Deadly Fate:
Klotho
variant might shorten life-span in humans as well as rodents (Premature aging)
Authors: Davenport R. John 
Published: August 2003

Five years ago, researchers created a mouse that turned gray and decrepit well before its time due to a defect in the
Klotho
gene. They proposed that the normal version of the gene might mimic its namesake, the Greek Fate who spun the thread of life. The mutant mice develop some of the same diseases seen in human aging: atherosclerosis, emphysema, and osteoporosis (see Genetically Altered Mice entry
tg4
). And their time is short; the animals live for only 8 or 9 weeks, about 10% as long as most lab strains do. Together, these results suggested that the
klotho
mouse resembles humans with premature aging disorders (see
"Of Hyperaging and Methuselah Genes"
). But the comparison had some flaws; for one, symptoms that strike the animals do not typically afflict old rodents. Controversy erupted over whether the creatures age too quickly or succumb more readily to disease. New work supports the idea that the
klotho
mouse might provide a model of human aging: A particular variant of the gene is more common in human infants than in the elderly, suggesting that the gene's effect on life-span extends to humans. The genetic alteration influences where the gene's protein product accumulates and might interfere with its function, but how such changes might influence aging is not yet clear.

The protein encoded by the
Klotho
gene resembles enzymes that nip sugar molecules off of proteins or lipids; the presence or absence of such sugar molecules can modify the function of those molecules. In addition, sequence analysis suggests that
Klotho
protein accumulates in the space surrounding cells, leading some researchers to suggest that it acts as an antiaging hormone that can move throughout the body. Humans carry their own version of
Klotho
, although until now, no one had identified mutations in it that might speed human aging.

To investigate that issue, Arking and colleagues sought variations in the human
KLOTHO
sequence that are more common in young people than in the old. This discrepancy, they reasoned, might hint at a version of the gene that is associated with accelerated aging: Genetic changes that speed aging are less prevalent in older populations because animals that carry the gene die young. The scientists sequenced the
Klotho
gene carried by newborns and elderly people (aged 75 and older) in Bohemian Czechs, chosen because this population hasn't mixed extensively with others over the last millennium. They identified a form of the gene carried by nearly 3% of the babies but only 1% of the elderly. In further studies, the researchers sequenced
Klotho
from infants and people 64 years of age and older in two other test populations: Caucasians and African Americans, both from Baltimore. Again, they noted that more than twice as many infants as older participants harbored the alternate form, named KL-VS. A statistical analysis that combined data from the three groups indicates that individuals with two copies of the KL-VS variant suffer a 2.6-fold greater chance of dying before age 65 than do people with two normal copies of
KLOTHO
.

Additional experiments suggest that the apparently harmful version of
KLOTHO
changes the protein in a way that thwarts its function. The KL-VS protein sequence differs from the normal version by two amino acids. The researchers genetically engineered a closely related and better studied enzyme so that it carries the KL-VS amino acids at those two sites; they manufactured this protein in
Escherichia coli
, then purified and characterized it. The genetically altered enzyme no longer recognized its substrate, which hints that the KL-VS protein is similarly crippled. Additional experiments indicated that KL-VS--but not normal Klotho--accumulates outside cultured human cells in large quantities. This observation suggests that cells might normally control the amount of exported protein--as expected if it acts as a hormone--and that perturbations in this process might underlie possible harmful physiological effects in people who carry the altered form. Further work is needed to discover whether the observed alterations in protein function influence the aging process and if so, how.

Some researchers question the statistical significance of the genetic differences between the young and old populations; more studies in other populations are necessary to confirm the link between
KLOTHO
and human aging. In addition, many questions remain about the biochemical function of the protein and its role in both mice and humans. But the new work could renew interest in the
klotho
mouse as a model of aging--and it spins intriguing new threads to follow.

--R. John Davenport

D. E. Arking, A. Krebsova, M. Macek Sr., M. Macek Jr., A. Arking, I. S. Mian, L. Fried, A. Hamosh, S. Dey, I. McIntosh, H. C. Dietz, Association of human aging with a functional variant of klotho.
Proc. Natl. Acad. Sci. U.S.A.
99
, 856-861 (2002).
[Abstract]
[Full Text]