AMPLIFICATION OF CFTR EXON-9 SEQUENCES TO MULTIPLE LOCATIONS IN THE HUMAN GENOME

Cloning and characterization of the cystic fibrosis transmembrane cond uctance regulator (CFTR) gene led to the identification and isolation of cDNA and genomic sequences that cross-hybridized to the first nucle otide binding fold of CFTR, DNA sequence analysis of these clones show ed that the cross-hybridizing sequences corresponded to CFTR exon 9 an d its flanking introns, juxtapositioned with two segments of LINE1 seq uences, The CFTR sequence appeared to have been transcribed from the o pposite direction of the gene, reversely transcribed, and co-integrate d with the L1 sequences into a chromosome location distinct from that of the CFTR locus, Based on hybridization intensity and complexity of the restriction fragments, it was estimated that there were at least 1 0 copies of the ''amplified'' CFTR exon 9 sequences in the human genom e. Furthermore, when DNA segments adjacent to the insertion site were used in genomic DNA blot hybridization analysis, multiple copies were also detected. The overall similarity between these CFTR exon 9-relate d sequences suggested that they were derived from a single retrotransp osition event and subsequent sequence amplification, The amplification unit appeared to be greater than 30 kb, Physical mapping studies incl uding in situ hybridization to human metaphase chromosomes showed that multiple copies of these amplified sequences (with and without the CF TR exon 9 insertion) were dispersed throughout the genome. These findi ngs provide insight into the structure and evolution of the human geno me. (C) 1997 Academic Press.