Structure and expression of the human FHIT gene in normal and tumor cells

The FHIT gene, encoded by 10 exons in a 1.1-kb transcript, encompasses approximately 1 Mb of genomic DNA, which includes the hereditary RCC t(3;8) translocation break at 3p14.2, the FRA3B common fragile region, and homozygous deletions in various cancer-derived cell lines. Because some of these genetic landmarks (e.g., the t(3;8) break between untranslated FHIT exons 3 and 4, a major fragile region that includes a viral integration site between exons 4 and 5, and cancer cell homozygous deletions in intron 5) do not necessarily affect coding exons and yet apparently affect expression of the gene product, we examined the FHIT locus and its expression in detail in more than 10 tumor-derived cell lines to clarify mechanisms underlying aberrant expression. We observed some cell lines with apparently continuous large homozygous deletions, which included one or more coding exons; cell lines with discontinuous deletions, some of which included or excluded coding exons; and cell lines that exhibited heterozygous and/or homozygous deletions, by Southern blot analysis for the presence of specific exons. Most of the cell lines that exhibited genomic alterations showed alteration of FHIT transcripts and absence or diminution of Fhit protein.