Deficiency of a novel retinoblastoma binding protein 2-homolog is a consistent feature of sporadic human melanoma skin cancer.

Using RNA arbitrarily primed PCR, the authors selected for transcripts with cell cycle-related differential expression in cultured human melanocytes. Among the partial cDNAs cloned, a novel cDNA was identified, which showed 54% identity to the recently cloned cDNA of the retinoblastoma binding protein-2 (RBP2). The 6.5-kB full-length cDNA of this RBP2-related gene, termed RBP2 homolog 1 (RBP2-H1), ), was obtained from a human teratocarcinoma cDNA library. Two independent libraries from human malignant melanomas were negative. A computerized sequence analysis revealed highly conserved motifs with possible functional meaning: two domains that, in the RBP2 homolog, mediate the binding and interaction with the proteins encoded by the retinoblastoma susceptibility gene, the TATA-binding protein, and the oncoprotein rhombotin 2; in addition, two DNA-binding zinc finger/leukemia-associated protein motifs were detected. Because a functional role in cell-cycle control and transcriptional activation can be envisioned, we investigated the expression of this novel transcript in normal fetal and adult tissues, as well as tissues of benign and malignant melanocytic tumors. By conducting multiple Northern blot, RT-PCR, and in situ hybridization analyses, the authors showed that the corresponding mRNA is expressed in virtually all normal tissues. Accordingly, they found RBP2-H1 expression in microdissected tissue samples from benign melanocytic nevi (n = 10). In contrast, the transcript is significantly down-regulated or even lost in tissue samples from human malignant melanomas (n = 13), melanoma metastases (n = 10), and melanoma cell lines (n = 7). The authors concluded that the loss or down-regulation of RBP2-H1 expression could be a useful molecular marker for a transformed phenotype in the human melanocytic system.