Mutation theory of carcinogenesis in retinoblastoma

Analysis of retinoblastoma age incidence data using a fully stochastic cancer model

Retinoblastoma (RB) is an important ocular malignancy of childhood. It has been commonly accepted for some time that knockout of the two alleles of the RB1 gene is the principal molecular target associated with the occurrence of RB. In this article, we examine the validity of the two-hit theory for RB by comparing the fit of a stochastic model with two or more mutational stages. Unlike many such models, our model assumes a fully stochastic stem cell compartment, which is crucial to its behavior. Models are fitted to a population-based dataset comprising 1,553 cases of RB for the period 1962-2000 in Great Britain (England, Scotland and Wales). The population incidence of RB is best described by a fully stochastic model with two stages, although models with a deterministic stem cell compartment yield equivalent fit; models with three or more stages fit much less well. The results strongly suggest that knockout of the two alleles of the RB1 gene is necessary and may be largely sufficient for the development of RB, in support of Knudson's two-hit hypothesis.

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.

A long view of fashions in cancer research

Despite the spectacular contributions to knowledge made by molecular biology during the last half century, cancer research has not delivered an agreed explanation of how malignant tumours originate. The models assiduously investigated in molecular terms largely reflect waves of fashion, and time has revealed their inadequacy: cancer is (1) not caused by the direct action of oncogenes, (2) not fully explained by the impairment of tumour suppressor genes, (3) not set in motion by mutations controlling the cell cycle, (4) not governed by the dependence of malignant tumours on an adequate blood supply and (5) not triggered by a failure of programmed cell death. But there is now strong evidence that cancers may have their origin in mutations that block the execution of critical steps in the process of normal differentiation. Cancer, thus seen, is not initially a disease of cell multiplication, but a disease of differentiation. The evidence for this point of view should now be explored.

Distribution of tumors in the retina in hereditary retinoblastoma patients

We have compared the location of the ocular tumors in hereditary retinoblastoma in relation to the age of the patients at time of diagnosis. Eighty fundus drawings were analyzed from 59 hereditary patients containing I 59 tumors. At the time of diagnosis, indirect ophthalmoscopy was performed under general anaesthesia in all patients and standard drawings of the retina were made depicting the number and relative location of all tumors. The distance between the center of the tumor and the center of the macula was measured and plotted against the age of the patients at time of diagnosis. The results show that the distance between the center of the tumor and the center of the macula at time of diagnosis increases with age during the first seven months after birth.

Familial and sporadic human renal cell carcinoma: evidence against a double-loss mechanism of carcinogenesis

It has been speculated that renal cell carcinoma (RCC) is an example of a double-loss mutation. We analyzed the age distribution of 71 cases of familial RCC and of 11 population-based cancer registries [German Democratic Republic, Denmark, Finland, Norway, Sweden, U.S.A. Whites, U.S.A. Blacks, Miyagi and Osaka Prefectures (Japan), Hong Kong, and Israeli Jews] according to the multi-hit and clonal growth models of carcinogenesis. The analysis rules out a double-loss mechanism for RCC. On both of the two models analyzed, carcinogenesis in the familial cases of RCC arises as a result of a three- to ten-fold increase in the average rate of mutation at the susceptible loci, as compared with the sporadic cases. In general, the clonal growth model provides a somewhat better fit to the age-distribution of RCC incidence than does the multi-hit model.

Retinoblastoma: a model for deriving the mutation rate without using any estimate of the size of the population at risk

Paediatric cancer of the retinae arises from a transient population of stem cells whose growth and decay are responsible for the development of the fully differentiated photo-receptors and nerve cells. A model is described which fits the data on the incidence of bilateral retinoblastoma and gives the somatic mutation rate without using an estimate of the size of the population at risk. It also gives the shape of the retinoblast growth/decay curve. The model has been tested on two independent sets of data from the USA, and given that both sets are representative of the USA as a whole, there seems to have been little change in the somatic mutation rate over the last 30 or so years. For a total retinoblast formation of 4 x 10(6) cells, the average mutation rate is 2.94 x 10(-7) per cell per year.

Distinct RB1 gene mutations with low penetrance in hereditary retinoblastoma

The interfamilial diversity in penetrance and expressivity of hereditary retinoblastoma was investigated in 29 families. By using a simple parameter for estimating the severity of the disease (diseased-eye-ratio), we were able to identify four families with a discrete low-penetrance phenotype. The underlying genetic defect was identified in three families. One family has a 3-bp deletion in exon 16 that results in the deletion of Asn480. In two further unrelated families, the identical missense mutation at codon 661 in exon 20 (CGG to TGG, Arg to Trp) was identified. These mutations are distinct from the majority of retinoblastoma gene alterations, as they do not result in the disruption of the gene product. We propose that reduced penetrance of retinoblastoma is the result of a residual function of these alleles in retinoblastoma precursor cells.

Patterns of risk of hereditary retinoblastoma and applications to genetic counselling

A registry including information about nearly 1,600 cases of retinoblastoma diagnosed in Britain has been created at the Childhood Cancer Research Group. Cases have been classified as 'old germ cell mutation', 'new germ cell mutation' or 'sporadic non-hereditary'. For a population-based group of 918 cases diagnosed between 1962 and 1985 we have calculated the proportions of unilateral/bilateral and hereditary/non-hereditary cases. Bilateral cases represent 40% of the total number over this period; the proportion known to be hereditary is 44%, a higher proportion than has been reported elsewhere. By following up selected groups of cases, an estimate has been made of the proportions of siblings of retinoblastoma patients and offspring of survivors from retinoblastoma who are themselves affected with the disease. Where there is no previous family history, the risk for siblings of retinoblastoma patients of developing the disease is approximately 2% if the disease in the affected child is bilateral and 1% if it is unilateral, assuming that there are no other siblings; if there are unaffected siblings the risks for subsequent children are lower. Children of patients with hereditary retinoblastoma have a one in two chance of carrying the germ cell mutation and for those who are carriers the probability of developing retinoblastoma is very close to the accepted figure of 90% if the parents have bilateral retinoblastoma but probably less if they have the unilateral form. For children of patients not known to be carriers, the probability of developing retinoblastoma is estimated to be about 1%, considerably lower than the previously accepted figure of about 5%. Retinoblastoma kindreds consist mainly of bilateral cases but there is evidence that some kindreds have a high proportion of unilateral cases. The ways in which these findings may be used in conjunction with modern techniques of molecular biology for prenatal and postnatal genetic counselling are discussed.

Testing and characterizing the two-stage model of carcinogenesis for a wide range of human cancers

The age dependence of incidence for 45 cancer types in three populations is analyzed on a two-stage kinetic model containing three determinable parameters: (i) the fraction of population at risk for a cancer; (ii) the product of the frequencies of cancer-producing mutations, and (iii) the growth rate of the transformed clone from which a cancer ultimately evolves. The model, simplifying that proposed by others, fits many cancers. Data are easily handled in terms of the derived parameters, providing the basis for epidemiological analysis, here applied in detail to liver, cervix and testis cancers for nearly 50 world-wide populations. We identify 12 cancers for which only a limited fraction of the population is at risk. We argue that the appearance of most cancers requires at least three mutational events. For child, youth, or early adult cancers, one mutation may be congenital. We arrange the various cancers in a descending scale, defining six groups that differ in the derived mutation frequencies. A cancer appearing later in life, for which the whole population can be at risk, shows a low mutation frequency, consistent with background spontaneous mutation. The other cancers require increases in mutation frequency, arising from an increased rate of cell division and/or mutation rate.

Types, rates, origin and expressivity of chromosome mutations involving 13q14 in retinoblastoma patients

A cytogenetic survey of 200 retinoblastoma (Rb) patients revealed that approximately 8.5% of the fresh germinal mutations were microscopically detectable chromosome mutations, either interstitial deletions or rearrangements, involving 13q14. They showed a strong bias toward paternal origin, indicating a significant contribution of errors in paternal meiotic processes. The incidence of patients with Rb due to such chromosome mutations was estimated to be 1.9 x 10(-6) of live births. Age-specific incidence of Rb tumors suggested that the Rb mutations by such chromosomal mechanisms had a lower carcinogenic potential, as indicated by the later onset of disease, than other Rb mutations of germinal origin.

Bilateral adrenal ganglioneuroblastoma with neuromelanin. Clinical and pathologic observations

A case of bilateral adrenal ganglioneuroblastoma in an adolescent boy is described. The clinical evolution was remarkable for its quiescent course, despite unfavorable prognostic indicators that included: intraabdominal primary site, large tumors on both sides of the midline, and "unfavorable" pathologic subtype. There was no family history of neuroblastomas. Production of neuromelanin was manifested by this unusual tumor. A review of published cases of multifocal neuroblastomas suggests that these tumors follow an uncommon natural history, at variance with the expected behavior of the usual unicentric and sporadic type of neuroblastoma.

Pineal malignant neoplasm in association with hereditary retinoblastoma

A patient with unilateral hereditary retinoblastoma who was successfully treated at the age of 7 weeks developed a tumour in the pineal region two and a half years later. The initial response to radiation treatment of the latter lesion was not maintained. Subsequent necropsy findings are described. Clinically and pathologically this case represents an example of the recently described trilateral retinoblastoma. The response to treatment after early recognition was disappointing.

Retinoblastoma mutation rate in New Zealand and support for the two-hit model

Age-specific incidence rates for 96 New Zealand patients with sporadic retinoblastoma peaked earlier for bilateral patients than for unilateral patients. The cumulative log survival until diagnosis for bilateral and unilateral patients followed linear and quadratic curves respectively, and supported the two-hit hypothesis for retinoblastoma. The germ cell mutation rate for retinoblastoma, assuming a single major gene, was calculated to be in the order of 9.3 X 10(-6) to 10.9 X 10(-6) for the New Zealand population.

The incidence and epidemiology of retinoblastoma in New Zealand: A 30-year survey

One hundred cases of retinoblastoma were diagnosed in New Zealand-born children between 1948 and 1977 inclusive. Five patients had an affected parent, and of the remaining sporadic cases 25 had bilateral and 70 unilateral tumours. The frequency of retinoblastoma, 1 in 17,500 births, was similar to that reported for most other countries. There was no evidence of an increase in the incidence of all cases of sporadic retinoblastoma during the 30-year period studied, nor was there any significant fluctuation in their incidence with space and time. There was an excess of bilateral sporadic cases in the southern-most districts of New Zealand, but this was of marginal significance. There was no significance evidence for any environmental influence on the occurrence of retinoblastoma.

Sister chromatid exchanges in patients with retinoblastoma

Spontaneous and mitomycin C(MMC)-induced sister chromatid exchanges were studied in 11 patients with retinoblastoma and 7 normal controls. Spontaneous rates were similar in patients and in controls. The MMC-induced rate was found to be significantly higher in bilaterally affected patients than in controls. It is suggested that this increase may be due to a DNA repair deficiency. However, it is not possible to clarify whether this abnormality is associated with the retinoblastoma gene or with another factor acting on the degree of expressivity of the disease in gene carriers.

Segregation analysis in hereditary retinoblastoma

Segregation analysis was performed in 211 nuclear families belonging to 166 pedigrees of hereditary retinoblastoma found in a number of series which have been gathered from the literature. Bilaterally affected carriers appear homogeneous. The segregation ratio in their offspring is 0.49 and the proportion of bilateral cases among affected offspring is 0.87. Both unilaterally affected and unaffected carriers appear heterogeneous. The very low segregation ratio (0.08) in the offspring of unilateral carriers who are not detected through an affected child, suggests the possibility of two types of carriers. "high" and "low transmitters." The proportions of "low transmitters" was estimated as 0.14 among all familial unilateral carriers and as 0.45 among all detected unaffected carriers. Unilateral and unaffected "high transmitters" give a significantly lower segregation ratio than bilaterally affected carriers. On the one hand, the existence of these two different types of carriers provides arguments in support of the hypothesis of delayed mutation. On the other hand, the differences in penetrance among "high transmitters." according to their phenotype, supports the hypothesis of host resistance. Under the two-mutation hypothesis, the possibility that the mutation rate is variable among individuals and partly genetically determined, is suggested.

An in vitro investigation of genetic susceptibility to cancer in diploid fibroblasts from retinoblastoma patients

Thirty-two diploid fibroblast strains from individuals with the three forms of retinoblastoma were examined with an in vitro clonogenic survival assay to determine their sensitivity to killing by X rays. Strains from sporadic unilateral retinoblastoma patients and normal controls were indistinguishable from one another (group D0's = 147 +/- 15 rads and 146 +/- 5 rads, respectively) while the strains from patients with the hereditary form of the disease were significantly more X-ray sensitive (D0 = 111 +/- 12). Strains derived from individuals with the D-deletion form of the disease resembled the hereditary strains with respect to sensitivity and heterogeneity, suggesting a possible, common etiology for these two forms of the disease. A DNA-repair defect is hypothesized as reflected by the observed hypersensitivity to X-irradiation in these cells. We suggest that this defect may be associated with the enhanced frequency of spontaneous and radiation-induced second tumors seen in some retinoblastoma patients.

Relationship between dose, time, and tumor yield in mouse dimethylhydrazine-induced colon tumorigenesis

Colorectal tumor yield and volume data were obtained using 355 CF1 mice serially sacrificed up to 84 weeks following various dose levels of the carcinogen 1,2-dimethylhydrazine dichloride (DMH). Several conclusions were reached: (a) With increasing doses of DMH, there was an increased tumor yield and decreased latency period. (b) With repeated doses, there was a rapidly cumulative tumor yield. (c) New tumors continued to accumulate in the colon and rectum even at long intervals after the DMH treatments. This was substantiated by a positive correlation between the number of tumors per colon and the delay after DMH. In addition, when several tumors were present in the same mouse, their sizes were graded rather than uniform. These observations are consistent with a 2- or multi-step carcinogenesis mechanism. The latter implies that DMH induces a permanent transmissible alteration within some cells which thereafter will be at risk for further alterations capable of initiating cancer growth.

Retinoblastoma: correlation of invasion of the optic nerve and choroid with prognosis and metastases

The histological sections of 74 retinoblastoma patients were reviewed. The extent of optic nerve invasion was correlated with choroidal/scleral extension. Choroidal invasion carries 100% survival provided that the sclera, iris, and whole optic nerve are not also involved. Invasion of the optic nerve beyond the lamina cribrosa also carries 100% survival provided that the resection line is free and that invasion does not involve the sclera or iris. Plentiful rosettes were usually found in those tumours that had not extended beyond the choroid or as far as the resection line of the optic nerve and were therefore associated with a good prognosis. The absence of rosettes did not necessarily indicate a poor prognosis. The site of metastases was related to optic nerve and scleral/extrascleral extension. The various systems of staging retinoblastoma are compared, and a new system is proposed to cover the whole spectrum of the disease and to offer a reliable guide to prognosis and treatment.