My latest Mind and Matter column for the Wall Street Journal is
about the possibility that some neurological conditions might be
caused by infectious agents -- of a sort
Might some forms of neurological illness, such as multiple
sclerosis and schizophrenia, be caused at least partly by bacteria,
viruses or other parasites? A largely Danish team has
recently publishedevidence of a strong association
between multiple sclerosis and a retrovirus, together with hints
that a gene called TRIM5, which is used by cells to fight viruses,
is especially active in people with MS.
Other illnesses have unexpectedly turned out to be caused by
parasites. In the 1980s, Barry Marshall of the University of
Western Australia ran into a brick wall of official disbelief for
suggesting that a bacterium caused stomach ulcers. Only by
deliberately infecting and then curing himself did he finally get
the medical establishment's attention (and eventually the Nobel
Prize).
The virus implicated in multiple sclerosis is called HERV-Fc1, a
bizarre beast called an "endogenous" retrovirus. What this means is
that its genes are part of the human genome. For millions of years,
they have been integrated into our own DNA and passed on by normal
heredity. It was one of the shocks of genomic science to find that
the human genome contains more retroviral than "human" genes: some
5% to 8% of the entire genome.
Normally, the genes of endogenous retroviruses remain dormant,
but-a bit like a computer virus that springs into action on a
trigger-something wakes them up sometimes, and actual viruses are
made from them, which then infect other cells in the body. The
Danish scientists suggest that this is what happens in multiple
sclerosis. Bjørn Nexø of Aarhus University writes that "retroviral
infections often develop into running battles between the immune
system and virus, with the virus mutating repeatedly to avoid the
immune system, and the immune system repeatedly catching up. One
can see the episodic nature of multiple sclerosis as such a running
battle."
The possibility that you can inherit the genes of a virus blurs
the distinction between a genetic and an infectious disease. The
HERV-Fc1 genes lie on the X chromosome. Since women have twice as
many X chromosomes as men, this might explain why some forms of MS
are more common in women. Dr. Nexø concludes hopefully: "The
finding that a disease is caused by an infectious agent is an
encouraging one. These are the diseases which we know best how to
treat.
Meanwhile, following some promising cases, a clinical trial
is now under way to find out if a cheap
antibiotic called minocycline might be used to treat schizophrenia.
Minocycline works against bacteria but can also kill a protozoan
parasite called toxoplasma, which has long been suspected of
causing at least some cases of schizophrenia. Toxoplasma can
reproduce sexually only in cats; to get from cat to cat, its egg
cells leave via the cat feces and wait to be eaten by other
mammals, especially rats. Inside a rat, it invades the brain, where
it alters the rat's behavior, disabling or even reversing the rat's
fear of cats. Cats eat foolhardy rats, facilitating the parasite's
entry into a new cat.
Human beings can also catch toxoplasma from cats, and it's known
to affect behavior: altering personalities, slowing reaction times
and increasing the risk of car accidents. More than 20 studies have
now found an association between schizophrenia and toxoplasma.
Schizophrenia is more common among those who had pet cats in their
childhood homes (but not in those who had pet dogs).
Indeed, some scientists think that schizophrenia only became common,
around 1870, when keeping cats as indoor pets became fashionable.
The parasite has genes for dopamine, a neurochemical found in
excess in schizophrenics.
For other kinds of disease, the search for an infectious agent
has not been so fruitful. The rise of autism in recent decades,
though partly attributable to better diagnosis, remains mysterious.
Many viruses have been accused of causing it, but so far none looks
especially guilty.
