From: Aubrey de Grey (ag24@gen.cam.ac.uk)
Date: Tue May 06 2003 - 16:22:11 MDT
Apologies for delayed reply -- been at the Foresight conference. Rafal
Smigrodzki wrote:
> ### So you are postulating a major chemical difference between fatty
> streak in apoE null mutants and in aged animals. Would it also apply to
> humans with hereditary hypercholesterolemia vs. aged humans?
Major by the metric of degradability, yes. Not nearly so much to humans
with hereditary hypercholesterolemia vs. aged humans, because the former
still take several years to form plaques, as opposed to weeks for the
apoE null mice.
> This is a legitimate hypothesis, but I'd need more data before I would
> accept it.
A key question is, are there any other comparably legitimate hypotheses?
> ### Interestingly, there are some lucky seniors who even at age 80 do not
> have atherosclerosis. If indeed there was an inexorable accumulation of
> indigestible material in most of us, how do they escape it? Would you say
> they do not make such supposedly insoluble plaque (hard to imagine,
> cholesterol oxidizes the same way in all humans), or do they have superior
> lysosomal enzymes?
Neither (well, not as the main factor anyway). Centenarians' arteries have
lots of fatty deposits. The main difference is that their vascular smooth
muscle is less prone to get inflamed and hyperplastic as a result of the
presence of this nasty stuff: i.e., they are better protected at the next
step.
> ### I went over Chinnery's articles (Am J Hum Gen, Lancet) and I didn't
> find any clear statement that hyperproliferation must precede COX
> negativity. Where do you think he says that it does?
Right, that's part of the problem, his model ineluctably predicts that
but he doesn't properly take it on board. The driving force that allows
mutants to take over is an acceleration of mitochondrial replication
when there are too few working mitochondria, with the mutant mitochondria
playing no part -- effectively they are just accumulating (or fluctuating)
detritus. But when they do accumulate, as a result of drift, the total
mitochondrial number increases to compensate, keeping the number of wild
type mitochondria optimal. This only breaks down when the mitochondrial
number reaches "alpha" times the optimal wild-type. So we should see hyperproliferation before the cell loses COX activity.
> In fact, hyperproliferation may well be triggered by COX negativity.
Absolutely -- it almost certainly is. But of course then it wil not
precede COX negativity.
> if indeed the mutated genome has a selective advantage, then cells with
> more mt genomes would be at a higher risk of suffering a mutation (this
> risk goes up as a simple function of the number of genomes), and once a
> single mutation is present, such cell would very quickly shift to 100%
> mutated genomes, as a result of selection.
Right -- other things being equal. But they aren't equal (see below).
> very large cells, like the ovum, and the syncytial muscle fiber, with
> 100 000 mtDNA's, and very small or quiescent cells, like chondrocytes,
> with only 1000 to 5000 mtDNA's, would differ in the accumulation of
> mutations by an order of magnitude or more. Is it indeed the case?
Not really, because the rate of accumulation is also affected by many
other things. The foremost is the mutation rate, which is probably
very affected by the respiration rate -- so in fact, oocytes shouldn't
get much mutant mtDNA because they are so quiescent. A second is the
ability for cells to die and be replaced once they become COX-negative;
this almost certainly happens, so it's not easy to do your comparison
except across postmitotic, rather apoptotis-resistant cells. Muscle
fibres are a bit strange because the mitochondria are not very mobile,
but the COX-negativity does spread slowly along the fibre.
> Chinnery was able to get simulation results consistent with experimental
> observations even without stem mitochondria.
Nope -- only to the superficial extent that he analysed his simulation.
He found parameters that gave a realistic number of COX-negative cells
in a lifetime, but he forgot to look at how many cells were highly
heteroplasmic (had lots of wild-type and lots of mutant). There are
far more of these than there are COX-negative cells -- but in real life,
including in (eg) Brierley et al, a paper from Turnbull's group that he
actually cites to the contrary!, we see many more totally or virtually
homoplasmic mutant cells than highly heteroplasmic ones. Ah yes, as
you note:
> Chinnery's model indicates that the number of cells with 50% mutated
> genomes will be actually pretty low, on the order of a few percent in
> aged humans
A few percent is actually a lot higher than the number of 100% mutant
cells seen in real life in any tissue except the substantia nigra and
one or two similarly stressed brain regions. There it's 10%; in the
skeletal or cardiac muscle it's down around 0.1%.
Aubrey de Grey
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