rs10751659 — PRG3 PRG3 Eosinophil Major Basic Protein Homologue Variant

PRG3 — The Eosinophil's Second Weapon in Allergic Tissue Damage

Buried inside the granules of every activated eosinophil lies a pair of cationic proteins — twin weapons in the allergic immune arsenal. The better-known of the two is eosinophil major basic protein (MBP, encoded by PRG2), a highly positively charged protein that disrupts cell membranes, triggers mast cell degranulation, and drives airway remodeling in asthma and atopic disease. Less appreciated is its structural cousin: eosinophil major basic protein homologue, MBPH¹¹ MBPH
Also known as MBP-2, encoded by the PRG3 gene (proteoglycan 3) on chromosome 11q12.1, immediately adjacent to PRG2. PRG3 was identified in 1999 as a distinct eosinophil granule protein with 63% amino acid identity to PRG2-encoded MBP. rs10751659 is an intronic variant in the PRG3 gene that tags variation in PRG3 expression and eosinophil granule protein activity.

The Mechanism

PRG3 encodes MBPH, a C-type lectin-like protein stored in the secondary (specific) granules of eosinophils — the same compartment as EPX (eosinophil peroxidase), ECP (eosinophil cationic protein), and EDN (eosinophil-derived neurotoxin). When eosinophils are activated by IgE-mediated signals, IL-5, or allergen stimulation, they degranulate, releasing MBPH along with its companion proteins into the surrounding tissue. MBPH carries a net cationic charge of +8 at neutral pH²² MBPH carries a net cationic charge of +8 at neutral pH
Compared to +16 for MBP1; the reduced charge of MBPH explains why it shows similar but less potent cytotoxic and stimulatory activities compared to MBP1 in vitro. Despite this charge difference, MBPH retains the capacity to kill parasites and tumor cells, stimulate basophil and neutrophil activation, and directly damage epithelial cells.

rs10751659 is located within an intron of PRG3 and does not alter the protein coding sequence directly. Intronic variants in eosinophil granule protein genes typically act as expression quantitative trait loci (eQTLs), influencing how much of the granule protein each eosinophil produces — directly setting the quantity of MBPH available for release upon activation. The C allele at this position (the minor allele, ~14.5% globally) marks variation in this expression regulation. The PRG3 and PRG2 genes share conserved GATA and STAT transcription factor binding sites in their promoters³³ conserved GATA and STAT transcription factor binding sites in their promoters
GATA-1 and STAT5 bind the proximal promoters of both PRG2 and PRG3, driving eosinophil-restricted transcription; IL-5 signals through STAT5, linking eosinophil maturation cytokine exposure to granule protein expression levels.

The Evidence

MBPH was characterized in 1999 when Plager and colleagues⁴⁴ Plager and colleagues
Plager DA et al. J Biol Chem 1999 — isolated hMBPH from eosinophil granules, detected transcript exclusively in bone marrow; demonstrated cytotoxic and basophil/neutrophil- stimulating activities with reduced potency relative to MBP1 identified a second cationic protein co-localizing with MBP1 in eosinophil secondary granules. The protein's disulfide bond architecture⁵⁵ disulfide bond architecture
Mass spectrometry mapping by Wagner et al. 2007 confirmed two disulfide bonds (Cys20-Cys115, Cys92-Cys107) conserved with MBP1 and C-type lectins, plus six free sulfhydryl cysteine residues; the structural conservation supports shared functional mechanisms is conserved with MBP1 and with C-type lectin-like proteins, indicating evolutionary optimization for the same biological role: direct cytotoxicity via cationic membrane disruption.

The eosinophil granule protein family — MBP1, MBPH, ECP, EDN, and EPX — constitutes the primary effector mechanism of eosinophil-mediated tissue damage in allergic disease. MBP knockout mice are protected from experimental colitis⁶⁶ MBP knockout mice are protected from experimental colitis
From Front Immunol 2021 review; eosinophilic infiltration also predicts poor response to medical therapy in ulcerative colitis, and elevated granule protein levels correlate with disease severity in asthma and atopic dermatitis. MBPH's contribution to this effector pool is at the emerging evidence stage for this specific intronic variant; the protein-level biology is well-established but the population-level genetics of this particular rs10751659 intronic tag SNP have not been characterized in large GWAS studies.

The notable population stratification of rs10751659 — with the C allele at ~20-25% in Europeans but approaching zero in East Asian populations — mirrors patterns seen in other eosinophil-related variants, where European populations carry greater diversity in eosinophil granule protein gene regulation.

Practical Actions

For CT heterozygotes and CC homozygotes, the actionable implications center on monitoring eosinophil activation and reducing triggers that cause eosinophil degranulation and MBPH release. MBPH released from eosinophils in airways, gut, and skin triggers mast cell degranulation and amplifies the local allergic response — creating a feedforward loop from the initial allergen exposure. The dominant allergic effector pathway driven by MBPH is eosinophil activation by IL-5 and IgE-mediated signaling; actions that reduce IL-5-driven eosinophil recruitment and activation address the specific mechanism at this locus.

Interactions

The PRG3 gene sits directly adjacent to PRG2 on chromosome 11q12.1, and both genes share regulatory machinery. rs11229030, a PRG2/PRG3 intergenic tag SNP, is associated with Crohn's disease susceptibility via eosinophil MBP-mediated gut epithelial damage — carriers of risk alleles at both PRG2 and PRG3 variants may face compounded eosinophil granule protein activity. The related coding variant rs34108746 in PRG3 (p.Tyr146Asp) directly alters the MBPH protein and is associated with eosinophil count at genome-wide significance across multiple large blood cell GWAS datasets.