Review Question - QID 214433

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RNA Processing Overview Process QID: 214433 (M1.OMB.1)

QID 214433 (Type "214433" in App Search)

A geneticist studies a family of patients who have a rare disease that causes early onset dementia in an autosomal dominant fashion. She finds that patients with the disease have a defective protease that does not catalyze normal degradation of certain lipid byproducts. Western blot analysis of this protein shows that the mutant allele produces a variant that is 10 kilodaltons larger than the wild type protein. Further structural studies show that an extra loop of unfolded amino acids is incorporated into the middle of the protein near the active site and blocks proper substrate binding. She sequences the wildtype and mutant genes and finds that there is a specific mutation responsible for the disease. Which of the following mutations is most likely seen in this family?

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AAA to AAG in the coding region

10%

20/192

AAA to AAG in the noncoding region

18%

34/192

CGA to TGA in the coding region

39%

75/192

CGA to TGA in the noncoding region

18/192

Deletion of TAG in the coding region

17%

33/192

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This mutation that leads to the insertion of an extra loop in the middle of the protein is most likely the result of a splice site mutation in an intron. AAA to AAG in the noncoding region would cause an alternate splice site because AG is the conserved sequence that is recognized.

Splicing is a step in RNA processing that leads to the removal of introns from pre-processed mRNA transcripts. This process is mediated by the spliceosome, which recognizes the conserved GU...AG sequence in transcripts. The GU occurs in the coding sequence and represents the start of the intron and the AG occurs in the noncoding sequence and represents the end of the intron. If multiple conserved AG sequences exist in an intron, alternative splicing can occur resulting in different protein products from the same transcript. Mutations that create aberrant splicing sites can, therefore, lead to insertion of additional protein loops into proteins. One disease where this mechanism is implicated is spinal muscular atrophy.

Incorrect Answers:
Answer 1: AAA to AAG in the coding region would be an example of a missense mutation; however, point mutations do not change the size of the protein and would not introduce a new loop.

Answer 3: CGA to UGA in the coding region would be an example of a nonsense mutation because the sequence UGA is a stop codon that terminates protein translation from the mRNA transcript. This mutation would result in a shorter protein rather than a longer protein.

Answer 4: CGA to TGA in the noncoding region would be an example of a silent mutation because the mutation exists in an intron that is spliced out during mRNA processing and is therefore not expressed in the final protein. These mutations can be used to track lineage but are generally not implicated in disease.

Answer 5: Deletion of TAG in the coding region would be an example of a nonstop mutation, which causes continued translation of a protein past the normal stopping point of the mRNA transcript. This would create a longer protein; however, the added sequence would be at one end rather than in the middle.

Bullet Summary:
Splice site mutations can lead to the insertion of extra amino acids into existing proteins due to the incorporation of intron sequences.

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