Genetic coding variant in complement factor B (CFB) is associated with increased risk of perianal Crohn’s disease and leads to impaired CFB cleavage and phagocytosis
Summary
Aim
Perianal Crohn’s disease ( pCD) occurs in up to 40% of patients with CD and is associated with poor quality of life, limited responses to treatment, and a poorly understood etiology. We performed a genetic association study comparing CD subjects with and without perianal disease and subsequently performed functional follow-up studies for a CDp-associated SNP in complement factor B (CFB).
A meta-analysis based on immunochip design was performed on 4056 pCD and 11,088 CD patients from three independent cohorts. Serological and clinical variables were analyzed by regression analysis. The risk allele of rs4151651 was introduced into the human CFB plasmid by site-directed mutagenesis. Binding of recombinant G252 or S252 CFB to C3b and its cleavage was determined in cell-free assays. Macrophage phagocytosis in the presence of recombinant CFB or CFB risk serum, or protective DC or healthy subjects was assessed by flow cytometry.
Results
Perianal complications were associated with colonic involvement, OmpC and ASCA serology, and serology quartile sum score. We identified a genetic association for pCD (rs4151651), a non-synonymous SNP (G252S) in CFB, in all three cohorts. Recombinant CFB S252 had reduced binding to C3b, its cleavage was impaired, and complement-driven phagocytosis and cytokine secretion were reduced compared to CFB G252. Serine 252 generates a de novo glycosylation site on CFB. Serum from homozygous risk patients showed significantly lower macrophage phagocytosis compared to non-risk serum.
Conclusion
The pCD-associated rs4151651 in CFB is a loss-of-function mutation that affects its cleavage, activation of the alternative complement pathway, and phagocytosis of pathogens, implicating the alternative complement pathway and CFB in the etiology of pCD.
Comments
Cedars-Sinai researchers have identified a genetic variant that increases people’s risk of developing perianal Crohn’s disease, the most debilitating manifestation of Crohn’s disease.
The variant generates changes in DNA that lead to a loss of protein function, which in turn alters the way the body recognizes and handles bacteria, making them less effective at fighting infections.
The discovery is published in the peer-reviewed journal GUT .
"Perianal fistulizing Crohn’s disease can be a really miserable condition," said study co-senior author Dermot McGovern, MD, PhD, director of Translational Research at the Cedars-Sinai F. Foundation Intestinal Immunobiology and Inflammation Research Institute. Widjaja and the L. and Lisa Z. Greer Chair in the Genetics of Inflammatory Bowel Disease. “Our current therapies really aren’t very good at treating it, so this study addresses a very important area of unmet medical need. By better understanding the underlying causes, we can begin to develop new treatment strategies for patients diagnosed with this chronic inflammatory condition, most of whom currently require surgery and often multiple surgeries."
Perianal Crohn’s disease is a complication of Crohn’s disease, a chronic inflammatory disorder that affects the digestive tract. The complication causes inflammation and ulceration of the skin around the anus, as well as other structures in the perianal area. Perianal Crohn’s disease occurs in up to 40% of people with Crohn’s disease and has limited responses to treatment, resulting in poor quality of life.
“We have had much more success in identifying genetic variants associated with the risk of developing diseases, but what we did here focused specifically on a very complicated and severe manifestation of Crohn’s disease. And that’s an unusual approach in genetics research,” said Talin Haritunians, PhD, a research assistant professor in the McGovern Laboratory and co-senior author of the study.
To discover genetic variants with a direct link to this severe manifestation, researchers analyzed genetic data from three independent cohorts of Crohn’s disease patients. The groups included a Cedars-Sinai cohort, an international genetics cohort recruited from more than 20 countries, and a cohort recruited from seven academic research medical centers across the United States. The three groups totaled 4,000 patients with perianal Crohn’s disease and more than 11,000 patients with Crohn’s disease without this complication.
The team of scientists compared the cohorts to see if they could detect genetic loci, which are areas of the genome associated with the development of this manifestation. The team identified 10 novel genetic loci and 14 known inflammatory bowel disease loci associated with the development of perianal complications.
During the functional characterization analysis, the team focused on a single change in a specific gene, called SNP, that was associated with perianal Crohn’s disease. This genetic variant affects a protein called Complement Factor B (CFB), leading to a loss of function of this protein that is important in fighting infections, so it is possible that patients with this genetic change may be more likely to have the condition.
Researchers performed multiple analyzes to confirm that there really is a loss of function in Complement Factor B (CFB), which can have a dramatic impact on the body.
"In the event that you have this mutation that leads to a non-functional protein, you do not get the normal signaling cascade and the body does not recognize that the bacteria are harmful and therefore those bacteria are not eliminated," co-said. Study lead author Kathrin Michelsen, PhD, research assistant professor of Medicine and Biomedical Sciences at Cedars-Sinai. “So, for those patients who have perianal Crohn’s disease, there are connections that form from the rectum to the skin area. And those tunnels are full of bacteria that are not eliminated.”
Michelsen also noted that the study demonstrates an important role for the alternative complement pathway and CFB in the development of perianal Crohn’s disease. The findings also suggest that targeting the alternative complement pathway may be a novel therapeutic approach to treat this disabling manifestation of Crohn’s disease.
This genetic variant may also be associated with other diseases.
“These genetic variants often predispose to more than one condition, and we believe this discovery also has potential ramifications for other diseases, not just Crohn’s disease,” McGovern said.
Researchers are now working to identify the role of additional genetic variants associated with perianal Crohn’s disease and other areas of unmet medical need in inflammatory bowel diseases.
Funding : The work was supported in part by grants from the National Institutes of Health (U01DK062413, P01DK046763), the F. Widjaja Foundation, the Leona M. And Harry B. Helmsley Charitable Trust, and the Fred L. Hartley Family Foundation.
