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It is a common misconception
that genetic diseases are, by their nature, untreatable. This
stems from a lack of understanding of the modifying effects of
the environment on expression of the genotype. As in all medical
therapy, the treatment of genetic disease depends on modifying
the environment so that any harmful expression of the mutant gene
can be counteracted. Thus, in the disease 
phenylketonuria
the expression of the mutant gene is counteracted by putting the
affected person on a diet low in phenylalanine. Similar dietary
manipulations can effectively control many other inborn errors
of metabolism.
Developments
in somatic
cell and molecular genetics, researchers have elucidated
the precise molecular defect in many genetic disorders, often
making it possible to neutralize the defect by application of
appropriate drugs or dietary changes. On occasion, surgeons can
counteract the consequences of a mutant gene by using bone marrow,
kidney, or even liver transplants.
The
increased understanding of immune mechanisms involved in organ
rejection has greatly increased the ability to transplant these
tissues and organs (see transplant). In other cases, surgical
repair of physical defects can be extremely effective treatment.
As alluded
to earlier, some genetic conditions can even be treated prenatally.
For example, specialists can treat fetuses in whom a certain kind
of vitamin B deficiency, thyroid deficiency, or heart failure
has been diagnosed. Fetal surgery can be performed on various
obstructive lesions involving the brain or urinary tract. Such
surgery permits the baby to be born without irreparable damage,
so that definitive repair can be performed after birth.
Although
it has not yet been used in humans, gene transfer therapy-the
introduction of a normal gene into an individual in whom that
gene is not functioning-would provide an effective way of treating
diseases due to mutant genes of major effect. The introduction
could be either into those tissue cells that normally express
the gene or into the early embryonic cell. Only the latter situation
would provide a full genetic cure, in that the genetic repair
would pass through the germ line to future generations. The former
situation would treat the diseased individual, but the mutant
gene would still be in the germ cell and hence could be passed
on to offspring.
Francis
Collins
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