Tumor suppressor genes: the puzzle and the promise

Expression of differentially phosphorylated Rb and mutant p53 proteins in myeloid leukemia cell lines

We studied the structure and expression of Rb and p53 genes in six myeloid leukemia cell lines (HL-60, KBM3, K562, KBM5, EM2, KBM7) in the light of the published reports that structural abnormalities of these genes are rarely seen in leukemic cells and also a recent finding that Rb gene expression can be regulated by the p53 protein. Except for HL-60 cells which have a truncated p53 gene, none of the other cell lines revealed any gross structural abnormalities in the Rb and p53 genes. KBM3, KBM5 and EM-2 expressed lower levels of Rb mRNA than HL-60, K562 and KBM7. The amount of Rb protein was lowest in KBM3 cells and in this and two other cell lines (KBM5, KBM7) Rb was markedly hypophosphorylated compared to the other three cell lines. HL-60 and K562 did not express p53 m-RNA, while the other four cell lines all expressed high levels of mutant p53 protein. Thus even in the absence of gross structural alterations, subtle abnormalities in the expression pattern of Rb and p53 genes occur in myeloid leukemia cells.

Genotypic classification of colorectal adenocarcinoma. Biologic behavior correlates with K-ras-2 mutation type

BACKGROUND

New measures enabling better prediction of biologic behavior of large bowel cancer are highly desirable. One hundred ninety-four consecutive primary, recurrent, and metastatic colorectal adenocarcinomas, accessioned during 1991 at Rhode Island Hospital, were classified according to the presence and specific type of K-ras-2 point mutation.

METHODS

An integrated histopathologic-genetic approach was used to detect mutations starting with minute, topographically selected, tissue samples from formaldehyde-fixed, paraffin-embedded specimens.

RESULTS

Each colorectal adenocarcinoma exhibited either no or only one of seven specific types of K-ras-2 mutation. The mutation type of each primary tumor was present consistently in its metastatic deposits. Thirty-five percent of primary colorectal adenocarcinomas were found to be mutated (42 of 119). A significantly higher mutation rate (65%) was seen in lymphogenous-hematogenous metastases as a group (35 of 54; P < 0.005). By contrast, 22% of anastomotic recurrences and transcoelomic metastasis were mutated (4 of 18). Twenty-eight percent of adenocarcinomas with invasion limited to muscularis propria (Tis, T1, T2) were mutated (16 of 57), compared to 41% for more deeply invasive tumors (T3, T4; 26 of 63). When colorectal adenocarcinomas were analyzed by specific K-ras-2 mutation type, it was found that codon 13 mutated tumors did not progress to local or distant metastasis (P < 0.01). Tumors having a codon 12 valine substitution did not metastasize beyond pericolonic-perirectal lymph nodes. In contrast, colorectal cancers with codon 12 aspartic acid substitutions accounted for most of the distant hematogenous deposits (P < 0.01). Tumors with normal K-ras-2 accounted for most intraperitoneal deposits.

CONCLUSIONS

Genotyping of colorectal adenocarcinoma by K-ras-2 status can identify subsets of patients likely to pursue indolent and aggressive forms of disease. The integrated histopathologic-genetic approach outlined is feasible for use in diagnostic pathology, providing information that together with clinicopathologic staging may individualize and optimize treatment.

Frequent loss of 11p13 and 11p15 loci in male germ cell tumours

Deletions within the short arm of the human chromosome 11 have been found to be involved in the genesis of several tumours, including different urogenital neoplasms. We have studied 31 male germ cell tumours (19 seminomas and 12 nonseminomas), and observed loss of heterozygosity at 11p loci in 40% (12/30) of these tumours [35% (9/26) at 11p13 and 31% (8/26) at 11p15]. Our data suggest that inactivation of one or more tumour suppressor genes on 11p are involved in the genesis of testicular cancer. In addition, identification of the parental origin of the allelic losses revealed a paternal loss in six patients and a maternal loss in one case.

Genetics of cancer predisposition and progression

The development of human cancer is a multistep process that entails a progressively more malignant phenotype through the evolution of cellular subsets with increasing numbers of genetic alterations. Here we review the molecular genetics of human cancer predisposition and progression and describe paradigmatic cancer types and cancer syndromes. We also briefly consider the future impact of molecular biology on cancer diagnosis and treatment.

Deletion of chromosome 22 without bcr rearrangement and without juxtaposition of c-abl in a case of acute myeloid leukemia

We describe a patient with acute myeloid leukemia (AML) who had a deletion of chromosome 22 at q11 as a sole chromosomal abnormality, resulting in the karyotype 46,XY,del(22)(q11). Southern blot analysis showed no bcr rearrangement and fluorescence in situ hybridization indicated no juxtaposition of c-abl. This study indicates that molecular events other than bcr rearrangement and c-abl juxtaposition were involved in leukemogenesis in this patient. We hypothesize that a tumor suppressor candidate gene may be located on the long arm of chromosome 22; its loss may lead to malignant transformation.

Loss of constitutional heterozygosity on chromosomes 5 and 17 in cholangiocarcinoma

It has been established that loss of tumour suppressor genes is crucial in carcinogenesis. There has been no reported study on searching for tumour suppressor genes in cholangiocarcinomas as yet. In order to investigate the loss of heterozygosity (LOH), which may represent such gene loss, in cholangiocarcinoma, we studied 14 patients with this tumour using restriction fragment length polymorphism analysis. Twenty-two probes assigned to chromosomes 1, 5, 7, 9, 11, 12, 13, 14, 16, 17 and 18 were used. Allelic losses were found in chromosomal regions 5q35-qter and 17p13. Loss of genetic material in these regions in cholangiocarcinoma was shared with hepatocellular carcinoma. Probes for other chromosomes have as yet shown no consistent LOH. In conclusion, this study for the first time showed LOH on chromosomes 5 and 17 in cholangiocarcinoma.

Diploid yeast cells yield homozygous spontaneous mutations

A leucine-requiring hybrid of Saccharomyces cerevisiae, homoallelic at the LEU1 locus (leu1-12/leu1-12) and heterozygous for three chromosome-VII genetic markers distal to the LEU1 locus, was employed to inquire: (1) whether spontaneous gene mutation and mitotic segregation of heterozygous markers occur in positive nonrandom association and (2) whether homozygous LEU1/LEU1 mutant diploids are generated. The results demonstrate that gene mutation of leu1-12 to LEU1 and mitotic segregation of heterozygous chromosome-VII markers occur in strong positive nonrandom association, suggesting that the stimulatory DNA lesion is both mutagenic and recombinogenic. In addition, genetic analysis of diploid Leu+ revertants revealed that approximately 3% of mutations of leu1-12 to LEU1 result in LEU1/LEU1 homozygotes. Red-white sectored Leu+ colonies exhibit genotypes that implicate post-replicational chromatid breakage and exchange near the site of leu1-12 reversion, chromosome loss, and subsequent restitution of diploidy, in the sequence of events leading to mutational homozygosis. By analogy, diploid cell populations can yield variants homozygous for novel recessive gene mutations at biologically significant rates. Mutational homozygosis may be relevant to both carcinogenesis and the evolution of asexual diploid organisms.

Chromosome analysis of adenomatous polyps of the colon: possible existence of two differently evolving cytogenetic groups

A chromosomal study of 42 colonic adenomatous polyps was performed using a technique of direct chromosome analysis derived from the prenatal procedure for diagnosing chromosomal alterations from chorionic villi sampling. Abnormal karyotypes were found in 22 cases. Trisomy 7, the most frequently found alteration, was found in 13 cases, followed by trisomy 13 (nine cases). Monosomy 18 was observed in two cases; in one of these, that of a polyp which had degenerated into an intra-mucosal adenocarcinoma, it was associated with 17p monosomy. Interestingly, these two types of alterations (trisomy 7 versus 18 and 17p monosomy) were not found together in the same lesion. This suggests that there could be two distinct chromosomal behaviors which might be related to the two cytogenetic groups described for colorectal adenocarcinoma. However, the respective frequencies of such cytogenetic groups varied inversely between adenomas and adenocarcinomas, thus suggesting that they evolve differently.

Genetic alterations in a malignant schwannoma from a patient with neurofibromatosis (NF1)

In a patient with neurofibromatosis (von Recklinghausen disease; NF1), normal lymphocytes, five cutaneous neurofibromas, and tumour tissue from a recurrence of a malignant schwannoma were analysed for genetic alterations. Eleven DNA markers located on chromosome 17 and nine randomly chosen markers representing chromosomes 1, 2, 3, 4, 5, 6, and 11, were analysed. High resolution Giemsa banding of lymphocytes revealed no chromosomal rearrangement. The DNA from the neurofibromas were all found to have the same restricted fragment length polymorphism pattern as the constitutional DNA from the patient. In the malignant schwannoma a complete loss of one allele was found at polymorphic loci on chromosome arm 17p. One gene copy of the TP53 gene (17p13.1) and the NF1 gene (17q11.2) was lost, as was one copy of the PGA gene (11q13). No mutations were detected in the mutational hotspots of the TP53 gene. Partial losses were detected at three loci on chromosomes 1, 2 and 6, indicating a clonal variation within the tumour since histological evaluation disclosed no normal tissue in the analysed specimen. Our data indicate that the NF1 gene may function as a tumour suppressor gene, and that, either by effect of dose reduction or complete inactivation, both the NF1 gene and the TP53 gene may be critical for the progression of a neurofibroma to a malignant schwannoma. The observations made are consistent with the concept of stepwise multigenetic changes in tumour progression.

Expression of the major protein kinase C substrate, the acidic 80-kilodalton myristoylated alanine-rich C kinase substrate, increases sharply when Swiss 3T3 cells move out of cycle and enter G0

The expression of the major protein kinase C (PKC) substrate, originally called "80K" for acidic SDS/PAGE-observed 80-kDa PKC substrate and now called "MARCKS" for myristoylated alanine-rich C kinase substrate, in Swiss 3T3 fibroblasts changes strikingly (15- to 22-fold) during transitions of cell growth. Quiescent cells in G0 express high levels of MARCKS mRNA and protein. However, plating these cells in fresh medium at low density to stimulate multiple rounds of cell division caused a striking down-regulation of MARCKS expression. The mRNA level declined to a minimum of 4.5% compared with quiescent control cells 6 hr after plating, and protein levels declined during the same period to 6.5% of the control value. This rapid down-regulation was independent of PKC activation and length of exposure to trypsin (1-10 min) but required plating in medium containing fresh serum. MARCKS mRNA and protein levels remained down-regulated for 3 days, during which time the cells were actively progressing through the cell cycle as judged by fluorescence-activated cell sorting analysis. However, on reaching quiescence, the expression of MARCKS mRNA and protein increased markedly. Furthermore, the rate of recovery of MARCKS mRNA and protein levels was shown to be dependent on the supply of serum-derived growth factors in the medium. Addition of hydroxyurea to arrest the cells in S phase or at the G1/S boundary rather than G0 completely prevented the recovery of MARCKS protein. The down-regulation of MARCKS following plating and its serum-dependent recovery was also demonstrated in tertiary cultures of mouse embryo fibroblasts. The results suggest that MARCKS may play a role in the regulation of entry and exit of cells from G0.

In-vivo assessment of DNA ligation efficiency and fidelity in cells from patients with Fanconi's anemia and other cancer-prone hereditary disorders

We developed a host cell DNA ligation assay, in which we transfected linearized plasmid pZ189 into human lymphoblasts or fibroblasts in order to assess the efficiency and accuracy of DNA ligation within these host cells. We used cell lines from patients with Fanconi's anemia and other chromosome breakage or instability syndromes (Bloom's syndrome, ataxia telangiectasia, Werner's syndrome). With the Fanconi's anemia lymphoblast line GM8010 we did not find a reduced, but a slightly hypermutable DNA ligation. Mutation analysis revealed a unique 7.9-12.5-fold increase in insertions or complex mutations. With cells from the other chromosome breakage/instability syndromes we also found a hypermutable and/or reduced DNA ligation. An impaired DNA ligation might be a common molecular mechanism of genetic instability in these disorders.

Oncogenes: present status

Oncogenes are genes associated with causation of cancer. They were originally associated with the ability of retroviruses to cause tumors in animals. These viral oncogenes (V-onc) have their cellular counterparts (C-onc) called Proto oncogenes. Function of Proto oncogenes is to maintain cellular growth and development. Activation of these proto-oncogenes can occur due to mutation which leads to uncontrolled cell growth. The Proto oncogenes can be grouped into different categories based on their protein products, i.e. protein kinases, growth factors, growth factor receptors, and DNA binding proteins. There are also genes that normally suppress malignant transformation and these are called anti oncogenes. Loss of their suppressor activity leads to unimpeded growth. Oncogene abnormalities are seen in pediatric leukemias, lymphomas, and various solid tumors. Anti oncogenes are associated with retinoblastoma (Rb gene), Wilms' tumor, rhabdomyosarcoma and neuroblastoma, etc. Identification of these abnormalities have diagnostic, prognostic and therapeutic implications. The utility of oncogenes in classification of human cancer and monitoring cancer therapy is quite clear, but the future of these for therapeutic interventions remains uncertain. Role of c-abl oncogene in chronic myeloid leukemia (CML), bcl-2, in lymphomas, N-myc in neuroblastomas and retinoblastoma (Rb) gene in retinoblastomas is well understood and used in designing proper therapeutic approaches. Since oncogenes also control normal cellular function, their use for therapy may be limited by the amount of damage to normal cells. Their maximum therapeutic benefit may be realized only when used in combination with other modalities.

Karyotypic analysis in primary myelodysplastic syndromes

Cytogenetics has provided new insights into the biology and pathogenesis of myelodysplastic syndromes. In patients with refractory anemia, it has provided proof of clonality and has helped differentiate chronic myelomonocytic leukemia from chronic myeloid leukemia. As a prognostic tool, cytogenetics has been predictive of duration of survival and leukemic transformation. However, its role as an independent prognostic factor compared with recent prognostic scoring systems remains to be determined. New techniques such as fluorescent in situ hybridization using chromosome-specific DNA probes may expand the usefulness of cytogenetics. The prognostic impact of cytogenetics may not be fully realized until more effective treatments become available.

Prostatic microcarcinomas in relation to cancer origin and the evolution to clinical cancer

BACKGROUND

In the prostate there is a uniquely high prevalence of microcarcinomas relative to larger cancers. The probability of metastasis has been found to correlate closely with increasing cancer volume. It is important to explore the relationship of microcarcinomas to these larger clinically significant cancers by comparing their histologic features and to trace their relationship to dysplasia, a proposed precursor lesion.

METHODS

The histologic features of 107 microcarcinomas were related to associated dysplasia lesions. Their histologic grade was quantitated and compared with the quantitative grade and volume distribution of 100 incidental autopsy cancers and 209 clinical cancers from radical prostatectomy.

RESULTS

Microcarcinomas were contiguous with dysplasia foci in 81% of the cases, and evidence of cancer origin was found in 48%. Only 3 of 107 microcarcinomas had Grade 4 (poorly differentiated) areas. Grade 4 frequency and extent increased with cancer volume, and the relationship was similar between the autopsy and prostatectomy series. Metastasis was found only with cancers larger than 4 cc and having Grade 4 areas.

CONCLUSIONS

The majority of prostatic microcarcinomas appeared to arise from dysplasia. Only 3% of these early cancers showed areas of high histologic grade. In the autopsy and clinical series, it appeared that poorly differentiated areas evolve from low-grade cancer with time and increasing volume. Probability of metastasis was a function of volume and grade.

Consistent interstitial chromosomal deletions in myeloid malignancies and their correlation with fragile sites

Chromosomal deletions occurring in myeloid malignancies have sometimes been reported either with no breakpoints or as terminal deletions. It is of importance to deduce whether these deletions are actually terminal or interstitial because this has implications for their biologic consequences and the mechanism of their development. Chromosomal deletions have been observed in 38 patients with myeloid malignancies. Two or more deletions occurred in six cases, and in seven cases this was part of a complex abnormality. In all, 45 deletions were observed. In all cases analyzed, the deletions consistently were interstitial. Of the 38 cases, 16 were myelodysplastic syndromes (MDS) [refractory anemia (RA), three; RA with ringed sideroblasts (RARS) three; RA with excess of blasts (RAEB) eight; RAEB in transformation (RAEB-t) one; and unclassified, one], 11 cases were acute nonlymphocytic leukemia (ANLL), and 11 were other myeloproliferative disorders [polycythemia rubra vera (PRV) seven; essential thrombocytopenia (ET), three; unclassified, one]. In general, no uniformity of breakpoints could be identified other than del(9)(q13q22.2) most of which occurred with t(8;21) and del(20)(q11.2q13.3 or 13.1). The breakpoints corresponded to or were adjacent to fragile sites in 49% (proximal 64%, distal 33%). These data emphasize that chromosomal deletions in myeloid malignancies are interstitial. The uniformity of breakpoints in del 9q and del 20q supports the concept that in some instances the exact breakpoints may be important through juxtaposition of genes rather than loss of critical regions. The data also suggest that there may be different mechanisms for the development of proximal and distal breakpoints.

Biological actions of oncogenes

Cancer, in many cases, results from multistep genetic mutation. Certain genes can have a predisposed susceptibility to mutations that lead to cancer because of chromosome location or their importance in the control of cell cycles. Mutations that deregulate the expression or activity of enzymes involved in the biochemical pathways of growth and differentiation or that suppress the expression of negative cell cycle control factors result in activation of oncogenesis. The study of oncogenes and tumor suppressor genes has greatly influenced our understanding of the molecular origins of cancer. We focus here on the normal biological action of proto-oncogenes compared with the transforming activities of oncogenes and tumor suppressor genes, and we discuss possible mechanisms of oncogenic transformation.

Cervical carcinoma: low frequency of allele loss at loci implicated in other common malignancies

Twenty cervical carcinomas were examined for loss of heterozygosity (LOH) using 22 RFLP markers, which mapped to regions of putative oncosuppressor gene loci, identified as candidates in other common solid tumours. Allele losses were identified in six of the eight chromosomal arms examined, but at a significantly lower frequency than that reported in other common solid tumours. No association was observed between allele losses at any chromosomal location and the presence or integration of 'high risk' types of HPV determined by a sensitive, specific PCR method. HPV 16, 18 or 33 were found in the majority (75%) of these tumours. We have looked at only a limited subset of chromosomal regions, but the results, so far, imply that carcinoma of the cervix may arise by different molecular events than other common solid tumours, and support the view that one of the distinctive events may be infection with HPV. Alternatively, similar molecular events may be occurring, but in regions of the genome not yet identified as targets in other solid tumours.

Mutant p53 expression in prostate carcinoma

The expression of the mutant p53 tumor suppressor gene was evaluated in 33 human prostate carcinomas. Using an immunohistochemical method with monoclonal antibodies PAb 1801 and PAb 240, 26 (79%) tumors demonstrated positive immunostaining for mutant p53. Only areas of glandular tumor were positive, with adjacent stromal elements and areas of glandular hyperplasia being negative. The predominant staining pattern was cytoplasmic. This pattern may be related to p53 binding to certain heat shock proteins (HSP 72/73), as a monoclonal antibody to these proteins demonstrated a cytoplasmic location as well. These results demonstrate that abnormal p53 expression is a frequent event in prostate cancer.

Molecular differential pathology of renal cell tumours

Recent application of molecular cytogenetic techniques to the evaluation of renal cell tumours revealed four subtypes, each with a characteristic combination of genetic alterations within the chromosomal and mitochondrial DNA. The most common, nonpapillary renal cell carcinomas are characterized by the loss of chromosome 3p sequences, rearrangement of the chromosome 5q region and loss of the chromosome 14q sequences. Papillary renal cell tumours can be divided into two groups. Tumours with a combined trisomy of chromosomes 7 and 17 as well as loss of the Y chromosome are papillary renal cell adenomas. Tumours with additional trisomies such as trisomy 16, 20 or 12 are papillary renal cell carcinomas. Chromophobe renal cell carcinomas show a combination of allelic losses, which do not occur in other types of renal tumours. In addition, they have a rearrangement in the mitochondrial DNA. Renal oncocytomas are benign tumours marked by normal or abnormal karyotypes with balanced or unbalanced translocations and an altered restriction pattern of the mitochondrial DNA. Although the major cytological characteristics of renal cell tumours, such as clear, granular, chromophobe and oncocytic cell phenotypes correspond to nonpapillary, papillary and chromophobe renal cell carcinomas and renal oncocytomas, there are many cases with overlapping phenotype. Therefore, a classification of renal cell tumours based on specific genetic alterations is proposed.

Disappearance of a highly unusual clone, 46,XY,del(7)(p12),t(9;22)(q34;q11) in chronic myeloid leukemia after treatment with recombinant interferon and cytosine arabinoside

A patient with the typical features of the stable phase of chronic myeloid leukemia (CML) displayed two karyotypically related subclones. In addition to the t(9;22), cells from one clone contained a deletion of the short arm of chromosome 7, del(7)(p12), [46,XY,del(7)(p12),t(9;22)(q34;q11)]; the other contained only the standard translocation [46,XY,t(9;22)(q34;q11)]. Cells with a deletion of the short arm of chromosome 7 at band p12 as the only additional abnormality have not been observed previously in CML. Conventional chemotherapy with hydroxyurea and then with recombinant interferon-alpha (rIFN-alpha) did not reduce the population of either subclone. However, after treatment with a combination of rIFN-alpha and low-dose cytosine arabinoside (LoDac) continuously infused subcutaneously (s.c.), cells from the clone with the deleted chromosome 7 disappeared and normal metaphases were demonstrable.

Definition of a tumor suppressor locus within human chromosome 3p21-p22

Cytogenetic abnormalities and high-frequency allele losses involving the short arm of human chromosome 3 have been identified in a variety of histologically different neoplasms. These findings suggest that a tumor-suppressor gene or genes may be located in the region of 3p14-p25, although there has been no definitive functional proof for the involvement of a particular region of 3p. We report a rapid genetic assay system that has allowed functional analysis of defined regions of 3p in the suppression of tumorigenicity in vivo. Interspecific microcell hybrids containing fragments of chromosome 3p were constructed and screened for tumorigenicity in athymic nude mice. Hybrid clones were obtained that showed a dramatic tumor suppression and contained a 2-megabase fragment of human chromosomal material encompassing the region 3p21 near the interface with 3p22. With these hybrid clones, we have defined a genetic locus at 3p21-p22 intimately involved in tumor suppression.

Activation of oncogenes and/or inactivation of anti-oncogenes by reactive oxygen species

Abundant evidence indicates that reactive oxygen species (ROS) are involved in mutagenesis and carcinogenesis. These chemical-generated or phagocyte-released ROS are known to cause a variety of genetic alterations which lie at the heart of the carcinogenic process. ROS have also been shown to cause malignant transformation of normal cells, and to increase expression of certain proto-oncogenes such as c-fos and c-jun. It is known that certain proto-oncogenes and anti-oncogenes may serve as the targets of carcinogens of various sorts. I hypothesize that ROS-mediated DNA damage may cause mutations and/or deletions in certain specific coding regions of tumor-related genes, and could be responsible for subsequent activation of oncogenes and/or inactivation of anti-oncogenes.

DNA sequence analysis of mutations induced by melphalan in the CHO aprt locus

In previous work, we established that treatment with melphalan (L-phenylalanine mustard) produced a predominance of A.T-->T.A transversions in the Simian virus 40 (SV40)-based shuttle vector pZ189 during replication in human 293 cells. Mutations were induced with varying doses (4-12 microM) melphalan in the aprt gene of the hemizygous Chinese hamster ovary (CHO) cell line D422 to determine whether a similar mutation spectrum would be observed in an endogenous gene. DNA sequence alterations were determined for 39 spontaneous and 41 melphalan-induced independent mutant clones. Other than a predominance of transversions in both systems, the spectrum of melphalan-induced aprt mutations bears little resemblance to the spectrum observed in the supF gene of the shuttle plasmid pZ189. In aprt, mutations at G.C base pairs (bp) predominated (29 of 41 base substitutions). Significantly enhanced mutagenesis was observed at 5' G-G-C 3' and 5' G-G-C-C 3' sites in the aprt gene. Almost half of the melphalan-induced base substitutions occurred at 5' G-N-C 3' sequences, which are believed to be potential interstrand crosslink sites.

Current status of adjuvant chemotherapy for colorectal cancer. Can molecular markers play a role in predicting prognosis?

BACKGROUND

Recent clinical trials establish a beneficial effect for adjuvant chemotherapy after surgical resection of the primary tumor (1) as single treatment for patients with colonic cancer and (2) combined with radiation therapy for patients with rectal cancer. Because adjuvant chemotherapy is not universally effective and is associated with toxicity and some degree of risk, it would be desirable to supplement standard pathologic staging criteria to define more precisely the subset of patients at high risk for tumor recurrence who would benefit most from adjuvant therapy. Tumor cell DNA content and cell proliferation measured by flow cytometry were identified as important and independent prognostic factors for patients undergoing curative resection of colorectal cancer. Basic laboratory investigations show a series of more specific molecular and genetic abnormalities that might provide better prognostic discrimination. Recent molecular studies suggest that the process of tumorigenesis in colorectal cancer proceeds through a series of genetic alterations that include both dominant and recessive protooncogenes. Characterization of these molecular genetic abnormalities may provide valuable prognostic information for use in patient management.

METHODS

Allelic loss was studied for chromosomes 5, 17, and 18, and immunohistochemical analysis was done of the p53 protein product in tumors from 91 patients with colorectal cancer.

RESULTS

Preliminary analysis of disease-free survival after surgical resection in 60 patients with Dukes' B or C tumors suggests a poorer prognosis associated with allelic loss on chromosome 18q (P = 0.08).

CONCLUSIONS

Additional studies involving a much larger population of patients with Dukes' B and C colorectal cancer are needed to define the true prognostic significance of these molecular markers.

The integration of molecular genetics into cancer management

Many of the phenotypes of cancer cells and tumors can now be traced to specific mutations in the genomes of these cells. These mutations may activate oncogenes, providing mitogenic stimulus to these cells. Alternatively, they may inactivate tumor-suppressor genes, relieving growth-inhibiting constraints placed on these cells. These genetic lesions together provide many of the explanations for the deregulated growth of tumor cells.

Gene therapy of cancer through restoration of tumor-suppressor functions?

The tools and concepts of gene therapy are being applied to the development of effective new treatments for human cancer. Most human cancers are associated with multiple interacting and cooperating mutations in protooncogenes and tumor suppressor genes. In several model systems, some features of the tumor phenotype can be suppressed in vitro through the restoration of expression of tumor suppressor genes such as Rb and p53. Before this phenomenon can serve as the basis for gene therapy of cancer, many conceptual and technical problems must be solved. Because such genetically modified cells continue to contain and express other mutations, it is important to determine the mechanisms and frequency of reversion to the tumor phenotype. To be clinically useful, highly efficient and targeted gene delivery vectors must be developed. The experimental evidence for tumor suppression by restored gene expression and the pivotal role played by tumor suppressor genes in the regulation of cell replication suggests that restored expression of some tumor suppressor genes in some tumor cells will eventually play a role in cancer gene therapy.

Transcriptional downregulation of gap-junction proteins blocks junctional communication in human mammary tumor cell lines

Subtractive hybridization, selecting for mRNAs expressed in normal human mammary epithelial cells (NMECs) but not in mammary tumor cell lines (TMECs), led to the cloning of the human gap junction gene connexin 26 (Cx26), identified by its sequence similarity to the rat gene. Two Cx26 transcripts derived from a single gene are expressed in NMECs but neither is expressed in a series of TMECs. Northern analysis using rat Cx probes showed that Cx43 mRNA is also expressed in the normal cells, but not in the tumor lines examined. Connexin genes Cx31.1, Cx32, Cx33, Cx37, and Cx40 are not expressed in either normal cells or the tumor lines examined. In cell-cell communication studies, the normal cells transferred Lucifer yellow, while tumor cells failed to show dye transfer. Both Cx26 and Cx43 proteins were immunolocalized to membrane sites in normal cells but were not found in tumor cells. Further analysis demonstrated that Cx26 is a cell-cycle regulated gene expressed at a moderate level during G1 and S, and strongly up-regulated in late S and G2, as shown with lovastatin-synchronized NMECs. Cx43, on the contrary is constitutively expressed at a uniform low level throughout the cell cycle. Treatment of normal and tumor cells with a series of drugs: 5dB-cAMP, retinoic acid, okadaic acid, estradiol, or TGFb had no connexin-inducing effect in tumor cells. However, PMA induced re-expression of the two Cx26 transcripts but not of Cx43 in several TMECs. Thus Cx26 and Cx43 are both downregulated in tumor cells but respond differentially to some signals. Modulation of gap-junctional activity by drug therapy may have useful clinical applications in cancer.

New models to define factors determining the growth and spread of human prostate cancer

The incidence of many cancers shows a sharp increase with age. This is particularly true of prostate cancer, which arises in many older males. Little or no morbidity is observed as the tumor develops in situ in the prostate. However, once malignant cells escape from the primary lesion and metastasize, the disease assumes a much more serious course. Here we report on the activity of human prostate cancer cells in culture as well as their behavior when transplanted into nude mice. In vitro, several lines of prostate carcinoma cells obtained from metastatic lesions were embedded in reconstituted basement membrane proteins (Matrigel) and found to exhibit highly invasive activity as observed with malignant cells from other types of tumors. Also, we report an improved method for obtaining an increased growth of human prostate cancer cells in nude mice by injecting these cells in Matrigel. Since there are no adequate animal models of prostate cancer, the systems described here may prove useful in defining events underlying the development and progression of the tumor cells to malignant status, as well as facilitate the analyses of novel therapeutic agents to prevent the growth and the spread of this cancer.

Genetics of transitional cell carcinoma

Several models of genetic events which define the origin and progression of human tumors have been elucidated over the last several years. These models suggest that the study of tumors at the level of both the chromosome and the gene can be useful in elucidating molecular events in tumor progression and in determining the biologic behavior of individual tumors. The genetics of transitional cell carcinomas are reviewed with emphasis on potential mechanisms of tumorigenicity and the clinical utility of genetic markers.

Expressivity of a common fragile site, fra(3)(p14.2), in patients with cancer and other diseases

A population survey of a common folate-sensitive fragile site, fra(3)(p14.2), has been carried out on PHA-stimulated peripheral lymphocytes of patients with cancer and other diseases, under both culture conditions of folate deprivation and aphidicolin treatment. Overall findings regarding variability of expressivity due to age and sex were very similar to those obtained in a healthy population. The expression of fra(3)(p14.2) by folate deficient condition appeared hardly influenced by such exogenous factors as tobacco smoking habit, past histories of radiotherapy and chemotherapy, while it was associated with the unfavorable prognosis of cancers. Furthermore, proportion of those with higher expression was slightly but significantly larger in both lung and breast cancer patients. These findings suggest that some factors relevant to the expression of fra(3)(p14.2) may be associated with development and progression of certain kinds of cancer.

Loss of heterozygosity on chromosomes 1 and 11 in carcinoma of the pancreas

Little is known of the molecular-genetic changes in carcinoma of the pancreas (CaP). In order to investigate the allele loss, or loss of heterozygosity (LOH), in CaP, we studied 13 patients with exocrine CaP and two with endocrine CaP using restriction fragment length polymorphism analysis. Twenty probes assigned to chromosomes 1, 5, 7, 9, 11, 12, 13, 14, 16, 17 and 18 were used. The frequency of LOH, or fractional allele loss (FAL), was found in two endocrine tumours to be 0.333 and 0.455 respectively; and FAL in 13 oxocrine tumours ranged from 0 to 0.25. Allele loss was shown in both exocrine and endocrine tumours by the probes Lambda MS1 at 1p33-35, and pMS51 at 11q13. Probes for other chromosomes have as yet shown no consistent LOH. In conclusion, the study showed LOH on chromosomes 1 and 11 in both exocrine and endocrine CaP.

Development of a flow-cytometric HLA-A locus mutation assay for human peripheral blood lymphocytes

A flow-cytometric technique was developed to measure the frequency of variant lymphocytes lacking expression of HLA-A2 or A24 allele products among donors heterozygous for HLA-A2 or A24. It was found that the variant frequency of lymphocytes in peripheral blood was of the order of 10(-4) and increased with donor age. Molecular analyses of mutant clones revealed that about one-third were derived from somatic recombinations and that the remaining two-thirds did not show any alterations after Southern blotting analysis. In contrast, mutants obtained after in vitro X-ray mutagenesis study were found to be mostly derived from large chromosomal deletions. A small-scale study on atomic bomb survivors did not show a significant dose effect.

Hepatitis B virus infection and primary hepatocellular carcinoma

For many years, epidemiological studies have demonstrated a strong link between chronic hepatitis B virus (HBV) infection and the development of primary hepatocellular carcinoma (PHC). Other hepatocarcinogens such as hepatitis C virus and aflatoxin also contribute to hepatocarcinogenesis either in conjunction with HBV infection or alone. Cellular and molecular biological studies are providing explanations for the HBV-PHC relationship, and models are now being formulated to further test the relative importance of various factors such as viral DNA integration, activation of oncogenes, genetic instability, loss of tumor suppressor genes, and trans-activating properties of HBV to the pathogenesis of PHC. Further research will probably define more than a single mechanism whereby chronic HBV infection results in PHC.

The cultured diploid fibroblast as a model for the study of cellular aging

The limited proliferative potential of the cultured human diploid fibroblast is now well established. A number of biological correlates suggest that this culture system is a model for the study of aging at the cellular level. The mechanism(s) that causes the loss of proliferative activity is unknown; the results of some recent studies indicate that specific genes may play a pivotal role in cellular aging in vitro. The extent to which changes in proliferative functions are causally related to aging in vivo is currently under investigation.

Loss of heterozygosity for 10q loci in human gliomas

Cytogenetic and RFLP studies have shown that chromosome 10 is frequently lost in tumor cells from glioblastomas, suggesting that a suppressor gene important in tumorigenesis is present on this chromosome. Forty-one tumors were examined for loss of heterozygosity at 23 loci on chromosome 10 to determine the smallest common deletion interval on this chromosome. Seven tumors did not lose heterozygosity for any of the markers. Twenty-three tumors lost an allele for all the informative loci. In 11 tumors heterozygosity was maintained at some loci and lost at other loci, indicating partial deletion of chromosome 10. The common region of deletion in these 11 tumors was located in 10q24-q26 between the markers pHUK-8 and pMCT122.2.

Chromosomal influence on hybrid cell proliferation

When normal cells and cancer cells (usually from the same species) are experimentally fused the resultant hybrid cells show loss of the tumorigenic phenotype. To examine the phenotypic phenomenon of growth suppression in hybrid cells in vitro, we examined the doubling times of somatic cell hybrids which contained single or multiple chromosomes derived from another species (inter-species hybrids). In all of the hybrid lines examined, the presence of transferred chromosomes prolonged the cell population doubling times in proportion to the number of such chromosomes found in the hybrid lines. These findings provide additional evidence to support the hypothesis that increasing the genetic burden of cells may reduce the division potential of cells cultured in vitro.

Nucleotide sequence analysis of human hypoxanthine phosphoribosyltransferase (HPRT) gene deletions

We have determined the nucleotide sequences of 10 intragenic human HPRT gene deletion junctions isolated from thioguanine-resistant PSV811 Werner syndrome fibroblasts or from HL60 myeloid leukemia cells. Deletion junctions were located by fine structure blot hybridization mapping and then amplified with flanking oligonucleotide primer pairs for DNA sequence analysis. The junction region sequences from these 10 HPRT mutants contained 13 deletions ranging in size from 57 bp to 19.3 kb. Three DNA inversions of 711, 368, and 20 bp were associated with tandem deletions in two mutants. Each mutant contained the deletion of one or more HPRT exon, thus explaining the thioguanine-resistant cellular phenotype. Deletion junction and donor nucleotide sequence alignments suggest that all of these HPRT gene rearrangements were generated by the nonhomologous recombination of donor DNA duplexes that share little nucleotide sequence identity. This result is surprising, given the potential for homologous recombination between copies of repeated DNA sequences that constitute approximately a third of the human HPRT locus. No difference in deletion structure or complexity was observed between deletions isolated from Werner syndrome or from HL60 mutants. This suggests that the Werner syndrome deletion mutator uses deletion mutagenesis pathway(s) that are similar or identical to those used in other human somatic cells.