Neurotrophin Receptors and Ligands Are Highly Expressed in Acute Leukemia and Promote Leukemogenesis.

The neurotrophins (NTs), which include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), NT-3, NT-4, and NT-6, play a major role in neuronal survival. NTs are unique in that they utilize two different classes of receptors: the TRK (tropomyosin-related kinase) receptor protein tyrosine kinase (TRKA, TRKB, TRKC) and the low affinity NGF receptor (LNGFR=p75NTR). Recently, we and others have obtained evidence for potential involvement of this receptor system in leukemia. In the present study, we demonstrate for the first time expression of the three TRKs on the protein level in blasts from patients with newly diagnosed de novo or secondary acute leukemia. TRK expression was detected by flow cytometry using monoclonal antibodies previously validated on cell lines expressing retrovirally encoded TRK and considered positive if >20% of leukemic blasts expressed the respective receptor. 93 adult patients (41 female, 52 male) with a mean age of 53.8 years and diagnosis of AML (87%), ALL (12%), or AUL (1%) were enrolled after informed consent. Positive expression for at least one TRK was found in 55% of the analyzed cases without statistically meaningful differences in expression rate between AML (42/81) and ALL (8/11). Interestingly, while TRKB was expressed alone in blasts, TRKA or TRKC expression always occurred concomitantly with TRKB. In contrast to a previous study, we established a clear correlation of TRK expression pattern and FAB classification. In particular, TRKA expression occurred in 19 of 32 myelo-monocytic/monocytic leukemias (59%) whereas only 5 of 39 non-myelo-monocytic/monocytic leukemias (13%) were positive (p<0.005). The same observation was made for TRKC expression (44% vs. 10%, p<0.02). However, TRKB expression was more widespread, but tended to correlate with secondary AML (p=0.05). While only very few point mutations were found in TRK sequences by RT-PCR, we observed co-expression of BDNF (ligand for TRKB) in over half of TRKB^pos cases (53%, 16/30) by flow cytometry. Expression of NGF or NT3 was also observed. Interestingly, high expression of TRKB or BDNF was associated with higher leukemic blast percentages (p< 0.0001). Remarkably, we found a trend for better mean overall survival in the BDNF^neg subgroup (908 days vs. 527 days for BDNF^pos). However, due to heterogeneity of clinical data and currently short follow-up, our results have not reached statistical significance. Activation of TRKA or TRKB by NGF and BDNF, respectively, efficiently rescued murine myeloid 32D cells from irradiation-induced apoptosis. Moreover, co-expression of TRKB/BDNF or TRKA/NGF in murine hematopoietic cells induced leukemia (n=6) in mouse models. Since BDNF is expressed by stromal cells in bone marrow, our findings suggest that TRKs might play an important role in leukemogenesis, with autocrine or paracrine stimulation as potential oncogenic mechanisms. Thus, TRKs and their downstream effectors might serve as novel therapeutic targets in acute leukemia.