by Dr. Jiwu Wang
Most cancer drugs target the cell cycle at various points, e.g. growth signals promoting entry into cell cycle and DNA replication and repair, because cancer cells tend to divide more rapidly than normal cells. Current cancer therapies such as chemotherapy or radiation therapy damage DNA and result in cell death following DNA replication during the S phase of the cell cycle. Functions of DNA repair pathways that remove lesions caused by DNA damaging drugs or treatments may modulate the killing of cancer cells. Therefore DNA repair genes have become important targets for inhibition as a means of enhancing the effects of chemotherapeutic agents or radiation.
Even without the effect from DNA damaging treatment in cancer therapy, DNA lesions occur spontaneously and constantly in normal cells. The DNA repair machinery is responsible for mending many types of common DNA damages, e.g. single strand nicks, double strand breaks, crosslinks, through multiple pathways specialized at fixing each type of problems. Different DNA repair pathways responsible for dealing with different types of DNA damage also overlap, enabling cells to avoid the fate of cell death when one repair or signaling factor is inactive due to mutation or inhibition.
Cell death may occur when two genes in the overlapping DNA repair pathways are both rendered non-functional even though each gene mutation alone is not lethal. The phenomenon is often described as synthetic lethal. The concept of synthetic lethal interactions has become the underlining principle for development of therapies based on targeting different DNA repair components. Several cancers are known to have defects in DNA damage response and repair pathways. For instance, BRCA 1 or BRCA 2 genes involved in homologous recombination are defective in familial breast and ovarian cancers. In a BRCA1- or BRCA2- background such as in breast cancer cells, inhibition of PARP1, a gene required for the efficient repair of single-stranded breaks of DNA during base excision repair, leads to persistent single-stranded gaps, double stranded breaks that are not fixed, and eventual death of cells.
Generally low target specificity of small molecules often makes it difficult to identify therapy lead compounds through large-scale synthetic lethal screening. In comparison, genetic synthetic lethality screening through RNA interference (RNAi) directly targeting genes in selected pathways or even all human genes can achieve relatively high specificity and success rate. Utilizing RNAi as a tool to screen for synthetic lethal in DNA damage signaling and repair pathways presents great opportunities to find novel cancer therapeutic targets. RNAi reagents such as siRNA and shRNA-expressing vectors used for screening can themselves be tested as putative therapeutics, as several clinical trials have so far demonstrated. Successfully performing high throughput RNAi screenings requires capabilities of efficient RNAi design, viral packaging, fluorescent proteins (as RNAi indicators and viral transduction markers), and advanced cell culture and analysis techniques. More details of setting up such screening can be found here.
Jiwu Wang, Ph.D.
CEO
Allele Biotechnology & Pharmaceuticals, Inc.
www.allelebiotech.com
