rs1545843 — SLC6A15 SLC6A15 depression risk variant

SLC6A15: The Hippocampal Amino Acid Transporter Linked to Depression Vulnerability

SLC6A15¹¹ SLC6A15
Solute Carrier Family 6 Member 15, a neuronal transporter in the same family as the serotonin and dopamine transporters is a transporter expressed almost exclusively in neurons that shuttles neutral branched-chain amino acids²² branched-chain amino acids
Including proline, leucine, isoleucine, and valine — several of which are used as precursors for glutamate, the brain's main excitatory neurotransmitter into brain cells. These amino acids serve as a substrate pool for glutamate synthesis, making SLC6A15 a key regulator of excitatory neurotransmission in the hippocampus — a brain region central to stress responses, memory, and the pathophysiology of depression.

Rs1545843 is an intronic variant in the SLC6A15 gene on chromosome 12q21.31. Carriers of the AA genotype express significantly less full-length SLC6A15 mRNA in hippocampal tissue, a finding that converges with animal data showing that stress-susceptible mice have 2.1-fold lower SLC6A15 expression in the hippocampus compared to resilient animals. This places rs1545843 at the intersection of genetic susceptibility and stress biology.

The Mechanism

Rs1545843 lies within an intron of SLC6A15 and affects pre-mRNA processing or transcriptional regulation rather than changing the protein sequence directly. The AA genotype is associated with reduced hippocampal SLC6A15 expression³³ reduced hippocampal SLC6A15 expression
Postmortem hippocampal tissue from AA carriers shows significantly lower full-length SLC6A15 mRNA than G-allele carriers, with an additive dose-response across GG, AG, and AA genotypes. When SLC6A15 is downregulated, neurons import fewer branched-chain amino acids, which reduces the availability of glutamate precursors in the hippocampus.

The downstream consequences appear to affect hippocampal circuit integrity. Brain imaging studies have found that AA carriers show reduced hippocampal volume and altered white matter microstructure⁴⁴ reduced hippocampal volume and altered white matter microstructure
Specifically, reduced fractional anisotropy in the left parahippocampal cingulum, a white matter tract connecting hippocampus to the cingulate cortex in the cortico-limbic network. The hippocampus also provides feedback inhibition of the HPA (hypothalamic-pituitary-adrenal) axis — the body's stress hormone system. When hippocampal function is compromised, cortisol levels can become dysregulated, creating a cycle linking genetic predisposition, stress sensitivity, and depression risk.

Interestingly, SLC6A15 knockout studies⁵⁵ SLC6A15 knockout studies
Loss of SLC6A15 protein in primary hippocampal neurons increases mitochondrial activity and neurite outgrowth suggest that SLC6A15 deficiency triggers compensatory metabolic responses in neurons — evidence that the brain attempts to adapt, though whether this compensation is sufficient under chronic stress conditions remains an open question.

The Evidence

The foundational study by Kohli et al. 2011⁶⁶ Kohli et al. 2011
"The neuronal transporter gene SLC6A15 confers risk to major depression." Neuron 2011 performed a genome-wide association study across 7 independent cohorts totaling approximately 3,800 depression cases and 10,600 controls, achieving genome-wide significance at rs1545843 (meta-analysis p = 1.41×10⁻⁹). This was one of the first depression GWAS hits to reach this threshold and is backed by convergent functional evidence.

The HPA-axis connection was established by Schuhmacher et al. 2013⁷⁷ Schuhmacher et al. 2013
"A variant of the neuronal amino acid transporter SLC6A15 is associated with ACTH and cortisol responses and cognitive performance in unipolar depression." IJNP 2013 in 248 depressed patients and 172 healthy controls. AA-genotype depressed patients showed significantly enhanced maximum ACTH and cortisol responses during a dexamethasone/CRH challenge (p = 0.03), along with impaired memory and sustained attention (p = 0.04). Crucially, these effects were seen only in patients, not in healthy controls — suggesting the variant amplifies the neurobiological consequences of depression rather than causing them in isolation.

Neuroimaging studies have documented structural brain changes associated with the risk allele. Choi et al. 2016⁸⁸ Choi et al. 2016
"Effects of a Polymorphism of the Neuronal Amino Acid Transporter SLC6A15 Gene on Structural Integrity of White Matter Tracts in Major Depressive Disorder." PLoS One 2016 (n=86 MDD + 64 controls) found that MDD patients carrying the A allele had significantly reduced white matter integrity in the left parahippocampal cingulum (p = 0.012), while GG homozygotes showed no significant difference between MDD and healthy controls. Wang et al. 2017⁹⁹ Wang et al. 2017
"A Combined Study of SLC6A15 Gene Polymorphism and Resting-State fMRI in First-Episode Drug-Naive Major Depressive Disorder." Genet Test Mol Biomarkers 2017 found that A-allele MDD patients also showed decreased regional brain activity in the medial cingulum, correlating with depression severity.

Practical Actions

The clearest implication of this variant is heightened vulnerability to HPA axis dysregulation under chronic stress. For AA carriers with a personal or family history of depression, early recognition of stress-related mood changes and proactive HPA-targeted interventions are especially relevant.

Phosphatidylserine is the best-studied supplement for blunting excessive cortisol responses to psychological stress. It works at the level of the HPA axis and has been shown in randomized trials to attenuate ACTH and cortisol surges. Given the documented HPA hyperreactivity in AA carriers with depression, this represents a mechanism-specific option.

Because SLC6A15 transports branched-chain amino acids that feed into hippocampal glutamate synthesis, ensuring adequate availability of these amino acids may partially compensate for reduced transporter activity. Leucine, isoleucine, and valine — collectively the BCAAs — are abundant in high-protein foods and also available as supplements.

Interactions

The most clinically relevant interaction involves BDNF rs6265 (Val66Met). BDNF is a neurotrophin that supports hippocampal neuron survival and neuroplasticity — the same hippocampal circuits affected by SLC6A15 downregulation. Carrying both the SLC6A15 AA genotype and the BDNF Met allele (rs6265) may compound hippocampal vulnerability, as both reduce hippocampal neurotrophic support from different angles.

COMT rs4680 (Val158Met) affects dopamine degradation in the prefrontal cortex and influences stress resilience through catecholaminergic pathways. Met/Met carriers have higher prefrontal dopamine but impaired stress-related dopamine mobilization. The combination of SLC6A15 AA and COMT Met/Met may affect hippocampal-prefrontal connectivity relevant to cognitive control under stress.

Serotonergic variants — including 5-HTTLPR/rs25531 (SLC6A4) and TPH2 rs4570625 — interact with HPA axis function and hippocampal plasticity through converging mechanisms. Accumulation of risk variants across these systems is thought to explain why most GWAS hits have modest individual effect sizes but considerable combined impact in polygenic risk models for depression.