In Search of Spinal Muscular Atrophy Disease Modifiers

The 5q Spinal Muscular Atrophy (SMA) is a hereditary autosomal recessive disease caused by defects in the survival motor neuron (SMN1) gene encoding survival motor neuron (SMN) protein. Currently, it is the leading cause of infantile mortality worldwide. SMA is a progressive neurodegenerative disease with "continuum of clinical severity", which can be modulated by genetic and epigenetic factors known as disease modifiers (DMs). Individuals (even siblings) with the same defects in SMN1 gene might have strikingly different types of SMA, supposedly due to the impact of DMs. There are several therapeutic options for SMA, all of them focusing on the restoration of the SMN protein levels to normal. Determining DMs and the pathways in which they are involved might aid in enhancing existing curative approaches. Furthermore, DMs might become novel therapeutic targets or prognostic biomarkers of the disease. This narrative review provides a brief overview of the genetics and pathobiology of SMA, and its bona fide modifiers. We describe novel, emerging DMs, approaches and tools used to identify them, as well as their potential mechanisms of action and impact on disease severity. We also propose several disease-modifying molecular mechanisms which could provide a partial explanation of the staggering variability of SMA phenotypes.