Molecular genetics in mastocytosis and hypereosinophilia

Advanced molecular genetic testing is gaining ground in the diagnosis of mast cell diseases and hypereosinophilia. In addition to the typical mastocytosis-driver mutation KIT D816 as well as PDGFRA, PDGFRB, FGFR1, and PCM1-JAK2 rearrangements, the MLL uses advanced Next-Generation Sequencing (NGS) mutation analysis to inform the diagnosis and prognostic stratification of these diseases. These NGS gene panels are continuously updated to reflect the latest advances in scientific understanding.

Over 90% of systemic mastocytosis patients have an activating KIT gene mutation. In most cases this involves the KIT D816V mutation, which can be detected from blood or bone marrow with a very sensitive quantitative PCR test performed at the MLL. Occasionally, sequencing techniques are used to detect other KIT mutations. The presence of a KIT mutation is a diagnostic criterion for systemic mastocytosis and clonal mast cell activation syndrome (Valent et al. Blood 2017).

A proportion of Mastocytosis patients have additional mutations in other genes as well as the KIT D816V mutation. These additional mutations are particularly common in advanced systemic mastocytosis cases. Advanced mastocytosis patients with SRSF2, ASXL1, and RUNX1 gene mutations have been found to have particularly unfavorable prognoses but additional mutations can also have an independent unfavorable prognostic impact in non-mastocytosis patient subgroups (indolent systemic mastocytosis patients with NRAS and DNMT3A mutations for instance have also been shown to be correlated with unfavorable prognoses). This has led to the inclusion of detection of mutations in these genes in prognostic scoring systems for systemic mastocytosis patients (Pardanani et al. Blood Adv. 2018, Jawhar et al. JCO 2019, Muñoz-Gonzalez et al. Blood 2019, Reiter et al. Blood 2020). This year's European Competence Network on Mastocytosis (ECNM) conference specifically highlighted the importance of advanced molecular genetic diagnostics in the prognostic stratification and management of mastocytosis patients. To address this need, the MLL now also offers a new mastocytosis prognostic panel that includes the molecular genetic workup of the SRSF2, ASXL1, RUNX1, NRAS, and DNMT3A genes.

Molecular genetic diagnostics are also essential for the diagnostic workup of hypereosinophilia. Neoplasias presenting with eosinophilia as well as PDGFRA-, PDGFRB-, FGFR1- or PCM1-JAK2-rearrangements have been included as a distinct entity in the WHO classification since 2017. The MLL can detect these typical disease gene rearrangements at the cytogenetic level with FISH as well as with PCR-based molecular techniques. The differential diagnosis of hypereosinophilia should also consider Chronic Eosinophil Leukemia (CEL). CEL is characterized by the presence of clonality, which distinguishes it from idiopathic hypereosinophilic syndrome. The MLL can test for molecular genetic clonality in CEL by using an NGS gene panel which includes the most frequently detected mutations in the ASXL1, DNMT3A, JAK2, SRSF2, and TET2 genes, and which has recently been expanded to include mutations in the STAT5B gene. Recurrent STAT5B mutations occur in 1.6% of cases presenting with ambiguous eosinophilia. The detection of clonality allows these cases to be classified as CEL. STAT5B mutations have also been correlated with an unfavorable prognosis (Cross et al. Leukemia 2019, Morisa et al. Am J Hematol 2020, Reiter & Gotlib Blood 2017).