Deadly proteins: prions Understand article

Since the epidemic of ‘mad cow disease’ in the 1980s and 90s, and the emergence of its human equivalent, variant Creutzfeld-Jacob disease, there has been a great deal of research into prions, the causative agents. Mico Tatalovic reviews the current state of knowledge.

There may be a hidden epidemic waiting to occur, with millions of people already infected; we cannot prevent or cure it, and we cannot even diagnose it until the fatal symptoms appear.

The disease is variant Creutzfeld-Jacob disease (vCJD), one of a group of diseases known as transmissible spongiform encephalopathies (TSEs), which are caused and transmitted by abnormal forms of prion proteins. Examples of TSEs include not only vCJD, but also scrapie in sheep, bovine spongiform encephalopathy (BSE or mad cow disease) in cattle and kuru^w1 in humans. These diseases create large fluid-filled holes in the brain tissue because the accumulation of aberrant prions causes the neurons (nerve cells) to die. It is these characteristic holes that give the diseases their names: spongiform (sponge-like) encephalopathies (brain diseases).

TSEs affect the central nervous system, with symptoms including problems with co-ordination and balance, shakiness and uncontrollable jerking movements. In humans, TSEs also cause personality changes and depression, and sufferers may experience confusion, memory problems and insomnia. As the diseases progress, most mental functions are lost, including the ability to speak. All TSEs are fatal, and as yet, there is no cure.

Prions are specific proteins found mainly in the nervous system, where – in their normal forms – they may have important functions. For example, studies on sea slugs, Aplysia, suggest that prions have a crucial role in memory formation (Si et al., 2010). Infectious prions are abnormal (aberrant) forms of prion proteins that replicate inside the host by forcing normal proteins of the same type to adopt the aberrant structure. This has a domino effect whereby a small number of aberrant prions can affect many normal ones and eventually lead to disease. As the aberrant prions form amyloids – aggregates of protein – in the cells, the cells die, creating holes in the brain.

Prions are the only known case of self-propagating pathogenic (disease-causing) proteins, and they are able to cause severe illness even though they seem to be just protein molecules: unlike bacteria, viruses or other known pathogens, they have no information encoded in nucleic acids (DNA or RNA) about how to invade and replicate within the host. There is still a veil of mystery around prions and exactly how they replicate, cross the blood-brain barrier and cross the species barrier – i.e. infect different species of host.

It was in the 1960s that investigators first found that TSE disease-causing agents appeared to lack nucleic acids; Tikvah Alper suggested that the agent was a protein. This idea sounded heretical because all other known disease-causing agents contained nucleic acids and their virulence and pathogenesis were genetically determined.

However, three decades of subsequent investigations, pursued most notably by Stanley Prusiner, who was awarded the Nobel Prize in Physiology or Medicine in 1997 for his work with prions and TSEs, resulted in the wide acceptance of this ‘protein-only hypothesis’^w2.

Nevertheless, there are still those who believe that prion diseases are actually caused by unconventional viruses and that prion proteins are just part of this mysterious virus. Koch’s postulates describe what is needed to prove that a certain agent causes a particular disease; one of the necessary steps is to use that agent to induce the disease in a healthy organism. To prove that a TSE is indeed caused by the prion protein itself, the isolated, purified aberrant prions must be used to transmit the disease. It wasn’t until February 2010 that exactly this was done, adding further substantial evidence for the protein-only hypothesis (Wang et al., 2010).

Why do some scientists fear a vCJD epidemic?

The most worrying prion strain is the one that causes vCJD – a form of mad cow disease that has crossed the species barrier to infect humans (see box below). The first case of vCJD appeared in the UK in 1996 and has since killed 168 people there, with a peak in 2000 when 28 people died^w3, and there have been small numbers of cases in other countries including France, Italy, Ireland, Canada and the USA. Exact numbers of people infected or dying from this disease around the world are not known and although there is still uncertainty about how people become infected, the main infection route is thought to be through eating infected beef.

Most other prion diseases, including ‘normal’ CJD, are genetic or occur for unknown reasons, and affect elderly people. Variant CJD differs from CJD in that it affects young people and the aberrant prions are found not only in the brain but also in tissues such as blood, tonsils and appendices; this opens new ways of transmission to the entire population and not only the elderly.

For example, prions could be spread through blood transfusions. In the UK, donated blood is leucodepleted (the leucocytes, which may harbour infectious prions, are removed). Many other countries (e.g. Germany) ban blood donations from people who have lived in the UK.

The disease may also be spread by surgical instruments: infectious prions are resistant to high temperatures, irradiation and common chemical treatments that destroy other known pathogens. Estimates of how serious the risk is vary.

Furthermore, prions have also been found in the epididymis and seminal plasma tissues of rams (Gatti et al., 2002). Such findings led to concern about prion infection via sperm donations and even resulted in a US ban on sperm donations by Europeans and men who have lived in Europe. Nonetheless, a survey of experts worldwide estimated that the chance of prion transmission via sperm was less than 1:10 000 000 (Mortimer & Barratt, 2006).

But tissue contact – whether via sperm or blood or on surgical instruments – is not the only potential source of transmission: most of us consume milk and dairy products.

In 2006, a team of scientists from Switzerland detected low levels of the normal form of prions in milk bought in European shops (Franscini et al., 2006). Another study found that aberrant prions were able to replicate within mammary glands of scrapie-infected sheep (Ligios et al., 2005). Taken together, these results suggest that aberrant prions might be present in the milk of animals suffering from prion diseases. As prion disease symptoms take several years to develop, aberrant prions may be present in milk that is sold before the animals are diagnosed with the illness. Nonetheless, although it is not yet possible to give a clear estimate of the risk, it is generally thought that milk is safe until proven otherwise.

It seems, therefore, that we could contract vCJD via several routes although the risk is probably low. But are we all equally at risk? Several studies have shown that vCJD only affects people with certain variants (alleles) of a particular gene (prnp); other people seem to be resistant or do not develop the symptoms as fast^w4. Worryingly, about 40% of the Western European and North American population and up to 92% of the Japanese population are homozygous for these susceptibility alleles. Furthermore, research on kuru suggests that even individuals without the susceptibility allele may become infected but simply take much longer to develop symptoms. So perhaps we are all susceptible to prion infection.

Because of all of these possible transmission routes and uncertainties in how important they are, the results of studies predicting the number of people dying from vCJD in the UK range from several hundred to more than ten million people. Even though it now looks as though the vCJD epidemic in the UK is diminishing, this may be just because many infected people are still developing symptoms. No one knows for sure. Ongoing research will elucidate just how risky prion diseases are to public health.

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