rs3756059 — SNCA

SNCA rs3756059 — The RBD Gateway Variant: Alpha-Synuclein's Earliest Warning Signal

REM sleep behavior disorder (RBD)¹¹ REM sleep behavior disorder (RBD)
A parasomnia in which the normal muscle paralysis during REM sleep fails, allowing people to physically act out their dreams — punching, kicking, or shouting while dreaming is not merely a sleep nuisance. It is the strongest known prodromal marker of synucleinopathy: roughly 80% of people with isolated (idiopathic) RBD eventually develop Parkinson's disease, dementia with Lewy bodies (DLB), or multiple system atrophy (MSA) — conditions that collectively define the synucleinopathy spectrum. The genetic variant rs3756059, sitting in an intronic region of the SNCA gene²² SNCA gene
Alpha-synuclein (SNCA) encodes the protein that forms Lewy bodies — the pathological hallmark of every synucleinopathy, is the most robustly replicated common genetic signal for RBD identified to date.

The A allele at rs3756059 was identified in the largest genome-wide association study of RBD ever conducted³³ the largest genome-wide association study of RBD ever conducted
Krohn et al. Genome-wide association study of REM sleep behavior disorder identifies polygenic risk and brain expression effects. Nature Communications, 2022 — a meta-analysis pooling 2,843 RBD cases and 139,636 controls across international cohorts. The odds ratio for the A allele is 1.26 (95% CI 1.19–1.33, p=3×10⁻¹⁶), a genome-wide significant association that robustly survives correction for multiple testing.

The Mechanism

rs3756059 is an intronic variant that does not change the alpha-synuclein protein sequence, but its location in the 5′ regulatory region of SNCA places it in a biologically active zone. The 2022 Krohn GWAS demonstrated that the RBD risk signal at this locus correlates most strongly with decreased expression of SNCA-AS1 — the long non-coding antisense RNA transcribed from the opposite strand of SNCA — in cerebellar tissue (p=9.9×10⁻¹⁹). SNCA-AS1 is thought to act as a natural suppressor of alpha-synuclein transcription; reduced SNCA-AS1 expression could therefore permit higher alpha-synuclein output, increasing the burden of protein available to misfold and seed Lewy body pathology.

A critical finding from fine-mapping studies⁴⁴ fine-mapping studies
Krohn et al. Fine-Mapping of SNCA in Rapid Eye Movement Sleep Behavior Disorder and Overt Synucleinopathies. Annals of Neurology, 2020 is that the RBD risk signal at the 5′ end of SNCA is genetically distinct from the classical Parkinson's disease risk signal, which maps to the 3′ end (rs356182, rs356219). Carriers of the 5′ RBD allele do not necessarily carry the 3′ PD alleles, and the two signals appear to influence different facets of SNCA biology: the 5′ signal preferentially associates with RBD and DLB, while the 3′ signal preferentially associates with classical PD. This dissociation provides molecular support for the clinical observation that RBD-to-PD conversion produces a distinct disease phenotype (DLB and PD with early autonomic and cognitive features) compared to classical PD.

The Evidence

The Krohn 2022 GWAS⁵⁵ Krohn 2022 GWAS
Krohn et al. Genome-wide association study of REM sleep behavior disorder identifies polygenic risk and brain expression effects. Nature Communications, 2022 represents a major advance in RBD genetics. Five independent loci reached genome-wide significance: SNCA, GBA, TMEM175, INPP5F, and SCARB2 — notably, GBA and TMEM175 are the two most established Parkinson's disease risk genes beyond SNCA, reinforcing that RBD sits on the same genetic risk spectrum. The SNCA signal was the strongest of the five, consistent with alpha-synuclein's central role in Lewy body formation. The effect size (OR=1.26) is modest by Mendelian standards but large by GWAS standards for a complex trait, and the p-value (3×10⁻¹⁶) far exceeds the genome-wide significance threshold.

The 2020 fine-mapping paper established that the 5′ RBD signal at SNCA is shared with dementia with Lewy bodies⁶⁶ dementia with Lewy bodies
DLB is the second most common dementia after Alzheimer's, defined by alpha-synuclein Lewy body deposits in cortical and limbic neurons but directionally opposite to some Parkinson's disease signals — providing a molecular basis for distinguishing RBD-associated synucleinopathies from classic PD at the genetic level.

Practical Actions

The clinical relevance of rs3756059 is its function as an early warning signal. RBD typically precedes overt synucleinopathy by a decade or more. For individuals who carry the A allele and also have clinical features suggestive of RBD (acting out dreams, vivid nightmares, falling out of bed), this variant places them in an elevated-risk category where early specialist evaluation can enable participation in prodromal cohort studies and access to emerging neuroprotective interventions as they become available.

The neuroprotective strategies supported by the alpha-synuclein literature — targeting the mitochondrial complex I dysfunction and oxidative stress that alpha-synuclein aggregation drives — are the same whether the entry point is PD or RBD.

Interactions

rs3756059 is genetically independent of the 3′ SNCA PD signals rs356182 and rs356219 and of the intron 4 variant rs2736990. Carrying risk alleles at multiple SNCA loci — particularly a 5′ RBD signal combined with a 3′ PD signal — likely confers additive risk of synucleinopathy, though no published study has directly quantified the combined OR for rs3756059 in combination with the other SNCA variants. The functional basis for their additivity would be multiple, independent perturbations to SNCA expression regulation — each pushing alpha-synuclein levels higher from a different regulatory region.

The co-identification of GBA and TMEM175 loci in the same GWAS is also significant: GBA encodes glucocerebrosidase, whose loss of function impairs lysosomal clearance of alpha-synuclein aggregates. Individuals who carry risk variants at both rs3756059 (more alpha-synuclein production) and GBA (impaired alpha-synuclein clearance) face combined perturbations to the synthesis-clearance balance that governs Lewy body formation.