γ-H2AX Expression Pattern in Non-Irradiated Neonatal Mouse Germ Cells and after Low-Dose γ-Radiation: Relationships Between Chromatid Breaks and DNA Double-Strand Breaks1

The DNA double-strand breaks (DSBs) are considered to be the most relevant lesions for the deleterious effects of ionizing radiation exposure. The discovery that the induction of DSBs is rapidly followed by the phosphorylation of H2AX histone at Ser-139, favoring repair protein recruitment or access, opens the possibility for a wide range of research. This phosphorylated histone, named gamma-H2AX, has been shown to form foci in interphase nuclei as well as megabase chromatin domains surrounding the DNA lesion on chromosomes. Using detection of gamma-H2AX on germ cell mitotic chromosomes 2 h after gamma-irradiation, we studied radiation-induced DSBs during the G(2)/M phase of the cell cycle. We show that 1) non-irradiated neonatal germ cells express gamma-H2AX with variable patterns at metaphase, 2) gamma-irradiation induces foci whose number increases in a dose-dependent manner, 3) some foci correspond to visible chromatid breaks or exchanges, 4) sticky chromosomes characterizing cell radiation exposure during mitosis are a consequence of DSBs, and 5) gamma-H2AX remains localized at the sites of the lesions even after end-joining has taken place. This suggests that completion of DSB repair does not necessarily imply disappearance of gamma-H2AX.

Molecular structure of double-minute chromosomes bearing amplified copies of the epidermal growth factor receptor gene in gliomas

Amplification of the epidermal growth factor receptor gene on double minutes is recurrently observed in cells of advanced gliomas, but the structure of these extrachromosomal circular DNA molecules and the mechanisms responsible for their formation are still poorly understood. By using quantitative PCR and chromosome walking, we investigated the genetic content and the organization of the repeats in the double minutes of seven gliomas. It was established that all of the amplicons of a given tumor derive from a single founding extrachromosomal DNA molecule. In each of these gliomas, the founding molecule was generated by a simple event that circularizes a chromosome fragment overlapping the epidermal growth factor receptor gene. In all cases, the fusion of the two ends of this initial amplicon resulted from microhomology-based nonhomologous end-joining. Furthermore, the corresponding chromosomal loci were not rearranged, which strongly suggests that a postreplicative event was responsible for the formation of each of these initial amplicons.

DNA methylation in mouse gametogenesis

DNA methylation is involved in many biological processes and is particularly important for both development and germ cell differentiation. Several waves of demethylation and de novo methylation occur during both male and female germ line development. This has been found at both the gene and all genome levels, but there is no demonstrated correlation between them. During the postnatal germ line development of spermatogenesis, we found very complex and drastic DNA methylation changes that we could correlate with chromatin structure changes. Thus, detailed studies focused on localization and expression pattern of the chromatin proteins involved in both DNA methylation, histone tails modification, condensin and cohesin complex formation, should help to gain insights into the mechanisms at the origin of the deep changes occurring during this particular period.

Characterization of a new cytogenetic subtype of ductal breast carcinomas

About 50% of ductal breast carcinomas do not yield analysable karyotypes after short-term culturing. Comparison of the cytogenetic subset to the whole data set of tumors revealed that slightly hyperdiploid tumors, that is, with DNA index between 1.05 and 1.3, were under-represented in tumors for which cytogenetic analysis was successful. The purpose of this study was to determine whether the pattern of chromosome imbalances in this subset differs from that generally reported. Comparative genomic hybridization (CGH) was used on 43 primary ductal breast carcinomas selected for slight hyperdiploidy. Microsatellite instability (MSI), TP53 mutation and expression were also investigated. All tumors were MSI negative. In all, 18 tumors (42%) presented mostly unbalanced chromosome rearrangements and DNA amplifications, with only few or no whole chromosome gains (WCG). This pattern of chromosome imbalances corresponds to that described in most breast tumors by previous cytogenetic and CGH analyses. It was associated with TP53 mutation in 17% of tumors. Another subset of 17 tumors (39%) displayed different and new features, characterized by recurrent gains of whole chromosomes 5, 7 and 8 with few chromosome rearrangements, rare DNA amplifications and no TP53 mutation. Eight tumors with as many rearrangements as WCG were left unclassified. We propose that, beside a major pathway characterized by multiple chromosome rearrangements, there is a minor pathway mainly characterized by WCG.

Comparative genomic hybridization analysis of hereditary swine cutaneous melanoma revealed loss of the swine 13q36-49 chromosomal region in the nodular melanoma subtype

Genetic alterations implicated in malignant melanoma are still poorly understood. Malignant melanomas present highly variable histologic and cytologic patterns. The aim of the present study is to define genomic imbalances associated with the development of 2 histologic types of swine hereditary cutaneous melanoma. We have investigated 11 swine tumors by comparative genomic hybridization (CGH), 4 superficial spreading melanomas (SSMs) and 7 nodular melanomas (NMs). Following laser capture microdissection and degenerate oligonucleotide primed-polymerase chain reaction, we were able to isolate and then amplify DNA from the 2 histologic subtypes. Consensus regions of chromosome gains were identified on both histologic subtypes, on swine chromosomes 3p13-p17 (75% of the SSMs and 71% of the NMs), 12q (100% of the SSMs and 57% of the NMs) and 14q11-q21 (75% of the SSMs and 42% of NMs). Chromosomal loss was restricted to NM lesions and the swine 13q36-49 region was lost in 100% of the NMs. Interphase fluorescence in situ hybridization with a probe mapping to the 13q41-q42 region indicates loss of the corresponding region on NM lesions. Taking into account this CGH analysis and the comparative genomic data between swine and human genomes, we suggest that a role for the human chromosomes 3p11-qter and chromosome 21 losses should be investigated in human nodular melanoma progression.

Homophilic anchorage of brain-hexokinase to mitochondria-porins revealed by specific-peptide antibody cross recognition

In brain tumors the main source of energy is from glycolysis, which is initiated by hexokinase 1 (HK1), an enzyme bound to the mitochondrial porin. Disruption of HK binding greatly affects tumor cell survival. Little is known about the acceptor site of HK1. Therefore, a polyclonal antibody (Pab) directed to MIAAQLLAYYFTELK (MK) peptide, corresponding to the 15-amino acids of the N-terminal sequence of brain HK1 was obtained. Anti MK antibody (aMK-Pab)bound specifically to HK as shown by ELISA. The aMK-Pab binding to MK peptide was antibody-concentration dependent and was completely abolished by its preincubation with the peptide at 6 x 10-8 M. The aMK-Pab recognized cytosolic HK (cHK) and HK solubilized (sHK)from rat-brain mitochondrial preparations, but not the yeast HK which does not have the MK sequence. An anti-brain HK Pab (aHK-Pab) directed to purified HK recognized the MK peptide; aHK-Pab bound to MK and this binding was inhibited by preincubation of the antibody with the MK peptide. It was previously demonstrated that brain HK anchors to mitochondria porins, also designated as voltage dependent-anion channels (VDAC) via the MK sequence. A specific anti-VDAC antibody (aVDAC-Pab) which specifically bound the N and C-terminal sequences of VDACwas found to bind to c-HK, sHK and MK-coated wells and this binding was abolished by aVDACPabpreincubation with MK peptide. These data suggest that the three Pabs cross-react with an epitope present in HK and VDAC, and which was presented in the MK peptide. Comparison of alignment of HK or VDAC sequences, available in the protein data bank (PDB), did not allow putative homologues responsible for the cross-reaction to be identified, suggesting that the epitope is conformational. This, added to inhibition of mitochondria-isolated HK binding by the MK peptide,suggests that there is an homophilic-type interaction between HK and porin, through a peptidic structure represented at least in part in the MK peptide.

Reconstruction of the ancestral karyotype of eutherian mammals

Applying the parsimony principle, i.e. that chromosomes identical in species belonging to different taxa were likely to be present in their common ancestor, the ancestral karyotype of eutherian mammals (about 100 million years old) was tentatively reconstructed. Comparing chromosome banding with all ZOO-FISH data from literature or studied by us, this reconstruction can be proposed with only limited uncertainties. This karyotype comprised 50 chromosomes of which 40-42 were acrocentrics. Ten ancestral pairs of chromosomes were homologous to a single human chromosome: 5, 6, 9, 11, 13, 17, 18, 20, X and Y (human nomenclature). Nine others were homologous to a part of a human chromosome: 1p + q (proximal), 1q, 2p + q (proximal), 2q, part of 7, 8q, 10p, 10q and 19p (human nomenclature). Finally, seven pairs of chromosomes, homologs to human chromosomes 3 + 21, 4 + 8p, part of 7 + 16p, part of 12 + part of 22 (twice), 14 + 15, 16q + 19q, formed syntenies disrupted in man.

Highly conserved chromosomes in an Asian squirrel (Menetes berdmorei, Rodentia: Sciuridae) as demonstrated by ZOO-FISH with human probes

The chromosomes of Menetes berdmorei (Rodentia, Sciuridae, Sciurinae) were studied by ZOO-FISH using whole human chromosome probes. All homoeologies between M. berdmorei and human chromosomes were determined, except for two small chromosome segments. Twelve human chromosomes are conserved in a unique block of synteny; ten are split into two and one into three blocks. Thus, a small number of interchromosomal rearrangements, about twenty, separates human from this squirrel karyotype. Homoeologies between human and the presumed ancestral chromosomes of Sciurinae could also be deduced, as well as those with the presumed ancestral chromosomes of eutherian mammals. Sciurinae chromosomes appear to be much closer to those of non-rodent mammals than those of Muridae and Cricetidae species studied so far. Thus, they provide an interesting tool to link the rodent genome to those of other mammals.

Potential rapid assessment of breast cancer prognosis using induced chromosome condensation performed on cytological specimens

We have previously shown that assessment of chromosome alteration rate by cytogenetics is well correlated with breast cancer prognosis factors. As karyotypes are usually difficult to obtain from solid tumors using conventional methods, a new approach is proposed. Metaphase-like chromosomes were directly obtained following chromosome condensation using calyculin A (okadaic acid) from cytologic specimens of breast cancers sampled by fine needle. Chromosome counts and rearrangement rates were established in a series of 45 tumors, as early as 24-48 h after sampling. A high rate of rearranged chromosomes was found to correlate with high histological grade, TNM stage and S-phase fraction, loss of estrogen receptor expression and DNA aneuploidy. The indication of genome alteration provided by this method constitutes a simple, potent and early potential prognostic factor in breast cancer directly applied on cytological specimens.

Overall DNA methylation and chromatin structure of normal and abnormal X chromosomes

DNA methylation patterns were studied at the chromosome level in normal and abnormal X chromosomes using an anti-5-methylcytosine antibody. In man, except for the late-replicating X of female cells, the labeled chromosome structures correspond to R- and T-bands and heterochromatin. Depending on the cell type, the species, and cell culture conditions, the late-replicating X in female cells appears to be more or less undermethylated. Under normal conditions, the only structures that remain methylated on the X chromosomes correspond to pseudoautosomal regions, which harbor active genes. Thus, active genes are usually hypomethylated but are located in methylated chromatin. Structural rearrangements of the X chromosome, such as t(X;X)(pter;pter), induce a Turner syndrome-like phenotype that is inconsistent with the resulting triple-X constitution. This suggests a position effect controlling gene inactivation. The derivative chromosomes are always late replicating, and their duplicated short arms, which harbor pseudoautosomal regions, replicate later than the normal late-replicating X chromosomes. The compaction or condensation of this segment is unusual, with a halo of chromatin surrounding a hypocondensed chromosome core. The chromosome core is hypomethylated, but the surrounding chromatin is slightly labeled. Thus, unusual DNA methylation and chromatin condensation are associated with the observed position effect. This strengthens the hypothesis that DNA methylation at the chromosome level is associated with both chromatin structure and gene expression.

'Side Population' cells in adult mouse testis express Bcrp1 gene and are enriched in spermatogonia and germinal stem cells

Stem cells in various somatic tissues (bone marrow, skeletal muscle) can be identified by the 'Side Population' marker based on Hoechst 33342 efflux. We show that mouse testicular cells also display a 'Side Population' that express Bcrp1 mRNA, the ABC transporter responsible for Hoechst efflux in hematopoietic cells. Inhibition of Hoechst efflux by specific BCRP1 inhibitor Ko143 show that germinal 'Side Population' phenotype is dependent on BCRP1 activity. Analysis of two well-defined models of altered spermatogenesis (W/Wv mutants and cryptorchid male mice) and RNA expression studies of differentiation markers demonstrate that germinal 'Side Population' contains spermatogonial cells. In addition, alpha 6-integrin and Stra8 germinal stem cell markers, are expressed in the 'Side Population'. In vivo repopulation assay clearly establishes that testis 'Side Population' in adult mice is highly enriched in male germ stem cells.

Phase II study of lonidamine and diazepam in the treatment of recurrent glioblastoma multiforme

Recurrent glioblastoma multiforme (GBM) is resistant to most therapeutic endeavours, with low response rates and survival rarely exceeding 6 months. There are no standard chemotherapeutic regimens and new therapeutic approaches have to be found. We report an open-label, uncontrolled, multicentre phase II trial of lonidamine (LND) and diazepam in 16 patients with GBM at first relapse and a Karnofsky performance status > or = 70. The treatment regimen consisted of LND 450 mg/day and diazepam 15 mg/day orally of every 28-day cycle until progression or unacceptable toxicity. Patients received a median of three cycles (range, 1-12). No complete or partial response was observed. Therefore, according to the design of the study, no additional patients were enrolled and the trial was closed. Nevertheless, seven stabilizations (50%) were observed. Median time to progression was 8 weeks (range, 5-19 weeks). Median overall survival from recurrence was 15 weeks (range, 14-61 weeks). No grade 3-4 toxicity, except somnolence, was observed and there were no therapy-related deaths. Dose reduction for diazepam due to somnolence (grade III) was performed in 9 patients. The combination of LND and diazepam is well tolerated. LND and diazepam, acting on two distinct mitochondrial sites involved in cellular energy metabolism, may exert a cytostatic effect on tumour growth as shown by the high percentage of stable patients. The LND-diazepam at the used dosing schedule did not show a complete or partial response. LND plus diazepam may be interesting in the adjuvant setting or associated to chemotherapy to act on different targets and increase the therapeutic index.

Segmental duplications in euchromatic regions of human chromosome 5: a source of evolutionary instability and transcriptional innovation

Recent analyses of the structure of pericentromeric and subtelomeric regions have revealed that these particular regions of human chromosomes are often composed of blocks of duplicated genomic segments that have been associated with rapid evolutionary turnover among the genomes of closely related primates. In the present study, we show that euchromatic regions of human chromosome 5-5p14, 5p13, 5q13, 5q15-5q21-also display such an accumulation of segmental duplications. The structure, organization and evolution of those primate-specific sequences were studied in detail by combining in silico and comparative FISH analyses on human, chimpanzee, gorilla, orangutang, macaca, and capuchin chromosomes. Our results lend support to a two-step model of transposition duplication in the euchromatic regions, with a founder insertional event at the time of divergence between Platyrrhini and Catarrhini (25-35 million years ago) and an apparent burst of inter- and intrachromosomal duplications in the Hominidae lineage. Furthermore, phylogenetic analysis suggests that the chronology and, likely, molecular mechanisms, differ regarding the region of primary insertion-euchromatic versus pericentromeric regions. Lastly, we show that as their counterparts located near the heterochromatic region, the euchromatic segmental duplications have consistently reshaped their region of insertion during primate evolution, creating putative mosaic genes, and they are obvious candidates for causing ectopic rearrangements that have contributed to evolutionary/genomic instability.

[A new model of human prostate cancer, the PAC120 xenograft]

Prostate cancer is the second cause of cancer death in men. Often, initialy hormono-independent, escape from anti-androgen therapy is a key event of tumoral progression showing an hormone-independent phenotype. To study morphological, genetic and molecular bases associated with the hormono-dependence escape, a new model of human adenocarcinoma prostate xenograft, PAC120, was established with its hormono-dependent and independent variants. Its growth was strongly inhibited by surgical castration or by administration of the new gonadotrophin-releasing hormone antagonist, FE 200486 (Ferring, San Diego, CA). Evolution to hormono-independence was frequently associated with a mucoid differentiation or a neuroendocrine-like pattern, with the apparition of new chromosomic alterations and variations of human gene expressions. PAC120 xenograft is a new model of hormone-dependent prostate cancer, opening the opportunity to study the hormone dependence escape mechanism and to evaluate the efficacity of new therapeutics.

Rapid prenatal diagnosis of Down syndrome using quantitative fluorescence in situ hybridization on interphase nuclei

OBJECTIVES

Presently, conventional cytogenetic analysis of metaphase chromosomes remains the reference approach in prenatal diagnosis. However, this method is labor-intensive and time-consuming. The first step toward the rapid identification of aneuploidies is achieved by interphase fluorescence in situ hybridization (FISH) with centromeric or locus-specific probes. Spot counting using this type of probes is a reliable approach, but is very time-consuming with some technical and biological limitations. In this study, we present a new FISH method using image cytometry for the detection of trisomy 21 within interphase nuclei.

METHODS

The method is based on a comparative quantitation of the fluorescence signals emitted by whole chromosome 21 and 22 painting probes cohybridized on interphase nuclei. The chromosomal imbalance was determined with an automated image cytometer by detecting an abnormal ratio of both fluorescence emissions when compared with the ratio obtained in normal cells.

RESULTS

Ten blood samples and twenty amniotic fluids were analyzed. Results from FISH and standard cytogenetics were compared and 100% correlation was achieved.

CONCLUSIONS

This method, which enables an easy detection of chromosomal imbalances without a need for metaphase preparations, can be applied to the diagnosis of trisomy 21 and extended to other disorders with chromosomal imbalances. Compared to other interphase FISH techniques, it avoids spot-scoring difficulties.

Chromosomal evolution of the Hapalemur griseus subspecies (Malagasy Prosimian), including a new chromosomal polymorphic cytotype

A cytogenetic study has been performed on Hapalemur griseus caught in different locations of the eastern and northern forests of Madagascar. This allowed the determination of the more precise distribution areas of the different subspecies and the isolation of a new cytogenetic polymorphic subspecies of H. griseus. The chromosomal changes distinguishing the different subspecies are Robertsonian translocations and gain of heterochromatin. The phylogeny established on chromosome comparison is compatible with the geographic distribution of the subspecies.

Unusual chromosome cleavage dynamic in rodent neonatal germ cells

At the metaphase/anaphase transition in the mouse and rat male germ lines during the perinatal period, sister centromeres separate before sister chromatids. This gives the chromosomes an unusual appearance that resembles the premature centromere division described in some human pathological conditions such as Roberts syndrome. At the same period, there is also an unusual pattern of DNA methylation, with strongly demethylated heterochromatin and methylated euchromatin. This suggests that chromosome DNA methylation may modulate chromatid and centromere splitting, without altering normal chromosome segregation.

Induction of multiple double-strand breaks within an hsr by meganucleaseI-SceI expression or fragile site activation leads to formation of double minutes and other chromosomal rearrangements

Gene amplification is frequently associated with tumor progression, hence, understanding the underlying mechanisms is important. The study of in vitro model systems indicated that different initial mechanisms accumulate amplified copies within the chromosomes (hsr) or on extra-chromosomal elements (dmin). It has long been suggested that formation of dmin could also occur following hsr breakdown. In order to check this hypothesis, we developed an approach based on the properties of the I-SceI meganuclease, which induces targeted DNA double-strand breaks. A clone containing an I-SceI site, integrated by chance close to an endogenous dhfr gene locus, was used to select for methotrexate resistant mutants. We recovered clones in which the I-SceI site was passively co-amplified with the dhfr gene within the same hsr. We show that I-SceI-induced hsr breakdown leads to the formation of dmin and creates different types of chromosomal rearrangements, including inversions. This demonstrates, for the first time, a direct relationship between double-strand breaks and inversions. Finally, we show that activation of fragile sites by aphidicolin or hypoxia in hsr-containing cells also generates dmin and a variety of chromosomal rearrangements. This may constitute a valuable model to study the consequences of breaks induced in hsr of cancer cells in vivo.

Study of the gonocyte cell cycle in irradiated TP53 knockout mouse foetuses and newborns

PURPOSE

To investigate the role of p53 in gonocyte cell-cycle arrest in rodents with or without radiation exposure.

MATERIALS AND METHODS

Pregnant p53 (+/-) mice, mated with p53 (-/-) males, were exposed to (137)Cs gamma-rays at day 18 postcoitum (p.c.) at doses ranging from 1.5 Gy to 3 Gy. The gonocyte cell cycle was studied in p53 (-/-) male foetuses and newborns after BrdU incorporation and DAPI staining, and compared with those of p53 wild-type animals.

RESULTS

The proliferation of gonocytes in wild-type mice is normally arrested between day 16 p.c. and birth, a period when p53 is strongly expressed in gonocytes; p53 high expression is prolonged in all gonocytes after 3 Gy irradiation. In p53 (-/-) mice, this period of gonocyte cell-cycle arrest is not modified, compared with wild-type mice. It is also prolonged after a 3 Gy exposure.

CONCLUSION

Two hypotheses are proposed. Either p53 is not involved in the control of gonocyte cell-cycle arrest in control and irradiated mice, or its role is redundant in this process.

Quantitative FISH analysis on interphase nuclei may improve diagnosis of DNA diploid breast cancers

The detection of DNA aneuploid cells using flow cytometry is an indication for the presence of tumor cells, but when DNA diploid cells are found in 25-33% of the cases, the diagnostic and prognostic significance of DNA ploidy is more limited. We analyzed interphase nuclei after in situ hybridization and using image cytometry on 50 breast tumors with diploid DNA content to investigate whether early chromosome rearrangements were detectable and if their occurrence was clinically significant. Imbalances between the two arms of chromosome 1 were found in 55% of the cases and values ranged from 1.5-3.0. Comparison with histological data showed that Grade I tumors mainly have imbalances (67%) and that Grade III tumors were mainly without the imbalance (67%), whereas Grade II tumors were intermediate (50% imbalance). These data suggest that the diagnosis of DNA diploid cases may be improved by using interphase FISH. In addition, the data also indicates that early breast tumors may have different genetic origins, which is important in the comprehension of tumor malignancy in early stages, especially for preinvasive lesions.

Comparative karyotype using bidirectional chromosome painting: how and why?

Rat is widely used in biomedical and pharmaceutical research but its genome has been significantly less studied than that of the mouse. This represents a major limitation for studying cytogenetic and molecular mechanisms in the rat model. As Muridae species underwent an intense chromosome evolution it is not possible to directly transpose knowledge of the mouse genome to that of the rat. For establishing a comparative karyotype between rat and mouse, painting probes of both species were prepared by PARM-PCR (Priming Authorizing Random Mismatches PCR) from a low copy number of sorted chromosomes, the mouse and rat specific painting probes being then hybridized on rat and mouse metaphases, respectively. The availability of rodent species chromosome painting probes as well as the information obtained by the comparative karyotype and comparative gene mapping data are of great interest to improve knowledge on species evolution but also to better understand carcinogenesis process, as illustrated by our data concerning the cytogenetic characterization of radon-induced rat lung tumors. Detailed methods for obtaining painting probes by PARM-PCR from sorted mouse and rat chromosomes and for their hybridization in homologous or heterologous conditions are described. Usefulness of chromosome painting is illustrated by the characterization of chromosomal abnormalities in a radon-induced rat lung tumor. Advantages and limitations of this technique as compared to classical cytogenetics, FISH and CGH are discussed.

Complex relationships between 5-aza-dC induced DNA demethylation and chromosome compaction at mitosis

A variety of treatments with 5-azadeoxycytidine (5-aza-dC) were applied to cultured human lymphocytes during one to four cell cycles. The effect of 5-aza-dC on DNA methylation was studied by using an antibody against 5-methylcytosine on mitotic chromosomes. 5-Azadeoxycytidine is known to induce strong and permanent demethylation of DNA. Unexpectedly complex relationships were observed between DNA methylation status and chromatid/chromosome compaction. The most dramatic alteration of compaction at mitosis was observed when pre-replicative chromosomes had unifilarly demethylated DNA. The compaction of chromosomes was found to depend only partially on the methylation of their DNA at the time of mitosis. Our results suggest that alteration of DNA methylation prevents the synchronization of chromatin compaction, inducing premature (or delayed) chromosome condensation, and that a crucial step is the DNA methylation status of the pre-replicative chromosome.

Recurrent allelic deletions at mouse chromosomes 4 and 14 in Myc-induced liver tumors

Transgenic mice expressing the c-Myc oncogene driven by woodchuck hepatitis virus (WHV) regulatory sequences develop hepatocellular carcinoma with a high frequency. To investigate genetic lesions that cooperate with Myc in liver carcinogenesis, we conducted a genome-wide scan for loss of heterozygosity (LOH) and mutational analysis of beta-catenin in 37 hepatocellular adenomas and carcinomas from C57BL/6 x castaneus F1 transgenic mice. In a subset of these tumors, chromosome imbalances were examined by comparative genomic hybridization (CGH). Allelotyping with 99 microsatellite markers spanning all autosomes revealed allelic imbalances at one or more chromosomes in 83.8% of cases. The overall fractional allelic loss was rather low, with a mean index of 0.066. However, significant LOH rates involved chromosomes 4 (21.6% of tumors), 14, 9 and 1 (11 to 16%). Interstitial LOH on chromosome 4 was mapped at band C4-C7 that contains the INK4a/ARF and INK4b loci, and on chromosome 14 at band B-D including the RB locus. In man, the homologous chromosomal regions 9p21, 13q14 and 8p21-23 are frequently deleted in liver cancer. LOH at chromosomes 1 and 14, and beta-catenin mutations (12.5% of cases) were seen only in HCCs. All tumors examined were found to be aneuploid. CGH analysis of 10 representative cases revealed recurrent gains at chromosomes 16 and 19, but losses or deletions involving mostly chromosomes 4 and 14 generally prevailed over gains. Thus, Myc activation in the liver might select for inactivation of tumor suppressor genes on regions of chromosomes 4 and 14 in a context of low genomic instability. Myc transgenic mice provide a useful model for better defining crosstalks between oncogene and tumor suppressor pathways in liver tumorigenesis.

[Update in radiation-induced neoplasms: genetic studies]

Radiation induced tumors are a possible (very) late complications of radiotherapy. The evaluation of the risks of radiation-induced tumors has been presented in different epidemiological studies, with the evaluation of the relative risk for different tissues. But, the genetic studies are rare, and no global theory exists. Two cytogenetic profiles are described, one with translocations and one with genetic material losses, evoking two different genetic evolutions. Two questions are stated. What are the radiation-induced genetic mechanisms? Is it possible to differentiate the radiation-induced and spontaneous tumors with genetic approaches? With 37 cytogenetic cases, 12 analyzed in our laboratory, the radiation-induced tumors were characterized by genetic material losses. An anti-oncogenic evolution is probable. A new molecularly study confirm these results. Only thyroid tumors do not have this evolution. For tumors with simple karyotype, like meningioma, radiation-induced tumors seem to be more complex than spontaneous tumors. But for the others, the differentiation is impossible to be done with cytogenetic. The mechanism of the chromosomic material losses in unknown, but some hypothesis are discussed.