Under-Expression of LINE-1 Retrotransposons in Chronic Myeloid Leukemia Patients: Effect of Tyrosine Kinase Inhibitors

Background: Transposable elements (TEs) are mobile pieces of DNA that can move ("jump") in our genome, thereby influencing a myriad of cellular processes. Long interspersed element-1 (LINE-1) retrotransposons comprise approximately 20 percent of the mammalian genome, and LINE-1 retrotransposition events can create genomic instability by unknown mechanism. In chronic myeloid leukemia (CML), BCR-ABL1 expression is associated with a high degree of chromosomal genetic instability. The aim of this study was to investigate LINE-1 retrotransposon ORF1 and ORF2 expression in CML patients and cell lines.

Materials and methods: LINE-1 quantitative RT-PCR was performed on 48 CML patients and 35 control subjects before therapy and during imatinib therapy. In addition LINE-1 expression was carried out on BCR-ABL1 positive and negative cell lines treated by kinase inhibitors, depleted in BCR-ABL1 by RNA interference. The effect of gamma irradiation (50 Gy) on CML and non CML cell lines was investigated.

Results: For CML patients at diagnosis, LINE-1 ORF1/ß-actin expression (median expression value 2.8 x 10²) was significantly lower than for healthy donors (3.9 x 10³) (P<0.0001). The same level of significance (P<0.0001) was observed in the comparison of LINE-1 ORF2 expression (median values of 1.7 x 10² for CML patients and 1.4 x 10³for healthy donors). The median expression of LINE-1 ORF1 and ORF2 in CML patients were lower than in control subjects by 14-fold and 8-fold, respectively. To exclude that the difference observed was not due to the different blood cell composition, we first analyzed LINE-1 ORF1 and ORF2 expression in patients with other diseases such as BCR-ABL1 positive Acute Lymphoblastic Leukemia (n=3), BCR-ABL1 negative Acute Lymphoblastic Leukemia (n=2), Acute Myeloblastic Leukemia (n=5), Essential Thrombocythemia (n=2), and other diseases as mastocytosis, myelofibrosis, lymphocytosis and myeloma, and second we analyze LINE-1 expression in the different cells from peripheral blood. Collectively, these data indicate that LINE-1 expression is significantly reduced in CML patients compared to patients with other diseases and cannot be due to the cellular composition.

Prospective analysis of LINE-1 expression in CML patients (22 responder, 5 non responder and 8 secondary resistant patients) during imatinib therapy showed a negative correlation between BCR-ABL1 and LINE-1 expression during the kinetics of response or relapse.

LINE-1 expression was investigated on BCR-ABL1 cell lines (K562 and LAMA-84) treated or not by imatinib, depleted in BCR-ABL1 or not by RNA interference. No modification of LINE-1 expression was observed in the different cell lines.

To try to modulate LINE-1 expression, BCR-ABL1 positive cell lines (K562 and LAMA-84) and BCR-ABL1 negative cell lines (HL60 and U937) were irradiated with gamma radiation (50Gy). After irradiation we notice a decrease of LINE-1 expression in the four cell lines. BCR-ABL1 expression was not modified in K562 and LAMA-84 cell lines after irradiation. In addition, we observed the appearance of a BCR-ABL1 transcript by quantitative PCR in HL60 and U937 cell lines as already reported by Deininger et al (Cancer Res. 1998;58:421).

Conclusions: Herein, we show that the levels of LINE-1 in CML patients at diagnosis were significantly lower than in control subjects. In spite of LINE-1 expression was inversely correlated with BCR-ABL1 expression in CML patients, no modification of LINE-1 mRNA expression was observed in CML cell lines treated by kinase inhibitors or depleted in BCR-ABL1 by RNA interference. Gamma irradiation induces the appearance of BCR-ABL1 transcripts and a decrease of LINE-1 expression. These two events could be concomitant or one could precede the other. However, we cannot exclude a link between LINE-1 activity and the BCR-ABL1 expression.Further studies will be necessary to determine if LINE-1 under-expression may precede the active disease or occurs as a result of the disease.