Abstract
Ubiquitin plays a vital role in neuronal proteostasis, as a major but often overlooked component of neurotoxic protein aggregates across neurodegenerative diseases. Although neuropathological changes can be present for years before clinical onset, early and accurate diagnosis remains an immense challenge in this disease category. The level of ubiquitin in cerebrospinal fluid (CSF) has been assessed as a biomarker for several disease entities. This systematic review compares current findings and evaluates the potential of CSF ubiquitin as a fluid biomarker. A systematic literature search identified studies comparing CSF ubiquitin levels between a control group and patients with one of the following diseases: Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD), Lewy body dementia (DLB), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS). All included studies were reviewed systematically by two independent authors. 171 studies were identified. A total of 17 studies met the eligibility criteria and were included. Nine out of 13 studies found a significant increase of CSF ubiquitin in AD patients compared with control groups. Correlations between CSF ubiquitin and other established biomarkers were demonstrated in seven studies. A single study was included for both HD and DLB respectively, each showing significantly higher CSF ubiquitin in patients compared to controls. In patients with PD, FTD or ALS, CSF ubiquitin levels were generally equal to those of the control groups, with two studies showing significantly decreased concentrations in a PD and an FTD cohort. Presently, the available body of research is insufficient to assess whether CSF ubiquitin could contribute to the clinical setting, alongside established markers of neurodegeneration. The correlation of elevated CSF ubiquitin with AD is well-founded, whilst validation of reduced or unchanged levels in the other neurodegenerative diseases will determine the usefulness of the biomarker in clinical practice.