Hereditary alpha tryptasemia and clinical implications

Abstract

Hereditary alpha tryptasemia was first described in 2016 and is the most common (up to 72%) cause of elevated serum basal tryptase (TPS). The clinical presentation of this condition, which is caused by copy number gains in the TPSAB1 gene encoding serum α TPS, is variable for each patient. Some patients are asymptomatic, whereas in others, especially those with increased mast cell activation, it has been associated with a higher risk of anaphylaxis. Better characterization of this entity is important to identify at-risk patients and to develop new treatment strategies. This review provided an overview of hereditary alpha tryptasemia and increased awareness of this condition by discussing the current information in the literature.

Key Words: Tryptasemia; Autosomal dominant; Hereditary alpha tryptasemia; Mast cell; Tyrosine kinase protein

Core Tip: Hereditary alpha tryptasemia was first described in 2016 and is the most common cause of elevated serum basal tryptase (TPS). The clinical presentation of this condition, which is caused by copy number gains in the TPSAB1 gene encoding serum α TPS, is variable for each patient.

INTRODUCTION

Tryptase (TPS) is a tetrameric serine protease molecule that is predominantly expressed and stored in tissue mast cells (MCs) and is also expressed in basophils in lesser amounts[1]. MCs are long-lived cells originating from CD34+/CD117+ pluripotent hematopoietic stem cells in the bone marrow and play a role in innate and acquired immunity as well as hypersensitivity reactions.

The majority of mature MCs are found in epithelial and mucosal tissues where the host encounters foreign antigens (such as skin, respiratory system, gastrointestinal system, and ocular tissue)[2]. Mature MCs contain cytoplasmic granules that store many mediators including TPS, histamine, heparin, leukotrienes, growth factors, cytokines, chemokines, serotonin, chymases, and cathepsins[3]. The activation of MCs is mainly initiated by high-affinity immunoglobulin E receptor Fc epsilon RI and a tyrosine kinase protein, c-KIT. Apart from these, there are many receptors such as complement receptors (CRs) (CR2, CR4, CR5), chemokine receptors (CCR) (CCR3, CCR5, and CXCR4), and toll-like receptors on the MC surface[4]. MCs are activated and degranulated through stimulation of these receptors. After degranulating MCs can remain in tissues for a long time by generating granules again[5]. Although basophils have similar granules to MCs, they contain 500 times less TPS than MCs[6]. Therefore, high TPS levels are mostly associated with excessive activation of MCs[7].

TPS FORMS, STRUCTURE, AND GENETICS

In MCs TPS is present in two different forms: (1) Monomeric pro-α TPS and pro-β TPS; and (2) Mature α TPS and β TPS in the tetrameric structure. Pro-TPSs are inactive TPS precursors and form mature active tetrameric TPSs after proteolysis and assembly. Mature TPSs are stabilized in MC secretory granules depending on heparin[6,8]. Serum basal TPS (sBT) is formed by inactive pro-α TPS and pro-β TPSs released continuously from unstimulated tissue MCs and then transported from the interstitial space to the peripheral circulation[9]. Mature forms of TPS are released only when MCs are activated[6]. The most commonly used clinical test measuring total TPS does not differentiate between pro-α TPS and pro-β TPS and cannot differentiate between mature and pro-TPSs; therefore, it is important to evaluate TPS measurement together with the clinical condition[3].

The TPS locus is located on the short arm of chromosome 16 and contains four TPS encoding genes (TPSAB1, TPSB2, TPSG1, and TPSD1). Although these four genes encode TPS, only the TPSAB1 and TPSB2 genes encode TPS isoforms measured as serum TPSs[9]. While the TPSB2 gene encodes β-2 and β-3 TPS isoforms, the TPSAB1 gene encodes α TPS and β-1 TPS. As a result of the competition of these α TPS and β-1 TPS alleles at the same locus, it is possible to observe individuals in which α TPS is completely absent[9-11]. The three defined β TPS isoforms are 99% similar to each other. The β TPS, which is the dominant form in MCs, is found in the blood both as an inactive monomer and as a heparin-bound tetramer[12,13]. The β TPS is the most effective TPS in the extracellular space and is assumed to play a role in physiological and pathological responses[14]. Two isoforms of α TPS, which is the second most important TPS after β TPS, have been defined as α-1 and α-2[15]. Although α TPS and β TPS have 95% similarity, they are not actually the same forms.

Mature β TPSs are neutral serine proteases with trypsin-like activity. This enzymatic activity of mature α TPSs is negligible, and it is a more positively charged molecule than β TPS[16]. In allergic reactions, the β TPS level increases in the blood with MC stimulation, but the α TPS level does not increase. Among other TPS forms, γ TPS is a transmembrane protein bound to the plasma membrane or secretory granule membrane. Delta TPS, on the other hand, is a TPS form that lacks a significant part of its active site and has very little catalytic activity. It is preferentially synthesized in MCs of the large intestine, lung, and heart[15].

In healthy individuals, the sBT level is generally stable and averages 5 ng/mL (< 1.0-30.7 ng/mL) in most individuals[17]. The upper limit (95 percentile value) of sBT level has been suggested as 11.4-15.0 ng/mL so far[17,18].

CONDITIONS ASSOCIATED WITH ELEVATED SBT

The sBT total serum TPS concentrations are used as the measure of body MCs burden in the clinic. The serum TPS level is important in the diagnosis of anaphylactic reactions in which MCs degranulate suddenly[19]. Since MCs are found in tissues rather than in the peripheral circulation, serum TPS will start to be highly detected in the blood approximately 30 min after the onset of symptoms and will return to the sBT value in time. For the diagnosis of anaphylaxis, a comparison of the serum acute TPS value taken 30 min after the onset of symptoms and within the first 2 h with sBT value has a high specificity and positive predictive value. The 1.2X sBT + 2 formula has been suggested for this purpose. If the patient does not have a previous sBT level, the TPS value 24 h after the symptoms completely disappear can be used as the sBT level[20-22].

More recently, Mateja et al[23] presented a new formula with higher specificity and sensitivity to confirm anaphylaxis in all patients [including patients with hereditary alpha tryptasemia (HAT) and systemic mastocytosis (SM)]. Accordingly, a serum acute TPS/sBT ratio of 1.685 or higher indicates anaphylaxis[23]. In addition to this information, it should be kept in mind that serum TPS levels may not always increase in anaphylaxis, especially in food-induced anaphylaxis, and a normal TPS value does not exclude anaphylaxis[24]. The sBT level may also be increased in various clinical conditions and diseases other than these conditions. TPS levels may be increased in patients with severe renal dysfunction[25,26] and parasitic infections[27,28]. TPS values are elevated in myeloid neoplasms (acute myeloid leukemia, chronic myeloid leukemia, myeloproliferative neoplasms, myelodysplastic syndrome, chronic myelomonocytic leukemia, chronic eosinophilic leukemia)[29,30] and MC diseases (SM, cutaneous mastocytosis), and the TPS value is used as a biomarker in the follow-up of these diseases (Table 1)[31]. In lymphoid neoplasms, TPS values generally remain stable[30].

Table 1 Conditions associated with high serum basal tryptase levels.

Etiology	Diagnostic methods	Serum basal tryptase range	Incidence
Hereditary alpha tryptasemia	ddPCR	8-50 ng/mL	≤ 67%
No disease is detected	Exclusion diagnosis	Variable	≤ 23%
Renal failure	Glomerular filtration rate < 60 mL/minute, elevated creatinine	10-50 ng/mL	≤ 16%
Systemic mastocytosis	Bone marrow aspiration, c-KIT D816V mutation by ddPCR, skin biopsy	20-200 ng/mL	≤ 5%
Other myeloid neoplasms, acute myeloid leukemia, myeloproliferative neoplasms, myeloid neoplasms with eosinophilia, myelodysplastic neoplasms	Complete blood count, bone marrow aspiration, molecular genetics, cytogenetics	15-50 ng/mL	Rare
Hypereosinophilic syndrome	Bone marrow aspiration, molecular genetics, cytogenetics	10-50 ng/mL	Rare
Chronic inflammatory diseases, rheumatoid arthritis, eosinophilic esophagitis	Rheumatological examinations, endoscopy, biopsy	5-25 ng/mL	Rare

ddPCR: Digital droplet PCR; KIT: Tyrosine kinase protein.

HAT

In 2014, Lyons et al[32] identified severe allergic phenotypes and elevated sBT levels in several families without evidence of any other underlying disease. These individuals had chronic and episodic symptoms such as cramping abdominal pain, flushing, diarrhea, urticaria, and a history of anaphylaxis (food anaphylaxis, venom anaphylaxis, idiopathic anaphylaxis) (Figure 1 and Figure 2A). The authors reported that these symptoms were induced by specific triggers such as stress, food, heat, vibration, exercise, trauma, and sometimes without any trigger. They initially suspected MC disease due to the similarity of symptoms and high levels of sBT; however, they failed to detect clonal MC disease and MC activation syndrome in most of the affected patients[32]. Two years later in 2016, Lyons et al[33] described HAT with high sBT levels resulting from congenital copy number gains in a single allele in the TPSAB1 gene encoding α TPS. Having identified a single aberration at the human TPS locus on chromosome 16p13.3 by exome and genome sequencing and linkage segregation analysis, Lyons et al[33] performed additional analysis of the TPSAB1 gene with a digital droplet PCR assay that validated the accurate detection of α TPS and β TPS sequences and direct measurement of copy numbers. Monoallelic extra alpha copies of the TPSAB1 gene were detected in the majority of patients, thus demonstrating an autosomal dominant inheritance pattern[34]. The prevalence of HAT has been reported in 4%-6% of the whole population in various studies[33,34]. On the other hand, HAT is the most common cause of high sBT, accounting for 64%-72% of cases in the general population with pathologically elevated sBT levels[35-37].

Figure 1 Approach in venom anaphylaxis. HAT: Hereditary alpha tryptasemia; c-KIT: Tyrosine kinase protein; sBT: Serum basal tryptase.

Figure 2 Relationship. The relationship between hereditary alpha tryptasemia (HAT), systemic mastocytosis (SM), hymenoptera venom anaphylaxis (HVA) and mast cell activation syndrome (MCAS). A: The relationship between HAT, SM, and HVA; B: Relationship between HAT and MCAS. HAT: Hereditary alpha tryptasemia; HVA: Hymenoptera venom anaphylaxis; MCAS: Mast cell activation syndrome; MMAS: Monoclonal mast cell activation syndrome; SM: Systemic mastocytosis; CM: Cutaneous mastocytosis.

HAT: CLINICAL FINDINGS

Not a single or uniform clinical phenotype is observed in patients with HAT. Some individuals may have no symptoms[38]. In one study, symptoms were reported in one-third of individuals with HAT with the same frequency as healthy individuals, while mild-to-moderate symptoms were reported in one-third, and significant symptoms were reported in the remaining individuals[33]. Functional gastrointestinal complaints are observed most frequently. Approximately half of the affected individuals reported irritable bowel syndrome (IBS)-like symptoms[39,40]. When samples taken from the duodenal and terminal ileum of patients with HAT were analyzed, it was observed that these individuals had more MCs than healthy individuals[41].

Recurrent skin symptoms including flushing and itching are observed in half of the symptomatic HAT cases. It has been observed that these symptoms usually occur spontaneously, but vibration and mild trauma are the most common triggers[37]. Mood changes (Table 2), chronic fatigue/pain, joint hypermobility, and autonomic dysfunction (orthostatic hypotension, tachycardia, presyncope, and syncope) are other reported clinical symptoms[40,42]. Multiple food intolerances, failure to thrive, and eosinophilic gastrointestinal disease have also been described in a few very symptomatic families with increased copies of TPSAB1[40].

Table 2 Clinical findings in hereditary alpha tryptasemia.

Clinical findings
Gastrointestinal symptoms	Nausea
	Abdominal pain
	Diarrhea
	Vomiting
	Dyspepsia
Neuropsychiatric findings	Cognitive disorders (such as memory disorders)
	Fatigue
	Sleep disorders
	Depressive disorders
	Nervousness
Cutaneous and allergic symptoms	Sudden hot flushes
	Pruritus
	Rash, urticaria
Pain	Muscle pain
	Headache
	Joint pain
Other findings	Systemic sudden hypersensitivity reaction
	Joint hypermobility

HAT, CUTANEOUS MASTOCYTOSIS, AND SM

The prevalence of HAT in patients with mastocytosis (systemic or cutaneous) is 2-3 times higher than in the general population (12%-21%) in Europe[34,43,44]. The prevalence of venom allergy in cases with SM and HAT is 3-fold higher (Figure 2) than in patients with SM alone[34,45].

HAT AND VENOM ANAPHYLAXIS

The rate of venom anaphylaxis in patients with HAT is not higher than in the general population[46,47]. However, more severe reactions are observed in patients with HAT as a result of insect bites[46]. Figure 1 shows the recommended approach algorithm according to the level of sBT and severity of anaphylaxis in patients who develop anaphylaxis as a result of insect bites (Figure 1 and Figure 2A).

HAT: DIAGNOSIS

Excluding individuals with HAT, the range of TPS in normal individuals was found to be mostly between 1.0 ng/mL and 11.4 ng/mL[37,48]. However, since lower basal TPS levels ranging from 8.0 ng/mL to 11.3 ng/mL (and rarely even lower) have been observed in some HAT-positive cases[18,38,40], a threshold value of 8.0 ng/mL has been recommended[18,38,40]. It was observed that higher sBT levels were observed in individuals with additional copy numbers of the TPSAB1 alpha gene. This resulted in sBT levels of 15.0 ng/mL ± 5.0 ng/mL in individuals with one extra gene copy number, 24.0 ng/mL ± 6.0 ng/mL in individuals with two extra gene copies, and 37.0 ng/mL ± 14.0 ng/mL in individuals with four extra copies[39].

Currently, the most reliable method (with 100% sensitivity and 90% specificity) for the detection of copy number and variations in the TPSAB1 gene is digital droplet PCR[3,40]. Commonly used genetic test indications for HAT are summarized in Table 3. If the HAT test is negative, the patient should be tested for the c-KIT D816V mutation in terms of SM, and bone marrow biopsy should be considered to exclude other bone marrow neoplasms. In asymptomatic individuals, there is no indication for basal TPS level measurement or genetic testing in most patients presenting with certain connective tissue abnormalities or symptoms of IBS[18]. In symptomatic patients with TPS levels higher than 15.0 ng/mL (Figure 2B and Figure 3), testing for the presence of the c-KIT D816V mutation in addition to the presence of HAT is important because of the high risk of developing MC activation syndrome in patients with SM, HAT, and immunoglobulin E-related allergy[49-51].

Figure 3 Clinical and laboratory approach to elevated serum basal tryptase. CBC: Complete blood count; GFR: Glomerular filtration rate; KIT: Tyrosine kinase protein; SBT: Serum basal tryptase; HAT: Hereditary alpha tryptasemia.

Table 3 Common indications for genetic testing for hereditary alpha tryptasemia.

sBT level ≥ 8 ng/mL
Recurrent signs of MC activation with unclear etiology (such as idiopathic anaphylaxis)
Suspected SM with negative c-KIT D816V, but very high TPS level
SM or MCAS with severe mediator-related symptoms
Serum TPS value > 15.0 ng/mL without underlying mastocytosis or myeloid neoplasm
Familial clustering of systemic MC activation symptoms
Higher levels of sBT according to the degree of bone marrow mast cell infiltration
Diagnosed or suspected idiopathic or secondary MCAS

MC: Mast cell; MCAS: Mast cell activation syndrome; sBT: Serum basal tryptase; SM: Systemic mastocytosis; TPS: Tryptase; KIT: Tyrosine kinase protein.

LIMITATIONS IN DIAGNOSIS

Since HAT is a newly defined disease, it is not well known by physicians, and it is difficult to differentiate it from other diseases without genetic testing. Genetic tests are not always available everywhere and can be confused with other allergic diseases with similar findings, making diagnosis difficult. Commonly used indications for genetic testing for HAT have been established as outlined in Table 3. Genetic testing for SM may be required if the HAT test is negative, and genetic testing may not be required in asymptomatic patients. In symptomatic patients with sBT levels higher than 15.0 ng/mL, it is important to test for the presence of the c-KIT D816V mutation in addition to the presence of HAT[49-51].

HAT: TREATMENT

In patients with HAT, treatment is recommended only in symptomatic patients with symptoms of MC activation (severe allergy or anaphylaxis)[3]. The World Allergy Organization and the European Mastocytosis Competence Network recommend that triggering factors should be well identified and avoided, recurrence should be prevented by allergen immunotherapy and desensitization if necessary, and a personalized emergency action plan should be established by ensuring the use of epinephrine in emergencies[52-55]. Since severe allergic reactions and anaphylaxis can be observed frequently in patients with HAT and SM together, it is important to establish an anaphylaxis emergency action plan, especially in these patients[33,53].

The combination of H1 and H2 antihistamines has been shown to provide relief, especially in gastrointestinal symptoms[56,57]. In some patients, it has been observed that intestinal symptoms were alleviated with MC stabilizers such as cromoglycate or flavonoid quercetin[40]. In the first study investigating the effect of omalizumab in symptomatic patients with HAT, the response rate to urticaria and anaphylaxis was found to be 100% in 13 patients, and the response rate to itching, rash, fatigue, nausea, and abdominal pain was found to be 50%.

In another study conducted with 18 patients, response to omalizumab was found to be 100% for urticaria and 80% for anaphylaxis[46,55]. Another candidate drug in the treatment of HAT and other MC-related disorders is AK002, an antibody that binds and inactivates the inhibitory receptor Siglec-8 on the surface of MCs, eosinophils, and to a lesser extent basophils. The safety and efficacy of this treatment continues to be investigated[58]. Maun et al[59] produced a non-competitive inhibitor antibody that binds to human α TPS and β TPS and induces dissociation of the active tetramer structure into its monomers. This antibody was shown to potently inhibit active β TPS in vivo. Since this antibody can also inhibit α/β TPS heterotetramers, it is thought to be a treatment option for patients with HAT[59].

LIMITATIONS IN TREATMENT

The most important limitation is the lack of different drugs and targeted therapies in addition to drugs such as antihistamines and omalizumab, which have been used since the past. Long-term management strategies for patients with mild or moderate symptoms are unclear. The efficacy and potential side effects of current older therapies are well known, but they remain palliative. Future development of targeted biological agent therapies and perhaps genetic therapies will result in more definitive and effective treatment.

CONCLUSION

HAT is inherited and is common in the population. It is the most common cause of elevated sBT. The recent discovery of HAT has led to a better understanding of the genetics, expression, and secretion mechanism of TPS. The demonstration that HAT increases the risk of severe allergic reactions and anaphylaxis, especially when associated with SM, has been important for the treatment of these patients. Further research is required to clarify how HAT plays a role in the pathogenesis of MC disorders. A comprehensive understanding of HAT may contribute to the development of new specific/targeted therapeutic methodologies for cases with MC disorders along with HAT.