Researchers at the Johns Hopkins University School of Medicine this month published in the journal Gastroenterology the first major, in-depth analysis of genetic risk factors of inflammatory bowel disease in African-Americans.
The NIH-funded study is an intensive evaluation of the genetics of inflammatory bowel disease (IBD) in the African-American population.
Crohn’s disease and ulcerative colitis affect as many as 1.6 million Americans. Patients with IBD have immune systems that attack their own intestines, resulting in inflammation. Recent years have seen a steady increase in reported cases of IBD in African-Americans.
“This study is the culmination of over a decade of work,” says Steven Brant, director of the Johns Hopkins Meyerhoff Inflammatory Bowel Disease Center and corresponding author of the study. “We hope it’s the first of many steps to better understand and treat these debilitating diseases.”
IBD genetics have been evaluated in more than 1,000 studies in white and Asian—primarily East Asian—populations. Although the risk is slightly lower than that of white Americans, African-Americans are at significant risk for IBD.
The study evaluated more than 1,500 African-American patients with IBD—including 1,088 with Crohn’s disease and 361 with ulcerative colitis—from 35 IBD centers across North America and used 1,797 African-Americans without IBD for comparison.
The primary goal of the study was to determine whether African-Americans share the same 163 separate genetic variations shown in white Americans as IBD genetic risk factors. A secondary goal was to identify novel regions of the genome, known as loci, causing IBD risk in African-Americans.
The researchers also examined if, within the majority of these IBD genetic loci, there are genetic variations that are of particular risk for causing IBD in African-Americans that are distinct from those identified in whites.
“We also studied whether there are regions of the genome that cause or protect IBD risk in African-Americans that arise from either their West African or European genetic ancestries,” Brant says.
The study revealed that in African-Americans, as in whites and Asians, the dominant region for ulcerative colitis genetic risk in is the human leukocyte antigen (HLA) region, a region that is a major determinant of immune regulation and risks for other immune diseases, like celiac disease and type 1 diabetes. However, the specific variant associated with African-Americans in this region is the same as the variant most highly associated with Japanese and Korean ulcerative colitis, and to a lesser degree ulcerative colitis in Europeans. It also is the same major risk variant in HLA for lupus in African-Americans, whose lupus risk is four times greater than white Americans.
The study did not find evidence for the major risk variant for European ulcerative colitis as having a role in the African-American population.
Identifying specific regions of the African-American genome that arise separately from the ancestral West African versus the ancestral European genome that are responsible for a portion of the genetic risk of IBD in the African-American population, the researchers found evidence for regions on four of the 22 autosomal (nonsex-linked) chromosomes as causative for IBD. For each of these regions, the IBD risk appears to come from the Caucasian ancestral genome that makes up 20 percent of the African-American genome overall.
In addition to Brant, the study was co-led by researchers at Emory University and Cedars-Sinai in Los Angeles.
THE STUDY BY THE NUMBERS
1,500 African-American patients from 35 IBD centers across North America
The primary goal: to determine whether African-Americans share the same 163 genetic variations shown in white Americans as IBD genetic risk factors.