A mutational theory of leukaemogenesis

Hypothesis: Bacterial induced inflammation disrupts the orderly progression of the stem cell hierarchy and has a role in the pathogenesis of breast cancer

BACKGROUND

The hierarchical model of stem cell genesis is based on the idea that the number of cell divisions between the zygote and fully differentiated epithelial cells is kept close to the minimum, which is log to the base 2 of the total number of cells produced in a human lifetime. The model assumes the orderly progression of stem cell divisions requires precise control at every stage in development. If the orderly progression is maintained then cancer will be rare. A prediction of the model is that if the orderly progression of the stem cell hierarchy is disturbed by trauma, ulceration or inflammation then cancer will occur.

HYPOTHESIS

Bacterial induced inflammation in breast ducts disturbs the stem cell hierarchy and is a cause of breast cancer.

EVIDENCE

Mammalian milk is not sterile. It contains a range of bacteria, derived endogenously by the entero-mammary circulation. The dominant flora consists of lactose fermenting bacteria. Pregnancy and breast feeding reduce the risk of subsequent breast cancer. The implication is that a lactose fermenting bacterial flora in breast ducts is protective. Malignant and benign breast tissue contains bacteria derived endogenously, but studies so far have not revealed a specific flora associated with malignancy. Periodontitis is associated with oral, oesophageal, colonic, pancreatic, prostatic and breast cancer. The pathogenic bacteria which cause periodontitis spread endogenously to cause inflammation at other epithelial sites. Meta-analysis of epidemiological studies shows that the consumption of yoghurt is associated with a reduction in the risk of breast cancer.

CONCLUSION

The hypothesis, although not proven, is supported by the available evidence. Lactose fermenting bacteria protect but pathogenic bacteria which induce inflammation raise the risk of breast cancer. The consumption of yoghurt also appears to be protective.

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.

A hypothesis to explain childhood cancers near nuclear power plants

Over 60 epidemiological studies world-wide have examined cancer incidences in children near nuclear power plants (NPPs): most of them indicate leukemia increases. These include the 2008 KiKK study commissioned by the German Government which found relative risks (RR) of 1.6 in total cancers and 2.2 in leukemias among infants living within 5 km of all German NPPs. The KiKK study has retriggered the debate as to the cause(s) of these increased cancers. A suggested hypothesis is that the increased cancers arise from radiation exposures to pregnant women near NPPs. However any theory has to account for the >10,000 fold discrepancy between official dose estimates from NPP emissions and observed increased risks. An explanation may be that doses from spikes in NPP radionuclide emissions are significantly larger than those estimated by official models which are diluted through the use of annual averages. In addition, risks to embryos/fetuses are greater than those to adults and haematopoietic tissues appear more radiosensitive in embryos/fetuses than in newborn babies. The product of possible increased doses and possible increased risks per dose may provide an explanation.

Childhood cancers near German nuclear power stations: hypothesis to explain the cancer increases

In early 2008, the very large Kinderkrebs in der Umgebung von Kernkraftwerken [Childhood Cancer near Nuclear Power Plants] (KiKK) study in Germany reported increases in leukaemias and solid cancers among children living near all German nuclear power plants (NPPs). This study, previously described in Medicine, Conflict and Survival, has triggered debates in many countries as to the cause or causes of these increased cancers. An accompanying article reports on the recent developments on the KiKK study including the responses by German radiation agencies, and the results of recent epidemiological studies near United Kingdom and French nuclear installations. This article outlines a possible explanation for the increased cancers. In essence, doses from environmental NPP emissions to embryos/foetuses in pregnant women near NPPs may be larger than suspected, and haematopoietic tissues may be considerably more radiosensitive in embryos/foetuses than in newborn babies. The article concludes with recommendations for further research.

Childhood cancer near German nuclear power stations

In 2008, the Kinderkrebs in der Umgebung von Kernkraftwerken (KiKK) study in Germany reported a 60% increase in solid cancers and a 120% increase in leukemias among children living within 5 km of all German nuclear power stations. The study has triggered debates as to the cause(s) of these increased cancers. This article reports on the findings of the KiKK study; discusses past and more recent epidemiological studies of leukemias near nuclear installations around the world, and outlines a possible biological mechanism to explain the increased cancers. This suggests that the observed high rates of infant leukemias may be a teratogenic effect from radionuclides incorporated by pregnant women living near nuclear reactors. Doses and risks from environmental emissions to embryos and fetuses may be larger than suspected. Hematopoietic tissues appear to be considerably more radiosensitive in embryos/fetuses than in newborn babies. Recommendations for advice to local residents and for further research are made.

Commentary: childhood cancer near nuclear power stations

In 2008, the KiKK study in Germany reported a 1.6-fold increase in solid cancers and a 2.2-fold increase in leukemias among children living within 5 km of all German nuclear power stations. The study has triggered debates as to the cause(s) of these increased cancers. This article reports on the findings of the KiKK study; discusses past and more recent epidemiological studies of leukemias near nuclear installations around the world, and outlines a possible biological mechanism to explain the increased cancers. This suggests that the observed high rates of infant leukemias may be a teratogenic effect from incorporated radionuclides. Doses from environmental emissions from nuclear reactors to embryos and fetuses in pregnant women near nuclear power stations may be larger than suspected. Hematopoietic tissues appear to be considerably more radiosensitive in embryos/fetuses than in newborn babies. Recommendations for advice to local residents and for further research are made.

Big babies and infant leukemia: a role for insulin-like growth factor-1?

Several epidemiologic studies have demonstrated that high birthweight is associated with an increased risk of infant leukemia; however, the reason for this relationship is unclear. Biologic data demonstrate that birth weight is correlated positively with circulating levels of insulin-like growth factor-1 (IGF-1). IGF-1 is important in blood formation and regulation and has been shown to stimulate the growth of both myeloid and lymphoid cells in culture. Since infants who develop leukemia are likely to have had at least one transforming event occur in utero, we hypothesize that high levels of IGF-1 may both produce a larger baby and contribute to leukemogenesis.

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.

Historical origins of current concepts of carcinogenesis

The first attempts to understand the causes of cancer were based on generalizations of what might now be termed a "holistic" nature, and hereditary influences were recognized at an early stage; these views survive principally through a supposed positive connection between psychological factors such as stress and diminished ability to combat the progressive development of tumors through some form of immunologically mediated rejection of potentially cancerous cells. While evidence for immunosurveillance is generally accepted, it is now widely regarded as almost wholly confined to instances where tumor viruses are involved as causative agents. The earliest theorists drew an analogy between the processes of carcinogenesis and of evolution; the cancer cells acquired the ability to outstrip their normal counterparts in their capacity for proliferation. This was even before evolution had been interpreted as involving a continuous succession of mutations. Evidence was already to hand before the end of the 18th century that exogenous agents, notably soot, a product of the "industrial revolution," could cause skin cancer. Somewhat over 100 years later, another industrial innovation, the manufacture of synthetic dyestuffs, implicated specific chemical compounds that could act systemically to cause bladder cancer. Meanwhile, the 19th century saw the establishment of the fundamentals of modern medical science; of particular relevance to cancer was the demonstration that it involved abnormalities in the process of cell division. The commencement of the 20th century was marked by a rediscovery of the concept of mutation; and it was proposed that cancer originated through uncontrolled division of somatically mutated cells. At around this time, two further important exogenous causative agents were discovered: X-rays and tumor viruses. In the late 1920s, x-radiation became the first established exogenous cause of mutagenesis. The discoverer of this phenomenon, H. J. Muller, suggested that while mutation in a single cell was the primary causative mechanism in carcinogenesis, its generally observed logarithmic increase in incidence with age reflected a "multihit" process, and that multiple successive mutations were required in the progeny of the original mutants. He also recognized that the rate of proliferation of potentially cancerous cells would markedly influence the probability of their subsequent mutation. These considerations are essentially the foundation of the generally accepted view of carcinogenesis that now seems unlikely to be superseded. However, this acceptance did not come about unopposed. The analogy between carcinogenesis and evolution was disliked by many biologists because it embodied the concept that cancer was an inevitable consequence of our evolutionary origins.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular proliferation and genetic events involved in the genesis of Burkitt lymphoma (BL) in immune compromised patients

A mathematical model simulating lymphomagenesis based on the two-hit theory of carcinogenesis is presented by contrasting the biologic variables responsible for a high risk of developing Burkitt lymphoma (BL) in three immunosuppressed groups with that of nonendemic BL. In this model, the pro-B lymphocyte is considered to be the target for BL-specific translocations such as t(8;14). With repeated mitosis, the target cell pool expands in the high-risk individual, and, thereby, the opportunities for a spontaneous translocation to arise are increased. The chromosomal translocation endows the target cell with survival advantages, and, hence, lymphoma develops. Modeling results demonstrate that this increased cell proliferation is sufficient in accounting entirely for the increase in tumor prevalence. Preventing enhanced cellular proliferation by obviating immune deficiency and treating patients with agents that restore immunity or have antiviral and antiproliferative properties prior to conversion from polyclonal B-cell proliferation to monoclonal malignancy could obviate the development of BL.

Ageing, information and the magical number seven

Concepts from information theory are applied to human ageing. An equation is derived, based on the general properties of information systems, which closely predicts the mortality rate in old age. The same equation predicts that infection will be increasingly severe when first exposure to micro-organisms is delayed leading to diseases which peak in early and middle age. A parameter of the equation is the level of redundancy in human information processing. Observed mortality rates show that this is sevenfold and it is suggested that seven is a fundamental dimensionless biological constant.

Alternative models for early onset of childhood leukaemia

This paper considers theoretical models for early-onset childhood leukaemia. The major focus of attention is the two-hit mutational model. A simple mathematical representation is used to explore mechanisms which might lead to onset of leukaemia at an unusually early age. Two such mechanisms are considered. The first of these, a germinal or very early embryonic first mutation is shown to imply that multiple independent leukaemic clones are likely to arise sequentially in very young patients. Clonal multiplicity could underlie the poor prognosis which has been associated with early onset childhood acute lymphoblastic leukaemia. It implies that curative therapy might require intensive treatment followed by bone marrow rescue to ensure eradication of all single-hit predisposed target cells. The prediction of multiple leukaemic clones might be tested in female patients by means of X-linked restriction fragment length polymorphisms and in patients with B-lineage neoplasms by determination of immunoglobin gene rearrangements. A second mechanism for early onset leukaemogenesis is the occurrence of a high cellular mutation rate in some patients. This is shown to result in leukaemia at significantly earlier age if the mutation rate is sufficiently high to influence target cell loss rate. This mechanism would enable more rapid clonal evolution of leukaemic cells and the early emergence of drug resistant variants. The prediction might be tested experimentally by sequential observation of genetic markers (e.g. Karyotypes, DNA fingerprint patterns) and the rate of emergence of drug resistant phenotypes. Other models, considered more briefly, include one-hit mutational 'dominants' in the developing embryo and faster growth kinetics in neoplasms of younger patients.(ABSTRACT TRUNCATED AT 250 WORDS)

The age incidence of childhood acute lymphoblastic leukaemia

A mathematical model of childhood acute lymphoblastic leukaemia (ALL) is presented which is based on the assumptions that: 1) malignant clones of lymphoblasts arise by spontaneous mutation; 2) this is normally prevented by repair systems which either cause intracellular DNA repair or eliminate the malignant clone; and 3) the repair systems are fallible and their performance decays at random with time. If the major drive to lymphocyte division is microbial antigens then the model produces a reasonable fit to published data on the age distribution of ALL. Furthermore the model offers an explanation for the apparent increase in the incidence of ALL which is associated with improved social conditions.

Spontaneous mutation rate in retinoblastoma

A novel approach was used to estimate the in vivo mutation rate of the retinoblastoma gene. A mathematical formula can be used to calculate the probability of neoplasia induced by one or more mutations in a population of dividing cells. This formula can then be applied to epidemiological data on hereditary and sporadic retinoblastoma. The analysis yields an estimate of the in vivo mutation rate of 8 x 10(-8)/gene/cell division (range 5.5 x 10(-8) to 1.3 x 10(-7]. The estimated non-replication association in vivo mutation rate is 2.4 x 10(-8)/gene/year (range 0 to 6.8 x 10(-8]. The formula is an improvement on previous attempts to produce a model of the process of mutation during cell generation. It can be applied to neoplastic disease in both children and adults.