Spontaneous mutation rate in retinoblastoma

The hierarchical model of stem cell genesis explains the man mouse paradox, Peto's paradox, the red cell paradox and Wright's enigma

The central dogma of carcinogenesis is that deleterious mutations accumulate in regularly cycling stem cells and eventually one of the cells will acquire a specific set of mutations which leads to uncontrolled cell proliferation. Each mutation is rare and the specific set is extremely rare so that even though there are millions of stem cells in a small area of mucosa the specific set of mutations to initiate the process of malignancy will only arise in one stem cell at most; hence neoplasia is clonal. But this model predicts that men, who are 1000 times larger than mice and live 30 times as long, should have a vastly increased risk of cancer compared with mice, but they don't (man-mouse paradox). The model also predicts that the prevalence of cancer in men should rise as power function of age and mutagen dose, the former is correct but not the latter (Peto's paradox). Furthermore there are more mitotic divisions in red cell precursors than in all other stem cells combined and yet erythroleukaemia is rare (red cell paradox). The central dogma is also challenged by Wright's enigma; the observation that some gastro-intestinal neoplasms are polyclonal in origin. The problem with the central dogma is the concept of a regularly cycling stem cell. In fact it is possible to produce all the cells that arise in a human lifetime with fewer than 60 rounds of DNA replication separating the zygote from mature differentiated cells in extreme old age. This hierarchical model of stem cell genesis leads to a very low prevalence of cancer, unless the orderly progression of the hierarchy is disturbed by inflammation, ulceration or trauma. This model explains the paradoxes and Wright's enigma. It is suggested that the number of cell divisions that separate the zygote from stem cells is a key variable in carcinogenesis.

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.

Rate, molecular spectrum, and consequences of human mutation

Although mutation provides the fuel for phenotypic evolution, it also imposes a substantial burden on fitness through the production of predominantly deleterious alleles, a matter of concern from a human-health perspective. Here, recently established databases on de novo mutations for monogenic disorders are used to estimate the rate and molecular spectrum of spontaneously arising mutations and to derive a number of inferences with respect to eukaryotic genome evolution. Although the human per-generation mutation rate is exceptionally high, on a per-cell division basis, the human germline mutation rate is lower than that recorded for any other species. Comparison with data from other species demonstrates a universal mutational bias toward A/T composition, and leads to the hypothesis that genome-wide nucleotide composition generally evolves to the point at which the power of selection in favor of G/C is approximately balanced by the power of random genetic drift, such that variation in equilibrium genome-wide nucleotide composition is largely defined by variation in mutation biases. Quantification of the hazards associated with introns reveals that mutations at key splice-site residues are a major source of human mortality. Finally, a consideration of the long-term consequences of current human behavior for deleterious-mutation accumulation leads to the conclusion that a substantial reduction in human fitness can be expected over the next few centuries in industrialized societies unless novel means of genetic intervention are developed.

Single genetic mutations can account for melanocytic naevi

BACKGROUND

The nature of melanocytic naevi is unknown notwithstanding their considerable significance for clinician and pathologist and despite the wealth of existing knowledge about melanocyte biology.

OBJECTIVES

To investigate how far a simple mutational model can explain the clinical and pathological features of melanocytic naevi, in particular their pattern of onset and frequency.

METHODS

I have constructed a model of the development of the adult melanocyte population from a single stem cell. The total cutaneous melanocyte population in a human adult is already known, as well as the range of spontaneous mutation rates at a given gene site. For each cycle of mitosis during the post stem-cell expansion of the melanocyte population, I calculate the accumulated number of cells likely to be mutated at a particular (although unknown) gene site. The results are interpreted in the light of a hypothesis that each of these mutant melanocytes will go on to form a melanocytic naevus. Comparisons are made with neurofibromas, occurring in type 1 neurofibromatosis and as sporadic lesions.

RESULTS

A single genetic mutation in melanocyte precursors is found to be sufficient to explain the clinical and pathological features of melanocytic naevi.

CONCLUSIONS

I propose that melanocytic naevi are a consequence of single spontaneous genetic mutations which inevitably occur during the development of the adult population of cutaneous melanocytes.

Synergistic interaction of heterozygous deletions impairs performance and confers susceptibility to disease at all ages

Rare recessive conditions are more common in the offspring of cousin unions than in the general population even in communities in which less than 0.5% of all marriages are between first cousins. It is shown that if the human genome has 30,000 genes in the haploid set then (Y+X)6/(Y)5 < 19, where X is the mean number of new mutations entering the human genome per generation and Y is the mean number of heterozygous deletions in the germ line of adults. This places an upper limit on X of 1.3. Functions specifying the frequency of homozygous deletions in the offspring of cousin and sibling unions are also derived in terms of X and Y. The best estimates with these three measures are that Y is between 6 and 10, and X between 0.5 and 1.3. Data on mutation rates in bacteria, worms, yeast, flies and mice are consistent with these estimates. It is argued that selection against heterozygous deletions must occur in order to prevent progressive build up in the genome; furthermore, it is postulated that the selection must be based on synergistic interaction. If heterozygous deletions interact synergistically to degrade biological performance then this has implications for human fertility, for polygenic disease, for the genetic basis of polygenic traits such as intelligence and the association between birth weight and adult disease (Barker's phenomenon) and the links between status and health inequality. The concept proposed is that a large set of genes specify each aspect of biological capability; the genes specify a redundant system so that one or two deletions will have minimal effect; but several deletions will interact synergistically to degrade performance and cause disease.

Can physical trauma cause breast cancer?

The objective of this study is to explore the effect of lifestyle on the risk of invasive breast carcinoma in women aged 50-65 years. A case-control study using a questionnaire and a semi-structured interview. Cases (n = 67) and controls (n = 134) were closely matched on known risk factors for breast cancer including age, family history, age at menarche, parity, age at first birth and menopausal status. Controls were chosen from a pool of 5600 women who attended for breast screening and filled in a questionnaire giving details to allow matching with cases. The study took place at the North Lancashire Breast Screening Service. Women were aged 50-65 years and presented with breast cancer or attended for breast screening. Women with breast carcinoma were more likely to report physical trauma to the breast in the previous 5 years than were the controls (odds ratio (OR) 3.3, 95% confidence interval (CI) 1.3-10.8, P < 0.0001). There were no significant differences in a wide range of other lifestyle indicators including factors relevant to social class, education, residence, smoking and alcohol consumption. In conclusion, recall bias is an unlikely explanation for these results in view of the nature and severity of physical trauma. Models of epithelial cell generation indicate that a causal link between physical trauma and cancer is plausible. A latent interval between cancer onset and presentation of under 5 years is also plausible. The most likely explanation of the findings is that physical trauma can cause breast cancer.

Putative non-Mendelian transmission of retinoblastoma in males: a phenotypic segregation analysis of 150 pedigrees

In a previous genotypic study of eight families, we described paternal segregation distortion favoring the transmission of mutant alleles at the retinoblastoma gene locus (RB1). In the current study, we reviewed all published retinoblastoma pedigrees with defined ascertainment (n = 150), to determine whether the phenotypic segregation frequency at the RB1 locus is in general influenced by the sex of the transmitting parent. Segregation analysis under complete ascertainment revealed that 49.1% of the offspring of male transmitters were affected, while 44.3% of the offspring of female transmitters were affected. While this difference is not statistically significant, it is consistent with the previous findings. No significant sex distortion could be detected among the progeny of carrier fathers and mothers. In order to quantify the transmission ratio more precisely further prospective molecular genetic analysis is warranted. We propose a biological mechanism to account for a putative segregation distortion, namely that genetic recombination creates clones of spermatogonia that are homozygous for the mutant RB1 allele leading to a non-Mendelian ratio of sperm. This model can be experimentally tested using amplification of DNA from single sperm cells.

Information and clonal concepts in ageing

A model based on information theory and one based on somatic mutation in an expanding pool of cells are combined to give a theory of ageing. A mathematical formula, based on concepts from information theory, gives a close fit to the age incidence of mortality and disease in middle and old age. The basic concept is that biological systems process information and make decisions to maintain their integrity. But the decision process is fallible and significant errors could lead to disease or death. Decay in the components of the system will lead to a rising error rate with age and the form of the curve will be influenced by redundancy which is a fundamental property of information systems. In addition it is suggested that an important part of the decay in the decision process is due to a subset of mutant stem cells which have a proliferative advantage and slowly expand in number throughout life. This idea is derived from application of a model of somatic mutation in expanding cells to data in cancer and on the increase of dominant mutations with paternal age. This composite view explains why ageing is less subject to the vagaries of the extrauterine environment than suggested by the simple information model and points to a significant role for the intrauterine environment in the ageing process.