The Non Receptor Transmembrane Tyrosine Kinase STYK1 Represents a Novel Lymphocyte Marker, Whose Expression Is Deregulated in Leukemia, and a Potential Molecular Druggable Target

Introduction Tyrosine kinases (TKs) play different roles either in physiological or in pathological cellular processes ranging from the regulation of intracellular signaling pathways leading for example to proliferation, differentiation and/or DNA syntheisis. The whole human genome encodes for approximately 95 TKs classified in two major families: transmembrane and non-transmembrane TKs. Transmembrane TKs are further splitted in two groups: Receptor Tyrosine Kinases (RTKs) and Non-Receptor Transmembrane Tyrosine Kinases. Despite in the last decade the role of RTKs has been deeply investigated very little is known about the second class of TKs, Non-Receptor Transmembrane Tyrosine Kinases. Recently, we began to investigate the roles played by these peculiar molecules in hematological tissues and preliminary data are herein presented and discussed.

Material & Methods and Results STYK1 is a Non-Receptor Transmembrane TK characterized by a very short extracellular domain, of approximately 25 aminoacid residues followed by a single transmembrane alpha-helix sequence and a conserved cytoplasmic region harboring the catalytic domain, splitted in two halves separated by a short kinase insert sequence, peculiar of class III RTKs, VEGFR and FGFR family members. Indeed, sequence analysis alignment reveals the higher homology with PDGFR and FGFR members. Surprisingly, data mining searches, performed with the Ensembl tools (http://www.ensembl.org) and using blast algorithm with human STYK1 protein as query, revealed that this protein possesses orthologues either in vertebrate model organisms, both mammalian and non-mammalian or in invertebrate metazoan, such as Ciona. Furthermore, a bioinformatic analysis carried-out at CBS (http://www.cbs.dtu.dk/services/NetPhos/) revealed 4 tyrosine residues putatively phosphorylated. Interestingly one of the tyrosine residue displays high homology towards a consensus sequence for the p85 regulatory subunit of the PI3K. Since our interest aims to understand the roles played by STYK1 in hemopoietic cells, we first investigated its expression in different blood cells populations. Therefore the different blood cellular components were separated by immunoaffinity sorting and total mRNA was extracted. Afterwards c-DNA was synthesized and STYK1 mRNA expression studied by RT-PCR. The results reveal that STYK1 mRNA is expressed predominantly in mature lymphocytes. To study more in details its expression within lymphoid population we attempted to separate B-from T-lymphocytes and STYK1 mRNA expression evaluated in the two populations. Furthermore, we also investigated STYK1 expression in leukemic hematological disorders. Nevertheless STYK1 mRNA expression is typical of mature hemopoietic cells populations, such as lymphocytes, its expression resulted deregulated in patients affected by different kind of leukemia (i.e. CML, ALL, AML). Currently a constitutively active form of STYK1 has been generated and its signaling is under investigation.

Conclusion The non-receptor transmembrane tyrosine kinase STYK1 represents a novel lymphocyte molecular marker whose expression results deregulated in patients affected by leukemia. Interestingly, due to its homology with class III RTKs like PDGFR, which are easily targeted by small molecules such as Imatinib mesylate, it may represents a novel druggable molecular target for leukemic cells.