By David N. Leff
Digestive Disease Week is a major medical congress, which this year staged a four-day meeting, ending today, in Atlanta. The conclave heard two trans-Atlantic scientists, from France and the U.S., report their latest discoveries on a compelling digestive theme ¿ Crohn¿s disease (CD).
This event led the journal Nature, dated May 31, 2001, to release both presentations over the web 10 days early. The two research articles convey the same long-awaited announcement ¿ identification of a gene that predisposes to CD ¿ perched on the long arm of human chromosome 16.
Pathologist Gabriel Nunez at the University of Michigan, Ann Arbor, is corresponding author of the American paper. Its title: ¿A frameshift mutation in Nod2 associated with susceptibility to Crohn¿s disease.¿
Medical geneticist Gilles Thomas, who heads a unit of INSERM, the French National Institute of Medical Science and Research in Paris, is senior author of the largely ¿ but not entirely ¿ parallel paper, headed, ¿Association of NOD2 leucine-rich repeat variants with susceptibility to Crohn¿s disease.¿
A sharp pang of pain in the southwest quadrant of the abdomen may hint at an attack of appendicitis. If so, the sufferer can count himself, or herself, lucky, because the same, sudden symptom can herald Crohn¿s disease. Appendicitis spells a quick, short trip to the operating table; CD, a lifelong sentence to diarrhea, often bloody, plus fever, abdominal cramps, weight loss ¿ all of which hang in there and intensify over the years.
Gastroenterologist Burrill Crohn at New York¿s Mount Sinai Hospital first described his eponymic malady in 1932, as a disease usually striking young adults before their 30s. Ever since then, CD has remained an inflammatory bowel disease ¿of unknown etiology.¿ But its subsequent epidemiology is all too well known. A scourge mainly of the Western World, it afflicts about half a million Americans ¿ one in a thousand ¿ and comparable numbers in Europe.
Bad Genes + Changing Lifestyle = Crohn¿s?
¿The incidence of Crohn¿s disease started increasing with the end of World War II,¿ Thomas told BioWorld Today in a telephone interview, ¿and kept going up into the 1980s. We don¿t know the exact reason for that, but we suspect that an environmental factor is causing that increase. This may be associated,¿ he suggested, ¿with changes in lifestyle that have taken place over the past half century. It may also be due to an unknown cryptomicroorganism that many investigators have attempted to identify, but have so far failed.
¿There is no cure for CD,¿ Thomas pointed out. ¿It¿s a chronic disease, and you have it for life. The therapies are based on nonsteroidal and steroidal anti-inflammatory drugs, also on immunosuppressors, and ultimately on surgery, which removes the ulcerated regions of the colon and intestine. But obviously,¿ he added, ¿these treatments have important side effects, and we hope that through our findings we will be able to propose more specific and less drastic therapies.
¿What we¿ve shown in our Nature paper,¿ Thomas continued, ¿is that three different genetic variants of the disease ¿ three nucleotide changes ¿ that occur in the human population are more frequently observed in CD patients than in unaffected individuals. And this increase of frequency of the mutant NOD2 gene demonstrates that these variations favor development of the disease.
¿We can say that these variations act in a recessive manner,¿ he went on. ¿That means, in order to be strongly disposed to CD, an individual has to receive variants from both his father and his mother. Only when both variants are present in the gene allele, the probability of developing CD is increased by a factor of 40, as compared to an individual who has none of these predisposing alleles.
¿Our strategy differs very significantly from that of the American group,¿ Thomas said. ¿We used positional cloning, which we have shown to be very effective in monogenic diseases. This approach has been questioned in regard to its effectiveness for polyfactorial diseases. The fact that we used it to identify NOD2 genes demonstrates ¿ for the first time, I think ¿ that indeed this strategy can be effective for some other polyfactorial diseases, such as CD.
¿Through positional cloning we reached and identified the NOD2 gene, which is indeed a very strong candidate to be involved in inflammation, and the etiology of CD. Now the fact that Gabriel Nunez has also found the same gene by other strategies is an independent confirmation of that.
¿The first thing that we did,¿ Thomas recounted, ¿was create a network of clinical enterologists in half a dozen European centers who supplied us 235 multiplex CD families with multiple cases. We studied these patients by linkage analysis, which resulted in localization of the NOD2 gene on chromosome 16, as we published in Nature in 1996.
¿At that time, the localization was precise to about 20 million base pairs. So all our efforts from 1996 up to now have focused more and more on defining a precise locus in that region. This we did by making a physical map of the region, using libraries of large DNA fragments known as Yacs and BACs ¿ yeast and bacterial artificial chromosomes.¿
20 Million Base Pairs Narrowed To 150,000
¿Then we searched these critical regions,¿ Thomas went on, ¿for variations of genes that would be associated with the disease. Once we found these associations, they became stronger and stronger, until we were sufficiently confident that the gene was lying in a region of about 150,000 base pairs. At that point, we sequenced this region, and found several variations associated with single genes. That led us to NOD2.¿
Now, Thomas and his co-authors are pursuing several CD research routes: ¿First we want to know exactly what is the consequence of the presence of the variant alleles in patients. Second, we want to identify additional variations that may be involved in CD ¿ not only NOD2, but also other genes. This will increase the power to search for additional susceptibility genes, which we know exist.
¿We also wish to develop an animal-model system, based on genetic manipulation of the mouse genome,¿ Thomas concluded, ¿that will be susceptible to inflammatory bowel disease, resembling CD.¿