Misfolded α-synuclein is a pathological hallmark of Parkinson’s disease (PD). Killinger et al. now report that the human appendix contains an abundance of misfolded α-synuclein and that removal of the appendix decreased the risk of developing PD. The appendix of both PD cases and healthy individuals contained abnormally cleaved and aggregated forms of α-synuclein, analogous to those found in postmortem brain tissue from patients with PD. Furthermore, α-synuclein derived from the appendix seeded rapid aggregation of recombinant α-synuclein in vitro. In two large-scale epidemiological studies, the authors demonstrated that an appendectomy occurring decades prior reduced the risk of developing PD, suggesting that the appendix may be implicated in PD initiation. The pathogenesis of Parkinson’s disease (PD) involves the accumulation of aggregated α-synuclein, which has been suggested to begin in the gastrointestinal tract. Here, we determined the capacity of the appendix to modify PD risk and influence pathogenesis. In two independent epidemiological datasets, involving more than 1.6 million individuals and over 91 million person-years, we observed that removal of the appendix decades before PD onset was associated with a lower risk for PD, particularly for individuals living in rural areas, and delayed the age of PD onset. We also found that the healthy human appendix contained intraneuronal α-synuclein aggregates and an abundance of PD pathology–associated α-synuclein truncation products that are known to accumulate in Lewy bodies, the pathological hallmark of PD. Lysates of human appendix tissue induced the rapid cleavage and oligomerization of full-length recombinant α-synuclein. Together, we propose that the normal human appendix contains pathogenic forms of α-synuclein that affect the risk of developing PD. INTRODUCTION Parkinson’s disease (PD) is clinically diagnosed on the basis of motor symptoms that result from the progressive loss of midbrain dopaminergic neurons of the substantia nigra . However, it is now recognized that PD is a multisystem disorder involving both motor and nonmotor features, involving the central and peripheral nervous system. Gastrointestinal (GI) dysfunction is a common nonmotor symptom of PD , often preceding the onset of motor symptoms by as many as 20 years. PD pathology, consisting of aggregated α-synuclein, has been detected in enteric neurons of the GI tract in patients with PD. α-Synuclein aggregates in enteric neurons can occur early in the disease, before motor symptom onset, during the prodromal phase of PD. Pathogenic accumulation of α-synuclein in the GI tract not only may contribute to the nonmotor symptoms of PD but also has been hypothesized to contribute to PD pathology in the brain . Experimental studies have shown that misfolded α-synuclein can spread in a prion-like fashion from cell-to-cell, triggering the formation of Lewy-like aggregates in neurons. A truncal vagotomy, in which the vagal nerve connecting the GI tract to the brain is severed, has been associated with lower PD risk in some, but not all, epidemiological studies. Because of the early appearance of α-synuclein aggregates in the GI tract of patients with PD and their capacity to ascend the vagal nerve to the brain it has been suggested that the GI tract could be the origin of PD pathology . Aberrant accumulation of α-synuclein in the GI tract occurs in response to toxins and bacteria that activate the immune system. This may signify that GI tract regions with regular interactions with environmental pathogens and enhanced immunosurveillance have a greater risk of developing α-synuclein abnormalities involved in PD. In agreement with this hypothesis, the appendix was recently shown to contain an abundance of α-synuclein in prodromal and clinical PD cases, as well as in neurologically intact individuals. Although the appendix is often considered to be a vestigial organ, its mucosa is rich in immune cells, and a primary function of the appendix is to assist the lymphatic system in the detection and removal of pathogens as well as to regulate intestinal bacterial composition. Consequently, the appendix may be prone to accumulating α-synuclein pathology that affects PD risk, although this has yet to be investigated in detail.

Pathogenic aggregation of α-synuclein is thought to involve the formation of kinetically stable fibrils of β sheet structure that can seed further aggregation. The aggregation of α-synuclein is a nucleation-dependent process that requires amino acids 61 to 95, referred to as the nonamyloid β component (NAC) . Truncated α-synuclein at the C terminus proximal to the NAC domain enhances nucleation-dependent aggregation . Truncated α-synuclein proteoforms are found in Lewy bodies and are enriched in the PD brain. Immune tissues, like the appendix, contain proteases capable of cleaving the C terminus of α-synuclein. However, it is unknown whether GI tract lymphoid tissues, like the appendix, have an enhanced capacity to generate truncated α-synuclein amylogenic seeds relevant to PD. In this study, we investigated the role of the vermiform appendix in PD. We analyzed two independent, yet complementary, epidemiological datasets: the nationwide Swedish National Patient Registry (SNPR) and the Parkinson’s Progression Markers Initiative (PPMI). The SNPR is unique in its magnitude, registering diagnoses in the entire population, covering over 91 million person-years. The PPMI data contain detailed information about PD diagnosis, age of onset, and other demographic factors, as well as genetic information. Analysis of the SNPR showed that an appendectomy was associated with a lower risk of developing PD. In addition, in both datasets, we found that an appendectomy occurring decades before PD was associated with a delayed age of PD onset/diagnosis. Next, we identified an abundance of aggregated α-synuclein in the mucosa and enteric plexuses of the healthy human appendix. These aggregates were present at all age groups, including young individuals (<20 years of age). Furthermore, the appendix contained soluble oligomeric α-synuclein and truncated forms of α-synuclein that are prone to rapid aggregation. Truncated α-synuclein was present in the healthy appendix but was more abundant in the appendix of patients with PD. Thus, our data suggest that the appendix may play a role in PD. The presence of pathogenic α-synuclein species in the appendix indicates a mechanism by which the appendix may contribute to, and possibly trigger, the development of PD. RESULTS Appendectomy is associated with a lower risk of PD We sought to understand the contribution of the GI tract lymphoid tissue and enteric neurons in the vermiform appendix to initiation and progression of PD. We first investigated whether removal of the appendix reduces the risk for PD in a large population. Our study examined the health records of a total of 1,698,000 individuals followed up to 52 years (91,888,589 person-years) to determine PD risk among individuals with and without a previous appendectomy, using data obtained from the SNPR and Statistics Sweden Тhe patient registry contains medical diagnoses and surgical codes for the Swedish population starting in 1964, which allowed for this long observation period. We identified a total of 551,647 individuals who had an appendectomy. Each of these was matched to two control individuals without an appendectomy (matched on the basis of birth year, sex, and geographical location). In the follow-up, we identified patients with PD (2252 in total) from the first hospitalization discharge or outpatient summary, excluding individuals with secondary parkinsonism (i.e., related to neuroleptic medications for schizophrenia) and diseases that might cause secondary parkinsonism, such as neurosyphilis. In individuals who had undergone an appendectomy, the overall risk for PD, as well as the age of PD diagnosis, was affected. First, the incidence of PD was lower in the appendectomy group compared to control individuals who did not have an appendectomy. PD incidence was 1.60 per 100,000 person-years among individuals who had an appendectomy [95% confidence interval (CI), 1.46 to 1.75] compared to 1.98 for controls (95% CI, 1.87 to 2.10;). This signifies that the overall risk of developing PD was significantly decreased by 19.3% in appendectomized individuals compared to the general population (95% CI, 10.4 to 27.2%; P < 10−4; ). Second, the cumulative prevalence of PD (total proportion of cases) in people who had an appendectomy was also lower by 16.9% relative to the general population (95% CI, 9.3 to 24.4%; P < 10−4; ). In total, PD was diagnosed in 1.17 out of every 1000 people who had an appendectomy (95% CI, 1.08 to 1.27) compared to 1.4 per 1000 in the general population (95% CI, 1.34 to 1.47). Third, we examined, among PD cases, whether a history of appendectomy was associated with a delayed age at first recorded diagnosis of PD. The age of PD diagnosis was, on average, 1.6 years later in individuals who had an appendectomy occurring 20 or more years prior than in cases without an appendectomy (95% CI, −0.09 to 3.34; P < 0.03;)

We also observed a significant delay in age of PD onset in individuals with an appendectomy 30 or more years prior (P < 0.006), but a limited size in this longer latency group precluded further analysis.