LONDON - Identification of the precise part of the wheat protein that triggers celiac disease in susceptible people could lead to new treatments for this condition, as well as for modified foods that sufferers could eat without ill effect. A new diagnostic test for celiac disease is likely to follow.
Bob Anderson, postdoctoral research fellow at the Institute of Molecular Medicine at the University of Oxford in the UK, told BioWorld International, "This discovery is very significant for people with celiac disease, but it could also benefit research into treatments for other diseases with an autoimmune component such as multiple sclerosis and diabetes, as well as aiding rational vaccine development for infectious diseases such as hepatitis B and C and HIV/AIDS."
Anderson and his colleagues report their findings in a paper in the March issue of Nature Medicine titled, "In vivo antigen challenge in celiac disease identifies a single transglutaminase-modified peptide as the dominant A-gliadin T-cell epitope." They are looking for partners to help develop their work further.
Celiac disease is very common, affecting about one in 300 people in Europe and North America. The highest incidence in Europe is in Ireland, where 1 in 100 people have the condition. Because many cases are relatively mild, however, only about 1 in 10 sufferers are ever diagnosed.
At its most severe, celiac disease presents in infancy with failure to thrive, gross malabsorption and deficiencies in various nutrients. More commonly, it is not picked up until adolescence or early adulthood, when it may cause vague symptoms such as lethargy, diarrhea, and, particularly in women, iron deficiency anemia.
Once diagnosed, sufferers are committed to a lifelong gluten-free diet, which means avoiding anything containing wheat, rye or barley. Anderson, who also works as a gastroenterologist in the Nuffield Department of Medicine at the John Radcliffe Hospital in Oxford, said he estimated the value of the global market in gluten-free foods for celiac disease alone at about #120 million (US$191 million) a year. "But if everyone who had celiac disease could be diagnosed, it could be worth about 10 times that," he added.
Over 90 percent of people diagnosed with celiac disease are positive for HLA-DQ2. The protein encoded by this gene is involved in presenting peptides of wheat protein to cells of the immune system, thus triggering the immune response that causes the disease. These peptides are derived from the group of proteins found in wheat called gliadins.
Anderson said, "There are about 50 of these gliadins, but until now it has not been clear which gliadin sequences are the cause of the disease." He and his colleagues decided to begin by focusing on A-gliadin, as its genetic sequence was already known.
The group also knew, from a study by Norwegian researchers published several years ago, that an enzyme called transglutaminase, which is present in the intestine, converts specific glutamine residues to glutamate in gluten peptides. This action of transglutaminase also enhances recognition of gluten peptides by gluten-specific lymphocytes purified from intestinal tissue biopsies taken from people with celiac disease.
"But what we did not know," Anderson said, "was which peptide of the gliadin was causing the immune reaction."
He decided to mount a search for the "dominant epitopes" of gliadin that were responsible for triggering the harmful immune response in celiac disease. "When someone is infected with a virus, the immune system initially recognizes one or two dominant peptides," Anderson said. "Then, over a period of weeks, the immune reaction spreads out to recognize other viral peptides. I reasoned that if you took people with celiac disease who were well and on a gluten-free diet, and challenged them with gluten, then you might pick up the consequent immune response."
The results are reported in the paper. Twelve people with celiac disease were invited to consume four slices of gluten-containing bread daily. One dropped out after half a day, because the bread made her vomit. But 10 continued for three days and one for 10 days.
Anderson said, "I followed their response, using cells from blood samples, over the following 12 days and, to my surprise and delight, 11 out of the 12 reacted very specifically. Six days after starting their bread, they all reacted against this one segment of 17 amino acids, but only if it had been treated with transglutaminase. It appeared that the immune response depended on transglutaminase converting one amino acid out of the 17 from a glutamine to a glutamate. This conversion appears to allow the peptide to bind to the DQ2 protein, and it is this which stimulates the immune response."
The group also showed in laboratory experiments that if they blocked DQ2, this halted the immune response. "We have also proved," Anderson said, "that this is a disease-specific immune response, because healthy control subjects with DQ2 did not mount this response."
Anderson has thought of many ways of exploiting the finding. He suggested that it may be possible to induce tolerance in the immune system by giving the peptide that is responsible for triggering celiac disease to people in large amounts. Another strategy could be to modify the peptide by altering the key amino acid, in the hope of desensitizing the immune system for several weeks at a time.
"You could also use this peptide sequence to make an antibody test that would tell you if food was toxic to people with celiac disease," Anderson added. "You could make wheat that does not have this particular sequence in it. You could make transgenic plants that could deliver therapeutic peptides as food. This is a long way off, but these are theoretical possibilities."
More broadly, he suggested, a similar approach could be used to study the early stages of infection with viruses such as hepatitis B, hepatitis C or HIV. "The technique we used could allow you to pick out a very small number of dominant peptide sequences that the immune system is responding to in vivo, to allow more rational vaccine development."
Anderson and his colleagues are trying to develop a new diagnostic test for celiac disease based on their findings, using a single peptide. This could revolutionize the diagnosis of celiac disease, which currently relies on a screening blood test followed by endoscopy and biopsy of the stomach lining.