MOLECULAR-CLONING OF PARTIAL CDNAS FOR RAT DNA TOPOISOMERASE-II ISOFORMS AND THEIR DIFFERENTIAL EXPRESSION IN BRAIN-DEVELOPMENT

cDNA segments for DNA topoisomerase II were amplified from rat brain RNA after reverse transcription by the polymerase chain reaction, using degenerate oligonucleotide primers deduced from the conserved regions of topoisomerase II of higher eukaryotes. The cDNA product from a successful amplification was homogeneous in length but heterogeneous in sequence. Restriction mapping of the cloned cDNA fragments revealed that they consisted of two distinct sequence groups. DNA sequencing of representative clones from each group, designated A and B, showed that they are highly homologous to cDNAs of human topoisomerase II isoforms, alpha and beta, respectively. Northern blot analysis indicated that the transcript level for rat topoisomerase IIalpha was high in embryonic brain and in the cerebellum of 2-day newborns, followed by rapid decrease to a undetectable level at 4 weeks after birth. In contrast, rat topoisomerase IIbeta transcript was present throughout the embryonic and postnatal stages. In the developing cerebellum, cells expressing topoisomerase IIalpha were confined exclusively to the outer mitotic zone of the external granular layer, whereas the transcript of topoisomerase IIbeta was detected over the entire cortical region. These results clearly indicate that the isoform alpha is expressed only in proliferating cells. The differential expression of topoisomerase II isozymes was also observed among developed tissues. Therefore, the isozymes are most likely to be involved in the following different physiological processes: topoisomerase IIalpha in cell proliferation, and topoisomerase IIbeta in some processes unrelated to cell proliferation.