To determine the prevalence of myositis-specific autoantibodies (MSAs) and myositis-associated autoantibodies (MAAs) and their clinical and immunogenetic correlations in Mediterranean patients with idiopathic inflammatory myopathies.
To determine the prevalence of myositis-specific autoantibodies (MSAs) and myositis-associated autoantibodies (MAAs) and their clinical and immunogenetic correlations in Mediterranean patients with idiopathic inflammatory myopathies.
Sera from 88 patients were studied for MSAs and MAAs by RNA and protein immunoprecipitation. HLA typing was performed by sequence-specific primer– and sequence-specific oligonucleotide–polymerase chain reaction and serology. Statistical analyses were performed with Student's t-test and Fisher's exact test. Cumulative survival probabilities were estimated by the Kaplan-Meier method and Cox regression analysis.
Twenty-eight patients (30%) had MSAs, most commonly antisynthetase antibodies (23.9%). Six patients (7.5%) had anti–Mi-2 antibodies. No anti–signal recognition particles were found. Arthritis, mechanic's hands, interstitial lung disease, and sicca syndrome were more prevalent in patients with antisynthetase antibodies. Dysphagia and the need for more treatment courses were more frequent in patients who were anti–Mi-2 positive. Forty-three patients (48%) had MAAs, 20 (22%) with anti–Ro 60 and 18 (20.4%) with anti–Ro 52. Ten patients (11.4%) were positive for anti–PM-Scl, 6 (6.8%) for anti-RNP, and 1 for anti-Ku antibodies. Patients with PM-Scl, RNP, or Ro antibodies were more often classified as having overlap syndrome. Immunogenetic studies found a significant association between HLA–DR3 and the presence of antisynthetase antibodies (P = 0.049), anti–PM-Scl antibodies (P = 0.017), and interstitial lung disease (P = 0.03). No statistically significant differences in mortality, survival, or clinical course were observed between patients positive for MSAs or MAAs and the remaining patients.
These results are consistent with those from other published series, although some differences warrant consideration. Autoantibody studies may be useful for defining more homogeneous groups of patients with idiopathic inflammatory myopathies.
Idiopathic inflammatory myopathies are a group of acquired, heterogeneous, systemic diseases characterized by progressive symmetrical muscle weakness, elevated serum levels of muscle enzymes, electromyographic abnormalities, and inflammatory infiltrates on muscle biopsy (1). Characteristic histopathologic features allow the classification of idiopathic inflammatory myopathies into polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM) (2). These conditions are commonly regarded as autoimmune disorders, and various autoantibodies directed to defined nuclear and cytoplasmic antigens are found in up to 55% of patients with PM or DM. Some of these autoantibodies are frequently encountered in rheumatic disorders associated with myositis and are referred to as myositis-associated autoantibodies (3–5). Other autoantibodies are specific for idiopathic inflammatory myopathies and target a subset of aminoacyl–transfer RNA (aminoacyl-tRNA) synthetase (6), components of the signal recognition particle (SRP) (7), and nuclear helicase-ATPase Mi-2 (8–11). In addition to the useful clinical classification of Bohan and Peter (12, 13), there is another approach to the classification of idiopathic inflammatory myopathies based on determination of autoantibodies specific to or associated with myositis (14). The prevalence of these autoantibodies may vary between geographic regions because of differences in the genetic background and/or exposure to environmental factors among different populations (15). Although the association of these autoantibodies with some specific clinical features has been suggested by several authors (14, 16–19), there are still many unanswered questions concerning their clinical applicability. Our working hypothesis for this study was that determination of these autoantibodies would be useful for clinical purposes in our population of patients with myositis. The goal of the present study was to determine the prevalence of myositis-specific autoantibodies (MSAs) and myositis-associated autoantibodies (MAAs) in a series of 88 adult patients with idiopathic inflammatory myopathies and to establish clinical, serologic, and genetic correlations in this Mediterranean population from a single hospital in Barcelona, Spain.
The study included 88 consecutive adult patients with idiopathic inflammatory myopathy. Patients were seen at Vall d'Hebron General Hospital in Barcelona, Spain, between 1983 and 2005. Vall d'Hebron is a 700-bed referral teaching hospital for a catchment population of nearly 450,000 inhabitants. Virtually all patients from the area with suspected myositis are referred to Vall d'Hebron, where they are diagnosed, treated, and followed up, whether or not the disease is severe. Patients included in the study gave informed oral consent to the use of their serum for research purposes. The study was approved by the institutional review board.
The diagnosis of PM/DM was based on the criteria of Bohan and Peter (12, 13), including symmetrical proximal muscle weakness, increased serum muscle enzymes, electromyography abnormalities, typical histologic findings on muscle biopsy, and characteristic dermatologic manifestations (heliotrope rash and Gottron's papules). Patients with idiopathic inflammatory myopathy who met the criteria for another defined connective tissue disease were included as patients with myositis overlap syndrome, and those with a diagnosis of cancer within 3 years of the myositis diagnosis were included as patients with cancer-associated myositis. Characteristic clinical and histologic features provided the diagnosis of IBM according to established criteria (2, 20).
Patients with myositis were categorized serologically into different groups based on the presence or absence of MSAs (antisynthetase, anti-SRP, or anti–Mi-2) and MAAs (anti–Ro 52, anti–Ro 60, anti-La, anti-RNP, or anti–PM-Scl). Patients with antisynthetase antibodies were divided into those positive for anti–histidyl–transfer RNA synthetase (anti–his-tRNA; Jo-1 group) and those positive for any of the other aminoacyl-tRNA synthetases (non–Jo-1 group). Moreover, patients testing positive for any of the MSA and MAA autoantibodies were further categorized into various subgroups. We also included a subgroup of patients with interstitial lung disease and myositis-specific antisynthetase antibodies in the absence of clinical myositis. Myositis was considered clinically absent if there was no significant proximal weakness by history and physical examination, serum creatine kinase was normal, and electromyography abnormalities were not observed. Patients were classified as having DM (n = 42); PM (n = 19); myositis overlap syndrome (n = 14); cancer-associated myositis (n = 8); IBM (n = 2); and interstitial lung disease and antisynthetase antibodies, but no myositis (n = 3). Patients with myositis overlap syndrome and cancer-associated myositis were further classified as having PM, DM, or IBM. In the group of patients with myositis overlap syndrome, 8 patients were diagnosed with PM and 6 were diagnosed with DM, systemic sclerosis was present in 8 patients, primary Sjögren's syndrome was present in 4, and systemic lupus erythematosus was present in 2. Patients with cancer-associated myositis (7 DM, 1 PM) presented with ovarian and breast cancer (2 patients each) and lung, colon, stomach, and nasopharyngeal cancer (1 patient each).
Clinical data were obtained by taking standardized history and conducting physical examination, and by reviewing patients' medical records. Data were recorded in a database that included demographic data, signs and symptoms, presence of associated disorders, systemic and organ involvement, clinical course, number and nature of treatment courses, and treatment responses.
Patients' overall clinical courses were defined as monocyclic (full recovery within 2 years without relapse, with or without drug therapy), chronic polycyclic (prolonged, relapsing course with ≥1 relapses occurring between periods of inactive disease), chronic continuous (persistent disease activity for >2 years, despite drug therapy, that is never inactive), and undefined (illness of <2 years' evolution). A drug trial was defined as a single course from the beginning of administration of a given drug to the time at which the drug was discontinued, or in the case of prednisone, the time at which the dose was reduced to one-quarter of the initial dose. Treatment response was defined as complete (patient totally recovered without evidence of active disease), partial (evidence of clinical improvement short of a complete clinical response), or none (no evidence of clinical improvement).
Serum samples from each patient were screened by indirect immunofluorescence for antinuclear antibodies using HEp-2 cells, and by enzyme-linked immunosorbent assay (ELISA) for antibodies directed against extractable nuclear antigens (Ro, La, RNP, Sm) and anti–his-tRNA synthetase (anti–Jo-1). Sera from all 88 patients in this series were tested by protein and RNA immunoprecipitation, which allows detection of other synthetases, MSA and MAA (anti–Mi-2, anti-SRP, anti–Ro 52, anti–Ro 60, anti-La, anti–PM-Scl, and anti-RNP), that may have been overlooked by the ELISA test and confirmation of ELISA results.
Immunoprecipitation analysis (IP) from HeLa cell extracts was performed as previously described (21). Briefly, for RNA analysis, HeLa cells growing in log phase at 2 × 105 were labeled with 32P orthophosphate (10 μCi/ml, New England Nuclear, Boston, MA) in 100 ml of phosphate-free minimal essential medium for 12–14 hours. Labeled whole-cell extracts were then prepared and incubated with 15 μl of sera in 2 mg/ml of protein A–Sepharose CL-4B (Amersham Pharmacia Biotech, Uppsala, Sweden) in 500 ml of IP buffer (10 mM Tris HCl, pH 8.0, 500 mM NaCl, 0.1% Nonidet P40). To extract bound RNAs, 30 ml of 3.0M sodium acetate, 30 ml of 10% sodium dodecyl sulfate, 2 ml of carrier yeast tRNA (10 mg/ml; Sigma, St. Louis, MO), and 300 ml of phenol–chloroform–isoamyl alcohol (50:50:1, containing 0.1% 8-hydroxyquinoline) were added to the Sepharose beads. After agitation in a vortex mixer and spinning for 1 minute, RNAs were recovered in the aqueous phase after ethanol precipitation and dissolved in 20 μl of electrophoresis sample buffer composed of 10M urea, 0.025% bromophenol blue, and 0.025% xylene cyanol-FF in Tris–borate–EDTA buffer (90 mM Tris HCl, pH 8.6, 90 mM boric acid, and 1 mM EDTA). The RNA samples were denatured at 65°C for 5 minutes and then resolved in 7M urea/10% polyacrylamide gel and localized in the gel by autoradiography.
For the protein studies, HeLa cells were allowed to incorporate 35S-methionine-cysteine (5 μCi/ml; Amersham, Arlington Heights, IL) in methionine-cysteine–free minimal essential medium (Sigma, St. Louis, MO) supplemented with 1% glutamine and antibiotics for 16–18 hours. Labeled whole-cell extracts were then prepared and incubated with 15 μl of sera in 2 mg/ml of protein A–Sepharose CL-4B (Amersham Pharmacia Biotech, Uppsala, Sweden) in 500 ml of IP buffer (10 mM Tris HCl, pH 8.0, 500 mM NaCl, 0.1% Nonidet P40). Immunoprecipitated proteins were separated by 10% sodium dodecyl sulfate–polyacrylamide gel electrophoresis and localized in the gel by autoradiography.
HLA class II was studied by means of sequence-specific primer– and sequence-specific oligonucleotide–polymerase chain reaction as routine determinations in our hospital laboratory. HLA class I was studied with serologic methods.
Univariate analysis was used to detect outlying values and to describe the distribution of variables within the groups. Qualitative data from this analysis are presented as the count (percentage) and quantitative data as the mean ± SD. Bivariate analysis was then performed to determine the relationships between clinical features and the presence of different antibodies: patients positive for MSA or MAA antibodies versus the reference group comprised of patients negative for MSA and MAA antibodies. Qualitative variables were compared using chi-square and Fisher's exact tests, and quantitative variables were compared using Student's t-test. When possible, associations were quantified with the odds ratio (OR) and 95% confidence interval (95% CI).
All survival analyses were developed in 2 ways: including all patients and including only cancer-free patients. Followup time was considered to be the interval between the date of diagnosis of the disease and the date of the most recent followup visit. MSA and MAA positive status were used as covariables. These data were also analyzed considering the time between the onset of the first symptoms and the date of the most recent followup visit. We analyzed survival at 5, 10, 15, and 20 years of followup, using the chi-square test or Fisher's exact test when appropriate. Cumulative survival times were estimated with the Kaplan-Meier method and were compared with the log rank test. All statistical analyses were performed with SPSS version 6.0 (SPSS, Chicago, IL). Significance was set at a P value less than 0.05. Holm's procedure was used to adjust P values to minimize the effect of multiple comparisons.
Between 1983 and 2005, 88 consecutive patients (61 women, 27 men; mean ± SD age 47.16 ± 17.77 years) were diagnosed with inflammatory myopathy in our department. Mean ± SD followup time was 7.1 ± 6.92 years. A total of 63 patients (71.6%) were positive for antinuclear antibodies (>1:160 dilution). Data from patients with and without MSAs and MAAs were statistically analyzed. The prevalence of these autoantibodies in the different groups of patients with myositis and the associations between them are shown in Tables 1 and 2. Clinical characteristics per serologic group and risk indicators according to MSA and MAA positive status are summarized in Tables 3, 4, and 5.
|Autoantibodies||PM sera (n = 27)||DM sera (n = 59)||IBM sera (n = 2)||Total (n = 88)|
|Anti–Jo-1||5 (18)||9 (15)||0||14 (16)|
|Other antisynthetases||4 (14)||4 (7)||0||8 (9)|
|Anti–Mi-2||0||6 (10)||0||6 (7)|
|Anti–PM-Scl||1 (4)||8 (14)||1 (50)||10 (11)|
|Anti–Ro 60/SSA||10 (37)||9 (15)||1 (50)||20 (22)|
|Anti–Ro 52||5 (18)||12 (20)||1 (50)||18 (20)|
|Anti-La/SSB||6 (20)||3 (5)||0||9 (10)|
|Anti–U1 snRNP||3 (11)||3 (5)||0||6 (7)|
|Anti–U5 snRNP||1 (4)||1 (2)||0||2 (2)|
|Anti-Ku||0||1 (2)||0||1 (2)|
|Anti-RNP (n = 8)||Anti-La (n = 9)||Anti–Ro 52 (n = 18)||Anti–Ro 60 (n = 20)||Anti–PM-Scl (n = 10)Anti–Mi-2||Anti–Mi-2 (n = 6)|
|Antisynthetase (n = 22)||1||3||5||6||0||0|
|Anti–Mi-2 (n = 6)||0||1||1||1||0|
|Anti–PM-Scl (n = 10)||0||0||1||1|
|Anti–Ro 60 (n = 20)||0||0||9|
|Anti–Ro 52 (n = 18)||2||5|
|Anti-La (n = 9)||2|
|Characteristic||MSA (n = 28)||MAA (n = 43)||MSA/MAA pos. (n = 12)||MSA or MAA (n = 59)||MSA/MAA neg. (n = 29)|
|Age at onset, mean ± SD years||46 ± 19||46.2 ± 17||43.9 ± 17||46.5 ± 18||48.3 ± 17|
|Nail fold changes, no.†||83.3||86.7||80||86.4||80|
|Cutaneous signs, no. (%)‡||1.6 (0.4)||1.7 (0.4)||1.7 (0.4)||1.8 (0.4)||1.7 (0.4)|
|Creatine kinase, IU/liter§||3,790||1,667||3,225||2,370||1,474|
|Drug trials, no.||1.6||1.2||1.17||1.4||1.4|
|Characteristic||Jo-1 (n = 14)||non–Jo-1 (n = 8)||Synthetase (n = 22)||Mi-2 (n = 6)||PM-Scl (n = 10)||Ro 52 (n = 18)||Ro 60 (n = 19)||RNP (n = 8)||MSA/MAA neg. (n = 29)|
|Age at onset, mean|
|± SD years||44 ± 19||41 ± 19||43 ± 17||56 ± 24||41 ± 15||49 ± 16||46 ± 17||49 ± 18||48 ± 17|
|Nail fold changes†||91.7||83.3||88.9||66.7||87.5||23.1||25||100||80|
|no. (%)‡||1.7 (0.4)||1.3 (0.5)||1.6 (0.5)||2 (0)||1.9 (0.3)||1.7 (0.4)||1.7 (0.4)||1.7 (0.4)||1.7 (0.4)|
|Drug trial, no.||1.4||0.6||1.1||3.3||1.2||1.2||1.3||1||1.4|
|Variables (reference category)||MSA+||Raw P||Adjusted P||MAA+||Raw P||Adjusted P|
|Age at onset (per year)||0.99 (0.97–1.02)||0.63||0.63||0.99 (0.97–1.02)||0.61||0.99|
|Male sex||1.26 (0.39–4.10)||0.70||0.70||1.68 (0.59–4.84)||0.33||0.67|
|Cancer associated (yes)||0.67 (0.10–4.33)||0.67||0.67||0.65 (0.12–3.47)||0.61||0.99|
|Overlap syndromes (yes)||1.62 (0.25–10.51)||0.61||0.61||4.64 (0.95–22.78)||0.06||0.12|
|Fever (yes)||1.43 (0.49–4.16)||0.52||0.52||1.43 (0.54–3.80)||0.48||0.96|
|Dry mouth/eyes (yes)||4.90 (1.17–20.48)||0.03||0.06||3.45 (0.87–13.62)||0.08||0.08|
|Raynaud's phenomenon (yes)||1.75 (0.55–5.51)||0.34||0.34||2.49 (0.88–7.05)||0.09||0.17|
|Nailfold changes (yes)||1.25 (0.27–5.79)||0.78||0.78||1.63 (0.36–7.43)||0.53||0.99|
|Arthritis (yes)||3.54 (1.17–10.68)||0.03||0.03†||3.27 (1.22–8.75)||0.02||0.04†|
|ILD (yes)||2.96 (1.00–8.78)||0.05||0.10||1.76 (0.65–4.74)||0.26||0.26|
|Mechanic's hands (yes)||4.50 (0.85–23.94)||0.08||0.16||2.19 (0.41–11.69)||0.36||0.36|
|Cutaneous signs (per sign)||0.79 (0.62–1.00)||0.04||0.04†||0.71 (0.55–0.91)||0.01||0.02†|
|Dysphagia (yes)||1.05 (0.34–3.22)||0.93||0.93||1.60 (0.59–4.32)||0.35||0.71|
|Creatine kinase, high (200–1,500 units)||0.86 (0.22–3.32)||0.82||0.82||1.31 (0.45–3.84)||0.63||0.99|
|Creatine kinase, very high (>1,500 units)||5.14 (1.30–20.36)||0.02||0.04†||2.22 (0.60–8.17)||0.23||0.23|
|Clinical course (nonmonocyclic)||0.79 (0.26–2.48)||0.69||0.99||1.17 (0.39–3.50)||0.79||0.79|
|Drug trials (per trial)||1.08 (0.78–1.50)||0.65||0.65||0.88 (0.60–1.31)||0.54||0.99|
|Treatment response (complete)||1.23 (0.43–3.49)||0.70||0.70||0.73 (0.28–1.90)||0.52||0.99|
|Survival (per year)||0.99 (0.92–1.08)||0.88||0.88||1.01 (0.94–1.08)||0.76||0.99|
|Mortality (yes)||0.78 (0.18–3.32)||0.95||0.95||2.58 (0.61–10.97)||0.20||0.39|
MSAs were detected in 28 patients (30%). The most frequently encountered were antisynthetase antibodies, which were present in 22 patients (23.9%), including anti–his-tRNA synthetase (anti–Jo-1) in 14 patients (16%) and other antisynthetases in the remaining patients. Patients with antisynthetases other than anti–Jo-1 presented with a clinical syndrome similar to the antisynthetase syndrome but with few skin lesions and hypomyopathic manifestations. Anti–Mi-2 autoantibodies were present in 6 patients (7.5%). Anti-SRP antibodies were not found in any patient.
We found no differences in the age at disease onset or sex ratio between patients positive for MSAs as compared with patients negative for MSAs or MAAs. Patients with MSAs had significantly more cases of sicca syndrome (dry mouth/eyes; OR 4.90, 95% CI 1.17–20.48), arthritis (OR 3.54, 95% CI 1.17–10.68), interstitial lung disease (OR 2.96, 95% CI 1.00–8.78), and higher creatine kinase values (OR 5.14, 95% CI 1.30–20.36) than patients without MSAs or MAAs, although only arthritis and higher creatine kinase values remained significant after correction with Holm's procedure. In addition, fewer patients with MSAs had characteristic cutaneous signs (OR 0.79, 95% CI 0.62–1.00) than patients without MSA or MAA antibodies. No differences were found in the remaining variables that were analyzed. Neither of the 2 patients with IBM was positive for any MSA.
MAAs were detected in 43 patients (48%), and the most frequently encountered were anti–Ro 60 and anti–Ro 52 antibodies, which were present in 20 (22%) and 18 (20.4%) patients, respectively. Ten patients (11.4%) had anti–PM-Scl antibodies, 6 (6.8%) had anti-RNP antibodies, and 1 patient had anti-Ku antibodies. MAAs were found in all the diagnostic groups, including IBM (anti–PM-Scl in one and anti–Ro 60 in the other) (22, 23).
No differences were found in the age at disease onset or sex ratio between the different MAA subgroups or between patients positive for any of the MAAs as compared with the remaining patients. MAA positive status was significantly associated with arthritis (OR 3.27, 95% CI 1.22–8.75) and with fewer classic skin manifestations of DM with respect to the rest of the patients (OR 0.71, 95% CI 0.55–0.91). There were no statistically significant differences in mortality, survival, or clinical course between patients who were MAA positive and the other patients.
Using Kaplan-Meier analysis, there were 24 censored patients out of 28 in the MSA- and MAA-negative group, with a mean ± SD survival time of 20.2 ± 2.5 years. These results were compared with patients who were positive for MSAs (23 censored out of 28; mean ± SD survival time 19.6 ± 2.2 years; log rank test: 0.08; P = 0.78) and positive for MAAs (29 censored out of 43; mean ± SD survival time 17.3 ± 2.3 years; log rank test: 1.83; P = 0.18), but no differences were found. In addition, survival analyses that included only cancer-free patients or that considered survival time as the time between the earliest date of symptoms and the most recent control visit or death did not show significant differences between the groups.
HLA typing was available in 62 of the 88 patients. An association was observed between HLA–DR3 and interstitial lung disease (P = 0.03), positive status to antisynthetase antibodies (P = 0.049), and positive status to anti–PM-Scl antibodies (P = 0.017). In addition, when patients positive for either antisynthetase or anti–PM-Scl (or both) were grouped together, a more highly significant association was found with HLA–DR3 (P < 0.001). There were no other associations.
This report describes the serologic findings and their clinical associations in a large series of patients from the Mediterranean basin diagnosed with idiopathic inflammatory myopathies in a single hospital in Barcelona, Spain. Groups of patients with myositis can be defined serologically by the presence of diagnostic autoantibodies, known as myositis-specific autoantibodies, which are directed against conserved conformational epitopes on cytoplasmic and nuclear components (4). MSAs include autoantibodies that bind to and inhibit the function of aminoacyl-tRNA synthetases (antisynthetases), those directed against proteins of the SRP (anti-SRP), and those that react with a 240-kd SNF2 superfamily helicase associated with the nucleosome remodeling and deacetylase complex, known as anti–Mi-2 autoantibodies (4–11). In contrast, MAAs have an association with myositis, but also occur in other related conditions and include anti–PM-Scl antibodies directed against the exosome (24–29); antibodies to small nuclear RNPs (snRNPs) (30–32); and other cytoplasmic antibodies such as anti-Ro, anti-La, and anti-Ku (33). The prevalence of MSAs and MAAs found in this study corresponded well with findings reported for patients from other countries in general trends (14, 16, 18, 19). There were, however, some differences. More patients were positive for PM-Scl antibodies in our study (11%) than in a European study (6%) (18), a North American study (2%) (14), or a Japanese study (0%) (34), although this factor probably depends on the percentage of overlap myositis syndromes in each series. Hausmanowa-Petrusewicz et al (16) also found a higher number of patients with myositis with anti–PM-Scl antibodies, most of them with scleromyositis.
Photosensitive rashes characterize DM and distinguish it from PM. In our study, anti–Mi-2 antibodies were found only in patients with DM (10%) and not in the other groups, supporting the idea that this autoantibody is characteristic of patients with DM. This finding is in conflict with some studies, mainly with the large European study (18) including 417 patients. Nevertheless, in the European study, anti–Mi-2 antibodies were determined by an ELISA test, which may have been responsible for the high detection of these antibodies in the sera of patients with PM. Results for the incidence and clinical characteristics associated with anti–Mi-2 antibodies in other published series are consistent with our findings (8, 14, 16). A recent study by Okada et al (15) has shed light on this issue and has found that ultraviolet radiation exposure may modulate the clinical and immunologic expression of inflammatory myopathies around the world. In that investigation, anti–Mi-2 autoantibodies were strongly related with the intensity of ultraviolet radiation and with the presence of DM, contributing to the idea that anti–Mi-2 autoantibodies are specific to DM, as we also found.
A relevant finding of our study was the lack of detection of anti-SRP antibodies. These antibodies have been classically related to a poor prognosis, with high levels of creatine kinase and cardiac involvement, mainly in black patients (7, 14, 35). It has also been found that patients with anti-SRP have a low frequency of pulmonary fibrosis, Raynaud's phenomenon, and inflammatory myopathy (18, 35). Moreover, a recent article has reported that anti-SRP autoantibodies are found in other connective tissue diseases and in white patients, and that cardiac involvement in patients with PM is less frequent and the prognosis is not as bad as has been previously reported (36). Anti-SRP autoantibodies are not common, with an incidence that ranges from 1% to 6% in different studies (14, 16, 18, 19, 24, 36), mainly in patients with PM. The absence of black patients and the higher number of DM cases in our series could explain the fact that anti-SRP autoantibodies were not found in our patients.
Among MSAs, the most prevalent are antisynthetase antibodies directed against aminoacyl-tRNA synthetase, mainly anti–Jo-1 (his-tRNA synthetase), which are usually associated with clinical manifestations such as interstitial lung disease, arthritis, fever, Raynaud's phenomenon, mechanic's hands, and other conditions (37). There are few exceptions, such as the French Canadian study, which did not find any patients with antisynthetase antibodies in a series of 30 patients with inflammatory myopathy (38). Some of the manifestations classically related to the antisynthetase syndrome are also frequently present in patients with anti–PM-Scl antibodies, raising the possibility that the immunogenetic background influences the autoantibody status of these patients. HLA–DR3 has been found in association with antisynthetase antibodies (39) and with anti–PM-Scl antibodies (28, 29). Our results agree not only with these clinical-serologic associations but also with the presence of interstitial lung disease, leading us to suggest that the constellation of signs and symptoms of the so-called antisynthetase syndrome could be more closely related with the HLA system (DR3) than with the antibodies. Recent advances in the pathogenesis of antisynthetase antibodies suggest that anti–Jo-1, an ubiquitous enzyme of cytoplasmic localization, is not only highly specific for PM/DM and not merely related to muscle inflammation, but actually acts as a chemokine with a potential role in the immune response of myositis (40–42). Patients with non–Jo-1 antisynthetase autoantibodies seem to have less muscle involvement (43), as was true in our 3 patients with interstitial lung disease and antisynthetase antibodies. In our study, antisynthetase antibodies were detected in patients with PM and in those with DM at similar rates, and were not specific for PM as other studies have suggested.
Other MAAs such as anti-Ro and anti-La autoantibodies, as well as anti-Ku and the different epitopes of anti-RNP antibodies (anti–U1 snRNPs and non–U1 snRNPs) are related with the presence of myositis. Anti–Ro 52 and anti–Ro 60 were both associated with antisynthetase antibodies, suggesting a coupled autoimmune response, as has been previously described (44, 45). The only patient with anti-Ku antibodies in our series had DM-antiphospholipid overlap syndrome and died due to breast cancer at the age of 35. Two of the 6 patients with anti–U1 snRNP antibodies included in the study were diagnosed as having gastric cancer and lung cancer, respectively, which is unusual, and the other 4 had a good clinical prognosis, as has been reported previously (31). Two patients with anti–U5 snRNP presented with scleroderma-PM overlap syndrome, which is in agreement with previous reports (32).
The results obtained in this study support the idea that different autoantibodies can help in the diagnosis of patients with inflammatory myopathies, and that different subgroups can be defined. Classic DM with characteristic skin lesions and dysphagia is associated with anti–Mi-2 antibodies, whereas nonerosive arthritis, interstitial lung disease, and mechanic's hands are associated with antisynthetase antibodies (the so-called antisynthetase syndrome). Some of these manifestations can also be found in patients with anti–PM-Scl antibodies, which are usually present in overlap syndromes of myositis and scleroderma.
Our study has certain limitations. First, we did not include patients age <18 years; therefore, the juvenile DM group was not represented in our study, because we usually only treat adults. Second, this study was retrospective in nature, but the low incidence of these rare diseases makes it very difficult to design and carry out prospective studies, even though recent efforts have been made by the International Myositis Assessment and Clinical Studies Group to improve this point. One drawback of this study was that the number of patients in some subgroups was rather small. Holm's procedure was used to correct for the effect of multiple comparisons, which may have masked some statistically significant results. In summary, our results support the idea that MSAs and MAAs are associated with different and characteristic clinical manifestations that could help in the diagnosis and management of inflammatory myopathies in our Mediterranean population.