With seed money from the Johns Hopkins
for Cell Engineering, a Johns Hopkins geneticist and
her team have discovered a critical link between the health
of stem cells and the length of the chromosome ends within
Chromosome ends, or telomeres, are repetitive
stretches of DNA that protect chromosomes in much the same
way as plastic tips on shoelaces prevent the fabric from
fraying. Each time a cell divides, its chromosome ends get
a little shorter, and eventually the cell can no longer
divide because its critical genetic information is exposed.
In stem cells, however, a protein called telomerase
normally maintains the telomeres' length, allowing the
cells to divide indefinitely.
Now, the Johns Hopkins researchers report that mice
engineered to have just half the normal amount of
telomerase can't maintain their stem cells' chromosome
ends, showing that a little telomerase isn't enough. In
these "half-telomerase" mice, their telomeres shortened
over time, bringing an early demise to stem cells that
replenish the blood supply, immune system and intestine.
Moreover, offspring of these mice bred to have normal
levels of telomerase still exhibited early loss of stem
cells, the researchers report in the Dec. 16 issue of
"These offspring have what we have called 'occult'
genetic disease; their genetic makeup is perfectly normal,
but they still have the physical problems of their
parents," said Carol Greider, director of
Molecular Biology and
Genetics in the Johns Hopkins Institute of Basic
Biomedical Sciences. "This phenomenon could complicate the
hunt for disease genes."
Scientists generally figure that inherited disease
accompanies an inherited mutation in one or more genes. In
the case of the genetically normal offspring of two
half-telomerase parents, however, the disease is still
present. The problem in these animals turns out to be the
animal's inherited telomere length, not the status of the
telomerase gene, Greider said.
"If you were to search for the genetic mutation behind
this mouse's disease, you wouldn't find it; there isn't
one," Greider said. "These mice develop disease only
because their telomeres are short, and having telomerase
doesn't lengthen them right away."
The condition of the mice is virtually identical to
the human disease dyskeratosis congenita, which has already
been linked to mutations in telomerase that hinder the
protein's telomere-maintaining activity. In both mice and
people with the condition, stem cells in bone marrow that
replenish the blood cells and those in the digestive system
that maintain the intestines can't divide as many times as
they should and die early.
Because sperm and egg arise from stem cells, too,
their telomeres gradually shorten, and each successive
generation starts out with chromosomes whose telomeres are
even shorter than their parents', the researchers report.
The failure of telomerase to lengthen these telomeres
explains why successive generations develop the physical
symptoms of the disease at younger ages than their parents
or grandparents, the researchers said.
In the Proceedings of the National Academy of Sciences
in October 2005, Greider and her team reported their study
of one family with dyskeratosis congenita. Mary Armanios,
now an assistant professor of oncology in the Johns Hopkins
Kimmel Cancer Center, discovered that the family carried a
genetic defect that caused the telomerase protein to be
half as effective as normal and that shortening telomeres
were to blame for earlier onset.
In this family, the affected grandmother developed
gray hair in her 20s and lung problems in her early 60s and
died at age 65. Her affected children developed signs of
the disease about 10 years earlier than she had, and
analysis of their cells revealed that 60 percent to 75
percent of their chromosomes had dangerously short
telomeres. In an affected grandson, signs of the disease
appeared 40 years earlier than in his grandmother and 20
years earlier than in his father. Roughly 90 percent of the
chromosomes in his cells had dangerously short
"We know it only takes one critically short telomere
to make a cell die, so it's clear that the more really
short telomeres a person has, the faster problems will
develop," said Greider, whose lab reported the role of the
critically short telomere in 2001.
The usual treatment for people with dyskeratosis
congenita is a bone marrow transplant from an unaffected
family member. But the team's new findings in mice suggest
that the family member chosen for the transplant — if
there's more than one option — should not only have normal
telomerase levels but also have long telomeres compared to
other family members.
"Normal levels of telomerase didn't lengthen short
telomeres in our mice, so the longer the telomeres are to
start with, the longer transplanted stem cells will be able
to divide and the more likely the transplant is to
succeed," Greider explained.
To engineer the half-telomerase mice, Ling-Yang Hao,
then a graduate student, knocked out one copy of the
telomerase gene in nonlaboratory mice, whose telomere
length is similar to humans'. (Typical laboratory mice have
very long telomeres.)
He then bred these half-telomerase mice to one
another, and the team studied offspring that also carried
just one telomerase copy. (Because one copy of each gene is
inherited from each parent, only 50 percent of the
offspring would be expected to end up with only one
telomerase copy, 25 percent would have no telomerase gene,
and 25 percent would have two copies of the telomerase
By the fifth generation, mice had severely shortened
telomeres and exhibited failure of organs that have high
turnover of their cells, the bone marrow and intestine
"We thought there might be some relationship between
telomerase, telomere length and the survival of stem cells,
but it was really exciting to see it," Greider said.
When the researchers looked at fifth-generation mice
that had by chance inherited their parents' good telomerase
copies, giving the animals a full complement of telomerase,
they were surprised to find that those mice had the same
symptoms and problems as their half-telomerase
The researchers are now mating these short-telomere
mice with mice with regular-length telomeres to see whether
telomere length goes back up. They're also studying the
affected stem cells to find out exactly how critically
short telomeres are affecting their survival.
The researchers were funded by the National Institutes
of Health and Johns Hopkins Institute for Cell Engineering.
Authors on the paper are Hao, Armanios, Greider, Margaret
Strong, Baktiar Karim, David Felser and David Huso, all of
the Johns Hopkins School of Medicine. Karim and Huso are
with the Department of Comparative Medicine.