Biochemical markers of bone and cartilage turnover may provide a potentially sensitive method for detection of active bone and cartilage degradation in RA [28], and it was suggested to reflect similar changes in the pre-clinical phase of RA, thus allowing to identify individuals who are at risk of developing the disease [16].
OPN and OPG are suggested to be related to the metabolic bone changes which occur in RA, but insufficient data are available on their changes in preclinical RA.
There are only two studies in the current literature which measure bone metabolism biomarkers in the preclinical phase of RA, but without definitive conclusions. One study reported the high prevalence of cartilage oligomeric matrix protein in the period near the diagnosis [29]. The other was performed by van Schaardenburg D et al. [16], who reported statistically significant increased mean levels of OPG and N-terminal type I procollagen pro-peptide in preclinical RA compared to controls.
In the current work, we measured serum OPN and OPG in RA and their FDRs compared to healthy controls as markers of pre-clinical disease.
OPN levels were higher in RA patients than in controls (without statistically significant difference). OPN levels were lower in FDRs than in controls (also non-significant). Several studies [30–33] reported the finding of significantly raised serum OPN in RA compared to healthy controls. When compared to systemic sclerosis patients, OPN levels were significantly higher in RA [33]. Meanwhile, there was no difference in OPN between systemic sclerosis and controls. As a result of these observations, OPN was suggested to be related to RA disease pathogenesis. Furthermore, a recent meta-analysis reported the significantly raised OPN in RA compared to osteoarthritis patients [34]. The detection of raised OPN in RA compared to other inflammatory and non-inflammatory arthritis strengthens the assumption that it is related to RA pathogenesis.
Although raised OPN levels in RA were confirmed in most studies, its relation to inflammatory parameters were inconsistent. Shaker et al. [30] reported that serum OPN was significantly correlated with parameters of disease activity such as the duration of morning stiffness, ESR and DAS28. Similarly, Bazzichi et al. detected that OPN level was significantly related to CRP and disease severity reflected by the HAQ score in RA patients [33].
In contrast, other studies [31–32] as well as the current work did not detect a significant correlation between serum OPN in RA patients and DAS28 as a measure of disease activity. In addition, the current work did not reveal any statistically significant correlation between serum OPN and the other assessed parameters in RA patients.
The second biochemical markers for bone turnover measured in this study was OPG. Significant differences in OPG levels were detected between the three studied groups, with significantly higher levels in RA patients compared to FDRs and controls.
Similarly, most studies reported raised OPG levels in RA compared to controls and correlated it with inflammatory activity in RA [35, 36]. A recent meta-analysis [37] revealed that, compared with the control group, OPG levels were significantly higher in the RA group (p < 0.001), and the disease activity score DAS28 was associated with OPG level in RA patients.
In the current study, although there was no correlation between OPG and laboratory parameters of active inflammation (DAS28, ESR and CRP), there was a statistically significant positive correlation between serum OPG and GS synovitis, which reflects joint inflammation. This may support the relation between OPG and inflammation in RA as detected by MSUS.
In order to test the hypothesis that FDRs of RA may have similarly altered bone metabolism, we measured serum OPN and OPG in FDRs without clinical symptoms. All included FDRs were examined by MSUS for assessing the presence of subclinical synovitis; ESR and CRP were also measured.
To the best of our knowledge, this is the first study to test serum OPN and OPG in FDRs, also in relation to MSUS.
Mean serum OPN was numerically lower in FDRs than in RA patients and controls. In addition, the mean serum OPG was significantly lower in FDRs than in RA patients. It was numerically higher in FDRs than in controls. There was no significant correlation between serum OPN or OPG and different measured parameters in FDRs.
The increased OPG in FDRs than controls could reflect an increased osteoclast activity in the preclinical phase, which was previously proposed by van Schaardenburg D et al. [16]. Liu YY et al. suggested that OPG levels vary at different rheumatoid disease stages [38] and found it to be elevated in early RA compared to longstanding disease. Thus, it seems logic to be also raised short before clinical disease onset in pre-RA, consequently to be raised in FDRs than in controls.
The difference in the levels of these markers of bone metabolism in FDRs compared to healthy controls (though, without statistically significant difference) denotes that FDRs possess an alteration from normal bone metabolism. Conforming with the same concept, when multiple cytokines in sera from unaffected FDRs of RA were previously compared to RA and healthy controls, they proved to have distinct cytokine profiles [39]. In order to confirm our findings and to find an explanation for these alterations, testing these biochemical markers on larger scale studies is mandatory.
Eight of 25 (32%) FDRs in the current study had arthralgia, while 17 FDRs (68%) were asymptomatic. This is not unusual, as it is reported that FDRs were significantly more likely to report joint symptoms compared to individuals with no family history of autoimmune disease [40].
In this study, the US7 score was performed for all RA patients (n = 20) and their FDRs (n = 25).
The detection of MSUS findings in 16/20 (80%) RA patients is an expected finding, but the detection of abnormal MSUS findings in 10/25 (40%) FDRs is noteworthy. Three of 10 (12%) FDRs with MSUS findings had arthralgia and 7/10 (28%) were asymptomatic. Synovitis was the commonest MSUS finding in RA (n = 16/20, 80%), and in their FDRs (n = 10/25, 40%). In addition, the joints most commonly involved in FDRs were the same as those involved in RA, being the wrist, MTP II and MCP II.
Erosions were detected in nine (45%) RA patients and in one (4%) FDR (in MCP II). The high percentage of bone erosions in RA is a further confirmation of the increased osteoclastogenesis. What was not expected is the detection of an erosion in an asymptomatic FDR which could be in favor of the presence of altered bone metabolism that is hypothesized to occur before clinical disease phase.
It was postulated that joint damage is not only the consequence of arthritis, but autoimmune processes that begin years before the clinical disease onset could play a role [41]. Thus, the finding of erosions in asymptomatic FDRs may be a result of an increased osteoclast activity in the preclinical phase which may explain the raised OPG in FDRs compared to controls.
Furthermore, mean ESR and CRP were statistically higher in FDRs than in controls. This conforms with what was previously reported of increased levels of acute phase reactants years before the onset of RA symptoms in blood donors [42, 43].
Of note, the mean ACPA was higher in FDRs than in controls, but without revealing statistical difference. It is well known that RA patients have raised RF and ACPA in the pre-clinical stage. This was first shown in a pivotal study by Nielen et al. [44] who observed the increased prevalence of autoantibodies over time in serum samples from RA patients collected serially in the preclinical phase before RA becomes clinically evident.
When FDRs with arthralgia were compared to those who were asymptomatic, several observations were done: FDRs with arthralgia had significantly higher ESR and CRP than asymptomatic FDRs (both p = 0.003). OPG was higher in those with arthralgia than in those without (though, without statistical difference). Serum OPN was higher in FDRs with arthralgia than in asymptomatics, also without significant difference. Furthermore, mean RF and mean ACPA were higher in FDRs with arthralgia.
Considering the EULAR formulated phases of RA development [45], FDRs of RA with arthralgia could represent the phase of symptoms without clinical arthritis; a more advanced phase in the timeline development of RA than asymptomatic FDRs. Accordingly, this can bring more insight into the pathogenesis of RA development.
All these altered findings assume that FDRs already “suffer” subclinical inflammatory, immunologic and imaging alterations in comparison to healthy controls.
The results of the current study are consistent with the hypothesis that FDRs are at an increased risk of developing RA than the general population [46], and represent a preclinical phase of RA. Considering this, timely follow-ups of FDRs for early initiation of treatment thus preventing progression to overt disease and disability is mandatory.
Study limitations:
The main limitation of this study is that the number of samples analyzed is relatively small to draw solid and definite conclusions. Another limitation might be that the US scans were performed by a single rheumatologist though highly experienced in MSUS (ES).