Blood biomarker for Parkinson disease: peptoids

Abstract

Parkinson disease (PD) is the second most common neurodegenerative disease. Because dopaminergic neuronal loss begins years before motor symptoms appear, a biomarker for the early identification of the disease is critical for the study of putative neuroprotective therapies. Brain imaging of the nigrostriatal dopamine system has been used as a biomarker for early disease along with cerebrospinal fluid analysis of α-synuclein, but a less costly and relatively non-invasive biomarker would be optimal. We sought to identify an antibody biomarker in the blood of PD patients using a combinatorial peptoid library approach. We examined serum samples from 75 PD patients, 25 de novo PD patients, and 104 normal control subjects in the NINDS Parkinson's Disease Biomarker Program. We identified a peptoid, PD2, which binds significantly higher levels of IgG3 antibody in PD versus control subjects (P<0.0001) and is 68% accurate in identifying PD. The PD2 peptoid is 84% accurate in identifying de novo PD. Also, IgG3 levels are significantly higher in PD versus control serum (P<0.001). Finally, PD2 levels are positively correlated with the United Parkinson's Disease Rating Scale score (r = 0.457, P<0001), a marker of disease severity. The PD2 peptoid may be useful for the early-stage identification of PD, and serve as an indicator of disease severity. Additional studies are needed to validate this PD biomarker.

Figure 1

Peptoid binding to pooled samples from PD and NC subjects. Each of the three peptoids bound markedly higher levels of IgG in the PD pool versus normal control (NC) pool (n=20 per pool).

Figure 2

PD2 binding to IgG subtypes. Notice that among the four IgG (immunoglobulin G) subtypes the binding is selectively higher for PD (Parkinson's disease) versus normal control (NC) pool primarily for the IgG3 isotype.

Figure 3

PD2 binding to individual PD and De novo patients. Left panel—binding levels are significantly higher in PD (n=75) versus vontrol (n=104) and de novo (n=25) versus control for PD2. Green bar=mean levels. Right panel—ROC curve for PD2 binding to PD versus Control subjects. AUC=0.74, P<0.001. PD, Parkinson's disease; AUC, area under the curve; ROC, receiver operating characteristic.

Figure 4

On-bead magnetic screening. A one-bead one-compound (OBOC) library of thousands of unique peptoid compounds bound to TentaGel beads is incubated with control serum, here serum pooled from normal control subjects. The library is then incubated with anti-human IgG-labeled magnetic nanoparticles so that beads having bound IgG from the serum can be sorted out using a strong magnet. The library is initially depleted of beads that bind IgG from the control serum, and then incubated with target serum, here serum pooled from PD subjects. After incubation with the magnetic nanoparticles again, the newly magnetized beads, called ‘hits’, are isolated. Peptoid compounds are cleaved from each of the ‘hit’ beads and their sequences are assessed by MS/MS. These ‘hit’ compounds are then resynthesized and validated on ELISA plates for their ability to detect target IgG. (Reprinted by permission from Macmillan Publishers: from Zaman et al.). ELISA, enzyme-linked immunosorben assay.