A short report of novel RARG-HNRNPM fusion gene in resembling acute promyelocytic leukemia

ABSTRACT Background Resembling acute promyelocytic leukemia (APL) is a unique subtype of APL who sharing clinical, morphological, and immunophenotypic features with typical APL, but lacking evidence of PML-RARA fusion gene and usually insensitive to arsenic trioxide (ATO) and all-trans retinoic acid (ATRA). For years, RARA, RARB and RARG rearrangement were found in resembling APL continually. The confirmed partner genes of RARG rearrangement included CPSF6, NUP98, NPM1, PML, and HNRNPC. These patients were a group of resembling APL with rare molecular genetic abnormality and unfavorable prognosis. They usually were resistant to ATO and ATRA but partially sensitive to anthracycline-based chemotherapy. Case presentation We reported a 25-year-old female patient with a novel fusion gene RARG-HNRNPM (RARG chr12:53606869: –; HNRNPM chr19: 8527413: + based on GRCh37/hg19 Assembly) through RNA-seq as resembling APL. The patient with RARG-HNRNPM was benefited from a combined chemotherapy homoharringtonine, cytarabine, and aclacinomycin (HAA) regimen with no relapse. Discussion and conclusions RARG rearrangement resembling APL are various. The treatment should be switched from ATRA/ATO to AML combined chemotherapy regimen early.


Background
Acute promyelocytic leukemia (APL) with typically PML-RARA fusion gene caused by t (15;17) (q22; q21) was distinguished from other types of acute myeloid leukemia (AML) according to the World Health Organization (WHO) 2016 criteria [1,2]. The clinical features of APL include severe hemorrhagic and coagulation disorder and sensitivity to arsenic trioxide (ATO), all-trans retinoic acid (ATRA), and anthracyclines. Nowadays, APL is considered 'curable leukemia' [1]. Resembling APL is referring to those rare cases that sharing clinical, morphological, and immunophenotypic features with APL, but lacking evidence of PML-RARA fusion gene and usually resistance to ATRA [3,4]. Herein, we reported a 25-year-old female case with a novel fusion gene involving the retinoic acid receptor gamma (RARG) gene and heterogeneous nuclear ribonucleoprotein M (HNRNPM) gene in a leukemic phenotype resembling APL.

Case presentation
A 25-year-old woman from China was admitted to Institute of Hematology and Blood Diseases Hospital with the symptoms of fever, pharyngodynia, and thoracalgia. Informed consent was obtained from the patient before bone marrow aspiration for further cytogenetic and molecular analysis.
Considering the morphology and immunophenotype were similar to APL, while we didn't find the molecular evidence of APL, we performed RNA-seq for a definitive diagnosis. Total RNA was extracted from BM mononuclear cells. RNA-sequencing was performed with Illumina NovaSeq 6000. The RNA-seq data was available in PubMed (PRJNA719216). The STAR-Fusion software (http://star-fusion.github.io) was applied to match the UCSC Genome Browser on Human February 2009 (GRCh37/hg19) Assembly and call the fusion gene. The total fusion gene list of the patient was shown as Supplementary Table  2. Reverse transcription-polymerase chain reaction (RT-PCR) and Sanger sequencing further verified the identified genes. The following primers were used to amplify RARG-HNRNPM mRNA: 5'-AGCCAGCCCTA-CATGTTCCC-3' for RARG, 5'-TGCTCTCCTGGCATGTT-CACC-3' for HNRNPM.

Discussion and conclusions
WHO Classification of Tumors of Hematopoietic and Lymphoid Tissues (2016) only ascribes APL with PML/ RARA to AML with a recurrent genetic abnormality. While 16 gene partners fused with RARA [4] (such as PLZF, NPM, NUMA, BCOR, FIP1L1, STAT5b, STAT3, PRKR1a, OBFC2A, GTF2I, TNRC18, NUP98, TFG, FNDC3B, IRF2BP2, and TBLR1) and cases with TBL1XR1-RARB rearrangement [5,6] have been found in the presence of APL cases without classic t (15;17). Variant APL with RARA and RARB rearrangement is characterized by its vast majority resistance to ATRA treatment as lacking target of ATO and similar morphological and molecular biology characteristics with typical APL [7].
It is reported that RARA breakpoint is highly fixed in variant APL which always occurs in exon 3 [7,13]. While, breakpoints harboring within RARG are different, including 5' UTR, exon1, exon 2, intron 3, exon 4, and exon 9 [4,13] . RARG breakpoint was also detected in exon 9 in our patient. Nevertheless, the functional domain of RARA and RARG were mostly remained in resembling APL cases regardless of different partner gene breakpoint.
RARG shares similar sequence and biological characteristics with RARA in the nucleus receptor family. It is responsible for regulating the healthy BM microenvironment and correcting hematopoiesis [3]. Specifically, RARG function contains of increasing the numbers of hematopoietic progenitors and eliciting HSC selfrenewal by affecting other gene expression [11].
HNRNPM is an RNA binding protein (RBP) that contains the common architecture of potential RNAbinding units: two RRM structures. HNRNPM is a subtype of heterogeneous nuclear ribonucleoproteins (hnRNPs) that plays a critical role in recognizing splice site and regulating alternative splicing [14].
Based on the cases reported above, patients with RARG rearrangement were expected to be resistant to ATRA/ATO but partially sensitive to anthracycline-based chemotherapy. Six out of eleven patients (including our patient) with RARG arrangement achieved CR with chemotherapy, while four out of eleven patients died during induction therapy. Although our patient showed intolerance to treatment with ATRA and no benefit from the standard DA regimen, she achieved CR after HAA regimen chemotherapy and kept in CR for 10 months. Therefore, RARG rearrangement APL should early switch from ATRA/ATO to AML combined chemotherapy regimen after distinguishing from typical APL. In fact, the proper treatment and prognosis for variant APL still needs to be accumulated.
Taken together, we reported a novel RARG-HNRNPM fusion gene in a patient resembling APL. It broadens the spectrum of variant APL genetics type.

Disclosure statement
No potential conflict of interest was reported by the author(s).