rs4950928 — CHI3L1 YKL-40 Promoter Variant (-131C>G)

YKL-40: The Inflammatory Thermostat You Inherit

YKL-40 — named after the amino acids tyrosine (Y), lysine (K), and leucine (L) and its 40-kDa molecular weight — is one of the most clinically informative biomarkers of tissue inflammation. Elevated circulating YKL-40 marks active inflammation in asthma, COPD, rheumatoid arthritis, and several cancers. The protein is encoded by the CHI3L1 gene on chromosome 1q32.1, and how much YKL-40 your body produces is largely predetermined by genetics. The rs4950928 variant in the CHI3L1 promoter is the single strongest genetic determinant of circulating YKL-40 levels yet identified.

The Mechanism

The rs4950928 variant sits 131 base pairs upstream of the CHI3L1 transcription start site (the "-131" in the classical C>G notation used in the literature, which describes the complementary coding strand). This position falls within a transcription factor binding site¹¹ transcription factor binding site
A region of DNA where regulatory proteins attach to control when and how much a gene is read and converted into protein. The common C allele (on the plus strand, ~79% of people) permits higher promoter activity, driving more CHI3L1 transcription and higher secretion of YKL-40 protein by macrophages, neutrophils, and epithelial cells during inflammatory stimulation. The rarer G allele (~21% globally) disrupts the binding site, reducing promoter efficiency and resulting in substantially lower baseline YKL-40 production — roughly half of what CC homozygotes produce.

In childhood birth cohorts involving 2,405 children, Guerra et al.²² Guerra et al.
Guerra S et al. Genetic and epigenetic regulation of YKL-40 in childhood. J Allergy Clin Immunol, 2018 found that methylation at five CpG sites³³ methylation at five CpG sites
DNA methylation is an epigenetic mechanism that adds a chemical tag to gene regulatory regions, typically reducing transcription without changing the underlying DNA sequence partially mediates the rs4950928 genetic effect on YKL-40, suggesting that environmental factors can modulate the gene's output on top of the inherited sequence.

The Evidence

The foundational study appeared in the New England Journal of Medicine in 2008. Ober et al.⁴⁴ Ober et al.
Ober C et al. Variation in the CHI3L1 gene influences serum YKL-40 levels, risk of asthma, and lung function. N Engl J Med, 2008 studied the carefully characterized Hutterite founder population alongside two independent case-control populations of European descent. The rs4950928 variant explained serum YKL-40 variation with extraordinary significance (P=1.1×10⁻¹³). The same variant predicted asthma in combined case-control analysis (P=1.2×10⁻⁵), and correlated with bronchial hyperresponsiveness (P=0.002) and FEV1/FVC ratio (P=0.002).

The dose-response relationship between genotype and YKL-40 was measured precisely by Hansen et al.⁵⁵ Hansen et al.
Hansen JW et al. YKL-40 and genetic status of CHI3L1 in a large group of asthmatics. Eur Clin Respir J, 2015 in 1,827 subjects: CC homozygotes averaged 45 µg/L, CG heterozygotes 32 µg/L, and GG homozygotes 19 µg/L — a more than twofold range driven by a single promoter nucleotide (P<0.0001).

In the Severe Asthma Research Program (SARP), Gomez et al.⁶⁶ Gomez et al.
Gomez JL et al. Genetic variation in CHI3L1 contributes to asthma severity and airway expression of YKL-40. J Allergy Clin Immunol, 2015 found that G allele carriers had lower YKL-40 airway expression and better FEV1% predicted, with this effect being statistically independent of a separate intronic CHI3L1 variant (rs12141494) that also influences asthma severity.

Beyond the airways: in adult cystic fibrosis patients⁷⁷ adult cystic fibrosis patients
Coriati A et al. YKL-40 as a clinical biomarker in adult CF patients. J Cyst Fibros, 2021, CC homozygotes had higher YKL-40, greater dysglycemia, lower lung function, and higher Pseudomonas aeruginosa colonization rates. And in a Chinese hypertension cohort, Xu et al.⁸⁸ Xu et al.
Xu T et al. Association of CHI3L1 gene variants with YKL-40 levels and hypertension incidence. J Cell Mol Med, 2021 found that G-allele carriers had 54% lower hypertension risk in men (OR 0.46, 95% CI 0.23–0.89), suggesting that chronically lower YKL-40 may protect against inflammation-driven vascular disease.

Practical Actions

For the common CC genotype, elevated baseline YKL-40 reflects a higher inflammatory setpoint. If respiratory symptoms are present, serum YKL-40 can serve as a useful monitoring biomarker alongside spirometry. Environmental triggers — allergens, pollutants, cigarette smoke — activate macrophage YKL-40 release and are especially worth avoiding in CC carriers whose baseline is already elevated. Heterozygous CG carriers have a meaningful advantage over CC but not the full protection of GG homozygotes.

For GG carriers, the very low YKL-40 baseline means that standard clinical reference ranges (calibrated to a CC-majority population) may classify their values as anomalously low — which is not pathological but simply reflects their genotype.

Interactions

The intronic variant rs12141494 (CHI3L1 intron 6) independently influences YKL-40 airway expression and asthma severity and was found statistically independent of rs4950928 in conditional analyses (Gomez et al. 2015, PMID 25592985). Together these two variants tag distinct regulatory mechanisms — the promoter (rs4950928) controls baseline circulating YKL-40, while the intronic variant modulates airway tissue expression specifically. The CHI3L1 locus eQTL cluster also includes rs10399931 and rs872129, which contribute additional signal to the YKL-40 quantitative trait locus at chromosome 1q32.1.