Intragenic amplification and formation of extrachromosomal small circular DNA

The PIP gene is expressed in exocrine glands and, in pathologic conditions, in breast cysts and breast cancers exhibiting apocrine features. It is localized on the long arm of chromosome 7, a region frequently alterated in mammary tumors. We previously described an abnormal restriction pattern of the PIP gene in 33% of prostate carcinomas analyzed. Here, we analyze the structure of the PIP gene in primary breast carcinomas. We report that part of the 3' end, including exon3, intron C, two-thirds of exon 4 and a small portion of intron B, is amplified and involved in the formation of extrachromosomal spcDNA molecules in 3/14 (21.4%) breast cancers analyzed. The involvement of a well-defined intragenic region of a gene in the formation of spcDNA appears to be unprecedented. Since spcDNA has been suggested to serve as an enhancer of genetic instability, the PIP gene may be the target of genomic variability processes in breast cancer.