Molecular Background of BCP-ALL Cases with an Early Switch to Monocytic Lineage

We identified a subset of BCP-ALL with switch towards the monocytic lineage within the first month of treatment (swALL)[Slámová et al Leukemia 2014]. During the switch cells gradually lose CD19 and CD34 expression and acquire CD33 and CD14 positivity. We proved clonal relatedness of switched monocytic blasts with the diagnostic leukemic cells based on identical Ig-TCR rearrangements. SwALL cases are not associated with MLL or BCR/ABL1 aberrancies and lack any known genetic markers of lineage ambiguity (detected by FISH or MLPA).

We analyzed transcriptomes of swALL samples at diagnosis (n=4) and at d8 (n=4) where the immunophenotypic switching was already apparent as well as control BCP-ALL (n=4). RNA was isolated form either FACS sorted cells or whole BM when blasts constituted >80% of cells. For RNA-Seq we used Illumina HiSeq 2000 paired-end or single end sequencing. Raw sequencing data were analyzed using adapted protocol from Anders at al [Anders et al Nature Protocols 2013] and custom scripts. For methylome analysis we used Enhanced Reduced Representation Bisulfite Sequencing (ERRBS)[Akalin et al PLoS Genetics 2012]. ERRBS quantitatively measures DNA methylation at ~3M CpGs genome-wide. Samples from swALL at diagnosis (n=7) and at d8 (n=4) and control BCP-ALL (n=4) were processed. Analysis was performed according to [Akalin et al Genome Biology 2012] and followed with custom analysis in R statistical language.

Comparison (generalized exact binomial test) of transcriptomes of B-lineage blasts from diagnosis between swALLs and control BCP-ALLs revealed a number of differentially expressed genes. Among 300 most significantly differentially expressed were KLF4, CEBPD, CLEC12A and CLEC12B (upregulated in swALL) and ANXA5, VPREB1, CD9 and IGHG3 (downregulated in swALL). Hierarchical clustering separated not only swALL and control BCP-ALL, but also swALL cells before and during the monocytic switch. Changes in gene expression during lineage switch included downregulation of ITGA6, Id2, EBF1, CD19, CD34, FLT3, MYB, CD79a, BCR, PAX5, GATA3 and TCF3 genes and upregulation of S100A10, AIF1, CD14, CD33, LGALS1, RNF130 and MNDA.

When comparing all three cell types (swALL B cell and monocytic blasts and control BCP-ALL blasts) we concentrated on 1) immunophenotype switch markers and 2) lineage related transcription factors (TF):

1) Both markers typical for B cell blasts (CD19, CD34) decreased during the switch. However while CD19 was expressed in swALL at diagnosis at same levels as in control BCP-ALL, CD34 was overexpressed in swALL compared to BCP-ALL at diagnosis. Both monocytic markers (CD33, CD14) increased their expression during the switch. CD14 showed no difference between swALL and control BCP-ALL at diagnosis. However CD33 was interestingly upregulated in swALL already at diagnosis and continued to rise during the switch. SwALL had therefore deregulated expression of lineage commitment markers already at diagnosis favoring stemness marker CD34 and myeloid marker CD33.

2) B lineage commitment related TFs (EBF1, TCF3, PAX5) were expressed in B lineage blasts in both swALL and control BCP-ALL. However they were all downregulated during the switch. On the other hand myeloid lineage related transcription factor CEBPA is overexpressed in diagnostic B lineage blasts in swALL compared to control BCP-ALL cases. Similarly CEBPD is overexpressed in swALL and its expression further rises during the switch. Other hematopoietic TFs upregulated in swALL cases include KLF4, NANOG and GATA3.

To confirm some of the epigenetic markers of swALL cases (demethylation of CEBPA promoter) and to widen epigenetic screening we used ERRBS. While some of the upregulated genes had expectedly hypomethylated promoters in swALL (CEBPA, GATA3) other genes (TCF3, PAX5) had demethylated promoters in all cases. While the whole DNA methylation picture is still a challenge to draw both omics method could clearly separate swALL cases from control BCP-ALL using principal component analysis.

In summary we show that immunophenotypic shift is associated with gene expression changes of surface markers, lineage specific transcription factors and other genes. Some of the genes have altered expression already at diagnosis. Expression of some key lineage genes is differentially regulated by DNA methylation.

Supported by: GAUK 914613, GAČR P301/10/1877, UNCE 204012, IGA NT13462-4