Neurodegenerative diseases are often associated with accumulation of misfolded proteins. For instance, amyotrophic lateral sclerosis (ALS) affects 5 out of 100000 people worldwide. The most notable ALS cases are Stephen Hawkin and Lou Gehrig, hence the disease is known as Lou Gehrig disease. In 20% of the ALS cases, it is due to a mutation in SOD1, while other cases without Sod1 mutation are associated with TDP-43 protein accumulation in the cytoplasm of spinal cord neurons. This study has shed light on treating ALS by accumulating RNA introns or simply delivering oligonucleotides.
Before you read on: this is a decent paper published in Nature; easy to understand; straight-forward protocols
In this study, the researchers performed a genome wide screen to see which genes are responsible for suppressing TDP-43 toxicity. They narrowed down on dbr1, which suppresses the TDP-43 and TDP-43 mutant toxicity. This suppression is, however, not due to a lowered expression of TDP-43. When human neuronal cell line is transfected with siRNA against dbr1, the toxicity caused by TDP-43 is relieved. The author also proved that Dbr1 knockdown reduces TDP-43 toxicity in primary rat neurons. Using DBR1 mutants that do not have lariat debranching enzymatic activity in yeast spotting assay (look at the figure below if you don’t know what DBR1 does: responsible for debranching the lariat, and subsequently degrading the introns to avoid accumulation of the junk DNA) , TDP-43 toxicity is reduced. As Dbr1 is responsible for debranching lariats following splicing, the knockdown of Dbr1 should increase the amount of introns in the cell. This group incorporated MS2 RNA binding protein into the intron and GFP-tagged MS2-CP protein to visualize the localization of intron. In Dbr1 null cell, intron is colocalized with TDP-43.
The accumulation of introns alleviated the TDP-43 toxicity, which suggest that the accumulation may be a way to treat ALS cases. There are currently no direct therapies against TDP43. So how can we go around this problem? Delivering oligonucleotides into ALS models may be a possible therapy in the near future. In fact, researchers had already used antisense oligonucleotide against SOD1 to treat animal models of ALS which showed slowed disease progression.
Take home message: Without Dbr1, a lariat debranching enzyme, introns accumulate in the cytoplasm which is correlated with lowered TDP-43 toxicity and TDP-43 colocalization with DBR1.Reference: Nat Genet. 2012 Oct 28. doi: 10.1038/ng.2434. [Epub ahead of print] Inhibition of RNA lariat debranching enzyme suppresses TDP-43 toxicity in ALS disease models. Armakola M, Higgins MJ, Figley MD, Barmada SJ, Scarborough EA, Diaz Z, Fang X, Shorter J, Krogan NJ, Finkbeiner S, Farese RV Jr, Gitler AD.