539 PRESUMPTIVE EVIDENCE FOR THE PRESENCE OF 2 ACTIVE X CHROMOSOMES IN A BALANCED X-AUTOSOLL TRANSLOCATION

Abstract

In human somatic cells, X chromosomes in excess of one are late replicating. In balanced X-autosome translocation, the general rule is that the normal X is late replicating. We are reporting the first such translocation in which there is no late replicating X in 1/3 of cells analyzed. A 6-year-old girl with the Beckwith-Wiedemann syndrome has a balanced, de novo, X;1 translocation (G-banded karyotype is 46,X,t(X;1) (Xpter→ Xq26::1q12→ lqter; lpter→ 1q12::Xq26→ Xqter). Using the BrdU terminal pulse method followed by acridine orange or Hoechst 33258 stain and giemsa in 2 successive lymphocyte cultures, the normal X was identified as late replicating in approximately 66% of metaphases. In the remaining 34% of cells, neither the normal X nor the Xt appeared to be late replicating. In these cells, regions of chromosomes 4 and 13, known to be late replicating, are clearly identifiable. In no case did the small Xq portion in the reciprocal translocation chromosome appear as late replicating. This finding serves as presumptive evidence for the presence of 2 active X chromosomes in a balanced X-autosome translocation. Supported in part by NIH grants # RR75 and CA 19834