The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

History of the Mitochondrial Free Radical Theory of Aging, 1954-1995
concentrations of mutations. He therefore concluded that, most probably, point mutations
never accumulate at all, and the contrary findings that had been published by other labs
were artifacts of their experimental techniques. The other possibility, of course, was that
there was something unusual about the particular region of the mtDNA that Schon had
assayed (within the gene for ATPase subunit 6); but nothing of that kind was apparent, so
this formal possibility was not—at that time—considered.
6.6.5. Inherited mtDNA-Linked Diseases; The Threshold Effect
Just as it is difficult to extrapolate confidently from observations in rapidly dividing
cells in vitro to the situation in non-dividing cells in vivo, so there are pitfalls in extrapolating
from the phenotypes caused by genetic abnormalities to those that exist in normal aging. It
is very tempting—and by no means always fruitless—to do this when the macroscopic
phenotype of the disorder resembles accelerated aging, such as in Werner’s syndrome. It is
equally tempting when the similarity is at a microscopic scale, and, indeed, several disorders
exist in which mutant mtDNA appears to accumulate in a way similar to that in normal
aging, but faster, often leading to early mortality. The similarities are great: the same tissues
(non-dividing ones, particularly those with high energy demand) are most affected, and the
levels of activity of mt-coded enzymes (such as cytochrome c oxidase) exhibit the same
mosaic, “all-or-none” distribution with some cells (or fiber segments) having no activity
while the rest have normal activity 79
The differences are also rather striking, however, and immediately give reason for
caution. One great potential value of these diseases for the study of mtDNA dysfunction is
that they are caused by the presence of the same mutation in every affected cell, so that the
total level of mutant mtDNA in a tissue sample can be assessed quite accurately, in contrast
to the situation in normal aging discussed in the previous section. But when this is done,
patients are often found to have very large amounts of mutant mtDNA—sometimes over
50% by the time their symptoms have become severe. 80,81 The shortcomings of quantitative
PCR (especially of point mutations) are considerable, but histochemical assays of enzymatic
activity (which of course detect point mutations just as well as deletions) had by the early
1990s clearly excluded the possibility that the total mtDNA mutation load reached such
levels in normal aging. This could be (and indeed has been: see Chapter 9) seen as a challenge
to the relevance of mtDNA decline in aging, since the survival—albeit in poor health—of
people with lots of mtDNA damage surely meant that the low levels seen in normal aging
were essentially harmless. But the alternative interpretation is that these inherited mutations
are not knockouts, but in fact only reduce the OXPHOS activity of cells by a rather small
degree. Most spontaneous mutations, by contrast, would be severe,* and would therefore
have a phenotypic effect when present at much lower levels.
This interpretation makes sense of the curious observation that all known inherited,
disease-causing mtDNA mutations in protein-coding genes are missense mutations, which
change one amino acid, as opposed to deletions or nonsense mutations, which remove or
truncate the encoded protein and are therefore generally more severe. (This is in contrast to
sporadic, non-inherited mtDNA-linked diseases, which exhibit similar symptoms and are
often associated with mtDNA deletions.) 82 These mutations (and those in tRNA genes) are
known to inhibit OXPHOS severely in vitro, 81 but again one must be cautious, because this
* If most spontaneous mutations are deletions then this is certainly true, but if most are point mutations then
it needs some defending, since typical amino acid substitutions in typical genes do not completely abolish
their function. But it must be taken into account that all the mt-coded proteins are subunits of complex
multimeric enzymes; proteins of this sort are more sensitive to amino acid changes than average.
79