The Chronicles of X-Linked Spinal Muscular Atrophy: The Linkage, The Gene and The SMN Complex

Spinal Muscular Atrophy (SMA) is an autosomal recessive motor neuron disease. SMA is associated with homozygous mutations in the Survival of Motor Neuron gene I (SMN1). SMN protein does not appear to exist in cells in isolation but associates with several proteins to form a large multi-protein complex. The functions of SMN complex include assembly, metabolism and transport of diverse classes of ribonucleoproteins. X- Linked Spinal Muscular Atrophy is a rare congenital disorder characterized by multiple joint contractures. It is associated with hypotonia, areflexia, chest deformities and congenital joint contractures. A candidate interval was defined for XL-SMA in Xp11.3-Xq11.2 in 1995. The purpose of this study was to refine the XL-SMA gene region and discover the XL-SMA gene. In addition to that, the gene product was investigated to delineate the genotype-phenotype correlation. My studies were focused on single nucleotide polymorphism (SNP) analysis. The candidate gene interval was refined by studying 14 SNPs in the three largest families. This analysis revealed a recombination event which allowed elimination of the NDP gene. Significantly positive LOD scores were obtained from these SNP studies. The exons and exon-intron boundaries of 12 genes were screened. No mutations were found in these genes in affected male samples. In late 2006, UBE1 (Ubiquitin activating enzyme 1) was discovered as the XL-SMA gene. UBE1 protein is responsible for the first step of ubiquitination of proteins in a cell. To investigate a possible common molecular mechanism between SMA and XL-SMA, proteins in the SMN Complex in XL-SMA patient cell lines were studied. SMN and Gemin3 protein levels were found to be consistently lower in XL-SMA patient cell lines (lymphoblasts) compared to healthy cell line. These results imply that there may be a common disease mechanism. To understand if the SMN and Gemin3 RNA levels decrease. RNA expression studies were performed. These studies confirmed that there is no difference of RNA expression of SMN and Gemin3 in XL-SMA cell lines when compared to healthy cell lines. As for UBE1, the same experimental procedure for SMN Complex proteins were repeated with antibodies to UBE1 to determine if there is any decline of UBE1 protein levels in XL-SMA patient cell lines compared to a healthy cell line. There was a decline in protein levels of UBE1 in XL-SMA patients. Two possible models are proposed for a molecular mechanism in XL-SMA: 1) UBE1 involves in degradation of a protein which downregulates SMN Complex (or a protein which stabilizes SMN Complex). When UBE1 is mutated, the protein in question is not degraded and this results in excess downregulation of SMN Complex (maybe via a pathway involving SMN-Gemin3 interaction). 2) UBE1 and UBA6 interact with the proteins of SMN Complex as they monoubiquinate them for different cellular processes. When UBE1 is mutated, UBA6 cannot compensate the deficiency of UBE1, which in turn disrupts normal cellular RNA metabolism required for motor neuron development and survival