Objective: To investigate serum biomarkers of progression in inactive primary progressive multiple sclerosis (PPMS).

Methods: We measured protein biomarkers (growth differentiation factor-15 (GDF-15), dickkopf-1 (DKK-1), neuron specific enolase (NSE) and cathepsin-D) in serum samples from 39 patients with inactive PPMS included in a clinical trial enrolling people with PPMS (clinicaltrials.gov identifier NCT02913157) and investigated the association of these biomarker levels with clinical disability at baseline and during follow-up. We then performed a meta-analysis of publicly available transcriptomic datasets to investigate the gene expression of these biomarkers in the CNS in progressive MS.

Results: When compared with healthy controls, people with PPMS had higher serum levels of GDF-15, DKK-1 and cathepsin-D at baseline. These findings match those in our meta-analysis which found increased expression of GDF-15 and cathepsin-D in the CNS in progressive MS. At baseline, elevated serum DKK-1 was associated with worse Expanded Disability Status Scale (EDSS) and nine-hole peg test (9HPT) scores. None of the other biomarkers levels significantly correlated with EDSS, Timed 25-Foot Walk Test (T25FWT), 9HPT, or cognitive measures. However, serum GDF-15 and cathepsin-D were higher at baseline in participants who developed worsening disability. Our receiver operating characteristic curve showed that higher serum GDF-15 and cathepsin-D at baseline significantly discriminated between participants who worsened in T25FWT and 9HPT and those who remained stable.

Conclusions: Patients with PPMS have altered levels of GDF-15, DKK-1 and cathepsin-D in serum, and GDF-15 and cathepsin-D may have predictive value in progression free of inflammatory activity in PPMS.