19. Allocyclic X Chromosome Visualized by Drug-Induced Premature Chromosome Condensation
Reiko Kanda, Yoko Yamagishi and Isamu Hayata
Keywords: calyculin A, chromosome painting, human lymphocytes, inactive X, premature chromosome condensation (PCC)
Okadaic acid and calyculin A, inhibitors of serine/threonine protein phosphatases, can induce premature chromosome condensation (PCC) at any stage of the cell cycle in human peripheral lymphocytes. Using this PCC technique, the authors previously attempted to develop a new biodosimetry for absorbed radiation dose, and they examined cytogenetical reactions of lymphocytes in different stages of the cell cycle to the phosphatase inhibitors mentioned above. In the course of PCC studies, one highly condensed chromosome per nucleus was frequently observed at early to middle S phases in female cells treated with calyculin, but not in male cells (Fig. 11). When these PCC spreads were hybridized with a whole chromosome-painting probe for the X chromosome, one FISH signal was located on the highly condensed chromosome. The other signals were diffused on the extended fibers or small particles. Judging from the Giemsa stained image, the former seemed to have finished DNA replication, while the latter was in the process of replication. Since such a highly condensed chromosome has seldom been observed in male PCC spreads, it was considered an inactive X chromosome, which may be identical to the sex chromatin body ("Barr body") of oral mucosal cells.
The present finding appears to be inconsistent with earlier reports that the inactive X chromosome replicates later than autosomes and the active homolog. One simple explanation for this finding could be that the inactive X chromosome in human female lymphocytes might precede other chromosomes in DNA replication at the end of the S phase in the cell cycle. Another hypothesis could be that the culture conditions, such as calyculin treatment, had an effect on the cellular factor that keeps the inactive X chromosome in late replication resulting in precocious replication of the inactive X chromosome. Recently, it was found that histone macroH2A1was concentrated in the inactive X chromosome of female mammals. This new histone might cause preferential condensation of the inactive X chromosome on calyculin treatment.
Kanda, R. and Hayata, I.: Somat. Cell Mol. Genet. 25, 173-176, 1999.
|Fig.11.||Prematurely condensed chromosomes (PCC) induced by calyculin A in female lymphocytes at early S phase. Cells are stained with Giemsa (left) and subsequently painted with whole chromosome painting probes for the X-chromosome (right). One hybridized chromosome was condensed to form a metaphase-like chromosome. The other hybridized signal was on the diffused fibers.|
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