Clinical and Health Affairs
Biomarkers in Rheumatologic Disease
A Review
By Irene Whitt, M.D., and Ann Reed, M.D.
Abstract
Rheumatologic disorders are in many ways heterogeneous diseases. They have an array of clinical manifestations, involve multiple organs, produce unpredictable flares, and have widely varying clinical outcomes. Diagnosing rheumatologic diseases and predicting outcomes in a reliable way could lead to new therapeutic strategies and optimization of care. This article reviews the current literature on biological and clinical markers related to diagnosing disease, assessing for disease activity, and predicting outcomes for selected rheumatologic conditions.
Clinical and biologic markers have led to advances in the care of patients with rheumatoid arthritis and a better understanding of the pathophysiology of psoriatic arthritis. Novel biologic markers in patients with systemic lupus erythematosus (SLE) and dermatomyositis promise better assessment of disease activity and a more accurate prediction of disease outcomes. However, the search for biomarkers needs to be intensified in order to catch up with the development of therapeutics that can treat these conditions.
Since 1996, 26 new therapies, including five biologic agents, have been approved by the FDA for use in treating rheumatologic diseases.1 This has resulted in earlier and more aggressive treatment paradigms for conditions such as rheumatoid arthritis and better patient outcomes. However, the cost of these new therapies, the biologic agents in particular, is high. (The average cost per year of treating a 165 lb. [75 kg] person with infliximab or adalimumab is estimated to be $22,000; the average cost of treating such a patient with etanercept is estimated at $18,400.) Given concerns about rising health care costs, this is a significant issue. Therefore, we need to make sure the patients who will get the most benefit from these effective-but-expensive treatments receive them. This is a challenge because our understanding of the pathophysiologic mechanisms that lead to disease manifestation, activity, and severity lags significantly behind the development of new treatments.
It is generally believed that rheumatologic disorders arise when an individual’s potentially hyper-reactive immune system (one predisposed to autoimmunity) interacts with the environment in a unique way at a certain point in time. The result is a bewildering array of disease manifestations, each with a wide clinical spectrum, varying potential for multiple organ involvement, unpredictable flares, and a wide variation in clinical outcomes. For example, one person with rheumatoid arthritis may have mild disease at onset and after a number of years of treatment be able to discontinue his or her medications. Yet another person may have severe disease with destructive arthritis, nodulosis, interstitial lung disease, and vasculitis and require aggressive lifelong treatment. If we were able to understand the level of “activation” of the immune system at a given point in time as it relates to a particular rheumatologic disease, we could better direct treatment.
Recent research has uncovered broad patterns of immune system activation for various rheumatologic diseases including rheumatoid arthritis, psoriatic arthritis, systemic lupus erythematosus, and idiopathic inflammatory myositis. However, the development of specific tools to measure disease activity has been slow.
The Role of Biological Biomarkers
The National Institutes of Health’s Biomarkers and Surrogate Endpoint Working Group defines a biological biomarker as a characteristic that can be objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacological responses to therapeutic intervention.2
Biological biomarkers potentially could be used at the time of diagnosis (more quantitative assessments better classify patients by severity of disease and lead to better assessments of prognosis), during treatment (more accurate assessments of flares lead to more targeted short-term treatment), and during evaluation of the treatment response (more accurate assessment leads to more targeted long-term treatment).3 Their greatest potential in clinical practice is for tailoring treatment to individual patients.
The Minnesota Autoimmunity Genomics Network, which is funded by a Minnesota Partnership for Biotechnology and Medical Genomics grant, has allowed for the creation of a unique biorepository that includes blood samples from patients with rheumatoid arthritis, psoriatic arthritis, dermatomyositis, and those with a positive ANA who do not meet the diagnostic criteria for systemic lupus erythematosus. This repository is being used for research that may lead to useful biomarkers for a number of diseases in the future.
Biological and Clinical Markers in Rheumatoid Arthritis
Rheumatoid arthritis (RA) is a heterogenous disease characterized by many different phenotypes. There is a wide variation in the range of joint involvement (mono- to polyarticular), the extent of joint damage, the course of disease (monophasic, cyclic, or relentlessly progressive), the presence of extra-articular manifestations, and the presence or absence of autoantibodies (namely rheumatoid factor and anti-cyclic citrullinated peptide antibodies). Currently, the natural history of RA is assessed as a combination of disease activity, joint damage, and functional impairment.
- Disease Activity
The clinical manifestations of RA are the consequences of synovial inflammation, which later leads to destruction of cartilage and bone in the joints. However, synovial inflammation is itself a heterogeneous process. We know from arthroscopic biopsy studies that the composition of the inflammatory cell infiltrate is heterogeneous and that the only consistent reflection of active disease in the involved joint is the presence of high numbers of macrophages in the synovium.4 Other cells present in the synovial inflammatory infiltrate do not reflect disease activity. For example, the degree of B-cell depletion in the synovium during rituximab therapy is not significantly associated with the clinical response, and B cells may be present in the synovium even if their level is depleted in the peripheral blood.5 In addition, study of the output from inflammatory cells, ie, measurement of cytokine levels, has been equally unfruitful. Both in the synovium and on serum measurements, cytokine expression does not always differ by disease and can vary almost 100-fold within the same individual.3 However, new measures of cytokine involvement and gene expression that have been developed and are being validated hold promise. Thus, insofar as our current knowledge of biomarkers is concerned, the periphery does not fully reflect what is happening in the joint and what is taking place in the joint may not predict disease activity overall.
In contrast, acute-phase reactants and clinical markers have been found to reflect disease activity and cumulative joint destruction; yet, they are subjective at best and are variable when present. Serum levels of c-reactive protein (CRP), a molecule induced by proinflammatory cytokines, especially interleukin 6 (IL-6), measured before the start of disease-modifying therapy can predict the degree of subsequent radiographic joint damage. Moreover, greater reduction in CRP levels from baseline is associated with less progression to joint damage.6 The erythrocyte sedimentation rate (ESR) is an alternative measure to CRP. But because ESR may be influenced by factors unrelated to inflammation, CRP remains the preferred biomarker of RA.
In contrast to biomarkers, clinical markers have proven useful in following disease activity over time and predicting joint damage in large groups of patients. But they are not as sensitive. Most useful are composite indices such as the Disease Activity Score (DAS-28), which uses 28 joint counts, the Simplified Disease Activity Index, and the Clinical Disease Activity Index. All of these indices use a “core” set of variables that correlate with disease activity. These subjective measures include swollen and tender joint counts, patient assessment of pain, the patient’s and physician’s assessment of disease activity, a measure of the acute-phase response (with CRP or ESR), and an assessment of function. Disease activity over time as assessed using these indices correlates significantly with progression of joint damage.7
Furthermore, large cohort and clinical trials have revealed that limiting disease activity or achieving remission early in the course of RA is the optimal treatment strategy. In one study, the probability of attaining remission or limited disease activity was more than 75% for patients who had active disease as indicated by the Simplified Disease Activity Index after three months of treatment, while it was only 25% for those with highly active disease at that same point in time.8 Therefore, the new standard has been to start treatment early, before initial damage occurs. However, this optimization strategy requires early referral and diagnosis.
- Joint Damage
Joint damage from RA is a consequence of ongoing synovial inflammation. It consists of cartilage damage, which radiographically appears as joint space narrowing, and bone destruction, which appears as areas of erosion. Therefore, many molecules representative of these two components of RA-related joint damage have been assessed as potential biomarkers (for example, components of the COL2 network of collagen proteins for cartilage destruction and CTX-II for bone destruction). Yet none of these markers has performed better than CRP, swollen joint counts, or composite disease activity indices at predicting disease progression and joint damage.
Autoantibodies are markers that are not derived from cartilage or bone but may predict joint damage. It has long been recognized that patients who test positive for rheumatoid factor (especially at higher titers) have a much higher rate of radiographic progression of joint destruction compared with patients who are seronegative. More recently, autoantibodies to citrullinated proteins (CCP antibodies) have also been shown to be predictive of joint damage.9
Therefore, the combination of autoantibody and acute-phase protein assessment, along with clinical assessment of early disease activity, may be the most reliable way to predict severe erosive RA. This approach allows us to institute and modulate the aggressiveness of therapy early in the course of the disease, which allows us to better achieve remission.
Biological and Clinical Markers in Psoriasis
Although psoriasis is a relatively common skin disease affecting approximately 2% of the Caucasian population in the United States, psoriatic arthritis (PsA) is much less common, affecting about 10% of patients with psoriasis. The development of PsA follows the development of skin disease in 80% of cases. Sometimes, however, it can precede the development of psoriasis, making the diagnosis difficult. Moreover, few serologic or proteomic biomarkers exist to aid in diagnosis or monitoring disease activity. Psoriatic arthritis is classified as a spondyloarthropathy (such as ankylosing spondylitis or reactive arthritis). From a pathophysiologic point of view, the synovitis at the level of the joint appears to be significantly different from RA, with activation of osteoblasts and the wingless protein pathway in PsA as opposed to osteoclast activation through the RANKL pathway in RA.
Biomarker research on PsA has been hampered by several issues. Unlike RA, there are not as many specific instruments for assessing disease activity or damage. Consequently, it has been difficult to study a homogeneous set of patients. In addition, candidate biomarkers have been borrowed from RA, even as emerging evidence suggests that the two diseases may be different enough to make the choice of those candidates inappropriate. However, recent studies using microarray technology to survey gene expression profiles have generated new hypotheses concerning the pathogenesis of PsA.
- Pathogenesis, Disease Activity, and Diagnosis
Unlike RA research, studies of the pathogenesis of PsA have been easier to do and more fruitful, as they can take advantage of the fact that psoriatic and uninvolved skin are accessible to repeated biopsy. They have also been useful for generating hypotheses for testing serum biomarkers. Several studies have shown important pathogenic roles of Th17 and Th1 T cells as well as plasmacytoid and myeloid dendritic cells and other cells. Researchers have focused not only on identifying members of the inflammatory infiltrate but also on their signaling messenger molecules (chemokines). Of interest are elevated levels of IL-6. IL-6 has also been found to be elevated in the serum of patients with active PsA.
In addition, whole-blood gene expression profiles from PsA patients have been found to be distinct from age- and sex-matched controls. In fact, the PsA gene expression profile shows little overlap with that of other inflammatory autoimmune diseases such as RA and systemic lupus erythematosus (SLE) and thus appears to be unique to PsA. Of 310 differentially expressed genes, a small subset of lymphocyte-specific genes were found to have the potential to discriminate PsA patients from controls and aid in diagnosis. Another small subset of genes is positively correlated with ESR, thus reflecting inflammation and disease activity.10
A few biomarkers associated with diagnosing PsA and distinguishing it from other diseases such as RA have been studied in clinical trials. These include increased expression of genes encoding for nucleoporin 63 kDa, which distinguishes PsA patients from controls and overexpression of MAP3K3 followed by CACNA1S, which can discriminate PsA from RA.11
- Disease Activity and Joint Damage
As with RA, molecular markers of either cartilage or bone damage in patients with PsA have been studied. None correlate well with joint damage, but serum levels of IL-612 and soluble IL-2 receptor do correlate with the development of erosive disease in PsA.13 In addition, TNF-alpha gene polymorphisms are associated with both joint erosion and progression of joint erosion in patients with early PsA.14
Therefore, these may be useful prognostic markers for damage, despite being cumbersome to test for. Most biomarkers studied for monitoring PsA do not increase when the disease is active, nor are they sensitive enough to change. As with RA, only ESR and CRP correlate well with swollen and tender joints, reflecting disease activity. In the synovium, more than 300 genes showed an altered expression in peripheral blood mononuclear cells obtained from patients with active PsA. These do not necessarily reflect clinical disease activity.
In conclusion, limited-but-promising data are available on biological biomarkers in psoriasis and psoriatic arthritis. The disease is difficult to study because of its heterogeneity and less-understood pathogenesis. At present, only high CRP and ESR levels along with swollen and tender joint counts may be predictive of erosive disease. However, serum levels of IL-6 and IL-2 soluble receptor as well as unique whole-blood gene expression profiles show promise in predicting active or erosive disease.
Biological and Clinical Markers in Systemic Lupus Erythematosus
Systemic lupus erythematosus is a chronic, inflammatory autoimmune disease that affects multiple organs and is characterized by nuclear antibodies, immune complexes, and systemic vasculitis. Targeted organs include the skin, joints, lungs, blood cells, kidneys, and the central and peripheral nervous systems. Because of the myriad clinical manifestations, SLE is difficult to diagnose. In addition, the disease course is characterized by unpredictable flares that can occur in any organ at any time. Therefore, biomarkers are greatly needed to facilitate more accurate diagnosis and better assessment of disease activity and organ involvement.
- Diagnosis, Disease Activity, and Organ Involvement
Multiple studies have shown that most patients with SLE have an elevated expression of a group of interferon (IFN)-regulated mRNA transcripts in the whole blood. The IFNs are a family of cytokines produced by various immune cells in response to intracellular pathogens (viruses, parasites, tumor cells, certain bacteria), thus mediating host defense against pathogens and tumorigenesis. Type I IFNs (which consist of 13 different IFN-alpha subtypes, IFN-beta, and IFN-gamma) are induced by viruses (double-stranded RNA), bacteria, and synthetic compounds and are produced primarily by plasmacytoid dendritic cells and macrophages.
There is a growing body of evidence linking type I IFN and lupus pathogenesis. For instance, about 20% of patients treated with INF-alpha developed positive antinuclear antibodies; some went on to develop SLE.15 In addition, 50% to 75% of adult patients with SLE exhibit a striking pattern of up-regulation of IFN-induced genes in peripheral blood cells. This IFN “signature” is strongly correlated with disease activity and certain clinical manifestations of SLE, including anti-dsDNA antibodies, low complement levels,16 Sm, Ro, and U1RNP autoantibodies, and lupus nephritis.17 Moreover, a serum “signature” comprising elevated levels of IFN-regulated chemokines also has been correlated with disease activity and, therefore, serves as a biomarker.18 In addition, the pattern of chemokine regulation has been associated with certain organ involvement such as the presence of active serositis or renal disease.
These findings have generated the hypothesis that, through an unknown but possible environmental trigger, type I IFN induces high levels of systemic chemokines in active SLE and leads to a state of “chemokine confusion” that alters the normal trafficking and chemotaxis of leukocytes in the body, setting the stage for autoimmunity.
Biological and Clinical Markers in Dermatomyositis
Dermatomyositis is a rare autoimmune disorder belonging to the group of idiopathic inflammatory myopathies, which is characterized by proximal muscle weakness, muscle inflammation, and a characteristic hyperkeratotic skin rash. Like other rheumatologic diseases, the exact cause is unknown, but it is believed that a combination of genetic and environmental factors (including viral infection) contribute to disease.
Type I interferon-regulated genes and proteins are elevated in patients with dermatomyositis, and a striking IFN signature has been found in most dermatomyositis patients studied, distinguishing them from controls. Furthermore, both the transcript and serum protein IFN signatures have been associated with disease activity in patients with dermatomyositis.19 Interestingly, the IFN signature pattern found in patients with dermatomyositis seems to be unique, yet has features that overlap those of SLE.
Conclusion
Clinical and biologic markers have led to advances in the care of patients with rheumatoid arthritis and to a better understanding of the pathophysiology of psoriatic arthritis. Novel biologic markers in SLE and dermatomyositis promise better assessment of disease activity and more accurate prediction of outcomes. However, the search for biomarkers needs to be intensified in order to keep pace with advances in therapy and lead to more targeted use of novel therapeutic agents. MM
Irene Whitt is in the second year of her fellowship in rheumatology at Mayo Clinic and has research interests in inflammatory myositis. Ann Reed is chair of the division of pediatric rheumatology at Mayo Clinic.
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