The Kids Are Alright (plus or minus 10%)
Every now and then the question of heredity pops up here, and a while back I mentioned I was planning to do a little research on the topic. Much as I love the Internets, I decided to spend some time in an actual library (a medical school library, though I am no medical student). I was looking at Crohn's in particular, but a lot of the research overlaps. I thought I'd share what I found, pointing out the stuff that is specific to Crohn's.
First let me say that every article and source I found describes CD/UC as complex and polygenic. Complex means that it does not follow the same pattern as simple genetic attributes, like eye color. Polygenic means that there are a number of genes associated with the disease. Moreover, all of the articles agreed that an environmental factor contributes to these diseases, although there is no scientific consensus as to what that factor is. A person has to have both the genetic susceptibility and the environmental factor to develop the disease.
That said, in the rest of the post I am talking about people who actually develop the disease; the figures for inheritance are not just for genetic susceptibility, but for diagnosis of the actual disease.
(Also, where appropriate I will refer to articles by number (#) behind the cut.)
In the U.S., CD affects approximately 4 people in 100,000; UC affects approximately 100 in 100,000.(3) So a person's random chance of having IBD is 1 in 25,000 for CD and 1 in 1000 for UC.
The single strongest risk factor for developing inflammatory bowel disease is having an affected relative with inflammatory bowel disease. If a family member is affected, the chance of developing Crohn's disease is 10%. (7)
The age-adjusted risk of inflammatory bowel disease is approx. 5% for siblings and 10% for offspring. (7)
Contrary to the above, one source reports risk to a sibling of an affected person ranging between 13% to 36% for CD and 7% to 17% for UC. (1) This sort of estimate is probably irrelevant to us, but scientists use it to demonstrate that genetics plays some role in the disease. Also, the high end of those estimates may come from studies of identical twins, so I'm not sure how pertinent they are.
A fairly rigorous study in Belgium found that for first-degree relatives (i.e. siblings or children) of CD affected people, the risk is 4.8% for IBD in general and 3.9% for CD specifically. The risk for offspring was highest, at 10.4%. (5) Based on the methodology and the large sample in this study, I think this is probably pretty accurate.
Several studies suggest that heredity is more important in CD than in UC, usually by a few percentage points. (1, 4, 5, 6)
One study found that children of parents who both have IBD run a 33% chance of developing the disease before age 28. (4) Those of us who come to this board trolling for dates should know that any child of two IBD affected parents has a 1 in 3 chance of developing the disease.
Oddly enough, the same study suggests there is a slightly higher risk - about 2 in 1000 - of developing IBD if your spouse already has or develops one of the diseases. Again, this points to the genetic/environmental interplay; it's not that IBD is necessarily contagious, but that couples living together for long periods face the same environmental triggers.
Studies also suggest that the child of a parent with CD is more likely to develop the disease at an earlier age than a patient with no family history of the disease, although the disease is not likely to be any more severe in the child. (1)
I hope this helps. I apologize to the UC folks here that this wasn't equally balanced between the two diseases, but I'm happy to help out anyone who thinks this needs a little more research.
(1) Ahmad, T. et al. Review article: the genetics of inflammatory bowel disease. Aliment Pharmacol Ther 2001; 15: 731-748.
(2) [omitted]
(3) Haigh, B. Inflammatory bowel disease and innate immune response genes: the challenge of complex polygenic disorders for the clinical geneticist. Clin Genet 2006; 69: 315-318
(4) Laharie, D. et al. Inflammatory bowel disease in spouses and their offspring. Gastroenterology 2001; 120: 816-819
(5) Peeters, M. et al. Familial aggregation in Crohn's Disease: increased age-adjusted risk and concordance in clinical characteristics. Gastroenterology 1996; 111: 597-603
(6) Orholm, M. et al. Familial occurrence of inflammatory bowel disease. New England Journal of Medicine Jan 10, 1991; 324:2, 84-88
(7) "Crohn's Disease". First Consult. (An online medical database accessed through the library.)
.
First let me say that every article and source I found describes CD/UC as complex and polygenic. Complex means that it does not follow the same pattern as simple genetic attributes, like eye color. Polygenic means that there are a number of genes associated with the disease. Moreover, all of the articles agreed that an environmental factor contributes to these diseases, although there is no scientific consensus as to what that factor is. A person has to have both the genetic susceptibility and the environmental factor to develop the disease.
That said, in the rest of the post I am talking about people who actually develop the disease; the figures for inheritance are not just for genetic susceptibility, but for diagnosis of the actual disease.
(Also, where appropriate I will refer to articles by number (#) behind the cut.)
In the U.S., CD affects approximately 4 people in 100,000; UC affects approximately 100 in 100,000.(3) So a person's random chance of having IBD is 1 in 25,000 for CD and 1 in 1000 for UC.
The single strongest risk factor for developing inflammatory bowel disease is having an affected relative with inflammatory bowel disease. If a family member is affected, the chance of developing Crohn's disease is 10%. (7)
The age-adjusted risk of inflammatory bowel disease is approx. 5% for siblings and 10% for offspring. (7)
Contrary to the above, one source reports risk to a sibling of an affected person ranging between 13% to 36% for CD and 7% to 17% for UC. (1) This sort of estimate is probably irrelevant to us, but scientists use it to demonstrate that genetics plays some role in the disease. Also, the high end of those estimates may come from studies of identical twins, so I'm not sure how pertinent they are.
A fairly rigorous study in Belgium found that for first-degree relatives (i.e. siblings or children) of CD affected people, the risk is 4.8% for IBD in general and 3.9% for CD specifically. The risk for offspring was highest, at 10.4%. (5) Based on the methodology and the large sample in this study, I think this is probably pretty accurate.
Several studies suggest that heredity is more important in CD than in UC, usually by a few percentage points. (1, 4, 5, 6)
One study found that children of parents who both have IBD run a 33% chance of developing the disease before age 28. (4) Those of us who come to this board trolling for dates should know that any child of two IBD affected parents has a 1 in 3 chance of developing the disease.
Oddly enough, the same study suggests there is a slightly higher risk - about 2 in 1000 - of developing IBD if your spouse already has or develops one of the diseases. Again, this points to the genetic/environmental interplay; it's not that IBD is necessarily contagious, but that couples living together for long periods face the same environmental triggers.
Studies also suggest that the child of a parent with CD is more likely to develop the disease at an earlier age than a patient with no family history of the disease, although the disease is not likely to be any more severe in the child. (1)
I hope this helps. I apologize to the UC folks here that this wasn't equally balanced between the two diseases, but I'm happy to help out anyone who thinks this needs a little more research.
(1) Ahmad, T. et al. Review article: the genetics of inflammatory bowel disease. Aliment Pharmacol Ther 2001; 15: 731-748.
(2) [omitted]
(3) Haigh, B. Inflammatory bowel disease and innate immune response genes: the challenge of complex polygenic disorders for the clinical geneticist. Clin Genet 2006; 69: 315-318
(4) Laharie, D. et al. Inflammatory bowel disease in spouses and their offspring. Gastroenterology 2001; 120: 816-819
(5) Peeters, M. et al. Familial aggregation in Crohn's Disease: increased age-adjusted risk and concordance in clinical characteristics. Gastroenterology 1996; 111: 597-603
(6) Orholm, M. et al. Familial occurrence of inflammatory bowel disease. New England Journal of Medicine Jan 10, 1991; 324:2, 84-88
(7) "Crohn's Disease". First Consult. (An online medical database accessed through the library.)
.