lecture 11

Spinal Muscular Atrophy

• Spinal Muscular Atrophy (SMA) is a neurodegenerative disease caused by mutations that disrupt the proper splicing of the Survival Motor Neuron (SMN) gene.
• SMA is the most common monogenic cause of death in infants.
• SMA results in a truncated (shortened), non-functional SMN protein.
• The SMN2 coding region is often duplicated in the genome.
• Having 3+ copies of the SMN2 gene leads to a mild disease phenotype.

SMN1 & SMN2

• SMN2 is processed differently than SMN1 due to a difference in a single nucleotide.
• The C in SMN1 is important for the 5' splice site of exon 7.
• The T in SMN2 removes a lariat branchpoint from the intron.

Potential treatments for SMA

• Antisplicing morpholino: blocks the splice site between exons 7 and 8
• Nusinersen: a short segment of nucleotides that blocks removal of exon 7
• ETS: an activator protein that binds the enhancer of the SMN1 gene
• Risdiplam: modifies splicing of the transcript to force inclusion of exon 7 in SMN2

mRNA degradation

• Prokaryotic mRNA has a half-life of 2-3 minutes.
• Eukaryotic mRNA stability varies from minutes to hours.
• Mammalian mRNAs have different lifetimes, ranging from less than 30 minutes to approximately 20 hours.
• The shorter-lived mRNAs code for regulatory proteins.

Eukaryotic RNA processing

• Eukaryotic pre-mRNAs are modified by the addition of a 5' 7-methylguanosine cap and a 3' poly-A tail.
• Introns are removed by splicing through the action of the large ribozyme complex called the spliceosome.
• The spliceosome recognizes splice sites in the pre-mRNA and catalyzes the splicing reaction.
• Exons can be joined in various combinations through alternative splicing.
• mRNA sequences can be modified by RNA editing (deamination of Cytosine or Adenosine).
• mRNAs in eukaryotic cells are degraded at different rates contributing to control of gene expression.

Eukaryotic RNA processing events

• Eukaryotic RNA processing begins while the RNA molecule is being transcribed.
• The tail of RNA polymerase II is phosphorylated, which allows processing proteins to assemble.
• The 5' cap protects the mRNA from degradation and helps initiate translation.
• The 3' polyA tail protects the mRNA from degradation and helps export it from the nucleus.
• Splicing of pre-mRNA removes introns, keeps exons.
• Splicing is carried out by the spliceosome.
• The spliceosome is a complex of proteins and snRNAs.

Alternative splicing

• Alternative splicing allows for different combinations of exons.

Protein Synthesis

• Translation is literally switching languages.
• tRNAs serve as adaptors that align amino acids on the mRNA template.
• Peptide bond formation is catalyzed by rRNA in the ribosome.
• Translation initiation is different in prokaryotes and eukaryotes.
• Translation begins at a start codon and ends at a stop codon.

Ribosomes

• Ribosomes are made of structural RNA and catalytic protein.

Ribosomes and Translation

• Prokaryotic translation initiation starts with the small subunit and initiator tRNA binding to the Shine-Dalgarno sequence.
• Eukaryotic translation initiation involves the small subunit and initiator tRNA binding to the 5' cap, followed by movement to the start codon.
• Prokaryotic mRNAs can be polycistronic.

Prokaryotic and Eukaryotic RNA Processing

• The difference between prokaryotic and eukaryotic RNA processing and ribosome recognition strategies is NOT a functional implication.