Haploinsufficiency in tumor predisposition syndromes: altered genomic transcription in morphologically normal cells heterozygous for VHL or TSC mutation

Tumor suppressor genes and their effector pathways have been identified for many dominantly heritable cancers, enabling efforts to intervene early in the course of disease. Our approach on the subject of early intervention was to investigate gene expression patterns of morphologically normal one-hit cells before they become hemizygous or homozygous for the inherited mutant gene which is usually required for tumor formation. Here, we studied histologically non-transformed renal epithelial cells from patients with inherited disorders that predispose to renal tumors, including von Hippel-Lindau (VHL) disease and Tuberous Sclerosis (TSC). As controls, we studied histologically normal cells from non-cancerous renal epithelium of patients with sporadic clear cell renal cell carcinoma (ccRCC). Gene expression analyses of VHLmut/wt or TSC1/2mut/wt versus wild-type (WT) cells revealed transcriptomic alterations previously implicated in the transition to precancerous renal lesions. For example, the gene expression changes in VHLmut/wt cells were consistent with activation of the hypoxia response, associated, in part, with the Warburg effect. Knockdown of any remaining VHL mRNA using shRNA induced secondary expression changes, such as activation of NF?B and interferon pathways, that are fundamentally important in the development of RCC. We posit that this is a general pattern of hereditary cancer predisposition, wherein haploinsufficiency for VHL or TSC1/2, or potentially other tumor susceptibility genes, is sufficient to promote development of early lesions, while cancer results from inactivation of the remaining normal allele. The gene expression changes identified here are related to the metabolic basis of renal cancer and may constitute suitable targets for early intervention.

Revisiting tissue specificity of germline cancer predisposing mutations

Heritable germline mutations in major cancer genes generally lead to a restricted pattern of tissue-specific malignancies, yet many of the same mutations frequently occur somatically in a broad range of spontaneous neoplasms affecting different organs. Might this reflect a difference in tumorigenesis in children and adults?

Meta-analysis identifies NF-κB as a therapeutic target in renal cancer

Objective

To determine the expression patterns of NF-κB regulators and target genes in clear cell renal cell carcinoma (ccRCC), their correlation with von Hippel Lindau (VHL) mutational status, and their association with survival outcomes.

Methods

Meta-analyses were carried out on published ccRCC gene expression datasets by RankProd, a non-parametric statistical method. DEGs with a False Discovery Rate of < 0.05 by this method were considered significant, and intersected with a curated list of NF-κB regulators and targets to determine the nature and extent of NF-κB deregulation in ccRCC.

Results

A highly-disproportionate fraction (~40%; p < 0.001) of NF-κB regulators and target genes were found to be up-regulated in ccRCC, indicative of elevated NF-κB activity in this cancer. A subset of these genes, comprising a key NF-κB regulator (IKBKB) and established mediators of the NF-κB cell-survival and pro-inflammatory responses (MMP9, PSMB9, and SOD2), correlated with higher relative risk, poorer prognosis, and reduced overall patient survival. Surprisingly, levels of several interferon regulatory factors (IRFs) and interferon target genes were also elevated in ccRCC, indicating that an ‘interferon signature’ may represent a novel feature of this disease. Loss of VHL gene expression correlated strongly with the appearance of NF-κB- and interferon gene signatures in both familial and sporadic cases of ccRCC. As NF-κB controls expression of key interferon signaling nodes, our results suggest a causal link between VHL loss, elevated NF-κB activity, and the appearance of an interferon signature during ccRCC tumorigenesis.

Conclusions

These findings identify NF-κB and interferon signatures as clinical features of ccRCC, provide strong rationale for the incorporation of NF-κB inhibitors and/or and the exploitation of interferon signaling in the treatment of ccRCC, and supply new NF-κB targets for potential therapeutic intervention in this currently-incurable malignancy.

Altered gene expression in morphologically normal epithelial cells from heterozygous carriers of BRCA1 or BRCA2 mutations

We hypothesized that cells bearing a single inherited "hit" in a tumor suppressor gene express an altered mRNA repertoire that may identify targets for measures that could delay or even prevent progression to carcinoma. We report here on the transcriptomes of primary breast and ovarian epithelial cells cultured from BRCA1 and BRCA2 mutation carriers and controls. Our comparison analyses identified multiple changes in gene expression, in both tissues for both mutations, which were validated independently by real-time reverse transcription-PCR analysis. Several of the differentially expressed genes had been previously proposed as cancer markers, including mammaglobin in breast cancer and serum amyloid in ovarian cancer. These findings show that heterozygosity for a mutant tumor suppressor gene can alter the expression profiles of phenotypically normal epithelial cells in a gene-specific manner; these detectable effects of "one hit" represent early molecular changes in tumorigenesis that may serve as novel biomarkers of cancer risk and as targets for chemoprevention.

Epidemiology of genetically determined cancer

Dominantly heritable susceptibility is known for virtually every cancer. Susceptibility is typically restricted to one or a few tumours. For some tumours there appear to be at least two different predisposing conditions. Some mutant gene carriers survive to old age without developing the expected tumour(s). Some cases are new germline mutations. None of the conditions is very common, because of natural selection against gene carriers. Two questions arise: What is inherited? What is the relationship between the hereditary and non-hereditary forms of the same tumour? Retinoblastoma is a prototypic tumour. Penetrance in humans is nearly complete by the age of five years in the heritable form, which usually affects both eyes. Rare cases in which there is a constitutional deletion of chromosomal band 13q14 permitted localization of the responsible gene. Tumour formation is clearly a rare event at the cellular level, suggesting the necessity of a second, somatic, event. The difference in ages at diagnosis between unilateral and bilateral cases also suggests that two somatic events occur in non-hereditary cases. One explanation is that the gene is recessive and the second event involves loss of the remaining normal allele by mutation, non-disjunction, deletion or somatic recombination. The normal allele may be regarded as anti-oncogenic.

Radiation dose-rate effects, endogenous DNA damage, and signaling resonance

We previously concluded, from our analysis of the published data of other investigators, that the yield of germ-line and somatic mutations after exposure to ionizing radiation is parabolically related to the logarithm of the dose-rate at which a given dose is administered. Here we show that other data reveal a similarly parabolic relationship for other ionizing radiation-associated phenomena, namely, genetic recombination, chromosomal translocation, cell inactivation and lethality, and human leukemogenesis. Furthermore, the minima for all effects fall in a relatively narrow range of the dose-rate logarithms. Because the only mechanism common to all of these phenomena is the double-strand break (DSB) in DNA, we refer to our previous analysis of the endogenous production of DSBs, from which we concluded that approximately 50 endogenous DSBs occur per cell cycle, although most are repaired without error. Comparison then reveals that their rate of production falls within the range of minima for the several end points pursuant to radiation-induced DSBs. We conclude that the results reflect a physiological principle whereby signals originating from induced DSBs elicit responses of maximal effectiveness when they are produced at a rate near that of the production of endogenous DSBs. We refer to this principle as "signaling resonance."

Improvements in tumor targeting, survivorship, and chemoprevention pioneered by tamoxifen. A personal perspective

Twenty years ago, antiestrogen therapy with tamoxifen played only a secondary role in breast cancer care. All hopes to cure metastatic breast cancer were still pinned on either the discovery of new cytotoxic drugs or a dose-dense combination of available cytotoxic drugs with bone marrow transplantation. A similar strategy with combination chemotherapy was employed as an adjuvant for primary breast cancer. Simply stated, the goal was to kill the cancer with nonspecific cytotoxic drugs while keeping the patient alive with supportive care. However, medical research does not travel in straight lines, and an alternative approach emerged to solve the problem of controlling tumor growth with minimal side effects: targeted therapy. The approach of using long-term antihormone therapy to control early-stage breast cancer growth would revolutionize cancer care by targeting the tumor estrogen receptor (ER). The success of the strategy would contribute to a decrease in the national mortality figures for breast cancer. More importantly, translational research that targeted the tumor ER with a range of new antiestrogenic drugs would presage the current fashion of blocking survival pathways for the tumor by developing novel targeted treatments. But a surprise was in store when the pharmacology of "antiestrogens" was studied in detail: The nonsteroidal "antiestrogens" are selective ER modulators--ie, they are antiestrogens in the breast, estrogens in the bone--and they lower circulating cholesterol levels. This knowledge would establish a practical approach to breast cancer chemoprevention for women at high risk (tamoxifen) and low risk (raloxifene).

A PERSONAL SIXTY-YEAR TOUR OF GENETICS AND MEDICINE

The past 60 years surely constitute a Golden Age for biomedical science, and for medical genetics in particular. A personal experience began with an encounter with inborn errors of metabolism, selection, and the incidences of hereditary diseases, and peaked with molecular biology, virology, and cytogenetics, finally focusing all three on the problem of cancer.

Optimized procedures for microarray analysis of histological specimens processed by laser capture microdissection

Analysis of cell-specific gene expression patterns using microarrays can reveal genes that are differentially expressed in diseased and normal tissue, as well as identify genes associated with specialized cellular functions. However, the cellular heterogeneity of the tissues precludes the resolution of expression profiles of specific cell types. While laser capture microdissection (LCM) can be used to obtain purified cell populations, the limited quantity of RNA isolated makes it necessary to perform an RNA amplification step prior to microarray analysis. The linearity and reproducibility of two RNA amplification protocols--the Baugh protocol (Baugh et al., 2001, Nucleic Acids Res 29:E29) and an in-house protocol have been assessed by conducting microarray analyses. Cy3-labeled total RNA from the colorectal cell line Colo-205 was compared to Cy5-labeled Colo-205 amplified RNA (aRNA) generated with each of the two protocols, using a human 10K cDNA array. The correlation of the gene intensities between amplified and total RNA measured in the two channels of each microarray was 0.72 and 0.61 for the Baugh protocol and the in-house protocol, respectively. The two protocols were further evaluated using aRNA obtained from normal colonic crypt cross-sections isolated via LCM. In both cases a microarray profile representative of colonic mucosa was obtained; statistically, the Baugh protocol was superior. Furthermore, a substantial overlap between highly expressed genes in the Colo-205 cells and colonic crypts underscores the reliability of the microarray analysis of LCM-derived material. Taken together, these results demonstrate that LCM-derived tissue from histological specimens can generate abundant amounts of high-quality aRNA for subsequent microarray analysis.

Altered gene expression in phenotypically normal renal cells from carriers of tumor suppressor gene mutations

BACKGROUND

The inherently complex signaling networks of tumors result from genetic and epigenetic alterations that occur during cancer initiation and progression.

METHODS

In an attempt to identify early molecular changes associated with dominantly inherited predisposition to "two-hit" renal tumors, the expression profiles of primary cultures of phenotypically normal renal epithelial cells from individuals bearing a germline mutation in either the von Hippel-Lindau (VHL) or the tuberous sclerosis complex (TSC) gene were compared to that of renal epithelial cells from control nonmutation carriers by microarray analysis.

RESULTS

Reliability of the microarray data from pooled samples was confirmed by real-time RT-PCR. Principal Component Analysis revealed substantial differences in the gene expression profiles of the renal epithelial cells from VHL and TSC mutation carriers. In several instances, the microarray data confirm our present knowledge of the cellular pathways affected by biallelic VHL and TSC mutations.

CONCLUSIONS

These findings demonstrate that heterozygosity for a mutant tumor suppressor gene may alter the expression profiles of phenotypically normal epithelial cells in a gene-specific manner. Detectable effects of "one-hit" represent early molecular changes in tumorigenesis that may serve as targets for chemopreventive intervention.

Use of RNA amplification in the optimal characterization of global gene expression using cDNA microarrays

Microarray analysis of human tissue is frequently hindered by the limited amount of RNA available. Although amplification protocols can be utilized, the relative representation of transcripts present in the starting material must remain unaltered. In this study, 200 ng of total RNA derived from cultured renal epithelial cells from tuberous sclerosis complex (TSC) carriers and control individuals was amplified by in vitro transcription with T7 RNA polymerase. The resulting Cy-labeled cDNAs (from total or amplified RNA (aRNA)) were analyzed as direct replicates and dye-flips on slides containing 10,000 human cDNAs. The Pearson correlation coefficients for the direct replicate experiments were 0.80 (20 microg total RNA), 0.85 (40 microg total RNA), and 0.93 (2 microg of aRNA). Comparisons between the array data revealed that the majority of genes expressed in total RNA (97% for 20 microg and 85% for 40 microg) were also detected in aRNA. The correlation coefficient of the expression ratios for genes detected in both total RNA (40 microg) and aRNA was 0.63. Further, Student's t-test indicated no significant difference (P = 0.83) between these ratios. These results indicate that the number of expressed genes detected with total RNA is proportional to the amount of RNA used and underscore the requirement of large amounts of total RNA for a comprehensive characterization of gene expression profiles. RNA amplification allows the detection of a large number of genes expressed in the starting RNA population without altering their relative intensities significantly. Thus, an RNA amplification step improves the quality of gene expression results obtained by microarray analysis. This study indicates that high quality microarray data can be generated from small amounts of RNA, including those extracted from limiting clinical samples and microdissected histological specimens.

Endogenous DNA double-strand breaks: production, fidelity of repair, and induction of cancer

This article extends our previous quantitative analysis of the relationship between the dynamics of the primary structure of DNA and mutagenesis associated with single-strand lesions to an analysis of the production and processing of endogenous double-strand breaks (EDSBs) and to their implications for oncogenesis. We estimate that in normal human cells approximately 1% of single-strand lesions are converted to approximately 50 EDSBs per cell per cell cycle. This number is similar to that for EDSBs produced by 1.5-2.0 Gy of sparsely ionizing radiation. Although EDSBs are usually repaired with high fidelity, errors in their repair contribute significantly to the rate of cancer in humans. The doubling dose for induced DSBs is similar to doubling doses for mutation and for the induction of carcinomas by ionizing radiation. We conclude that rates of production of EDSBs and of ensuing spontaneous mitotic recombination events can account for a substantial fraction of the earliest oncogenic events in human carcinomas.

Cancer genetics

Cancer is a genetic disease of somatic cells. Tumor karyotypes are rarely normal, and most show multiple abnormalities of both number and structure. The first direct evidence for this concept of cancer came from studies of tumor-specific translocations in leukemias and lymphomas, revealing the importance of oncogenes and the regulation of gene transcription in cancer. A second major source of information about human cancer genes is hereditary cancer. Genetic predisposition of the autosomal dominant type imposes a high relative risk for one or more kinds of cancer. In the past decade or so, more than 30 mutant genes for such hereditary cancers have been cloned. Penetrance depends upon additional, somatic, mutations. A few of the genes are oncogenes or DNA repair genes, but most are tumor suppressor genes. Some tumor suppressors regulate transcription, while others operate in signal transduction pathways that are involved in regulating processes of cell birth, differentiation, and death. The knowledge gained is stimulating new approaches to the treatment and prevention of cancer.

Two genetic hits (more or less) to cancer

Most cancers have many chromosomal abnormalities, both in number and in structure, whereas some show only a single aberration. In the era before molecular biology, cancer researchers, studying both human and animal cancers, proposed that a small number of events was needed for carcinogenesis. Evidence from the recent molecular era also indicates that cancers can arise from small numbers of events that affect common cell birth and death processes.

Chasing the cancer demon

Boveri's idea that somatic mutations are at the root of cancer found its first specific support with the investigation of leukemia and Burkitt's lymphoma, and the discovery of the mechanism of oncogene activation by balanced translocation. The study of retinoblastoma later led to the cloning of the first antioncogene, or tumor suppressor gene, and to understanding the mechanisms by which the wild-type genes lose activity. Only a small subset of cancer involves simple mechanisms. A category of hereditary disorders called the phakomatoses provide a perspective on the chain of oncogenic events in such cancers because of two-hit precursor lesions that have a low probability of malignant transformation. The common carcinomas are much more complex and are typically genetically unstable, owing either to mutational instability or chromosomal instability.

Mechanism and relevance of ploidy in neuroblastoma

Neuroblastoma has a broad spectrum of clinical behavior, ranging from spontaneous regression to dissemination and fatality. The heterogeneity that has long puzzled many investigators has been shown by more recent studies to be closely correlated with various clinical and genetic factors. Tumor cell ploidy is one of the factors; diploid and near-triploid neuroblastomas show poor and excellent clinical outcomes, respectively. We offer a hypothesis that explains how the ploidy state of the tumor plays a fundamental role in this heterogeneity, and why various prognostic factors are correlated with each other. This hypothesis may be applicable to tumors other than neuroblastoma.

Inverse radiation dose-rate effects on somatic and germ-line mutations and DNA damage rates

The mutagenic effect of low linear energy transfer ionizing radiation is reduced for a given dose as the dose rate (DR) is reduced to a low level, a phenomenon known as the direct DR effect. Our reanalysis of published data shows that for both somatic and germ-line mutations there is an opposite, inverse DR effect, with reduction from low to very low DR, the overall dependence of induced mutations being parabolically related to DR, with a minimum in the range of 0.1 to 1.0 cGy/min (rule 1). This general pattern can be attributed to an optimal induction of error-free DNA repair in a DR region of minimal mutability (MMDR region). The diminished activation of repair at very low DRs may reflect a low ratio of induced ("signal") to spontaneous background DNA damage ("noise"). Because two common DNA lesions, 8-oxoguanine and thymine glycol, were already known to activate repair in irradiated mammalian cells, we estimated how their rates of production are altered upon radiation exposure in the MMDR region. For these and other abundant lesions (abasic sites and single-strand breaks), the DNA damage rate increment in the MMDR region is in the range of 10% to 100% (rule 2). These estimates suggest a genetically programmed optimatization of response to radiation in the MMDR region.

Hereditary predisposition to cancer

Both hereditary and environmental factors influence the risk of cancer. Four risk categories, or oncodemes, can exist for a particular kind of cancer, depending upon the presence of neither, one, or both factors: (1) spontaneous, or background; (2) hereditary; (3) environmental; (4) interactive. In the second, mutation imparts a high relative risk, but a generally low attributable risk; in the fourth, the opposite obtains. The second oncodeme contains genes that are also important for the non-hereditary forms of the same cancer. Probably all forms of cancer exist in a dominantly heritable form. Most of the genes are tumor suppressors, although a few are oncogenes or DNA repair genes. The mutations are in most, if not all cases, maintained in a population by an equilibrium between mutation and selection. Most of the cloned genes are expressed widely among tissues, yet there is typically some tumor specificity. Somatic mutations in second alleles at the relevant loci are necessary, but generally not sufficient for carcinogenesis, although they, in some instances, lead to the formation of benign precursor lesions. Further events are necessary for carcinogenesis. This is particularly true for carcinomas. The benign lesions appear to involve an increase in number of long-lived cells that can accumulate other mutations. For some tumors, physiologic events, such as tissue growth at puberty or proliferation of embryonic stem cells, may produce this effect. Mutations of DNA mismatch repair genes underscore the effect that changes in somatic mutation rates can have, especially in the risk for multi-event carcinomas. Conversely, these are the tumors that offer the greatest opportunity for prevention.

Hereditary cancer: two hits revisited

According to a "two-hit" model, dominantly inherited predisposition to cancer entails a germline mutation, while tumorigenesis requires a second, somatic, mutation. Non-hereditary cancer of the same type requires the same two hits, but both are somatic. The original tumor used in this model, retinoblastoma, involves mutation or loss of both are somatic. The original tumor used in this model, retinoblastoma, involves mutation or loss of both copies of the RB1 tumor-suppressor gene in both hereditary and non-hereditary forms. In fact, most dominantly inherited cancers show this relationship. New dominantly inherited cancers show this relationship. New questions have arisen, however. When a tumor-suppressor gene is ubiquitously expressed, why is there any specificity of tumor predilection? In some instances, it is clear that two hits produce only a benign precursor lesion and that other genetic events are necessary. As the number of necessary events increase, the impact of the germline mutation diminishes. The number of events is least for embryonal tumors, and relatively small for certain sarcomas. Stem-cell proliferation evidently plays a key role early in carcinogenesis. In some tissues it is physiological, as in embryonic development and in certain tissues in adolescence. In adult renewal tissues, the sites of the common carcinomas, mutation may be necessary to impair the control of switching between renewal and replicative cell divisions; the APC gene may be the target of such a mutation.

Predisposition to renal carcinoma in the Eker rat is determined by germ-line mutation of the tuberous sclerosis 2 (TSC2) gene

Genetic predisposition to neoplasia often involves tumor suppressor genes. One such model of hereditary renal carcinoma was described in the rat by Eker. These tumors share morphologic similarities with human renal cancer. Linkage analysis localized the inherited mutation to rat chromosome band 10q12. This region is syntenic with human chromosome band 16p13.3, the site of the tuberous sclerosis 2 (TSC2) gene. A specific rearrangement of the rat homologue of TSC2 was found to cosegregate with carriers of the predisposing mutation. Tumors with or without loss of heterozygosity expressed only the mutant allele, consistent with the two-hit hypothesis. This mutation gave rise to an aberrant transcript that deletes the 3' end normally containing a region of homology with the catalytic domain of rap1GAP.

Genetic predisposition to transplacentally induced renal cell carcinomas in the Eker rat

N-Ethyl-N-nitrosourea-induced transplacental renal carcinogenesis in the rat results primarily in Wilms' tumors, apparently because primitive nephroblasts are the preferred target. Our question is whether N-ethyl-N-nitrosourea-induced mutations in the fetal kidney would increase the number of adult-type renal cell carcinomas in the Eker rat, which is heterozygous for a mutation that predisposes to renal cell carcinoma. Surprisingly, renal cell tumors but no Wilm's tumors began to appear from as early as 1 week after birth. Thus, the inheritance of a renal cell carcinoma mutation determines the specificity of tumor histology even with in utero carcinogenesis.

The retinoblastoma-related gene, RB2, maps to human chromosome 16q12 and rat chromosome 19

A retinoblastoma-related human gene, referred to as RB2, has been cloned based on sequence homology of the E1A-binding domain of the retinoblastoma gene. Structural homology with the retinoblastoma gene suggests a possible function of RB2 as a tumor suppressor gene. In this study, we have mapped this gene to human chromosome 16q12.2 and rat chromosome 19, using fluorescence in situ hybridization and somatic hybrid cell analysis, respectively. Based on known syntenic relationships among human, rat and mouse, the data suggest that the mouse homolog resides on chromosome 8. Deletions of chromosome 16q have been found in several human neoplasias (including breast, ovarian, hepatic, and prostatic cancers) which is in support of an involvement of RB2 in human cancer as a tumor suppressor gene.

Antioncogenes and human cancer

The antioncogenes, or tumor suppressor genes, as negative regulators of cell division, stand in contrast to oncogenes. For most human cancers, the more frequently mutated genes are the antioncogenes, the principal exception being the leukemias and lymphomas. Persons heterozygous for germ-line mutations in antioncogenes are strongly predisposed to one or more kinds of cancer, and most dominantly inherited cancer is attributable to such heterozygosity. Seven antioncogenes have been cloned through the study of these persons, and several others have been mapped. An eighth one was mapped and cloned through the investigation of tumors and is not yet known in hereditary form. Three dominantly inherited forms of cancer are not attributable to mutations in antioncogenes. The corresponding nonhereditary forms of most cancers generally reveal abnormalities of the same antioncogenes that are found in the hereditary forms but may also show additional ones. Some cancers, especially the embryonal tumors of children, have a small number of antioncogene mutations; some others, such as most sarcomas, have more, and the common carcinomas have the most, reflecting a hierarchy of controls over growth of stem cell populations. Still more members of this gene category remain to be mapped and cloned through the study of cancer families and of tumors. The genes that have been cloned act at diverse points in the signal transduction pathway in cells, from the outer cell membranes to sites of gene transcription, in some cases as negative regulators of oncogene expression.

Assignment of 22 loci in the rat by somatic hybrid and linkage analysis

Twenty structural genes and two unique anonymous DNA fragments have been mapped in the rat (Rattus norvegicus) with a panel of mouse x rat hybrids and linkage analysis. Ten of the 20 autosomes are represented by at least one of these markers. A new syntenic relationship among rat Chromosome (Chr) 16, mouse Chr 14, and human Chr 10q was established. Results of this study further support the extensive conservation of synteny between the rat and mouse and, to a lesser degree, between rat and human.

Susceptibility to renal carcinoma in the Eker rat involves a tumor suppressor gene on chromosome 10

Germ-line mutations of tumor suppressor genes confer strong predisposition to tumor formation. In the rat, a form of dominantly inherited renal carcinoma (RC) results in multiple chromophobe cell tumors that resemble the human disease, and heterozygous carriers (RC/+) are highly susceptible to environmental agents (radiation and chemical carcinogens), making it a desirable model to study epithelial carcinogenesis. By linkage analysis, the locus of the inherited RC mutation was mapped to rat chromosomal band 10q12, near the protamine locus (logarithm of odds score = 17.96). Renal tumors also showed a loss of heterozygosity at this locus, lending support to the recessive nature of this putative tumor suppressor gene. Our result suggested that the human homolog of the RC gene may reside on human chromosome 16, not known to be altered commonly in human RC.

Pediatric molecular oncology. Past as prologue to the future

The great successes in the treatment of childhood cancers have been followed in recent years with a new understanding of the molecular genetic abnormalities that underlie their origin. For some cancers these genetic changes are in oncogenes; in others, they are in antioncogenes, or tumor suppressor genes. Initiating aberrations in the former are associated characteristically with chromosomal translocations, the analysis of which should soon lead to the identification of oncogenes other than those in the myc family. Several tumor suppressor genes of importance in the childhood cancers have been cloned and others should soon follow. It is reasonable to expect that during the current decade molecular understanding of the genetic defects in the major pediatric cancers will be achieved. The knowledge gained through these studies, past and future, may provide new approaches to prevention and treatment and lead to additional progress.

Introduction to the genetics of primary renal tumors in children

Wilms tumor can be explained only partially by the "two hit" model that was originally developed for retinoblastoma. Heterogeneity of two kinds operates. The first is that four other primary tumors are regularly observed in children, and the second is that Wilms tumor itself appears to represent more than one genetic entity. All five of these primary renal tumors arise from primary or secondary mesenchyme, renal blastema, or renal epithelium. Mesoblastic nephroma, and possibly clear cell sarcoma, may have some genetic affinity with Wilms tumor, but rhabdoid tumor of the kidney and renal carcinoma do not. At least three different genes seem to be important in the origin of Wilms tumor. One, WT1, whose mutations may be associated with aniridia, may follow the "two hit" model in that there are cases in which both copies of the gene are defective or lost, as expected for a tumor suppressor gene. A second gene, which is associated with Beckwith-Wiedemann Syndrome (BWS) and which has not been cloned, appears to be imprinted in females, and may have an oncogene function. It is evidently activated by gain of a paternal allele or by loss of the inactive, but possibly trans-sensing, maternal allele. Activation of the insulin-like growth factor II gene may be a final common pathway for mutation in both WT1 and BWS. A third gene is unlinked to either of the other two, but its location and function are unknown. It shares with WT1 specificity for Wilms tumor, which is not true of the BWS gene.

New markers, D16FC1 and Tp12, differentiate between rat chromosomes 16 and 17

Problems in differentiating rat chromosomes 16 and 17 cytogenetically can be resolved with unique probes mapped to these chromosomes. Using somatic cell hybridization and nonisotopic in situ hybridization, probes D16FC1 and Tp12 were localized to 16p16-->p15 and 17q12.1-->q12.2, respectively. The locations of these probes can serve as reference points to facilitate mapping of future probes to rat chromosomes 16 and 17.

Spontaneous and radiation-induced renal tumors in the Eker rat model of dominantly inherited cancer

Hereditary renal carcinoma (RC) in the rat, originally reported by R. Eker in 1954, is an example of a Mendelian dominant predisposition to a specific cancer in an experimental animal. At the histologic level, RCs develop through multiple stages from early preneoplastic lesions (e.g., atypical tubules) to adenomas in virtually all heterozygotes by the age of 1 year. The homozygous mutant condition is lethal at approximately 10 days of fetal life. Ionizing radiation induces additional tumors in a linear dose-response relationship, suggesting that in heterozygotes two events (one inherited, one somatic) are necessary to produce tumors, and that the predisposing gene is a tumor suppressor gene. No genetic linkage has yet been found between the Eker mutation and rat DNA sequences homologous to those in human chromosome 3p, the presumed site of the putative tumor suppressor gene responsible for human RC. Nonrandom loss of rat chromosome 5 in RC-derived cell lines is sometimes associated with homozygous deletion of the interferon gene loci at rat chromosome bands 5q31-q33. Since this locus is not linked with the predisposing inherited gene in the Eker rat, it probably represents a second tumor suppressor gene involved in tumor progression.

Stem cell regulation, tissue ontogeny, and oncogenic events

The number of necessary oncogenic events is a function of tissue ontogeny. A minimum of two events appears to suffice for certain embryonal tumors, leukemias, and lymphomas for which the target tissues normally show stem cell proliferation. Other tumors, including those featured in the Li-Fraumeni syndrome, arise in target tissues whose stem cells are conditionally stimulated to proliferate, as in response to hormones, and involve more events. The most complex cancers include most carcinomas, which arise in renewal tissues whose stem cells do not normally proliferate. The number of necessary oncogenic events appears to increase with the number of controls on proliferation.

Localization of the interferon-alpha gene cluster to rat chromosome bands 5q31----q33 by fluorescence in situ hybridization

The chromosomal location of the rat interferon-alpha (IFNA) gene cluster was determined by fluorescence in situ hybridization. The fluorescent signals were localized to 5q31----q33. A previous report, using somatic cell hybrids, had suggested that the IFNA locus maps at a different region on rat chromosome 5.