Low expression of human tubulin tyrosine ligase and suppressed tubulin tyrosination/detyrosination cycle are associated with impaired neuronal differentiation in neuroblastomas with poor prognosis

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Abstract
Neuroblastoma (NBL), one of the most common childhood solid tumors, has a distinct nature in different prognostic subgroups. However, the precise mechanism underlying this phenomenon remains largely unknown. To understand the molecular and genetic bases of neuroblastoma, we have generated its cDNA libraries and identified a human ortholog of tubulin tyrosine ligase gene (hTTL/Nbla0660) as a differentially expressed gene at high levels in a favorable subset of the tumor. Tubulin is subjected to several types of evolutionarily conserved posttranslational modification, including tyrosination and detyrosination. Tubulin tyrosine ligase catalyzes ligation of the tyrosine residue to the COOH terminus of the detyrosinated form of α‐tubulin. The measurement of hTTL mRNA expression in 74 primary neuroblastomas by quantitative real‐time reverse transcription‐PCR revealed that its high expression was significantly associated with favorable stages (1, 2 and 4s; p = 0.0069), high TrkA expression (p = 0.002), a single copy of MYCN (p < 0.00005), tumors found by mass screening (p = 0.0042), nonadrenal origin (p = 0.0042) and good prognosis (p = 0.023). The log‐rank test showed that high expression of hTTL was an indicator of favorable prognosis (p = 0.026). Immunohistochemical analysis using specific antibodies generated by us demonstrated that tyrosinated tubulin (Tyr‐tubulin), detyrosinated tubulin (Glu‐tubulin) and hTTL as well as Δ2‐tubulin were positive in favorable tumors, whereas only Δ2‐tubulin was positive in the tumors with MYCN amplification. In an RTBM1 neuroblastoma cell line, hTTL was increased after treating the cells with bone morphogenetic protein 2 (BMP2) or all‐trans retinoic acid (RA), which induced neuronal differentiation. These results suggest that the deregulated tubulin tyrosination/detyrosination cycle caused by decreased expression of hTTL is associated with inhibition of neuronal differentiation and enhancement of cell growth in the primary neuroblastomas with poor outcome.

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