Identification of novel human neuronal leucine-rich repeat (hNLRR) family genes and inverse association of expression of Nbla10449/hNLRR-1 and Nbla10677/hNLRR-3 with the prognosis of primary neuroblastomas.

Abstract

To search for novel prognostic indicators, we previously cloned >2,000 novel genes from primary neuroblastoma (NBL) cDNA libraries and screened for differential expression between the subsets with favorable (stage 1 or 2 with a single copy of MYCN) and unfavorable (stage 3 or 4 with amplification of MYCN) prognosis. From them, we have identified 3 genes of human neuronal leucine-rich repeat protein (NLRR) family: Nbla10449/hNLRR-1, Nbla00061/hNLRR-2/GAC1 and Nbla10677/hNLRR-3. An additional family member, hNLRR-5, was also found by homology search against public database. NLRR family proteins have been proposed to function as a neuronal adhesion molecule or soluble ligand binding receptor like Drosophila toll and slit with multiple domains including 11 sets of extracellular leucine-rich repeat (LRR)-motifs. However, the functional role of the NLRR protein family has been elusive. Our present study shows that hNLRR mRNAs are preferentially expressed in nervous system and/or adrenal gland. In cancer cell lines, hNLRR-1, hNLRR-3 and hNLRR-5 are expressed at high levels in the neural crest-derived cells. Most remarkably, in primary NBLs, hNLRR-1 is significantly expressed at high levels in unfavorable subsets as compared to favorable ones, whereas the expression pattern of hNLRR-3 and hNLRR-5 is the opposite. In order to understand the function of these receptors, we have used newborn mouse superior cervical ganglion (SCG) cells which are dependent on nerve growth factor (NGF) for their survival. Expression of the mouse counterparts of hNLRR-2 and hNLRR-3 is up-regulated after NGF-induced differentiation and down-regulated after NGF depletion-induced apoptosis. On the other hand, expression of hNLRR-1 and hNLRR-5 is inversely regulated in the same system. These results have suggested that the regulation of the hNLRR family genes may be associated with NGF signaling pathway in both SCG cells and neuroblastoma. Our quantitative real-time RT-PCR analysis using 99 primary NBLs has revealed that high levels of hNLRR-1 expression are significantly associated with older age (>1 year, p=0.0001), advanced stages (p=0.0007), low expression of TrkA (p=0.011), and MYCN amplification (p=0.0001), while those of hNLRR-3 expression are significantly correlated with the favorable prognostic indicators. Furthermore, multivariate analysis reveals that expression of hNLRR-1 is an independent prognostic indicator in human neuroblastoma. Thus, our results demonstrate that, despite being members of the same family, hNLRR-1 and hNLRR-3 may share different biological function among the NBL subsets, and that their expression level becomes novel prognostic indicators of NBL.