DNA demethylation and tri-methylation of H3K4 at the TACSTD2 promoter are complementary players for TROP2 regulation in colorectal cancer cells

TROP2 is a powerful cancer driver in colorectal cancer cells. Divergent epigenetic regulation mechanisms for the corresponding TACSTD2 gene exist such as miRNAs or DNA methylation. However, the role of TACSTD2 promoter hypermethylation in colorectal cancer has not been investigated yet. In this study, TROP2 expression strongly correlated with promoter methylation in different colorectal tumor cell lines. Treatment with 5-Azacytidine, a DNMT1 inhibitor, led to demethylation of the TACSTD2 promoter accompanied by an increase in TROP2 protein expression. TROP2 expression correlated with promoter methylation in vivo in human colon tumor tissue, thereby verifying promoter methylation as an important factor in the regulation of TROP2 expression in colorectal cancer. When performing a ChIP-Seq analysis in HCT116 and HT29 cells, we found that TACSTD2 promoter demethylation was accompanied by tri-methylation of H3K4. In silico analysis of GSE156613 data set confirmed that a higher binding of histone mark H3K4me3 around the TACSTD2 promoter was found in TACSTD2 high expressing tumors of colon cancer patients compared to the corresponding adjacent tumor tissue. Moreover, the link between TROP2 and the H3K4me3 code was even evident in tumors showing high intratumoral heterogeneity for TROP2 staining. Our data provide novel evidence for promoter demethylation and simultaneous gains of the active histone mark H3K4me3 across CpG-rich sequences, both being complementary mechanisms in the transcriptional regulation of TACSTD2 in colon cancer. The functional consequences of TROP2 loss in colorectal cancer needs to be further investigated.

Influence of a nonfragile FHIT transgene on murine tumor susceptibility

FHIT, at a constitutively active chromosome fragile site, is often a target of chromosomal aberrations and deletion in a large fraction of human tumors. Inactivation of murine Fhit allelessignificantly increases susceptibility of mice to spontaneous and carcinogen-induced tumorigenesis. In this study, transgenic mice, carrying a human FHIT cDNA under control of the endogenous promoter, were produced to determine the effect of Fhit expression, from a nonfragile cDNA transgene outside the fragile region, on carcinogen-induced tumor susceptibility of wildtype and Fhit heterozygous mice. Mice received sufficient oral doses of N-nitrosomethybenzylamine (NMBA) to cause forestomach tumors in >80% of nontransgenic control mice. Although the level of expression of the FHIT transgene in the recombinant mouse strains was much lower than the level of endogenous Fhit expression, the tumor burden in NMBA-treated male transgenic mice was significantly reduced, while female transgenic mice were not protected. To determine if the difference in protection could be due to differences in epigenetic changes at the transgene loci in male versus female mice, we examined expression, hypermethylation and induced re-expression of FHIT transgenes in male and female mice or cells derived from them. The transgene was methylated in male and female mice and in cell lines established from male and female transgenic kidneys, the FHIT locus was both hypermethylated and deacetylated. It is likely that the FHIT transgene is more tightly silenced in female transgenic mice, leading to a lack of protection from tumor induction.

Fhit modulation of the Akt-survivin pathway in lung cancer cells: Fhit-tyrosine 114 (Y114) is essential

The Fhit tumor suppressor binds and hydrolyses diadenosine polyphosphates and the Fhit-substrate complex has been proposed as a proapoptotic effector, as determined by infection of susceptible cancer cells with adenoviruses carrying wild-type fragile histidine triad (FHIT) or catalytic site mutants. The highly conserved Fhit tyrosine 114 (Y114), within the unstructured loop C-terminal of the catalytic site, can be phosphorylated by Src family tyrosine kinases, although endogenous phospho-Fhit is rarely detected. To explore the importance of Y114 and identify Fhit-mediated signaling events, wild-type and Y114 mutant FHIT-expressing adenoviruses were introduced into two human lung cancer cell lines. Caspase-dependent apoptosis was effectively induced only by wild-type but not Y114 mutant Fhit proteins. By expression profiling of FHIT versus mutant FHIT-infected cells, we found that survivin, an Inhibitor of Apoptosis Protein (IAP) family member, was significantly decreased by wild-type Fhit. In addition, Fhit inhibited activity of Akt, a key effector in the phosphatidylinositol 3-OH kinase (PI3K) pathway; loss of endogenous Fhit expression caused increased Akt activity in vitro and in vivo, and overexpression of constitutively active Akt inhibited Fhit-induced apoptosis. The results indicate that the Fhit Y114 residue plays a critical role in Fhit-induced apoptosis, occurring through inactivation of the PI3K-Akt-survivin signal pathway.

Fhit-deficient normal and cancer cells are mitomycin C and UVC resistant

To identify functions of the fragile tumour suppressor gene, FHIT, matched pairs of Fhit-negative and -positive human cancer cell clones, and normal cell lines established from Fhit −/− and +/+ mice, were stressed and examined for differences in cell cycle kinetics and survival. A larger fraction of Fhit-negative human cancer cells and murine kidney cells survived treatment with mitomycin C or UVC light compared to matched Fhit-positive cells; ∼10-fold more colonies of Fhit-deficient cells survived high UVC doses in clonigenic assays. The human cancer cells were synchronised in G1, released into S and treated with UVC or mitomycin C. At 18 h post mitomycin C treatment ∼6-fold more Fhit-positive than -negative cells had died, and 18 h post UVC treatment 3.5-fold more Fhit-positive cells were dead. Similar results were obtained for the murine −/− cells. After low UVC doses, the rate of DNA synthesis in −/− cells decreased more rapidly and steeply than in +/+ cells, although the Atr–Chk1 pathway appeared intact in both cell types. UVC surviving Fhit −/− cells appear transformed and exhibit >5-fold increased mutation frequency. This increased mutation burden could explain the susceptibility of Fhit-deficient cells in vivo to malignant transformation.

Development of spontaneous tumours and intestinal lesions in Fhit gene knockout mice

The fragile histidine triad (FHIT) gene is frequently inactivated in various types of tumours. However, the system-wide pathology caused by FHIT inactivation has not been examined in detail. Here we demonstrate that Fhit gene knockout mice develop tumours in the lymphoid tissue, liver, uterus, testis, forestomach and small intestine, together with structural abnormalities in the small intestinal mucosa. These results suggest that Fhit plays important roles in systemic tumour suppression and in the integrity of mucosal structure of the intestines.

Common fragile genes

Common chromosome fragile sites show susceptibility to DNA damage, leading to alterations that contribute to cancer development. The cloning and characterization of fragile sites have demonstrated that fragile sites are associated with genes that relate to tumorigenesis. Identification of the basis of instability at fragile sites and the related genes provides an entree to understanding of important aspects of chromosomal instability, a prominent feature of neoplastic genomes. FHIT/FRA3B and WWOX/FRA16D, the most sensitive common fragile genes in the human genome, function as tumor suppressor genes. The common features of these two common fragile genes are summarized, and suggest clues to understanding the relation between genomic instability and tumor biology.

Cancer and the FRA3B/FHIT fragile locus: it's a HIT

The FHIT gene encompassing the most active common human chromosomal fragile region, FRA3B, was discovered in 1996 and proposed as a tumour suppressor gene for important human cancers. Seven years and more than 350 reports later, early questions concerning its tumour suppressor role have been answered. Recent studies on the role of Fhit loss in major types of human cancers report association with high proliferative and low apoptotic indices, node positivity, loss of mismatch repair protein, likelihood of progression and reduced survival.

Fhit protein expression in endometrial cancers: no correlation with histological grade

The genes specifically involved in endometrial cancers have not yet been discovered. The FHIT gene, a tumour suppressor located at 3p14.2, is altered in many human tumours, including those derived from the female genital tract. We have thus investigated the status of Fhit protein expression in endometrial carcinomas (EC), and its association with histological grade of malignancy in order to determine if Fhit expression is inactivated in EC and if so, whether it is inactivated during initiation or progression. Recent studies have reported that alteration in the FHIT locus detected by DNA and RNA analysis is well correlated with loss of Fhit protein expression in tumours. Thus, we characterised Fhit protein expression as an indication of FHIT gene status in 35 cases of EC of different histological grade (G1: 13 cases; G2: 14 cases; G3: 8 cases). In our group of cancers, Fhit protein expression was absent or reduced in 37% (13/35) of EC. The first 13 cases, judged as G1, showed Fhit deficiency in approximately 38.5% of cases (5/13). For G2 and G3 tumours these numbers were similar and accounted for approximately 35.7% (5/14) and approximately 37.5% (3/8), respectively. No statistical difference was found for Fhit expression among the various groups of tumours, which allowed us to conclude that morphological grade does not seem to be an important factor. Our results suggest that Fhit inactivation is an early event in carcinogenesis of the endometrium. As this observation is contrary to some already published reports, another independent study with larger amounts of material is necessary to determine this issue definitely.

Characterization of a familial RCC-associated t(2;3)(q33;q21) chromosome translocation

A Polish family was identified in which multifocal clear cell renal carcinoma segregated with a balanced constitutional chromosome translocation, t(2:3)(q33;q21), similar to the renal cell cancer-associated t(2;3)(q35;q21) reported in a Dutch family. Bacterial artificial chromosome (BAC) contigs encompassing the 2q and 3q breakpoints were constructed and BACs crossing the breakpoints were partially sequenced. All known regional markers, genes, and expressed sequence tags (ESTs) were mapped relative to the contigs, as well as to the breakpoint sequences. Two single ESTs mapped within the 2q breakpoint BAC, whereas the repeat-rich 3q breakpoint region was gene poor. Physical mapping suggested that the 3q break was in 3q13, possibly near the border with 3q21. Physical mapping illustrated that the 2q break was closely telomeric to the 2q31 FRA2G site, consistent with the G-band assignment. Characterization of full-length cDNAs for the ESTs near the 2q break will determine if a gene(s) is altered by this familial translocation.

Fhit protein expression in hereditary and sporadic colorectal cancers

The majority of hereditary nonpolyposis colorectal cancer (HNPCC) is caused by mutations in DNA mismatch repair genes, especially in MLH1 and MSH2. Tumours in such patients also show microsatellite instability characteristic for DNA repair defects. The FHIT gene, a candidate tumour suppressor gene located at 3p14.2 has been shown to be involved in carcinogenesis of many human tissues, including digestive tract tissues. In our study, we characterized Fhit protein expression in hereditary and sporadic colorectal cancers (CRC). Our intention was to determine if cancers with mutations in the mismatch repair genes, MSH2 and MLH1, would show more frequent inactivation of the FHIT gene. Sixteen HNPCC and 28 sporadic CRC cases were examined by standard immunohistochemical analyses. Both study groups comprised carefully and selectively chosen cases. We have observed higher frequency of loss or reduction of Fhit protein expression in hereditary CRC than in sporadic cases (44% vs. 25%). Although this difference was not statistically significant (p = 0.17), possibly due to the small number of available tumour specimens, the tendency is interesting. More extensive studies on a larger number of cases should be done in the HNPCC group to confirm statistical significance. Our results suggest that the FHIT gene plays an important role in carcinogenesis of at least one fourth of all colorectal cancers.

The DIRC1 gene at chromosome 2q33 spans a familial RCC-associated t(2;3)(q33;q21) chromosome translocation

A reciprocal, balanced, constitutional chromosome translocation, t(2;3)(q33;q21), which is associated with familial clear cell renal cancer, has been described and the genomic regions surrounding the 2q and 3q breakpoints have been characterized. Based on the genomic map of the 2q break, EST AI468595 was positioned near the 2q33 translocation and the full-length gene and cDNA were isolated. This 57-kb gene, designated the DIRC1 gene, was disrupted between exons 1 and 2 by the familial translocation. The 1.5-kb mRNA encodes an 11-kDa predicted protein of 104 amino acids. Low-level expression of DIRC1 was detected by reverse transcriptase-polymerase chain reaction amplification in adult placenta, testis, ovary, and prostate and in fetal kidney, spleen, and skeletal muscle. A GFP-Dirc1 fusion protein was expressed in vitro and a polyclonal anti-Dircl peptide serum was prepared. A panel of cancer and cancer-derived cell line DNAs was examined for DIRC1 mutations, but only a rare polymorphism was observed. Two familial tumors showed loss of the derivative 3 chromosome, as observed in a Dutch kindred with t(2;3)associated renal cancers. Mutations in the second DIRC1 allele were not detected. Further studies will be required to determine if disruption of the DIRC1 gene contributed to development of the associated familial clear cell renal cancers.

FRA3B and other common fragile sites: the weakest links

In 1979, the first chromosome alteration associated with familial cancer was reported. Five years later, a fragile site was observed in the same chromosome region. The product of the fragile histidine triad (FHIT) gene, which encompasses this fragile site, is partially or entirely lost in most human cancers, indicating that it has a tumour-suppressor function. Inactivation of only one FHIT allele compromises this suppressor function, indicating that a 'one-hit' mechanism of tumorigenesis is operative. Are genes disrupted at other fragile sites? And, are these genes also tumour suppressors?

Potential cancer therapy with the fragile histidine triad gene: review of the preclinical studies

CONTEXT

The fragile histidine triad gene (FHIT) encompasses a human common fragile site, FRA3B, that is susceptible to environmental carcinogens. Deletion and inactivation of FHIT have been seen in a number of human premalignant and malignant lesions.

OBJECTIVE

To review and evaluate preclinical studies of cancer therapy using the FHIT tumor suppressor gene and related studies involving Fhit protein expression.

DATA SOURCES

A MEDLINE search of articles published from 1996 to June 2001 was performed; article reference lists were used to retrieve additional relevant articles.

STUDY SELECTION

Immunohistochemical studies of primary tumors or relevant lesions were selected to evaluate Fhit expression in premalignant or malignant stages. Preclinical studies on antitumorigenic or therapeutic introduction of FHIT were reviewed for the effects of exogenous Fhit expression. For the immunohistochemical analyses, 26 studies were included that analyzed at least 15 cases of a single type of tumor. For precancerous lesions, 9 studies were included that analyzed at least 4 cases. For studies of FHIT introduction, 9 published studies were included.

DATA EXTRACTION

Using primary data from each of the studies, we assessed the rationale and potential contribution of FHIT cancer therapy. Data was independently abstracted by 2 authors and study quality was assessed by 2 other authors.

DATA SYNTHESIS

Overall, 60% (1162/1948 cases) of primary tumors showed absent or markedly reduced Fhit protein expression in cancer cells. Studies of preneoplastic lesions or early-stage cancer showed absence or marked reduction of Fhit protein expression in 0% to 93% of samples (overall, 31% [127/408 cases]). Preclinical studies using 26 cancer-derived cell lines from human lung, head and neck, esophageal, gastric, cervical, pancreatic, and kidney cancers, showed that reintroduction of FHIT resulted in inhibition of in vitro tumor cell growth or of in vivo tumorigenicity in 17 (57%) of 30 cell line experiments. Model systems for human preventive cancer therapy suggested that oral introduction of viral vector-mediated FHIT into Fhit-deficient mice may prevent carcinogen-induced tumor development in some cases.

CONCLUSION

These findings show that FHIT gene therapy may potentially be clinically useful for treatment of cancer and also prevention of carcinogen-induced tumor development, suggesting a rationale for further research involving FHIT introduction.

The tumor spectrum in FHIT-deficient mice

Mice carrying one inactivated Fhit allele (Fhit +/- mice) are highly susceptible to tumor induction by N-nitrosomethylbenzylamine, with 100% of Fhit +/- mice exhibiting tumors of the forestomach/squamocolumnar junction vs. 25% of Fhit +/+ controls. In the current study a single N-nitrosomethylbenzylamine dose was administered to Fhit +/+, +/-, and -/- mice to compare carcinogen susceptibility in +/- and -/- Fhit-deficient mice. At 29 weeks after treatment, 7.7% of wild-type mice had tumors. Of the Fhit -/- mice 89.5% exhibited tumors (average 3.3 tumors/mouse) of the forestomach and squamocolumnar junction; half of the -/- mice had medium (2 mm diameter) to large (>2 mm) tumors. Of the Fhit +/- mice 78% exhibited tumors (average 2.4 tumors/mouse) and 22% showed medium to large tumors. Untreated Fhit-deficient mice have been observed for up to 2 years for spontaneous tumors. Fhit +/- mice (average age 21 mo) exhibit an average of 0.94 tumors of different types; Fhit -/- mice (average age 16 mo) also showed an array of tumors (average 0.76 tumor/mouse). The similar spontaneous and induced tumor spectra observed in mice with one or both Fhit alleles inactivated suggests that Fhit may be a one-hit tumor suppressor gene in some tissues.

Fhit protein expression in oral epithelium: immunohistochemical evaluation of three antisera

BACKGROUND

A number of studies have shown that the Fhit tumour suppressor protein is abundantly expressed in normal epithelial cells of human organs and that this expression is lost or reduced in the majority of cancers arising in these epithelial tissues. A variety of antiFhit sera have been used but a systematic comparison of the different antisera has not yet been reported.

MATERIALS AND METHODS

We compared the Fhit expression pattern in the epithelium of fibrous epuli, oral lichen planus, oral epithelial dysplasia and oral squamous cell carcinomas (OSCC) using three different Fhit antisera.

RESULTS

The antigstFhit sera from two sources gave very similar results for all types of oral lesions except for lichen planus and showed that about 60% of OSCCs have lost Fhit expression.

CONCLUSION

Although different staining patterns were found for the three antisera, all three could be used for evaluation of Fhit expression in OSCC.

FHIT RNA and protein expression in oral squamous cell carcinomas

BACKGROUND

To investigate the possible role of FHIT, a possible tumour suppressor gene, in oral carcinogenesis, we examined 17 oral squamous cell carcinomas (OSCCs) for genetic alterations.

MATERIALS AND METHODS

Fresh tissue was obtained during surgery, snap-frozen in liquid nitrogen and stored at -70 degrees C. Nested PCR amplification to examine the integrity of FHIT mRNA was performed on the reverse transcribed complementary DNA obtained from the frozen normal and tumour tissue. Immunohistochemistry was done on formal in-fixed paraffin-embedded tissue protein from the same cases using a polyclonal antiserum against the full length Fhit.

RESULTS

Twelve out 17 (71%) OSCCs showed reduced or absent Fhit protein and half of the cases with reduced Fhit protein exhibited aberrant RT-PCR products.

CONCLUSION

Immunohistochemical detection of Fhit protein expression in OSCCs is the more sensitive method to determine the status of Fhit in these tumours, in agreement with previous studies of other tumour types.

Fragile histidine triad expression delays tumor development and induces apoptosis in human pancreatic cancer

The fragile histidine triad (FHIT) gene is a tumor suppressor gene that is altered by deletion in a large fraction of human tumors, including pancreatic cancer. To evaluate the potential of FHIT gene therapy, we developed recombinant adenoviral and adenoassociated viral (AAV) FHIT vectors and tested these vectors in vitro and in vivo for activity against human pancreatic cancer cells. Our data show that viral FHIT gene delivery results in apoptosis by activation of the caspase pathway. Furthermore, Fhit overexpression enhances the susceptibility of pancreatic cancer cells to exogenous inducers of apoptosis. In vivo results show that FHIT gene transfer delays tumor growth and prolongs survival in a murine model mimicking human disease.

Altered gene expression in immunogenic poorly differentiated thyroid carcinomas from RET/PTC3p53-/- mice

Cancers develop and progress via activation of oncogenes and loss of tumor suppressor genes, a progression that can be recapitulated through cross breeding mouse strains harboring genetic mutations. To define the role of RET/PTC3, p53 and Fhit in thyroid carcinogenesis, we intercrossed RET/PTC3 transgenics with p53-/- mice. This new strain, RET/PTC3p53-/-, succumb to rapidly growing and strikingly large multilobed thyroid tumors containing mixtures of both well and poorly differentiated, highly proliferative follicular epithelial cells. Interestingly, transplanted tumors from RET/PTC3p53-/- mice grew in SCID but not syngeneic immunocompetent mice indicating that these advanced tumors were immunogenic. RET/PTC3 protein expression was reduced to undetectable levels in tumors of older mice suggesting that the continued elevated expression of RET/PTC3 may not be necessary for tumor progression. Similarly, expression of Fhit protein was reduced in early tumors and undetected in older tumors irrespective of tumor histopathology. In contrast to RET/PTC3p53-/- mice, RET/PTC3Fhit-/- mice did not develop advanced thyroid carcinomas. These studies support a model of human thyroid cancer whereby thyroid epithelium expresses RET/PTC3 protein at early stages of tumor development, followed by the reduction of RET/PTC3 and loss of p53 function with progressive reduction of Fhit protein expression coincident with malignant progression.

Sequence conservation at human and mouse orthologous common fragile regions, FRA3B/FHIT and Fra14A2/Fhit

It has been suggested that delayed DNA replication underlies fragility at common human fragile sites, but specific sequences responsible for expression of these inducible fragile sites have not been identified. One approach to identify such cis-acting sequences within the large nonexonic regions of fragile sites would be to identify conserved functional elements within orthologous fragile sites by interspecies sequence comparison. This study describes a comparison of orthologous fragile regions, the human FRA3B/FHIT and the murine Fra14A2/Fhit locus. We sequenced over 600 kbp of the mouse Fra14A2, covering the region orthologous to the fragile epicenter of FRA3B, and determined the Fhit deletion break points in a mouse kidney cancer cell line (RENCA). The murine Fra14A2 locus, like the human FRA3B, was characterized by a high AT content. Alignment of the two sequences showed that this fragile region was stable in evolution despite its susceptibility to mitotic recombination on inhibition of DNA replication. There were also several unusual highly conserved regions (HCRs). The positions of predicted matrix attachment regions (MARs), possibly related to replication origins, were not conserved. Of known fragile region landmarks, five cancer cell break points, one viral integration site, and one aphidicolin break cluster were located within or near HCRs. Thus, comparison of orthologous fragile regions has identified highly conserved sequences with possible functional roles in maintenance of fragility.

FHIT gene therapy prevents tumor development in Fhit-deficient mice

The tumor suppressor gene FHIT spans a common fragile site and is highly susceptible to environmental carcinogens. FHIT inactivation and loss of expression is found in a large fraction of premaligant and malignant lesions. In this study, we were able to inhibit tumor development by oral gene transfer, using adenoviral or adenoassociated viral vectors expressing the human FHIT gene, in heterozygous Fhit(+/-) knockout mice, that are prone to tumor development after carcinogen exposure. We therefore suggest that FHIT gene therapy could be a novel clinical approach not only in treatment of early stages of cancer, but also in prevention of human cancer.

FHIT gene expression in human urinary bladder transitional cell carcinomas

BACKGROUND

Our purpose was to investigate the expression of FHIT (Fragile Histidine Triad) gene product in a series of 110 urinary bladder TCCs, and its eventual relationship with histological grade, clinical stage, recurrences and patients' survival.

MATERIALS AND METHODS

We performed immunohistochemistry in archival material of formalin-fixed, paraffin-embedded tissues, using the anti-Fhit antibody and the Streptavidin-biotin peroxidase method.

RESULTS

In 30 out of 110 cases (27.27%) Fhit protein was absent whereas in 32 cases (29.08%) it was abnormally expressed. In 48 cases (43.63%) Fhit protein was diffusely expressed in all tumor cells. A statistically highly significant correlation (p < 0.001) was noticed between Fhit protein absence or reduction and clinically advanced tumors. Conversely, abnormal Fhit protein expression was not associated with age, histological grade, tumor size, number of recurrences, and clinical outcome in terms of patients' survival.

CONCLUSIONS

These results confirm that FHIT gene inactivation is a late event in urinary bladder carcinogenesis. Fhit protein reduced expression or complete absence correlates with advanced clinical stage of the disease, and does not seem to correlate with tumor recurrences and patient survival.

Effect of adenoviral transduction of the fragile histidine triad gene into esophageal cancer cells

Reintroduction of a tumor suppressor gene product in cancer cells is a promising strategy for cancer gene therapy. The fragile histidine triad (FHIT) gene has been identified in a region at chromosome 3p14.2, which is deleted in many tumors, including esophageal cancer. Previous studies have shown frequent biallelic alterations of the FHIT gene in numerous tumors, and have demonstrated a tumor suppressor function of Fhit. We have studied the biological effects of adenoviral-FHIT transduction in esophageal cancer cell lines. Results showed suppression of cell growth in vitro in three of seven esophageal cancer cell lines, all seven of which showed abundant expression of the transgene. Adenoviral-FHIT expression, but not control adenoviral infections, induced caspase-dependent apoptosis in two esophageal cancer cell lines, TE14 and TE4, which express no or very little Fhit, respectively. Treatment of TE14 cells with adenoviral-FHIT vectors resulted in abrogation of tumorigenicity in nude mice. A third esophageal cancer cell line, TE12, without detectable endogenous Fhit, showed accumulation of cells at S to G2-M and a small apoptotic cell fraction after adenoviral-FHIT transduction. Thus, adenoviral-FHIT expression can inhibit the growth of esophageal cancer cells, at least in part through caspase-dependent apoptosis, suggesting that adenoviral-FHIT infection should be explored as a therapeutic strategy.

Primary cervical carcinomas show 2 common regions of deletion at 3P, 1 within the FHIT gene: evaluation of allelic imbalance at FHIT, RB1 and TP53 in relation to survival

Chromosome arm 3p is re-arranged in many tumor types, including cervical carcinomas. Putative tumor-suppressor genes on 3p have been proposed, including the FHIT gene, which maps to chromosome band 3p14.2. We have analyzed 79 primary cervical carcinomas for allelic imbalance (AI) at 17 chromosome 3 loci, including 3 within the FHIT gene. Expression of the FHIT gene was evaluated after immunohistochemistry with an antibody against the pFHIT protein. Previously determined human papillomavirus status, defined after in situ hybridization, showed type 16 or 18 in 56/77 tumors. Tumors were also analyzed for AI at loci within the RB1 (chromosome band 13q14.2) and the TP53 (17p13) genes for AI. AI was found at 1 or more 3p loci in 50/79 tumors, at frequencies ranging from 30% to 52% at the individual loci. Two smallest regions of overlapping deletion (SROs) were found, 1 including parts of the FHIT gene (SRO flanked by D3S1481 and D3S1313) and another more distal SRO between D3S32 and D3S1286. FHIT protein expression was reduced in 57/69 (83%) tumors but not associated with AI at FHIT loci (p = 0.56). AI was found in TP53 and RB1 in 18% and 29% of the samples, respectively. Relapse-free survival was associated with AI in the TP53 gene in both a univariate (p = 0.0003) and a multivariate (p = 0.004) analysis. This study confirms a high frequency of AI at chromosome arm 3p in primary cervical carcinomas. The AI results and the reduced FHIT protein staining indicate that FHIT alterations are important in cervical carcinogenesis.

Expression of Fhit protein during mouse development

Fhit protein has a putative tumor suppressor function in several types of human and experimental cancers. To assess whether Fhit is involved in fetal development we have examined the distribution of Fhit protein in the 12- through 16-day postcoitum mouse fetus and in postnatal day 0 mouse pups by immunocytochemistry. High levels of Fhit protein were observed in the endodermal derivatives, namely, bronchi, trachea, esophagus, stomach, and intestine, in the 12- to 16-day postcoitum mouse fetus and in the postnatal day 0 pup. Other tissues showed a more restricted pattern of Fhit protein expression. These results suggest that Fhit may play a role in the development of specific tissues during mouse development.

Early deregulation of the the p16ink4a-cyclin D1/cyclin-dependent kinase 4-retinoblastoma pathway in cell proliferation-driven esophageal tumorigenesis in zinc-deficient rats

The p16ink4a-cyclin D1/cyclin-dependent kinase 4 (Cdk4)-retinoblastoma (Rb) pathway has emerged as a critical target in oncogenesis. The zinc-deficient (ZD), N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal cancer model provides a tool to study cell proliferation and cell cycle control in cancer initiation. Weanling rats were fed a ZD or zinc-sufficient (ZS) diet for 5 weeks, and then given a dose of NMBA. After 14 weeks, esophageal tumor incidence was 88% in ZD rats with highly proliferative esophagi versus 0% in ZS rats. Expression of p16ink4a, cyclin D1, Cdk4, and Rb in relation to that of proliferating cell nuclear antigen was characterized in esophagi by immunohistochemistry at 0, 24, and 48 h, and 1, 3, 7, 10, and 14 weeks after NMBA treatment. As early as 24 h, proliferating cell nuclear antigen-positive focal hyperplastic lesions were detected in the suprabasal layers of ZD esophagi. At the same time, overexpression of cyclin D1, Cdk4, and Rb was found in the corresponding lesion in adjacent esophageal sections. By contrast, p16ink4a expression was reduced or absent. At all time points, p16ink4a showed reduced nuclear staining in ZD esophagi compared with that in ZS esophagi. In addition, increased expression of the hyperphosphorylated forms of Rb was detected in ZD esophagi by immunoblotting. Importantly, tumors were consistently observed in ZD esophagi at very early time points. These data, obtained using a unique in vivo model for esophageal cancer with rapid tumor induction, provide strong evidence for a link between deregulation of the p16ink4a-cyclin D1/Cdk4-Rb pathway and the initiation of esophageal tumors.

Analysis of the fragile histidine triad (FHIT) gene in lobular breast cancer

The fragile histidine triad (FHIT) gene is a candidate tumour suppressor gene in breast and other cancers. We investigated deletions within the FHIT gene in lobular breast cancer and found that 16% of cases showed loss of heterozygosity (LOH) within the gene. We compared LOH within FHIT in lobular and ductal breast tumours and found a significant association between LOH at FHIT and the ductal histological type (P<0.001). To determine whether genomic alteration of the FHIT gene in lobular breast cancer leads to Fhit inactivation we have assessed the level of Fhit expression by immunohistochemical detection and determined that 27% (15 of 55) consecutive sporadic lobular tumours showed negative or reduced Fhit expression. A significant association was found between LOH at the FHIT gene and reduced Fhit expression in lobular and ductal tumours (P=0.025 and P=0.001, respectively). Thus, genetic alterations within the FHIT gene, leading to loss of Fhit protein, may play an important role in the carcinogenesis of a significant number of sporadic lobular breast cancers, even though the apparent frequency of genomic alterations within the gene is lower than in ductal breast cancer.

Differential susceptibility of renal carcinoma cell lines to tumor suppression by exogenous Fhit expression

Hemizygous deletions of the fragile histidine triad (FHIT) gene at human chromosome band 3p14.2 and down-regulation of its gene product are found in the majority of renal cell carcinomas (RCCs). Functional tumor suppressive activity of Fhit in renal cancer cells previously was observed in RCC cell line RC48, which lacks endogenous Fhit expression. To further investigate the potential role of FHIT as a tumor suppressor gene in RCC, we transfected FHIT cDNA expression constructs into RCC cell lines RCC-1 and SN12C, which show low-level expression of endogenous Fhit and reveal an intact von Hippel-Lindau (VHL) gene. Stable transfectants of both cell lines showed no alterations of cell morphology, proliferation kinetics, or cell cycle parameters in vitro. The FHIT gene transfer rate, however, was significantly lower in RCC-1 cells compared with SN12C cells, suggesting a selection against exogenous Fhit expression. In addition, in nude mouse assays, a significant delay of tumor formation was observed for FHIT-transfected RCC-1 cell lines, with outgrowing tumors demonstrating loss of Fhit expression in the majority of cells. In contrast, tumorigenicity of FHIT-transfected SN12C cell clones was not suppressed, despite stable transgene expression. In conclusion, our results demonstrate a selective tumor suppressive activity of Fhit in RCC cells in vivo and suggest that the susceptibility to suppression is not restricted to cancer cells with complete loss of Fhit expression.

Muir-Torre-like syndrome in Fhit-deficient mice

To investigate the role of the Fhit gene in carcinogen induction of neoplasia, we have inactivated one Fhit allele in mouse embryonic stem cells and produced (129/SvJ x C57BL/6J) F(1) mice with a Fhit allele inactivated (+/-). Fhit +/+ and +/- mice were treated intragastrically with nitrosomethylbenzylamine and observed for 10 wk posttreatment. A total of 25% of the +/+ mice developed adenoma or papilloma of the forestomach, whereas 100% of the +/- mice developed multiple tumors that were a mixture of adenomas, squamous papillomas, invasive carcinomas of the forestomach, as well as tumors of sebaceous glands. The visceral and sebaceous tumors, which lacked Fhit protein, were similar to those characteristic of Muir-Torre familial cancer syndrome.

Altered expression of Fhit in carcinoma and precarcinomatous lesions of the esophagus

The FHIT gene, located at chromosome 3p14.2, is a tumor suppressor gene often involved in tumors resulting from exposure to environmental carcinogens. We studied 46 pairs of esophageal primary tumors and corresponding normal squamous mucosa specimens by molecular genetic and immunohistochemical methods to investigate the role of the FHIT gene in esophageal carcinoma. In addition, we studied several different types of lesions, such as carcinoma in situ or dysplasia by immunohistochemistry. Loss of heterozygosity at or around the FHIT gene was observed in 35 (76%) primary tumors. Immunohistochemical detection of Fhit protein in the primary tumors demonstrated that 14 (30%) were positive and 32 (70%) were negative. We observed concordance between loss of Fhit protein and loss of heterozygosity and between loss of Fhit protein and RNA abnormalities. Because the FHIT/FRA3B locus is susceptible to damage by environmental carcinogens, we investigated the correlation between Fhit expression and smoking or alcohol habits. In this relatively small study, the patients who were both heavy users of tobacco and alcohol showed a significantly higher frequency of loss of Fhit expression than those who were light users. Noncarcinomatous squamous epithelium showed positive Fhit reactivity in most cases; however, five showed negative Fhit reactivity. Interestingly, all of these five patients had habits of heavy use of tobacco and alcohol. Eight of 12 carcinomas in situ, 2 of 4 severe dysplasias, 4 of 8 moderate dysplasias, and 3 of 9 mild dysplastic lesions showed negative Fhit reactivity. These findings indicated that loss of Fhit expression may be an early event in the development of human esophageal carcinoma and may occur even in normal-appearing squamous epithelium in some patients heavily exposed to environmental carcinogens.

Loss of FHIT expression in transitional cell carcinoma of the urinary bladder

Cytogenetic and loss of heterozygosity (LOH) studies demonstrated chromosome 3p deletions in transitional cell carcinoma (TCC). We recently cloned the tumor suppressor gene FHIT (fragile histidine triad) at 3p14.2, one of the most frequently deleted chromosomal regions in TCC of the bladder, and showed that it is the target of environmental carcinogens. Abnormalities at the FHIT locus have been found in tumors of the lung, breast, cervix, head and neck, stomach, pancreas, and clear cell carcinoma of the kidney. We examined six TCC derived cell lines (SW780, T24, Hs228T, CRL7930, CRL7833, and HTB9) and 30 primary TCC of the bladder for the integrity of the FHIT transcript, using reverse transcriptase-polymerase chain reaction (RT-PCR) to investigate a potential role of the FHIT gene in TCC of the bladder. In addition, we tested expression of the Fhit protein in the six TCC-derived cell lines by Western blot analysis and in 85 specimens of primary TCCs by immunohistochemistry. Three of the six cell lines (50%) did not show the wild-type FHIT transcript, and Fhit protein was not detected in four of the six cell lines (67%) tested. Fhit expression also was correlated with pathological and clinical status. A significant correlation was observed between reduced Fhit expression and advanced stage of the tumors. Overall, 26 of 30 (87%) primary TCCs showed abnormal transcripts. Fhit protein was absent or greatly reduced in 61% of the TCCs analyzed by immunohistochemistry. These results suggested that loss of Fhit expression may be as important in the development of bladder cancer as it is for other neoplasms caused by environmental carcinogens.

Fhit expression in gastric adenocarcinoma: correlation with disease stage and survival

BACKGROUND

The FHIT gene is inactivated by deletion in a large fraction of human tumors, including gastric carcinomas, and the Fhit protein has been proposed to act as a tumor suppressor in multiple tumor types. A large fraction of gastric adenocarcinomas have lost expression of the candidate tumor suppressor protein, Fhit, whereas normal gastric epithelial cells are strongly positive and Fhit loss has been found to correlate with alterations of the FHIT locus. Because the majority of gastric tumors in the current study were found to be entirely negative for Fhit protein, it is possible that alteration of the carcinogen-susceptible fragile region within the FHIT gene is an early event in gastric carcinoma, as it is in lung carcinoma.

METHODS

To determine whether the absence of Fhit protein correlates with expression of tumor markers or with clinical parameters, such as grade, stage, and survival time, the authors assessed Fhit expression using immunohistochemistry in a well characterized set of 55 gastric adenocarcinomas resected over several years, with longitudinal follow-up of patients for outcome.

RESULTS

In this set of 55 gastric cancers, the absence of Fhit protein correlated with higher tumor stage (P = 0.003) and higher histologic grade (P = 0.007). In addition, patients whose tumors had lost expression of Fhit died of disease significantly earlier than those with Fhit positive tumors (P = 0.017). The absence of Fhit expression did not correlate with the expression of any tumor markers.

CONCLUSIONS

Larger studies will be required to elucidate further the relation between tumor stage, grade, and Fhit loss and to determine whether inclusion of Fhit antiserum in immunophenotyping of gastric adenocarcinomas will be a useful indicator of post-diagnosis prognosis.

The histidine triad superfamily of nucleotide-binding proteins

Histidine triad (HIT) proteins were until recently a superfamily of proteins that shared only sequence motifs. Crystal structures of nucleotide-bound forms of histidine triad nucleotide-binding protein (Hint) demonstrated that the conserved residues in HIT proteins are responsible for their distinctive, dimeric, 10-stranded half-barrel structures that form two identical purine nucleotide-binding sites. Hint-related proteins, found in all forms of life, and fragile histidine triad (Fhit)-related proteins, found in animals and fungi, represent the two main branches of the HIT superfamily. Hint homologs are intracellular receptors for purine mononucleotides whose cellular function remains elusive. Fhit homologs bind and cleave diadenosine polyphosphates (Ap(n)A) such as ApppA and AppppA. Fhit-Ap(n)A complexes appear to function in a proapoptotic tumor suppression pathway in epithelial tissues. In invertebrates, Fhit homologs are encoded as fusion proteins with proteins related to plant and bacterial nitrilases that are candidate signaling partners in tumor suppression.

Loss or reduction of Fhit expression in renal neoplasias: correlation with histogenic class

Involvement of the 3p14.2 region of chromosome 3 in kidney cancers was suggested 20 years ago, when a reciprocal constitutional translocation, t(3;8)(p14.2;q24), was shown to segregate with bilateral clear cell renal carcinoma in 3 generations of 1 family. The FHITgene that is interrupted at 3p14.2 by the t(3;8) translocation has been isolated, characterized, and shown to be frequently altered, mainly by internal deletion, in carcinomas or cancer-derived cell lines of the lung, stomach, pancreas, esophagus, cervix, and colon. Although up to 90% of sporadic clear cell renal carcinomas, representing 70% of adult renal carcinomas, exhibit loss of FHIT alleles, FHIT gene alterations have been documented for only a few renal cell carcinoma-derived cell lines. Nevertheless, more than 50% of clear cell carcinomas were recently shown to express little or no Fhit protein, unlike the normal kidney tubule epithelium, which is uniformly strongly positive for Fhit expression. We have extended our immunohistochemical study of expression of Fhit protein to the spectrum of histopathologic subtypes of adult renal tumors. There is an apparent continuum of Fhit expression from the 100% strongly positive oncocytomas through mostly positive papillary and chromophobe to the mostly negative clear cell and sarcomatoid to the negative or predominantly negative collecting duct renal carcinomas. This pattern of diminishing Fhit expression correlates with reported frequency of 3p allele loss in renal carcinomas and may parallel the potential for aggressive behavior of tumors, as suggested by the abundant Fhit expression in the benign oncocytomas and the near absence of Fhit expression in sarcomatoid and collecting duct RCCs.

Immunohistochemical evaluation of Fhit protein expression in oral squamous cell carcinomas

The expression of Fhit (fragile histidine triad) protein in oral squamous cell carcinoma (OSCC) and adjacent oral epithelium was evaluated by immunohistochemistry on formalin-fixed paraffin-embedded blocks of 32 cases of OSCC. Rabbit polyclonal anti-GST-Fhit antiserum at 1:600 was used, after antigen enhancement in a microwave pressure cooker, in a saturated lead thiocyanate solution. This antiserum has been shown specifically to detect human Fhit by immunohistochemistry at dilutions up to 1:10,000. The Fhit protein expression was evaluated using both the intensity and extent of staining. Normal stratified squamous epithelium showed strong positivity, especially in the stratum spinosum and areas of keratinisation. Basal and parabasal cells were negative or expressed low levels of Fhit relative to the squamous epithelium. Mild and moderate epithelial dysplasia showed Fhit expression in the superficial layers, while Fhit expression was absent from severely dysplastic lesions. A reduction or loss of Fhit expression was found in 21 (66%) of the OSCC. The alterations in Fhit protein expression in OSCC, and not in normal tissues, are consistent with the proposal that Fhit inactivation plays a role in oral carcinogenesis.

Loss of FHIT expression in acute lymphoblastic leukemia

Loss of expression of the FHIT tumor suppressor gene is common in epithelial malignancies such as lung, kidney, esophageal, gastric, and cervical cancers. To assess the role of FHIT in acute leukemias, we examined 18 primary acute lymphoblastic leukemias (ALLs), 8 ALL-derived cell lines, 7 cell lines from other hematological malignancies, 14 lymphoblastoid cell lines, and 5 peripheral blood lymphocyte samples for expression of FHIT mRNA and protein by reverse transcription-PCR and Northern and Western blots. Fhit protein expression was detected in only 24% of primary ALLs and leukemia/lymphoma cell lines, but it was detected in all lymphoblastoid cell lines and peripheral blood lymphocyte samples. Interestingly, Fhit protein expression was lost in all T-cell ALLs but was lost in only half of the B-cell ALLs. Northern blotting of 7 normal lymphoblastoid cell lines and 13 of the neoplastic cell lines confirmed the results obtained by Western blotting regarding FHIT expression. The high frequency of loss of Fhit expression in ALLs suggests that inactivating alterations at the FHIT locus contribute to development of the leukemias.

Cancer-specific chromosome alterations in the constitutive fragile region FRA3B

We have sequenced 870 kilobases of the FHIT/FRA3B locus, from FHIT intron 3 to intron 7. The locus is AT rich (61.5%) and Alu poor (6. 2%), and it apparently does not harbor other genes. In a detailed analysis of the 308-kilobase region between FHIT exon 5 and the telomeric end of intron 3, a region known to encompass a human papillomavirus-16 integration site and two clusters of aphidicolin-induced chromosome 3p14.2 breakpoints, we have precisely mapped 10 deletion and translocation endpoints in cancer-derived cell lines relative to positions of specific repetitive elements, regions of high genome flexibility and aphidicolin-induced breakpoints. Conclusions are (i) that aphidicolin-induced breakpoint clusters fall close to high-flexibility sequences, suggesting that these sequences contribute directly to aphidicolin-induced fragility; (ii) that 9 of the 10 FHIT allelic deletions in cancer cell lines resulted in loss of exons, with 7 deletion endpoints near long interspersed nuclear elements or long terminal repeat elements; and (iii) that cancer-specific deletions encompass multiple high-flexibility genomic regions, suggesting that fragile breaks may occur at these regions, whereas repair of the breaks involves homologous pairing of flanking sequences with concomitant deletion of the damaged fragile sequence.

Reduced Fhit expression in sporadic and BRCA2-linked breast carcinomas

Evidence for alteration of the FHIT gene in a significant fraction of breast carcinomas has been reported, in apparent concordance with loss of heterozygosity (LOH) at chromosome region 3p14.2 in breast cancer and benign proliferative breast disease. A significantly higher frequency of LOH at the FHIT locus was reported for BRCA2-/- tumors, possibly due to misrepaired double-strand breaks at this common fragile region. To determine whether such genomic alterations lead to Fhit inactivation, we have assessed the level of Fhit expression by immunohistochemical detection in sporadic tumors and cancers occurring in BRCA2 999del5 carriers. To determine whether Fhit inactivation may have prognostic significance, we have also assessed expression of breast cancer markers and clinical features in sporadic tumors relative to Fhit expression. Of 40 consecutive sporadic breast carcinomas studied for tumor markers, 50% showed reduced Fhit expression. In these sporadic cancers, loss of Fhit expression was not correlated significantly with the presence or absence of other tumor markers. In a study of 58 sporadic and 34 BRCA2 999del5 Icelandic invasive cancers, there was a significant association of LOH at 3p14.2 with reduced expression of Fhit (P = 0.001); also the lower expression of Fhit and higher LOH at 3p14.2 in BRCA2 999del5 tumors relative to sporadic cancers was significant (P = 0.002). Thus, genetic alteration at the fragile site within the FHIT gene leads to loss of Fhit protein in a significant fraction of sporadic breast cancers and a much larger fraction of familial breast cancers with an inherited BRCA2 mutation, consistent with the idea that loss of BRCA2 function affects stability of the FHIT/FRA3B locus.

Role of FHIT in human cancer

Through investigation of hemizygous and homozygous deletions in common human cancers, including lung cancer, we have cloned and characterized a gene at chromosome region 3p14.2, FHIT, that is inactivated in epithelial tumors, particularly in tumors resulting from exposure to environmental carcinogens. In some tumors, particularly those associated with environmental carcinogens, alterations in the FHIT gene occur quite early in the development of cancer. In other cancers, Fhit inactivation seems to be a later event, possibly associated with progression to more aggressive neoplasias. Thus, detection of Fhit expression by immunohistochemistry in premalignant and malignant tissues may provide important diagnostic and prognostic information.

The FHIT gene is expressed in pancreatic ductular cells and is altered in pancreatic cancers

We examined 2 normal pancreata, 21 primary pancreatic ductal cancers, and 19 pancreatic cancer cell lines for Fhit expression and FHIT gene status. The normal pancreas expressed Fhit protein in the cytoplasm of ductular cells, whereas interlobular and larger ducts, acini, and insulae of Langerhans were negative. Fhit protein was detected by immunoblot assay in 11 pancreatic cancer cell lines; of the 8 cell lines lacking Fhit protein, 7 lacked FHIT mRNA and 1 showed an abnormally sized transcript. DNA from five of these eight cell lines showed homozygous loss of FHIT exon 5. In 8 of the 21 primary cancers, Fhit expression was detected by immunohistochemistry. Reverse transcription-PCR analysis of 6 of the 13 cases lacking Fhit showed normal-sized FHIT product in 3 cases and a mixture of normal and abnormal products in the other 3. Sequencing showed that abnormal bands were missing variable numbers of exons. Loss of microsatellite DNA markers internal to the FHIT gene was observed in 10 of 13 primary cancers lacking Fhit protein (homozygous in two cases) and in only 1 of the 8 cancers expressing Fhit protein. In nine primary cancers, four expressing and five lacking Fhit protein, it was possible to obtain pure cancer DNA by microdissection. Three of the five microdissected cases lacking Fhit protein exhibited homozygous deletion of FHIT exon 5. In conclusion, the lack of Fhit protein in pancreatic cancers correlated with absence or alteration of FHIT mRNA and was often associated with FHIT gene anomalies.

FHITness and cancer

In early 1996, the Fragile Histidine Triad or FHIT gene (pronounced FIT) was cloned and shown to straddle the most fragile human chromosome site at chromosome band 3p14.2. The exceptionally large FHIT locus also encompasses a hereditary renal carcinoma associated chromosome translocation breakpoint and is very frequently altered by internal deletions in the most common human cancers. Germline alteration of one allele in familial cancer and deletions within the gene in sporadic cancers are hallmarks of tumor suppressor genes. Some of the DNA and RNA alterations exhibited by the FHIT gene in cancers showed features not previously encountered for known tumor suppressor genes, prompting a number of investigators to reject FHIT as a suppressor gene. However, evidence continues to accumulate, demonstrating that FHIT inactivation occurs in the majority of lung, gastric, cervical, and kidney carcinomas and that replacement of Fhit expression in such cancer cells suppresses their tumorigenicity.

The role of deletions at the FRA3B/FHIT locus in carcinogenesis

The FHIT gene, which encodes a 1-kb message and a 16.8-kDa protein that hydrolyses diadenosine triphosphate (ApppA) to ADP and AMP in vitro, covers a megabase genomic region at chromosome band 3p14.2. The gene encompasses the most active of the common human chromosomal fragile regions, FRA3B. Over the years, it has been suggested that fragile sites might be especially susceptible to carcinogen damage and that chromosomal regions of nonrandom alterations in cancer cells may coincide with defined fragile sites. Within the FRA3B region, the characteristic induced chromosome gaps can occur across the entire region, but 60% of the gaps are centered on a 300-kb region flanking FHIT exon 5, the first protein-coding exon. Numerous hemizygous and homozygous deletions, translocations and DNA insertions occur within FHIT in cancer cell lines, uncultured tumors, and even in preneoplastic lesions, especially in tissues such as lung that are targets of carcinogens. This supports the proposed cancer-fragile site connection and suggests that the FHIT gene, expression of which is frequently altered in cells showing FHIT locus damage, is a tumor suppressor gene whose inactivation may drive clonal expansion of preneoplastic and neoplastic cells. Replacement of Fhit expression in Fhit-negative cancer cells abrogates their tumorigenicity in nude mice. Analysis of the approximately 300-kb DNA sequence encompassing FHIT exon 5 in the FRA3B epicenter has provided clues to the mechanism of repair of the fragile site double strand breaks. The mechanism involves recombination between LINE 1 elements with deletion of the intervening sequence, often including FHIT exons. These studies have also shown that FHIT alterations generally entail independent deletion of both FHIT alleles. Future studies will focus on two objectives: study of (1) the in vivo function of the Fhit protein and (2) mechanisms of break and repair in the FRA3B fragile region.