rs391300 — SRR SRR rs391300

The Amino Acid Signal That Shapes Insulin Secretion

Serine racemase (SRR) is an enzyme best known for its role in the brain, where it converts the common amino acid L-serine into D-serine — a mirror-image molecule that acts as a critical co-agonist of NMDA receptors¹¹ NMDA receptors
N-methyl-D-aspartate receptors are ion channels gated by both glutamate and a co-agonist such as glycine or D-serine; without co-agonist binding, glutamate alone cannot open the channel. In 2016, researchers discovered that SRR is also highly expressed in human and mouse pancreatic beta cells — the cells that produce insulin — meaning the same D-serine signaling pathway that modulates synaptic transmission also shapes how your pancreas responds to glucose.

The rs391300 variant is an intronic SNP in SRR. It does not change the SRR protein directly, but it likely alters gene expression or splicing efficiency in a way that affects how much serine racemase your beta cells produce. The C allele (reported as G in older literature using minus-strand notation) was identified as a risk allele for type 2 diabetes in a 2010 genome-wide association study²² 2010 genome-wide association study
Tsai et al. A genome-wide association study identifies susceptibility variants for type 2 diabetes in Han Chinese. PLOS Genetics, 2010 of Han Chinese populations.

The Mechanism

SRR-produced D-serine activates NMDA receptors in pancreatic beta cells, and this signaling modulates glucose-stimulated insulin secretion. The mechanism is acutely sensitive to D-serine levels. Acute D-serine exposure³³ Acute D-serine exposure
Lockridge et al. Acute D-serine co-agonism of beta-cell NMDA receptors potentiates glucose-stimulated insulin secretion. Cells, 2021 enhances insulin release by amplifying the depolarization signal in beta cells, but chronic elevation of D-serine⁴⁴ chronic elevation of D-serine
Suwandhi et al. Chronic D-serine supplementation impairs insulin secretion. Molecular Metabolism, 2018 paradoxically blunts beta-cell responsiveness, leading to diet-independent hyperglycemia.

Mice lacking serine racemase show improved glucose tolerance and enhanced insulin secretory capacity⁵⁵ improved glucose tolerance and enhanced insulin secretory capacity
Lockridge et al. Serine racemase is expressed in islets and contributes to the regulation of glucose homeostasis. Islets, 2016, along with reduced fasting insulin and blood glucose. This suggests that higher SRR activity — potentially driven by the C allele — can chronically desensitize beta-cell NMDA receptors and reduce the acute insulin secretion response to glucose over time.

The Evidence

The original Han Chinese GWAS⁶⁶ Han Chinese GWAS
Tsai FJ et al. A genome-wide association study identifies susceptibility variants for type 2 diabetes in Han Chinese. PLOS Genetics, 2010 identified rs391300 as a novel T2D susceptibility locus with an odds ratio of 1.28 (95% CI 1.18-1.39, p=3.06×10⁻⁹) across a discovery cohort of 995 cases and 894 controls with replication in 1,803 cases and 1,473 controls.

A Chinese gestational diabetes study⁷⁷ Chinese gestational diabetes study
Wang Y et al. Association of six single nucleotide polymorphisms with gestational diabetes mellitus in a Chinese population. PLoS One, 2011 of 1,764 pregnant women (725 GDM cases, 1,039 controls) found rs391300 associated with gestational diabetes with an OR of 1.20 (95% CI 1.02-1.42, p=0.028) under an additive model, rising to OR 1.82 (95% CI 1.23-2.70, p=0.003) under a recessive model. When combined with other T2D risk variants, the per-allele OR for GDM risk was 1.196 (p=1.08×10⁻⁴).

A Chinese metformin pharmacogenomics study⁸⁸ Chinese metformin pharmacogenomics study
Dong ZL et al. Serine racemase rs391300 G/A polymorphism influences the therapeutic efficacy of metformin in Chinese patients with diabetes mellitus type 2. Clin Exp Pharmacol Physiol, 2011 of 402 T2D patients found that those carrying the T allele (GA/AA in minus-strand notation) showed significantly better improvements in fasting plasma glucose, postprandial glucose, and cholesterol after 12 weeks of metformin monotherapy, suggesting the C allele may blunt metformin response.

Replication in 11,530 Japanese individuals⁹⁹ Replication in 11,530 Japanese individuals
Imamura M et al. Replication study for the association of rs391300 in SRR and rs17584499 in PTPRD with susceptibility to type 2 diabetes in a Japanese population. J Diabetes Investig, 2013 failed to confirm the association (OR 0.97, p=0.44), indicating the effect may be population-specific and most relevant to Han Chinese. The C allele frequency is highest in East Asians (~70%), meaning CC homozygosity is most prevalent in this population where the original effect was detected.

Practical Actions

For carriers of two C alleles (CC genotype), the primary concern is monitoring glucose regulation, particularly fasting glucose and 30-minute post-load glucose responses, which one study found elevated even in non-diabetic CC carriers. During pregnancy, CC women should ensure gestational diabetes screening is completed promptly given the elevated GDM risk.

The pharmacogenomics finding warrants attention: if you are CC and require diabetes treatment, the data suggest metformin may be less effective than in T-allele carriers. Your prescribing physician should track HbA1c response carefully and consider alternative or combination therapy earlier if metformin monotherapy does not achieve target glycemic control.

Interactions

rs391300 was identified in the same GWAS as rs17584499 in PTPRD (protein tyrosine phosphatase receptor delta), which influences insulin resistance. PTPRD and SRR represent distinct pathways to T2D — insulin resistance versus impaired insulin secretion — and carrying risk alleles in both may compound risk additively.

Separately, a 2018 study¹⁰¹⁰ 2018 study
Girard et al. Faster progression from MCI to probable AD for carriers of a single-nucleotide polymorphism associated with type 2 diabetes. Neurobiol Aging, 2018 found rs391300 associated with faster progression from mild cognitive impairment to Alzheimer's disease, consistent with the overlapping role of D-serine in both peripheral glucose metabolism and central NMDA receptor neurotransmission. Carriers of the C allele with additional Alzheimer's risk factors (APOE4, family history) may face compounded neurocognitive risk through dual dysregulation of this pathway.