Humans come in many shapes, colours and sizes, but at the DNA level, the genomes of any two people are more than 99% the same. The DNA contained within the chromosomes of each of your cells carries the instructions needed to build and maintain the many different types of cells that make us unique. This complete set of DNA instructions is called a genome.
Every human has:
- 1 genome
- 75 – 100 trillion cells in the human body
- 46 chromosomes in each human cell
- 20,500 genes in a human genome [1]
The Human Genome Project (HGP) was one of the great feats of exploration in history. It was an international research effort to sequence and map all of the genes – together known as the genome – of members of our species, Homo sapiens. Completed in April 2003, the HGP gave us the ability, for the first time, to read nature’s complete genetic blueprint for building a human being [1].
Medical science agrees that our genetics make up one of three reasons why we have diseases such as psoriasis and highlight that our environment and diet are the two other key players in our health or severity of disease. The point being made here is that while genetic similarities exist between Crohn disease and psoriasis, and to a lesser degree other autoimmune diseases, the function of our 20,500 genes is only one third of a problem. We can control to our best ability, the other factors attributed to these diseases. For example, if members of the same family and household experience Crohn disease, psoriasis or another form of autoimmune disease, then environmental and dietary factors need to be considered.
Psoriasis, Crohn disease (CD) and ulcerative colitis (UC) are chronic inflammatory disorders each with genetic markers that overlap one another. A Danish study of 75 209 patients with psoriasis had shown a psoriasis-associated increased risk of CD and UC, which was higher in severe psoriasis, and an increased risk of psoriasis in patients with inflammatory bowel disease. This calls for a need to increase our focus on gastrointestinal symptoms when affected by psoriasis [2].
The dance between Crohn and psoriasis
Psoriasis and Crohn disease both affect the thin tissue layers of a surface of our body, called epithelial layers. With psoriasis it is the skin epithelial layer and with Crohn disease, the intestinal epithelial cells are affected. Both psoriasis and Crohn disease are triggered by an activated cellular immune system, carry a risk of a sibling also being affected, and are prevalent in populations of European ancestry.
Psoriasis is a common hyperproliferation disorder of the skin, whilst Crohn disease is primarily a gut disorder affecting any aspect of the gastrointestinal tract but may also manifest beyond the gut and onto the skin in cases such as erythema nodosum and pyoderma gangraenosum.
Although psoriasis and Crohn disease are clinically distinct diseases, they are observed together more frequently than expected by chance. Recent genome-wide association studies have searched the human genome for small variations and identified shared susceptibility genes for both psoriasis and Crohn disease. This provides evidence for a genetic overlap of both diseases and shared genetic factors linked to the cause of both diseases.
The shared susceptibility genes that were identified in the genome study have names such as IL23R (MIM 607562), IL12B (MIM 161561), REL (MIM 164910), and TYK2 (MIM 176941), however one of the strongest genetic markers of both Crohn disease and psoriasis is IL23R, a pro-inflammatory cytokine, and considered to be a key player driving autoimmunity in human disease [3].
It is likely that future studies will identify more shared genetic markers for Crohn Disease and psoriasis by increasing the size of the genetic data being analysed.
For us in the meantime, we can focus our attention on creating a nurturing environment and healthy dietary habits.
Reference
[1] National Human Genome Research Institute. 2016. An Overview of the Human Genome Project. [ONLINE] Available at: https://www.genome.gov/12011238/an-overview-of-the-human-genome-project/. [Accessed 20 August 2018].
[2] Egeberg A, Mallbris L, Warren RB, Bachelez H, Gislason GH, Hansen PR, Skov L. Br J Association between psoriasis and inflammatory bowel disease: a Danish nationwide cohort study. Dermatol. 2016 Sep;175(3):487-92. doi: 10.1111/bjd.14528. Epub 2016 Jun 20.
[3] Ellinghaus D, Ellinghaus E, Rajan P. Nair,Philip E. Stuart,Tõnu Esko, Andres Metspalu, Sophie Debrus,John V. Raelson,Trilokraj Tejasvi, Majid Belouchi,Sarah L. West,Jonathan N. Barker, Sulev Kõks,Külli Kingo,Tobias Balschun,Orazio Palmieri et al. 2012. Combined Analysis of Genome-wide Association Studies for Crohn Disease and Psoriasis Identifies Seven Shared Susceptibility Loci. American Journal of Human Genetics, 90(4), 636–647.
