Prevalence of Autoantibodies in Patients with Hereditary Alpha-Tryptasemia

Hereditary alpha-tryptasemia (HαT) is an autosomal dominant trait estimated to occur in approximately 5% of the Caucasian population and is especially prevalent among patients diagnosed with clonal mast cell (MC) disorders [1]. Individuals with HαT have increased copies of the gene encoding alpha-tryptase at TPSAB1, on one or both chromosomes [2]. Genotypes with as many as four extra alpha-tryptase-encoding loci have been reported [3].

There are several phenotypic traits associated with HαT. Individuals with HαT are at increased risk of both severe Hymenoptera venom anaphylaxis as well as unprovoked anaphylaxis, as compared to individuals without HαT [4]. For this reason, HαT is regarded a as a modifier of MC disorders, including anaphylaxis [5]. Given the minimal proteolytic activity associated with alpha-tryptase homotetramers, it is unclear why patients with HαT experience a greater risk of urticaria and anaphylaxis. It may be due to the formation of alpha-/beta-tryptase heterotetramers, which have been shown to have greater stability and proteolytic activity compared to beta-tryptase homotetramers [6]. The former cleave the mechanosensing adhesion G protein-coupled receptor E2, resulting in MC degranulation which triggers cutaneous flushing, pruritus, and hives for many HαT-positive patients [6]. Heterotetrameric alpha-/beta-tryptase also cleaves and activates protease-activated receptor 2 (PAR2) in endothelial cells inducing vascular permeability in a PAR2-dependent manner [6]. A positive association exists between the number of gene copies encoding alpha-tryptase and the proportion of alpha-/beta-tryptase heterotetramers to total tryptase [6]. This alpha-tryptase gene-dose effect might account for increased symptoms of MC activation in individuals with HαT.

Postural orthostatic tachycardia syndrome (POTS), hypermobile Ehlers-Danlos syndrome (hEDS), and mast cell activation syndrome (MCAS) represent a disease cluster which may cosegregate, although the data are not yet entirely clear [2, 7‒9]. While none of these conditions is currently understood to be autoimmune in nature, there appears to be a higher prevalence of autoimmunity in individuals with POTS and those with hEDS relative to the general population. In a large cohort of patients with POTS, 16% had one or more autoimmune diseases, including Hashimoto’s thyroiditis (6%), celiac disease (3%), rheumatoid arthritis (RA) (2%), and systemic lupus erythematosus (SLE) (2%), all greater than the prevalence in the general population [10]. hEDS has been associated with celiac disease and eosinophilic esophagitis [11, 12]. Several studies have also examined the prevalence of autoantibodies, as opposed to established autoimmune diseases, in patients with POTS. Thyroid autoantibodies, antiphospholipid antibodies (APLA), and parietal cell antibodies are present in patients with POTS [13, 14]. Unlike the cohort studies above which have reported associations between POTS, hEDS, and autoimmune disease, to our knowledge, only one case series has reported this association in the MCAS population [15]. That study described 3/5 patients with preexisting diagnoses of either RA, SLE, or Sjogren’s disease. Since patients with HαT may also have features of MCAS, hEDS, and POTS, we undertook an investigation of the prevalence of autoantibodies in individuals with HαT.

We studied a cohort of patients who were referred to a tertiary-care allergy clinic in a university teaching hospital. These individuals were referred for investigation of immunologic phenomena including anaphylaxis, chronic urticaria, or other ill-defined symptom clusters. The study was approved by the St. Michael’s Hospital Research Ethics Board. Patients seen in clinic between 2018 and 2022 were included in the study. Serum tryptase levels were ordered as part of the routine investigations for symptoms of MC mediator release or were available with the referral. Individuals with baseline serum tryptase equal to or greater than 8 μg/L underwent tryptase genotyping for HαT (Gene by Gene, Houston, TX). Patients aged 15 years or older in whom tryptase genotyping was diagnostic for HαT were included. Patients who were HαT-negative were excluded. Demographic data including HαT genotype status and specific allergic history were extracted on chart review.

The following autoantibody serology results were extracted from the allergy clinic’s medical record: thyroid peroxidase antibody (anti-TPO), thyroglobulin antibody (anti-TG), APLA, antinuclear antibody (ANA), rheumatoid factor (RF), anticardiolipin antibody, parietal cell antibody, and anti-transglutaminase IgA (anti-tTG). Total serum IgA was also measured to ensure patients were not deficient (defined as IgA <LLN). Patient-reported history of the following autoimmune diseases was also extracted: Hashimoto’s thyroiditis, celiac disease, RA, and SLE.

We performed a thorough literature search to determine the reported prevalence in the general population of the autoantibodies and autoimmune diseases noted above. OVID Medline and EMBASE were searched using key words including “autoantibodies,” “population-based,” and “prevalence” to identify the largest, most diverse general population cohorts for each autoantibody (online suppl. material, available at https://doi.org/10.1159/000541880) and autoimmune disease investigated, with autoantibody reference ranges close to those employed in our diagnostic laboratories (anti-TPO <35 IU/mL, anti-TG <40 IU/mL, ANA titer <1:80, RF <14 IU/L, anticardiolipin antibody IgG <20 U/mL, parietal cell antibody titer <1:20, anti-tTG IgA <12 U/mL). Having comorbid disease was not an exclusion criterion for any cohort.

We used descriptive statistics including frequency with percentage, mean with standard deviation, and median with interquartile range, when appropriate. We used N-1 Chi-squared test to compare the proportions of autoantibody positivity and diagnosed autoimmune diseases in our cohort of HαT individuals against the reported proportions in the general population. All statistical analyses were performed using Microsoft Excel. A cutoff p value <0.05 was considered statistically significant.

We identified a total of 101 patients with HαT confirmed through tryptase genotyping. The median age at the time of HαT diagnosis was 43 years old (range 15–75), and most were female (87/101; 86.1%). The mean serum tryptase level was 15.3 μg/L ± 5.0, and the proportions of HαT genotypes were as follows: 42.6% 3α/2β, 46.5% 2α/3β, 6.9% 4α/2β, and 4% 3α/3β. On history taking with the allergist, self-reported drug hypersensitivity was present in 52/101 (52.5%) patients. POTS and hEDS were diagnosed in 15/101 (14.9%) and 13/101 (12.9%) patients, respectively. No patients had cutaneous or systemic mastocytosis.

Among patients with established diagnoses of autoimmune diseases by their respective specialists, the prevalence of Hashimoto’s thyroiditis, SLE, RA, and celiac disease were 3/101 (3%), 1/101 (1%), 0/101 (0%), and 5/101 (5%), respectively (Table 1). Autoantibody results completed within our cohort included anticardiolipin and anti-tTG (61/101) and ANA (78/101) (Table 2). The specific proportions of anti-TPO, anti-TG, APLA, ANA, RF, parietal cell antibody, and anti-tTG were 8/70 (11.4%), 5/66 (7.6%), 1/61 (1.6%), 10/78 (12.8%), 2/64 (3.1%), 2/63 (3.2%), and 3/61 (4.9%), respectively (Table 2). There were no statistically significant differences in the frequency of positive autoantibodies in males versus females or between duplication and triplication genotypes.

Prioritizing general populations, the available reference estimates of ANA, anti-TPO, anti-TG, RF, and anti-tTG prevalence were from the US National Health and Nutrition Examination (NHANES) (Table 2) [16, 17]. Similarly, for APLA and parietal cell antibody, the best available reference cohorts were from Germany and Italy, respectively [18, 19]. In applying similar search criteria, we identified four reference cohorts which reported prevalence of Hashimoto’s thyroiditis, SLE, RA, and celiac disease (Table 3) [20‒23]. All reference cohorts reported confirmed diagnoses of autoimmune disease with the exception of the SLE cohort which reported self-reported diagnoses.

We compared the prevalence of autoantibodies between our study cohort and that of the general population. The proportion of individuals with HαT with positive tTG antibody (3/61, 4.9%) was significantly higher than that reported in the general population (133/16,667, 0.8%) (p < 0.001) (Table 2). Two of the three individuals who were seropositive had preexisting diagnoses of celiac disease.

We also compared the prevalence of established autoimmune diseases within our cohort to the prevalence of these autoimmune diseases in general populations. The proportions of previously diagnosed SLE (1/101, 1%) and celiac disease (5/101, 5%) in our cohort were found to be significantly higher than the prevalence in the general population (194/96,996, 0.2% [p = 0.035] and 26/2,845, 0.9% [p < 0.001], respectively) (Table 3). Of the five individuals with celiac disease, two tested positive for tTG antibody, two declined testing and one had an undetectable tTG antibody (gluten-free diet).

In the course of clinical practice, we had noted that individuals with HαT tended to have comorbid diagnoses of autoimmune disease. We set out to formalize these observations in a large cohort of patients diagnosed with HαT. We found a marked increase in the prevalence of self-reported drug hypersensitivity, celiac disease, SLE, and anti-tTG positivity relative to estimates in the general population.

Of the panel of autoantibodies and previously diagnosed autoimmune disorders screened for in our cohort of HαT individuals, we found a significantly higher proportion of positive tTG antibody, comorbid SLE, and celiac disease compared to general population estimates. Furthermore, our finding of increased proportion of tTG-positive antibodies in individuals with HαT is likely underestimated given that two individuals with confirmed celiac disease declined anti-tTG testing and 1 other patient with confirmed celiac disease was on a gluten-free diet, leading to a false-negative result. One cohort of HαT patients did not identify an increased proportion of anti-tTG antibodies relative to controls; however, biopsies showed intestinal epithelial cell changes with signs of immunologic activation and barrier dysfunction that were histologically similar to inflammatory conditions such as Crohn’s disease or celiac disease but distinct from IBS or functional GI disorders [24]. One possible mechanism underlying these changes causing barrier dysfunction may be the ability of heterotetrameric alpha-/beta-tryptase to cleave and activate PAR2, increasing intestinal permeability during episodes of inflammation [5, 25]. This mechanism may play a role in the pathogenesis of autoantibody production as well. Our results do not show a correlation between tryptase levels or numbers of alpha-tryptase copies and the presence of tTG antibody. Overall, these findings suggest that those with diagnosed or suspected HαT may benefit from increased screening and longitudinal follow-up for celiac disease and SLE.

Comorbid autoimmunity in HαT is in keeping with that described in the POTS and hEDs populations [10‒14]. A recent cohort of patients with POTS was found to have a similar prevalence of HαT to that of the general population [26]. In contrast, our cohort of HαT patients appears enriched in comorbid POTS (14.9%) versus estimates in the general population (0.2%), in keeping with prior cohorts of HαT [2, 27].

We have characterized a cohort of individuals with HαT. In this patient population, we note a marked increase of self-reported drug hypersensitivity (52.5%) as compared to the general population [28] (8.3%) and to patients with clonal MC disorders, such as systemic mastocytosis (18.6%) [29‒33]. Individuals with these MC disorders are generally advised to avoid medications that act as MC secretagogues [34]. Fewer studies have studied drug hypersensitivity in HαT populations. One case report described a HαT-positive woman who developed recurrent anaphylaxis and an episode of cardiac arrest following administration of paclitaxel and PEGylated liposomal doxorubicin for treatment of ovarian cancer [35]. Recurrent anaphylaxis to additional MC secretagogues was refractory premedication with cetirizine and montelukast and was ultimately successfully treated with omalizumab 300 mg q4 weeks, suggesting its efficacy as a first-line adjunct treatment for those with severe reactions. A large cohort of 101 patients with HαT described 30% with self-reported drug anaphylaxis [9]. Eleven of these patients with recurrent and refractory anaphylaxis were treated with omalizumab, ten of whom achieved suppression of anaphylaxis during their follow-up period. Mechanistically, one hypothesis for this increased risk is simultaneous activation of adhesion G protein-coupled receptor E2 or PAR2 by drugs and heterotetrameric alpha-/beta-tryptase, pathways that are postulated to drive MC activation in HαT.

This study has limitations due to its retrospective and single-center design. There is likely a significant referral bias, as patients may be referred because of the presence of symptoms of MC activation or because of coexistent POTS or hEDS. Completion of autoantibody results varied widely among patients due to several patient factors including failure to complete ordered laboratory investigations, inability to pay out-of-pocket for certain tests not covered by public health insurance, or comorbid conditions rendering tests unreliable (i.e., tTG antibody in patients on a gluten-free diet). Despite the relatively large size of our cohort, these incomplete panels make our estimates less accurate. Furthermore, given the sampling methods within our comparator autoantibody cohorts, it is possible that patients within these general populations have undiagnosed comorbid disease. Future prospective studies describing the prevalence of (i) autoantibodies in a larger cohort of individuals with HαT will help confirm these results; this question should be investigated in populations who fulfill the diagnostic criteria for MCAS as well. Conversely, tryptase genotyping for HαT in cohorts with confirmed autoimmune diseases will be informative. Finally, longitudinal monitoring of seropositive individuals for the subsequent diagnosis of an autoimmune disease will be important.

In conclusion, our study showed an increased prevalence of self-reported drug hypersensitivity, celiac disease, SLE, and anti-tTG positivity relative to reference general populations. These findings suggest that patients with HαT would benefit from screening and longitudinal follow-up for autoimmune diseases, namely, celiac disease and SLE as well as for drug hypersensitivity reactions. These findings offer insight into comorbid autoimmune conditions that might contribute to the broad constellation of symptoms seen in this patient population.

This study protocol was reviewed and approved by the Research Ethics Board of St. Michael’s Hospital (Approval No. 18-022). Written informed consent from participants was not required for the study presented in this article in accordance with local/national guidelines.

The authors have no conflicts of interest to declare.

There was no funding acquired for the current study.

C.S., E.L., and P.V. contributed to study design and data interpretation and collectively wrote the manuscript. C.S. performed data collection. C.S. and E.L. conducted data analysis. All authors approved the final manuscript and vouch for the integrity of the work.

Edited by: H.-U. Simon, Bern.

The data that support the findings of this study are not publicly available due to their containing information that could compromise the privacy of research participants but are available from Dr. Peter Vadas upon reasonable request.