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.

Hereditary cancers: from discovery to intervention.

This conference concerned hereditary cancers of the breast, ovary, and colon, which are the common, often fatal, cancers with the greatest heritability in their causation. Four genes whose mutations impart dominantly heritable predisposition to one or more of these cancers have been cloned and one more has been mapped. The most molecular details are known for colon cancer. The APC gene of familial polyposis coli leads to the accumulation of numerous polyps, but the probability of transformation of the latter to cancer is low. This provides the opportunity to monitor putative preventive measures with an intermediate end point. In hereditary nonpolyposis colon cancer, transformation of the polyp to cancer is accelerated by an inherited mutation in either of two DNA mismatch repair genes. The discovery of an intermediate end point could be very helpful for breast cancer. Testing persons at risk for predisposing mutations depends heavily on the availability of promising measures for prevention or treatment.

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.