A clinically normal 28-year-old woman had three conceptuses with trisomy 21 and one normal child. She showed minimal cytogenetic evidence of mosaicism: 4% of her blood cells and 6% of skin fibroblasts had trisomy 21. Also, 7% of her blood cells showed aneuploidy of the X chromosome which was associated with premature centromere division (PCD,X); 6% of fibroblasts showed trisomy 18, 10% of fibroblasts showed PCD,21, and 1% PCD,18. It is unlikely that this woman is a constitutional mosaic for trisomies X, 18, and 21, all at low levels. We suggest that she has a predisposition to irregular centromere separation and that chromosomes X, 18, and 21 are most susceptible to its action.
Six adult patients presented with clinical features of essential thrombocythaemia. Five of the patients, although Ph-positive, have maintained these features without evidence of leukaemia; in one case for 9 years. A sixth patient developed leukaemic blast crisis following a persistently high platelet count over 4 years. Her cells were Ph-negative, but hybridization of gene probes to chromosomes in situ and to leukaemic DNA showed that the abl oncogene had moved to the breakpoint cluster region (bcr) on the normal chromosome 22. This patient has the same molecular gene change as occurs in some cases of Ph-negative chronic myeloid leukaemia (CML) whose leukaemic cells likewise show no evidence of chromosomal translocation. Molecular studies are essential for the correct diagnosis of these patients. The Ph genomic lesion appears to have a range of leukaemic expression which includes thrombocythaemia as well as chronic myeloid leukaemia and acute lymphatic leukaemia.
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