Epigenetics: the neglected key to minimize learning and memory deficits in Down syndrome.

Down syndrome (DS) is the most common genetic intellectual disability, caused by the triplication of the human chromosome 21 (HSA21). Although this would theoretically lead to a 1.5 fold increase in gene transcription, transcript levels of many genes significantly deviate. Surprisingly, the underlying cause of this gene expression variation has been largely neglected so far. Epigenetic mechanisms, including DNA methylation and post-translational histone modifications, regulate gene expression and as such might play a crucial role in the development of the cognitive deficits in DS. Various overexpressed HSA21 proteins affect epigenetic mechanisms and DS individuals are thus likely to present epigenetic aberrations. Importantly, epigenetic marks are reversible, offering a huge therapeutic potential to alleviate or cure certain genetic deficits. Current epigenetic therapies are already used for cancer and epilepsy, and might provide novel possibilities for cognition-enhancing treatment in DS as well. To that end, this review discusses the still limited knowledge on epigenetics in DS and describes the potential of epigenetic therapies to reverse dysregulated gene expression.