Therapy

siRNA’s are capable of functioning in vivo, as “smart,” targeted drugs. As mentioned before, genome-wide linkage analysis has determined various susceptibility genes in asthmatic patients. These include genes for atopy, asthma, airway hyperresponsiveness, and IgE overproduction. We will survey the classes of possible targets for RNAi for asthma: Fc receptors, cytokines, chemokines, adhesion molecules, cell membrane receptors (i.e. proinflammatory receptors), protein kinases, and transcription factors.7 The advantage to RNAi is that it is a transient therapeutic, unlike gene therapy, and it acts more like a drug, altering mRNA rather than DNA.

IgE
The gene encoding for the Fc epsilon RI receptor on mast cells that interacts with IgE has been identified. Targeting these receptors can prevent Fc receptor cross-linking, which can prevent mast cell degranulation, which can curb the initial inflammatory response in asthma.

Cytokines TNF-α, an inflammatory cytokine, has been the target of some siRNA approaches. In asthma, we often see the Th2 cytokine response overwhelming the Th1 cytokine response. The RNAi silencing of Th2 activity (i.e. silencing of translation of IL-4, IL-5, and IL-13) can be a possible option for treatment.

Cell membrane receptors
The receptors for cytokines (both Th1 and Th2 in origin) and chemokines have also been discussed as targets for RNAi. Researchers have development monoclonal antibodies that block TNF-α receptors. Now, RNAi could possibly inhibit the upregulation of these receptors.

Chemokines
Chemokines (RANTES and eotaxin 1) that recruit T cells to the inflammation site as well as chemokine receptors (CCR3) that bind these chemokine ligands may also be a route for RNA interference.

Adhesion molecules
Certain cell adhesion molecules such as ICAM-1 mediate the adhesion of leukocytes to the endothelial cells of the vasculature of the inflammatory tissue. CAM’s on the endothelium mediate a strong adhesion with integrins (i.e. LFA) on leukocytes. Blocking this interaction can prohibit the recruitment of Th2 cells to the site of inflammation, thereby curbing asthma.

Protein kinases and Transcription Factors
RNAi may direct the shutdown of certain tyrosine-protein kinases involved in the inflammatory cascade. Most cytokine receptors transduce an extracellular signal (involving binding of the cytokine to the cytokine receptor) to the cell nucleus via the Jak/STAT pathway.8 Potential tyrosine protein kinase targets include Lyn, Syk, and Btk.

The first work on targeting protein kinases using RNAi in the context of asthma was done by Heinonen et al on silencing the mRNA for Bruton’s tyrosine kinase (Btk). Btk is involved in histamine release by mast cells. It was shown that histamine release was reduced 20-25% by Btk silencing. 9

Approaches to block the MAP kinase pathway, as well as the Jak-STAT pathway (specifically blocking the translation of STAT 6) have been investigated.

NF-κB plays a ubiquitous role as a transcription factor in inflammatory processes. In the context of asthma, they direct the transcription of genes for cytokines such as IL-4 and IL-13. In airway epithelial cells, NF-κB can also direct the transcription of inducible nitric oxide synthase (iNOS) which perpetuates the inflammatory response in asthma, by creating NO, which leads to the generation of harmful radicals, notably peroxynitrite, which damages cells and tissues. Blocking the activation of NF-κB by RNAi has been reviewed by Guo et al.10 The gene for the p65 subunit of NF-κB and can be a target for RNAi silencing.

We have presented a myriad of targets for RNA interference therapy for asthma. Finding the most specific targets with the least side effects is the greatest challenge on this end. However, an equally challenging avenue of research is the delivery of RNA interference.