An Italian Multicenter Study on Anti-NXP2 Antibodies: Clinical and Serological Associations

The identification of anti-NXP2 antibodies is considered a serological marker of dermatomyositis (DM), with calcinosis, severe myositis and, in some reports, with cancer. Historically, these associations with anti-NXP2 antibodies have been detected by immunoprecipitation (IP), but in the last few years commercial immunoblotting assays have been released. The aim of this collaborative project was to analyse the clinical features associated to anti-NXP2 antibodies, both with commercial line blot (LB) and IP. Myositis-specific and myositis-associated autoantibodies were detected in single centres by commercial line blot (LB); available sera were evaluated in a single centre by protein and RNA immunoprecipitation (IP), and IP-Western blot. Sixty patients anti-NXP2+ (NXP2+) positive by LB were compared with 211 patients anti-NXP2 negative with idiopathic inflammatory myositis (IIM). NXP2+ showed a younger age at IIM onset (p = 0.0014), more frequent diagnosis of dermatomyositis (p = 0.026) and inclusion-body myositis (p = 0.009), and lower rate of anti-synthetase syndrome (p < 0.0001). As for clinical features, NXP2+ more frequently develop specific skin manifestations and less frequently features related with overlap myositis and anti-synthetase syndrome. IP confirmed NXP2 positivity in 31 of 52 available sera (62%). Most clinical associations were confirmed comparing NXP2 LB+/IP+ versus NXP2-negative myositis, with the following exceptions: inclusion-body myositis diagnosis was not detected, whilst dysphagia and myositis were found more frequently in NXP2 LB+/IP+ patients. The 21 LB+ /IP-myositis patients did not show differences in clinical features when compared with the NXP2-myositis patients and more frequently displayed multiple positivity at LB. Risk of developing cancer-associated myositis was similar between NXP2-positive and NXP2-negative myositis patients, either when detected by LB or IP. Protein-IP confirmed NXP2 antibodies in nearly 60% of sera positive for the same specificity with commercial assay. Double-positive cases rarely occurred in myositis patients with a clinical diagnosis other than dermatomyositis. Patients only positive by LB (LB+/IP−) did not display clinical features typical of NXP2. NXP2 positivity by LB should be confirmed by other methods in order to correctly diagnose and characterize patients affected by idiopathic inflammatory myositis. Supplementary Information The online version contains supplementary material available at 10.1007/s12016-021-08920-y.


Introduction
Idiopathic inflammatory myopathies (IIM) are a group of heterogeneous autoimmune inflammatory diseases primarily involving muscle and skin [1]. IIM subtypes include dermatomyositis (DM), polymyositis (PM), anti-synthetase Fredi Micaela, Cavazzana Ilaria, and Ceribelli Angela equally contributed as first author. syndrome (ASS), overlap myositis (OM), immune-mediated necrotising myopathy (IMNM) and sporadic inclusion-body myositis (IBM) [1]. Approximately 60-65% of IIM patients have detectable serum autoantibodies, namely myositisspecific autoantibodies (MSA), which are unique to IIM and usually mutually exclusive to one another, or myositisassociated autoantibodies (MAA) which can occur in other connective tissue diseases (CTD) or be present along with MSA [2]. Although only anti-Jo-1 antibodies have been included in the recent EULAR/ACR classification criteria for IIM [3], it was acknowledged that several other MSA also carry clinical value, due to their ability to stratify specific disease subsets [2,4].
The aim of this collaborative project was to analyse the clinical features associated with anti-NXP2 antibodies in a large Italian IIM cohort, including the occurrence of concomitant cancer, using both commercial line blot (LB) and homemade IP.

Patients
This study was conducted in the frame of the FIRMA group (Forum Interdisciplinare per la Ricerca nelle Malattie Autoimmuni). This project started in 2018 with the aim to collect clinical and serological data from several Italian centres belonging to the FIRMA group: 13 centres collected data on 61 patients locally positive for anti-NXP2 antibodies (NXP2+) by LB.
As control group we collected the same data on 211 patients with a diagnosis of IIM negative for anti-NXP2 (NXP2−) followed-up by two third-level centres (Brescia and Pavia University).
DM, PM and OM were defined according to Bohan and Peter Criteria [27], whereas ASS [28], IBM [29] and IMNM [30] were diagnosed according to the currently used definitions.
Clinical data were obtained from clinical charts. Disease onset was considered as the first skin, joint, muscle or constitutional symptom/sign related to IIM. For all the patients, demographic and epidemiological data, extramuscular findings, including skin manifestations (heliotrope rash, Gottron's papule, mechanic's hands, sclerodactyly, cutaneous ulcerations), calcinosis, arthritis, Raynaud's phenomenon, dysphagia and myocarditis were collected. Muscle involvement was defined when patients presented at least one condition amongst muscle enzymes' elevation, muscle weakness, presence of typical electromyography (EMG) alterations and/or inflammatory findings on muscle biopsy. Interstitial lung disease (ILD) was defined by high-resolution computed tomography (HRCT) chest scan. Finally, data regarding the presence of a neoplastic disease during patient's life were retrieved: cancer-associated myositis (CAM) was defined as neoplastic disease onset 3 years before or after myositis' diagnosis, according to Yang et al. [31].
Patients' sera were isolated from whole blood through centrifugation at 2000 g for 15 min, and then stored at −20 °C until use.

Statistical Analysis
Categorical variables were expressed as number or percentage, continuous variables as median and interquartile range (IQR). Comparisons between groups were performed by chi-square test, Fisher's exact test, Student's t test and Mann-Whitney test when appropriate. A logistic regression model was built to identify clinical manifestations independently associated with NXP2+ with the inclusion of the variables that were significant at univariate analysis. A p-value < 0.05 was considered statistically significant. The odd ratio (OR) with 95% confidence interval (95% CI) was calculated. Survival from cancer onset was estimated using Kaplan-Meier method and differences between groups were compared using the log-rank test. The GraphPad4 version was used for statistical analysis. The study was approved by the Ethical Committee of the leading centre (ASST Spedali Civili of Brescia, NP3511).

Results
Anti-NXP2 antibodies were found in 61 IIM patients, 42 females and 19 males, with a median age at disease onset of 46 years (IQR 28.7-59.2), and a median followup of 26 months (IQR 12-120). Demographic data are reported in Table 1: DM was the most common diagnosis, occurring in 42 cases, followed by PM (11 cases), IBM (4 cases), ASS, OM and IMNM in one case, each. Most of the patients were Caucasian (95%). Clinical data were available for 60 patients.

Cancer Association
Both anytime cancer and CAM myositis were more rarely diagnosed in LB NXP2+ compared with NXP2− , without any significant difference ( Limiting the analysis to LB+/IP+ NXP2+ group (Table 3), 3 patients presented an anytime cancer, with 2 cases of CAM (a melanoma skin cancer diagnosed 7 months before IIM diagnosis, and a multicentric ductal carcinoma breast cancer found simultaneously at the diagnosis of IIM). Moreover, the exclusion of the 4 patients with juvenile onset did not change the results: anytime cancer occurred in 3 adult NXP2+ (3/27, 11.1% vs 40/190, 21.5%; p = 0.30) and CAM in 2 (2/27, 7.4% vs 20/190, 10.5%; p = 1). Again, even if a lower cancer prevalence in NXP2+ occurred, this was not significant. Anti-NXP2 antibodies were not associated with concomitant cancer development, either when positive by LB alone or by both IP and LB methods, as shown by survival analysis (Fig. 1 Supplemental).
A multivariate analysis of all 60 NXP2+ (detected by LB only) does not confirm the association with DM.

Analysis of Discordant Sera (LB Positive, IP Negative)
As reported above, 21 NXP2+ sera by LB were not confirmed by IP. These discordant patients showed the same clinical features of 211 NXP2-negative patients; in particular, no differences were found regarding features associated with NXP2 clinical phenotype (i.e. DM rash, calcinosis, dysphagia) ( Table 1 supplemental material).
These clinical associations are (mostly) based on the detection of anti-NXP2 by IP [8,10,11]. However, the availability of IP assay is confined to few research centres, whereas, nowadays, laboratories are widely using commercially available immunoassays, such as LB, for the detection for MSA or MAA.
This large Italian multicentre study analysed the clinical spectrum associated with anti-NXP2 antibodies detected by LB, confirming previously described associations, namely skin rash, facial rash, heliotropic rash, periorbital oedema and calcinosis [6,10,11]. In contrast, symptoms associated with overlap myositis or ASS were not found to be associated with NXP2 antibodies, including ILD, thus confirming previous data [6].
When detected only by LB, we found anti-NXP2 antibodies significantly associated with IBM: this unusual observation was not reported in other case series, reviewed in a recent meta-analysis [6], and was not confirmed when anti-NXP2 antibodies were searched by IP. In fact, double-positive NXP2 patients were mostly affected by DM. IBM is a different type of IIM, with peculiar clinical and histological features, recently characterized by a new autoantibody specificity, known as anti-cN1A [36]. Anti-NXP2 antibodies were confirmed by IP in 31 on 52 available sera (60%). When the 31 double-positive patients were compared with IIM NXP2− patients, the previously described clinical associations were confirmed with higher significance, and dysphagia and myositis were also added as associated with anti-NXP2 antibodies. Most of the papers reported a strict correlation between anti-NXP2 antibodies and dysphagia [2,6,10,11,13,33], whilst the occurrence of myositis in anti-NXP2-positive patients is less evident in different cohorts. Nevertheless, a severe muscle weakness with myalgia [6,11], with higher levels of CK [11] or necrotizing histological features [37] have been described in some NXP2+ case series.
Multivariate analysis, performed on 31 double-positive sera, confirmed the association between NXP2 and DM, and a negative association with ILD was also assessed.
The description of a possible link between anti-NXP2 antibodies and cancer was reported by some authors [13,15,38], but two recent meta-analyses did not confirm this hypothesis [7,12]. In 2012, Ichimura and colleagues firstly reported the association between CAM and anti-NXP2+ IIM on 8 patients, 3 of whom had cancer within 3 years from diagnosis [38]. Fiorentino and colleagues in 2013 confirmed this association in 37 anti-NXP2-positive patients with DM: at multivariate analysis, cancer was associated with either NXP2 or TIF1gamma antibodies, older age, and male sex [15]. Finally, in 2017, Albayda reported an increased risk for cancer in a cohort of 56 NXP2+ DM patients compared with the general population (3.68-fold increased risk), whereas no differences were found between NXP2+ and NXP2− IIM cases [13]. No association with increased risk of cancer was otherwise reported in several other multicentric studies [2,8,39]. In the present study we also did not confirm an increased risk of cancer associated with anti-NXP2 antibodies, although we did not perform a direct comparison with the general population. These discrepant data could be due to the low number of patients considered and/or the different ethnic background of different cohorts [15, 38, present study]. Although MORC3 protein is known to be involved in cellular senescence, p53 recruitment [40] and oncogenesis [41], the clear demonstration of the oncogenic potential of anti-NXP2 antibodies was not reported, so far, in contrast to what has been described for anti-TIF1gamma [42] or anti-RNAPol-III antibodies, specific for SSc [43].
The reliability between the different methods of detection of MSA and/or MAA remains a topic of discussion. Immunoprecipitation has historically been used to identify MSA and MAA, but it is an impractical method for widespread diagnostic use. Conversely, commercial multiplex assay represents an easier, low-cost method that can simplify the detection of these autoantibodies in clinical practice. In our cohort the gold standard IP did not confirm the result of the multiplex assay technique in nearly 40% of cases, and moreover, these patients did not present clinical features included in the clinical spectrum of NXP2 antibody. It has been already demonstrated that multiple testing, whereas increasing efficiency could also increase false-positive results. Several reports described how the agreement between commercial assay and IP may be influenced by the rarity of the autoantibody or by the antigen specificity and no conclusive data are currently available [16][17][18][19][20][21][22][23][24][25][26]44]. In 2019, Fiorentino et al. compared results from LB and IP in DM patients, and only a fair concordance was found for NXP2 specificity [26] differently from the moderate concordance shown by a previous study [17]. Otherwise, in a recent paper of Tansley et al. a high agreement was found between LB, dot blot and IP [44]. Linked to the problem of false-positive results is the frequency of double-or multiplepositive MSA. In literature, the presence of a concomitant MSA is considered a rare event, with a prevalence reported between 0 and 0.2% [2,17] in studies using IP. When using commercial assays, the prevalence of a co-existing MSA is much higher: in a previous Italian work, the prevalence using LB was above 15% [17], more recently reported as 6% in an English study [25]. In our analysis we have found that discordant sera more frequently presented an associated MSA compared with double-positive NXP2, confirming the idea that when multiple MSAs are found in commercial testing, results should be carefully managed. In fact, even if multiple MSAs appear to be associated with IIM, the clinical subset cannot be clearly defined.
NXP2 antibody is not associated with a unique IIF pattern: in a previous study the 60% of NXP2 sera positive with IP displayed a nuclear dot pattern consistent with the presence of PML bodies [9]. In the present study, this pattern was found only in the 35% of double-positive patients, but in none of the sera not confirmed by IP. Therefore, even if we can state that IIF could not be considered as a confirmation method, the presence of multiple nuclear dot pattern could help to discriminate true NXP2 patients [45].
The present study has some limitations. First, serum was not available neither for all NXP2 patients positive with commercial assay nor for the control group; therefore not all the patients were centrally tested with IP, thus limiting the analysis of concordance between assays. Moreover, only a single sample was available, not allowing longitudinal analysis. Recently, a longitudinal evaluation of 14 NXP2 patients confirmed how this MSA, similar to others, can fluctuate along with changes in disease activity, suggesting a possible role as a biomarker for monitoring during follow-up [46]. Finally, data obtained with the LB were not compared with other commercial assay; however, it has been recently demonstrated a good agreement between the results of LB and dot blot assay regarding NXP2 [20].
The main strengths are the number of clinical cases from different Italian centres and the prospective evaluation of sera collected by IP and IIF on HEp-2 cells.
In conclusion, double-positive sera for LB and IP describe a clinical spectrum characteristically associated with anti-NXP2 antibodies. Whilst we recognize the importance of the routine use of multiple testing to diagnose patients affected by IIM, the only use of commercial LB assay seems to not adequately identify anti-NXP2-positive patients falling in the NXP2 classical clinical spectrum.

Availability of Data and Materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
Code Availability Not applicable.

Declarations
Ethics Approval This retrospective study has been approved by local ethic committee (ASST-Spedali Civili of Brescia, Italy), protocol number: 3511.

Consent to Participate and Publication
Data were obtained from medical charts: every patient signed our institutional consent for the management of personal data.

Conflict of Interest The authors declare no competing interests.
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