Torque teno mini virus driven childhood acute promyelocytic leukemia: The third case report and sequence analysis

In this manuscript, we report torque teno mini virus (TTMV) as a cause of acute promyelocytic leukemia (APL) lacking PML::RARA in a 3-year-old boy. Astolfi et al. firstly identified partial integration of the TTMV genome into RARA intron 2, which resulted in in-frame TTMV::RARA fusion in two APL-like pediatric cases without PML::RARA in November 2021. This fascinating report identified an unexpected exogenous genetic cause of APL and could be of great importance for diagnosing and managing APL. Here we report the third childhood APL-like case caused by TTMV integration and investigate the location and structure of the integrated TTMV sequence. These findings suggest TTMV::RARA is a recurrent cause of APL lacking PML::RARA. Considering the widespread prevalence of TTMV in the population, more TTMV::RARA positive APL-like cases might remain to be identified. Establishing a bioinformatic analysis strategy optimized for the highly variable TTMV genome sequence may facilitate the identification of TTMV::RARA by whole transcript sequencing. An effective PCR protocol to identify TTMV::RARA based on a profound analysis of the conservation of TTMV segments in the fusion transcript is also expected. Also, further investigation is needed to elucidate the oncogenic mechanisms of TTMV integration and the clinical features of TTMV::RARA positive patients.

Acute Promyelocytic Leukemia With Torque Teno Mini Virus::RARA Fusion: An Approach to Screening and Diagnosis

Acute promyelocytic leukemia (APL) with variant RARA translocation is linked to over 15 partner genes. Recent publications encompassing 6 cases have expanded the spectrum of RARA partners to torque teno mini virus (TTMV). This entity is likely underrecognized due to the lack of clinician and pathologist familiarity, inability to detect the fusion using routine testing modalities, and informatic challenges in its recognition within next-generation sequencing (NGS) data. We describe a clinicopathologic approach and provide the necessary tools to screen and diagnose APL with TTMV::RARA using existing clinical DNA- or RNA-based NGS assays, which led to the identification of 4 cases, all without other known cytogenetic/molecular drivers. One was identified prospectively and 3 retrospectively, including 2 from custom automated screening of multiple data sets (50,257 cases of hematopoietic malignancy, including 4809 acute myeloid leukemia/myeloid sarcoma/APL cases). Two cases presented as myeloid sarcoma, including 1 with multiple relapses after acute myeloid leukemia-type chemotherapy and hematopoietic stem cell transplant. Two cases presented as leukemia, had a poor response to induction chemotherapy, but achieved remission upon reinduction (including all-trans retinoic acid in 1 case) and subsequent hematopoietic stem cell transplant. Neoplastic cells demonstrated features of APL including frequent azurophilic granules and dim/absent CD34 and HLA-DR expression. RARA rearrangement was not detected by karyotype or fluorescent in situ hybridization. Custom analysis of NGS fusion panel data identified TTMV::RARA rearrangements and, in the prospectively identified case, facilitated monitoring in sequential bone marrow samples. APL with TTMV::RARA is a rare leukemia with a high rate of treatment failure in described cases. The diagnosis should be considered in leukemias with features of APL that lack detectable RARA fusions and other drivers, and may be confirmed by appropriate NGS tests with custom informatics. Incorporation of all-trans retinoic acid may have a role in treatment but requires accurate recognition of the fusion for appropriate classification as APL.

Case report of pediatric TTMV-related acute promyelocytic leukemia with central nervous system infiltration and rapid accumulation of RARA-LBD mutations

TTMV::RARA is a recently reported fusion gene associated with acute promyelocytic leukemia (APL), caused by the integration of torque teno mini virus (TTMV) genomic fragments into the second intron of the RARA gene. Currently, there have been only six documented cases, with clinical presentations showing significant variability. Although initial responses to all-trans retinoic acid (ATRA) treatment may be observed in patients with TTMV::RARA-APL, the overall prognosis remains unfavorable among infrequent reported cases. This article presents a pediatric case that manifested as PML::RARA-negative APL with central nervous system involvement at onset. The patient experienced both intramedullary and extramedullary relapse one year after undergoing allogeneic hematopoietic stem cell transplantation. Upon identification as TTMV::RARA-APL and subsequent administration of two rounds of ATRA-based treatment, the patient rapidly developed multiple RARA ligand-binding domain mutations and demonstrated extensive resistance to ATRA and various other therapeutic interventions. Additionally, the patient experienced ARID1A mutant clone expansion and progressed MYC-targeted gene activation. This case represents the first documentation of extramedullary involvement at both the initial diagnosis and relapse stages, emphasizing the intricate clinical features and challenges associated with the rapid accumulation of multiple ATRA-resistant mutations in TTMV::RARA-APL, characterizing it as a distinct and complex sub-entity of atypical APL.