Tumor suppressor genes: does FHIT fit?

What is a tumor suppressor gene? From a functional point of view, it is a gene whose product can restrict the tumorigenic and metastatic processes at a certain step. From a genetic point of view, both alleles of a tumor suppressor gene are inactivated directly through deletion, inactivating mutations, or epigenetic modifications (e.g., methylation) in neoplastic cells but not in their normal counterparts. The FHIT (fragile histidine triad) gene at chromosome 3p14 was initially identified by positional cloning in 1996 (1) and is considered a candidate tumor suppressor gene in multiple tumor types. Dinucleoside 5 8, 5--P, P-triphosphate (Ap 3A) hydrolase activity is the only known function of pFHIT, and all four of the conserved histidines in the protein are required for full activity of the enzyme(2). The discovery of the FHIT gene drew great attention in the cancer research community because loss of DNA sequences from chromosome 3p (including the 3p14 region) was recognized as one of the most frequent genetic and cytogenetic abnormalities present in a broad range of solid tumors. At least three tumor suppressor gene loci are suspected to be present on the short arm of chromosome 3. However, despite a rigorous search over the past two decades, only one tumor suppressor gene had been previously identified on chromosome 3, the VHL (von Hippel-Lindau disease) gene at 3p25, which was found to play a role in the tumorigenesis of renal cancer and hemangioblastoma(3). Excitement grew when aberrant transcripts of FHIT, which exhibited deletions of exons or insertions of intronic sequences, were observed frequently in a variety of solid tumors, including cancers of the lung, stomach, breast, cervix, and the head and neck (1,4–6). These aberrant transcripts were concordant with deletions of DNA within the gene and the lack of detectable FHIT protein (pFHIT) in the tumors or cell lines analyzed(6–8). However, soon after the gene was reported as a candidate tumor suppressor, evidence began to accumulate against the hypothesis. Genomic deletions did not always affect exons of the gene and were not always associated with a lack of detectable full-length FHIT messenger RNA. Inactivating mutations were not commonly observed in the remaining FHIT alleles in tumors and cell lines. At least some genomic alterations were not completely clonal (i.e., present in only a small fraction of the cells), even in tumor-derived cell lines(7). In some studies(9,10), altered transcripts could not be consistently observed when using one-step polymerase chain reaction (PCR) analysis instead of two-step (nested) PCR, arguing that the aberrant transcripts observed by nested PCR might even represent a PCR artifact. Despite these puzzling findings, frequent abnormalities have been observed in other studies (6,11) using one-step PCR, suggesting that at least some of the abnormalities are real. These discrepancies may be due to the preferential amplification of certain complementary DNA (cDNA) fragments under different PCR conditions. The aberrant transcripts often coexist with the wild-type, full-length transcripts in tumors and even in cell lines derived from single clones. Since partial deletions of the FHIT gene are often observed in these samples, they may affect transcription fidelity and result in varying levels of aberrant transcripts. On the other hand, because the gene contains one of the most common fragile sites in the human genome, FRA3B, the aberrant transcripts may derive from subclones that occur randomly and are not associated directly with tumor initiation or progression. The presence of this fragile site may also account for the propensity to see deletions and no point mutations of the second allele. The fact remains that most tumor cell lines and primary tumors with FHIT abnormalities do not express—or express only low levels of—FHIT protein (6,8). One could speculate that the aberrant transcripts may inhibit protein translation or trigger protein degradation. Alternatively, another as yet unidentified gene within or close to FHIT may be the critical target while FHIT is an innocent bystander. Finally, some investigators have noted that aberrant FHIT transcripts can be observed in many nontumor tissues (12,13) and that they also become more predominant in aging cells (14). These observations suggest that FHIT abnormalities are not tumor specific but represent the result of randomly occurring deletion events at a fragile site or reduced RNA-splicing fidelity in aging cells. However, loss of FHIT could be a very early clonal event in tumorigenesis. In fact, frequent loss of heterozygosity (LOH) at 3p14 is observed in oral premalignant lesions (15) and in normal-appearing bronchial epithelia of smokers (16). These findings support the notion that loss of the 3p14 region occurs early and frequently in tissues exposed to carcinogens. However, the direct role of the loss in tumorigenesis needs further investigation. In this issue of the Journal, Muller et al. (17) report a study in which they have assessed LOH at FHIT, FHIT expression pat-