Ian C. Eperon

Analysis of an almost complete mammalian mitochondrial DNA sequence has identified 23 possible tRNA genes and we speculate here that these are sufficient to translate all the codons of the mitochondrial genetic code. This number is much smaller than the minimum of 31 required by the wobble hypothesis. For each of the eight genetic code boxes with four codons for one amino acid we find a single specific tRNA gene with T in the first (wobble) position of the anticodon. We suggest that these tRNAs with U in the wobble position can recognize all four codons in these genetic code boxes either by a "two out of three" base interaction or by U.N wobble.

The multiplicity of proteins compared with genes in mammals owes much to alternative splicing. Splicing signals are so subtle and complex that small perturbations may allow the production of new mRNA variants. However, the flexibility of splicing can also be a liability, and several genetic diseases result from single-base changes that cause exons to be skipped during splicing. Conventional oligonucleotide strategies can block reactions but cannot restore splicing. We describe here a method by which the use of a defective exon was restored. Spinal muscular atrophy (SMA) results from mutations of the Survival Motor Neuron (SMN) gene. Mutations of SMN1 cause SMA, whereas SMN2 acts as a modifying gene. The two genes undergo alternative splicing with SMN1, producing an abundance of full-length mRNA transcripts, whereas SMN2 predominantly produces exon 7-deleted transcripts. This discrepancy is because of a single nucleotide difference in SMN2 exon 7, which disrupts an exonic splicing enhancer containing an SF2ASF binding site. We have designed oligoribonucleotides that are complementary to exon 7 and contain exonic splicing enhancer motifs to provide trans-acting enhancers. These tailed oligoribonucleotides increased SMN2 exon 7 splicing in vitro and rescued the incorporation of SMN2 exon 7 in SMA patient fibroblasts. This treatment also resulted in the partial restoration of gems, intranuclear structures containing SMN protein that are severely reduced in patients with SMA. The use of tailed antisense oligonucleotides to recruit positively acting factors to stimulate a splicing reaction may have therapeutic applications for genetic disorders, such as SMA, in which splicing patterns are altered.