Partial Xp duplication in a girl with dysmorphic features: the change in replication pattern of late-replicating dupX chromosome.

In this paper we present the case of a girl at the age of 32 months with dysmorphic features, including general muscular hypotonia, developmental delay and mental retardation. The cytogenetic analysis revealed de novo partial duplication of Xp: 46,X,dup(X)(p11.23-->p22.33: :p11.23-->p22.33). To characterize the duplication, X painting, Kallman (KAL), yeast artificial chromosomes (YACs) and bacterial artificial chromosomes (BACs) covering Xp11.23-->Xp22.33 region were used. Selective inactivation of the abnormal X chromosome using HpaII digestion of the AR gene was evident. After BrdU incorporation the abnormal X was late-replicating in all lymphocytes examined. There was one peculiar exception observed: the break-point region was consistently early replicating. The replicating pattern of this region corresponded to the active X chromosome. Methylation pattern of late replicating X chromosome was studied also using antibodies against 5-methylcytosine. The pattern corresponded to the normally inactive X chromosome, with the exception of the previously observed break-point region which revealed an early replicating pattern with strong fluorescent signal, similar to the pattern of the active X chromosome. The observed phenomenon could lead to the abnormal phenotype of the patient, with some normally inactive genes of the break-point region escaping the inactivation process. The abnormal clinical findings could also be due to tissue-dependent differences in the inactivation pattern.