Molecular Genetics of Autosomal Recessive Spinal Muscular Atrophy

Molecular basis of genetic heterogeneity: role of the clinical neurologist

Advances in molecular genetics have disclosed many different explanations for allelic heterogeneity, how different clinical syndromes arise from mutations in the same gene. The converse, how similar clinical syndromes arise from mutations of different genes on different chromosomes is called locus heterogeneity. Both, however, give rise to some disease-defining mutations, as in childhood spinal muscular atrophy or Duchenne muscular dystrophy. Nevertheless, new problems have been created, including what might be called "diagnosis by the number," diverse syndromes from mutations in the same gene without current explanation, or siblings with different clinical syndromes. These discoveries have transformed the clinical neurology of heritable diseases. They also provide clinicians with new responsibilities and opportunities in defining clinical syndromes and influencing the evolution of our clinical language.

Missense mutation clustering in the survival motor neuron gene: a role for a conserved tyrosine and glycine rich region of the protein in RNA metabolism?

The Survival Motor Neuron (SMN) gene shows deletions in the majority of patients with Spinal Muscular Atrophy (SMA), a disease of motor neuron degeneration. To date only two missense mutations have been reported in SMN in patients with SMA. The fact that no SMN-homologues have been forthcoming from data-base searching has resulted in a lack of hypotheses concerning the structural and functional consequences of these mutations. Recently SMN has been shown to interact with heterogeneous nuclear ribonucleoproteins (hnRNPs) suggesting a role in mRNA metabolism. We describe a novel missense mutation and the subsequent identification of a triplicated tyrosine-glycine (Y-G) peptide sequence at the C-terminal of SMN which encompasses each of the three predicted amino acid sequence substitutions. We have identified apparent orthologues of SMN in Caenorhabditis elegans and Schizosaccharomyces pombe. These sequences retain the highly conserved Y-G motif and provide additional support for a role of SMN in mRNA metabolism.