Background

B-cell precursor acute lymphoblastic leukemia (BCP-ALL) is a heterogeneous lymphoproliferative malignancy. Despite novel therapeutic strategies ranging from immunotherapies [1] to targeting mutational driver lesions, e.g. BCR-ABL1 [2], prognosis remains poor for refractory and relapsed BCP-ALL, in particular for adult patients [3].

BCP-ALL can be molecularly classified into various genetic subtypes with differences in clinical outcome and age-dependent prevalence [4]. The subtypes are defined by structural chromosomal alterations, e.g. translocations or recurrent aneuploidy patterns, with secondary events, such as sequence mutations and copy number alterations, in pathways related to epigenetic regulation, cell cycle, lymphoid differentiation, cytokine receptor, kinase and RAS signalling [5]. They show distinct mRNA expression [6] and methylation profiles [7] likely reflecting different leukemogenic mechanisms underlying each subtype.

Near Haploid (NH) ALL (24–30 chromosomes) and high hyperdiploid (HeH) ALL (51–67 chromosomes) as defined by conventional cytogenetics [8, 9] are two different subtypes with distinct clinical outcomes in childhood BCP-ALL [10,11,12,13]. They are defined by non-random patterns of chromosomal losses and gains as well as cooperating single nucleotide variants [14, 15]. Virtual karyotyping by SNParrays showed in pediatric patients that a near haploid karyotype with retained chromosomes (e.g. 10, 14, 18, 21) could also be observed in a duplicated manner, resulting in a high hyperdiploid karyotype with trisomies or tetrasomies of the otherwise retained chromosomes [16, 17]. In our adult patient cohort these duplicated karyotypes or karyotypes with the same non-random gain of chromosomes were identified by virtual karyotyping (WES; SNParrays) [18]. RNAseq analysis revealed shared gene expression profiles of near haploid and high hyperdiploid samples with a clear distinction to other subtypes across pediatric and adult subgroups [14, 18, 19] Secondary mutations in RAS-pathway genes and epigenetic regulators such as CREBBP [14, 15, 18] and a shared DNA methylation profile [18] have been observed in both subtypes. Due to these biological similarities and our RNAseq-based subtype classification, NH and HeH samples have been grouped together (NH/HeH) despite the clinical importance of differentiating between NH and HeH samples.

In NH and HeH BCP-ALL, underlying causes of chromosomal instability, i.e. TP53 mutations [20] or aberrant RAG activity [21], have thus far not been identified. It remains unclear if aneuploidy resembles a driver event or an epiphenomenon. Additionally, the most frequent secondary mutations are only seen in a subset of patient samples and are inconsistently gained or lost at relapse [14, 22,23,24]. Functional studies are limited by the lack of appropriate models [25]. Thus, leukemogenesis of these subtypes is incompletely understood, and additional genetic alterations may contribute to pathogenesis and therapy resistance.

In addition to DNA-based genetic alterations, altered transcripts arising from alternative transcription start sites (TSS) may contribute to leukemogenesis in lymphoid neoplasms [26, 27]. In the present study, we describe the alternative transcript of Cytochrome B5 Type A (CYB5A), CYB5Aalt, starting from a previously undescribed TSS. CYB5A wildtype (WT) is located on chromosome 18q22.3 and encodes for the 15.2 kDA hemeprotein Cytochrome B5, which reduces methemoglobin to ferrous hemoglobin and provides reducing equivalents in steroid biogenesis, lipid biosynthesis and to members of the cytochrome P450 system [28,29,30,31]. It is physiologically expressed in human B-lymphocyte lineage [32]. In drosophila, mutations in CYB5A WT homologue dappled cause the formation of melanotic tumors and dysregulation of hematopoiesis [33]. In humans, mutations in CYB5A WT cause type IV methemoglobinemia [34]; low CYB5A mRNA and protein expression is associated with shorter survival in pancreatic cancer [35].

We identify CYB5Aalt to be a frequent event and highly specific to the NH/HeH gene expression cluster in a cohort of 50 matched initial diagnosis-relapse samples. Further, its expression was increased in relapse. Specificity and frequency of CYB5Aalt was confirmed in a cohort of 140 BCP-ALL initial diagnosis samples from adult patients. Additionally, we present first evidence for a potential role of CYB5Aalt in apoptosis and viability by overexpressing CYB5Aalt in vitro in BCP-ALL cell line NALM-6.

Methods

Patient material

This manuscript extends analyses from two previously published cohorts including adult and pediatric BCP-ALL patients enrolled into trials of population-based German study cohorts (GMALL, AIOP-BFM, COALL) [18, 23].

For the exploratory cohort (Additional file 1), methylation, transcriptome and whole exome analysis has been performed by Schroeder et al. [23]. The cohort was designed to include relapsed BCP-ALL patients with paired initial diagnosis (ID) -relapse (REL) samples, solely including patients lacking driver fusion genes detected by routine clinical diagnostics (BCR-ABL1, MLL rearrangements, ETV6-RUNX1). Eighty-six samples had sufficient RNA material for gene fusion detection and were used in this study for the detection of CYB5Aalt (cohort 1, n = 86) and 80 of these samples had sufficient RNA quality for gene expression analysis in transcripts per million (TPM). The validation cohort, cohort 2 (n = 140, Additional file 2), represents a subset of a patient cohort of ID BCP-ALL samples with sufficient RNA data, previously published by Bastian et al. [18]. RNAseq pipeline algorithms for both cohorts are shown in Additional file 3.

Transcript validation by reverse transcriptase polymerase chain reaction and sanger sequencing

After Ficoll density separation, RNA and DNA isolation of patient samples was performed following standard procedures (AllPrep, Quiagen, Hilden, Germany; Trizol, Life Technologies, Carlsbad, CA). RNA was transcribed to complimentary DNA (cDNA) using MMLV Reverse Transcriptase (Epicentre, Chicago, USA). For validation, Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) on patient cDNA was performed using CYB5Aalt-specific primers (forward: 5’-TCCAGCTCCTACCTGTTACCTT-3’, reverse: 5’-GGAGGTGTTCAGTCCTCTGC-3’). PCR bands were extracted using QUIAquick Gel Extraction Kit (Quiagen, Hilden, Germany) and bilateral Sanger Sequencing using the CYB5Aalt-specific primers was performed. Geneious version 5.4.3 software (Biomatters Ltd., Auckland, NZ) was used for analysis.

Cell lines and culture

The human cell line NALM-6 (BCP-ALL; ACC-128) was purchased from the DSMZ (Braunschweig, Germany). Cells were maintained in RPMI 1640 medium containing 25 mM HEPES, 2 mM L-glutamine, 1 mM sodium pyruvate, 100 U/ml and 100 mg/ml streptomycin (all from Merck Millipore, Darmstadt, Germany). 10% Fetal bovine serum (Linaris, Bettingen, Germany) was added to medium. The medium has not been changed for any experiment. Cell culture was routinely checked for mycoplasma contamination by PCR (Merck Millipore).

Plasmid constructs and transfection

cDNA derived from CYB5Aalt positive patient RNA served as template for a standard PCR to amplify the alternative open reading frame of CYB5Aalt (CYB5Aalt-ORF) using the primers 5′-GCCACCATGTCCAAAACATTCATCATTGGGGAG-3′ and 5′-TGTTCAGTCCTCTGCCATGTATAGGC-3′. The 191 base pairs (bp) PCR product was cloned via TOPO vector pCR 2.1 (Invitrogen, Karlsruhe, Germany) into vector pcDNA3.1-IRES-GFP for eukaryotic overexpression (pCYB5Aalt). BCP-ALL cell line NALM-6 was transfected with pCYB5Aalt or empty vector control (pEmpty) by electroporation (Neon Transfection System, Invitrogen, Basel, Switzerland). After 24 h, transfected cells were treated with neomycin (0.6 mg/ml, Merck Millipore) for 4 weeks. Cells were cloned by single cell sorting for Green Fluorescent Protein (GFP) and kept in culture for additional 4 weeks. Stable integration of pCYB5Aalt was confirmed by standard PCR on genomic DNA, expression was confirmed by RT-PCR using cDNA of clones and Glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) as internal control. Before conduction of experiments, cells were thawed, cultured for 4 weeks and expression was reconfirmed by RT-PCR.

RNA extraction and quantitative real-time PCR

RNA of pCYB5Aalt- and pEmpty-NALM-6 clones was isolated using the Rneasy Kit (Quiagen, Hilden, Germany) and cDNA was synthesized using MMLV Reverse Transcriptase (Epicentre, Chicago, USA). Quantitative real-time PCR (qRT-PCR) was performed using the Sybr Green PCR assay (Invitrogen, Karlsruhe, Germany) following the instructor’s manual. CYB5A mRNA expression was measured using CYB5A- forward primer 5′-TGAGGATGTCGGGCACTCTA-3‘and CYB5A-reverse primer 5′-GAGGTGTTCAGTCCTCTGCC -3′. GAPDH was used as internal control (forward: 5′-GAGTCAACGGATTTGGTCGT-3′, reverse: 5′-GATCTCGCTCCTGGAAGATG-3′). Relative expression values were indicated as Δ CT value (GAPDHCT-CYB5ACT).

WST-1 viability assays

pCYB5Aalt NALM-6 and empty vector controls were treated with Venetoclax (SelleckChem, München, Germany) and seeded in quintets for each concentration of Venetoclax (0, 0.2, 0.5, 1, 5, 10 μM). 1 × 105 cells per well were seeded in 90 μl medium and incubated at 37 °C, 5% CO2. After 72 h, 10 μl of WST-1 reagent was added to each well and cells were incubated at same conditions for 3 h to allow for reduction of WST-1 to formazan by the electron transport chain of viable cells. Then, absorbance was measured using a Sunrise microplate absorbance reader (Tecan, Männerdorf, Switzerland) at 450 nm with a reference wavelength of 620 nm. Raw absorbance measurements were normalised to untreated control samples, after subtracting WST-1 absorbance measurements in wells only containing medium.

Cell death assays

pCYB5Aalt NALM-6 and empty vector controls were treated with Venetoclax and seeded in duplicates for each concentration of Venetoclax (0, 0.2, 0.5, 1, 5, 10, 20 μM). 2.5 × 105 cells per well were seeded in 500 μl medium and incubated at 37 °C, 5%CO2. After 48 h, 300 μl of cells per well were transformed to FACS tubes and washed with 4 °C Phosphate Buffered Saline. Working on ice, 50 μl Annexin V binding buffer (1:10 dilution) (BD Pharmingen, Heidelberg, Germany) and 1 μl Propidium Iodide (PI) (BD Pharmingen, Heidelberg, Germany) were added to each tube. Cells were then incubated for 15 min at room temperature in the dark. Dead cells were then analyzed for PI positivity and GFP negativity by fluorescence activated cytometry using FACSCalibur (BD Pharmingen, Heidelberg, Germany).

High throughput sequencing analyses

Sample preparation, sequencing and bioinformatic analysis of the matched, multiomics ID-REL BCP-ALL cohort and the validation cohort have been performed as previously published. To identify CYB5Aalt and to determine its mRNA expression, ‘stringtie‘package [36] and ‘defuse‘package [37] were used. To increase transcript specificity, only CYB5A transcripts spanning a genomic distance exceeding 120,000 bp and partial alignment to CYB5A were used for downstream analysis of CYB5Aalt expression and frequency. Only samples passing quality control by RNA-SeQC [38] were used for expression analysis. Normalized mRNA Expression was reported in TPM.

pCYB5Aalt NALM-6 and empty vector controls

For RNAseq, 6 samples per lane were sequenced with an average of approximately 30 million mapped reads per sample (MMRS). All sequences were aligned to the human genome build GRCh38 [39] using STAR-aligner [40]. Samples were further processed using the DESEQ2 pipeline for differential gene expression analysis [41]. Hierarchical clustering of the 500 most variably expressed genes based on DESEQ2-regularised-logarithm transformation (rlog) gene expression values and the heatmap were constructed using the ‘pheatmap’ package [42]. Principal component analysis (PCA) was performed with rlog-normalised gene expression values using DESEQ2. Gene set enrichment analysis was performed using the ‘fgsea’ package [43]. As input for ranked gene lists, mean log2-foldchange of genes between pCYB5Aalt NALM-6 and empty vector controls was used. Gene sets “Hallmark “and “KEGG subset of canonical pathways “from the ‘misgdbr’ package were used for analysis. Data visualization has been done using ‘ggplot2’ [44].

Statistical analysis

Subtype frequency of CYB5Aalt was analysed by Fisher’s exact test. Inverse correlation between CYB5Aalt and CYB5A gene expression was evaluated by performing linear regression analysis. Euclidean distance with average linkage was used for unsupervised clustering. Fisher’s exact test was used to assess enrichment of CYB5Aalt samples in CYB5A low expressers in patient cohorts. Wilcoxon rank sum test was performed to compare differences in CYB5A gene expression as well as expression of BH3-motif genes between CYB5Aalt positive and negative patient samples. Group differences in PI-uptake to evaluate cell death and group differences in WST-1 reduction to formazan to assess viability were evaluated by Mann-Whitney-U-Test. All statistical tests were both-sided. Adjustment of p-values for multiple comparison was performed by the Benjamini-Hochberg method.

Results

Transcriptomic characterisation of BCP-ALL identifies CYB5Aalt as novel transcript

Fifty BCP-ALL samples at initial diagnosis and relapse, 26 adult and 24 pediatric patients, lacking cytogenetic rearrangements identified by conventional diagnostics (BCR-ABL1, KMT2A-AFF1, ETV6-RUNX1, TCF3-PBX1) were analysed as exploratory cohort (cohort 1, Additional file 4) [23]. ALL patients were previously classified into molecular BCP-ALL subtypes based upon their mRNA expression and methylation profiles in addition to specific chromosomal rearrangements and mutations [23]. Twelve patients (24 samples) were allocated to the Ph-like and to the DUX4r subtype, respectively. Fourteen patients (28 samples) were characterised by an aneuploid karyotype defined by 3 or more whole chromosomes affected by loss of heterozygosity (LOH) or hyperdiploidies identified by virtual karyotyping (Additional file 5) [18]. Ten out of fourteen aneuploid patients showed gains in chromosomes 4, 14, and 21, distinct methylation and mRNA expression profiles and were defined as NH/HeH BCP-ALL. Four NH / HeH samples showed LOH of most disomic chromosomes and gains in chromosome 14 and 21 suggesting a “masked” near-haploid phenotype [17]. The remaining 4 aneuploid samples all had TP53 mutations and a masked low hypodiploid karyotype (LH) with whole chromosomal gains in 1 and 22. Further patients were assigned to the PAX5mut (2 patients), PAX5r (1 patient), BCL2r (2 patients), ZNF384f (2 patients), MLLr (1 patient) and MEF2Dr subtype (1 patient). The remaining seven samples could not be allocated to a specific subtype.

Sufficient RNA material was available for 86 out of 100 patient samples to analyse subgroup specific fusion transcripts and transcripts with non-canonical exon usage at initial diagnosis and relapse (Additional file 1). This analysis revealed a previously undescribed alternative transcript of CYB5A, CYB5Aalt, in 13 out of 86 patient samples. The alternate TSS is currently not annotated in publicly available CAGE-Seq [45] and RNAseq data [46] and thus regarded a novel finding. In antisense direction, the novel TSS of CYB5Aalt (hg19, chr18:72,084,437) is located 125,268 bp upstream of the wildtype TSS. RNAseq coverage and junction reads allowed for further characterisation of CYB5Aalt (Fig. 1A). Towards its 5’end, CYB5Aalt contains a novel sequence resulting in two new exons with alternative exon 2 splicing into exon 2 of CYB5A WT, thereby skipping exon 1 of the wildtype. Since exon 1 encodes the start of the open reading frame (ORF) of CYB5A WT, skipping exon 1 by CYB5Aalt gives rise to an alternative open reading frame, starting from an ‘ATG’-codon in exon 2 of CYB5A WT. This alternative ORF maintains the reading frame of the wildtype ORF and may result in a truncated version of CYB5A WT lacking its heme-binding domain (Additional file 6). Expression and sequence of CYB5Aalt was confirmed by RT-PCR followed by Sanger Sequencing (Additional file 6).

Fig. 1
figure 1

CYB5Aalt starts from novel TSS and CYB5Aalt is increased in relapse. A Comparison of RNAseq Read coverage (in RPKM log10) between CYB5Aalt positive and CYB5A WT samples reveals a novel TSS for CYB5Aalt and usage of two non-canonical exons upstream of CYB5A WT Exon 1 in representative patient samples (PL09 ID, PL09 REL). Arcs represent split reads between exons. Alternative Exon 2 skips CYB5A WT Exon 1 and splices into WT Exon 2. Exons of CYB5Aalt and CYB5A WT are illustrated beneath according to read coverage and genomic coordinates. B CYB5Aalt gene expression (in TPM log2) in matched ID-REL patient samples, which are CYB5Aalt positive. Colours represent matched patient samples. Connected dots represent matched expression values of patients at ID and REL

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CYB5Aalt frequency is increased in relapsed BCP-ALL and enriched in NH/HeH gene expression cluster

Next, CYB5Aalt expression was compared between initial diagnosis and relapse of matched BCP-ALL patient samples. CYB5Aalt expression was detected in 13 out of 86 samples (Fig. 1B). It occurred in 8 out of 43 REL samples (18.6%) and in 5 out of 43 ID samples (11.6%). Four of five samples at ID maintained CYB5Aalt expression in relapse. Four samples gained CYB5Aalt expression at relapse without detectable expression at ID. Summarised in Table 1, median bone marrow blast count of CYB5Aalt-positive patients (n = 9) was 88%. CYB5Aalt-negative patients (n = 41) had a median blast count of 92%. CYB5Aalt was detected in 7/37 adult and 6/49 pediatric samples (5/19 adult and 4/25 pediatric patients respectively). CYB5Aalt was identified in 11 out of 18 NH/HeH samples, in Ph-like (1/20) and LH (1/6) samples. Overall, the frequency of CYB5Aalt samples was 61.1% (11/18) in NH/HeH samples compared to 2.7% (2/68) in other BCP-ALL samples (p = 2 × 10− 4, two tailed Fisher’s exact test). Within the NH/HeH gene expression cluster, four CYB5Aalt-positive samples showed a masked near-haploid phenotype (4/6) and seven samples were hyperdiploid (7/12).

Table 1 Basic clinical characteristics and subgroup frequencies of CYB5Aalt positive and negative patients in cohort 1
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The specificity towards the NH/HeH gene expression cluster was further validated in an independent validation cohort of 140 BCP-ALL RNA ID patient samples, analysed by RNAseq [18], including 15 molecular subtypes including recurrent cytogenetic rearrangements identified by conventional diagnostics (BCR-ABL1, KMT2A-AFF1, ETV6-RUNX1, TCF3-PBX1) (Additional file 8). CYB5Aalt expression could be detected in 9 samples. 80% (4/5) NH/HeH cases were positive for CYB5Aalt compared to 3.7% (5/135) of non-NH / HeH samples (p = 3.5 × 10− 5, two tailed Fisher’s exact test). The five CYB5Aalt cases, that were not assigned to NH/HeH BCP ALL, comprised four Ph-like (n = 27) and one sample that could not be categorised by a subtype (n = 17). Although CYB5Aalt was not exclusively identified in NH/HeH samples, we observed statistically significant enrichment for CYB5Aalt in NH/HeH samples in both cohorts. CYB5Aalt-expression across molecular subtypes is shown in Additional file 9.

CYB5A WT mRNA expression is decreased in CYB5Aalt samples

To examine the impact of CYB5Aalt on CYB5A WT expression, gene expression analysis was performed. In cohort 1, 80 samples passed quality control (Additional file 1) and mean CYB5A WT gene expression was significantly lower in CYB5Aalt samples (0.29 vs. 3.13 TPM log2, p = 5.19 × 10− 5, Wilcoxon rank sum test). In cohort 2 (n = 140), mean CYB5A WT expression was also lower in CYB5Aalt samples (0.004 vs. 3.759 TPM log2, p = 6.1 × 10− 5, Wilcoxon rank sum test). A total of 220 samples passed quality control and were used for expression analysis (combined cohort 3, n = 220). In combined cohort 3, mean gene expression of CYB5A WT in CYB5Aalt positive samples was 0.17 TPM log2 and 3.54 TPM log2 in CYB5Aalt negative samples (p = 2.1 × 10− 9, Wilcoxon rank sum test). CYB5A WT expression was mostly stable across molecular subtypes in combined cohort 3 apart from CYB5Aalt positive samples (Fig. 2A). CYB5Aalt samples were highly overrepresented among the 10% of samples with the lowest CYB5A WT expression (68.2%, 15/22) in comparison with the rest of the cohort (3.5%, 7/198, p = 2.23 × 10− 13, two-tailed Fisher’s exact test). Further, an inverse relation between CYB5A WT and CYB5Aalt gene expression was observed (Fig. 2B) suggesting that a higher CYB5Aalt expression contributes to a decrease in CYB5A WT gene expression (R2 = 0.25, p = 0.01, linear regression analysis).

Fig. 2
figure 2

CYB5A WT transcript is decreased in CYB5Aalt-samples and shows an inverse correlation with CYB5Aalt mRNA expression. A CYB5A WT gene expression (TPM log2) in combined cohort 3 (n = 220) is shown across molecular subgroups. Circles resemble CYB5Aalt negative samples (n = 198), triangles resemble CYB5Aalt positive samples (n = 22). Subgroups with CYB5Aalt positive samples include NH/HeH (15/25), Ph-like (5/48), LH (1/12) and unknown (1/26). B Relation between CYB5A and CYB5Aalt gene expression (both in TPM log2) in CYB5Aalt positive samples (n = 22). Blue line represents linear regression line, grey area indicates 95% confidence interval

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Overexpression of CYB5Aalt-ORF induces a distinct transcriptional program in NALM-6 cells

Since CYB5Aalt was acquired and higher expressed in relapse, we explored its role in therapy resistance using a cell line overexpression model. CYB5Aalt-ORF was overexpressed in the BCP-ALL cell line NALM-6 (Fig. 3A) by stably integrating pcDNA3.1-CYB5AaltORF-IRES-GFP (pCYB5Aalt). Similarly to CYB5Aalt patient samples, downregulation of CYB5A WT mRNA was observed in pCYB5Aalt NALM-6 clones compared to empty vector controls by qRT-PCR (Additional file 10).

Fig. 3
figure 3

pCYB5Aalt Nalm 6 showed a distinct transcriptional program and resistance to Venetoclax induced cell death. A CYB5Aalt-ORF is stably overexpressed in NALM-6 cells. NALM-6 cells were transfected with pCYB5Aalt (+) or pEmpty (−). RT-PCR was used to confirm overexpression of CYB5Aalt-ORF mRNA. Two representative samples of each are shown. GAPDH was used as a control. NTC = non-template control. Full length electrophoretic gel is displayed in Additional file 17. B pCYB5Aalt NALM-6 show a distinct transcriptional program compared to empty vector controls. Samples were grouped according to the 500 most variably expressed genes between samples. Columns indicate pCYB5Aalt clones and empty vector controls, rows represent gene expression in rlog for each sample. Colour of the heatmap indicates relative expression strength according to the deviation from the mean of all samples in rlog. C Viability of pCYB5Aalt NALM-6 cells and pEmpty upon Venetoclax treatment (n = 3). Cells were incubated with different concentrations of Venetoclax (0 μM, 0.2 μM, 0.5 μM, 5 μM, 10 μM) for 72 h. Viability was assessed photometrically after 3 h incubation time with WST-1. Absorbance values were normalised to untreated control samples and shown as viability (%). Bars represent mean viability, error bars ±standard deviation of three independent experiments each with five technical replicates per concentration. Mann-Whitney-U-Test, ***P-value< 0.001, ns = non-significant. D Cell death of pCYB5Aalt NALM-6 and pEmpty upon Venetoclax treatment (n = 4). Cells were incubated at different concentrations of Venetoclax (0 μM, 0.2 μM, 0.5 μM, 5 μM, 10 μM, 20 μM) for 48 h. Cell death was assessed by staining cells with PI. Bars represent percentage of PI positive cells, error bars ±standard deviation of four independent experiments each with technical duplicates for each concentration. Mann-Whitney-U-Test, ***P-value< 0.001, **P-value< 0.01, *P-value< 0.05, ns = non-significant

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Further, RNA sequencing was carried out for four pCYB5Aalt and two empty vector NALM-6 clones. Principal component analysis using the top 500 variably expressed genes showed two clusters separating empty vector clones from pCYB5Aalt clones (Additional file 11). One of the pCYB5Aalt clones clustered separately from all other clones. It was excluded from downstream analysis since this difference is most probable explained by random genomic integration of the overexpression vector and not by the expression of CYBalt-ORF. Using the top 500 variable expressed genes, hierarchical clustering of the genes between samples was carried out. The resulting heatmap of differentially expressed genes (Fig. 3B) showed clustering of pCYB5Aalt clones and empty vector controls suggesting the induction of a distinct transcriptional program by the overexpression of CYB5Aalt-ORF in NALM-6 cells.

To get insights into how these differences in the transcriptional program may affect biological mechanisms in pCYB5Aalt NALM-6, gene set enrichment analysis was carried out (Additional file 12). To explore possible involved pathways, MsigDB Hallmark and KEGG pathway gene sets comparing pCYB5Aalt NALM-6 and empty vector controls were used. Among significantly differentially regulated gene sets (false discovery rate (FDR) < 0.05), there were several gene sets related to the endoplasmic reticulum such as the Unfolded Protein Response (normalised enrichment score (NES) = − 1.65, FDR = 0.006), Xenobiotic metabolism (NES = − 1.57, FDR = 0.009), Peroxisome (NES = -1.82, FDR = 0.006) and Protein secretion (NES = − 1.82, FDR = 0.006). Similarly, these gene sets were significantly downregulated (FDR < 0.05) in CYB5Aalt patient samples compared to CYB5A WT samples in the NH/HeH gene expression cluster in combined cohort 3 (Additional file 13). Additionally, the DNA repair gene set was significantly downregulated (NES = − 2.05, FDR = 0.006). Cytochrome b5, encoded by CYB5A WT, is anchored in the endoplasmic reticulum membrane and the localisation domain is retained by the CYB5Aalt-ORF. A murine CYB5A−/− model induced changes in gene and protein expression in multiple CYP-enzymes [29, 30] involved in xenobiotic metabolism. The unfolded protein stress response is interlinked with the key regulator of apoptosis BCL2 [47] suggesting a possible contribution of CYB5Aalt towards therapy resistance via this mechanistic link. Interestingly, the comparison of mRNA expression of genes belonging to the BCL2-family (Additional file 14) showed an upregulation of anti-apoptotic genes, e.g. BCL2, BCL2L2 and MCL1 and downregulation of pro-apoptotic genes, e.g. BAD and BID, in pCYB5Aalt-NALM-6. Similarly, pro-apoptotic BH3-motif genes such as BID (FDR = 0.01) were downregulated and anti-apoptotic genes, i.e. BCL2L2 (FDR = 0.04) were upregulated in CYB5Aalt-positive samples in combined cohort 3 (Additional file 14).

pCYB5Aalt Nalm-6 is more resistant to Venetoclax induced cell death

To assess, whether the observed trend towards upregulation of anti-apoptotic mediators in pCYB5Aalt-NALM-6 has a functional effect upon BCL2-regulated apoptosis, pCYB5Aalt NALM-6 and empty vector controls were incubated with the selective BCL2 inhibitor Venetoclax [48]. After 48 h, pCYB5Aalt-Nalm 6 and empty vector controls were stained with WST-1 as a surrogate marker for viability. pCYB5Aalt-Nalm 6 showed significantly (p < 0.05, Mann-Whitney U Test) higher viability (Fig. 3C) at different concentrations of Venetoclax (0.2 μM:0.67 vs. 0.29 absorbance at 450 nm, 0.5 μM: 0.61 vs. 0.24, 1 μM: 0.45 vs. 0.19) compared to empty vector controls in three independent experiments. Further, the cells were stained with PI to assess cell death. FACS-Analysis gating for PI positive and GFP negative cells revealed significantly (p < 0.05, Mann-Whitney U Test) less cell death in pCYB5Aalt-Nalm 6 cells at different concentrations of Venetoclax (0.2 μM: 9.6% vs. 16.6%, 0.5 μM: 19.6% vs. 31.9%, 5 μM: 37.2% vs. 64.5%, 10 μM: 47.7% vs. 71.8%, 20 μM: 47.7% vs. 71.8%) in four independent experiments (Fig. 3D).

Discussion

The underlying mechanisms of leukemogenesis in NH and HeH BCP-ALL are incompletely understood and a mutational driver event remains to be identified. Similarly to chromophobe renal cell carcinoma and pancreatic neuroendocrine tumors [49], malignancies with a reported high fraction of unidentified drivers, a uniformly occurring aneuploidy pattern is present in NH and HeH BCP-ALL, which arises early in leukemogenesis [15, 50]. However, the functional role of this pattern as a driver or passenger event is disputed [51]. In addition, secondary mutational events are often volatile and lost or gained at relapse [15, 23]. Despite cytogenetic and outcome differences between NH and HeH BCP-ALL, they share a common gene expression and methylation profile likely reflecting a common leukemogenic mechanism with differences in clinical outcome likely dependent on a simple gain of chromosomes (HeH) or a loss of chromosomes (NH) with a possible consequential duplication of chromosomes and widespread uniparental disomies (masked-NH). Here, we add to the genetic complexity of NH and HeH BCP-ALLby introducing the alternative transcript of CYB5A, CYB5Aalt, and provide first evidence pointing towards a possible role of CYB5Aalt in therapy resistance. CYB5Aalt was firstly discovered analysing RNAseq data in a matched ID-REL cohort of 50 BCP-ALL patients (cohort 1) showing enrichment in relapse and increased occurrence in the NH/HeH gene expression cluster. In this for relapse selected cohort, CYB5Aalt was detected in samples with a masked near-haploid phenotype and in samples with a virtual high hyperdiploid karyotype. Relapse in HeH-BCP ALL is not common and associated with a less favourable prognosis [16, 22, 52, 53]. Interestingly, CYB5Aalt was the most frequent genetic event secondary to aneuploidy in relapsed HeH BCP-ALL (Additional file 15). The specificity towards the NH/HeH gene expression cluster was consequently confirmed in cohort 2, a RNAseq-cohort of 140 BCP-ALL ID patient samples and cross validated via qRT-PCR and Sanger Sequencing.

The comparison of the accompanying Exonseq data of samples positive or negative for CYB5Aalt did not show any differences in regional coverage suggesting that no genomic deletion detectable by exon coverage can explain de novo transcription of CYB5Aalt. Rather, the HeH subtype has been described to be widely hypomethylated (23) and first Hi-C experiments suggest a wide dysregulation of 3D-chromatin architecture in this subtype compared to ETV6-RUNX1 BCP-ALL [54], making epigenetic dysregulation a probable cause for this genetic event. This is further supported by enrichment for tri-methylation of H3K27 near the TSS of CYB5Aalt [55, 56], a histone modification frequently associated with gene silencing [57], and the incidence of comparable non-canonical exon usage starting from cryptic TSS after treating a lung cancer cell line with a combination of a histone deacetylase inhibitor and a DNA methyltransferase inhibitor [58].

To gain first insights into biological mechanisms related to BCP-ALL, that CYB5Aalt may influence, pCYB5Aalt was overexpressed in BCP-ALL cell line NALM-6. In concordance to the patient cohorts, pCYB5Aalt overexpression caused a decrease in expression of wildtype CYB5A mRNA in this functional model. Further, it induced a distinct transcriptional program in NALM-6 cells compared to empty vector controls. Gene set enrichment analysis revealed several significantly altered gene sets related to the endoplasmic reticulum, i.e. the unfolded protein stress response, xenobiotic metabolism and protein secretion as well as fatty acid metabolism and adipogenesis. These findings are in line with previously reported roles for CYB5A WT [28, 29, 31] and a downregulation of these pathways may be  partly explained by the observed decreased expression of wildtype CYB5A. CYB5Aalt leads to an alternative ORF lacking the coding sequence for the heme-binding domain, which is necessary for Cytochrome B5’s reducing capability [31]. This lack raises the possibility of a dominant-negative effect of CYB5Aalt upon Cytochrome B5 and further studies are needed to investigate protein interactions. Interestingly, DNA repair was also significantly downregulated. Aneuploidy induces DNA damage response pathways via several proposed mechanisms. Consequent activation of p53 suppresses aneuploidy-induced tumorigenesis in mice models [59]. High hyperdiploid BCP-ALL shows prolonged metaphase which triggers a p38 and p53-mediated G1 arrest and blocks proliferation [60]. In contrast to other aneuploid BCP-ALL subtypes, e.g. hypodiploid BCP-ALL, mutations in TP53 are no frequent event in the NH or HeH subtype. Therefore, a downregulation of DNA damage response by CYB5Aalt in the context of aneuploidy may contribute to a survival and proliferative advantage of leukemic cells. Further, hyperdiploidy has been associated with increased proteotoxic stress resulting in an increase in apoptosis [61]. pCYB5Aalt NALM-6 also show a significantly downregulated protein stress response pathway. The unfolded protein stress response controls cell fate decision via the mitochondrial pathway of apoptosis controlled by the BCL-2 protein family [47], which play a dominant role in survival of lymphoid malignancies [62]. We observed a trend towards mRNA upregulation of the anti-apoptotic BCL2, MCL1, BCL2L1 and BCL2L2 in pCYB5Aalt NALM-6 compared to empty vector controls. Notably, the same trend was observed in the combined patient cohorts (Additional file 13). To gain first insights whether BCL2-regulated apoptosis is functionally impacted in NALM-6 overexpressing pCYB5Aalt, pCYB5Aalt NALM-6 and empty vector controls were incubated with Venetoclax, a specific BCL2-inhibitor, and assessed for viability and apoptosis. Although BCL2 expression was higher in pCYB5Aalt NALM-6, which has been associated with higher sensitivity towards Venetoclax [63, 64], pCYB5Aalt NALM-6 showed both, increased viability assessed by WST-1 and reduced cell death measured by PI-staining. This may be explained by the higher expression of MCL1, which has been reported to sequester BCL2, thus preventing Venetoclax from binding BCL2 and inducing apoptosis [64,65,66]. The observed resistance towards Venetoclax provides first evidence for a role of pCYB5Aalt in mediating resistance towards BCL2-dependent apoptosis. This might be of clinical significance, although Venetoclax is still under clinical investigation as therapeutic option in refractory and relapsed (R/R) BCP-ALL [67], because preclinical data derived from patient-derived xenografts and primary BCP-ALL cells as well as a case report of three R/R T-ALL patients suggested efficacy of Venetoclax in overcoming resistance towards components of induction therapy [68,69,70,71]. Further, patients resistant to Venetoclax with chronic lymphocytic leukemia and acute myeloid leukemia, where Venetoclax has already been approved, show dismal outcomes and worse response towards standard antineoplastic therapy [72, 73]. However, further mechanistic studies are needed to gain a deeper understanding of how CYB5Aalt may be implicated in therapy resistance of BCP-ALL.

Conclusion

We report the occurrence of CYB5Aalt, an alternative transcript of CYB5A arising from a previously undescribed TSS, to be highly specific for NH and HeH BCP-ALL in two independent cohorts. It is the most frequent secondary genetic event in relapsed NH and HeH BCP-ALL as assessed by RNAseq, Exonseq and gene panel sequencing analysis in a matched ID-REL cohort. Overexpressing CYB5Aalt in BCP-ALL cell line NALM-6 provides first hints for a functional implication in BCL2-mediated apoptosis.