Lyons 2019 — HαT genotyping and the clinical phenotype

One-paragraph summary

Hereditary alpha-tryptasemia (HαT) is an autosomal-dominant trait caused by germline copy-number variation — duplications or triplications — of the alpha-tryptase encoding portion of the TPSAB1 gene. Lyons et al established a droplet-digital-PCR genotyping assay capable of resolving alpha-tryptase copy number and applied it to 35 multi-generation families. Population frequency of HαT is approximately 5% (1 in 20 individuals of European ancestry), making it the most common known genetic determinant of elevated baseline serum tryptase. The phenotype is multi-system: hypersensitivity reactions including anaphylaxis, dysautonomia (POTS, orthostatic intolerance), gastrointestinal symptoms (IBS-like), connective-tissue features (joint hypermobility, EDS-spectrum overlap), chronic widespread pain, and fatigue. The phenotypic overlap with FM, ME/CFS, and POTS is large and clinically recognized. For this project, HαT functions as a heritable genetic stratifier for the H2-MC-active subtype that is independent of the variable serum-tryptase measurement: a TPSAB1 genotype either shows the duplication/triplication or it doesn't, removing the dynamic-range problem of clinical tryptase assays. Critically, downstream literature (Lyons group and others) reports that ~80% of HαT/MCAS patients have reduced intraepidermal nerve fiber density on skin biopsy — i.e. small fiber neuropathy. This puts HαT at the intersection of H2 (mast-cell) and the SFN clinical axis.

Claims as triples

• TPSAB1_duplication — causes → elevated_serum_tryptase [evidence: ddPCR genotype-phenotype correlation across 35 families; confidence: established]
• TPSAB1_duplication — predicts → fm_autoimmune [evidence: phenotypic overlap with FM/MCAS/POTS; confidence: bridging]
• TPSAB1_duplication — correlates_with → mast_cell_activation [evidence: tryptase elevation + activation symptoms; confidence: emerging]
• TPSAB1_duplication — correlates_with → small_fiber_neuropathy [evidence: ~80% reduced IENFD in HαT/MCAS per follow-up literature; confidence: emerging]
• TPSAB1_duplication — correlates_with → autonomic_dysregulation [evidence: POTS phenotype prevalence in HαT cohorts; confidence: established]
• TPSAB1_duplication — correlates_with → ibs [evidence: GI symptom prevalence in HαT cohorts; confidence: emerging]

Methods note

Cross-sectional family-based study, n=35 multi-generation families ascertained for elevated baseline serum tryptase. Droplet-digital-PCR assay developed for TPSAB1 alpha-tryptase copy number (resolving 2, 3, 4, or 5 copies). Clinical phenotyping via standardized symptom questionnaire across 96 affected individuals. Population frequency estimated via genotyping of 125 unselected blood-bank donors. No interventional arm.

Limitations

• HαT is not synonymous with MCAS. Many MCAS patients are HαT-negative; many HαT carriers are clinically minimally affected. The genotype is a risk modifier, not a diagnostic.
• Phenotypic overlap with FM is observational and unstratified. The study did not enroll FM patients per se; the FM-overlap inference is from subsequent literature applying the assay to FM/POTS/EDS cohorts.
• The 80% SFN-in-HαT figure is from follow-up literature, not this paper; should be verified separately before treating it as a load-bearing claim.
• Mechanism is upstream. Lyons et al show a genetic cause for elevated tryptase but do not demonstrate that elevated tryptase is causal for downstream pain or SFN — those connections rest on other mast-cell biology.

Open questions raised

• Does TPSAB1 genotyping in an FM cohort identify a cleanly definable MC-active subgroup? (Operationalizes Q24.)
• Are HαT-positive FM patients a distinct subtype, or are they distributed across the project's H1/H2/H3 chains? Tractable in any FM cohort with stored DNA.
• Does TPSAB1 copy number predict response to high-dose continuous cromolyn? Christoforou 2026 protocol enrolled MCAS-stratified ME/CFS without TPSAB1 genotyping; adding it would refine the responder prediction.

Triangulation notes

• Provides the heritable substrate the H2 chain needed. White paper v0.2 §6.2 cited HαT as "the genetic substrate for the MC-active subtype" without a backing paper file; ingestion closes that gap.
• Operationalizes Q24 (overlap of anti-SGC IgG positive vs. MC-active FM patients) with a genotypic stratifier independent of the variable tryptase assay. The biomarker-mapping cohort can include TPSAB1 ddPCR at minimal cost (~$30/sample).
• Compatible with Christoforou 2026's MCAS-stratified high-dose cromolyn protocol. TPSAB1 genotyping as the stratification gate would tighten the inclusion criteria and improve effect-size detection.
• Bridges the MC and SFN axes. ~80% SFN in HαT/MCAS aligns the H2-MC subset with the established SFN clinical phenotype in FM (40-60% prevalence). Suggests the SFN phenotype itself may be partly HαT-driven in a subset of patients.

Bridges

• Strengthens B9 (endometriosis ↔ FM via estrogen-MC activation) by adding a heritable factor that pre-disposes to MC activation independent of estrogen signalling — i.e. estrogen-MC and HαT-MC are two routes to the same H2-active phenotype.
• New candidate B12 — HαT/MCAS ↔ FM via TPSAB1-mediated MC over-reactivity. Genotype-driven, population-frequency-defined, and clinically tractable. The bridge closes if TPSAB1 genotyping enriches the FM-cohort signal for MC-active features (tryptase, chromogranin A, H2-pattern symptoms).