16 replies to this topic

[jaydfox]

With all this debate about using WILT to prevent cancer, no serious consideration seems to be given to DNA repair. Citing Potten's work, the lower GI tract has been offered as evidence that WILT will not work, because stem cell turnover would deplete the telomeres in far less than the 10 years proposed between reseeding of stem cell reservoirs.

Here is my analysis of Potten's data. I previously said that the telomerase-knockout study speaks for itself, and that we don't need to understand the underlying science. However, that was hasty. I believe that regardless of the validity of that study, the gut stem cell dilemma presents a problem for WILT. Either WILT won't work (stem cells turn over daily), or DNA repair will work better than is being represented by de Grey.

The Small Intestine as a Model of DNA Integrity and Longevity

Nuclear DNA integrity and the incidence of malignant cancers are strongly correlated; thus, any treatment which hopes to dramatically reduce cancer rates and slow the exponential increase of those rates must address the issue of nuclear DNA integrity. Slowing the exponential increase of cancer incidence rates can be restated as increasing the doubling period of cancer incidence rates—increasing the period of time that cancer incidence rates require to double.

In pursuing a body-wide treatment for the problem of DNA integrity in an aging mammal, it is proposed that the mucosal lining of the small intestine—as a model tissue—suggests that DNA integrity can be maintained much longer than currently anticipated. This would achieve the two goals of reducing cancer incidence rates and slowing the exponential increase of cancer incidence rates.

Resistance to Mutation and Tumorigenesis

The intestines offer an interesting dichotomy: both the large and small intestines are exposed to high concentrations of mutagenic toxins, and both have a physiology of crypts that maintain villi, with stem cells located at or near the base of the crypts. However, cancer rates in the large intestine are much higher than in the small intestine. In fact, certain regions of the small intestine have virtually no known disposition for cancer.

Studies of gene expression find that relative expression rates of tumor suppressors and apoptosis suppressors can account for at least part of this dichotomy. The large intestine express tumor suppressors to a lesser degree, and express apoptosis suppressors to a higher degree. While the reason for the dichotomy in gene expression is not understood, the effects on cancer incidence speak for themselves.

The proliferative cells of the small intestine—especially the stem cells, and most especially the ultimate stem cells—have a high sensitivity to mutagens (radiation, etc.), and will undergo apoptosis and replacement by neighboring proliferative cells. The cells with the lower sensitivity to mutation also display an increased capacity for repair.

Given the hostile environment of the small intestine, the relative balance of mutation-induced apoptosis and mutation repair achieved by various tiers of stem cells in the small intestine indicate that ablation by mutation-sensitivity (a tumor suppression mechanism) and replacement by cells with higher relative DNA integrity is an effective cancer prevention method.

Cellular Proliferation from Stem Cells

The cells of the small intestine offer another insight into the potential for body-wide cancer prevention (i.e. reduction in cancer incidence rates and the exponential growth of those rates).

Through experimental procedures, it has been determined that the crypts of the small intestine maintain 6 ultimate stem cells. This number is very precisely balanced: if a seventh ultimate stem cell mistakenly appears, one of the seven stem cells will undergo apoptosis. If one of the six ultimate cells dies (e.g. through mutation-induced apoptosis), it is replaced by a cell in the next tier of stem cells—a stem cell that has had higher repair capacity, and thus a typically similar or higher DNA integrity.

Likewise, as the number of second-tier stem cells falls below the normal range, it is replaced by a stem cell from the next tier—a stem cell that has had yet higher repair capacity, and thus a typically similar or higher DNA integrity.

While the tiered structure of these stem cells is indeed interesting, what is critical to understand is the total proliferative capability of the ultimate stem cells. The six ultimate stem cells must service three villi, each of which has 3,500 cells. Those cells turn over (ablation through apoptosis and/or release into the small intestine) about once every 1-3 days. This means that 10,500 cells must be replaced on a near-daily basis by only six ultimate stem cells. That works out to about 1,750 cells every 24-72 hours, per ultimate stem cell.

This workload is staggering. It implies that cells can be extremely proliferative, a metabolic activity that is very stressful on the genome, with a very high level of tissue fidelity. DNA integrity at the tissue level remains very high.

Whether individial cells can maintain such a near-perfect level of fidelity is not clear, but the turnover that is accomplished will ablate any tumorigenic cells, unless the cells in question have low turnover. Turnover seems to be the key. Cells with the lowest turnover can be kept mutation- and cancer-free by tumor suppression, and cells with high turnover can be kept mutation- and cancer-free by ablation and replacement. DNA repair and maintenance also plays a role in cells with an intermediate level of turnover.

Not all body tissues can have their turnover rates increased. Certainly the skin, blood, muscles, and many internal organs are condusive to such a model. The brain, and perhaps other organ systems, might not be. However, the DNA-repair strategy seen in the tissues with intermediate levels of turnover (e.g. the second- and third-tier stem cells of crypts) seems to also be effective in preventing cancer.

Conflicting Interpretations of Cell Division Rates of “Ultimate” Stem Cells

Concern has been raised about how rapidly the ultimate stem cells divide. In one interpretation, the ultimate stem cells must divide daily. In another interpretation, the ultimate stem cells may divide far less often, matching the division rates of blood and skin stem cells. In yet another interpretation, the ultimate stem cells are replenished by sequestering blood stem cells.

Daily division of ultimate stem cells

In the study conducted by Potten, cell velocity in the crypts was measured, and division was carefully measured. Given the experimental data, the conclusion was that the ultimate stem cells divide approximately every 24 hours.

Monthly division of ultimate stem cells

In a study conducted on telomerase-knockout mice, it was noted that in generations of the mice in which telomeres were short enough to induce tissue dysfunction, the onset of intenstinal dysfunction did not precede dysfunction in blood and skin tissues. From this study, it was concluded that the ultimate stem cells of the small intestine cannot be dividing more frequently than blood or skin stem cells. In other words, the division rate of the intestinal stem cells is on the order of a month, not a day.

Sequestration of Blood Stem Cells

Yet another hypothesis, which seeks to reconcile Potten’s data with the telomerase-knockout study, proposes that the intestinal crypts sequester a blood stem cell to refresh the stem cell pool. An implication of such a radical idea is that it seems highly likely that any such system would have evolved, rather than being a consequence of chance. Given the sheer number of crypts in the small intestine, the simplest explanation that a very large percentage of the crypts would engage in such periodic sequestration is if such a system were by design.

Converging Conclusions Drawn from Divergent Interpretations

Given the divergent interpretations of Potten's data and the telomerase-knockout study, one might expect divergent conclusions to be drawn with respect to DNA integrity and WILT as cancer prevention methods. However, I find little such divergence, but rather a convergence.

Daily division of ultimate stem cells

If the stem cells of the small intestine divide daily, this implies that cells have a much higher capacity for division without tumorigenesis than under the current paradigm. The perceived combination of cellular ablation (programmed and responsive) and replacement, balanced with DNA integrity (repair and maintenance), is suggested as a model for other body tissues.

Monthly division of ultimate stem cells

If the stem cells of the small intestine divide monthly, this implies that the successive tiers of stem cells are much more proliferative than previously concluded. The proliferative capacity of the stem cell reservoir has now shifted to a tier of stem cells that relies on DNA repair/maintenance, not ablation, for DNA integrity in the crypt. This bolsters the case for DNA repair/maintenance as a suitable cancer prevention strategy.

Sequestration of Blood Stem Cells

If the crypts replenish themselves by sequestering blood stem cells, this implies an evolutionary pathway. However, it has further implications. Blood stem cell reservoirs must be more proliferative than previously thought, to support such replenishment. This implies that DNA integrity of the blood stem cells is higher than previously concluded. This has far-reaching implications for the rest of the body.

This also implies that periodic ablation of the ultimate stem cells, with replenishment by a secondary source of stem cells with higher DNA integrity, is a sufficient means to achieve DNA integrity within a tissue. This is in keeping with the principle of tumor suppression and replacement by stem cells with higher DNA integrity. It is also in keeping with WILT. However, the implication here is that the periodic reseeding would be a sufficient, but not necessary, means of achieving increased DNA integrity.

In WILT, the reseedings are necessary, as the WILT strategy is tantamount to programmed death in the absense of reseedings. In the DNA-integrity model, the absense of reseedings would merely leave one potentially subject to a "normal" aging process (albeit a greatly slowed process, due to the increased DNA repair/maintenance, ablation, and cellular proliferation suggested earlier).

Finally, this implies a method for reseeding the stem cells of the intestines: simply reseed the blood stem cell reservoirs.

Final Conclusion

The small intestine offers a model that suggests that ablation of cells with low DNA integrity, with a consequent replacement by cells with higher DNA integrity, is a viable approach to reduce cancer rates and preserve long-term DNA integrity. Various interpretations of experimental data each suggest that tumorigenesis can be prevented through a balance of increased apoptosis (through tumor suppression), increased DNA repair maintenance, and increased cell replacement (through proliferation).

The genetic framework for such a system is already within the genome, requiring not an engineering of new genetics, but of existing genetics.

[jaydfox]

I wanted to do more research on this, but I feel that feedback from actual biologists might be more effective than my trying to get caught up on the entire field of intestinal stem cell dynamics and oncology.

So, please feel free to rip my conclusions apart. It's the only way for me to improve them, or to find out if they're just plain wrong. Don't spare my feelings. Feel free to attack my logic and my interpretations, but please don't attack me.

[olaf.larsson]

Yes very interesting but the idea that there would be a special smallintestine stemcell population consisting of only 6(!) cells sounds little absurd to me.

[jaydfox]

Yes, sorry for the lack of references. I'm looking for people to break my logic, not my data. But if you have better data, please don't hesitate to cite it.

I will cite references as I get access to them (I currently have only abstracts, so I'm drawing conclusions from the abstracts, not the original data, of a few different studies. I apologize in advance. The only hard data I had was from Potten, which Prometheus already posted as he said, so that's the data I used for the numbers and the description of the tiered-model of stem cells in the crypts).

[jaydfox]

I should point out that Potten's paper is six or seven years old, so I would not be surprised if new data existed which might answer the puzzle of the ultimate stem cells.

I'm not the first to suggest that the small intestine offers a model for DNA integrity, as Potten himself made that assertion at least as early as 1998, and he apparently was referencing others who made the assertion even earlier.

However, what I am pointing out, which may or may not be original, is that regardless of the interpretation of the ultimate stem cells (which Potten concluded to divide daily), the small intestine still serves as a model. Whether the ultimate stem cells divide daily, monthly, or are even replaced periodically by blood stem cells, it doesn't matter. The small intestine still offers a model for DNA integrity as a preventative measure for cancer.

[kevin]

jay,

very lucid.. even I could follow the logic..

one might be able to increase apoptosis sensititivity and proliferation indefinitely and thus by natural selection keep a clean DNA code...

Would you say that the reason why high rates of turnover do not exist in other tissues is because it was never required as it is in the small intestine? Can you imagine any tissues where such a system might not work other than the brain? What stresses of high cellular turnover might be placed on the system which would prevent it from being useful? The cleanup of apoptotic processes would have to be stepped up a few notches if it were going to keep up I think.

[jaydfox]

Kevin,

The skeletal muscle is the first tissue I would have doubts about. Following Aubrey's reasoning in why ablation of COX-negative muscle fiber segments could kill an entire muscle fiber (1), we must be concerned about the proverbial "throwing out the baby with the bathwater". Given the length of muscle fibers, they would probably be difficult, but by no means impossible, to turn over rapidly. Besides, in SENS, in the topic of cell depletion, Aubrey already covered use of exercise and hormones to induce growth in muscle tissue, so my concerns here are probably premature. Also, gene suppression of the gene(s) the causes muscle fiber catabolism can be used to counter the size-diminshing aspects of cell ablation.

I doubt most of the internal organs of the torso would be unamenable to rapid turnover: the liver serves as a model there. The fact that stem cell tissue is hoped to cure so many problems with internal organs is a testament to the scientific viewpoint that internal organs can be rejuventated: cell replacement. However, rather than ablating just senescent cells, we can tune DNA integrity through selective ablation.

Blood and skin already turnover rapidly.

So that leaves bone and nerve tissue for the most part. I don't know if the brain is the only post-mitotic component of the nervous system, or if it extends down the spinal cord, or if it indeed encompasses the entire peripheral nervous system as well.

And I know nothing really about bone tissue.

Because I already pointed out the inherent difficulties in turning over skeletal muscle, I'm wondering how this applies to the heart, which is a distinct cousin of skeletal muscle. The heart may be the main exception of the torso organs. But again, stem cell research in this area is robust: the problem may not be that the heart is post-mitotic by necessity, but merely by design/lack of stem cell abundance/rejuvenative capacity. This can probably be fixed.

1) de Grey ADNJ. The reductive hotspot hypothesis of mammalian aging: membrane metabolism magnifies mutant mitochondrial mischief. Eur J Biochem 2002; 269(8):2003-2009. Relevant quote: "Moreover, OXPHOS-negative muscle fibre segments that were prevented from using the PMRS to survive might atrophy and potentially kill the entire fibre, risking severe sarcopenia"

[jaydfox]

Would you say that the reason why high rates of turnover do not exist in other tissues is because it was never required as it is in the small intestine?

That's part of it. High turnover (e.g. through extreme upregulation of p53) has been shown to be very effective at preventing cancer, but without an appreciable, and sometimes even a negative impact on lifespan. The reasons are probably many, but one is that the increased proliferation required exhausts the rejuvenative capabilities of stem cells, and postmitotic tissues don't have sufficient stem cell resources to fall back on for cell replacement. Thus, tumor suppressors accomplish the ablation aspect of the proposal more than adequately (in the gut lining, Potten pointed out that tumor suppression was sufficient to detect as little as a single DNA damage event).

It's the replacement part that poses a problem. Whether under "little" WILT or tumor suppression, replacement becomes the key issue.

However, under WILT, the replacement issue becomes decidedly lethal. Under advanced tumor suppression, replacement merely becomes the same issue that we already have today, and one amenable to stem cell therapy. Ablation through tumor suppression would not pose a hard limit of 10 years, but a much softer limit that can even be alleviated through methylation modulation as Prometheus describes. Thus, with adequate research, the tumor suppression + upregulation of kDRM factos + methylation rejuvenation, while decidedly more complex, actually provides at least as technically efficient a means of dealing with cancer, while eliminating the problems inherent in programmed death and frequent mandatory reseedings. Reseedings will be necessary to make the solution more effective in the long term, but they would not be nessecary to prevent the sudden onset of programmed death.

That's not to say that little WILT is simple. While the concept is simple in the extreme (delete telomerase), the technical issue are many and challenging, I've discovered upon further investigation of some of the relevant published work.

[manofsan]

Wow, I only just stumbled onto this thread. Very cool ideas.

Frivolous question -- why the sacred number of 6 for these ultimates? Is it cell geometry-based at all, or alternatively a product of the statistical mechanics of genome mutation? Perhaps an optimal number to deal with the vareity and frequency of environmental threats in the intestine?

So one could make analogies with the autophagy discussions. Ablation turnover is analogous to autophagy turnover, because mitochondria are relatively more mutation-prone, just like the intestinal surface cells. But mitochondria have the cauldron within, while the intestinal surface has the cauldron going on around it. Gee, no wonder fiber is a necessary part of our diet.

With respect to what you've said on skeletal, neural, and even muscle fibers -- so anything where tissue structural integrity is important, you don't want to have a high rate of cellular apoptosis turnover, so as not to degrade the tissue structure. You then want to resort to the less disruptive methods of autophagy/proteolysis/biogenesis, tumor suppression, repair systems (DNA repair, etc)
But where tissue structural integrity is less critical, then bombs away -- demolish the entire cells with apoptosis to more thoroughly clean house.

But rather than have these enhanced mechanisms acting on a constant ongoing basis throughout your life, wouldn't it be better to do it as the discrete-event periodic treatment? Heck, we can all last a decade or so without appreciable degradation in mitochondrial or nuclear DNA quality, before having to check in for treatment.

So in fantasizing about some future medical treatment -- you'd probably want to make the patient convalescent and even unconscious for awhile, to minimize any usage and/or strain on their physical tissue structures, allowing you to more heavily resort to the apoptosis than you otherwise could. (Heck, just suspend them floating in the sci-fi healing tank, like in the movies)

Then after the emaciated-but-purified body has had a chance to build itself back up again with cell replacement and biogenesis, the patient can then leave and resume their normal stressful life.

What's WILT, by the way?

[jaydfox]

What's WILT, by the way?

Heh, depending on whom you ask, it's either "programmed death" or the "ultimate cure for cancer". Personally, I figure it's a little bit of both.

Okay, but seriously, you can get more info here, and a more detailed technical writeup here (there's a PDF link there).

[manofsan]

It sounds like WILT is basically about increasing the telomere counter length across the board, so that all your cells have a longer lease on life in regards to the programmed time limit.

But given all the other non-programmed limits or problems that hit you as your cells get older, it sounds like WILT is only part of the solution.

[jaydfox]

It sounds like WILT is basically about increasing the telomere counter length across the board, so that all your cells have a longer lease on life in regards to the programmed time limit.

WILT stands for Whole-body Interdiction of the Lengthening of Telomeres.

Interdiction basically means "preventing".

It prevents your telomeres from getting any longer, something intended in this case to prevent cancers from growing big enough to be harmful. As Prometheus says, WILT will activate the death countdown timer in all cells that are affected (and with WILT, we're aiming for affecting a very high percentage of all cells).

Also, in the case of certain tissue types (blood, gut, and skin, for example), WILT would presumably prevent cancers from forming in the first place, since they would exhaust that countdown timer in a decade or so, far less than the "lifetime" it usually takes for cancers to develop.

[jaydfox]

since they would exhaust that countdown timer in a decade or so

This may or may not be the case with gut stem cells. See my first posts in this thread on this subject.