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

i. From the given information, it appears that the mutation is affecting the recruitment of a factor involved in translation. The factor could bind to the mRNA in a sequence-specific manner and inhibit the recruitment of ribosome. This factor could be expressed specifically be in the muscle cells so that the mutation has a cell-type specific phenotype.

ii. Since the binding site for a factor that inhibits the recruitment of ribosome to the mRNA is mutated, ribosome recruitment to the mRNA is increased resulting in the elevated production of the protein.

iii. Intron-exon junctions exhibit a characteristic conserved junction sequence (AG-GT). If the intron-exon junction sequence is mutated, either the intron is retained leading to the production of longer mRNA or alternative splicing occurred resulting in the skipping of introns/exons. In the given case, it appears that skipping of few introns/exons resulted in the production of truncated mature mRNA.

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