Background

Lung cancer is responsible for the highest cancer-associated mortality rate worldwide. Only 16% of patients affected with Non-small cell lung cancer (NSCLC), which is the most common subtype, are alive 5 years after diagnosis, and this number has hardly improved over several decades [1]. One reason for this poor prognosis is that only 15% of lung cancers are diagnosed at an early stage. Till recently the standard of care for NSCLC at stages I-IIIA was surgery, resulting in patient survival rates of 23% in stage IIIA, 33% in stage IIB, and up to 89% in stages IA [2]. Adjuvant chemotherapy after radical resection of localized NSCLC improves survival at 5 years by about 5% [3]. However, there is still a relatively high risk of relapse, and up to 40% of all stage IB and 60% of stage II patients die from their disease despite receiving adjuvant chemotherapy [4]. The integration of prognostic and predictive biomarkers has the potential of identifying patients who are at a low-risk of relapse following surgery and do not need further therapy, and conversely, patients who are at a high risk of relapse and who potentially may derive the greatest benefit from adjuvant treatment, including chemotherapy or personalized treatment based on individual tumor profiling. Therefore, an effort to identify more robust prognostic and predictive biomarkers is needed [5].

The Helicase-like Transcription Factor (HLTF) is a member of the yeast mating SWItch/Sucrose Non Fermenting (SWI/SNF) family of proteins involved in chromatin remodeling. Several studies demonstrated its function in gene transcription [6], cell cycle [7], DNA repair [8, 9], and genome stability maintenance [10], supporting its tumor suppressor role. In cancer, two different alterations in HLTF expression were reported: (i) an epigenetic silencing by hypermethylation of its promoter and (ii) an alternative splicing of its mRNA, leading to the production of several shorter forms of the protein lacking DNA repair domains. The hypermethylation of HLTF promoter was first identified in colon cancer [11] and was reported in other types of cancers, including gastric cancers [12,13,14,15,16]. It was shown in HeLa cells that HLTF mRNA was alternatively spliced in the exons 19 to 22 region, resulting in the expression of shorter truncated protein forms. The distinctive character of the HLTF spliced mRNA variants (I21R) is that they contain the intron 21 between exons 21 and 22. To date, the expression of HLTF protein forms was reported in head and neck, cervix and thyroid [17,18,19,20] cancers and associated with a poor prognosis [16].

In lung cancer, one study assessed the hypermethylation of HLTF in a cohort of 54 patients with NSCLC [21]. Promoter hypermethylation was found in 21 patients (39.6%), including 9/20 squamous cell carcinoma (SCC) and 12/33 adenocarinoma (ADC). Patients whose tumors harboured HLTF hypermethylation had shorter survival, in comparison with patients whose tumors had a hypomethylated HLTF promoter (log-rank, p = 0.035). So far, to our knowledge, there are no published data about the expression of HLTF (wild-type and its truncated forms) in lung cancer.

The purpose of this study is to assess the expression of wild-type (WT) and spliced variants (I21R) of HLTF mRNAs in NSCLC and evaluate their clinical relevance. We analyzed publicly available databases for HLTF in lung cancer and assessed its expression in NSCLC cell lines and in a clinically annotated cohort of 171 patients with resected stage I-II NSCLC.

Methods

In silico analyses

Available genomic profiling data (mutation, copy number, DNA methylation [correlation only], and mRNA expression) for HLTF were downloaded from cBioPortal, an online portal for accessing data from The Cancer Genome Atlas (TCGA) project and other cancer genome profiling initiatives (http://www.cbioportal.org/public-portal). Additional cancer genome profiling data were obtained from the Catalogue of Somatic Mutations in Cancer (http://cancer.sanger.ac.uk/cosmic; [22]).

To obtain gene expression estimates for individual mRNA forms, paired-end RNAseq raw read data from the TCGA project were downloaded from the Cancer Genomics Hub (https://cghub.ucsc.edu/). In brief, reads were mapped to the latest human reference assembly, hg19, using TopHat, a splice-aware short-read aligner (http://ccb.jhu.edu/software/tophat). Alignment output was then supplied to Cufflinks (http://cufflinks.cbcb.umd.edu/), which was run in the reference-guided mode to quantify the abundance of known transcripts as well as predict and estimate expression of novel isoforms. Pre- and post-alignment quality control was performed with FastQC (http://www.bioinformatics.babraham.ac.uk/projects/fastqc/) and RSeQC (http://rseqc.sourceforge.net/), respectively.

Patient characteristics

A total of 171 patients with resected stage I-II NSCLC collected at University Health Network (Toronto, Canada) were included in this study. These patients had surgery between 1996 and 2005. The length of follow-up: median 5.4 years, range 0.1–12 years. As these patients all underwent surgery before 2005, none of them received adjuvant chemotherapy as it did only become standard after 2005. The clinical and demographic characteristics of the patient cohort are listed in Table 2.

Cell lines and cell culture

NSCLC cell lines were purchased from the American Type Cell Collection (ATCC, http://www.atcc.org), and cultured according to ATCC recommendation. Among these, there were 33 of the adenocarcinoma (ADC) subtype (H1693, H2122, H2228, H2279, H1573, H1395, H522, H1792, H838, H1819, H4011, H2291, H2073, H1568, H920, H1993, H4006, HCC827, H3255, H23, H4019, H2126, H1437, H1944, H2009, H2405, H1373, H1355, H1975, HCC2935, A549, H650, H1650), two large cell carcinoma (H661, H4017), one mixed adenosquamous carcinoma (H647) and two of undefined histology (DFC1032, DFC1024). MGH7 cells (squamous cell carcinoma, SCC) were cultured as described [23].

mRNA expression

Total RNA was extracted from cell lines with RNeasy Mini Kit (QIAGEN) according to the manufacturer’s instructions. RNA purity and concentration were assessed with Nanodrop (Thermo Scientific). Total RNA (input 150 ng) extracted from cell lines and patient tumours were reverse transcribed into cDNA (SuperScript III, Invitrogen). Droplet Digital polymerase chain reaction (ddPCR) was performed based on the manufacturer’s recommendations (QX200, Bio-Rad). ddPCR is a highly sensitive qPCR due to a step of sample fractioning (limiting dilutions) by generation of droplets (water:oil emulsion). It allows retrieving an absolute count of RNA copies for each sample, and is particularly indicated for low-expressed targets. Each ddPCR was performed with 22.5 ng cDNA in triplicates. Reaction conditions were as follow: ddPCR cycle was set up at 95 °C for 5 min, 40 cycles of [30 s at 95 °C and 1 min at 58 °C], 5 min at 4 °C, and finally 5 min at 90 °C. Results were analyzed with QuantaSoft (Bio-Rad), and the cut-off to define positive and negative droplets was set up at 10,000 arbitrary units of fluorescence amplitude. This signal is then used in the calculation of HTLF copy number by a Poisson regression (QuantaSoft, Bio-Rad).

Primers to detect either WT HLTF mRNA (F: 5’-GTTCAAAGATTAATGCGCT-3′ and R: 5’-AAAGACAGGAATGTTGTAAACTGAGA-3′) or HLTF mRNA variants I21R (F: 5’-TCCAGTTTCAAAGGTAAAGTACTC-3′ and R: 5’-GCCAGTGGTCAACAACAGAA-3′) by ddPCR were designed with Primer3 and purchased from Eurogentec. Primers were tested for nonspecific amplicons and primer dimers by visualizing PCR products on 1% agarose gels and droplet distribution profile (QuantaSoft, Bio-Rad).

Statistical analyses

Expression levels of different variants of HLTF were measured in triplicates. The reliability was assessed by calculating the intra-class correlation coefficient (ICC) based on the within and between variances estimated using the variance component analysis. For the outcome analysis, the three replicates were averaged for each sample. Two outcome variables were assessed: overall survival (OS) and disease-free survival (DFS). Both were measured from surgery date. For OS, the time was calculated up to the date of death or last follow-up with death of any cause as an event; for DFS, the time was calculated up to the date of relapse, death or last follow-up with death or relapse as events. There were 71 deaths (number of events for OS) and 81 events for DFS in the cohort. The averages of the three replicates of WT and I21R HLTF expressions were tested for their associations with OS and DFS using the Cox proportional hazards regression. Both variants of HLTF were also dichotomized at their respective medians and were tested as categorical variables using the log-rank test. The percentages for OS and DFS for the high and low values of each of these covariates were calculated using the Kaplan-Meier method. A composite covariate was created by combining WT and I21R HLTF expression levels and a data-driven covariate was defined as “Low WT HLTF and High I21R HLTF” vs. the rest. This new covariate was also tested for its association with OS and DFS by employing the log-rank test. These covariates were tested for their association with outcome, adjusting the model for age (≤65, > 65), sex, stage (I vs. II), and histology (ADC vs. the rest) using Cox regression. All p-values were based on the Wald test. HLTF expressions (continuous) were also tested for their associations with the clinical factors (age, sex, histology, and stage) using the Mann-Whitney test. A cut-off of p ≤ 0.05 was used for statistical significance.

Results

In silico analysis of HLTF alterations in NSCLC

We collected available online data for HLTF alterations in NSCLC from TCGA (Lung ADC and SCC, TCGA Provisional 2015/02/04) and COSMIC, focusing on mutations, copy number alterations (CNAs), and methylation data. We analyzed these data in association with HLTF expression. The type and the frequency of the different HLTF alterations in ADC and SCC are reported in Table 1. When all types of molecular alteration were considered, HLTF was more frequently altered in SCC (438/504 cases, 83%) than ADC (266/578 cases, 46.0%; p < 0.0001). While a high expression of HLTF was more frequently reported in SCC than in ADC (28.6% vs 7.5%, p < 0.0001), a negative correlation between HLTF expression and its methylation status was found in both SCC (Pearson: − 0.475 and Spearman: − 0.484; Fig. 1) and ADC (Pearson: − 0.473 and Spearman: − 0.420; Fig. 1). HLTF copy number gain was more frequent in SCC (57% vs 23.3%, p < 0.0001) as well as high amplifications (26.1% s 2.1%, p < 0.0001). Conversely, loss of heterozygosity was more frequent in ADC than in SCC (22.3% vs 2.9%, respectively, p < 0.0001), along with diploidy (52% vs 13.5%, respectively, p < 0.0001). HLTF mutation is a rare event both in ADC and SCC, 2.2 and 1.5%, respectively (Table 1). The majority of the 13 mutations (12 missense and 1 splice site mutation) reported in ADC and 8 mutations (6 missense, 1 splice site and 1 nonsense mutations) in SCC are found in regions encoding functional domains involved in DNA binding (HIRAN domain), Sp1/Sp3 interaction (carboxyl-terminal domain), and DNA repair (SNF2_N/helicase-ATPase and RING finger domains) (Fig. 2).

Table 1 HLTF alterations in lung adenocarcinoma and squamous cell carcinoma
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Fig. 1
figure 1

Distribution of NSCLC tumors associated with HLTF expression. a Adenocarcinoma. b Squamous cell carcinoma. HLTF expression according to HLTF methylation (left panels) and copy number alterations (CNA) (right panels)

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Fig. 2
figure 2

HLTF mutations in NSCLC. a. Lung adenocarcinoma. b. Lung squamous cell carcinoma. Mutation data were retrieved from TCGA and COSMIC databases. HLTF protein is depicted as a grey line with its functional domains: DNA binding domain (DBD, orange box), HIRAN (brown box), SNF2 (blue box), Helicase/ATPase I-III (yellow boxes), zinc finger RING (green box). Under the protein is a scale showing the amino-acid size. Mutation are depicted by colored dots (missense: green; spliced or stop mutation: red) with their position and the residue change

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In vitro screening of HLTF mRNA expression in a panel of NSCLC cell lines

We assessed HLTF mRNA expression in 39 NSCLC cell lines. Measurements for WT and I21R HLTF were considered reliable based on ICC values (0.878 and 0.933, respectively). Overall, the level of WT HLTF expression was significantly higher than the level of I21R HLTF expression (Fig. 3) (median 71.5 vs. 18.3 respectively, Wilcoxon signed-rank test, p = 4.5 × 10− 7).

Fig. 3
figure 3

Distribution of WT and I21R HLTF expressions (number of copy) in lung adenocarcinoma (ADC) (left panel) and squamous cell carcinoma (SCC) (right panel) patients and cell lines. RNA from cell lines and tumors was extracted and reverse transcribed. A ddPCR was performed to detect WT and I21R HLTF expression by using specific primer sets. The high sensitivity of ddPCR provides an absolute count of RNA copies for each sample, displayed in Y-axis

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Assessment of HLTF expression in a cohort of 171 patients with NSCLC

HLTF expression was assessed by RT-ddPCR in 171 tumours from patients with surgically resected stage I-II NSCLC. Patient data are summarized in Table 2. Measurements for WT and I21R HLTF were considered reliable based on ICC values (0.984 and 0.846, respectively). As in NSCLC cell lines, the level of WT HLTF expression was significantly higher than the level of I21R HLTF expression in tumours of patients with NSCLC (Fig. 3) when considering all patients (median 20.6 vs. 2.2 respectively, Wilcoxon signed-rank test, p = 2.2 × 10− 16). There was no difference between SCC and ADC for WT and I21R HLTF expressions (p = 0.09 and 0.17, respectively). Overall, the level of HLTF expression was lower in tumours from patients than in cell lines (WT HLTF: 20.6 vs. 71.5, p = 9.3 × 10− 11 and I21R HLTF: 2.2 vs. 18.3, p = 2.8 × 10− 12, Wilcoxon rank-sum test; Fig. 3).

Table 2 Association of WT HLTF and I21R HLTF expressions with patient clinical characteristics
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HLTF and clinical characteristics

WT and I21R HLTF expressions were first tested for their association with patient clinical characteristics (age, sex, histology, and stage). There was no association with these characteristics and HLTF expression (Table 2).

Second, we considered a composite covariate combining WT and I21R HLTF expressions dichotomized at their median levels. Four groups were built accordingly. There was no association with patient clinical characteristics (Table 3).

Table 3 Association of the composite covariate combining WT HLTF and I21R HLTF expression levels (n, %) with patient clinical characteristics
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HLTF expression and outcome

A univariate analysis (Cox proportional hazard regression model and log-rank test) was first performed to assess the association between WT and I21R HLTF expression levels, and OS and DFS. The mRNA expression measures were sequentially considered as continuous and as dichotomous variables. There was no significant association of each variable with OS and DFS (Table 4).

Table 4 Univariate analyses of the association of HLTF expression with overall survival and disease-free survival
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The association of the combined covariates of WT and I21R HLTF expression with OS and DFS were analyzed by the log-rank test. When considering the four groups, the « Low WT HLTF-High I21R HLTF » group showed a trend for a poorer DFS, but did not reach statistical significance (DFS at 5 years = 25%, log-rank p = 0.067), compared with the three other groups. We thus compared this group (Low WT HLTF-High I21R HLTF) to the three other groups combined. There was no statistical difference in OS (HR 1.26, CI 0.67–2.34; p = 0.48), but the DFS was significantly worse in this group (HR 1.96, CI 1.17–3.30; p = 0.011) (Table 4 and Fig. 4d).

Fig. 4
figure 4

Association of HLTF expression with OS (a, c) and DFS (b, d). Four groups of patients were built, based on the combined covariates of WT and I21R HLTF expression levels. WT: wild-type HLTF. Mut: I21R HLTF. In A and C, the four groups were considered independent from each other. In C and D, the group “low WT HLTF-High I21R HLTF” was compared with the other ones, which were combined in one group called “Other”

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A multivariate analysis (Cox proportional hazard regression model) was performed to include age, sex, stage, histology, and HLTF expression; WT alone (model 1), I21R alone (model 2) and the 2 composite groups (model 3) were considered (Table 5). HLTF expression (for each model) was not associated with OS. The expressions of WT (model 1) and I21R HLTF (model 2) were not associated with DFS. However, the shorter DFS of the group « Low WT HLTF-High I21R HLTF » (model 3) as compared with the other groups remained significant (HR 1.98, CI 1.15–3.4, p = 0.014; Table 5).

Table 5 Multivariate analyses of the association of HLTF expression with survival and disease-free survival
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Discussion

The purpose of this study was to assess the expression of WT and variant forms of HLTF mRNAs in NSCLC and evaluate their clinical relevance. Our hypothesis was that the expression of HLTF mRNA variant I21R has a poor prognosis on patients with NSCLC. In head and neck, cervix and thyroid cancers, the expression of HLTF truncated protein has been associated with poor outcome [16,17,18,19,20]. The present study showed that in a cohort of 171 patients, the combination of low expression of WT HLTF transcript and high expression of I21R HLTF transcript was associated with poor prognosis in early stage NSCLC.

Overall, in silico analysis showed that HLTF alterations, including gene amplifications, high expression, and methylation occurred more frequently in SCC than in ADC. Mutations in HLTF were rare in both ADC and SCC; however, the mutations observed in ADC were different from those found in SCC. In ADC, mutations occur in DNA binding domain and DNA repair domains (Fig. 2), which might alter HLTF transcriptional and DNA repair abilities. Conversely, in SCC, mutations did not occur in functional domains but there are 2 nonsense mutations leading to the expression of a shorter protein containing only the DBD. This suggests that these shorter proteins would only have transcriptional activity. Further investigations are required to assess the functional consequence and potential clinical impact of these mutations in cancer. Copy number alterations were also found to be different between ADC and SCC; high amplifications were rare in ADC, but 83% of SCC have either a gain (57%) or an amplification (26%) of HLTF. These observations are consistent with the fact that HLTF is located on chromosome 3q, which is frequently amplified in SCC. We also analyzed the association of HLTF expression with its methylation status in both NSCLC types. In both ADC and SCC, there was a negative correlation between methylation and HLTF expression, but a high expression was more frequently seen in SCC, which might be related to the higher frequency of gene copy number. Intriguingly, we did not notice any difference in HLTF expression levels (WT and I21R) between ADC and SCC by RT-ddPCR. This discrepancy may be possibly explained by the fact that we assessed HLTF mRNA variants separately, while data reported in cBioportal considered only WT HLTF expression without distinguishing the variants.

In the available online data, only WT HLTF expression was assessed. To our knowledge, to date the expression of the HLTF spliced variants with intron 21 retention (I21R) has not been assessed. Using the RT-ddPCR with specific primers that we constructed, we were able to evaluate the expression of WT HLTF mRNA and its spliced variants I21R. Spliced variants I21R lead to the expression of shorter protein forms, which are thought to disturb WT HLTF function and act as oncogene proteins [16]. Studies in head and neck, cervix and thyroid cancers showed that the expression of such shorter proteins was associated with poor prognosis [17,18,19,20]. They replace WT HLTF progressively and accumulate along the carcinogenic process, most likely due to their higher stability compared with WT HLTF. It was reported that the I21R transcripts have a lower abundance than the WT HLTF transcript in mouse heart and brain transcriptomes [7, 24]. We analyzed RNA-seq data from TCGA for the presence of the intron 21 sequence and found that its expression was a rare event in NSCLC. In both the NSCLC cell lines and the 171 resected NSCLC from patients, WT HLTF levels were significantly higher than I21R HLTF.

Castro et al. studied the methylation for several genes including HLTF in NSCLC and reported that patients with HLTF methylation have shorter survival [21]; this study represents the only study of HLTF in lung cancer. They reported HLTF methylation frequency for NSCLC and did not observe any significant difference for HLTF methylation between ADC and SCC (12/33 vs. 9/20, respectively; p = 0.57, Fisher exact test). cBioportal does not provide gene methylation frequency but only correlations with the expression of a given gene. In both ADC and SCC, we observed a negative correlation between HLTF expression and methylation. Interestingly, HLTF expression was affected more by the variation in HLTF copy number than its promoter methylation status.

Conclusion

So far to our knowledge, our study is the first to assess the clinical impact of WT and variant forms of HLTF expression in patients with NSCLC. TCGA in silico analysis of HLTF alterations including mutations, amplification, and mRNA expression modifications were more frequent in SCC than in ADC. In NSCLC cell lines and patient samples, both the expressions of WT and spliced I21R HLTF mRNAs were detected, but with the latter at lower levels. In a cohort of 171 patients with resected stage I-II NSCLC, the combination of a low WT HLTF expression with a high I21R HLTF expression was associated with shorter DFS both in univariate and multivariate analyses. Surgically resected early stage NSCLC are very heterogeneous and no prognostic factor has been clinically validated for the risk of relapse. Very likely, a panel of several biomarkers will be necessary to predict tumour with poor prognostic; that would therefore require more intensive follow-up and treatment. If validated in independant cohorts, the combination of low WT and high I21R HLTF might belong to this biomarker panel for the prognostic of surgically resected NSCLC. As detailed in a review article we published recently [16], the HLTF gene could be involved in various ways during the stages of tumour initiation and progression, by its ability to alternatively express proteins of different sizes with distinct functions ranging from tumour suppressor to oncoprotein. The involvement of alternative RNA splicing in producing tumour promoting proteins is a process that does not require inactivating mutation of a tumour suppressor gene and might be an underestimated carcinogenic mechanism. Further studies should precisely investigate the functions of these HLTF protein forms and their role in cancer development.