The nonsense mediated mRNA decay (NMD) pathway targets and degrades mRNAs containing premature termination codons (PTCs). NMD often exacerbates the effects of nonsense mutation containing alleles by preventing the expression of partially-functional truncated proteins. As such, inhibition of NMD is considered to be a broadly applicable approach for treating numerous inherited diseases. In cases in which the truncated protein does not retain function, NMD inhibition may be synergistic with "readthrough" therapy: the administration of drugs that stimulate ectopic translation of PTCs, thus producing full length proteins.
However, before such an approach can be realized, the endogenous roles of the NMD pathway must be understood. In particular, loss of NMD is in itself completely lethal to most organisms, including Drosophila, probably because of a role of the NMD pathway in inhibiting the expression of a number of endogenous (non-mutated) transcripts. Recently, we have shown that this requirement of NMD for viability is due to degradation of only a single endogenous NMD target: the mRNA of the stress response gene Gadd45 (Growth Arrest and DNA Damage)*. In particular, we have found deletion of Gadd45 can rescue Drosophila carrying null mutations in core NMD genes to adulthood.
Based on these findings, we are now in the position to test whether administration of readthrough drugs does indeed work synergistically with loss of NMD in restoring functional protein expression of PTC-containing genes. To do this, we will combine known stop-codon containing alleles with mutations in NMD genes, while simultaneously suppressing NMD mutant lethality using the Gadd45 deletion. This will be done both with and without drug-induced stop codon readthrough. Ultimately, our data should reveal the underlying relationship between RNA degradation and translation in regulating gene expression of mutated genes, in both an experimental and clinical setting.
*"Degradation of Gadd45 mRNA by nonsense-mediated decay is essential for viability". Nelson et al. (2016) eLife, In press.