Protein tyrosine phosphatase receptor-type? (PTPRG) is down regulated in CML patients at diagnosis and restored upon TKI treatment: Preliminary results

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Background. Chronic myelogenous leukemia (CML) is the most common myeloproliferative disease accounting for ~15% to 20% of all cases of leukemia (1-1.5/100.000 cases per year). It originates from a pluripotent bone marrow stem cell in which a t(9;22) results in the production of BCR/ABL fusion protein which has a constitutive tyrosine kinase activity and deregulates signal transduction pathways. Phosphorylation of key residues is required for the full transforming activity of BCR/ABL; for this reason much attention has been focused on the role of phosphatases, natural regulators of the tyrosine kinase signaling. We have previously reported that protein tyrosine phosphatase receptor type ? (PTPRG) is a tumor suppressor gene which interacts with BCR/ABL, inhibits downstream signaling events and is downregulated in CML. Whitin the QNRF research project NPRP 4-157-3-052 we analyzed the expression levels of PTPRG gene in 32 CML patients at diagnosis and following TKI treatment aiming at the evaluation of the clinical impact of PTPRG dowregulation in CML. Methods: Thirtytwo patients diagnosed with CML in chronic phase and 13 untreated patients diagnosed with philadelphia-negative myelod disorders as control group were included in the study. 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 the PTPRG gene was evaluated by a sybr-green absolute quantification RT-PCR assay in 2 samples from each patient, taken at diagnosis and following TKI treatment using the beta-Actin (ACTB) housekeeping gene for normalization. Results were expressed as PTPRG/ACTB ratio and were validated using predesigned TaqMan quantitative RT-PCR assays for PTPRG and ABL1 genes. BCR-ABL1 transcript was quantified by realtime RT-PCR according to the European Leukemia Net guidelines. Statistical analysis and comparisons were performed using the SPSS-software. Results: PTPRG transcript was undetectable in 11/32 (34,3%) CML samples at diagnosis and the median levels of PTPRG mRNA 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=0.02). On the contrary, PTPRG mRNA was detected at variable levels, ranging from 0.17 to 30%, in 29/32 follow up samples, taken at different time points of treatment. Differences in PTPRG gene expression levels between CML samples before and after treatment were statistically significant (p=0,027). Quantitative RT-PCR for PTPRG has been also set up at the Hematology center, NCCCR, Doha, Qatar. Preliminary results showed that mean levels of PTPRG mRNA were comparable to the italian group of patients. No statistically significant correlation was observed between PTPRG levels and clinical/biological factors. Interestingly, 2 of the 3 patients showing higher level of PTPRG mRNA at diagnosis than in the follow up sample showed resistance to TKI treatment. Conclusions: We found a down regulation of PTPRG in a high percentage of CML patients and a recovery of its expression 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.