Heterogeneous distribution of Retinal Degeneration Protein 3 in normal human fetal tissues: Exploring the possible relevance to neuroblastoma genesis

Background: Our studies identified the loss of Retinal degeneration Protein 3 (RD3) in the predominant infant tumor, neuroblastoma, indicated its novel regulatory function in neuroblastoma pathogenesis and, showed its localization in tissues beyond retina. To explore the possible physiological role of RD3 in regulating the genesis of neuroblastoma, we examined its distribution in human fetal normal tissues. Methods: Constitutive mRNA levels (QPCR, RNAScope), protein expression (immunohistochemistry), and sub-cellular localization of RD3 were investigated in the array (20 sites) of human (n=5) fetal normal tissues. In silico RNA sequencing data from seven independent studies (total n = 407) of human fetal tissues (101 sites) was utilized to validate the differential expression of RD3 in developing tissues. Results: RNAscope and QPCR analysis indicated a steady state of RD3 transcription across the fetal tissues with measurable inter-tissue differences (For e.g. high in GI tissues vs low in adrenal gland). RNA-seq data on varying stages of development clearly indicated stage-dependent dynamic fluctuations in RD3 transcription as the development progress. Notably, RD3 transcription is stably expressed in choroid plexus and are relatively absent in spinal cord through the developmental stages. IHC analysis recognized the presence of RD3 protein in array of fetal normal tissues, the tissue- and cell-specific distinctions in protein expression, and the variances in sub-cellular localization with each tissue. Conclusion: For the first time, the results presented here revealed the tissue-specific transcription, expression and localization of RD3 in fetal tissues. Recognizing the heterogeneous expression of RD3 between and within the fetal tissues signify its possible function beyond photoreceptor cell survival and could shed light on its functional relevance in neuroblastoma genesis and/or evolution.