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Gene silencing by siRNA (short interfering RNA) is a still developing field in biology and has evolved as a novel post-transcriptional gene silencing strategy with therapeutic potential. With siRNAs, virtually every gene in the human genome contributing to a disease becomes amenable to regulation, thus opening unprecedented opportunities for drug discovery. Besides the well-established role for siRNA as a tool for target screening and validation in vitro, recent progress of siRNA delivery in vivo raised expectations for siRNA drugs as the up-and-coming 'magic bullet'. Whether siRNA compounds will make it as novel chemical entities from 'bench to bedside' will probably depend largely on improving their pharmacokinetics in terms of plasma stability and cellular uptake. Whereas locally administered siRNAs have already entered the first clinical trials, strategies for successful systemic delivery of siRNA are still in a preclinical stage of development. Irrespective of its therapeutic potential, RNAi (RNA interference) has unambiguously become a valuable tool for basic research in biology and thereby it will continue to have a major impact on medical science. In this review, we will give a brief overview about the history and current understanding of RNAi and focus on potential applications, especially as a therapeutic option to treat human disease.

Cytogenetics has not only precipitated the discovery of several oncogenes, but has also led to the molecular classification of numerous malignancies. The correct identification of aberrations in many tumors has, however, been hindered by extensive tumor complexity and the limitations of molecular cytogenetic techniques. In this study, we have investigated five malignant melanoma (MM) cell lines from at least three different passages using high-resolution R-banding and the recently developed methods of comparative genomic hybridization and multicolor or multiplex fluorescence in situ hybridization. We subsequently detected nine consistent translocations, seven of which were novel: dic(1;11)(p10;q14), der(9)t(3;9)(p12;p11), der(4)t(9;4;7)(q33:p15-q23:q21), der(14)t(5;14)(q12;q32), der(9)t(9;22)(p21;q11), der(19)t(19;20)(p13.3;p11), der(10)t(2;12;7;10)(q31:p12-->pter:q11.2-->q31:q21), der(19)t(10;19)(q23;q13), and der(20)t(Y;20)(q11.23;q13.3). Furthermore, using the human HG-U133A GeneChip, positive expression levels of oncogenes or tumor-related genes located at the regions of chromosomal breakpoints were identified, including AKT1, BMI1, CDK6, CTNNB1, E2F1, GPNMB, GPRK7, KBRAS2, LDB2, LIMK1, MAPK1, MEL, MP1, MUC18, NRCAM, PBX3, RAB22A, RAB38, SNK, and STK4, indicating an association between chromosomal breakpoints and altered gene expression. Moreover, we also show that growth of all five cell lines can be significantly reduced by downregulating CDK6 gene expression with small interfering RNA (siRNA). Because the majority of these breakpoints have been reported previously in MM, our results support the idea of common mechanisms in this disease.

Malignant melanoma is a prime example of a treatment-resistant tumor with poor prognosis. Even with innovative treatment regimens, response rates remain low, and the duration of responses is short. More than 90% of all melanomas express the antiapoptotic protein Bcl-2, shown to contribute to a chemoresistant phenotype in melanoma. We previously demonstrated that antisense-mediated inhibition of Bcl-2 sensitizes malignant melanoma to apoptosis-inducing treatment modalities. In the present study, we evaluated synthetic small interfering RNA (siRNA) compounds targeting Bcl-2 as a novel approach to downregulate Bcl-2 expression in melanoma cells. siRNA treatment led up to a 19-fold reduction of bcl-2 mRNA levels and only barely detectable Bcl-2 protein expression at low nanomolar concentrations. Silencing of Bcl-2 in melanoma cells by specific siRNA led to a moderate increase in apoptotic cell death and inhibition of cell growth. However, if siRNA compounds targeting Bcl-2 were combined with the apoptosis-inducing chemotherapeutic agent cisplatin, a massive increase in apoptotic cell death compared with controls was observed. Notably, the combination of Bcl2 siRNA and low-dose cisplatin resulted in a supra-additive effect, with nearly complete suppression of cell growth, whereas cell growth in cisplatin-only-treated cells was only moderately affected (96% vs. 25%, p < 0.001). These findings underline a key role for Bcl-2 in conferring chemoresistance to melanoma and highlight Bcl-2 siRNA strategies as novel and highly effective tools, with the potential for future targeted therapy of malignant melanoma.

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