Specific terminal DNA replication sequence of X chromosomes in different tissues of a live-born triploid infant

Using the thymidine pulse method, DNA replication kinetics were studied on cells derived from cartilage, gonad, lymphocytes, and skin of a live-born triploid (69,XXY) infant with typical clinical findings. Replication studies showed that 3% of the lymphocytes had one early and one late replicating X, and 97% of the lymphocytes, and cartilage, gonad, and skin cells had two early replicating X's. Asynchronous DNA replication between the two early replicating X's was observed in all tissues (range 25-40%). The predominant terminal replication sequence of X chromosomes from chondrocytes, gonad, and skin fibroblast differed from that of the lymphocytes. Thus, a tissue-specific DNA replication pattern of the early-replicating X chromosome may be present. In every tissue, the last band to complete DNA replication was Xq21. Polymorphisms of metaphase chromosomes of parents and the patient were studied by Q-banding. The possible origin of the extra haploid set of chromosomes is discussed.