH extending to markers 1 centimorgan or more from the DSB. In addition, crossing over, which can lead to LOH of a whole chromosome arm, was not observed, implying that there are key differences between mitotic and meiotic recombination mechanisms. These results indicate that extensive LOH is normally suppressed during DSB-induced allelic recombination in dividing mammalian cells.