Commentary highlights diagnostic ambiguity in reported FGFR1-rearranged acute mast cell leukemia case

A 2026 commentary argues that a previously reported case of primary acute mast cell leukemia with an FGFR1 rearrangement cannot be classified confidently from the published data alone, because flow cytometry, bone marrow immunohistochemistry, morphology and molecular findings do not fully resolve whether the prominent mast-cell proliferation was an independent leukemia or part of an FGFR1-rearranged myeloid/lymphoid neoplasm.

The article, published in Frontiers in Oncology, is a commentary rather than a new clinical study or case series. Its authors, Huizhen Du, Congcong Geng, Na Zhu, Zhao Li, Yalu Liu, Chunyan Liu, Jing Niu, Qian Wang and Hongwei Ren, introduce no additional patient data. Instead, they reassess the diagnostic evidence in a case report by Liu, Gao, Du, Ji and Li. The commentators state that the molecular association between the reported disease and FGFR1 rearrangement was already known and was not presented as a new observation.

The original report described two distinct abnormal populations in bone marrow flow cytometry. One accounted for 17.54% of cells and was identified as abnormal mast cells. A second population comprised 63.5% of nucleated cells, expressed strong CD33, weak CD11b and CD2, and lacked CD13, CD34, CD117 and CD203c. Because of this unusual phenotype, flow cytometry could not confidently establish the cellular origin of the larger population.

Bone marrow biopsy immunohistochemistry produced a different immunophenotypic pattern: the abnormal cells were reported to express CD117, CD25, CD2 and tryptase. The commentary treats this cross-platform discordance as the central diagnostic issue rather than as proof that the flow-cytometric population absolutely lacked CD117. On retrospective visual inspection of the published flow-cytometry plot, the authors say it may not be possible to distinguish complete CD117 negativity from very dim expression. The original case report also did not describe a distinct dual, dim-and-strong CD117 staining pattern on immunohistochemistry.

The authors emphasize that CD117 is useful but should not be interpreted alone. Citing a review of flow-cytometric pitfalls, they note that no single antigen is completely specific and that mast-cell identification requires a multiparameter approach incorporating marker combinations, gating strategy and awareness of interfering populations. In a case with an unusual immature phenotype, reported absence or very dim expression of CD117 or CD203c in flow cytometry should therefore prompt reconciliation with morphology, histology and complementary immunophenotyping before mast-cell differentiation is excluded. CD68 staining in the original report is not treated by the commentary as independent proof of mast-cell lineage; the interpretation instead relies on morphology and the mast-cell-associated findings, including tryptase.

FGFR1 rearrangement raises a separate classification issue. Current World Health Organization and International Consensus Classification frameworks recognize FGFR1-rearranged disease within the genetically defined spectrum of myeloid/lymphoid neoplasms with tyrosine-kinase gene fusions. These neoplasms may have heterogeneous myeloid and/or lymphoid manifestations, and eosinophilia, although present in many cases, is not obligatory. The original case included leukocytosis, splenomegaly and mildly FDG-avid enlarged hepatotactic and retroperitoneal lymph nodes. The main article did not report a lymph-node biopsy or provide an eosinophil count, leaving those aspects incompletely characterized.

The reported marrow morphology showed no dyserythropoiesis, ring sideroblasts, multinuclearity or myeloid blasts, and the original report used those observations to exclude an associated hematologic neoplasm. The commentary argues that these findings do not settle the classification question in the presence of an FGFR1 rearrangement. The complex karyotype included dic(8;11)(p12;q12); involvement of 8p11–12 is cytogenetically compatible with the FGFR1 rearrangement later demonstrated by break-apart fluorescence in situ hybridization, although the specific fusion partner was not established. The published data consequently cannot distinguish definitively between primary mast-cell leukemia carrying an FGFR1 rearrangement and an FGFR1-rearranged myeloid/lymphoid neoplasm with prominent mast-cell differentiation.

The molecular interpretation also has bounded limits. Illumina high-throughput sequencing reportedly found a wild-type KIT sequence as well as SETD2 and GNB1 mutations, while the FGFR1 rearrangement was demonstrated by FISH. The commentary says the report does not provide enough assay-level information to verify coverage of individual non-D816V KIT variants. Absence of the D816V mutation alone therefore should not be interpreted as excluding all activating KIT mutations, including reported non-D816V variants such as K509I, N819Y and I817V. The report also did not identify the CD117 antibody clone used for immunohistochemistry, confirm the flow-cytometric CD117 assessment with isotype controls, or report a separate RNA assay defining the FGFR1 fusion transcript.

Clonal identity between the phenotypically different cell populations also remains unestablished because the population reported as CD117-negative was not separately sorted for molecular analysis. The commentary’s conclusion is therefore cautious: when morphology, tissue immunophenotyping, flow cytometry and a defining genomic abnormality appear discordant, lineage assignment and disease classification require integrated review rather than reliance on CD117 or any single platform alone. Its practical contribution is to delineate what the published case establishes, what remains unresolved and which distinctions cannot be verified from the available assay and clinical information.

Subject of Research: Diagnostic and classification ambiguity in a reported FGFR1-rearranged acute mast cell leukemia case

Article Title: Commentary: Case Report: Experience of a rare case of primary acute mast cell leukemia with FGFR1 gene rearrangement

Article References: Du, H., Geng, C., Zhu, N., Li, Z., Liu, Y., Liu, C., Niu, J., Wang, Q., & Ren, H. (2026). Commentary: Case Report: Experience of a rare case of primary acute mast cell leukemia with FGFR1 gene rearrangement. Frontiers in Oncology, 16, Article 2001012. https://doi.org/10.3389/fonc.2026.2001012

Image Credits: AI Generated

DOI: 10.3389/fonc.2026.2001012

Keywords: FGFR1 rearrangement, acute mast cell leukemia, flow cytometry, CD117, immunohistochemistry, myeloid/lymphoid neoplasm, mast-cell diagnosis, case report commentary, molecular classification, Commentary, Case, Report