The Impact Of Novel Splicing Abnormalities Of BCR-ABL On The Pathogenesis Of CML

Chronic myeloid leukemia (CML) is effectively treated with tyrosine kinase inhibitors (TKIs), but reactivation of BCR-ABL frequently occurs through acquisition of kinase domain point mutations. The mechanism of resistance in patients without BCR-ABL kinase domain point mutation is still elusive. Previous studies have revealed the abnormal splicing of BCR-ABL kinase domain, including Exon8/9 junction 35bp insertion and exon-skipping of Exon 7 (T O'Hare et al, Blood 2011. Gaillard JB, et al. Mol Cancer Ther 2010). The insertion of 35 intronic nucleotides at the exon 8/9 splice junction introduces a stop codon after 10 intron-encoded residues and inactive tyrosine kinase activity. The effect of these splicing abnormalities on susceptibility of cells against TKIs is still controversial. Furthermore, the conventional direct sequence techniques could not evaluate splicing abnormalities in major molecular response (MMR)- complete molecular response (CMR) CML patients, who achieved clinically leukemia-free state with a small number residual CML stem cells.

The aim of this study is to evaluate the frequency and the patterns of splicing abnormalities of BCR-ABL in CML patients, especially who achieved MMR-CMR by TKI treatment.

We analyzed peripheral blood samples from healthy individuals and CML chronic phase patients. We extracted total RNA from these samples and synthesized cDNA, and then performed PCR-amplification of BCR-ABL kinase domain in CML patients and ABL kinase domain in healthy individuals, respectively. PCR products were subjected to the amplicon sequence: We deeply sequenced BCR-ABL fusion gene transcripts, and evaluated splicing forms of BCR-ABL by using HiSeq 2000 (illumina).

We successfully established a novel analysis method, which can detect the pattern of splicing abnormalities even in MMR-CMR patients. Using the amplicon sequence technique, we detected abnormal splicing patterns of BCR-ABL in 5 out of 15 CML patients. We also found that the splicing abnormalities were not restricted to 35bp insertion at the exon8/9 junction, thus intronic retention of intron 8 and intron 9 could be frequently detected with or without the 35bp insertion in CML patients (Table 1). Of note, these abnormal splicing patterns always co-existed with wild type BCR-ABL transcripts in all 5 cases analyzed. In addition to the novel splicing abnormalities in CML, we unexpectedly found in healthy individuals that splicing abnormalities such as 35bp insertion at the exon8/9 junction and intronic retention could be detected in ABL1 transcripts (Table 1). This result suggests that this sort of splicing abnormalities could occur at a certain frequency in steady state human hematpoiesis, and is not specific to BCR-ABL.

We have newly established an analysis system to efficiently detect splicing abnormalities of BCR-ABL even in MMR-CMR CML patients. Using this highly efficient amplicon sequence technique, we identified novel splicing abnormalities both in healthy individuals and CML patients, and found that the wild type BCR-ABL transcripts always co-exist with abnormally spliced BCR-ABL transcripts. These results collectively suggest that splicing abnormalities within the ABL1 kinase domain are not specific to CML patients treated with TKIs, and that the detection of such kinase domain splicing abnormalities do not reflect insusceptibility of the remaining cells during TKI treatment.

No relevant conflicts of interest to declare.