Mutations that introduce stop codons cause a number of genetic diseases. For exa

Question

Mutations that introduce stop codons cause a number of genetic diseases. For example, from 2% to 5% of the people who have cystic fibrosis possess a mutation that causes a premature stop codon in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). This premature stop codon produces a truncated form of CFTR that is nonfunctional and results in the symptoms of cystic fibrosis. One possible way to treat people with genetic diseases caused by these types of mutations is to trick the ribosome into reading through the stop codon, inserting an amino acid into its place. Although the protein produced may have one altered amino acid, it is more likely to be at least partly functional than is the truncated protein produced when the ribosome stalls at the stop codon. Indeed, geneticists have conducted clinical trials on people with cystic fibrosis with the use of a drug called PTC124, which interferes with the ribosome's ability to correctly read stop codons (C. Ainsworth, 2005, Nature 438:726–728). On the basis of what you know about the mechanism of nonsense-mediated mRNA decay (NMD), would you expect NMD to be a problem with this type of treatment? Yes, NMD would be a problem, as it requires ribosome to recognize a specific stop codon. Yes, NMD would be a problem because even although the ribosome would read through the stop codon, premature degradation of the mRNA would take place. None of the above is true. No, NMD would not be a problem because the ribosome would read through the mutated stop codon, removing exon-junction proteins that would normally signal premature degradation of mRNA. No, NMD would not be a problem because it only occurs in prokaryotes. Mutations that introduce stop codons cause a number of genetic diseases. For example, from 2% to 5% of the people who have cystic fibrosis possess a mutation that causes a premature stop codon in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). This premature stop codon produces a truncated form of CFTR that is nonfunctional and results in the symptoms of cystic fibrosis. One possible way to treat people with genetic diseases caused by these types of mutations is to trick the ribosome into reading through the stop codon, inserting an amino acid into its place. Although the protein produced may have one altered amino acid, it is more likely to be at least partly functional than is the truncated protein produced when the ribosome stalls at the stop codon. Indeed, geneticists have conducted clinical trials on people with cystic fibrosis with the use of a drug called PTC124, which interferes with the ribosome's ability to correctly read stop codons (C. Ainsworth, 2005, Nature 438:726–728). On the basis of what you know about the mechanism of nonsense-mediated mRNA decay (NMD), would you expect NMD to be a problem with this type of treatment? Yes, NMD would be a problem, as it requires ribosome to recognize a specific stop codon. Yes, NMD would be a problem because even although the ribosome would read through the stop codon, premature degradation of the mRNA would take place. None of the above is true. No, NMD would not be a problem because the ribosome would read through the mutated stop codon, removing exon-junction proteins that would normally signal premature degradation of mRNA. No, NMD would not be a problem because it only occurs in prokaryotes.

Explanation / Answer

The Nonsense-mediated mRNA decay is present in all most all eukaryotic organisms and it is a major surveillance pathway. In this process, the putative mRNA, which have premature-termination codons, like UAA, UGA and UAG would be degraded before protein synthesis. The Upf proteins degrade the mRNA, which is obtained from abnormal splicing with premature-termination codons.

However, sometimes the inhibition of Nonsense-mediated mRNA decay results the dominant-negative activity or deleterious gain-of-function of resulting proteins. Therefore, they functions as a normal ones.

Thus, the correct option is (C) None of the above.

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