Chemosensitivity Genes Research Articles

Expression heterogeneity of ABC-transporter family genes and chemosensitivity genes in gastric tumor, carcinomatosis and lymph node metastases

Introduction. Metastatic tumors (particularly gastric cancer) have been found to be characterized by heterogeneity between the primary tumor and metastases. This type of heterogeneity comes to the fore when treating primary-metastatic forms of tumor and is an important reason for the low effectiveness of their treatment. In this regard, comparative analysis of ABC-transporter gene expression and chemosensitivity genes will allow to characterize to a certain extent the resistance and sensitivity of primary tumor, carcinomatosis and metastases to therapy and provide the basis for personalized treatment approach.Aim. To evaluate expression heterogeneity of ABC-transporter genes and chemosensitivity genes in gastric tumor, carcinomatosis and lymph node metastases.Materials and methods. Overall 41 patients with disseminated gastric cancer stage IV with carcinomatosis of peritoneum were included in the investigation. All patients underwent surgery according to Roux palliative gastrectomy. After surgery patients underwent chemotherapy depending on indications. RNA was isolated using RNeasy Plus mini kit (Qiagen, Germany). The expression level of ABC transporter genes (ABCB1, ABCC1, ABCC2, ABCC5, ABCG1, ABCG2) and chemosensitivity genes (BRCA1, RRM1, ERCC1, TOP1, TOP2α, TUBβ3, TYMS, GSTP1) was assessed by reverse transcription polymerase chain reaction (RT-PCR) in primary tumor, carcinomatosis and lymph node metastases.Results. The expression levels of the genes under study were shown to vary widely. For ABC transporter genes, ABCG1 (3.1 ± 1.1; max 32.0), ABCG2 (7.9 ± 2.3; max 54.1), ABCG2 (9.6 ± 3.8; max 101.0) were the most expressed genes in gastric tumor tissue, carcinomatosis and lymph node metastasis, respectively. Hyperexpression among chemosensitivity genes at all three sites was characteristic only of TOP2α (17.2 ± 6.0; max. 161.9; 10.8 ± 4.1; max. 105.1; 35.3 ± 0.8; max. 439.6, respectively). We found that TOP2α and BRCA1 gene expression levels were higher in lymph node metastasis compared with gastric tumor tissue and carcinomatosis (at p = 0.005 and p = 0.001). Whereas ABCC1 gene expression was statistically significantly higher in carcinomatosis (p = 0.03).Conclusion. Thus, a high level of expression heterogeneity is observed in gastric cancer, which affects the expression patterns of various genes in different localizations. The expression profile can be used to determine the level of heterogeneity and approach to personalized therapy tactics.

PERSONALIZED APPROACH TO ADJUVANT CHEMOTHERAPY IN PATIENTS WITH NON-SMALL CELL LUNG CANCER

Numerous studies have shown that differential expression and/or co-expression of several chemosensitivity genes, such as ERCC1, RRM1, TUBB3, TOP2A, and some others, in the tumor tissue are closely associated with chemoresistance and prognosis in cancer patients. Despite the fact that more and more data comes to support the idea on the important role of the presented genes in the assessment of chemosensitivity (CS), at present time there is ample room remaining for further research of the combined estimation of the CS gene expression for a personalized approach to chemotherapy in patients with lung cancer.

Predictive and Prognostic Significance of mRNA Expression and DNA Copies Aberrations of ERCC1, RRM1, TOP1, TOP2A, TUBB3, TYMS, and GSTP1 Genes in Patients with Breast Cancer.

Increasingly, many researchers are focusing on the sensitivity in breast tumors (BC) to certain chemotherapy drugs and have personalized their research based on the assessment of this sensitivity. One such personalized approach is to assess the chemotherapy’s gene expression, as well as aberrations in the number of DNA copies—deletions and amplifications with the ability to have a significant effect on the gene’s activity. Thus, the aim of this work was to study the predictive and prognostic significance of the expression and chromosomal aberrations of eight chemosensitivity genes in breast cancer patients. Material and methods. The study involved 97 patients with luminal B breast cancer IIB–IIIB stages. DNA and RNA were isolated from samples of tumor tissue before and after treatment. Microarray analysis was performed for all samples on high-density microarrays (DNA chips) of Affymetrix (USA) CytoScanTM HD Array and Clariom™ S Assay, human. Detection of expression level of seven chemosensitivity genes—RRM1, ERCC1, TOP1, TOP2a, TUBB3, TYMS, and GSTP1—was performed using PCR real-time (RT-qPCR). Results. The expression of the RRM1 (AC scheme), TOP2α, TYMS, and TUBB3 genes in patients with an objective response to treatment (complete and partial regression) is higher than in patients with stabilization and progression (p < 0.05). According to our results, the presence of a high level of GSTP1 in a tumor biopsy is associated with the low efficiency of the NAC CP scheme (p = 0.05). The presence of RRM1 deletion is associated with complete and partial regression, as for the TOP1 and TUBB3 genes (p < 0.05). Higher rates of metastatic survival are associated with a high level of expression and amplification of the GSTP1 gene (log-rank test p = 0.02 and p = 0.05). Conclusion. Thus, a complex assessment of the chemotherapy’s gene expression is important not only for understanding the heterogeneity and molecular biology of breast cancer but also to obtain a more accurate disease prognosis.

Model constructions of chemosensitivity and prognosis of high grade serous ovarian cancer based on evaluation of immune microenvironment and immune response

BackgroundThe prognosis of high grade serous ovarian cancer (HGSOC) patients is closely related to the immune microenvironment and immune response. Based on this, the purpose of this study was to construct a model to predict chemosensitivity and prognosis, and provide novel biomarkers for immunotherapy and prognosis evaluation of HGSOC.MethodsGSE40595 (38 samples), GSE18520 (63 samples), GSE26712 (195 samples), TCGA (321 samples) and GTEx (88 samples) were integrated to screen differential expressed genes (DEGs) of HGSOC. The prognosis related DEGs (DEPGs) were screened through overall survival analysis. The DEGs-encoded protein–protein interaction network was constructed and hub genes of DEPGs (DEPHGs) were generated by STRING. Immune characteristics of the samples were judged by ssGSEA, ESTIMATE and CYBERSORT. TIMER was used to analyze the relationship between DEPHGs and tumor-infiltrating immunocytes, as well as the immune checkpoint genes, finally immune-related DEPHGs (IDEPHGs) were determined, and whose expression in 12 pairs of HGSOC tissues and tumor-adjacent tissues were analyzed by histological verification. Furthermore, the chemosensitivity genes in IDEPHGs were screened according to GSE15622 (n = 65). Finally, two prediction models of paclitaxel sensitivity score (PTX score) and carboplatin sensitivity score (CBP score) were constructed by lasso algorithm. The area under curve was calculated to estimate the accuracy of candidate gene models in evaluating chemotherapy sensitivity.Results491 DEGs were screened and 37 DEGs were identified as DEPGs, and 11 DEPHGs were further identified. Among them, CXCL13, IDO1, PI3, SPP1 and TRIM22 were screened as IDEPHGs and verified in the human tissues. Further analysis showed that IDO1, PI3 and TRIM22 could independently affect the chemotherapy sensitivity of HGSOC patients. The PTX score was significantly better than TRIM22, PI3, SPP1, IDO1 and CXCL13 in predicting paclitaxel sensitivity, so was CBP score in predicting carboplatin sensitivity. What’s more, both of the HGSOC patients with high PTX score or high CBP score had longer survival time.ConclusionsFive IDEPHGs identified through comprehensive bioinformatics analysis were closely related with the prognosis, immune microenvironment and chemotherapy sensitivity of HGSOC. Two prediction models based on IDEPHGs might have potential application of chemotherapy sensitivity and prognosis for patients with HGSOC.

Identification of synthetic chemosensitivity genes paired with BRAF for BRAF/MAPK inhibitors

Molecular-targeted approaches are important for personalised cancer treatment, which requires knowledge regarding drug target specificity. Here, we used the synthetic lethality concept to identify candidate gene pairs with synergistic effects on drug responses. A synergistic chemo-sensitivity response was identified if a drug had a significantly lower half-maximal inhibitory concentration (IC50) in cell lines with a pair of mutated genes compared with those in other cell lines (wild-type or one mutated gene). Among significantly damaging mutations in the Genomics of Drug Sensitivity in Cancer database, we found 580 candidate synergistic chemo-sensitivity interaction sets for 456 genes and 54 commercial drugs. Clustering analyses according to drug/gene and drug/tissue interactions showed that BRAF/MAPK inhibitors clustered together; 11 partner genes for BRAF were identified. The combined effects of these partners on IC50 values were significant for both drug-specific and drug-combined comparisons. Survival analysis using The Cancer Genome Atlas data showed that patients who had mutated gene pairs in synergistic interaction sets had longer overall survival compared with that in patients with other mutation profiles. Overall, this analysis demonstrated that synergistic drug-responsive gene pairs could be successfully used as predictive markers of drug sensitivity and patient survival, offering new targets for personalised medicine.

A clear cancer cell line (150057) derived from human endometrial carcinoma harbors two novel mutations

BackgroundCell lines are extremely useful for both basic and clinical research. Thus, establishing endometrial cancer cell lines with malignant histology is important. This study aimed to extensively characterize an endometrial clear cell carcinoma cell line.MethodsThis cell line, named 150,057, was derived from the endometrial clear cell cancer of a 63-year-old woman. The morphology, chromosomes, chemosensitivity, tumor markers, xenotransplantation characteristics, and cancer-related genes of the cell line were characterized.ResultsThis cell line exhibited adequate growth, being passaged more than 70 times. The morphology of the cells was polygonal with a cobblestone-like appearance. Karyotyping of the cell line revealed a hypodiploid chromosomal number. 150057 cells expressed CA19–9 and CA125. The cell line was sensitive to doxorubicin, paclitaxel, carboplatin, and cisplatin. After the cells were transplanted into the subcutaneous region of non-obese diabetic-severe combined immunodeficiency mice, they generated xenograft tumors with similar histology as the original tumor. A total of 59 somatic nucleotide mutations were identified in 25 of the 53 examined tumor suppressor genes and oncogenes. Two novel mutations were found in FGFR3 and ARID1A.ConclusionWe established and characterized an endometrial clear cell carcinoma cell line that may be useful in carcinogenesis and treatment research for endometrial cancer.

MRNA expression of chemosensitivity genes as a predictor of response to neoadjuvant chemotherapy in non-small cell lung cancer

The objective response rate to neoadjuvant chemotherapy (NAC) in patients with non-small cell lung cancer (NSCLC) is approximately 16 %. Therefore, the search for new predictive factors and markers that could precisely predict the NAC response and be relevant to the choice of the adequate treatment policy is of utmost importance. the purpose of the study was to analyze the relationship between the expression levels of chemosensitivity genes and NAC response in patients with non-small cell lung cancer. material and methods. The study included 30 patients with stage IIAIIIB NSCLC. Total RNA was isolated from normal and tumor lung tissue samples prior to NAC. The expression level of chemosensitivity genes, such as BRCA1, RRM1, ERCC1, TOP1, TOP2a, TUBB3, TYMS, and GSTP1 was evaluated by real-time reverse transcriptase quantitative PCR (RT-qPCR). results. A significant difference in the expression levels of the studied genes between patients with different NAC response was found. The objective response to therapy was observed in 67 % (10/15) of patients having no ERCC1 expression compared to those with ERCC1 expression (p&lt;0.05). Low (less than 0.2) and high (more than 1.2) BRCA1 expression levels were associated with low rates of NAC response compared with patients whose expression level was between the lower and upper quartiles. Statistically significant differences were shown for the GSTP1 and RRM1 genes. conclusion. A comprehensive assessment of the expression of chemosensitivity genes is important not only in terms of understanding heterogeneity and complexity of the molecular biology of NSCLC, but also for more accurate prediction of response to NAC and identification of potential drug targets.

The identification of induction chemo-sensitivity genes of laryngeal squamous cell carcinoma and their clinical utilization.

To identify potential molecular markers for induction chemotherapy of Laryngeal squamous cell carcinoma (LSCC). Differently expressed genes between chemo-sensitive group (seven cases) and chemo-insensitive (five cases) group after induction chemotherapy by TPF were identified by microarrays. Bayes network and Random forest analyseswere employed to identify core genes for induction chemotherapy. The diagnostic value of these core genes was also evaluated by ROC analysis. Six genes (SPP1, FOLR3, KYNU, LOC653219, ADH7 and XAGE1A) are highly expressed, while seven gene (CADM1, NDUFA4L2, CCND2, RARRES3, ERAP2, LYD6 and CNTNAP2) present significantly low expression. Among these genes, genes CADM1, FOLR3, KYNU, and CNTNAP2 are core candidates for LSCC chemo-sensitivity. And that the low expression of CADM1 may result in chemo-sensitivity, which leads to high expression of gene FOLR3 and KYNU, and low expression of gene CNTNAP2. Besides, ROC analysis shows that these four genes exhibit effective diagnostic value for induction chemo-sensitivity. CADM1 may be a potential molecular marker for LSCC induction chemotherapy, while CADM1, FOLR3, KYNU, and CNTNAP2 may provide essential guidance for LSCC diagnosis and follow-up treatment strategies.

Characterization of a Novel Cell Line (HCH-3) Derived from a Human Ovarian Clear Cell Carcinoma

Objective: Established cell lines are important materials helping a medical basic research. The reports listed in detail of the cell line derived from an ovarian clear cell carcinoma were only 14, so far. Owing to little information, the establishment of malignant tumor cell line with individual characteristics is especially significant to research this disorder. Therefore, this study was carried out to establish and characterize a new human cell line derived from ovarian clear cell carcinoma. Methods: The cell line HCH-3 was established from a left ovarian tumor of a 41-year-old woman. Characters of the cell line investigated included morphology, chromosome analysis, heterotransplantation, tumor markers, chemosensitivity, and cancer genes. Results: This cell line has been growing well for 206 months and subcultured more than 50 times. Monolayer cultured cells were multipolar in shape, showing a cobble stone appearance and a tendency of multilayering without contact inhibition. They showed a human karyotype with a modal chromosomal number in the hypotetraploid range. The cells could be transplanted into the subcutis of SCID mice and made tumors looking like the original tumor. HCH-3 cells demonstrated both CA 125 and CA19-9 which were detected immunohistochemically in the original tumor and the heterotransplanted tumor. The cells were not sensitive to agents generally managed in the treatment of gynecological cancers by MTT assay. KRAS and TP53 mutations were discovered in hotspot locations of 50 cancer genes. Conclusion: HCH-3 is an ovarian clear cell carcinoma cell line in which CA 125 and CA19-9 expression have been clarified. Mutations were discovered in KRAS and TP53 genes. This newly established cell line may be helpful in basic study on ovarian clear cell carcinoma, the etiology of which is not yet entirely recognized.

P1.30 Intratumoral Heterogeneity in Expression of Chemotherapy Response Markers in Invasive Ductal Breast Carcinoma Nos

ABSTRACT Background Chemotherapy response is composed of drug resistance and sensitivity. Previously, we found that intratumoral heterogeneity of invasive ductal breast carcinoma not otherwise specified (IDBC NOS), presented by the different types of morphological structures (tubular, trabecular, solid, alveolar structures, and discrete groups of tumor cells) displaying architectural arrangements of tumor cells may influence on chemotherapy response (Zavyalova et al., 2008); however, the mechanism of this effect is unknown. Aim We aimed to analyze expression of the multidrug resistance (MDR) and chemosensitivity genes in the different types of morphological structures and their microenvironment of tumor in IDBC NOS. Materials and methods Tubular, trabecular, alveolar structures, and their microenvironment (local and distant) were isolated by laser microdissection PALM (Carl Zeiss, Gemany) from 10 um sections of FFPE breast tumor. The isolated and whole amplified cDNA (RNA) was used for analysis of the MDR (ABCB1, ABCC1, ABCC2, ABCC5, ABCG2, GSTP1, and MVP) and chemosensitivity (TOP1, TUBB3, and TYMS) genes by the real-time TaqMan PCR. Results As seen in Table 1, expression of the MDR and chemosensitivity genes was significantly different between both the studied types of morphological structures and their microenvironment. Moreover, we observed the interesting phenomenon consisting in the fact that different types of morphological structures and their local microenvironments are supplemented each other in expression of the MDR and chemosensitivity genes. Tubular structures had a significantly higher activity of the MDR and chemosensitivity genes in comparison with other types of morphological structures, whereas their microenvironment was fully inactive in respect of the expressing genes. In contrast, a low expression level of the MDR and chemosensitivity genes in alveolar structures was compensated by high activity of these genes in the surrounding microenvironment. Interestingly that trabecular structures displaying moderate expression of the analyzed genes had microenvironment with a similar level of activity of these genes. Conclusion Together, the data obtained demonstrates that within IDBC NOS tumor there are subclones of tumor cells and their specific microenvironment with different expression of chemotherapy response markers.The study was supported by the Russian Federation President Grant (MK-1259.2012.7) and grant of the OPTEK company.

P1.31 Snp Q787Q of Egfr Gene and Efficacy of Egfr-Tki in Patients With Non-Small Cell Lung Cancer

ABSTRACT Background Many activating mutations in epidermal growth factor receptor (EGFR) gene have been correlated with sensitivity to EGFR tyrosine kinase inhibitors (TKIs). Single nucleotide polymorphism (SNP) in exon 20 of EGFR gene (2361G > A transition) does not alter the amino acids of glutamine at codon 787 (Q787Q). We reviewed the relationship between this SNP and EGFR-TKI sensitivity. Patients and Methods Twenty five patients (male 19, female 6) with only SNP Q787Q in EGFR exon 20 were analyzed. Tissue or cytologic specimens were used for analysis of mutations in exon 18 to 21 of EGFR gene by direct sequencing. Treatments with EGFR-TKIs were performed in 16 patients and response evaluations were eligible in 15 cases. Results In 15 response evaluable patients, there were 2 partial responses (PR) and 5 stable diseases (SD) [response rate: 13.3%, disease control rate: 46.7%]. Median time to progression (TTP) was 392 days (range: 105 ∼ 511). Five patients showed longer than 3 months of TTP (PR 2 and SD 3, adenocarcinoma 4 and squamous cell carcinoma 1, gefitinib 4 and erlotinib 1). Conclusion Thirteen percent of NSCLC with 2361G > A transition without other activating mutations responded to EGFR-TKIs. Table 1 . Expression level of MDR and chemosensitivity genes in the different types of morphological structures and their microenvironment ABCB1 ABCC1 ABCC2 ABCC5 ABCG2 GSTP1 MVP TOP1 TUBB3 TYMS Tubular structures 0 0 0.712 0.301 0 1.000 0.009 0.270 0.294 0.939 - microenvironment 0 0 0 0 0 0 0 0 0 0 Alveolar structures 0 0 1.063 0 0 0 0 0 0 0.240 - microenvironment 0 0 0 0 0.392 0.899 0 4.221 0.215 0 Trabecular structures 0.296 0 0.838 0 0 1.610 0 0 0.110 0 -microenvironment 0.329 0.002 0.668 0.749 0 0 0 0.733 0.011 0 Distant microenvironment 0 0.001 0 0.017 0 0 0 0.361 0 1.216

Abstract LB-349: ZO1 gene represents a chemosensitivity gene in multiple myeloma

Abstract Chemotherapy agents are extremely important in the treatment of liquid malignancies, such as multiple myeloma (MM). Unfortunately, chemotherapy resistance in MM therapy is the most significant cause of treatment failure. The ability to predict, treat, or circumvent resistance is extremely likely to improve clinical outcomes. Thus, identification of novel genes that play a crucial role in MM progression and chemosensitivity is necessary to understand this disease better at the molecular level. Moreover, these genes and their products may serve as new therapeutic targets for MM, whose expression could improve patient outcomes or served as a predictor for chemotherapy outcome. To identify potential chemosensitivity genes, establishing a high-throughput method for validation of targets becomes urgently needed. Toward this purpose, we have successfully developed a high-throughput siRNA based functional target validation approach and identified a group of potential chemosensitivity genes. Our preliminary studies focusing on one of the candidate gene, ZO1 (zonula occludens 1), suggested that targeting ZO1 led to tumor cell resistant to several chemotherapy agents, including doxorubicin (Dox), cisplatin (Cis), methotrexate (MTX), and bortezomib. Further analysis with 264 bortezomib treated MM patients indicated that expression level of ZO1 correlated with patient response to bortezomib treatment. Two clones and pooled RPMI 8226 MM cell line, which were developed against bortezomib treatment in our lab, showed loss of ZO1 expression, suggesting a role of ZO1 may play in bortezomib resistance development. More importantly, ZO1 targeting in myeloma cells resulted in cell resistance to bortezomib treatment. Taken together, our studies reveal a novel chemosensitivity gene, which may lay the foundation for further clinical studies. Moreover, in the clinical arena, it may form a basis for algorithms to help predict chemosensitivity based on baseline gene expression profiles in the future, and may provide the strategies aimed at enhancing chemosensitivity by increasing expression of target genes of interest.[[Unsupported Character - &amp;#8194;]] Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-349. doi:1538-7445.AM2012-LB-349

Validation of Potential Chemosensitivity Genes in Multiple Myeloma

Abstract 1818Chemotherapy agents are extremely important in the treatment of liquid malignancies, such as multiple myeloma (MM). Unfortunately, chemotherapy resistance in MM therapy is the most significant cause of treatment failure. The ability to predict, treat, or circumvent resistance is extremely likely to improve clinical outcomes. Thus, identification of novel genes that play a crucial role in MM progression and chemosensitivity is necessary to understand this disease better at the molecular level. Moreover, these genes and their products may serve as new therapeutic targets for MM, whose expression could improve patient outcomes or served as a predictor for chemotherapy outcome. To identify potential chemosensitivity genes, establishing a high-throughput method for validation of targets becomes urgently needed. Toward this purpose, we have successfully developed a high-throughput siRNA based functional target validation approach and identified 34 potential chemosensitivity genes.Our preliminary studies focusing on one of the candidate gene, TJP1 (tight junction protein 1), suggested that targeting TJP1 led to tumor cell resistant to several chemotherapy agents, including doxorubicin (Dox), cisplatin (Cis), methotrexate (MTX), and bortezomib. Further analysis with 264 bortezomib treated MM patients indicated that expression level of TJP1 correlated with patient response to bortezomib. Two clones and pooled RPMI 8226 MM cell line, which were developed against bortezomib treatment in our lab, showed loss of TJP1 expression, suggesting a role of TJP1 may play in bortezomib resistance development. More importantly, TJP1 targeting in myeloma cells resulted in cell resistance to bortezomib treatment.Protein tyrosine phosphatase, receptor type, O1 (PTRPO) was identified in our screening as well. We first examined the expression of PTPRO in a panel of myeloma cell lines, and found that PTPRO expression was dramatically decreased in several drug resistant lines compared to their parental sensitive cells. Moreover, using a Lentiviral cDNA expression system to overexpress PTPRO, we observed that PTPRO significantly inhibited cell growth and sensitized drug sensitivity of several drug lines by triggering apoptosis. Finally, evaluation of the publicly available gene expression profiling data gathered from primary plasma cells on the Multiple Myeloma Genomics Portal, and the linked clinical annotation describing patient outcomes, has shown that over-expression of PTPRO is associated with a superior overall survival compared to patients whose myeloma show low levels of PTRPO expression, suggesting this may be a prognostic marker as well.Together, we have successfully identified a group of potential chenmosensitivity genes, in which two of the genes (TJP1 and PTPRO) have been further validated to sensitize the drug treatment in multiple myeloma cells. Disclosures:No relevant conflicts of interest to declare.

Lymphoma Regression Induced by Ganciclovir in Mice Bearing a Herpes Thymidine Kinase Transgene

The dose limitations imposed on cancer chemotherapeutic agents by their lack of selectivity can, in theory, be circumvented by a strategy entailing the prophylactic insertion into hosts of drug-sensitivity genes that are acquired or expressed in some but not all cells. This strategy predicts that neoplasms arising from drug-sensitive cells might be safely treatable with tumor-eradicative drug doses because the presence of a modicum of drug-insensitive stem cells will protect vital tissues from lethal depopulation. To test this prediction, lymphomas were induced with Abelson leukemia virus in mice bearing a herpes simplex virus thymidine kinase (HSV-TK) transgene selectively expressed in lymphoid cells. Of 12 transgenic mice treated with the HSV-TK-specific substrate ganciclovir (GCV), 11 exhibited complete tumor regressions; 5 of these mice remained tumor-free over observation periods that exceeded 100 days. Among the lymphomas that recurred, most appeared to represent mutant subpopulations that were GCV-insensitive because they had lost HSV-TK, implying that independent insertion of multiple HSV-TK gene copies might provide a means of preventing recurrences. The results of this study demonstrate that chemosensitivity genes can enhance the efficacy of treatment in hosts who subsequently develop a neoplasm. While the use of a germ-line gene insertion model precludes direct human application, the results also imply the merits of exploring an alternative version of the strategy in which somatic insertion of chemosensitivity genes in mosaic fashion is used prophylactically to enhance the prospect that a subsequent tumor will respond to therapy.