Prenatal anatomical and molecular changes in a mouse model of spinal muscular atrophy
Spinal Muscular Atrophy (SMA) is monogenic disease caused by deletion of the Survival Motor Neuron 1 (SMN1) gene, with an incidence of about one in 10,000 live births. In the most severe and common type of SMA, symptoms often appear within three months after birth and, historically, infants would not live past the age of two. In recent years, the approval of three SMN replacement therapies has caused a therapeutic revolution, by increasing patient life expectancy and helping them reach previously unachievable motor milestones. SMA is characterised by the loss of lower motor neurons and muscle atrophy and, as such, is defined as a neurodegenerative disorder. However, a growing amount of evidence from preclinical and human studies show that SMA causes a broad range of non-neuromuscular phenotypes, such as cardiac, and fatty acid metabolism defects, with implications for treatment administration. Evidence from preclinical and clinical research also suggests the existence of an early, limited therapeutic time window. These aspects of disease manifestations raise a fundamental question of whether SMA pathogenesis has systemic, pre-symptomatic components. In this thesis, I address this question by investigating potential embryonic effects of the disease in a mouse model of severe SMA. Anatomical analyses using micro-computed tomography (μCT) revealed that SMA embryos were significantly smaller than littermate controls, indicative of general developmental delay. Additionally, SMA cardiac ventricles were smaller, whilst the liver was bigger, hinting at organ-specific effects of low levels of SMN. A comparative proteomic screen showed significant molecular perturbations in all organs examined, with very little overlap between organs. Using quantitative western blots, I characterised SMN protein expression patterns, from early embryonic to postnatal symptomatic time points. Collectively, this thesis provides experimental evidence that high levels of SMN are required during prenatal stages of development and may help to explain some systemic phenotypes observed in SMA. My work suggests that disease mechanisms are established long before overt symptoms appear, in embryos.