I am a first year graduate student doing a rotation in
Vicki Lundblad's lab. She discovered Est1, the yeast
gene which holds the RNA template for the telomeres.
It is a part of this "telomerase" complex.
When it is disrupted, resulting yeast show a senescence phenotype
mimicking eukaryotic senescence, but not exactly paralleling
it - the yeasts die due to chromosome loss, whereas
senescent fibroblasts can sit for years, and do not suffer
massive chromosome loss or immediate death. I don't think
they lose their telomeres, it's just that they stop dividing
about the time their telomeres get real short.
As far as I can tell, there is no way to tell if telomere
shortening is a cause or effect of cell aging. For example, if the
genome knows that humans don't live past a hundred years,
it seems to me that it might just turn the telomerase off
in somatic cells to save energy. It makes sense that
cancer cells have turned it back on, because they are planning
on dividing many more times than they're supposed to, since it has
been shown in yeast that telomere loss leads to chromosome
loss, which is definitely bad news for the cell.
I think the big theory goiing around
is that mammalian cells are supposed to sense the short
telomeres, and exit the cell cycle, so they don't lose
chromosomes perhaps. Cancer cells will die if they lose their
chromosomes, so they have to turn the telomerase on.
This can't be proven, though, or it
would be on the cover of Time and Newsweek.
In any case,
I don't think it is yet possible to say whether short telomeres
are an upstream or parallel occurrence with respect to cell senescence.
When the genes involved in both processes are isolated and characterized it
will be possible to say for certain.
(I know I'm doing my part!)
I pulled up a few abstracts on medline addressing questions posed
in previous postings. It seems like the Canadian guys are really
sold on this.
AN 93066190. 93021.
AU Allsopp-R-C. Vaziri-H. Patterson-C. Goldstein-S. Younglai-E-V.
Futcher-A-B. Greider-C-W. Harley-C-B.
IN Department of Biochemistry, McMaster University, Hamilton, ON,
Canada.
TI Telomere length predicts replicative capacity of human fibroblasts.
SO Proc-Natl-Acad-Sci-U-S-A. 1992 Nov 1. 89(21). P 10114-8.
JT PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES
OF AMERICA.
PT JOURNAL-ARTICLE (ART).
AB When human fibroblasts from different donors are grown in vitro, only
a small fraction of the variation in their finite replicative
capacity is explained by the chronological age of the donor. Because
we had previously shown that telomeres, the terminal guanine-rich
sequences of chromosomes, shorten throughout the life-span of
cultured cells, we wished to determine whether variation in initial
telomere length would account for the unexplained variation in
replicative capacity. Analysis of cells from 31 donors (aged 0-93
yr) indicated relatively weak correlations between proliferative
ability and donor age (m = -0.2 doubling per yr; r = -0.42; P = 0.02)
and between telomeric DNA and donor age (m = -15 base pairs per yr;
r = -0.43; P = 0.02). However, there was a striking correlation,
valid over the entire age range of the donors, between replicative
capacity and initial telomere length (m = 10 doublings per kilobase
pair; r = 0.76; P = 0.004), indicating that cell strains with shorter
telomeres underwent significantly fewer doublings than those with
longer telomeres. These observations suggest that telomere length is
a biomarker of somatic cell aging in humans and are consistent with
a causal role for telomere loss in this process. We also found that
fibroblasts from Hutchinson-Gilford progeria donors had short
telomeres, consistent with their reduced division potential in vitro.
In contrast, telomeres from sperm DNA did not decrease with age of
the donor, suggesting that a mechanism for maintaining telomere
length, such as *telomerase* expression, may be active in germ-line
tissue. Author-abstract.
AN 92309418. 92000.
AU Levy-M-Z. Allsopp-R-C. Futcher-A-B. Greider-C-W. Harley-C-B.
IN Department of Biochemistry, McMaster University Hamilton, Ontario,
Canada.
TI Telomere end-replication problem and cell aging.
SO J-Mol-Biol. 1992 Jun 20. 225(4). P 951-60.
JT JOURNAL OF MOLECULAR BIOLOGY.
PT JOURNAL-ARTICLE (ART).
AB Since DNA polymerase requires a labile primer to initiate
unidirectional 5'-3' synthesis, some bases at the 3' end of each
template strand are not copied unless special mechanisms bypass this
"end-replication" problem. Immortal eukaryotic cells, including
transformed human cells, apparently use *telomerase,* an enzyme that
elongates telomeres, to overcome incomplete end-replication.
However, *telomerase* has not been detected in normal somatic cells,
and these cells lose telomeres with age. Therefore, to better
understand the consequences of incomplete replication, we modeled
this process for a population of dividing cells. The analysis
suggests four things. First, if single-stranded overhangs generated
by incomplete replication are not degraded, then mean telomere length
decreases by 0.25 of a deletion event per generation. If overhangs
are degraded, the rate doubles. Data showing a decrease of about 50
base-pairs per generation in fibroblasts suggest that a full deletion
event is 100 to 200 base-pairs. Second, if cells senesce after 80
doublings in vitro, mean telomere length decreases about 4000
base-pairs, but one or more telomeres in each cell will lose
significantly more telomeric DNA. A checkpoint for regulation of
cell growth may be signalled at that point. Third, variation in
telomere length predicted by the model is consistent with the abrupt
decline in dividing cells at senescence. Finally, variation in
length of terminal restriction fragments is not fully explained by
incomplete replication, suggesting significant interchromosomal
variation in the length of telomeric or subtelomeric repeats. This
analysis, together with assumptions allowing dominance of *telomerase*
inactivation, suggests that telomere loss could explain cell cycle
exit in human fibroblasts. Author-abstract.
AN 91054430. 91000.
AU Greider-C-W.
IN Cold Spring Harbor Laboratory, New York 11724.
TI Telomeres, *telomerase* and senescence.
SO Bioessays. 1990 Aug. 12(8). P 363-9.
JT BIOESSAYS.
PT JOURNAL-ARTICLE (ART). REVIEW (REV). REVIEW-TUTORIAL (TUT).
AB Eukaryotic chromosomes end with tandem repeats of simple sequences.
These GC rich repeats allow telomere replication and stabilize
chromosome ends. Telomere replication involves an equilibrium of
sequence loss and addition at the ends of chromosomes. Repeats are
added de novo by *telomerase,* an unusual DNA polymerase. *Telomerase*
is an RNP in which an essential RNA component provides the template
for the added telomere repeats. Telomere length maintenance plays an
essential role in cell viability. Author-abstract. 58 Refs.
AN 90259099. 90000.
AU Harley-C-B. Futcher-A-B. Greider-C-W.
IN Department of Biochemistry, McMaster University, Hamilton, Ontario,
Canada.
TI Telomeres shorten during ageing of human fibroblasts.
SO Nature. 1990 May 31. 345(6274). P 458-60.
JT NATURE.
PT JOURNAL-ARTICLE (ART).
AB The terminus of a DNA helix has been called its Achilles' heel. Thus
to prevent possible incomplete replication and instability of the
termini of linear DNA, eukaryotic chromosomes end in characteristic
repetitive DNA sequences within specialized structures called
telomeres. In immortal cells, loss of telomeric DNA due to
degradation or incomplete replication is apparently balanced by
telomere elongation, which may involve de novo synthesis of
additional repeats by novel DNA polymerase called *telomerase.* Such
a polymerase has been recently detected in HeLa cells. It has been
proposed that the finite doubling capacity of normal mammalian cells
is due to a loss of telomeric DNA and eventual deletion of essential
sequences. In yeast, the est1 mutation causes gradual loss of
telomeric DNA and eventual cell death mimicking senescence in higher
eukaryotic cells. Here, we show that the amount and length of
telomeric DNA in human fibroblasts does in fact decrease as a
function of serial passage during ageing in vitro and possibly in
vivo. It is not known whether this loss of DNA has a causal role in
senescence. Author-abstract.
