A new article

Suresh Rattan rattan at imsb.au.dk
Tue Jul 29 07:22:05 EST 1997


I am posting here the text of a newly published article in EUROPEAN JOURNAL
OF GENETICS IN SOCIETY, VOL. 3, ISSUE 2, PAGES: 27-29; 1997.

_____________________

Gene Therapy for Ageing: Mission Impossible?

Suresh I. S. Rattan, PhD, DSc
Laboratory of Cellular Ageing, Department of Molecular and Structural
Biology, University of Aarhus, DK-8000  Aarhus - C, Denmark.


Key words: ageing, gene therapy, gerontogenes, virtual gerontogenes, defence,
maintenance, homeostasis.


The modulation of ageing and lifespan has occupied the human mind since its
inception. Throughout human history, the search for means to prevent or
retard ageing has followed three main directions: (i) cleansing from
impurities and wastes; (ii) nutritional supplements, including the use of
medicinal plants; and (iii) replacement therapy. The immense popularity of
various spas and water therapies even today, is an example of the first
type of anti-ageing approach. The claims made for various herbal and other
medicinal plant products, such as ginseng, ginkgo biloba and garlic, as
nutritional supplements and anti-ageing drugs have received some support,
however preliminary, from laboratory and/or clinical tests. Replacement
therapy, especially hormonal replacement therapy as an anti-ageing
treatment has been used and misused for quite some time.
	Various therapeutic procedures have been followed to replace the
lost hormones, such as gland transplantation, secretory-cell injections and
hormone injections. Whereas many of these approaches, such as monkey
testicle transplants in the 1950s, have been frauds [1], others have been
refined to some extent and are still in use. For example, the injection of
biosynthetic human growth hormone for six months in elderly men resulted in
some increase in lean body mass, a decrease in adipose-tissue mass, and a
slight increase in vertebral bone density [2]. Similarly, there have been
some claims regarding the anti-ageing effects of dehydroepiandrosterone
(DHEA), which is the primary precursor of sex steroids [3]. Recently, the
so-called sleep hormone melatonin has received much attention for its role
as an anti-ageing hormone [4]. However, several questions remain unanswered
regarding the wider applicability of such approaches because of our present
lack of understanding of the regulation of synthesis of various hormones,
their modes of action, metabolism and interrelations with other hormones.
	Experimentally, the most effective anti-ageing and life-prolonging
strategy has proved to be calorie restriction. A large number of studies
have established that calorie restriction delays ageing and prolongs the
lifespan of various animals [5]. Although most of the studies on dietary
restriction and ageing have been performed on rats and mice, results have
started to emerge from studies on long-term dietary restriction of
non-human primates, rhesus monkeys [6, 7]. However, at this stage, it is
very difficult to say what kind of voluntary dietary-restriction regimes
will have similar anti-ageing and life-prolonging effects in human beings,
considering that there are significant differences in the biology and,
perhaps more importantly, the sociology of human beings as compared with
other animals.

The nature of ageing genes
Ageing has many facets and almost all the experimental data suggest that
ageing is not controlled by a single mechanism. Individually no tissue,
organ or system becomes functionally exhausted, even in very old organisms,
yet it is their combined interaction and interdependence that determines
the survival of the whole. With respect to the genes involved in the ageing
process, termed gerontogenes [8], there is much ongoing debate concerning
their nature and number.
	Since evolutionary theories of ageing and longevity discount the
notion of adaptive nature of ageing [9, 10], the concept of gerontogenes is
linked with the genes involved in homeostasis and longevity assurance,
instead of certain special genes for ageing. Therefore the term
gerontogenes does not refer to a tangible physical reality of real genes
for ageing but refers to an emergent functional property of a number of
genes which influence ageing. For this purpose, the term "virtual"
gerontogenes has been suggested [11]. The concept of virtual genes refers
to the emergent property of several genes whose functions are tightly
coupled and whose combined action and interaction resemble the effect of
one gene. Treating such a group as a virtual gene is a useful conceptual
tool while the search continues for the genetic elements of regulation of
complex biological processes, such as ageing. This idea of virtual
gerontogenes is in line with the evolutionary explanation of the ageing
process as being an emergent phenomenon caused by the absence of eternal
maintenance and repair instead of being an active process caused by
evolutionary adaptation.
	Obviously, not every gene is potentially a virtual gerontogene.
However, potentially every gene can affect the survival of an organism.
Therefore, a distinction must be made between immediate survival or death
on the one hand and the process of ageing on the other. The inactivation of
any essential gene will result in the death of an organism without having
anything to do with the process of ageing. The set of possible virtual
gerontogenes can be narrowed down to sets of genes involved in the
maintenance and repair of the cellular and sub-cellular components.
	Evidence for the hypothesis that candidate virtual gerontogenes
operate through one or more of the mechanisms of somatic maintenance and
repair comes from experiments performed to retard ageing and to increase
the lifespan of organisms. For example, anti-ageing and life-prolonging
effects of calorie restriction are seen to be accompanied by the
stimulation of various maintenance mechanisms. These include increased
efficiency of DNA repair, increased fidelity of genetic information
transfer, more efficient protein synthesis, more efficient protein
degradation, more effective cell replacement and regeneration, improved
cellular responsiveness, fortification of the immune system, and enhanced
protection from free-radical- and oxidation-induced damage [11, 12].
Similarly, anti-ageing effects of a dipeptide, carnosine on human diploid
fibroblasts [13] and a cytokinin, kinetin on human fibroblasts and on
insects [14, 15] also appear to be due to the effect of these chemicals on
maintaining the efficiency of defence mechanisms, including efficient
protein synthesis and turnover and the removal of oxidative damage.
	Genetic selection of Drosophila for longer lifespan also appears to
work mainly through an increase in the efficiency of maintenance
mechanisms, such as antioxidation potential. An increase in lifespan of
transgenic Drosophila containing extra copies of Cu-Zn superoxide dismutase
(SOD) and catalase genes is due primarily to enhanced defences against
oxidative damage [16]. The identification of long-lived mutants of the
nematode Caenorhabditis elegans, involving various genes [17] may provide
other examples of virtual gerontogenes because in almost all these cases
increased lifespan is accompanied by an increased resistance to oxidative
damage, an increase in the activities of SOD and catalase enzymes, and an
increase in thermotolerance [18, 19].

Perspectives for gene therapy
Estimates of the number of genes which might qualify as being a part of the
virtual gerontogene network of mammals run up to a few hundred out of about
one hundred thousand genes, and their allelic variants. Direct gene therapy
directed towards the overall ageing process seems to hold little promise.
This is because gene therapy for ageing will require methods to improve
upon the "genetic hand of cards" which determines the ageing and longevity
of a "genetically normal" individual.
	Assuming that there are 50 longevity assurance genes which
constitute the virtual gerontogene network in which they interact with each
other, this gives rise to 250 or 1015  (a million billion) possibilities of
their interacting and influencing each other. Not considering billions of
cells in an adult, even at the level of a single cell zygote, interfering
with such a complex network and improving upon what is already a "normal"
combination for that particular individual (in the absence of any obvious
genetic diseases) is a mission impossible.
	The question of total gene therapy for ageing is linked with the
issue of defining what is a normal combination of genes in a so-called
normal and healthy individual. It is in the very nature of genetic
polymorphism and the interactive nature of the genome that each individual
is unique and the term "normal" is extremely wide ranging. Therefore,
improving upon an already optimal or normal situation with respect to the
genetic constitution related to ageing and lifespan is a no-win situation.
	What is more likely to be achieved in the not-so-distant is that
experimental manipulation of certain genes will fine-tune or re-tune the
network and will prevent the onset of various age-related diseases and
impairments by maintaining the efficiency of homeostatic processes. Major
age-associated diseases which can be the first target of gene therapy
include cardiovascular and cerebrovascular diseases, cancer, diabetes,
osteoarthritis, osteoporosis, Alzheimer's disease, Parkinson's disease and
loss of renal function.
	In the short term, such studies will also result in the development
of a variety of so-called anti-ageing products, by concentrating on
individual members of the gerontogene family and maintenance network. Some
of the genetic mechanisms expected to be crucial in this regard are those
involved in maintaining: (i) the structural and functional integrity of the
nuclear and mitochondrial genome; (ii) the accuracy and speed of transfer
of genetic information from genes to gene products; (iii) the turnover of
defective and abnormal macromolecules; and (iv) the efficiency of
intracellular and extracellular communication and responsiveness.
	Much is known about each of the above-mentioned categories of
maintenance mechanisms. The application of modern, more sensitive methods
can further establish in detail what happens to these processes during
ageing. However, the ultimate aim of biogerontological research is to
understand why these changes occur, how they affect various other
constituents of the network, and how these can be modulated in order to
maintain the healthy span of life. The attainment of this goal will require
the development of experimental approaches in which the interactions
between mechanisms of maintenance at various levels is studied and the
reasons for their failure are understood. Elucidating the nature and
components of the virtual gerontogene network will open up new
possibilities of interfering with the network and re-tune the system for
both successful ageing and a prolonged survival. Immortality will,
nevertheless, remain unattainable.

References
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2.	Rudman, D., Feller, A.G., Nagraj, H.S., Gergans, G.A., Lalitha,
P.Y., Goldberg, A.F., Schlenker, R.A., Cohn, L., Rudman, I.W. and Mattson,
D.E. (1990) Effects of human growth hormone in men over 60 years old. New
Eng. J. Med., 323: 1-6.
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Dr. Suresh I. S. Rattan, PhD; DSc
Laboratory of Cellular Ageing
c/o Kemisk Institute
University of Aarhus
DK-8000 Aarhus - C
Denmark
Tlf: +45 89 42 39 56	Fax: +45 86 19 61 99






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