Altered Co-Translational Processing Plays a Role in Huntington's Pathogenesis-A Hypothesis

Abstract

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by the expansion of a CAG codon repeat region in the HTT gene's first exon that results in huntingtin protein aggregation and neuronal cell death. The development of therapeutic treatments for HD is hindered by the fact that while the etiology and symptoms of HD are understood, the molecular processes connecting this genotype to its phenotype remain unclear. Here, we propose the novel hypothesis that the perturbation of a co-translational process affects mutant huntingtin due to altered translation-elongation kinetics. These altered kinetics arise from the shift of a proline-induced translational pause site away from Htt's localization sequence due to the expansion of the CAG-repeat segment between the poly-proline and localization sequences. Motivation for this hypothesis comes from recent experiments in the field of protein biogenesis that illustrate the critical role that temporal coordination of co-translational processes plays in determining the function, localization, and fate of proteins in cells. We show that our hypothesis is consistent with various experimental observations concerning HD pathology, including the dependence of the age of symptom onset on CAG repeat number. Finally, we suggest three experiments to test our hypothesis.

Keywords: Huntington's disease; biophysics; kinetics; neurodegenerative disease; protein aggregation; protein biogenesis; translation; translation regulation.

Figure 1

The proposed co-translational mechanism of HD pathology. (1) A CAF (orange) recognizes N17 (blue rectangle) of nascent Htt (top, nascent proteins shown in green, ribosome in gray). In the case of mHtt (bottom), the poly-proline region is not correctly positioned to slow translation as N17 emerges, reducing the ability of the CAF to bind. (2) In the case of Htt, the CAF directs the subcellular localization of Htt to the Golgi, ER, and mitochondria. Enzymatic modifications (not shown) may occur between targeting and localization, as is known to occur for some proteins in E. coli (Sandikci et al., 2013). mHtt is largely directed to the cytosol, where proteolysis (3) produces short, Exon 1-containing fragments (short green line segments) that form amyloid. Proteolysis can also result in C-terminal mHtt fragments that interfer with ER function (El-Daher et al., 2015).

Figure 2

A simple chemical-kinetic model explains how the age of onset of HD symptoms could arise from disruption of co-translational processing of huntingtin. (A) The CAF is assumed to bind N17 irreversibly in the region of optimal binding with rate k_on. (B) Within this model, the CAF can only bind when the nascent Htt molecule is between 52 and 71 amino acids in length (i.e., when the ribosome starts translating the poly-proline region). (C) As N_CAG increases the number of glutamines (Q) in the binding region increases and the number of prolines (P) decreases, leading to a decrease in the time available for CAF binding (τ_AFB). τ_AFB has units of τ_A (see Methods). (D) The fraction of Htt that is co-translationally misprocessed depends on τ_AFB and, thereby, on N_CAG, as expressed by Equation 1. (E) The fraction of Htt which is misprocessed (f_mp in Equation 1) strongly correlates with N_CAG when realistic values for k_on and CAF concentration are used (see Methods for a complete description of Equation 1). (F) The age of HD symptom onset shows strong negative correlation with the fraction of Htt misprocessed predicted by Equation 1. Age of onset vs. N_CAG data were extracted from Figure 1C of Lee et al. (2012) with PlotDigitizer (plotdigitizer.sourceforge.net).