<i>TCF3::HLF</i> Fusion Gene in a Pediatric B‐Cell Lymphoblastic Lymphoma With Multifocal Osteolytic Lesions: Expanding the Clinical and Molecular Landscape

To the editor TCF3::HLF is a fusion gene resulting from the t(17;19)(q22;p13) translocation, associated with a rare (<1% of cases) and highly aggressive subtype of pediatric B-cell acute lymphoblastic leukemia (B-ALL) with dismal prognosis [1-3]. Due to the high resistance to standard chemotherapeutic regimens and the frequent occurrence of early relapses, these patients are eligible for experimental treatment, with a combination of chemotherapy, BCL-2 inhibitor, and immunotherapies targeting CD19 or CD22 showing particularly promising results [4, 5]. To date, all the reported cases of TCF3::HLF-associated malignancies presented as disseminated diseases. In this report, we describe the first case of a B-cell lymphoblastic lymphoma harboring the TCF3::HLF fusion gene, highlighting the diagnostic challenges and potential implications for treatment. A previously healthy 4.5-year-old female patient was admitted to the pediatric haemato-oncology unit for evaluation of multiple osteolytic bone lesions with associated soft-tissue components involving the skull, vertebrae, pelvis, ribs, and long bones (Figure 1A). She had a 5-week-long history of back pain in the lumbar region and torticollis 3 weeks prior to admission. Parents noticed palpable masses in the parietal and frontal head regions (Figure S1). One week prior to admission, she developed low-grade fevers and symptoms of a mild upper respiratory tract infection. She had no history of night sweats, polyuria, or polydipsia. Positron emission tomography-computed tomography (PET/CT) demonstrated disseminated FDG-avid lesions in the skull, including the skull base, and extensive involvement of both axial and appendicular skeleton. The metabolic activity corresponded to Deauville score 5 (Figure 1B). The clinical presentation raised suspicion for Langerhans cell histiocytosis; however, the characteristic BRAF V600E mutation was not detected in either circulating cell-free DNA (cfDNA) or in the subsequently obtained tumor tissue. Histopathological evaluation of a biopsy from one of the subcutaneous lesions unexpectedly suggested a centroblastic polymorphic variant of diffuse large B-cell lymphoma (DLBCL) with a germinal center B-cell-like (GCB) immunophenotype. This diagnosis was later challenged by flow-cytometric analysis of the same specimen, which demonstrated a lack of surface kappa and lambda light chain expression on the tumor cells, suggestive of an immature B-cell malignancy. Concurrently, targeted RNA sequencing for recurrent fusion genes in pediatric B-cell malignancies identified a TCF3::HLF fusion gene, type I [6]. A pathogenic variant in the NRAS gene (NM_002524.5:c.183A>C; p.(Q61H)) with a variant allele frequency (VAF) of 51% was also identified. Similar NRAS variants have been previously reported in association with the TCF3::HLF fusion [6]. Extended immunohistochemical and flow cytometry analysis of an additional biopsy sample confirmed nuclear expression of terminal deoxynucleotidyl transferase (TdT), further supporting the diagnosis of a precursor B-cell neoplasm (Figure 2). Based on this integrated immunophenotypic and molecular profiling, the diagnosis was revised to B-cell lymphoblastic lymphoma (B-LBL) with atypical morphology. Morphological analysis of the bone marrow (BM) aspirate did not reveal infiltration by malignant cells. However, minimal disseminated disease (MDD) analysis later demonstrated positivity at a level of 10−3 for both the TCF3::HLF fusion and lymphoma-specific IG/TR gene rearrangements. Analysis of cerebrospinal fluid (CSF) showed no evidence of malignant cell infiltration. At diagnosis, blood counts were within normal range, and biochemistry revealed only mildly elevated LDH levels (1.2 ULN), with no evidence of tumor lysis syndrome (TLS), and she had no coagulopathy. Guided by the molecular findings, the patient received an individualized treatment regimen. Initial induction therapy with dexamethasone, vincristine, and daunorubicin was adjusted to a combination of venetoclax and asparaginase (Figure S2A) based on the previously reported results of drug-response profiling of TCF3::HLF-positive acute lymphoblastic leukemias [5, 6]. With this tailored approach, the patient achieved molecular remission in BM (minimal residual disease (MRD) negativity for both IG/TR and TCF3::HLF) at the end of induction. She subsequently received one cycle of inotuzumab ozogamicin (Figure S2B) followed by a combination of dexamethasone, vincristine, and venetoclax as a bridge to transplantation (Figure S2C). A PET/MRI scan performed after the inotuzumab cycle demonstrated a complete metabolic response. Allogeneic hematopoietic stem cell transplantation (HSCT) from a matched unrelated donor was performed 4 months after diagnosis, following conditioning with etoposide and total body irradiation (TBI). During the first year after HSCT, she experienced recurrent flares of acute graft-versus-host disease (GvHD) grade II and is currently receiving immunosuppressive therapy with mycophenolate mofetil (MMF) for moderate chronic GvHD. The most recent follow-up, performed one year after HSCT, confirmed negative MRD in the BM aspirate and a negative PET/CT scan. The patient currently remains in complete remission 15 months after HSCT. We report the first documented case of a B-cell lymphoma with the TCF3::HLF fusion gene, expanding the spectrum of malignancies associated with this fusion. Our findings emphasize the importance of comprehensive molecular analysis in pediatric B-cell lymphomas with unusual presentations or diagnostic uncertainties. Although follow-up remains limited, this case illustrates that early molecular diagnosis and timely initiation of tailored treatment strategies can lead to remission even in highly aggressive disease subtypes. The authors were supported by the Ministry of Health of the Czech Republic in cooperation with the Czech Health Research Council under project No. NU23-05-00353. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.