Is HLA-B27 the only gene responsible for Ankylosing spondylitis?

We have already seen in the previous posts that HLA-B27 is the most common gene associated with Ankylosing spondylitis (AS). However, HLA-B27 does not seem to be the only gene associated with AS.

Strong indicators of this fact include-
1. AS can occur in individuals who do not carry HLA-B27 gene.
2. Amongst the HLA-B 27 individuals, only about 1-5% individuals develop AS.
3. HLA-B27 positive relatives of AS patients have a risk of developing AS that is 5.6 to 15 times that of HLA-B27 positive individuals in general population. This would mean that there are other non HLA-B27 familial genetic factors involved in causation of AS.

There has been some major work by the Wellcome trust case Control Consortium
& Australo-Anglo-American Spondyloarthritis Consortium to look into the genetics of AS. These (& other) studies have revealed that there are other genes & genetic loci responsible for Ankylosing spondylitis as well—
1. HLA-B60 seen in HLA-B27 positive as well as negative AS patients.
2. HLA-B 39 seen in HLA-B27 negative patients
3. ERAP-1— endoplasmic reticulum aminopeptidase-1
4. Interleukin-23 receptor gene—IL-23R
5. RUNX3
6. KIF21B
7. 2p15
8. IL12B
10. 21q22
11. ANTXR2
12. PTGER4
13. CARD9
14. TBKBP1

Out of these genes, ERAP-1 & IL-23 R have generated maximum interest. The researchers have found that some variants of ERAP1 protect against the development of Ankylosing spondylitis. For individuals who carry HLA-B27, their risk of developing Ankylosing spondylitis decreases by a factor of four if they carry two copies of the protective variant of ERAP1.

HLA-B27 presents the pathogen antigen to the immune cells. The ERAP-1 gene interacts with HLA-B27 to affect how these peptides are presented to the immune system. The researchers have found that some variants of ERAP1 protect against the development of Ankylosing spondylitis by reducing the amount of peptide available to HLA-B27 within cells. This could prove to be a target for treatment in the future.

Tests for these genetic markers are not available routinely as of now. But, then, if they are found to be clinically useful; tests should be available in the future.

1. Investigating the genetic association between ERAP1 and ankylosing spondylitis. Harvey D & colleagues. Hum Mol Genet. 2009 Nov 1;18(21):4204-12.
2. Progress in the genetics of ankylosing spondylitis. Matthew A brown. Briefings in Functional Genomics (2011) 10 (5): 249-257.
3. Interaction between ERAP1 and HLA-B27 in ankylosing spondylitis implicates peptide handling in the mechanism for HLA-B27 in disease susceptibility. The Australo-Anglo-American Spondyloarthritis Consortium (TASC), the Wellcome Trust Case Control Consortium 2 (WTCCC2), Nature Genetics 43, 761–767 (2011)

Ankylosing Spondylitis: Have you missed your diagnosis, as you were HLA-B27 negative?

Doctors as well as patients have equated ankylosing spondylitis with HLA-B27 since a long time. In fact, many of the patients call Ankylosing spondylitis ‘HLA-B27 disease’

This is good as far as awareness is concerned. But, on the flip side, many patients have missed the diagnosis, as they were HLA-B27 negative. This is particularly important, as the average delay in the diagnosis of Ankylosing spondylitis is 8- 11 yrs. The delay for women is even more than that of men. This is quite unacceptable as the first 10 years are the most important for a patient as the treatment can be initiated before permanent limitations of spinal mobility and deformity has set in.

Although HLA-B27 gene is the most important gene predisposing to Ankylosing spondylitis, studies have shown that it contributes only 20-30% of the total genetic risk. No doubt HLA-B27 is a strong risk factor for the development of Ankylosing spondylitis, but that does not mean that it is a must for diagnosis.

So, then how important is HLA-B27 for a diagnosis of Ankylosing spondylitis?
Only about 80-90% of the patients with AS have the HLA-B27 gene. That means that the rest could miss the diagnosis if one would totally depend on HLA-b27 for a diagnosis.
The converse of this is also interesting. Only 1% of people with HLA-B27 develop the disease. So, HLA-B 27 alone cannot be equated with AS in somebody with backache.

Low back pain is a relatively common symptom that may be associated with a variety of conditions other than AS. The single most important feature that raises the suspicion of AS is inflammatory backache. The characteristics of inflammatory backache are –(1) morning stiffness of > 30 minutes, (2) improvement in back pain with exercise but not with rest, (3) awakening because of back pain during the second half of the night only, and (4) alternating buttock pain.

Role of MRI
MRI of the sacroiliac joints is one of the best investigations for a definitive & early diagnosis of AS. HLA-B27 is not diagnostic of AS, but can only guide us towards a diagnosis. MRI of the SI joints can give a definite diagnosis by actually showing the inflamed SI joints. Though, MRI is a costly, time-consuming investigation; its utility in confirming the diagnosis in a particular individual cannot be understated.

All in all, let us not diagnose AS with HLA-B27 alone. Definitive history of inflammatory backache/ other features & MRI of the sacroiliac joints can be the best guide for the diagnosis.

1. Brown MA, Kennedy LG, MacGregor AJ, et al. Susceptibility to ankylosing spondylitis in twins: the role of genes, HLA, and the environment. Arthritis Rheum 1997; 40: 1823–28.
2. Feldtkeller E, Khan MA, van der Heijde D, van der Linden S, Braun J. Age at disease onset and diagnosis delay in HLA-B27negative vs. positive patients with Ankylosing spondylitis. Rheumatol Int 2003; 23:61–6.
3. Blum U, Buitrago-Tellez C, Mundinger A, et al. Magnetic resonance imaging (MRI) for detection of active sacroiliitis – a prospective study comparing conventional radiography, scintigraphy, and contrast enhanced MRI. J Rheumatol 1996; 23:2107–2115.

Do have a look at this video to understand this better–

How does HLA-B27 lead to ankylosing spondylitis?

HLA-B27 gene is closely related to Ankylosing spondylitis. It is found in almost 70-90% of patients with Ankylosing spondylitis.

HLA B27 stands for Human Leukocyte Antigen 27. The HLAB 27 gene produces the HLA B27 molecule, which belongs to the family of MHC class I molecules. The function of this family of molecules is to present antigenic peptides to the T cells.

Let us understand this process of antigen presentation further. Whenever a pathogen (bacterium/ virus) enters the body, body’s immunity recognizes these as foreign. The next step is to inform the other immune cells about these pathogens & provide them with sufficient information about the pathogens. With this information, immune cells then launch an attack against these pathogens. This information about the pathogens consists of parts of the pathogen (called antigens) & is presented by the antigen presenting cells to the T cells. These parts are the signatures of these particular virus/ bacteria. The T cells can trace the location of these pathogens based on the signature antigens they have & launch an attack on them.

Now, let us focus on what happens inside these antigen presenting cells. The signature antigens are given final touches in a structure called proteosomes. The final antigens are carried to another structure called the Endoplasmic reticulum where they are mounted onto the MHC class I molecule. The MHC molecule folds & is then taken to the surface of the cell. This complex is presented to the T cells, which then recognize it & mount an attack on the pathogen. The T cells do not recognize the antigens in absence of the HLA molecule.

A few pathogens have antigens that are similar to proteins in our joints. When such pathogens (eg. Yersinia, Chlamydia) enter the body (generally the gut & cause loose motions), their antigens are picked up by the antigen presenting cells. It is hypothesized that HLA-B27 tends to pick up the particular antigens in the pathogen that are similar to the joint proteins & present them to the T cells. T cells recognize these as foreign & attack any structure with these proteins. The pathogen is definitely taken care of; but as I said, T cells also start considering our joints as foreign & attack them as well. This is precisely what happens in reactive arthritis (ankylosing spondylitis & reactive arthritis belong to the same group of arthritis). In reactive arthritis, one has loose motions followed by joint inflammation. This is called the ‘Arthritogenic peptide hypothesis’

There is an alternative hypothesis to explain the cause of joint inflammation. This is called the HLA-B27 folding hypothesis. The difference between HLA-B27 & the other HLA molecules is that B27 has a slower rate of folding & is prone to misfolding. When this happens on a large scale, the endoplasmic reticulum malfunctions & triggers generation of cytokines (TNF-α, IL-1, IL-6). These cytokines attack the joint & cause inflammation. This sequence of events has been shown in cells of the synovial fluid in patients.

This is how the HLA-B27 positivity; in the presence of infections/ environmental factors translates into inflammation of the joints (spine & other joints) & ultimately manifests as ankylosing spondilytis/ reactive arthritis.

Mear JP, Schreiber KL, Munz C, Zhu X, Stevanovic S, Rammensee HG, et al: Misfolding of HLA-B27 as a result of its B pocket suggests a novel mechanism for its role in susceptibility to spondyloarthropathies. J Immunol 163:6665–6670, 1999

Smith JA, Märker-Hermann E, Colbert RA: Pathogenesis of Ankylosing spondylitis: current concepts. Best Pract Res Clin Rheumatol 20:571–591, 2006

Turner MJ, Sowders DP, DeLay ML, Mohapatra R, Bai S, Smith JA, et al: HLA-B27 misfolding in transgenic rats is associated with activation of the unfolded protein response. J Immunol 175: 2438–2448, 2005

Robert A., Monica L., Erin I.: From HLA-B27 to spondyloarthritis: a journey through the ER Immunol Rev. 2010 January ; 233(1): 181–202

Benjamin R, Parham P. Guilt by association: HLA-B27 and ankylosing spondylitis. Immunol Today 1990: 11: 137–42.