Prenatal diagnosis of trisomy 21 by i(21q): a rare case of fetoplacental chromosomal discrepancy.

OBJECTIVE: A study was conducted to explain the mechanism of an unusual discrepancy between short- and long-term culture examination methods of chorionic villus sampling (CVS). METHOD: In a 29-year-old Caucasian woman, transabdominal CVS was carried out at 12 weeks of gestation. Non-mosaic karyotype 46,XX,i(21q) was found on long-term CVS culture but number and morphology of chromosomes were normal on short-term culture, amniocyte culture, hygroma colli fluid and fetal fibroblast. RESULTS: Chromosomal aberration probably appeared after the trophoblast cell line differentiation, four days after fertilization, by means of a 21 centromere misdivision and formation of a i(21q) with secondary positive selection of the 46,XX,i(21q) cell line and loss of the 46,XX in the fetus. CONCLUSION: The restricted number of cases with this type of discrepancy limits the possibility of drawing generalised conclusions. In case of discrepancy, we recommend confirmation by amniocentesis or by fetal blood combined with sonographic examination to provide a more definitive diagnosis.

Cartographic study: breakpoints in 1574 families carrying human reciprocal translocations.

Reciprocal translocations (rcp) are among the most common constitutional chromosomal aberrations in man. Using a European database of 1574 families carrying autosomal rcp, a cartographic study was done on the breakpoints involved. The breakpoints are non-randomly distributed along the different chromosomes, indicating "hot spots". Breakpoints of rcp that result in descendants that are unbalanced chromosomally at birth are more frequent in a distal position on chromosomal arms, and 65% of them are localised in R-bands. Among the R-bands, bands rich in GC islands and poor in Alu repetitive sequences are more frequently the site of breakpoints, as well as bands that include a fragile site. This result suggests that the variation in degree of methylation in GC islands could be involved in chromosomal breakage and hence in chromosomal rearrangements. The heterogeneity of the human chromosomal structure has been demonstrable by metaphase banding techniques since 1970. In contrast to G-bands, R-bands are sites of high gene concentration (Korenberg et al. 1978), are relatively rich in cytosine plus guanine (GC), and in Alu repetitive DNA sequences (Korenberg and Rykowski 1988). More recently Holmquist (1992) has proposed four types of R-bands, depending on their relative richness in GC and Alu DNA sequences. R-bands rich in GC correspond almost exactly to T-bands (Dutrillaux 1977). They contain 65% of all genes while they represent only 15% of the genome (Holmquist 1992). The aim of this study is to analyse the distribution of the breakpoints along chromosomes from a European database of autosomal rcp in order to relate it to the specificity of different chromosomal regions.

[Human familial autosomal reciprocal translocations]. / Les translocations réciproques autosomiques familiales humaines.

Reciprocal translocations are one of the most frequently observed structural chromosome abnormalities. They are defined by a segment exchange between two non-homologous chromosomes. A great number of different translocations exist since any chromosome can be involved in the translocation and the position of the breakpoint can vary. Though generally silent these translocations can be expressed in the form of reproduction failure or, more seriously, as offspring showing mental retardation/malformation syndromes. Since the risk of malformation varies from one translocation to the next, genetic counselling and prenatal diagnosis strategies should be adopted to suit the particular malformation risks of each individual translocation. This is currently not the case. Different prediction methods (for the most probable mode of unbalance at birth, the risk of unbalance at term) are presented. A computer system, called Reci-Conseil brings these different functionalities together to create a new aid for genetic counselling. The data base on which it is founded (approx 2000 families) offers interesting perspectives for genomic mapping of partial trisomies and monosomies.