7. Your friend just started working in a new lab studying cell differentiation.

Question

7. Your friend just started working in a new lab studying cell differentiation. She has been seeing a weird phenotype in smooth muscle and fibroblast cells, and, after much work, has determined that the issue is a mutation in the 3' end of the alpha tropomyosin gene outside of the coding region. She determined that the mutation greatly increases the expression of this protein. To her surprise, when she sequenced her striated muscle and brain cell lines, many of them also had the same mutation, but they didn't show a change in phenotype due to the mutation. At first she thinks that it's just due to miRNA differences between cell lines, but it turns that the levels of miRNA are (somewhat surprisingly) all the same in the cell types

Explanation / Answer

A. From the given data, it is clear that a mutation in the 3'-UTR is causing elevated levels of protein. This can happen only when the mutation is in the cis site of a translation inhibition factor.
B. Since the binding site for translation repression factor is mutated, now the mRNA can produce ample amounts of protein.
C. Intron-exon junction = AG-GT. If an intron-exon junction sequence is mutated, splicing becomes defective. As the given mRNA size is longer than the WT mRNA, it appears that mutation led to the skipping of one or more introns/exons during splicing.

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