Investigating the physiological role of S199A and S199D mutants of PHF6 protein in T-cell acute lymphoblastic leukemia

Background/aim: T-cell acute lymphoblastic leukemia (T-ALL) is a form of leukemia characterized by the proliferation of immature T lymphocytes. NOTCH1 is one of the most frequently mutated genes in T-ALL. NOTCH1 expression in T-cell development depends on plant homeodomain finger protein 6 (PHF6), which plays a tumor suppressor role in T-ALL. Several studies have shown that PHF6 expression is essential for NOTCH1 expression. Therefore, whether posttranslational modification of PHF6 plays a role in the regulation of NOTCH1 expression and T-ALL cell line proliferation was investigated herein. Materials and methods: The amino acid sequence of PHF6 was analyzed and it was found that a putative protein kinase A (PKA) phos-phorylation motif RDRS199 was conserved in several vertebrate species and the S199 site was expected to be phosphorylated accord-ing to the PhosphoSite database. Therefore, an eukaryotic expression vector of human PHF6 was constructed, and the codon 199 was changed to the codon encoding the nonphosphorylatable alanine and the phosphorylation-mimicking aspartic acid via site-directed mutagenesis. After confirming the ectopic expressions of the PHF6 vectors by western blot analysis, the effects of these proteins were identified on the NOTCH1 expression using western blot analysis, leukemic cell proliferation using MTT assay, and expressions of the cell surface markers of T-cells using flow cytometry. Results: The ectopic expression of wild-type PHF6 stimulated the formation of CD4 + T-cells. While the expression of the wild-type PHF6 suppressed the growth of the leukemic cell line, this effect was diminished in both the alanine and aspartic acid mutants of PHF6. In addition, both mutants also seemed to negatively affect the NOTCH1 expression, although the effect of the alanine mutant was more severe. Conclusion: Taken together, the different biological activities exerted by the conserved S199 phosphorylation-site mutants shown in this study implicate that signaling pathway(s) leading to differential phosphorylation of this residue may have a substantial effect on the activity of PHF6, and thus may constitute a potential therapeutic target in T-ALL.