Clinical and Molecular Characteristics of TCF3-Hlf Gene Fusion-Positive B-Cell Precursor Acute Lymphoblastic Leukemia: A Series of 24 Cases from a Large Cohort of Leukemia Patients
Authors: Chen Xue , Wang Fang , Zhang Yang , Ma Xiaoli , Yuan Lili , Fang Jiancheng , Panxiang Cao , Chen Jiaqi , Zhou Xiaosu , Liu Ming , Liu Mingyue , Liu Hongxing 
Published: November 2021

Abstract
Introduction
B-cell acute lymphoid leukemia (B-ALL) with t(17;19)(q22;p13)/TCF3-HLF is very rare and has a dismal prognosis even with the application of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Due to the rarity of this fusion, only a few cases have been described in the literature. In this study, we retrospectively analyzed 24 cases with TCF3-HLF from a large cohort of B-ALL, which constituted the largest cohort of TCF3-HLF-positive ALL reported to date and reported in detail the laboratory characteristics and prognoses of this group of patients.
Methods
From Apr. 2012 to Feb. 2020, a total of 3287 cases were diagnosed with B-ALL in our hospital. All of them underwent afusion gene screening test including TCF3-HLF through multiplex-nested reverse transcription-PCR. Whole transcriptome sequencing (WTS) was performed using RNA extracted from the bone marrow (BM) samples by HiSeq 2500. The gene expression signature for TCF3-HLF-positive B-ALL was investigated by comparing with healthy controls, B-ALL with TCF3-PBX1/TCF3-ZNF384, and B-ALL negative for pathogenic fusion genes (Chen X et al., Blood Cancer J 2021).
Results
A total of 24 cases with TCF3-HLF were identified, accounting for 0.73% of all B-ALL cases. Among them, 22 (91.67%) were children (≤18 years), and 2 (8.33%) were adults. Length of follow-up varied. The endpoint of the follow-up was Jul. 1st, 2020. Overall survival (OS) was defined as the time from diagnosis to death or the time of the last follow-up.
Of the 24 cases with TCF3-HLF, 8 had type I, 12 had type II, 4 had both type II and III chimera isoforms. Karyotype was available in 22 cases, 19 showed abnormal karyotypes and most of them (12/19, 63%) harbored further structural and/or numerical aberrations besides t(17;19)(q22;p13) translocation. Gene mutation screening of 58 genes was performed on 17 cases at the time of diagnosis. Eight (47%) of them showed mutations and 7 of them had mutations involving RAS signaling pathway genes (NRAS, KRAS, FLT3, and PTPN11). Immunophenotypic examination showed 12 (50%) and 19 (79%) patients exhibited aberrant expression of CD13 and CD33, only 4 patients (17%) were negative of both CD13 and CD33.
Gene expression clustering revealed apparent separation of TCF3-HLF-positive cases from TCF3-PBX1/TCF3-ZNF384-positive cases and those without pathogenic fusions. The differential expression of 535 genes (up: 207, down: 328) was identified in TCF3-HLF-positive cases compared to TCF3-PBX1-positive cases. The differential expression of 471 genes (up: 281, down: 190) was identified in TCF3-HLF-positive cases compared to TCF3-ZNF384-positive cases. TCF3-HLF-positive patients displayed a significantly up-regulated expression of HLF, which was almost not expressed in other cases (Figure 1).
The median OS of the 24 patients was 18.5 months (range 6-75 months). Thirteen of them underwent allogeneic HSCT (allo-HSCT) and the median OS was 23 months (range 13-75 months). Eight of them were in complete remission (CR) until the last follow-up; 2 of them relapsed after a first allo-HSCT and survived in CR after a second allo-HSCT; 3 of them died (2 died of relapse and 1 died of lung infection under CR). Eleven cases did not receive allo-HSCT, and the median OS was 9 months (range 6-29 months). Seven of them died (6 died after relapse, 1 died without achieving remission); 3 of them relapsed and re-induction was failed; only one case has survived in CR for 19 months till the last follow-up. Twelve cases underwent chimeric antigen receptor T-cells (CAR-T) therapy. Nine of them achieved CR after CAR-T therapy and bridged to allo-HSCT; one case achieved CR after CAR-T therapy but relapsed and lost opportunity for allo-HSCT; the other 2 patients achieved CR after the first application of CAR-T therapy but failed to achieve CR again by CAR-T therapy when relapsed.
Conclusions
We provide systematic insights into the laboratory characteristics and prognoses of B-ALL cases with TCF3-HLF in a large cohort. TCF3-HLF-positive B-ALL has a characteristic gene expression profile that differs markedly from TCF3-PBX1 and TCF3-ZNF384-positive B-ALL and shows a dismal prognosis. TC3F-HLF-positive ALL remains an incurable disease, although CAR-T therapy and allo-HSCT can improve the prognosis to some extent. Advanced therapeutic approaches, including novel drug discovery and development, are urgently required to improve the outcome of this ALL subtype.
Figure 1 Figure 1.

Disclosures
No relevant conflicts of interest to declare.