rs1495377 — TSPAN8

Intronic variant in TSPAN8 (Tetraspanin 8) associated with reduced insulin secretion and type 2 diabetes risk; the G allele impairs pancreatic beta-cell and alpha-cell function, reducing insulinogenic response to glucose

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TSPAN8 rs1495377 — A Hidden Regulator of Insulin Secretion

Tetraspanin 8 (TSPAN8) is a four-pass transmembrane glycoprotein that organizes proteins and lipids at the cell surface into signaling microdomains. While it is perhaps best known for its role in tumour biology, TSPAN8 is expressed in pancreatic islets where it plays a less-heralded but clinically meaningful role in regulating the insulin secretory response to glucose. rs1495377 is an intronic variant in the TSPAN8-LGR5 locus on chromosome 12, identified as a type 2 diabetes susceptibility locus by large-scale genome-wide association studies. The G allele carries an odds ratio of 1.28 for type 2 diabetes and measurably impairs the pancreatic response to dietary glucose.

The Mechanism

Tetraspanins11 Tetraspanins
A large superfamily of four-transmembrane-domain scaffold proteins that cluster cell-surface receptors, integrins, and signaling molecules into functional microdomains called tetraspanin-enriched microdomains (TEMs) serve as molecular organisers at the cell membrane. TSPAN8 is expressed in gastrointestinal epithelium and pancreatic islets, where it forms complexes with integrins and growth factor receptors. In beta cells, the tetraspanin scaffold is thought to regulate the surface organization of glucose-sensing machinery and the vesicle fusion events required for insulin exocytosis.

The intronic position of rs1495377 suggests it acts as a regulatory variant22 regulatory variant
intronic variants frequently alter splicing efficiency, create or destroy transcription factor binding sites in regulatory elements, or tag causal variants through linkage disequilibrium rather than directly changing the TSPAN8 protein sequence. The precise molecular mechanism — whether altered splicing, expression levels, or LD with a coding variant in LGR5 (a nearby Wnt co-receptor also at this locus) — has not been experimentally resolved. Both TSPAN8 and LGR5 are plausible effectors given their roles in beta-cell biology and Wnt signalling.

The Evidence

The type 2 diabetes association was first reported in the Wellcome Trust Case Control Consortium 2007 GWAS33 Wellcome Trust Case Control Consortium 2007 GWAS
Wellcome Trust Case Control Consortium, Nature 2007; ~2,000 T2D cases vs ~3,000 controls on the Affymetrix 500K array; rs1495377-G emerged at p=7×10⁻⁶, OR 1.28 (95% CI 1.11–1.49).

The association was confirmed and strengthened in the Zeggini et al. 2008 meta-analysis44 Zeggini et al. 2008 meta-analysis
Zeggini E et al., Nature Genetics 2008; meta-analysis of 10,128 European-ancestry individuals with up to 53,975 in replication; the TSPAN8-LGR5 locus reached genome-wide significance at p=1.1×10⁻⁹. This placed TSPAN8 in the established tier of T2D loci alongside TCF7L2 and PPARG.

Critically, Grarup et al. 200855 Grarup et al. 2008
Grarup N et al., Diabetes 2008; n=4,516 glucose-tolerant Danish participants underwent oral glucose tolerance testing; TSPAN8 C-allele at rs7961581 (a correlated marker) associated with 4.5% reduced corrected insulin response (CIR), 3.9% reduced AUC-insulin/AUC-glucose ratio, and 5.2% reduced insulinogenic index, all p≤0.03 showed that the TSPAN8 diabetes risk allele acts through impaired insulin secretion rather than insulin resistance — the beta cell simply produces less insulin in response to glucose stimulation.

Jonsson et al. 201366 Jonsson et al. 2013
Jonsson A et al., Diabetes 2013; 4,654 normoglycemic Finnish PPP-Botnia Study participants plus human islet experiments; examined 43 T2D-associated SNPs for effects on both alpha- and beta-cell function in vivo and in vitro added a further dimension: the TSPAN8 risk allele was associated with decreased fasting and 2-hour glucagon concentrations both in vivo and in vitro, indicating that alpha-cell dysregulation (not just impaired insulin secretion) contributes to TSPAN8-mediated diabetes risk. Impaired glucagon suppression after meals compounds the glucose excursion seen in early type 2 diabetes.

Practical Actions

The TSPAN8 T2D association operates through the insulin secretory axis: G-allele carriers produce a blunted insulin response to glucose. This makes carbohydrate quantity and glycaemic load the primary dietary levers — reducing the demand on a beta-cell apparatus that is already under-responding. Strategies that support insulin secretory function (specific nutrients like magnesium and inositol) and that reduce postprandial glucose excursions (dietary fibre, meal timing, and monitoring) are the most directly relevant interventions.

Monitoring fasting glucose and HbA1c gives visibility into whether the secretory deficit is translating into glycaemic impairment. Early identification of impaired fasting glucose (5.6–6.9 mmol/L) or impaired glucose tolerance allows intervention before the progression to overt diabetes.

Interactions

rs1495377 sits in the TSPAN8-LGR5 locus near rs7961581, which is in linkage disequilibrium and was the marker used in the Grarup 2008 insulin secretion study. These are not independent signals but rather correlated markers tagging the same functional haplotype.

The TSPAN8 secretory deficit compounds with TCF7L2 rs7903146, the strongest common genetic predictor of T2D, which also acts through reduced insulin secretion. Carriers of both risk variants face a multiplicative reduction in beta-cell response to glucose. Multi-variant risk profiling that includes both loci provides substantially better T2D risk stratification than either alone.

Nutrient Interactions

carbohydrate increased_need

Genotype Interpretations

What each possible genotype means for this variant.

CC Normal Secretory Response Normal

Common protective genotype; standard insulin secretion at this locus

You carry two copies of the C allele at rs1495377. The C allele is the common variant worldwide (~57% globally, ~52% in Europeans) and is associated with normal TSPAN8-mediated insulin secretory response to glucose. Studies comparing TSPAN8 risk allele carriers to non-carriers found that the C/C genotype maintains standard insulinogenic index and corrected insulin response. About 33% of people carry this genotype.

The TSPAN8 locus does not add measurable type 2 diabetes risk at this genotype; other genetic and lifestyle factors govern your metabolic health from this point.

CG One Risk Allele Intermediate

One G allele modestly reduces insulin secretory capacity and raises T2D risk

TSPAN8's contribution to T2D risk specifically comes from the insulin secretion pathway rather than insulin resistance — the beta cell produces less insulin in response to glucose stimulation. This distinction is important because interventions that reduce carbohydrate load (by lowering peak glucose) are more relevant than interventions targeting insulin sensitivity (like those recommended for PPARG or ADIPOQ variants).

The Jonsson 2013 study also found that the TSPAN8 risk allele reduces glucagon suppression after meals. This dual effect — blunted insulin response AND reduced glucagon suppression — compounds postprandial glucose excursions. Monitoring HbA1c, which captures integrated glycaemia over three months, is more informative than fasting glucose alone for this phenotype.

GG Two Risk Alleles High Risk

Homozygous for the TSPAN8 T2D risk allele; measurably reduced insulin secretory capacity

Two mechanistic deficits converge in GG homozygotes. First, the insulinogenic index — the ratio of insulin released in the first 30 minutes after glucose ingestion relative to glucose rise — is measurably reduced, consistent with impaired first-phase insulin secretion. First-phase insulin release is the critical initial spike that suppresses hepatic glucose output and initiates peripheral glucose uptake; even a 10% deficit meaningfully extends postprandial glucose excursions. Second, glucagon concentrations are higher than expected after meals, adding a hepatic glucose output signal that a normal beta-cell would counteract with higher insulin. Together, these create a glucose environment more likely to cause prolonged postprandial hyperglycaemia.

The practical upshot is that GG carriers particularly benefit from dietary strategies that lower peak postprandial glucose (glycaemic load reduction, meal composition), monitoring strategies that catch early glucose dysregulation (HbA1c, not just fasting glucose), and magnesium supplementation to support the secretory machinery that does exist.