A guide to diagnosis and prognostic tools for AdvSM

Introduction 

Systemic mastocytosis (SM) comprises a heterogeneous spectrum of diseases characterized by neoplastic mast cell (MC) accumulation in extracutaneous organs.^1,2 Advanced SM (AdvSM) is rare, with a prevalence of 5.2–7 cases per million,³ and includes aggressive SM (ASM), SM with an associated hematologic/myeloid neoplasm (SM-AHN/AMN), and MC leukemia (MCL), with differences in terminology and classification between the World Health Organization (WHO) and International Consensus Classification (ICC) criteria.^4,5 The pathogenesis of SM is characterized by mutations in the KIT gene, which encodes a tyrosine kinase receptor that plays a key role in regulating MC function.⁶ The most common KIT mutation among patients with SM is the D816V mutation, which is present in ~90% of adult patients.⁶  

While patients with non-AdvSM have a near-normal life expectancy, those with AdvSM experience poorer outcomes.⁷ The rarity and variable clinical presentation of SM pose diagnostic challenges for healthcare professionals. Diagnostic delays are common in SM; in a survey of 56 patients with SM, a mean of 5.8 physicians were consulted before a diagnosis was reached, with an average of 5.6 years between symptom onset and diagnosis.⁸ A diagnosis of AdvSM is critically delayed in ~30–40% of patients and is likely to be overlooked if the patient is not referred to a specialist center.⁹ With the advent of KIT-targeted therapies such as midostaurin and avapritinib, the treatment landscape for AdvSM has evolved beyond palliative measures and chemotherapeutic agents.¹⁰ This has also created a need to reassess prognostic models developed in earlier treatment eras. Earlier recognition and accurate diagnosis are therefore required to enable timely treatment with targeted therapies, with reliable and validated tools to assess prognosis.  

When should AdvSM be suspected? 

As multiple organ systems can be affected, patients with AdvSM may present with various signs and symptoms, some of which are nonspecific.² For example, common allergic and hypersensitivity symptoms, such as pruritus, anaphylaxis, and gastrointestinal symptoms, are triggered by MC-derived mediators in patients with SM. MC accumulation can result in hepatosplenomegaly and fibrosis, while fatigue and weight loss are also common. Maculopapular mastocytosis lesions, typically observed skin lesions in SM, are less common in AdvSM. Osteopenia and osteoporosis can occur, while osteolysis is typically seen in AdvSM. Ascites is common, whereas liver cirrhosis and variceal bleeding are rare.² 

If any of the following are observed during routine examination, a diagnosis of AdvSM should be considered:² 

• Unexplained cytopenias/cytoses 
• Unexplained weight loss  
• Hepatosplenomegaly/lymphadenopathy  
• Mediator-related symptoms  
• Skeletal abnormalities 
• Ascites, portal hypertension, or other evidence of organ dysfunction  
• Absence or presence of maculopapular cutaneous mastocytosis  

Establishing a diagnosis of SM 

Prior to classification, suspected SM should be assessed using established diagnostic criteria (Figure 1). These criteria rely on MC infiltration, morphology and immunophenotype, KIT mutation testing, and serum tryptase levels. Although the WHO and ICC have the same diagnostic categories, the requirements for diagnosis differ. The WHO requires one major and one minor criterion, or ≥3 minor criteria, whereas the ICC requires one major or ≥3 minor criteria for diagnosis of SM.^4,5  

Diagnostic work-up 

Bone marrow biopsy and aspiration are central to the diagnosis of SM, with the biopsy considered the most important diagnostic step.³ A bone marrow examination is typically the first investigation undertaken because the bone marrow is almost universally involved, and histologic diagnostic criteria for non-bone marrow, extracutaneous organ involvement are not standardized.¹¹ Nevertheless, peripheral blood testing is playing an increasingly important role as a less invasive method for detecting the KIT D816V mutation prior to bone marrow biopsy.¹² However, a negative peripheral blood result should not exclude SM, and a bone marrow biopsy should be conducted when clinical suspicion persists.¹³ 

Bone marrow evaluation provides important diagnostic information from both the biopsy and aspirate. In SM, MCs form large aggregates, which may be associated with increased eosinophils, lymphoid aggregates, and/or fibrosis. The most common MC infiltration pattern in SM is focal/multifocal, characterized by aggregates of >15 MCs. Expression of CD2, CD25, and CD30 indicates an abnormal immunophenotype and constitutes a minor diagnostic criterion. These markers can be identified in the bone marrow, peripheral blood, or extracutaneous organs.^3,4 An aspirate provides information on MC morphology; the diagnostic criteria require ≥25% of MCs to be type I or type II or spindle-shaped, a feature which can also be assessed in extracutaneous sites.^3,11  

In addition to bone marrow findings, peripheral blood and serum assessments provide complementary information. Peripheral blood assessments provide evidence of cytopenias or cytoses.³ Elevated serum ALP is a common, but nonspecific, abnormality in AdvSM.² A persistently elevated serum tryptase level (>20 ng/mL) is considered a minor diagnostic criterion in both the ICC and WHO guidelines. In AdvSM, tryptase levels may exceed 200 ng/mL.¹¹ However, in the presence of an AHN/AMN, elevated serum tryptase levels may be unreliable and therefore do not qualify as a diagnostic criterion, as increased levels (>15 µg/L) are seen in patients with AML, CML, and MDS without SM.^11,14 In patients with hereditary α-tryptasemia (HαT), caused by increased TPSAB1 gene copy numbers encoding α-tryptase, WHO guidelines suggest that basal serum tryptase levels be adjusted, as these patients typically experience elevated levels (8–50 ng/mL).^4,15,16 However, there is a lack of consensus on the appropriate correction method.² A higher prevalence of HαT is observed among patients with SM, with 15–20% of patients with SM carrying TPSAB1 replications, which should be taken into consideration.¹⁶  

As the pathogenesis of SM is strongly associated with gain-of-function KIT mutations, genetic testing is warranted in SM diagnosis and detection of an activating KIT mutation is a minor diagnostic criterion.¹⁶ Due to the relatively low burden of MCs, assays with high sensitivity must be used.¹⁷ Allele-specific oligonucleotide quantitative polymerase chain reaction (ASO-qPCR) and digital droplet PCR (ddPCR) are considered gold-standard techniques, with a sensitivity of 0.01%.^3,13,17 False-negative results can arise due to the low level of nucleic acid material when using peripheral blood, which falls below the level of detection.³ A sample with a higher mutation burden, such as a bone marrow aspirate, should be used in such cases.³ Although typically prevalent, a negative KIT D816V mutation result detected by a highly sensitive assay in a suspected patient should prompt investigation of other activating KIT mutations.³ Molecular profiling of mutations associated with AHN/AMN, which are more prevalent in AdvSM compared with non-AdvSM, can be achieved using next-generation sequencing (NGS).¹⁷ As an AHN/AMN is associated with poor prognosis, early recognition of SM-AHN/AMN is critical for timely intervention.¹⁷ 

Classifying AdvSM 

Once a diagnosis of SM has been confirmed using the criteria and methods previously mentioned, the next step is to determine the SM subtype (Figure 2). Of the AdvSM subtypes, ASM is defined by ≥1 C-finding; C-findings reflect organ damage, and include cytopenia(s), hepatosplenomegaly, and bone lesions.^11,17,18 SM-AHN/AMN is characterized by clonal MC proliferation that meets the diagnostic criteria for SM and another myeloid/hematologic neoplasm.^2,19 The ICC classification criteria specify SM-AMN, while the WHO criteria specify SM-AHN.^4,5 MCL is defined by ≥20% neoplastic immature MCs in the bone marrow.¹⁹ B-findings (indicative of high MC burden) are typically observed in non-AdvSM.¹⁷ 

Prognostic assessment 

Risk stratification in AdvSM can dictate first-line treatment choice and ultimately improve outcomes for patients. Several scores have been developed to assess the prognosis of patients with AdvSM,¹ including the Global Prognostic Score for Mastocytosis (GPSM), International Prognostic Score for Mastocytosis (IPSM), Mayo Alliance Prognostic System (MAPS), Mutation-adjusted Risk Score (MARS), and Revised MARS (MARS-R) (Table 1).^20–24 Several scores exist due to the heterogeneity of AdvSM, which encompasses different subtypes with variable clinical features and molecular abnormalities. For example, IPSM, MARS, and GPSM were developed in different patient populations and incorporate different combinations of clinical, laboratory, and molecular risk factors.²⁵ The aforementioned scores do not include the presence of a KIT mutation for prognostication; instead, other high-risk somatic mutations are investigated (e.g. SRSF2, ASXL1, or RUNX1) which have been associated with poor prognosis.^20–24 Selection of a scoring system in clinical practice may involve balancing factors such as minimal invasiveness and low burden for the patient, (for example, using blood-based monitoring) against the stability and reliability of mutation-based assessments.²³ 

The MAPS clinical risk model uses five easily attainable clinical variables to allow prediction of survival: age >60 years, diagnosis of AdvSM, thrombocytopenia <150 × 10⁹/L, anemia, and increased ALP.²¹ Survival is proportionally correlated with the number of risk factors, with more favorable outcomes for patients with ≤1 risk factor (median survival, not reached) and poorer outcomes for patients with ≥4 risk factors (median survival, 9–27 months).²¹ A clinical-molecular model was also developed based on the presence of adverse mutations (ASXL1, RUNX1, and NRAS); which replaces anemia in the aforementioned model.²¹ The GPSM-Progression Free Survival (GPSM-PFS) and GPSM-Overall Survival (GPSM-OS) models were developed based on prognostic factors for PFS (platelet count, serum β2-microglobulin, and tryptase levels) and OS (hemoglobin [Hb], serum ALP, and ≥1 mutation in SRSF2, ASXL1, RUNX1, or DNMT3A), respectively.²³ The IPSM considers age, tryptase level, leukocyte count, Hb level, platelet count and skin involvement as variables for prognostication of AdvSM.²⁰ The total risk point score dictates the risk group (AdvSM 1–4). The MARS score is a 5-parameter, WHO-independent scale, based on age >60 years, anemia, thrombocytopenia, the presence of 1 high molecular risk gene mutation (SRSF2, ASXL1, or RUNX1), all assigned one point each; and ≥2 high-risk mutations, assigned two points.²⁴  

Prognosis in the era of KIT-targeted therapy 

The development of KIT-targeted treatments, such as midostaurin and avapritinib, has improved the prognosis of patients with AdvSM.²⁶ However, the current risk scores for AdvSM do not reflect the current treatment landscape; for example, MARS was developed and validated in patients treated with midostaurin or cladribine.²⁴ Existing scores also rely on categorical variables, which do not provide information on relationships between risk factors and outcomes.²² The MARS-R was therefore developed as a continuous scale risk scoring system to overcome these limitations.²² MARS-R comprises three continuous variables (age, monocyte count, and platelet count) and five categorical variables (sex, skin involvement, and presence of ASXL1, RUNX1, and SETBP1).²² An individual value is provided for each patient, with negative or positive scores reflecting better or worse OS compared to the average-risk patient with AdvSM.²² The score was developed and internally validated in patients with KIT D816V-positive AdvSM who were treated with midostaurin (n = 120) or avapritinib (n = 167), from the German Registry on Disorders and Eosinophils and Mast Cells (GREM) and EXPLORER and PATHFINDER studies, respectively, independent of WHO-defined subgroups.²² Internal validation demonstrated that the MARS-R outperformed MARS and IPSM in terms of risk assessment, independent of treatment, and showed that avapritinib demonstrated longer OS compared with midostaurin across all three MARS-R risk groups.²² In clinical practice, the MARS-R can be independently applied to KIT D816V-positive patients with AdvSM when initiating midostaurin or avapritinib treatment.²² MARS-R has been externally validated in a separate real-world cohort of KIT D816V-positive patients with AdvSM (n = 103).²⁷ Further validation in larger real-world cohorts, including patients with KIT D816V-negative AdvSM, is needed. 

Table 1. Comparison of prognostic tools for AdvSM* 

● = 1 point; ●● = 2 points; ●●● = 3 points; ↑, higher; ↓, lower. 
*Data from Sperr, et al.²⁰, Pardanani, et al.²¹, Lübke, et al.²², Muñoz-González, et al.²³, and Jawhar, et al.²⁴ 
AdvSM, advanced SM; cont, continuous; GPSM, Global Prognostic Score for Mastocytosis; HR, high-risk; IPSM, International Prognostic Score for Mastocytosis; IR, intermediate-risk; LR, low-risk; MAPS, Mayo Alliance Prognostic System; MARS, Mutation-adjusted Risk Score; MARS-R, Revised MARS; OS, overall survival; PFS, progression-free survival; SM, systemic mastocytosis. 

Future perspectives 

Applying machine learning models to electronic health record data may help identify patients who should be screened for SM, allowing earlier diagnosis.²⁸ Routine use of highly sensitive molecular assays is required to confidently detect the presence of genetic mutations associated with AdvSM.²⁵ For high-risk patients in particular, single-cell genomics would allow characterization of the molecular signature and bone marrow microenvironment to guide treatment responses.²⁹ The Flow-SuperRCA assay has demonstrated higher sensitivity than ASO-qPCR for detection of KIT D816V in bone marrow and peripheral blood samples, which could reduce false-negative rates and streamline diagnosis.¹² Finally, dynamically assessing the clinical course of patients with AdvSM using baseline and on-treatment parameters, such as additional somatic mutations and serum ALP, may optimize prognosis and aid in treatment decision-making.³⁰ 

Conclusion 

AdvSM diagnosis requires active clinical suspicion followed by integrated morphologic, immunophenotypic, biochemical and molecular assessment. As a rare disease with a broad spectrum of clinical and molecular findings, this can be challenging to diagnose. However, a correct and timely diagnosis ensures that patients receive appropriate treatment.³ While the KIT D816V mutation drives the majority of AdvSM cases, its absence does not preclude diagnosis or assessment of prognosis. There are several risk scores available to assess prognosis, which combine different variables to stratify patients. As KIT-targeted therapies reshape outcomes, prognostic assessment will continue to evolve, as demonstrated by development of the MARS-R score. Advances in diagnostic methods and monitoring may enable earlier diagnosis and accurate prognostication, potentially improving outcomes for patients with AdvSM. 

This educational resource is independently supported by Blueprint Medicines. All content is developed by SES in collaboration with an expert steering committee. Funders are allowed no influence.