Expression of the Cell Polarity Protein PTK7 in Acute Myeloid Leukaemia.

The planar cell polarity pathway plays a major role in embryogenesis and tissue organisation. Recent genetic studies have highlighted the role of novel receptors and signaling molecules implicated in this pathway. Amongst the receptors, the pseudo tyrosine kinase receptor 7 (PTK7) is an orphean tyrosine kinase receptor with kinase-dead activity. Knock-out of PTK7 in mice strongly affects embryonic development leading to a major neural tube defect. Presence of PTK7 was previously investigated in epithelial and endothelial cells that both express the receptor. In normal donors, we found no expression of PTK7 in peripheral blood (n=5) whereas PTK7 expression was found with low levels in PBPC after G-CSF stimulation (n=3) and high levels in normal myeloid progenitors and CD34⁺ CD38⁻ bone marrow cells (n=3). Overexpression of PTK7 was already described in solid tumors including breast, lung and pancreatic cancers. We decided to study the potential implication of PTK7 in haematological malignancies. We performed a wide range multicolour immunophenotyping screen on more than 240 patient samples treated at Institut Paoli-Calmettes and 10 leukemia cell lines. In hematologic malignancies, we demonstrated that PTK7 was widely expressed in AML (136 of 195 patients) and in the most immature subsets of Acute lymphoblastic (5 of 20 patients) or biphenotypic leukaemia (3 of 3 patients). We found no expression of PTK7 in chronic disorder such low grade NHL (n=7), CLL (n=6) or Chronic Myelomonocytic Leukemia (n=3). In AML, we demonstrated that PTK7 expression mostly correlates with granulocytic lineage differentiation and that it could be partially expressed in AML 4 or 5 subsets. Flow cytometry analysis confirmed the co-expression of PTK7 with granulocytic lineage markers and that PTK7 expression in myelomonocytic leukaemia was limited to the myeloid subset of blasts. The strongest immunophenotyping correlation was found with CD117/c-Kit expression (p<0.001) and PTK7 Mean Fluorescence Intensity directly correlates with c-Kit MFI (p=0.001). Interestingly, stimulation of cultured TF1 cells (that endogenously express c-Kit and PTK7) with SCF triggered an increased expression of PTK7. Correlation between PTK7 expression and biological or clinical features was also evaluated. We demonstrated that PTK7 expression clustered with some cytogenetic subsets (high levels in CBF AML (n=19) or APL (n=13), low levels in FLT3 mutated AML(n=17) or complex karyotype (n=20)). We also found that PTK7 expression was associated with a lower WBC count at diagnosis (p=0.001) and a lower frequency of extramedullary disease (p<0.001) in whole population and in both AML1–3 and AML 4–5 subgroups. We report here novel findings that potentially implicate ptk7, a PCP gene, in hematopoiesis and AML. In vitro, we showed that ectopic expression of PTK7 promotes cell migration, cell survival and resistance to anthracyclin-induced apoptosis. Ongoing works are currently investigating which molecular mechanisms regulate PTK7 functions in normal and pathological situations.