rs6589702 — PRG2 PRG2 Eosinophil Major Basic Protein Variant

PRG2 — When the Eosinophil's Primary Weapon Fires in the Wrong Place

Inside every eosinophil sits a crystalline granule containing one of the immune system's most potent cytotoxic proteins: major basic protein (MBP)¹¹ major basic protein (MBP)
The predominant constituent of the eosinophil secondary granule crystalloid core, encoded by the PRG2 gene at chromosome 11q12.1. MBP comprises approximately 50% of all eosinophil granule protein by mass and is one of the principal mediators of tissue injury in eosinophil- driven inflammatory conditions. Under normal conditions, eosinophils patrol mucosal surfaces and deploy MBP against parasites — an ancient and effective anti-helminthic weapon. The problem arises when eosinophils activate in the airways, skin, gut, or esophagus in response to allergens: MBP then targets host tissues with the same destructive force it would use on a worm. rs6589702 (merged to the current canonical form rs2508922) is an intergenic regulatory variant that has been annotated to the PRG2/PRG3 eosinophil major basic protein gene cluster based on its proximity to this locus and its context in immune disease genetics.

The Mechanism

PRG2 encodes pro-eosinophil major basic protein (proMBP), a precursor that is processed to the mature 13.8 kDa MBP cationic protein with a pI of 11.4. Its high arginine content endows MBP with a strongly positive charge that enables electrostatic membrane disruption²² electrostatic membrane disruption
MBP's positive charge interacts with negatively charged phospholipids in cell membranes, increasing membrane permeability and causing direct cytotoxicity to epithelial cells, airway smooth muscle, and nerve sheaths at concentrations reached in the local tissue microenvironment during eosinophil degranulation.

The M2 muscarinic receptor pathway adds a second mechanism for airway damage. Vagal nerve endings in the lung express M2 muscarinic autoreceptors that normally brake acetylcholine release — limiting bronchoconstriction after each nerve firing. MBP is an endogenous allosteric antagonist of these M2 receptors³³ MBP is an endogenous allosteric antagonist of these M2 receptors
MBP binds allosterically to the M2 receptor at a site distinct from the acetylcholine binding pocket, preventing the receptor from terminating nerve firing and leading to unchecked acetylcholine release and sustained bronchoconstriction. This was demonstrated in isolated guinea pig airway preparations by Jacoby et al. 1993. When degranulating eosinophils release MBP onto the airway wall, M2 receptor blockade directly amplifies bronchospasm — a distinctly genotype-specific mechanism that generic bronchodilators do not fully address.

The regulatory role of rs6589702 at this locus is inferred from its intergenic position within the genomic neighborhood of the PRG2/PRG3 cluster. PRG2 gene expression is under tight eosinophil-specific regulatory control via GATA-1 and STAT5 elements in its promoter, with two alternative promoters (P1 and P2) switching usage during eosinophil differentiation. Variants that alter these regulatory elements or modulate chromatin accessibility in eosinophil progenitors could influence how much MBP a person's eosinophils produce — with downstream consequences for allergen-triggered tissue damage.

The Evidence

The direct functional evidence for rs6589702 itself is emerging. The strongest mechanistic anchor comes from a Sardinian GWAS⁴⁴ Sardinian GWAS
Concas et al. 2023, Human Molecular Genetics — 869 individuals from the Sardinian general population plus 11,822 INTERVAL UK blood donors as replication cohort examining white blood cell morphological parameters, which found that the PRG2 gene region contains multiple independent genetic signals influencing eosinophil granule content and cellular morphology. The lead PRG2 variant (rs769591668, p.Ser148Pro) showed the largest effect on eosinophil scatter of any locus in the genome (β=−1.65, p=8.3×10⁻³²), and conditional analysis revealed a second independent signal in the broader PRG2/PRG3 cluster at 11:57110622 — demonstrating that multiple independent regulatory and coding variants at this locus influence eosinophil biology.

Independent support for the locus comes from the broader IBD genetics literature: an Ashkenazi Jewish Crohn's disease GWAS identified rs11229030⁵⁵ rs11229030
The primary PRG2/PRG3 GWAS hit for Crohn's disease; located approximately 45 kb downstream of PRG2 at chr11:57,435,536 (GRCh38); OR 1.15, p=8×10⁻⁹ in 10 cohorts combining 1,878 cases and 4,469 controls at the same 11q12.1 PRG2/PRG3 cluster, confirming that regulatory variation near the eosinophil major basic protein genes contributes to immune-mediated mucosal disease. rs6589702 appears to represent an additional variant within or near this region that may tag a distinct regulatory signal.

MBP biology in disease has been extensively validated independent of the specific genetics. Sputum MBP levels are specifically elevated in asthma versus non-asthma chronic cough, and MBP concentration correlates with eosinophil count in asthmatic patients (r=0.88, p<0.0001) but not in non-asthmatic patients, supporting eosinophil degranulation — not mere eosinophil presence — as the pathologically relevant event in Kim et al. 2011 (n=46)⁶⁶ Kim et al. 2011 (n=46). In eosinophilic esophagitis, MBP granule protein deposition correlates with symptom severity (p=0.0001) independently of eosinophil counts, and was present in all symptomatic EoE patients studied — making it a more reliable disease activity marker than eosinophil enumeration alone.

Practical Actions

For AG and AA carriers, the clinical implication centers on eosinophil monitoring and recognition of eosinophil-mediated tissue damage as the effector mechanism. When MBP-driven inflammation is active — whether in the airway, gut, or skin — standard anti-inflammatory approaches that reduce eosinophil recruitment and activation are the most targeted interventions. Sputum and serum MBP levels, where available, provide a window into how much eosinophil degranulation is actually occurring, beyond what eosinophil counts alone reveal.

Interactions

rs6589702 and rs11229030 are two regulatory variants annotated to the PRG2/PRG3 gene cluster at chromosome 11q12.1. Conditional analyses in the Sardinian GWAS confirm that at least two independent signals coexist within the broader PRG2/PRG3 locus, each tagging distinct regulatory elements. Carriers of risk alleles at both rs6589702 and rs11229030 may face compounding variation in PRG2 regulatory control.

The PRG3 intronic variant rs10751659 adds the related MBPH (major basic protein homologue) gene to this cluster's interaction network. PRG3 shares conserved GATA/STAT promoter elements with PRG2 and is expressed in parallel in eosinophil granules. Regulatory variation across the cluster (PRG2 + PRG3) compounds via two protein effectors with shared cytotoxic mechanisms.

Upstream, variants affecting eosinophil recruitment — IL-5 signaling (rs1295686, rs2243290), IL-33/ST2 axis (rs11229030's co-located Crohn's signals), or TSLP pathway — determine how many eosinophils arrive at tissue sites. rs6589702 may influence what happens once they get there: how much MBP each eosinophil produces and releases.