PRECISE LOCALIZATION OF THE FHIT GENE TO THE COMMON FRAGILE SITE AT 3P14.2(FRA3B) AND CHARACTERIZATION OF HOMOZYGOUS DELETIONS WITHIN FRA3BTHAT AFFECT FHIT TRANSCRIPTION IN TUMOR-CELL LINES

Chromosomal or allelic losses at 3p14 are common in a variety of human tumors, including those of the lung, breast, kidney, and head and nec k. This suggests the existence of a tumor suppressor gene in this band . A promising candidate is the recently cloned FHIT gene, which spans the common fragile site, FRA3B, at 3pl4.2. We previously identified a region of fragility at 3pl4.2 (FRA3B) of >85 kb by cloning DNA flankin g pSV2nec integrations and constructed a partial genomic contig of the region. Using probes from the contig, we tested for deletions within this region in DNA from 105 human tumor cell lines, predominantly deri ved from lung cancers. We identified one gastric and four lung cancer cell lines with homozygous interstitial deletions involving the FRA3B region. The deletion in one lung cancer cell line lies entirely within our contig and is <65 kb. We have identified, cloned, and sequenced t his breakpoint junction. We have also shown that our probes lie within intron 5 of the FHIT gene and, furthermore, that exon 5 is located si milar to 1 kb from one of our probes and, thus, lies within the region of fragility. Two lines with entirely intronic deletions yield FHIT t ranscripts of normal size. In one of these, this was the sole transcri pt identified. In the other line, an FHIT transcript completely normal in sequence was accompanied by two larger abnormal transcripts. These results leave open the possibility that some homozygous deletions wit hin the FHIT gene are without phenotypic effect and result from geneti c instability of this region. However, taken together, our results pro vide evidence that breakage and rearrangement within the FRA3B fragile site sequences result in alterations of FHIT and are likely to be inv olved in carcinogenesis. (C) 1997 Wiley-Liss, Inc.