Molecular analysis of therapy-related acute promyelocytic leukemia following langerhans cell histiocytosis

Among Japanese and Italians, the close association was demonstrated between the treatment with etoposide for Langerhans cell histiocytosis (LCH) and the development of therapy-related acute promyelocytic leukemia (t-APL), which is characterized by a reciprocal translocation that involves the PML gene on chromosome 15 and the RAR a gene on chromosome 17. To reveal the mechanism of the tran s location, we analyzed the breakpoints of the PML and RARa genes in three patients with t-APL following etoposide therapy for LCH. Southern blot analysis of the PML gene showed similar patterns of rearrangement among them. The breakpoints of the PML gene in patients with t-APL were determined in a 170 bp fragment of intron 6 of the PML gene, much as with de novo APL. The breakpoints of the RARa gene were clustered in a 670 bp fragment of intron 2. which is 10kb in sixe. Moreover, they were only 4 bp apart in two patients. It was in contrast to de novo APL in which the breakpoints of the RARa gene were randomly distributed in intron 2. However, any topoisomerase II cleavage sites were not identified near the breakpoints. Because etoposide inhibits the normal religation of double-strand breaks in DNA, it would cause increased nonhomologous DNA recombination and leukemogenesis in vivo. Thus, the underlying mechanism of etoposide-re fated APL may be different from those of de novo APL Chromatin structure might be more important than specific consensus sequence in the distribution of breakpoints in eloposide-related APL.