1 In no other case of autosomal inv dups two alphoid signals have been reported this seems to be due to the fact that very large duplications are unviable or that the second centromere has not been detected due to the use of staining techniques inappropriate in highlighting the centromere. In the case of the inv dup(8p), the breakage of the original dicentric chromosome 8qter → 8p23.1 ∷8p23.1 → 8qter occurs in different positions between the two centromeres with a preferential breakpoint at the level of the second centromere that became inactive (six out of 16 cases). The dicentric, due to its intrinsic instability, would undergo an asymmetric breakage between the two centromeres leading to the final inv dup. 16 It has been assumed 1, 5 that the first product of the abnormal meiotic recombination on the basis of this type of rearrangement was a dicentric chromosome, either p → q ∷q → p or q → p ∷p → q. 1, 2 Other examples are those concerning 1q, 3, 4 2q, 5 3p 6, 7 4p, 8 5p, 9, 10 9p, 11 10p and 10q, 12 18p 13 18q, 14 21q 15 and the X X or the Y Y rearrangements leading to duplications of parts of the short or the long arm with concomitant deletion. The best-known case is that of the inv dup(8p). The molecular characterization of several rearrangements interpreted as simple duplications led to the discovery that most of them were in fact inverted duplications associated with the deletion of the portion distal to the duplication. Phenotype/karyotype relationships in inv dup rearrangements must also take into account the influence of the other abnormal cell line during embryogenesis. Selection versus the most viable cell line(s) results either in a confined placental mosaicism with the inv dup cell line as the only one present in the embryo or in children with both the deleted and the inv dup cell lines. Although we had previously assumed that the dicentric underwent a breakage at meiosis II so that the zygote inherited the inv dup chromosome, our findings and those of others indeed indicate that the dicentric may be inherited in the zygote and that it might persist as such in early postzygotic stages, then undergoing different breakages in different cells leading to different abnormal chromosomes, either deleted or duplicated. Its breakage leads to the formation of a deleted and an inv dup chromosome. ![]() We had already demonstrated that the first product of the abnormal meiotic recombination at the basis of the inv dup rearrangements is a dicentric chromosome. We analysed the case of a mosaicism del(8p)/inv dup(8p) found in prenatal diagnosis. Here, we describe a second situation that might also be indissolubly linked to a mosaic condition or at least to a confined placental mosaicism. The mosaicism normal/abnormal cell lines may remain confined to placenta or persist in the embryo. ![]() The best-known situation indissolubly linked to mosaicism is the uniparental disomy where a trisomic or monosomic zygote develops at least one cell line with 46 chromosomes.
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