Newsgroup: it.scienza.biologia
Titolo: Che senso ha il cancro? II (lungo)

 cancro e mutazioni?
E` difficile dirlo.  Una specie piu` resistente al cancro dovrebbe avere,
da un lato, un piu` efficace meccanismo di riparazione dei piccoli danni
al DNA e/o un piu` potente sistema immunitario (per distruggere le cellule
cancerose), e dall'altro un'efficace attivazione dell'apoptosi nel caso che
i danni al DNA siano gravi o comunque non riparabili.  Sono tre variabili
che agiscono in maniera contrastante (la prima con la terza) oppure non
correlata (la seconda), senza contare che la radioresistenza puo` essere
diversa a seconda del tipo di cellula (es. germinale, staminale, somatica).
Pertanto credo che non sia facile trovare una correlazione.
Comunque ho trovato un abstract molto interessante relativo alla domanda
(originariamente di G. Bruno) nel subject ("Che senso ha il cancro?"):
  (4) Hum Mutat 1994;3(2):166-9
 Does cancer kill the individual and save the species?
 Sommer SS
  I have argued that most germline mutations are due to endogenous process.
 I speculate that endogenous control of germline mutation serves an important
 biological function. A certain rate of mutation is required to generate
 sufficient variation for adaptation during evolutionary time. Sexual
 reproduction and recombination may serve to enhance variation, but
 ultimately new germline mutation is required to replenish variant alleles
 lost secondary to negative selection, genetic drift, and population
 bottlenecks. Unfortunately, the requisite mutation rates carry a terrible
 price: for each advantageous mutation, there are many disadvantageous ones.
 Thus, all mammals are plagued with substantial Mendelian and multifactorial
 disease. Consequently, the optimal mutation rate should be at a level
 just sufficient to maintain the requisite variation needed for adaptation.
 In this view, mechanisms for negative selection are necessary to keep the
 mutation rate in check. If a high germline mutation rate produces a high
 mutation rate in somatic tissues, cancer may be an important mediator of
 negative selection. The multiple mutations necessary to produce cancer
 serve to amplify relatively small differences in the mutation rate, thereby
 providing an efficient selection against individuals with germline mutations
 that result in a high mutation rate. This hypothesis can account for the
 general similarity of the longevity-corrected cancer incidence profile
 and the small but significant prevalence of cancer before and during the
 reproductive period. While this hypothesis must presently be viewed as
 speculative, it integrates certain previously disjointed observations and
 suggests an alternative to the general assumption that cancer represents
 a breakdown in normal physiology.
 PMID: 8199598, UI: 94258168
E` certamente un modo di pensare anticonvenzionale: il cancro come normale
strategia evolutiva per eliminare gli individui portatori di troppe
mutazioni negative, e cosi` proteggere la specie.  Penso che tale ipotesi
potrebbe davvero dare un "senso" al cancro.
Riguardo al meccanismo molecolare impiegato, esso potrebbe consistere
nella riattivazione di geni embrionali normalmente silenti, e la cosa
potrebbe derivare dal ricorso a un ancestrale meccanismo di adattamento
(e quindi il cancro si potrebbe vedere come una regressione filogenetica).
Infatti negli eucarioti unicellulari da cui noi deriviamo, come i protozoi
ciliati (es. parameci), esistono un macronucleo e un micronucleo: il primo
viene utilizzato per le normali attivita` della cellula, mentre il secondo
e` mantenuto inattivo (si trova eterocromatinizzato) e viene impiegato solo
per la riproduzione, e (credo) anche per la sopravvivenza della cellula
nel caso che il macronucleo sia stato danneggiato ad es. da radiazioni.
In entrambi i casi il micronucleo viene riattivato e si utilizzano solo
i suoi geni anziche` quelli del macronucleo (che viene sempre distrutto
al termine della riproduzione sessuata, quando il micronucleo risultante
ha generato un nuovo macronucleo):
  (5) Microbiol Rev 1994 Jun;58(2):233-67
 The DNA of ciliated protozoa.
 Prescott DM
 Department of Molecular, Cellular and Developmental Biology, University
 of Colorado, Boulder 80309-0347.
  Ciliates contain two types of nuclei: a micronucleus and a macronucleus.
 The micronucleus serves as the germ line nucleus but does not express its
 genes. The macronucleus provides the nuclear RNA for vegetative growth.
 Mating cells exchange haploid micronuclei, and a new macronucleus develops
 from a new diploid micronucleus. The old macronucleus is destroyed.
 This conversion consists of amplification, elimination, fragmentation,
 and splicing of DNA sequences on a massive scale. Fragmentation produces
 subchromosomal molecules in Tetrahymena and Paramecium cells and much
 smaller, gene-sized molecules in hypotrichous ciliates to which telomere
 sequences are added. These molecules are then amplified, some to higher
 copy numbers than others. rDNA is differentially amplified to thousands
 of copies per macronucleus. Eliminated sequences include transposonlike
 elements and sequences called internal eliminated sequences that interrupt
 gene coding regions in the micronuclear genome. Some, perhaps all,
 of these are excised as circular molecules and destroyed. In at least
 some hypotrichs, segments of some micronuclear genes are scrambled in
 a nonfunctional order and are recorded during macronuclear development.
 Vegetatively growing ciliates appear to possess a mechanism for adjusting
 copy numbers of individual genes, which corrects gene imbalances resulting
 from random distribution of DNA molecules during amitosis of the
 macronucleus. Other distinctive features of ciliate DNA include
 an altered use of the conventional stop codons.
 PMID: 8078435, UI: 94359432
E questa ricerca piu` vecchia si conclude con una considerazione che e`
l'inverso di quella presumibilmente corretta: vale a dire che la particolare
riproduzione sessuale nei protozoi si e` evoluta come risposta adattativa
alla pressione mutagenica delle radiazioni ionizzanti.
  (6) Radiat Res 1986 Nov;108(2):215-21
 Ionizing radiation promotes protozoan reproduction.
 Luckey TD
  This experiment was performed to determine whether ionizing radiation is
 essential for maximum growth rate in a ciliated protozoan. When extraneous
 ionizing radiation was reduced to 0.15 mrad/day, the reproduction rate of
 Tetrahymena pyriformis was significantly less (P less than 0.01) than it was
 at near ambient levels, 0.5 or 1.8 mrad/day. Significantly higher growth
 rates (P less than 0.01) were obtained when chronic radiation was increased.
 The data suggest that ionizing radiation is essential for optimum
 reproduction rate in this organism.
 PMID: 3097750, UI: 87068293
E poiche` nelle nostre cellule somatiche si trovano eterocromatinizzati
(inattivi) i geni embrionali, questi ricercatori cinesi hanno pensato che
la loro riattivazione nel cancro possa rappresentare un tentativo disperato
della cellula di sopravvivere alla distruzione del DNA dei geni somatici:
  (7) Acta Biotheor 1995 Sep;43(3):279-80
 Why animals have tumours.
 Niu DK, Wang YF
 Department of Biology, Lanzhou University, Gansu, China.
  From the viewpoint of an evolutionary biologist, carcinogenesis should
 be looked upon as a protective mechanism against destruction of DNA.
 Because genes expressed in embryonic cells are covered and protected by
 heterochromatinization, they are the most appropriate 'alternate genes'
 compared to genes that are expressed already in somatic cells.
 When DNA-damage occurs, the embryonic genes can be activated.
 Some somatic cells exhibit some features of embryonic cells.
 PMID: 7483963, UI: 96033295
Ritornando alla tua domanda su che legame puo` esserci tra cancro e
mutazioni, ci sono delle cose molto interessanti da rilevare.  Infatti
nelle cellule cancerose e` presente un numero elevatissimo di mutazioni,
che non sembra spiegabile in base al tasso di mutazione delle normali
cellule somatiche.  Pertanto e` stata avanzata di recente l'ipotesi
che le cellule tumorali esprimano un fenotipo attivamente mutatore:
  (8) Adv Cancer Res 1998;72:25-56
 Cancer cells exhibit a mutator phenotype.
 Loeb LA
 Joseph Gottstein Memorial Cancer Research Laboratory, Department of
 Pathology, University of Washington, Seattle 98195-7705, USA.
  This review analyzes the concept and evidence in support of a mutator
 phenotype in human cancer. The large number of mutations reported in tumor
 cells cannot be accounted for by the low mutation rates observed in normal
 somatic cells; rather, it must be a manifestation of a mutator phenotype
 present early during the tumorigenic process. The interaction between
 increased mutagenesis and clonal selection provides a mechanism for the
 selection of cells with increased proliferative advantage. The concept of
 a mutator phenotype in cancer has gained considerable support from the
 findings of enormous numbers of somatic mutations in repetitive sequences
 in human tumors. Moreover, cell lines exhibiting microsatellite instability
 demonstrate an increased mutation frequency in expressed genes. A knowledge
 of mechanisms that generate multiple mutations in cancer cells has important
 implications for prevention. For many tumors, a delay in the rate of
 accumulation of mutations by a factor of two could drastically reduce
 the death rates from these tumors.
 PMID: 9338073, UI: 97479421
  (9) Semin Cancer Biol 1998 Dec;8(6):421-9
 On the origin of multiple mutations in human cancers.
 Jackson AL, Loeb LA
 Department of Pathology, University of Washington, Seattle, WA, 98195, USA.
  Tumor progression is a multi-step process, proceeding by multiple
 alterations from a normal cell to a localized tumor, and finally to one that
 acquires the ability to invade and metastasize. Tumors are characterized by
 many mutations in the form of base substitutions, deletions, chromosomal
 translocations, and gene amplifications, and these mutations are found to
 accumulate as tumors progress. In contrast, spontaneous mutations are very
 rare events. Considering the high fidelity of DNA replication in normal
 cells, it seems improbable that spontaneous mutations could be the source
 of the large numbers of genetic alterations that are observable in cancer
 cells. The question of how multiple mutations accumulate in tumor cells
 is one of considerable interest, since understanding the source of these
 mutations may facilitate the detection of tumors and may provide new
 approaches to cancer prevention. We have proposed that the multiple
 mutations detectable in cancer cells result from a mutator phenotype,
 in which loss of a genome stability function occurs early during tumor
 development and predisposes the tumor cell to the accumulation of further
 mutations. We will first consider the evidence that cancer cells manifest
 a mutator phenotype, and subsequently discuss the possibility that a
 mutator phenotype can be selected and can be transient as tumors progress.
 PMID: 10191176, UI: 99208576
Questa ipotesi sembra essere confermata dal riscontro sperimentale che
il gene TP53 (che codifica per la proteina p53, il famoso "guardiano del
genoma") e` ipermutabile in qualche stadio dello sviluppo tumorale:
  (10) Genetics 1998 Apr;148(4):1619-26
 Hypermutability in carcinogenesis.
 Strauss BS
 Department of Molecular Genetics and Cell Biology, The University of
 Chicago, Illinois 60637, USA. 
bs...@midway.uchicago.edu
  The presence of numerous chromosomal changes and point mutations in tumors
 is well established. At least some of these changes play a role in the
 development of the tumors. It has been suggested that the number of these
 genetic changes requires that tumorigenesis involves an increase in mutation
 rate. However, the presence of numerous changes can also be accounted for
 by efficient selection. What is required to settle the issue is some measure
 of nonselected mutations in tumors. In order to determine whether the tumor
 suppressor TP53 (coding for the protein p53) is hypermutable at some stage
 of carcinogenesis, the frequency of silent and multiple mutations in this
 gene has been examined. Silent mutations make up approximately 3% of the
 total recorded but constitute 9.5% of the mutations found in tumors with
 multiple mutations. Multiple closely linked mutations are also observed.
 Such multiple mutations suggest the operation of an error-prone replication
 process in a subclass of cells. The published data indicate that TP53 is
 hypermutable at some stage of tumor development. It is not yet clear whether
 TP53 is unique or whether other genes display a similar pattern of silent
 and multiple mutations.
 PMID: 9560381, UI: 98240726
E dalla sua frequenza pare che le cellule tumorali abbiano un tasso di
mutazione 100 volte superiore a quello delle cellule normali:
  (11) Semin Cancer Biol 1998 Dec;8(6):431-8
 Hypermutability and silent mutations in human carcinogenesis.
 Strauss BS
 Department of Molecular Genetics and Cell Biology, The University of
 Chicago, 920 East 58th Street, Chicago, IL, 60637, USA.
  Silent or synonymous mutations are nucleotide changes which do not result
 in an amino acid change. Such alterations make up approximately 4% of the
 total TP53 mutations in a database of almost 10,000 mutations. Insofar as
 these changes do not confer a selective advantage, silent mutations make it
 possible to estimate an unbiased mutation frequency. Since this frequency
 is at least 100 times higher than would be expected for normal tissue,
 tumors must be hypermutable. In addition, a class of tumors may be
 supermutable with a mutation frequency comparable to that observed
 in the generation of immune diversity.
 PMID: 10191177, UI: 99208577
Una possibile spiegazione di questo strano fatto e` stata ipotizzata
dall'osservazione che nei batteri si verifica un aumento del tasso di
mutazione a seguito del venir meno delle risorse alimentari.  Stranissimo
e` che le mutazioni che si verificano sono adattative, vale a dire che
si verificano proprio quelle mutazioni che rendono la cellula batterica
adatta a sopravvivere in quell'ambiente divenuto inospitale.  Si tratta
di un fenomeno apparentemente lamarckiano, ma che sembra mediato da geni
regolatori evolutivamente preposti a cio`:
  (12) J Bacteriol 1998 Jun;180(11):2862-5
 Adaptive mutagenesis at ebgR is regulated by PhoPQ.
 Hall BG
 Biology Department, University of Rochester, Rochester, New York
 14627-0211, USA. 
dr...@uhura.cc.rochester.edu
  Adaptive mutations are mutations that occur in nondividing or very slowly
 dividing microbial cells during prolonged nonlethal selection and that are
 specific to the challenge of the selection in the sense that the only
 mutations that can be detected are those that provide a growth advantage
 to the cell. The phoPQ genes encode a two-component positively acting
 regulatory system that controls expression of at least 25 to 30 genes in
 Escherichia coli and Salmonella typhimurium. PhoPQ responds to a variety
 of environmental stress signals including Mg2+ starvation and nutritional
 deprivation. Here I show that disruption of phoP or phoQ by Tn10dCam
 significantly reduces the adaptive mutation rate to ebgR, indicating that
 the adaptive mutagenesis machinery is regulated, directly or indirectly,
 by phoPQ. The finding that it is regulated implies that adaptive mutagenesis
 does not simply result from a failure of various error correction mechanisms
 during prolonged starvation.
 PMID: 9603873, UI: 98268999
  (13) Curr Biol 1999 Dec 16-30;9(24):1477-80
 Environmentally constrained mutation and adaptive evolution in salmonella.
 Massey RC, Rainey PB, Sheehan BJ, Keane OM, Dorman CJ
 Department of Microbiology, Department of Plant Sciences, Moyne Institute
 of Preventive Medicine, University of Dublin, Trinity College, University
 of Oxford, Dublin 2, Oxford, OX1 3RB, Ireland, UK.
  The relationship between environment and mutation is complex [1].
 Claims of Lamarkian mutation [2] have proved unfounded [3] [4] [5];
 it is apparent, however, that the external environment can influence the
 generation of heritable variation, through either direct effects on DNA
 sequence [6] or DNA maintenance and copying mechanisms [7] [8] [9] [10],
 or as a consequence of evolutionary processes [11] [12] [13] [14] [15] [16].
 The spectrum of mutational events subject to environmental influence is
 unknown [6] and precisely how environmental signals modulate mutation is
 unclear. Evidence from bacteria suggests that a transient recombination-
 dependent hypermutational state can be induced by starvation [5].
 It is also apparent that changes in the mutability of specific loci can
 be influenced by alterations in DNA topology [10] [17]. Here we describe
 a remarkable instance of adaptive evolution in Salmonella which is caused
 by a mutation that occurs in intermediate-strength osmotic environments.
 We show that the mutation is not 'directed' and describe its genetic basis.
 We also present compelling evidence in support of the hypothesis that the
 mutational event is constrained by signals transmitted from the external
 environment via changes in the activity of DNA gyrase.
 PMID: 10607591, UI: 20076699
Quali che siano i meccanismi delle mutazioni adattative, l'ipotesi brillante
(sempre del solito cinese) e` che l'anossia dovuta al loro stesso massivo
sviluppo induca un analogo cambiamento nelle cellule tumorali, che cosi`
manifestano il fenotipo mutatore:
  (14) Acta Biotheor 1998-99;46(4):379-81
 Ideas in theoretical biology. Origin of cancerous cells from tumours.
 Niu DK, Chen JK
 School of Life Sciences, Wuhan University, Wuhan, Hubei, China.
  With a previous paper (Niu & Wang, 1995), a general, hypothetical outline
 of the mechanism of carcinogenesis was proposed. With reference to the fact
 of starvation-induced hypermutation in micro-organisms, we propose that
 the hypoxia commonly seen in the cells at the centre of solid tumours might
 also result in hypermutation, and then p53-dependent programmed cell death.
 Like the apparently adaptive mutations in micro-organisms, only those genes
 (e.g. p53) that enable the cells to escape from apoptosis may be selected.
 PMID: 10423829, UI: 99352628
In definitiva, il cancro si originerebbe da una certa sequenza di eventi,
il cui mantenimento e` stato favorito da necessita` evolutive.  Infatti
negli unicellulari essa si mostra utile alla sopravvivenza della specie
stimolando mutazioni adattative, ma anche nei multicellulari risulta utile
alla specie eliminando gli individui portatori di troppe mutazioni nocive.
Ecco dunque come la vedo io:
                  ,--> apoptosi
  Danno al DNA --<
                  '--> riattivazione dei geni embrionali -->
  --> fenotipo embrionale --> angiogenesi 1 --> proliferazione -->
  --> anossia --> induzione ipermutabilita` e angiogenesi 2 -->
  --> mutazioni incontrollate --> progressione tumorale
E puo` darsi che il DNA a maggior rischio di dare inizio alla sequenza di
trasformazione tumorale (anziche` a quella opposta dell'apoptosi) sia quello
delle cellule staminali.
Notare che l'anossia da` luogo anche alla ripresa dell'angiogenesi, un altro
importantissimo evento nello sviluppo del cancro.  L'angiogenesi si attiva
molto presto gia` in una prima fase (probabilmente perche` viene indotta
da un gene embrionale):
  (15) Cancer Res 1996 Dec 1;56(23):5391-6
 Up-regulation of vascular endothelial growth factor/vascular permeability
 factor in mouse skin carcinogenesis correlates with malignant progression
 state and activated H-ras expression levels.
 Larcher F, Robles AI, Duran H, Murillas R, Quintanilla M, Cano A,
 Conti CJ, Jorcano JL
 Department of Cell and Molecular Biology, CIEMAT, Madrid, Spain.
  Angiogenesis is a crucial process for tumor growth and metastasis regulated
 by the balance of positive and negative factors. Vascular endothelial growth
 factor (VEGF/VPF) is a specific mitogen for endothelial cells that has been
 shown to be overexpressed in a variety of tumors and other inflammatory
 diseases. To analyze the implication of VEGF/VPF during tumorigenesis,
 we have studied its expression at different stages of tumor development
 using the mouse skin carcinogenesis model. VEGF/VPF mRNA was induced in skin
 in vivo after 12-O-tetradecanoylphorbol-13-acetate treatment. Constitutive
 up-regulation of VEGF/VPF at the mRNA and protein levels was also observed
 in premalignant papillomas and, at a higher level, in squamous carcinomas,
 suggesting a correlation between VEGF/VPF expression and tumor progression.
 A direct positive correlation between VEGF/VPF mRNA expression and the level
 of activated H-ras gene was found in a series of cell lines representing
 different stages of epidermal tumor development. Consequently, a clone of
 one of these cell lines, HaCa4, which has lost most of its v-ras expression,
 down-regulated VEGF mRNA expression concomitantly with its metastatic
 potential. Direct evidence of H-ras involvement in VEGF induction was
 obtained when an immortalized mouse keratinocyte cell line transduced with
 a retrovirus carrying v-H-ras showed highly increased VEGF/VPF mRNA levels.
 These data show that in mouse skin carcinogenesis, the VEGF/VPF angiogenic
 stimulus occurs early during premalignant papilloma development and further
 increases at later stages. Moreover, we demonstrate that increasing the
 activated H-ras dose, a phenomenon that takes place sequentially throughout
 mouse skin tumor development, may play an additional role by facilitating
 malignant in vivo progression through the modulation of VEGF/VPF-mediated
 angiogenesis.
 PMID: 8968091, UI: 97122497
  (16) Gan To Kagaku Ryoho 1998 Feb;25(3):365-9
 [Analysis of tumor vascular density using the window chamber technique].
 Nasu R, Kimura H, Uokawa K, Nagata K, Akagi K, Murata T, Tanaka Y
 Dept. of Radiology, Kansai Medical University.
  Vascular structure is indispensable for a tumor to maintain its growth.
 The structure also affects drug delivery over chemotherapy, and oxygen
 tension in the tissue during irradiation. Therefore, measuring the vascular
 density in the tumor will help to reach an expected level or to evaluate
 the effect of those therapies. Through previous observation, changes in
 vascular structure, like the vascular density, have been reported. However,
 the tissue was usually removed from the deceased animal and examined in the
 microscope. Not many studies have shown changes in one and the same animal's
 tissue. In this study, we used the dorsal flap window chamber technique,
 and observed changes in the development of vascular structure and vascular
 density in a tumor. Window chambers were attached to the backs of 3 rats,
 and they anesthetized every 24 hours for microscopic observation. Vascular
 growth in the tumor started 8 to 9 days after implantation. Their average
 vascular density was about 25%, and this ratio continued till necrosis
 started in the tumor. "Necrosis occurs in a tumor because its growth is
 so rapid that the vascular development can not maintain the same speed."
 This has been a kind of established theory. During observation, we
 recognized blood stagnation in vessels as the tumor grew, and the
 relationship between tumor size and its vascular density was significant
 (r = 0.926). Our data lead us to another conclusion that "Vascular growth
 is as fast as tumor growth, keeping a ratio of about 25%, but necrosis is
 caused by deficiency of blood flow circulation."
 PMID: 9492830, UI: 98153829
Infine osservo che il noto promotore tumorale tetradecanoilforbolo acetato
(alias forbolo miristato acetato) si e` rivelato un potente angiomitogeno
(Cancer Res 1992 Sep 1;52(17):4821-3; Growth Factors 1995;12(4):289-301;
Oncogene 1997 May 22;14(20):2475-83; Blood 1997 Nov 15;90(10):4153-61;
J Biol Chem 1999 May 28;274(22):15407-14).  Probabilmente la promozione
tumorale ha a che vedere appunto con l'induzione dell'angiogenesi (mentre
l'iniziazione consiste proprio nel danno al DNA).
E` tutto.  Scusate per l'eccessiva lunghezza di questo post.
Saluti.
  -= Giorgio Brandi =-

regolatori evolutivamente preposti a cio`:
Mi piacerebbe sapere come i ricercatori possono provare ciò, come
possono dimostrare che un microbo posto in un ambiente senza cibo
prima muta e poi sopravvive, come possono provare che non ha subito
una mutazione casuale (e altri 999.999 nel frattempo sono morti) o
che era un mutante da prima.
Saluti.
Roberto