Novel Fusion Genes Involving Hnrnpc and Rarg in Acute Myeloid Leukemia Mimicking Acute Promyelocytic Leukemia
Authors: Su Zhan , Liu Xin , Xu Yan , Hu Weiyu , Zhao Chunting , Zhao Hongguo , Feng Xianqi , Zhang Shuchao , Yang Jie , Shi Xue , Qiu Lugui , Peng Jun 
Published: October 2019

The roles of Heterogeneous nuclear ribonucleoproteins(hnRNPs) in regulating tumor development and progression, either as oncogenes or as tumor suppressors, were well documented. HnRNP C is one of the members of hnRNPs,and differential expression of hnRNP C has been found in series of tumor cells. However, the role of hnRNP C in leukemia has not been reported to date. Here, we report the first novel gene fusion event between HNRNPC and retinoic acid receptor gamma (RARG) in acute myeloid leukemia mimicking acute promyelocytic leukemia. This translocation produced the HNRNPC-RARG fusion gene and its reciprocal, RARG-HNRNPC.

A 43-year-old man was referred to our hospital with fever and a sore throat.Laboratory investigations revealed the following patient characteristics: (1) white blood cell count 12 × 109/L (blasts 1% and abnormal promyelocytes 86%). (2) Morphologic analysis of the bone marrow aspirate showed 86.5% microgranular atypical promyelocytes (Figure 1a, 1b). (3) Analysis from flow cytometry showed that the blasts were positive for CD33, CD13, CD45, and cMPO and negative for CD14, CD34, CD16, CD56, HLA-DR, B- or T-cell markers. Thus, the patient started all-trans retinoic acid (ATRA) treatment immediately. Afterwards, chromosomal analysis revealed 47 metaphases, and most of them were involved in t(14;17). Fluorescence in situ hybridization and RT-PCR assays did not identify the PML/RARA, NPM-RARA, PLZF-RARArearrangement. ATRA therapy lasted for 3 weeks, but no response was observed. Next, the patient received 2 cycles of induction chemotherapy until a complete response. Afterwards, he received 6 cycles of chemotherapy. Unfortunately, the leukemia relapsed 1 year later, and all treatments (including ATRA and arsenious acid) failed to produce any effects. The patient died from sepsis.

To identify molecular alterations, transcriptome sequencing analysis was performed. A 213-bp RARG-HNRNPC fusion product was specifically amplified from the patient's cDNA, as predicted (Figure 1c). Sanger sequencing showed that RARG exon 9 was fused in-frame to HNRNPC exon 3(Figure 1d). The RARG 5'-region encoding the ligand-binding domain was fused to the HNRNPC3'-region, where a cluster of phosphorylation sites is located(Figure 1e). We also found a reciprocal chimeric transcript. The amplicon size of HNRNPC-RARG fusion was 186-bp (Figure 2a). Sanger sequencing demonstrated that HNRNPC exon 3 was fused in-frame to RARG exon 5 (Figures 2b). The HNRNPC 5'-region encodes an RNA recognition motif (RRM), and the segment from RARG encodes a DNA binding domain (DBD, Figure 2c).

HnRNP C ubiquitously expressed RNA-binding protein (RBP) which are believed to influence pre-mRNA metabolism such as splicing, polyadenylation, stability, transport, andtranslation mediated by internal ribosome entry site. HnRNP C also plays an essential role in cell progression and the regulation of several DNA repair proteins. Retinoic acid receptors (RARs) are transcription factors that belong to the nuclear hormone receptor family.RARA, RARB, and RARG are three RARs subtypes which share highly similar sequences and functions. A study showed RARG seems to act as a major regulator maintaining the balance between HSC self-renewal and differentiation.

Acute myeloid leukemias mimicking acute promyelocytic leukemia, or acute promyelocytic-like leukemias (APLL), share the same morphology and immunocytochemistry features with typical acute promyelocytic leukemia (APL) except the RARA rearrangements, and little is known about the molecular mechanisms of APLL. The sequences and function of the RARG and RARA are highly alike, and therefore can logically explain the similarity of biological characteristics between the two entities. Three other fusion genes harboring RARG ( including NUP98-RARG , PML-RARG and CPSF6-RARG) have been found in APLL. Unfortunately they showed resistance to treatment with ATRA or ATRA plus arsenic. Moreover, poor prognosis was observed likewise. All the above confirm that RARG rearrangements are not random but recurrent genetic abnormalities.

In conclusion, we present a novel HNRNPC-RARG fusion gene and its reciprocal in APLL, and suggest that at least a portion of APLLs have RARG gene rearrangements. We propose that RARG-rearranged APLL may be a novel candidate subtype of acute myelocytic leukemia, or even of APL.

Disclosures

No relevant conflicts of interest to declare.