rs3787345 — PTPN1 PTPN1 LD Block Co-Variant

Intronic PTPN1 tag SNP within the 100-kb PTP1B insulin-resistance haplotype block; the minor C allele co-segregates with the risk haplotype associated with elevated PTP1B expression, impaired insulin signaling, and lipid dysregulation

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The PTP1B Insulin Brake — PTPN1 rs3787345

PTP1B — the protein encoded by PTPN1 — functions as one of the most precisely validated negative regulators of insulin signaling in the human body. When insulin binds its receptor and triggers the tyrosine kinase cascade that drives glucose uptake into muscle and fat cells, PTP1B dephosphorylates the activated insulin receptor11 dephosphorylates the activated insulin receptor
removes phosphate groups from tyrosine residues on the insulin receptor kinase domain, directly terminating downstream glucose-uptake signaling and its immediate substrates, ending each insulin pulse. In animal models, complete deletion of PTP1B22 complete deletion of PTP1B
PTP1B knockout mice remain lean on high-fat diets, show insulin hypersensitivity, and resist diet-induced obesity — providing proof-of-concept that PTP1B expression level is a key determinant of whole-body insulin sensitivity produces profound insulin hypersensitivity and resistance to obesity — establishing PTPN1 as both a drug target and a genetic risk locus for type 2 diabetes and metabolic syndrome.

rs3787345 is an intronic variant at GRCh38 chr20:50568886, situated within the ~100-kb PTPN1 linkage disequilibrium block33 ~100-kb PTPN1 linkage disequilibrium block
A genomic region where recombination is rare, so all variants within it tend to be inherited together as a fixed haplotype package; every PTPN1 variant associated with insulin resistance and T2D falls within this same block that has been the subject of repeated association studies across European, Hispanic, and Asian cohorts. The variant itself does not alter the PTP1B amino acid sequence; its clinical significance comes from co-segregation with the risk haplotype across this regulatory block.

The Mechanism

The functional driver of this haplotype block is not rs3787345 itself but a functional insertion in the PTPN1 3'-UTR44 functional insertion in the PTPN1 3'-UTR
A variant in the untranslated tail of the PTPN1 mRNA that stabilizes the transcript in skeletal muscle, increasing the amount of PTP1B protein produced per transcription event; this was identified by Bento et al. 2004 as the likely causal mechanism within the LD block that stabilizes PTPN1 mRNA in skeletal muscle, raising PTP1B protein levels. Higher PTP1B means each insulin pulse is terminated faster — the kinase window is shorter, less glucose transporter (GLUT4) is translocated to the muscle cell surface, and more circulating insulin is required to achieve the same glucose uptake. In the liver, PTP1B-impaired insulin signaling reduces LDL receptor recycling and increases VLDL assembly, connecting this locus to dyslipidemia independent of adiposity.

rs3787345 sits between rs941798 (chr20:50546698) and rs6020611 (chr20:50578070), two other well-characterized tag SNPs for this same haplotype block. All three variants — rs941798, rs3787345, and rs6020611 — carry the same biological message: minor allele carriage marks the risk haplotype with elevated PTP1B expression.

The Evidence

The haplotype block containing rs3787345 was systematically characterized by Bento et al. 200455 Bento et al. 2004
Bento JL et al. Association of protein tyrosine phosphatase 1B gene polymorphisms with type 2 diabetes. Diabetes. 2004 Nov;53(11):3007-12, who genotyped 23 noncoding PTPN1 SNPs across 161 kb in Caucasian cohorts and found that all associated variants clustered in a single ~100-kb block with OR ~1.3 for type 2 diabetes and a population-attributable risk of 17–20%. Importantly, the functional mechanism — mRNA stabilization via a 3'-UTR variant — was directly demonstrated, distinguishing this from purely statistical associations.

Palmer et al. 200466 Palmer et al. 2004
Palmer ND et al. Association of PTPN1 gene polymorphisms with measures of glucose homeostasis in Hispanic Americans: the IRAS Family Study. Diabetes. 2004 Nov;53(11):3013-9 replicated these findings in 811 Hispanic Americans, showing that all 20 common PTPN1 LD-block SNPs were significantly associated with the insulin sensitivity index (p=0.003) and fasting glucose (p<0.001), extending the signal beyond European populations.

rs3787345 was included in the 14-SNP PTPN1 panel examined by Cheyssac et al. 200677 Cheyssac et al. 2006
Cheyssac C et al. Analysis of common PTPN1 gene variants in type 2 diabetes, obesity and associated phenotypes in the French population. BMC Med Genet. 2006;7:44, which studied 1,227 T2D cases and 1,047 controls in a French cohort. Haplotype analysis identified the CACG haplotype — incorporating multiple LD-block SNPs including rs3787345 — with marginal association with T2D (unadjusted p=0.02), while rs941798 and rs2426159 showed the most consistent individual associations with fasting insulin, HOMA-B, and lipid markers in normoglycemic controls.

The lipid connection was established by Bauer et al. 201088 Bauer et al. 2010
Bauer F et al. PTPN1 polymorphisms are associated with total and low-density lipoprotein cholesterol. Eur J Cardiovasc Prev Rehabil. 2010;17(1):28-34, who found that minor allele carriers of closely co-segregating PTPN1 tag SNPs in this same block showed elevated LDL and total cholesterol specifically in lean men (BMI <26 kg/m²; p<0.05) — a BMI-stratified finding consistent with PTP1B-driven insulin resistance elevating cholesterol through direct hepatic mechanisms even in the absence of obesity.

Evidence level is moderate: the T2D signal for this entire LD block is replicated and mechanistically grounded, but the specific individual contribution of rs3787345 — as opposed to its co-varying block companions — has not been isolated at the genome-wide significance level. The signal is strongest at the haplotype level.

Practical Actions

Carrying the C allele at rs3787345 — particularly the CC genotype — marks the PTPN1 risk haplotype that confers chronically elevated PTP1B activity. The most actionable implications are metabolic monitoring and lifestyle choices that offset PTP1B-mediated insulin resistance.

Fasting insulin, HOMA-IR, and fasting LDL are the most informative biomarkers for this haplotype: PTP1B-driven insulin resistance can be present at normal body weight, so these labs add information beyond BMI. Resistance training and aerobic exercise directly downregulate PTP1B expression in skeletal muscle, providing a partially specific countermeasure. Reducing dietary saturated fat below 10% of calories limits hepatic LDL production through the PTP1B-impaired insulin signaling pathway.

Interactions

rs3787345 is in strong LD with rs941798, rs6020611, rs3787348, and rs914458 — all members of the same PTPN1 100-kb risk haplotype block. Carriers of the C allele at rs3787345 are likely co-carriers of the risk alleles at these other loci. If multiple PTPN1 tag SNPs are genotyped, the combination provides stronger haplotype resolution than any single variant alone. At the pathway level, PTPN1 interacts with the insulin receptor (INSR), IRS1, and PIK3R1 — downstream components of the insulin signaling cascade whose variants can compound PTP1B-driven impairment.

Nutrient Interactions

dietary carbohydrate altered_metabolism
dietary fat altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

TT Common Form Normal

Common genotype — T allele homozygote, not tagged for the PTPN1 risk haplotype

rs3787345 TT represents the population-major genotype at this intronic PTPN1 locus. The T allele does not co-segregate with the haplotypes across the PTPN1 100-kb LD block that have been associated with elevated cholesterol, impaired insulin sensitivity, or type 2 diabetes risk in multiple cohort studies. The T allele is the non-reference (alternate) allele by dbSNP convention but is the population-major allele (~58% globally in gnomAD v4). No specific metabolic or monitoring actions are indicated by this genotype at this locus.

CT Heterozygous Carrier Intermediate

One C allele — moderate haplotype tagging for the PTPN1 insulin resistance risk block

Heterozygous CT individuals carry one copy of the haplotype that is associated with higher PTPN1 mRNA stability and, consequently, higher PTP1B protein levels in skeletal muscle and liver. Bento et al. (2004) established that the PTPN1 risk haplotype confers an OR of approximately 1.3 for type 2 diabetes with additive dose-response, so each C allele incrementally raises the association signal. Bauer et al. (2010) found that minor allele carriers of co-segregating PTPN1 tag SNPs showed elevated LDL and total cholesterol specifically in lean men (BMI <26 kg/m²), suggesting the lipid effect is a direct hepatic consequence of impaired insulin signaling rather than secondary to adiposity. At a single copy, the effect size is modest. Palmer et al. (2004) confirmed the block-level insulin sensitivity association in 811 Hispanic Americans.

CC Risk Homozygous High Risk

Two C alleles — strongest PTPN1 haplotype signal for elevated PTP1B expression and metabolic risk

CC homozygotes carry two copies of the haplotype most strongly associated with elevated PTPN1 mRNA stability and PTP1B protein production. The proposed functional mechanism — a 3'-UTR insertion in the same LD block stabilizing PTPN1 mRNA in skeletal muscle (Bento et al. 2004, PMID 15504984) — is additive: two copies produce more PTP1B than one, which produces more than none. The downstream consequences are twofold: (1) peripheral insulin resistance from faster termination of insulin receptor kinase activity, and (2) hepatic dyslipidemia from reduced insulin-dependent LDL receptor recycling and increased VLDL assembly.

The population-attributable risk estimate from Bento et al. (2004) for the entire PTPN1 risk haplotype block is 17–20% for T2D in Caucasians — one of the highest figures reported for any noncoding diabetes locus. Across the same block, Palmer et al. (2004) confirmed in 811 Hispanic Americans that all 20 common haplotype SNPs associated significantly with insulin sensitivity index (p = 0.003) and fasting glucose (p < 0.001), making this the best-replicated noncoding locus for insulin resistance genetics. Cheyssac et al. (2006) studied rs3787345 directly in the French population, including it in haplotype blocks showing marginal T2D association before permutation correction.

The lipid signal from Bauer et al. (2010) is BMI-stratified: in lean men (BMI <26 kg/m²), minor allele carriers of closely co-segregating tag SNPs showed significantly elevated LDL and total cholesterol (p<0.05). This makes lean CC carriers a particularly important group to monitor — they carry the full haplotype signal without the adiposity-driven insulin resistance that masks the PTP1B mechanism in overweight individuals.