A prospective case–control study was performed in outpatient clinics and inpatient departments of Neurology, Rheumatology and Rehabilitation of our university hospitals. Initially, 73 consecutive dermatomyositis patients were enrolled between January 2021 and January 2023 who fulfilled the 2017 European League Against Rheumatism and American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies [6]. The patients with previous history of diabetes mellitus, malignant tumor, infectious diseases, thyroid disorders, hypertension, ischemic heart disease, cardiac failure, chronic obstructive pulmonary disease, liver and kidney failure, in use of statins were excluded. Thus, only twenty-five out of the initial 73 dermatomyositis patients (group A) were included in the current study along with 25 healthy participants (group B) selected in the same period, and matched for age, gender, ethnicity, weight, height, and ethnicity.

A written informed consent was obtained from the included participants, and the ethics committee on research involving human subjects of our faculty of medicine approved this study.

Each patient was subjected to a complete history taking with special stress on age, age at disease onset, gender, ethnicity, disease duration, and tobacco use. Complete general examination including current waist circumference, weight, height, calculation of body mass index (BMI), and measurement of blood pressure was done for all participants. Neurological clinical examination with special stress on skin manifestations was done.

After 12 h fasting period, 15 ml venous blood sample was drawn in the morning from all participants, immediately (less than 30 min) centrifuged at 3000 rpm for ten minutes at 4 °C, and processed.

Levels of serum fasting insulin and plasma glucose levels were analyzed. We calculated insulin resistance using a homeostasis model assessment of insulin resistance (HOMA-IR) that was measured according to the following formula: fasting plasma glucose (mmol/l) × fasting plasma insulin concentration (μIU/ml)/22.5 and it was defined as a HOMA-IR more than four [7].

We additionally, estimated serum level of myostatin using a commercially available ELISA kit (R and D Systems DGDF eighty, Minneapolis, USA). Serum level of myostatin was estimated fully blinded to clinical or other laboratory data of the participants. We measured its absorbance at 450 nm using a Spark plate reader (Tecan, Switzerland), with an intra-assay coefficient of variation below twelve percent, inter-assay coefficient of variation below fourteen percent, and limit of blank LoB is 0.370 ng/ml according to the manufacturer’s data. Han and colleagues reported that the serum myostatin level was 7 ± 2.5 ng/ml in healthy subjects [8]

Total extraction of RNA from 1 × 106 cells was done following the manufacturer’s instructions using the RNeasy Mini kit (Qiagen). We performed quantitative real-time reverse transcriptase–polymerase chain reaction (RT-PCR) for two IFN-inducible genes: interferon induced protein with tetratricopeptide repeats 1(IFIT1) and Mx1(MxA).

Primers used were as follows: for MxA, F.5′-CGGCTAACGGATAAGCAGAG-3′ and R. 5′-ACCTACAGCTGGCTCCTGAA-3′; for IFIT1, F. 5′-AAAAGCCCACATTTGAGGTG-3′ and R. 5′- GAAATTCCTGAAACCGACCA-3′ (reverse). Reverse transcription of the RNA by QuantiTect Reverse Transcription Kit (Qiagen) was performed. Analysis of IFIT1 and MxA genes expression was done using cDNA products. Quantitative real-time PCR analysis was carried out in which each run consisted of after an initial denaturation step at 95 °C for 5 min, conditions for cycling were 40 cycles of denaturation at 95 °C for 30 s, annealing at 57 °C for thirty seconds, and extension at 72 °C for 1 min. The expressed data were normalized by GAPDH that was used as a housekeeping gene.

All variables including demographic, clinical and laboratory indicator data were expressed as count (%) or mean ± SD. All statistical analyses were done by utilizing the statistical package for the social sciences version 24 which is released in 2016 by the International Business Machines Corporation, USA. We utilized Chi-square test, Fisher's exact test, independent samples t-test, Pearson correlation analysis, Spearman correlation analysis and multivariate regression analysis between serum levels of myostatin and other variables when needed. A value of P was set at less than 0.05 for significant results.