In this paper, the authors discuss RNA interference (RNAi) as the basis for therapies to treat transthyretin amyloidosis. In this condition, as in other forms of amyloidosis, solid deposits of misfolded or otherwise damaged proteins known as amyloid accumulate with age, causing various forms of dysfunction in tissues. In the case of transthyretin amyloid, these solid aggregates contribute to heart failure and other cardiovascular conditions, and are thought to be involved in a range of other, less immediately pressing age-related conditions. Studies of supercentenarians suggest that transthyretin amyloidosis leading to heart failure is the predominant cause of death in that population. Any comprehensive toolkit of rejuvenation therapies must include a way to clear amyloids, and thus remove their contribution to aging and age-related disease.
Transthyretin (TTR) is a transport protein that is primarily expressed in the liver. Its primary function is to transport Vitamin A (retinol) through its interaction with the retinol binding protein. Although the majority of newly synthesized TTR protein folds and functions properly, TTR protein misfolding can occur. TTR misfolding is exacerbated by destabilizing mutation and proteolysis and, if left uncorrected, misfolded TTR has a propensity to form pathologic amyloid fibrils. TTR-mediated amyloidosis (ATTR amyloidosis) is a progressive, systemic and ultimately fatal disease resulting from the damage caused by the deposition of insoluble TTR fibrils. TTR-containing amyloid fibrils can deposit in peripheral and central nervous systems, the gastrointestinal tract, eye, kidney and/or the heart. In contrast to hereditary ATTR amyloidosis, wild-type ATTR amyloidosis results from the misfolding of wild-type TTR protein with deposition occurring predominantly in the heart. Clinical presentation of wild-type ATTR amyloidosis, which includes carpal tunnel syndrome and cardiomyopathy, typically occurs much later in life relative to the hereditary forms, and likely reflects the fact that wild-type TTR is less prone to misfolding than other, mutated variants present in the hereditary form of the condition.
Although disease manifestation varies across the different forms of ATTR amyloidosis, the common feature of these diseases is the misfolding of TTR protein that ultimately results in amyloid formation and deposition. As such, mitigation of TTR amyloid deposition is crucial to the development of any successful therapeutic treatment for all forms of ATTR amyloidosis. Tetramer stabilizers are a class of small molecule therapies currently under development and even approved in certain geographic locales for the treatment of ATTR amyloidosis. These modalities aim to limit TTR aggregation by binding and stabilizing the properly folded tetramer, thereby decreasing the concentration of aggregation-prone species. To date, data from clinical trials suggests that this approach can slow the rate of ATTR amyloidosis progression. Regardless, there is still a need for effective treatments for ATTR amyloidosis.
RNA interference (RNAi) is a naturally occurring biological process by which small interfering RNA (siRNA) can direct sequence-specific degradation of mRNA, leading to inhibition of synthesis of the corresponding protein. Recent advances in the efficient and specific delivery of siRNA to the liver have paved the way for development of RNAi-based therapeutics for disease targets expressed in the liver. As such, an RNAi therapeutic strategy is well-suited to the treatment of ATTR amyloidosis. Specifically, the therapeutic hypothesis behind this strategy predicts that silencing TTR gene expression will reduce the total amount of TTR protein, both folded and misfolded, that becomes a substrate for amyloid fiber formation, thereby reducing tissue burden and the consequences thereof. Given the vast majority of pathogenic protein in ATTR amyloidosis originates in the liver, liver specific gene silencing enabled by current delivery technologies should result in nearly complete reduction of systemic TTR levels. Finally, given the ability to silence all known disease causing TTR variants, including wild-type, an RNAi therapy may be not only an effective approach for the treatment of ATTR amyloidosis but also more generally applicable.
siRNA formulations targeting TTR resulted in robust knockdown of hepatic TTR mRNA and serum protein in transgenic mice. Further, RNAi-mediated knockdown of hepatic TTR inhibited TTR protein deposition and promoted the regression of existing TTR deposits in pathologically relevant tissues. Finally, the extent of regression of TTR tissue deposits correlated with the extent of reduction in serum TTR exposure. Together, these data suggest that RNAi-mediated knockdown of hepatic TTR expression, by virtue of significantly reducing the systemic concentration of the precursor to the protein aggregate, can prevent the formation of new deposits and thereby allow an otherwise overwhelmed endogenous repair process to reverse the consequences of protein misfolding. Further, while maximal protein knockdown would be ideal, the data suggests that lower levels of knockdown also have potential to result in clinical benefit.