IDENTIFICATION OF ILLEGITIMATE RECOMBINATION HOT-SPOT OF THE RETINOICACID RECEPTOR-ALPHA GENE INVOLVED IN 15,17-CHROMOSOMAL TRANSLOCATION OF ACUTE PROMYELOCYTIC LEUKEMIA

Acute promyelocytic leukemia (APL) has been characterized by 15;17 chr omosomal translocation, which involves the retinoic acid receptor alph a (RARA) gene on chromosome 17 and the PML gene on chromosome 15. The breakpoints have been mapped to three cluster regions in the PML gene, and to RARA gene intron 2. We have examined the distribution of break points within RARA gene intron 2. An extremely restricted region (ERR) of 50 bp within RARA gene intron 2 was identified as the duster regio n of breakpoints by polymerase chain reaction and sequence analysis of DNA from APL patients. To study experimentally the mechanism involved in the translocation, ERR was tested in NIH3T3 cells by in vitro tran sfection-recombination assay, in which target sequences were placed ei ther downstream of the SV40 promoter or upstream of the neo gene. Cell s were conferred resistance to G418 only when the promoter was fused t o the neo gene by recombination of two target sequences during transfe ction. The molecular junctions were analysed in five clones, and all o f them were shown to be confined within a 20 bp region in a 148 bp DNA fragment containing ERR. This suggests that ERR might be the illegiti mate recombination hot spot in mammalian cells.