Testing a candidate composite serum protein marker of skin severity in systemic sclerosis

Objectives Using an integrated multi-omic analysis, we previously derived a candidate marker that estimates the modified Rodnan Skin Score (mRSS) and thus the severity of skin involvement in SSc. In the present study we explore technical and biological validation of this composite marker in a well-characterized cohort of SSc patients.Methods Cartilage oligomeric matrix protein (COMP), collagen type IV (COL4A1), tenascin-C (TNC) and spondin-1 (SPON1) were examined in serum samples from two independent cohorts of patients with dcSSc. The BIOlogical Phenotyping of diffuse SYstemic sclerosis cohort had previously been used to derive the composite marker and Molecular Determinants to Improve Scleroderma (SSc) treatment (MODERNISE) was a novel validation cohort. Multiple regression analysis derived a formula to predict the mRSS based on serum ELISA protein concentration.Results The serum concentration of two of the proteins-COMP and TNC-positively correlated with the mRSS, particularly in early dcSSc patients. Interpretable data could not be obtained for SPON1 due to technical limitations of the ELISA. COL4A1 showed a correlation with disease duration but not overall mRSS. Patients receiving MMF showed lower serum concentrations of COMP, COL4A1 and TNC and a lower composite biomarker score not established on treatment. A revised ELISA-based three-protein composite formula was derived for future validation studies.Conclusions Although more validation is required, our findings represent a further step towards a composite serum protein assay to assess skin severity in SSc. Future work will establish its utility as a predictive or prognostic biomarker. What does this mean for patients?Systemic sclerosis (SSc), also called scleroderma, is a severe but uncommon autoimmune rheumatic disease. Skin thickening occurs in almost every case and is severe in patients with diffuse disease. Clinical assessment is difficult and so a blood test that reliably assesses the severity of skin thickening would be very helpful for routine care and clinical trials. We have previously shown that four proteins in the blood can be used to estimate the severity of skin thickening. In this study we used an enzyme-linked immunosorbent assay (ELISA) test to directly measure levels of protein in blood samples from patients with diffuse SSc. We found that two proteins (TNC and COMP) remain strongly correlated with skin severity score, while one (COL4A1) showed a more complex relationship and was influenced by disease duration. The fourth protein (SPON1) could not be detected by available ELISA. This work is another step towards developing a robust blood test to assess skin severity. Further work in additional groups of patients is needed to achieve this.