Ptprg and BCR/ABL1 Expression In CML Patients At Diagnosis and Upon TKI Treatment: Preliminary Results

Chronic Myeloid Leukemia (CML), is a biologically and clinically heterogeneous disease that originates in a pluripotent bone marrow stem cell. The hallmark of CML is the presence of an aberrant gene, BCR-ABL1, which encodes for a constitutively activated tyrosine kinase. Protein tyrosine phosphatases can negatively regulate ABL1 mediated signaling by dephosphorylating the kinase and/or its substrates and impaired phosphatase activity has been linked to several solid cancers and leukemias. We have previously reported the role of the receptor-type phosphatase PTPRG as tumor suppressor gene and demonstrated a significant decrease in the PTPRGexpression level in CML.

Here we have investigated the expression of PTPRG gene in 32 CML patients aiming at the evaluation of the clinical relevance of the PTPRGdown regulation.

All 32 CML patients included in this study were diagnosed in chronic phase. Thirteen untreated patients diagnosed with philadelphia-negative myelod disorders were included in the study as control group. The study was approved by the Local Ethics Committee, and informed consent in accordance with declaration of Helsinki was obtained from each patient. The expression level of PTPRG gene was evaluated by a sybr green real-time RT-PCR assay in 2 samples from each patient, taken at diagnosis and following TKI treatment. The beta-Actin (ACTB) housekeeping gene was used for normalization. Results were validated using predesigned TaqMan quantitative RT-PCR assays for PTPRG (intracellular domain) and ABL1 genes. Quantitative RT-PCR was used to quantify BCR-ABL1 gene fusion transcripts according to the European Leukemia Net guidelines. Statistical analysis and comparisons were performed by Mann-Whitney's and Student paired T tests using the SPSS software.

The mean levels of PTPRG transcript, expressed as PTPRG/ACTB ratio, were significantly lower in CML samples at diagnosis compared to the non-CML control group (0,44%, range 0-0,37 vs 6,29%, range 0,09-52; p=0.020). Recovery of PTPRG mRNA expression at variable levels, ranging from 0.17 to 30%, was detected in 29/32 follow up samples, taken at different time points of treatment (from 3 months to 10 years). Differences in PTPRG gene expression levels between CML samples at diagnosis and after treatment were statistically significant (p=0,027). No statistically significant correlation was observed between PTPRG/ACTB and BCR-ABL/ABL1 ratios nor with deepness of molecular response or SOKAL and EUTOS scores, even though 2 of the 3 patients showing higher level of PTPRGmRNA at diagnosis than in the follow up sample showed resistance to TKI treatment.

In this study we confirm a down regulation of PTPRG in a high percentage of CML patients and show that its expression is restored upon treatment with TKIs. Deregulated expression of PTPRG phosphatase underline its role as a tumor suppressor gene in CML and highlights its potential use as a new bio-marker of disease potentially usable in association with BCR/ABL1, allowing for a better understanding of the molecular mechanism underlying the development of CML, and propose possible new avenues for therapeutic treatment.

Yassin: qatar national research fund: Patents & Royalties, Research Funding.