rs146527530 — CTSS CTSS AD Risk Signal (Budu-Aggrey 2023)

CTSS rs146527530 — The Strongest Independent Cathepsin S Locus Signal for Atopic Dermatitis

Cathepsin S¹¹ Cathepsin S
A lysosomal cysteine protease encoded by CTSS on chromosome 1q21.3. Expressed almost exclusively in professional antigen-presenting cells — dendritic cells, macrophages, and B cells — where it catalyses the final, rate-limiting step of MHC class II peptide loading by removing the invariant chain CLIP fragment sits at the intersection of two major atopic dermatitis mechanisms: adaptive immune priming and histamine-independent itch. rs146527530 is the third of three statistically independent atopic dermatitis risk signals identified at the CTSS locus in the Budu-Aggrey 2023 genome-wide association meta-analysis — and with an odds ratio of 1.25, it is the strongest of the three CTSS-locus signals.

The variant falls at chromosome 1:151,086,720 (GRCh38) within the GABPB2 gene, located in the same 1q21.3 chromosomal neighbourhood as CTSS itself. The G risk allele is rare globally (overall allele frequency ~1%), with the highest frequency in non-Finnish European populations (~1.5%) and Ashkenazi Jewish populations (~2.1%). It is essentially absent in East Asian populations.

The Mechanism

CTSS is the only protease capable of completing the final step of MHC class II antigen loading. In the lysosomal compartment of dendritic cells and B cells, newly assembled MHC class II molecules arrive with the invariant chain CLIP fragment blocking the peptide groove²² newly assembled MHC class II molecules arrive with the invariant chain CLIP fragment blocking the peptide groove
Without CTSS activity, CLIP cannot be removed and no antigenic peptide can load — severely impairing CD4+ T cell activation. With excess CTSS activity, antigen presentation is amplified, lowering the threshold for immune sensitisation to environmental antigens.

In the skin, cathepsin S has a second independent mechanism: it is secreted extracellularly by skin-resident dendritic cells, where it cleaves and activates protease-activated receptor 2 (PAR2) on TRPV1-expressing sensory nerve endings³³ it cleaves and activates protease-activated receptor 2 (PAR2) on TRPV1-expressing sensory nerve endings
PAR2 activation by extracellular CTSS triggers TRPV1 calcium signalling in cutaneous nociceptors, generating itch that is entirely independent of histamine release. This explains why antihistamines often fail to adequately control atopic dermatitis itch — the PAR2/TRPV1 pathway is not histamine-dependent.

rs146527530 is intronic and does not alter the CTSS protein directly. Like the other two CTSS-locus signals (rs187080438 and rs115161931), it most likely acts through a cis-regulatory mechanism that alters CTSS expression levels in skin-resident immune cells. The three signals were identified as conditionally independent — each retains genome-wide significance after statistically conditioning on the others — indicating they tag distinct molecular regulatory elements rather than the same causal variant.

The Evidence

Budu-Aggrey et al. (Nature Communications, 2023)⁴⁴ Budu-Aggrey et al. (Nature Communications, 2023) conducted a multi-stage GWAS meta-analysis that is the largest atopic dermatitis GWAS to date: a European discovery phase of 864,982 individuals (60,653 cases), a multi-ancestry expansion to 1,086,394 individuals, and independent replication in 23andMe European cohorts. Rs146527530-G showed an odds ratio of 1.25 (95% CI 1.22–1.28) with p=2×10⁻⁸⁸ in the multi-ancestry analysis — one of the most statistically robust atopic dermatitis association signals at the locus. The effect allele frequency of ~2% means this signal is rare but detectable in standard consumer genotyping chips and whole-genome sequencing datasets.

Causal evidence for CTSS as the functional gene at this locus comes from animal models. Kim et al. (Journal of Investigative Dermatology, 2012)⁵⁵ Kim et al. (Journal of Investigative Dermatology, 2012) showed that transgenic mice overexpressing CTSS spontaneously develop chronic skin disease resembling atopic dermatitis — PAR-2 upregulation in skin dendritic cells triggered CD4+ T-cell priming, increased MHC class II surface expression, and scratching behaviour. Elevated CTSS activity alone is sufficient to cause the phenotype without any external allergen challenge.

The itch mechanism was dissected by Chung et al. (Neurobiology of Pain, 2019)⁶⁶ Chung et al. (Neurobiology of Pain, 2019): intradermal cathepsin S injection induced dose-dependent scratching in mice through PAR2 on TRPV1-expressing sensory neurons. TRPV1 knockout reduced scratching by 50%; PAR2 antagonists abolished it completely. These data establish cathepsin S as a molecular pruritogen operating through a non-histaminergic channel — directly relevant to treatment selection in CTSS-locus risk allele carriers with difficult-to-control itch.

Practical Actions

For G allele carriers, the two actionable consequences of altered CTSS activity are: (1) lowered threshold for immune sensitisation through amplified antigen presentation, which makes skin barrier integrity especially important since more antigen penetrating the barrier translates to more antigen available for CTSS-mediated MHC class II loading; and (2) histamine-independent itch via PAR2/TRPV1, which means standard antihistamine therapy may be insufficient and treatments targeting the downstream immune cascade or PAR2 pathway are more mechanistically appropriate.

With an OR of 1.25, the individual risk increment is meaningful — larger than the other two CTSS signals — but context matters: the absolute risk depends on baseline atopic risk in the individual, and many G allele carriers will not develop clinically significant atopic dermatitis. The variant's value is in explaining the biological pathway if atopic disease is present, and in guiding treatment selection toward mechanism-aligned options.

Interactions

rs146527530 is one of three conditionally independent signals at the CTSS locus identified by Budu-Aggrey 2023, alongside rs187080438 and rs115161931. Each signal tags a distinct regulatory element; co-carriage of risk alleles at multiple signals compounds the CTSS expression effect additively. Within the broader atopic architecture, CTSS operates downstream of epithelial barrier variants (FLG, SPINK5): antigen penetrating through a disrupted barrier is then amplified by elevated CTSS-driven MHC class II presentation. Carriers of both barrier-disruption variants and CTSS-locus signals face a mechanistic double hit — more antigen entering and a more sensitive antigen-presentation machinery processing it.