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
alpha-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 Delta2-tubulin were positive in favorable
tumors, whereas only Delta2-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.