rs1495377 — TSPAN8

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.