A hypothetical anti-neoplastic mechanism associated to reserve cells

The telomere complex and the origin of the cancer stem cell

Exquisite regulation of telomere length is essential for the preservation of the lifetime function and self-renewal of stem cells. However, multiple oncogenic pathways converge on induction of telomere attrition or telomerase overexpression and these events can by themselves trigger malignant transformation. Activation of NFκB, the outcome of telomere complex damage, is present in leukemia stem cells but absent in normal stem cells and can activate DOT1L which has been linked to MLL-fusion leukemias. Tumors that arise from cells of early and late developmental stages appear to follow two different oncogenic routes in which the role of telomere and telomerase signaling might be differentially involved. In contrast, direct malignant transformation of stem cells appears to be extremely rare. This suggests an inherent resistance of stem cells to cancer transformation which could be linked to a stem cell’specific mechanism of telomere maintenance. However, tumor protection of normal stem cells could also be conferred by cell extrinsic mechanisms.

Developmental genes and cancer in children

Childhood tumours are associated with congenital abnormalities suggesting that disruption of normal developmental processes may be linked with oncogenesis. Genetic and environmental exposures may combine to disrupt critical epigenetic processes during development, thus affecting gene-related signalling pathways and cellular function. This review examines the role of critical genes and processes regulating development such as the polycomb family and sonic hedgehog (SHH) as well as the Wnt signalling pathways and epigenetic variations (Snf5), methylation and loss of heterozygosity in controlling homeotic gene transcription and intracellular chromatin structure. The developmental and perinatal periods appears important as a window of opportunity for cancer research.

Cancer as an evolutionary and ecological process

Neoplasms are microcosms of evolution. Within a neoplasm, a mosaic of mutant cells compete for space and resources, evade predation by the immune system and can even cooperate to disperse and colonize new organs. The evolution of neoplastic cells explains both why we get cancer and why it has been so difficult to cure. The tools of evolutionary biology and ecology are providing new insights into neoplastic progression and the clinical control of cancer.

Modification of p53 protein profile by gamma irradiation followed by methyl donor starvation

The possible beneficial radio-protective effects of one-carbon transfer agents namely folate, choline and methionine have been the subject of extensive investigation. Ionizing radiation is known to extensively damage the DNA. One-carbon transfer agents have been proposed to have important role in context of DNA repair via their role in purine and thymidylate synthesis and in DNA methylation. Sufficient dietary availability of one-carbon transfer agents therefore, might have ability to modify radiation effects. In present study modifications in level of tumor suppressor protein p53 by gamma irradiation followed by methyl donor starvation was observed. Experiments showed an increase in nuclear and cytoplasmic p53 protein concentration in liver, spleen and thymus. The overall rise in the level of p53 protein in liver was found to be less than that in spleen and thymus. Moreover significant heterogeneity in the basal level of expression of the p53 protein in liver, spleen and thymus was observed as the level of p53 protein in spleen and thymus was found to be 7-8 fold more than that in liver. Results indicated that radiation stress followed by methyl donor starvation could significantly induce p53 protein in spleen and thymus where there was a dramatic accumulation of p53 following irradiation, while in other tissues, particularly the liver, no such dramatic response was seen. Folate contribution of intestinal bacteria was found to influence p53 protein levels. Our observations indicated a prominent role played by the methyl donors in protecting the cell against harmful effects of ionizing radiation.

Interpreting epithelial cancer biology in the context of stem cells: tumor properties and therapeutic implications

Over 90% of all human neoplasia is derived from epithelia. Significant progress has been made in the identification of stem cells of many epithelia. In general, epithelial stem cells lack differentiation markers, have superior in vivo and in vitro proliferative potential, form clusters in association with a specialized mesenchymal environment (the 'niche'), are located in well-protected and nourished sites, and are slow-cycling and thus can be experimentally identified as 'label-retaining cells'. Stem cells may divide symmetrically giving rise to two identical stem cell progeny. Any stem cells in the niche, which defines the size of the stem cell pool, may be randomly expelled from the niche due to population pressure (the stochastic model). Alternatively, a stem cell may divide asymmetrically yielding one stem cell and one non-stem cell that is destined to exit from the stem cell niche (asymmetric division model). Stem cells separated from their niche lose their stemness, although such a loss may be reversible, becoming 'transit-amplifying cells' that are rapidly proliferating but have a more limited proliferative potential, and can give rise to terminally differentiated cells. The identification of the stem cell subpopulation in a normal epithelium leads to a better understanding of many previously enigmatic properties of an epithelium including the preferential sites of carcinoma formation, as exemplified by the almost exclusive association of corneal epithelial carcinoma with the limbus, the corneal epithelial stem cell zone. Being long-term residents in an epithelium, stem cells are uniquely susceptible to the accumulation of multiple, oncogenic changes giving rise to tumors. The application of the stem cell concept can explain many important carcinoma features including the clonal origin and heterogeneity of tumors, the occasional formation of tumors from the transit amplifying cells or progenitor cells, the formation of precancerous 'patches' and 'fields', the mesenchymal influence on carcinoma formation and behavior, and the plasticity of tumor cells. While the concept of cancer stem cells is extremely useful and it is generally assumed that such cells are derived from normal stem cells, more work is needed to identify and characterize epithelial cancer stem cells, to address their precise relationship with normal stem cells, to study their markers and their proliferative and differentiation properties and to design new therapies that can overcome their unusual resistance to chemotherapy and other conventional tumor modalities.