Pathologic Features of Anti-Mi-2 Dermatomyositis.

OBJECTIVE: To identify the characteristic pathologic features of dermatomyositis (DM) associated with anti-Mi-2 autoantibodies (anti-Mi-2 DM). METHODS: We reviewed 188 muscle biopsies from patients (1) pathologically diagnosed with DM through the sarcoplasmic expression for the myxovirus-resistant protein A and (2) serologically positive for 1 of 5 DM-specific autoantibodies (DMSAs) (anti-Mi-2, n = 30; other DMSAs, n = 152) or negative for all 5 DMSAs (n = 6). We then compared the histopathologic and immunohistochemical features of patients with anti-Mi-2 DM to those with non-Mi-2 DM and patients with anti-synthetase syndrome (ASS) (n = 212) using the t test, Fisher exact test, and a logistic regression model. RESULTS: Patients with anti-Mi-2 DM showed significantly higher severity scores in muscle fiber and inflammatory domains than non-Mi-2 DM patients. The presence of perifascicular necrosis, increased perimysial alkaline phosphatase activity, and sarcolemmal membrane attack complex deposition was more frequent in patients with anti-Mi-2 DM (p < 0.01). After Bonferroni correction, there were no significant differences in the percentages of the features mentioned above between the patients with anti-Mi-2 DM and those with ASS (p > 0.01). CONCLUSION: Perifascicular necrosis and perimysial pathology, features previously reported in ASS, are common in patients with anti-Mi-2 DM. Our findings not only assist in differentiating anti-Mi-2 DM from other DM subtypes but also suggest the possibility of an overlapping mechanism between anti-Mi-2 DM and ASS. CLASSIFICATION OF EVIDENCE: This study provides Class II evidence that the muscle biopsies of DM patients with anti-Mi-2 autoantibodies are more likely to demonstrate higher severity scores in muscle fiber and inflammatory domains.

Severe muscle damage with myofiber necrosis and macrophage infiltrates characterize anti-Mi2 positive dermatomyositis.

OBJECTIVE: The aim of our study was to investigate clinical and histopathological findings in adult DM patients positive for anti-Mi2 (anti-Mi2+) antibodies compared with DM patients negative for anti-Mi2 (anti-Mi2-). METHODS: Clinical data of adult DM patients, who fulfilled EULAR/ACR 2017 classification criteria, were gathered from electronic medical records of three tertiary Rheumatology Units. Histopathological study was carried out on 12 anti-Mi2+ and 14 anti-Mi2- muscle biopsies performed for diagnostic purpose. Nine biopsies from immune mediated necrotizing myopathy (IMNM) patients were used as control group. RESULTS: Twenty-two anti-Mi2+ DM [90.9% female, mean age 56.5 (15.7) years] were compared with 69 anti-Mi2- DM patients [71% female, mean age 52.4 (17) years]. Anti-Mi2+ patients presented higher levels of serum muscle enzymes than anti-Mi2- patients [median (IQR) creatine-kinase fold increment: 16 (7-37)vs 3.5 (1-9.9), P <0.001] before treatment initiation. Moreover, a trend towards less pulmonary involvement was detected in anti-Mi2+ DM (9.1% vs 30.4%, P =0.05), without any case of rapidly progressive interstitial lung disease. At muscle histology, anti-Mi2+ patients showed more necrotic/degenerative fibres than anti-Mi2- patients [mean 5.3% (5) vs 0.8% (1), P <0.01], but similar to IMNM [5.9% (6), P >0.05]. In addition, the endomysial macrophage score was similar between anti-Mi2+ and IMNM patients [mean 1.2 (0.9) vs 1.3 (0.5), P >0.05], whereas lower macrophage infiltration was found in anti-Mi2- DM [mean 0.4 (0.5), <0.01]. CONCLUSIONS: Anti-Mi2+ patients represent a specific DM subset with high muscle damage. Histological hallmarks were a higher prevalence of myofiber necrosis, endomysial involvement and macrophage infiltrates at muscle biopsy.

The nucleosome remodeling and deacetylase complex has prognostic significance and associates with immune microenvironment in skin cutaneous melanoma.

PURPOSE: The Nucleosome Remodeling and Deacetylase (NuRD) complex is an important marker in multiple biological processes whose clinical significance has rarely previously been reported in cancers. In this study, we proposed to estimate the potential of NuRD complex as prognostic signature in skin cutaneous melanoma (SKCM) patients. METHODS: SKCM samples were obtained from the Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO). Sample clustering was performed based on the mRNA levels of core subunits of NuRD complex. Survival analysis was carried out by using Kaplan-Meier method. SKCM samples were grouped into prognostically good and poor groups according to their overall survival (OS). Logistic regression analysis was adopted to construct a model based on the optimal subunits of NuRD complex to estimate prognosis of SKCM samples. RESULTS: Samples from TCGA were grouped into four clusters which were then divided into good and poor prognostic groups. Significant differences existed in immune microenvironment and mutational rates of frequently mutated genes between good and poor prognostic groups. Besides, several immune-related pathways were significantly activated in good prognostic group. A logistic regression model was constructed by using patients' prognostic group and mRNA expressions of NuRD complex from TCGA as categorical responsive values and continuous predictive variables, respectively, which could independently distinguish prognostically different SKCM patients from another three independent GEO datasets. CONCLUSION: In conclusion, we first reported the potential prognostic value and roles in immune microenvironment of NuRD complex in SKCM, which should be helpful for experimental and clinical research in SKCM.

Distinct tissue injury patterns in juvenile dermatomyositis auto-antibody subgroups.

INTRODUCTION: Juvenile dermatomyositis (JDM) can be classified into clinical serological subgroups by distinct myositis-specific antibodies (MSAs). It is incompletely understood whether different MSAs are associated with distinct pathological characteristics, clinical disease activities, or response to treatment. METHODS: We retrospectively reviewed clinicopathological data from consecutive JDM patients followed in the pediatric rheumatology clinic at a single center between October 2016 and November 2018. Demographics, clinical data, and laboratory data were collected and analyzed. Detailed muscle biopsy evaluation of four domains (inflammation, myofiber, vessels, and connective tissue) was performed, followed by statistical analysis. RESULTS: Of 43 subjects included in the study, 26 (60.5%) had a detectable MSA. The most common MSAs were anti-NXP-2 (13, 30.2%), anti-Mi-2 (7, 16.3%), and anti-MDA-5 (5, 11.6%). High titer anti-Mi-2 positively correlated with serum CK > 10,000 at initial visit (r = 0.96, p = 0.002). Muscle biopsied from subjects with high titer anti-Mi-2 had prominent perifascicular myofiber necrosis and perimysial connective tissue damage that resembled perifascicular necrotizing myopathy, but very little capillary C5b-9 deposition. Conversely, there was no positive correlation between the levels of the anti-NXP-2 titer and serum CK (r = - 0.21, p = 0.49). Muscle biopsies from patients with anti-NXP-2 showed prominent capillary C5b-9 deposition; but limited myofiber necrosis. Only one patient had anti-TIF1γ autoantibody, whose muscle pathology was similar as those with anti-NXP2. All patients with anti-MDA-5 had normal CK and near normal muscle histology. CONCLUSIONS: Muscle biopsy from JDM patients had MSA specific tissue injury patterns. These findings may help improve muscle biopsy diagnosis accuracy and inform personalized treatment of JDM.

[Dermatomyositis: new antibody, new classification]. / Dermatomyosites Nouveaux anticorps, nouvelle classification.

Dermatomyositis are rare chronic auto-immune diseases characterized by cutaneous involvement. Diagnosis could be made in childhood or in aldult. There are some different clinical and histological presentation associated with different myositis specific antibody. There are five dermatomyositis specific autoantibodies, anti-Mi2, anti-Tif1-γ, anti-NXP2, anti-MDA5, and anti-SAE. Anti-Mi2 are associated with "classical form" of DM with cutaneous and muscular involvement. Anti-Tif1γ and anti-NXP2 are found in juvenile and adult dermatomyositis, and are associated with recurrent diseases with cutaneous involvement at the forefront. In adults, they are associated with cancer. Anti-MDA5 antibodies are associated with a systemic involvement and an interstitial lung disease. Finally, anti-SAE have been found only in adults, with a classic form.

TITLE: Dermatomyosites Nouveaux anticorps, nouvelle classification. ABSTRACT: Les dermatomyosites (DM) sont des maladies auto-immunes rares du groupe des myopathies inflammatoires idiopathiques, définies par une atteinte cutanée caractéristique. Elles peuvent survenir dans l'enfance, ou chez l'adulte. Il existe des variations phénotypiques entre les DM concernant la présentation cutanéomusculaire (ex: amyopathique) mais aussi la présentation extra-cutanéomusculaire (ex: atteinte pulmonaire ou articulaire associée). Le caractère auto-immun de ces pathologies est souligné dans 60 % des cas par la présence d'anticorps spécifique de myosite. Ces derniers sont associés à la présence de caractéristiques cliniques, histologiques, mais aussi pronostiques. Ils sont au nombre de cinq, les anti-Mi2, anti-Tif1-γ, anti-NXP2, anti-MDA5 et anti-SAE. Les anti-Mi2 sont associées à une forme clinique cutanée classique, une atteinte musculaire souvent sévère au diagnostic et une bonne évolution sous traitement. Les deux suivants, fréquents chez l'enfant et l'adulte, sont associés à des formes récidivantes cutanées et sont fortement associés aux cancers chez l'adulte. Les anti-MDA5 sont les anticorps associés aux formes les plus systémiques avec une atteinte pulmonaire interstitielle rapidement progressive pouvant être très grave. Enfin, les anti-SAE n'ont été décrits que chez l'adulte, avec une atteinte classique.

More prominent muscle involvement in patients with dermatomyositis with anti-Mi2 autoantibodies.

OBJECTIVE: To define the clinical phenotype of dermatomyositis (DM) with anti-Mi2 autoantibodies. METHODS: In this longitudinal cohort study, the prevalence and severity of clinical features at disease onset and during follow-up in patients with anti-Mi2-positive DM were compared to patients with anti-Mi2-negative DM, antisynthetase syndrome (AS), and immune-mediated necrotizing myopathy (IMNM). Longitudinal anti-Mi2 autoantibody titers were assessed. RESULTS: A total of 58 patients with anti-Mi2-positive DM, 143 patients with anti-Mi2-negative DM, 162 patients with AS, and 170 patients with IMNM were included. Among patients with anti-Mi2-positive DM, muscle weakness was present in 60% at disease onset and occurred in 98% during longitudinal follow-up; fewer patients with anti-Mi2-negative DM developed weakness (85%; p = 0.008). Patients with anti-Mi2-positive DM were weaker and had higher creatine kinase (CK) levels than patients with anti-Mi2-negative DM or patients with AS. Muscle biopsies from patients with anti-Mi2-positive DM had prominent necrosis. Anti-Mi2 autoantibody levels correlated with CK levels and strength (p < 0.001). With treatment, most patients with anti-Mi2-positive DM had improved strength and CK levels; among 10 with multiple serum samples collected over 4 or more years, anti-Mi2 autoantibody titers declined in all and normalized in 3, 2 of whom stopped immunosuppressant treatment and never relapsed. Patients with anti-Mi2-positive DM had less calcinosis (9% vs 28%; p = 0.003), interstitial lung disease (5% vs 16%; p = 0.04), and fever (7% vs 21%; p = 0.02) than did patients with anti-Mi2-negative DM. CONCLUSIONS: Patients with anti-Mi2-positive DM have more severe muscle disease than patients with anti-Mi2-negative DM or patients with AS. Anti-Mi2 autoantibody levels correlate with disease severity and may normalize in patients who enter remission.

Clinical significance of autoantibodies in dermatomyositis and systemic sclerosis.

Autoimmune connective tissue diseases, including dermatomyositis and systemic sclerosis, have a heterogeneous clinical presentation and prognosis; moreover, their clinical features are often incomplete and overlap with other rheumatic disorders, which can make diagnosis and prognostic stratification challenging. Specific autoantibodies have been associated with certain clinical findings as well as prognostic implications, and many new associations have been made over the last decade. Although patient populations manifest considerable heterogeneity, autoantibodies can be used to help predict clinical features, prognosis, and response to therapy. In this review, the clinical and prognostic implications associated with disease-specific autoantibodies in dermatomyositis and scleroderma are summarized with an emphasis on how the clinician can use this information for patient care.

[Dermatomyositis and Autoantibodies].

Recent studies have identified novel dermatomyositis-specific autoantibodies and revealed that disease-specific autoantibodies become positive at a high rate in this disease. Moreover, these autoantibodies have been demonstrated to show a strong correlation with distinct clinical manifestations and complications such as interstitial lung disease and malignancy. Thus, these autoantibodies are now recognized as useful tools to classify this varied disease into more homogeneous subsets. In this review, the clinical significance of five dermatomyositis-specific autoantibodies, anti-Mi-2, anti-MDA5, anti-TIF1, anti-NXP2, and anti-SAE, was described.

Dermatomyositis Clinical and Pathological Phenotypes Associated with Myositis-Specific Autoantibodies.

PURPOSE OF REVIEW: Dermatomyositis is an idiopathic inflammatory myopathy with a variety of systemic and cutaneous manifestations. The myositis-specific autoantibodies (MSAs) are associated with phenotypic features and provide a tool for sub-classification of dermatomyositis patients. This review focuses on recent work characterizing the clinical features that accompany the MSAs in dermatomyositis. RECENT FINDINGS: There is increasing recognition of the distinct clinical and pathological phenotypes associated with each MSA. Most of these features display considerable overlap between MSA groups. Despite this, there are notable differences between the typical combinations of cutaneous and systemic manifestations, response to therapy, prognosis, and disease sequelae that define each dermatomyositis MSA group. The MSAs may ultimately improve diagnosis and sub-classification of dermatomyositis patients. However, more work is needed to understand the pathologic basis for much of the heterogeneity found within these subgroups.

Autoantibodies in children with juvenile dermatomyositis: A single centre experience from North-West India.

The objective of this study is to determine autoantibody profile in children with juvenile dermatomyositis (JDM). Children who were diagnosed with JDM (either recently diagnosed during the study period or follow-up patients) were included in the study. Autoantibodies were detected with commercially available Immunodot kit. Thirty patients were included in the study. Nine out of thirty patients (30%) were positive for one of the 12 autoantibodies tested. Anti-SRP antibody was most common antibody detected in 3 patients followed by anti-MDA-5 antibody in 2 patients; while anti-Jo1 antibody, anti-TIF1-γ antibody, anti-Mi-2 antibody, and anti-PM-Scl antibody were positive in 1 patient each. A different disease phenotype was observed with each autoantibody. The patient with anti-Jo1 antibody had a severe systemic disease in the form of interstitial lung disease; patients with anti-MDA-5 antibody and anti-Mi2 antibody had more severe skin disease with mild muscle disease and patients with anti-SRP antibody had significant skin and muscle disease. Anti-TIF1-γ and anti-PM-Scl antibodies were seen in patients with features of overlap syndrome (myositis-scleroderma). Estimation of autoantibodies may serve as an adjunct tool in delineating and defining distinct clinical phenotypes in children diagnosed with juvenile dermatomyositis. They may also help in prognostication.

Specific autoantibodies in dermatomyositis: a helpful tool to classify different clinical subsets.

Autoantibodies are important in the diagnosis of dermatomyositis. They can be divided in two different groups: myositis-associated autoantibodies (MAA) prevailing in overlap syndromes, and myositis-specific autoantibodies (MSA), with diagnostic specificity exceeding 90%. Our purpose was to detect retrospectively the prevalence of the most common MSAs in a group of 19 adult DM patients (13 women, 6 men). A severe DM (SDM), with extensive cutaneous and muscular manifestations, dysphagia, and sometimes pneumopathy, was detected in ten cases. Three patients had a mild DM (MDM), with little muscle and skin impairment, and a short course. Four patients suffered from amyopathic DM (ADM), two from paraneoplastic DM (PDM). Each serum was tested for ANA, ENA, MAAs, MSAs. Myositis-specific autoantibodies were detected in 15 cases. The most frequent was anti-TIF1γ, associated with SDM or PDM in four out of seven cases. Anti-MDA5 antibodies were recorded in a SDM and in a ADM with lung fibrosis. Anti-Mi2 and anti-SRP antibodies were both detected in a MDM and in a SDM, whereas anti-SAE1 in a amyopathic form. Other antibodies (anti-NXP2, -Jo1, -PL7, -PL12, -OJ) were found in single patients with SDM. Our series confirmed that specific autoantibodies could be helpful to classify different clinical subsets, particularly in the case of paraneoplastic forms or association with pneumopathy. Moreover, they can help in predicting the disease evolution and influence therapeutic strategies. A greater number of cases should be useful to highlight the clinical and pathogenic role of these antibodies, and develop a homogeneous protocol for diagnosis and treatment.

Ikaros fingers on lymphocyte differentiation.

The Ikaros family of DNA-binding proteins are critical regulators of lymphocyte differentiation. In multipotent, hematopoietic progenitors, Ikaros supports transcriptional priming of genes promoting lymphocyte differentiation. Ikaros targets the Nucleosome Remodeling Deacetylase (NuRD) complex to lymphoid lineage genes, thereby increasing chromatin accessibility and transcriptional priming. After lymphoid lineage specification, Ikaros expression is raised to levels characteristic of intermediate B cell and T cell precursors, which is necessary to support maturation and prevent leukemogenesis. Loss of Ikaros in T cell precursors allows the NuRD complex to repress lymphocyte genes and extends its targeting to genes that support growth and proliferation, causing their activation and triggering a cascade of events that leads to leukemogenesis. Loss of Ikaros in B cell precursors blocks differentiation and perpetuates stromal adhesion by enhancing integrin signaling. The combination of integrin and cytokine signaling in Ikaros-deficient pre-B cells promotes their survival and self-renewal. The stages of lymphocyte differentiation that are highly dependent on Ikaros are underscored by changes in Ikaros transcription, supported by a complex network of stage-specific regulatory networks that converge upon the Ikzf1 locus. It is increasingly apparent that understanding the regulatory networks that operate upstream and downstream of Ikaros is critical not only for our understanding of normal lymphopoiesis, but also in placing the right finger on the mechanisms that support hematopoietic malignancies in mouse and human.

Autoantibodies in polymyositis and dermatomyositis.

Inflammatory myopathies are a group of acquired diseases, characterized by immunoflogistic processes primarily involving the skeletal muscle. According to recent classification criteria, four major diseases have been identified: polymyositis (PM), dermatomyositis (DM), sporadic inclusion body myositis (IBM), and necrotizing autoimmune myositis (NAM). Autoantibodies can be found in the sera of most patients with myositis. Myositis-specific autoantibodies (MSAs) are markers of very specific disease entities within the spectrum of myositis, and target proteins involved in key processes of protein synthesis. Myositis autoantigens comprise the well-defined aminoacyl-tRNA synthetases, the Mi-2 helicase/histone deacetylase protein complex, and the signal recognition particle (SRP) ribonucleoprotein, together with novel targets such as TIF1-γ, MDA5, NXP2, SAE, and HMGCR. Recent studies suggest that autoantigens drive a B cell antigen-specific immune response in muscles. Interestingly, an increased expression of Jo-1 and Mi-2 in regenerating fibers in muscle biopsies from PM and DM patients compared to normal was demonstrated. Myositis autoantigen up-regulation was observed in neoplastic tissues, thus representing a potential link between cancer and autoimmunity in myositis. Non-immunological mechanisms seem to participate to the pathogenesis of inflammatory myopathies; induction of endoplasmic reticulum stress response in response to abnormal muscle regeneration and inflammation has recently been reported in patients with myositis. This review article provides an update of new emerging insights about the clinical and pathophysiologic role of principal autoantibodies in myositis.

Myositis autoantibodies.

PURPOSE OF REVIEW: To review recent advances in our understanding of autoantibodies associated with dermatomyositis and the autoimmune necrotizing myopathies. RECENT FINDINGS: Autoantibodies preferentially associated with dermatomyositis include those recognizing Mi-2, MDA5, TIF1γ, and NXP-2. Each of these is associated with a distinct clinical phenotype. Autoantibodies found in patients with autoimmune necrotizing myopathies recognize signal recognition particle and 3-hydroxy-3-methylglutaryl-coenzime A reductase (HMG-CoA) reductase. The latter are found in patients with statin-associated autoimmune muscle disease. SUMMARY: As these are helpful both diagnostically and prognostically, a rheumatologist should be familiar with autoantibodies found in patients with dermatomyositis and the autoimmune necrotizing myopathies.

[How can we diagnose and better understand inflammatory myopathies? The usefulness of auto-antibodies]. / Comment faire le diagnostic et mieux comprendre les myopathies inflammatoires? L'utilité des auto-anticorps.

The inflammatory myopathies are a group of quite proteiform, systemic auto-immune diseases which include polymyositis, dermatomyositis and inclusion body myopathies. To facilitate the diagnosis, classification criteria (Bohan and Peter, 1975) have been proposed, based essentially on clinical criteria. In addition, over the past fifteen years, auto-antibodies characterizing certain forms of inflammatory myopathy have been identified. One distinguishes schematically: auto-antibodies specific for myositis and auto-antibodies sometimes associated with myositis. Concerning the myositis specific auto-antibodies (MSA), schematically there are a dozen specificities which are classed according to the cellular distribution of the auto-antigen. The most characteristic are certainly the auto-antibodies directed against cytoplasmic antigens: the anti-tRNA synthetases (anti-Jo-1 (PL-1), anti-PL-7, PL-12, EJ, OJ, JS, KS, ZO, YRS), anti-SRP (signal recognition particle), anti-Mas and anti-KJ, anti-Fer (eEF1), anti-Wa and anti-CADM p140. Other auto-antibodies are directed against nuclear auto-antigens: the anti-Mi-2, anti-PMS (PMS1, PMS2) and related antibodies (MLH1, DNA PKcs…), anti-56 kDa, anti-MJ (NXP-2), anti-SAE and anti-p155/p140 (TIF-1γ). Concerning the auto-antibodies sometimes associated with myositis (myositis associated auto-antibodies or MAA), they can also be observed in other auto-immune diseases. These auto-antibodies are directed against nuclear or nucleolar auto-antigens: the anti-PM-Scl, anti-Ku, anti-RNP (U1 RNP and U2 RNP, U4/U6 RNP and U5 RNP), anti-Ro 52 kDa and more rarely, anti-Ro 60 kDa and anti-La. The auto-antibodies related to myositis are biological tools which are of interest in two main ways. They have allowed us to sort out the nosology of these inflammatory myopathies, in particular by defining anti-tRNA synthetase syndrome. It now remains to determine how they might be employed to complement the classical clinico-biological diagnostic criteria. In this perspective, it will be indispensable first of all to diffuse and standardize the methods of detection. The latter are at the moment very heterogeneous as they use techniques and above all antigenic preparations which are extremely diverse. These antibodies are also very interesting "physiopathological" tools to try to better understand myositis. The example of anti-tRNA synthetases is a particularly original model of auto-immunization, which allows one to establish a link between an initial, probably poorly specific muscular lesion and the appearance of auto-antibodies which maintain and aggravate the muscular disease.

Expression of the dermatomyositis autoantigen Mi-2 in regenerating muscle.

OBJECTIVE: Autoantibodies against the chromatin remodeler Mi-2 are found in a distinct subset of patients with dermatomyositis (DM). Previous quantitative immunoblotting experiments demonstrated that Mi-2 protein levels are up-regulated in DM muscle. This study was undertaken to define the population of cells expressing high levels of Mi-2 in DM muscle and to explore the regulation and functional role of Mi-2 during muscle regeneration. METHODS: The expression of Mi-2 was analyzed by immunofluorescence microscopy in human muscle biopsy specimens. In an experimental mouse model, cardiotoxin was used to induce muscle injury and repair, and expression of Mi-2 during muscle regeneration was studied in this model by immunofluorescence and immunoblotting analyses. In addition, a cell culture system of muscle differentiation was utilized to artificially modulate Mi-2 levels during proliferation and differentiation of myoblasts. RESULTS: In human DM muscle tissue, increased Mi-2 expression was found preferentially in the myofibers within fascicles affected by perifascicular atrophy, particularly in the centralized nuclei of small perifascicular muscle fibers expressing markers of regeneration. In injured mouse muscle tissue, Mi-2 levels were dramatically and persistently up-regulated during muscle regeneration in vivo. Premature silencing of Mi-2 with RNA interference in vitro resulted in accelerated myoblast differentiation. CONCLUSION: Expression of Mi-2 is markedly up-regulated during muscle regeneration in a mouse model of muscle injury and repair. It is also up-regulated in human DM myofibers expressing markers of regeneration. Results of the in vitro studies indicate that this protein may play a role in modulating the kinetics of myoblast differentiation. Our findings thus suggest that high levels of Mi-2 expression in muscle biopsy tissue from patients with DM reflect the presence of incompletely differentiated muscle cells.