DISTURBING new evidence suggests that nearly three times as many people may
be at risk from BSE than previously thought.
So far, all victims of vCJD, the human form of BSE, have been from a minority
of the population with a particular genetic make-up. This raised hopes that most
of the population was immune to the infection.
But the discovery that a closely related disease called Kuru is more
widespread among people in Papua New Guinea than previously thought, suggests
vCJD may pose a threat to everyone exposed to the infectious agent that causes
it.
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According to vCJD expert and British government adviser John Collinge of St
Mary鈥檚 Hospital, London, this could indicate that the mean incubation period in
people is 30 years or more. 鈥淲e may well see [cases] well into the second half
of the century,鈥� he says.
So far, all the 88 victims of vCJD are believed to have inherited identical
copies of a critical gene from both parents. The critical gene makes a protein
called PrPC. This is the normal version of the rogue 鈥減rion鈥� protein which
causes BSE in cattle and vCJD in humans.
At a particular site in the gene called codon 129, the victims have DNA which
loads the amino acid methionine into the PrPC protein. Victims inherited the
same gene variant from both parents, giving them this 鈥淢M鈥� combination. About 37
per cent of the population share the MM combination.
In other people, at least one of their two prion genes builds the amino acid
valine into the PrPC protein instead of methionine. Most have the 鈥淢V鈥�
combination, while the rest are 鈥淰V鈥� (see Table).
But tests on 11 elderly victims of a different form of human CJD, called
Kuru, suggest otherwise. Presented last week by Collinge at the Millennium
Festival of Medicine in London, the findings show that all 11 of them had the
鈥渉eterozygous鈥� or 鈥淢V鈥� combination, which is considered most protective.
Although Kuru is not the same as vCJD, the findings suggest that people with
the MV combination can鈥檛 assume they鈥檙e immune to prion diseases. 鈥淎ll these
latest [cases] are heterozygous, and we鈥檙e seeing lifetime incubation periods,鈥�
says Collinge, who is a member of the British government鈥檚 Spongiform
Encephalopathy Advisory Committee (SEAC).
The implication is that people with the 鈥減rotective鈥� gene combinations are
not immune from the disease. They鈥檒l simply have to wait longer for the disease
to strike.
Peter Smith, who chairs SEAC, agrees the new findings are cause for concern.
鈥淚t doesn鈥檛 rule out the possibility that the incubation period is longer in
people of that genotype,鈥� he says.
Kuru has killed over 2500 members of the Fore tribe in Papua New Guinea in
the past 100 years. The practice of eating dead relatives鈥� brains as a mark of
respect caused the rapid spread of Kuru from a single individual who probably
developed it through a spontaneous gene mutation.
The practice was banned in 1957, and the number of cases had
dwindled鈥攗ntil the latest, worrying emergence of new cases among the
elderly.