Insights into the Molecular Pathology of SBMA
Summary: We have recently undertaken a proteomics study, coupled to a comparative systems biology analysis to characterize the protein composition of polyQ-Androgen receptor (polyQ-AR) complexes vs. wild-type-AR (WT-AR) complexes. The data shows that AR-interacting partners change dynamically between polyQ-AR and the WT-AR. Our initial results indicate that the polyQ-AR associates with several RNA-binding proteins that participate in RNA metabolism, presumably through an alternative RNA-splicing pathway. The possibility that a transcription factor protein like the AR could also bind to RNA is very plausible; and has been previously described by pioneers in the AR field.
We have employed a unique and powerful genetic approach of using a “humanized” polyQ-AR Drosophila (fly) model that allows us to ectopically express the polyQ-AR in the developing fly embryo. By crossing polyQ-AR flies to RNAi flies of homologous genes encoding candidate polyQ-AR RNA-binding protein interactors, we have been able to identify a number of ligand-dependent enhancers and suppressors.
As neuronal survival has been area of investigation for trinucleotide repeat diseases, and is thought to contribute to disease etiology and pathology, we have specifically focused on mitochondrial dysfunction and apoptosis, and proteasome inhibition. We will study the impact of overexpressing and silencing selected interactors from our fly genetic interaction screen, on cellular processes associated with SBMA such as androgen ligand- and polyQ-AR-induced mitochondrial dysfunction and proteasome inhibition. To do so, we will be using mouse motor neuron-derived cell lines expressing 65Q- and 24Q-AR, and transfected HEK-GFPu cells, respectively.
Our approach of combining proteomics and systems biology is extremely well suited to unraveling complex biological systems and has already proven extremely useful. In the future, we will be interested how polyQ-AR would also impact on the global alternative RNA splicing profile of SBMA patients, and determine the correlation to neuronal survival. These experiments will enable us to infer potentially new molecular mechanisms of polyQ-AR function and give us insight into the mechanisms underlying neurodegeneration in SBMA.
