PHILADELPHIA CHROMOSOME-NEGATIVE CHRONIC MYELOID LEUKEMIA WITH BCR::ABL1 FUSION

Abstract

Introduction: Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm characterized by the chromosome philadelphia (Ph) in 90-95% of cases. The Ph chromosome results from the reciprocal t(9;22)(q34.1;q11.2) and encodes the fusion gene BCR::ABL1. The chimeric tyrosine kinase oncoprotein is constitutively activated, leading to continuous cell proliferation and driving leukemogenesis. Case report: Herein, we present a rare case of a 36-years-old male patient diagnosed with Ph-negative CML. The patient was admitted to the Hematology Department of Instituto Nacional de Câncer (INCA) in September 2019 presenting persistent fever, splenomegaly, weight loss and night sweats. Blood workup showed significant leukocytosis (229.450/μL), thrombocytosis (639.000/μL) and eosinophilia (16.000/μL, 7%). At diagnosis, the patient exhibited 5% of myeloid blasts in blood and the b2a2 isoform of BCR::ABL1 was detected by qualitative reverse-transcription polymerase chain reaction (RT-PCR). The patient was classified as high risk according to the Sokal Index (2.1) and his initial treatment with hydroxyurea was followed by Imatinib (600 mg/day). Molecular monitoring was performed by qualitative RT-PCR for p210, because ABL control gene failed to amplify in our quantitative RT-PCR (qPCR) assay in several samples, despite the successful amplification of BCR::ABL1 fusion gene. After 18 months, and again after 42 months, clinical worsening due to the patient's low adherence to treatment led to a change in therapy to Nilotinib (800 mg/dia) and Dasatinib (100 mg/dia), respectively. In 2023, the patient was hospitalized with accelerated phase CML (AP-CML) characterized by basophilia and no detectable myeloid blast. Conventional cytogenetic analysis was performed in bone marrow sample from relapse and revealed a normal karyotype (46, XY[20]), however, FISH using LSI BCR::ABL1 Dual Color/Dual Fusion Probe identified one BCR signal; two ABL signals and a single fusion signal in 86/100 nuclei analyzed, consistent with the BCR::ABL1 detection by RT-PCR. Additionally, the F317L mutation was detected by the BCR::ABL1 mutational testing realized at relapse. The mutation in AP-CML scenario led to the recommendation for hematopoietic stem cell transplantation. Three months post-transplantation, the patient rejected the graft, showing complete autologous recovery. Nilotinib treatment (800 mg/dia) was reintroduced with satisfactory clinical response. Discussion: This case describes a rare instance of CML with a normal karyotype but positive for BCR::ABL1, often referred to as Ph-negative CML. Literature suggests that those cases may arise from complex chromosomal abnormalities, including three-way or multi-way translocations involving chromosomes 9, 22, and additional chromosomes. In this case, the normal karyotype by conventional cytogenetics together to the FISH pattern implies the possibility of a cryptic insertion between chromosomes 9 and 22, where a partial ABL1 segment from chromosome 9 may have inserted into the BCR locus on chromosome 22, thus hindering detection by conventional cytogenetics. This highlights the importance of integrating molecular cytogenetics and molecular biology techniques with conventional cytogenetics for comprehensive CML diagnosis, as cases with a normal karyotype may still harbor complex chromosomal abnormalities only detectable by FISH and PCR testing.