Metformin to treat Huntington disease: A pleiotropic drug against a multi-system disorder

Huntington disease (HD) is a neurodegenerative disorder produced by an expansion of CAG repeats in the HTT gene. Patients of HD show involuntary movements, cognitive decline and psychiatric impairment. People carrying abnormally long expansions of CAGs (more than 35 CAG repeats) produce mutant huntingtin (mHtt), which encodes tracks of polyglutamines (polyQs). These polyQs make the protein prone to aggregate and cause it to acquire a toxic gain of function. Principally affecting the frontal cortex and the striatum, mHtt disrupts many cellular functions. In addition, this protein is expressed ubiquitously, and some reports show that many other cell types are affected by the toxicity of mHtt. Several studies reported that metformin, a widely-used anti-diabetic drug, is neuroprotective in models of HD. Here, we provide a review of the benefits of this substance to treat HD. Metformin is a pleiotropic drug, modulating different targets such as AMPK, insulin signalling and many others. These molecules regulate autophagy, chaperone expression, and more, which in turn reduce mHtt toxicity. Moreover, metformin alters gut microbiome and its metabolic processes. The study of potential targets, interactions between the drug, host and microbiome, or genomic and pharmacogenomic approaches may allow us to design personalised medicine to treat HD.