Background
Acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) are two myeloid malignancies [
1] which share similar somatic gene mutations [
2]. A portion of MDS patients eventually progresses to AML as the disease progresses. Several prognostic models have been developed to better risk stratify AML and MDS patients, such as European Leukemia Net (ELN) risk classification for AML [
3] and international prognostic scoring system (IPSS) or revised IPSS (IPSS-R) for MDS [
4‐
8]. However, patients may have different prognosis even in the same risk group [
9‐
12]. Exploration of more markers that have prognostic significance are warranted to better risk stratify patients with the diseases.
Long non-coding RNAs (lncRNAs) are transcripts longer than 200 nucleotides without protein coding ability. The functions of most lncRNAs remain poorly characterized, but some of them have been demonstrated to be involved in the hematopoiesis [
13‐
15]. For example, the expression levels of
HOTAIRM1 and
NEAT1 are elevated during myeloid differentiation, and downregulation of
HOTAIRM1 or
NEAT1 delays myeloid maturation [
13,
14]. Expression of
XIST, a lncRNA at X chromosome, inactivates X chromosome in the female cells. The female mice with deletion of
Xist develop a highly aggressive disease mimicking MDS/MPN [
15]. However, the roles of lncRNAs in MDS remain largely unknown [
16], and only a few research investigated the role of lncRNAs in de novo AML [
17‐
19]. In the research aimed to find prognostic biomarkers in acute myeloid leukemia (AML), we found expression of
HOXB-AS3, a lncRNA located at human
HOXB cluster, is a potential risk factor. However, its clinical relevance and pathogenesis in AML and MDS remain to be determined.
Here, we demonstrate that high expression of HOXB-AS3 is an adverse prognostic factor for both de novo AML and primary MDS patients. Furthermore, the expression of HOXB-AS3 promotes cell proliferation in myeloid cells.
Methods
Patients
We retrospectively included the adult patients with newly diagnosed primary MDS and de novo AML at the National Taiwan University Hospital (NTUH) from 1992 to 2010. Among them, 157 MDS and 193 AML patients, who had available cryopreserved bone marrow (BM) cells for RNA array analysis and comprehensive clinical information, were recruited for this study. The Cancer Genome Atlas (TCGA) AML cohort available on the TCGA website (
https://cancergenome.nih.gov/) and an independent cohort of 30 MDS patients subsequently diagnosed between January 2011 and May 2012 at the NTUH was served as the validation cohorts.
All patients with AML other than acute promyelocytic leukemia (non-APL AML,
n = 174) underwent standard induction chemotherapy (Idarubicin 12 mg/m
2 per day for two to 3 days and Cytarabine 100 mg/m
2 per day for five to 7 days), and consolidation chemotherapy with two-to-four courses of high-dose Cytarabine (2000 mg/m
2 every twelve hours for 4 days, total eight doses), with or without an anthracycline (idarubicin or mitoxantrone), after they achieved complete remission (CR) as described in our previous studies [
20]. Nineteen APL patients received concurrent all-trans retinoic acid (ATRA) and Idarubicin or Mitoxantrone as induction chemotherapy and ATRA with Idarubicin, Mitoxantrone or high dose Cytarabine as consolidation chemotherapy when they achieved CR. If the patients had relapsed or refractory AML, or adverse prognostic factors at diagnosis, such as adverse-risk cytogenetic abnormalities or somatic gene mutations, they underwent allogeneic hematopoietic stem cell transplantation (allo-HSCT) when they had feasible hematopoietic stem cell donors.
In the NTUH MDS training cohort, most MDS patients (70.7%) only received supportive care. Eight patients (5.1%) received AML-directed intensive chemotherapies, 18 (11.5%) received hypomethylating agent (azacitadine or decitabbine), and 20 patients (12.7%) underwent allo-HSCT.
The BM samples from AML and MDS patients were collected at diagnosis, and the mononuclear cells were isolated by Ficoll-Hypaque gradient centrifugation and cryopreserved as previously described [
21,
22]. The BM cells from 20 healthy transplantation donors were used as the normal controls to compare the gene expressions with those of AML and MDS patients. This study was approved by the Institutional Review Board of NTUH (IRB number: 201507084RINA and 201503072RINC). All the patients have signed informed consents for the collection of samples and clinical information.
Analysis of cytogenetic abnormalities and gene mutations
The BM cells were harvested directly or after one to 3 days of un-stimulated cultures. The metaphase chromosomes were banded by the G-banding method as previously described. [
23] The determination of mutations in
NPM1 [
21,
24],
AML1 (
RUNX1) [
25],
ASXL1 [
26],
DNMT3A [
20],
EZH2 [
27],
IDH2 [
28],
NRAS [
29],
KRAS [
29],
TP53 [
30],
SETBP1 [
31],
SRSF2 [
32],
TET2 [
33],
MLL/PTD [
34],
SF3B1 [
35],
U2AF35 [
36], and
ZRSR2 [
35] was performed as described previously.
Microarray experiments and analysis
The raw data of TCGA AML cohort was downloaded from TCGA website (
https://cancergenome.nih.gov/). The detail methods of microarrays for NTUH AML and NTUH MDS cohorts were described in Additional file
1.
The expression levels of two transcript clusters, TC17002254.hg.1 and TC17002858.hg.1, on Affymetrix GeneChip® HTA 2.0 represent HOXB-AS3 (NCBI Reference Sequence: NC_000017.11) expression. TC17002254.hg.1 detects variants 1 to 5 of HOXB-AS3, and TC17002858.hg.1 detects all variants of HOXB-AS3. Because TC17002858.hg.1 detects all HOXB-AS3 variants and the expression pattern was similar between the two transcript clusters, expression of TC17002858.hg.1 was used to stratify patients.
Cell lines, cell cultures, and associated experiments
OCI/AML3 and TF-1 were human myeloid leukemia cell lines. TF-1 cell line (BCRC number 60323) was purchased from Bioresource Collection and Research Center (BCRC), Hsinchu, Taiwan on Sep 29, 2014. BCRC (
http://www.bcrc.firdi.org.tw/) is a nation-wide cell bank in Taiwan, and it provides the service of preservation, identification, and selling of cell lines. OCI/AML3 was a gift from Dr. Minden (Ontario Cancer Institute/Princess Margaret Hospital, Canada) in 2008. The detailed methods of cell cultures, constructions of lentiviral vectors with shRNA and lncRNA, lentiviral production, lentiviral infections, quantitative real time PCR, proliferation assay and nuclear-cytoplasm fractionation were described in Additional file
1.
BrdU flow assay
BD Pharmingen™ APC BrdU Flow Kits (Cat. NO. 552598) was used for BrdU flow assay. Cells were incubated with 10 μM BrdU at 37 °C for three hours, and then harvested for BrdU flow assay. The detailed method was described in the user manual of the manufacture. The flow cytometry was performed on LSR II (BD Bioscience, San Jose, CA) through the service provided by the Flow Cytometric Analyzing and Sorting Core Facility at the NTUH, and on FACS Canto II (BD Bioscience, San Jose, CA) through the service provided by the Molecular and Immune Function Laboratory at Tai Cheng Stem Cell Therapy Center at the National Taiwan University.
Statistical analysis
Mann-Whitney test was used to calculate the significance if the continuous data were not normally distributed, and Kruskal-Wallis test was used for comparing the difference between more than two groups. Chi-square test was used to calculate the significance of association between HOXB-AS3 expression and other categorical parameters, including sex, IPSS risk groups, 2016 WHO subtypes, cytogenetic abnormalities, and gene mutations. Fisher exact test was used if any expected value of the contingency table was less than five.
For patients with de novo AML in the NTUH AML cohort, overall survival (OS) was measured from the date of diagnosis to the date of last follow-up or death. The patients were censored on the date of last follow-up if they were alive. Relapse free survival (RFS) was defined from the date of complete remission to the date of relapse, last follow-up, or death. Relapse and death were defined as events in the RFS analysis. If the patients were alive and in complete remission, they would be censored. For patients with primary MDS, OS was measured from the date of diagnosis to the date of last follow-up or death. The patients were censored on the date of last follow-up if they were alive.
Kaplan-Meier (KM) estimation was used to plot survival curves, and log-rank tests were used to calculate the difference of OS and RFS between different groups in AML patients and OS in MDS patients. Median follow-up duration was calculated by reverse KM estimation. Multivariate Cox proportional hazard regression analysis was used to investigate independent prognostic factors for OS. A
P value less than 0.05 was considered statistically significant. All statistical analyses were performed with MedCalc® 15.6.1 software (
https://www.medcalc.org/).
Discussion
In this study, we reported the clinical relevance of lncRNA HOXB-AS3 in de novo AML and primary MDS. We demonstrated that higher HOXB-AS3 expression was an adverse prognostic factor for AML and MDS patients. To our knowledge, HOXB-AS3 represents the first reported lncRNA whose expression is able to predict the prognosis of MDS patients. We also found that HOXB-AS3 expression could further stratify IPSS lower risk patients into two subgroups with distinct prognosis. This would help us identify the IPSS lower risk patients who need to be treated aggressively.
In addition to identifying the prognostic value of
HOXB-AS3 expression in myeloid malignancies, we explored the biological function of
HOXB-AS3. We revealed that
HOXB-AS3 promoted the proliferation of myeloid cells. Previous studies showed that anti-sense lncRNAs in
HOX clusters influence the expressions of
HOX genes by
trans- or
cis-regulation [
13,
42,
43]. For examples,
HOTAIR trans-regulates the expression of
HOXD genes through PRC2 complex [
42], whereas
HOTAIRM1 cis-regulates the expressions of
HOXA1 and
HOXA4 genes [
13]. Given that expressions of
HOX genes are important in cell proliferation, hematopoiesis and leukemogenesis [
36,
38,
39], it is possible that
HOXB-AS3 promotes cell proliferation by regulating the expressions of certain
HOX genes. However, our microarray analysis indicated that downregulation of
HOXB-AS3 in OCI/AML3 cells did not significantly alter the expression of any
HOX gene (Fig.
3a). Instead,
HOXB-AS3 potentiated the expressions of a set of genes critical for cell cycle progression and DNA replication (Fig.
3c and Additional file
1: Figure S4), and this finding was consistent with its ability to increase the S-phase cell population (Fig.
2c, e, and g). A recent study showed that knocking down
HOXB-AS3 reduces the cells in S phase and the ability in colony formation, which is consistent with our findings [
46]. Further, they demonstrated that
HOXB-AS3 binds EBP1 to increase the EBP1-NPM1 complex [
46]. Accordingly, overexpression of
HOXB-AS3 increases the transcription of rRNA and de novo protein synthesis [
46]. This mechanism might explain the effect of
HOXB-AS3 on cell proliferation.
The clinical relevance of
HOXB-AS3 in hematopoietic diseases remains poorly characterized. A previous study reported a positive correlation of
HOXB-AS3 expression with
NPM1 mutations in AML patients [
47]. In our study, we not only illustrated high
HOXB-AS3 expression as a poor prognostic biomarker in AML and MDS, but also disclosed its promotion effect on cell proliferation in two myeloid cell lines (Fig.
2). Further, we showed that higher
HOXB-AS3 expression was an adverse prognostic marker in IPSS lower risk patients, but not higher risk ones. It may indicate that higher expressions of
HOXB-AS3 influences the prognosis through enhancing proliferation of the abnormal hematopoietic cells in IPSS lower-risk patients, but has no implication when the patients already have many risk factors as in IPSS higher risk patients. These findings imply that
HOXB-AS3 may have distinct clinical relevance in different myeloid malignancies.
Of note, a recent study reported that
HOXB-AS3 can encode a small peptide to influence the alternative splicing of pyruvate kinase M, thereby inhibiting the proliferation of colon cancer cell lines [
48]. In addition,
HOXB-AS3 expression is downregulated in colorectal cancer (CRC) tissues and is correlated with favorable prognosis for CRC patients [
48]. The seemingly discrepancies between this previous study and our findings are likely due to the different variants of
HOXB-AS3 used. The previous study investigated exclusively
HOXB-AS3 variant 1 (NR_033201.2), and the small peptide is encoded from the last two exons of this variant [
48]. However, in leukemia cell lines, variant 1 is expressed at a very low level, and the majority of
HOXB-AS3 transcripts are variants 2/3/5 (Additional file
1: Figure S16), which do not contain the last two exons (Fig.
1b) and therefore cannot encode the small peptide. Because of the abundant expression of variants 2/3/5, we used the longest variant, variant 2, for the overexpression studies and results are consistent with the conclusions derived from the knockdown studies, in which the two
HOXB-AS3 shRNAs target variants 2, 3, 5, 8 and variants 2, 3, 5, 6, 7, 8, respectively. Therefore, the current and previous studies suggest the existence of variant-specific functions of
HOXB-AS3. The relative abundance of
HOXB-AS3 variants could determine its context-dependent roles in different cancer types.
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