Marguerite Prieur

Molecular mapping of twenty-four features of Down syndrome on chromosome 21.

To determine which regions of chromosome 21 are involved in the pathogenesis of specific features of Down syndrome, we analysed, phenotypically and molecularly, 10 patients with partial trisomy 21. Six minimal regions for 24 features were defined by genotype-phenotype correlations. Nineteen of these features could be assigned to just 2 regions: short stature, joint hyperlaxity, hypotonia, major contribution to mental retardation and 9 anomalies of the face, hand and foot to the region D21S55, or Down syndrome chromosome region (DCR), located on q22.2 or very proximal q22.3, and spanning 0.4–3 Mb; 6 facial and dermatoglyphic anomalies to the region D21S55-MX1, including the DCR and spanning a maximum of 6 Mb on q22.2 and part of q22.3. Thus, the complex phenotype that constitutes Down syndrome may in large part simply result from the overdosage of only one or a few genes within the DCR and/or region D21S55-MX1.

Classification of Human Chromosome 21 Gene-Expression Variations in Down Syndrome: Impact on Disease Phenotypes

Down syndrome caused by chromosome 21 trisomy is the most common genetic cause of mental retardation in humans. Disruption of the phenotype is thought to be the result of gene-dosage imbalance. Variations in chromosome 21 gene expression in Down syndrome were analyzed in lymphoblastoid cells derived from patients and control individuals. Of the 359 genes and predictions displayed on a specifically designed high-content chromosome 21 microarray, one-third were expressed in lymphoblastoid cells. We performed a mixed-model analysis of variance to find genes that are differentially expressed in Down syndrome independent of sex and interindividual variations. In addition, we identified genes with variations between Down syndrome and control samples that were significantly different from the gene-dosage effect (1.5). Microarray data were validated by quantitative polymerase chain reaction. We found that 29% of the expressed chromosome 21 transcripts are overexpressed in Down syndrome and correspond to either genes or open reading frames. Among these, 22% are increased proportional to the gene-dosage effect, and 7% are amplified. The other 71% of expressed sequences are either compensated (56%, with a large proportion of predicted genes and antisense transcripts) or highly variable among individuals (15%). Thus, most of the chromosome 21 transcripts are compensated for the gene-dosage effect. Overexpressed genes are likely to be involved in the Down syndrome phenotype, in contrast to the compensated genes. Highly variable genes could account for phenotypic variations observed in patients. Finally, we show that alternative transcripts belonging to the same gene are similarly regulated in Down syndrome but sense and antisense transcripts are not.

The cell cycle of lymphocytes in Fanconi anemia

SummaryBrdU-incorporation techniques were used to study the cell cycle in 18 cases of Fanconis anemia (FA).By comparison with controls, a significant slowing of the cell cycle of lymphocytes in vitro was observed in all FA patients, and possibly in FA heterozygotes, although to a lesser degree. It is probable that the demonstration of the slowing is dependent on the culture conditions. No slowing was observed in other patients affected by at least one of the symptoms of FA. The slow cell cycle of FA cells is mostly due to a very long G2-phase. A relationship between slow cell cycle and chromatid anomalies exists, the slower cells being significantly more frequently carriers of radial figures than the faster cells, in the same patient.

Systematic analysis of 95 reciprocal translocations of autosomes.

SummaryThe statistical analysis of 95 cases of reciprocal translocations involving autosomes detected among about 10,000 patients studied with the R-banding technique gives the following information:1.An excess of break points exists for chromosome arms 4p, 9p, 10q, 21q, and 22q and a deficiency for 1p, 2p, and 6q. Furthermore, there are relatively more break points in the small arms than in the large arms, when the translocation is ascertained through an unbalanced translocation carrier. Except for chromosome 22, and ascertainment bias explain this non random distribution.2.An excess of telomeric break points exists in all cases of translocations ascertained through unbalanced carriers, and an excess of centromeric break point exists in the case of 3:1 and 1:3 segregations only. These excesses are also explained by an ascertainment bias.3.The break points are located usually at the junction of the bands (interfaces).4.The size of the chromosomal imbalance varies in the ascertainment classes. It is very large in cases ascertained through balanced carriers (at least one break point is far from the telomere), large in cases ascertained through abortion, and relatively moderate in cases ascertained through unbalanced translocation carriers (at least one break point is juxta telomeric).5.An excess of balanced reciprocal translocations exists in our sample of mentally retarded and malformed children (position effect?).6.An excess of balanced reciprocal translocations (not involving chromosome 21) exists among the trisomics 21 and their parents (interchromosomal effect?).7.A large excess of maternal transmission exists in cases of 3:1 segregation of reciprocal translocation. Deleterious effects of the reciprocal translocations are widely known, but their relation to the topologic changes of the chromatids needs further investigation. Thus, it seemed useful to analyze carefully the 95 reciprocal translocations observed among the 9183 patients studied since banding techniques became available in our laboratory.Our intention was to seek a correlation among the localization of break points, the chromosomes or segments there of involved in the rearrangements, and the types of segregation observed in the families ascertained.

Breast cancer genetic evolution: I. Data from cytogenetics and DNA content.

SummaryA general scheme of chromosome alterations occurring during tumor progression is proposed from the cytogenetic study of 113 breast carcinomas. For 76 of these tumors, chromosome numbers and rate of chromosome rearrangements were correlated with DNA content studied by flow cytometry. A series of 536 cases was used as control for flow cytometry. The following evolution can be proposed: 1. occurrence of unbalanced rearrangements decreasing chromosome number and DNA content; 2. correlatively to the rate of chromosome rearrangements, formation of endoreduplications leading to hyperploid sidelines; 3. persistence of the near diploid cells and decrease of chromosome number to about 35 and of DNA index to .85; 4. more frequently, elimination of the near diploid cells and complete passage to hyperploidy; 5. further losses of chromosomes in the hyperploid tumors, whose karyotypes can decrease to about 55 chromosomes and a DNA index of 1.35; 6. eventually, occurrence of a second endoreduplication, leading to an apparent near tetraploidy. The rate of rearranged chromosomes may reach 80% in both near diploid tumors with 35–40 and hyperploid tumors with 55–65 chromosomes which can be regarded as those with the highest degree of tumor progression. It is shown that the increase of chromosome number and DNA index above diploidy is very limited, and that all tumors with more than 50 chromosomes and 1.35 DNA content passed through endoreduplication. This results in many possible losses of heterozygosity in these cases.

Large-Scale Deletions and SMADIP1 Truncating Mutations in Syndromic Hirschsprung Disease with Involvement of Midline Structures

Hirschsprung disease (HSCR) is a common malformation of neural-crest-derived enteric neurons that is frequently associated with other congenital abnormalities. The SMADIP1 gene recently has been recognized as disease causing in some patients with 2q22 chromosomal rearrangement, resulting in syndromic HSCR with mental retardation, with microcephaly, and with facial dysmorphism. We screened 19 patients with HSCR and mental retardation and eventually identified large-scale SMADIP1 deletions or truncating mutations in 8 of 19 patients. These results allow further delineation of the spectrum of malformations ascribed to SMADIP1 haploinsufficiency, which includes frequent features such as hypospadias and agenesis of the corpus callosum. Thus, SMADIP1, which encodes a transcriptional corepressor of Smad target genes, may play a role not only in the patterning of neural-crest-derived cells and of CNS but also in the development of midline structures in humans.

Analyse des échanges de chromatides dans les cellules somatiques humaines

A new technique (BUdR treatment followed by acridine orange staining) allowing a differentiation of sister chromatids is described. A statistical analysis of 91 human karyotypes gives an estimate of the frequency of exchanges. The mean of sister chromatid exchanges is 27,3 and the minimum number is 11 per cell. — The frequency of these exchanges is proportional to the relative length of each chromosome, and the accumulation of several exchanges in some segments evokes the possibility of a “negative interference”. — The analysis of endomitoses treated with BUdR during at least two generations is not in disagreement with the model of semi-conservative replication of chromosomal DNA, but the modifications of the chromatids may result from a completely different process. — The frequency of endomitoses is increased by the treatment. These endomitoses allow a very precise analysis of the evolution of the sister chromatid exchanges, during two successive cellular generations.

Automated fluorescent genotyping detects 10% of cryptic subtelomeric rearrangements in idiopathic syndromic mental retardation

Recent studies have shown that cryptic unbalanced subtelomeric rearrangements contribute to a significant proportion of idiopathic syndromic mental retardation cases. Using a fluorescent genotyping based strategy, we found a 10% rate of cryptic subtelomeric rearrangements in a large series of 150 probands with severe idiopathic syndromic mental retardation and normal RHG-GTG banded karyotype. Fourteen children were found to carry deletions or duplications of one or more chromosome telomeres and two children had uniparental disomy. This study clearly shows that fluorescent genotyping is a sensitive and cost effective method that not only detects cryptic subtelomeric rearrangements but also provides a unique opportunity to detect uniparental disomies. We suggest giving consideration to systematic examination of subtelomeric regions in the diagnostic work up of patients with unexplained syndromic mental retardation.

Comparaison de la structure fine des chromatides d'Homo sapiens et de Pan troglodytes

Prometaphasic chromosomes of man and chimpanzee are compared, using R, Q, T and H-bands techniques. — Six pericentric inversions, one telomeric fusion-translocation, 4 intercalar deletions or insertions, 16 deletions or additions of terminal Q-bands, and an important variation of heterochromatin distinguish the 2 species. — The evolutive role of chromosomal rearrangements separating the 2 species is discussed with particular reference to the formation of human chromosome No 2.- The simultaneous analysis of human chromosomal pathology and of the chromosomal structures in primates should contribute to the understanding of accidental modifications of the genome and to the interpretation of their consequences.

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.