Martine Lombard

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.

The rate of chromosome breakage is age dependent in lymphocytes of adult controls.

SummaryChromosome breaks and chromatid-type lesions from a prospective study of more than 1000 lymphocyte karyotypes from each of six controls were analysed. These lesions were more frequent in older (75 years old on average) than in younger (29 years old on average) controls, especially after 72h cultures. All controls were found to be carriers of fragile sites. The most frequent were 3p14.3 and 16q23, especially in older controls. At least one fra(X)(q27) mitosis was found in each control. Most deletions occurred after breakage in heterochromatin or in late-replicating euchromatin. As almost all radials were either “mitotic chiasmata” or triradials (branched chromosomes), it is concluded that chromatid exchanges between non-homologous segments are very rare, and indicate chromosomal instability syndrome or recent exposure to a mutagen.

Acquired chromosome rearrangements in human lymphocytes: effect of aging.

SummaryA prospective study of structural rearrangements occurring in normal lymphocytes was carried out. For each of two newborns and four young and two old adults, about 1000 metaphases from 72-h and 120 from 48-h cultures were studied. The frequency of rearrangements between bands 7p14, 7q35, 14q11.2 or 14q12 and 14qter, which is on the average about 0.003, is higher in newborns (0.0043) than in adults (0.0024). Conversely, the rearrangements involving other bands, which have a frequency of 0.025 on the average, are more frequent in old adults (f=0.038) than in young adults (f=0.025) and newborns (f=0.013). The first type of rearrangement, which occurs in utero, may correspond to immunoglobulin and related gene rearrangements. The other rearrangements seem to accumulate progressively and may reflect exposure to mutagens. It is import to discriminate these two types of rearrangements when studying the effect of low doses of mutagens.

Aneuploidy in human lymphocytes: an extensive study of eight individuals of various ages.

Data on aneuploidy from a prospective study on a large number of lymphocyte metaphases (over 1000 in 72-h and 100 in 48-h cultures) per individual from eight healthy donors of various ages are reported. Chromosome losses were dependent on culture time, being significantly more frequent in 72-h than in 48-h cultures. All donors exhibited various degrees of aneuploidy which increased with age in women. This increase resulted essentially from X chromosome losses, as previously reported. Although the rate of aneuploidy limited to autosomes was similar in newborns and in adults, the distributions of the missing autosomes were different. In the two newborns studied, autosome aneuploidy was random. In the adults, a significant inverse correlation with autosome lengths was observed. The inverse correlation between chromosome lengths and losses may be explained by selective pressure against monosomic cells in the adults.

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.

Common methylation characteristics of sex chromosomes in somatic and germ cells from mouse, lemur and human

DNA methylation of sex chromosomes was analysed using anti-5-methylcytosine antibodies on metaphase chromosomes of somatic cells from three species: human, lemur and mouse. Germ cells were also studied in male mouse. In female cells (human and mouse), the late replicating X was always the less methylated chromosome. Compared with autosomes, the methylation of both X chromosomes was always lower in fibroblasts than in lymphocytes and the difference was always greater in mouse than in human. In human, mouse and lemur male cells, the labelling of the unique X chromosome was quite similar to that of the early replicating X from female cells. Except for the heterochromatic region of the human Y chromosome, strongly methylated, the overall methylation of the Y chromosome was low. In mouse testicular cells, a variety of DNA methylation patterns was observed according to the cell type and the state of differentiation. Finally, the only structures of sex chromosomes which remain methylated in all conditions correspond to their pseudoautosomal regions.

Chromosome imbalance in endometrial adenocarcinoma

The results of karyotypic analysis by R-banding after short-term culture of eight new cases of endometrial adenocarcinomas are presented and compared to previously published data. Among a total of 25 cases reported that had a diploid or near-diploid chromosome number, 72% contained a trisomy or tetrasomy 1q, often as the only abnormality. An excess of the long arm of chromosome 1 is, therefore, shown to be the predominant feature of endometrial adenocarcinoma. Trisomies 10, 2, 7, and 12 were, in decreasing order, the most frequently associated abnormalities, but trisomy 10, found in 40% of the cases, can also exist as the only imbalance. Because breakpoints in chromosome 1 are generally centromeric, a position effect with oncogene activation seems unlikely. It is suggested that the observed chromosome imbalances are secondary and are the result of the adaptation of the cancer cell to disturbed metabolic pathways.

Inversion (14)(q12qter) or (q11.2q32.3): the most frequently acquired rearrangement in lymphocytes.

SummaryIn a large study of chromosome rearrangements occurring in human lymphocytes from normal subjects, inv (14)(q12qter) or (q11.2q32.3) is found to be the most frequent, affecting 0.15% of mitoses. The same inversion is observed in the lymphocytes of the chimpanzee, indicating the ancestry of this inversion. It is not induced by ionizing radiations, and its frequency may be increased in Fanconi anemia, but not in ataxia telangiectasia. It may represent one of the steps of the process of leukemogenesis.

Constitutional balanced translocations in patients with solid tumors.

In a sample of 329 patients with a solid tumor (colon and breast adenocarcinoma, cervical carcinoma, and meningioma), four balanced constitutional translocations were observed. Two were t(13q14q), and two were reciprocal translocations. Comparison with surveys of newborns showed a significant excess of translocations in our sample.

Chromosomal aberrations induced by low-dose γ-irradiation study of R-banded chromosomes of human lymphocytes

The effect of low-dose (0-0.5 Gy) gamma-radiations was studied on R-banded chromosomes from lymphocytes of healthy donors of various ages. In cells from newborns, an increase of chromosome damage roughly proportional to the dose was found. In lymphocytes from young adults chromosomal aberrations were not detected at doses of 0.05 and 0.1 Gy, and in lymphocytes from old adults chromosomal aberrations were not detected at doses of 0.05 and 0.1 Gy, and in lymphocytes from old adults not even at 0.2 Gy. The difficulty in detecting aberrations in lymphocytes from adults is largely due to a considerable background of chromosomal anomalies which should be borne in mind in dosimetry studies. The rate of induction largely depends on the types of rearrangements. One-break terminal deletions are efficiently induced at 0.1 and 0.2 Gy and are the best indicators of exposure at these doses. At 0.5 Gy, the frequencies of 2-break lesions, i.e., dicentrics and reciprocal translocations, increase, whereas that of deletions decreases.