Gene A is normally expressed in both liver cells and kidney cells. A patient is found to have a mutation in the regulatory region of Gene A. The clinical result is that Gene A is expressed at normal levels in the liver, but is completely absent in the kidney. Where is the mutation most likely located?

Question

Gene A is normally expressed in both liver cells and kidney cells. A patient is found to have a mutation in the regulatory region of Gene A. The clinical result is that Gene A is expressed at normal levels in the liver, but is completely absent in the kidney. Where is the mutation most likely located?

BlackTom AI · Accepted Answer

We are given a gene normally expressed in both liver and kidney, but a regulatory-region mutation causes normal expression in liver yet complete loss of expression in kidney. This pattern points to a tissue-specific regulatory element rather than a global component of transcription or mRNA processing.
Option 1: In a distal enhancer element specific for kidney-specific activators. This fits the scenario: an enhancer that responds to kidney-specific transcription factors drives expression in kidney. If this enhancer is mutated, kidney expression would drop while liver expression (driven by liver-specific factors and other regulatory elements) could remain normal. So this option explains tissue-specific loss.
Option 2: In the poly-A signal sequence. The poly-A signal affects mRNA 3' end processing and stability; a mutation here would more likely impact mRNA maturation broadly, potentially affecting transcripts in multiple tissues, not selectively abolishing kidney expression while leaving liver expression intact. This doesn't explain tissue-specific silencing.
Option 3: In the TATA Box of the promoter. A promoter TATA box mutation would generally reduce or abolish transcription across all tissues where the gene is active, not leave liver expression intact. The observed pattern suggests a regulatory element controlling tissue specificity rather than core promoter function.
Option 4: In the protein-coding exon region of the gene. A coding sequence mutation would typically alter the protein in all tissues where the transcript is produced (and may be lethal or severely deleterious), not selectively eliminate kidney expression while preserving liver expression. This mismatch makes it unlikely.
In summary, the kidney-specific enhancer mutation (Option 1) best accounts for normal liver expression with absent kidney expression due to loss of a tissue-specific regulatory input, whereas all other options would produce broader or different defects not aligned with the described phenotype.

Bio 93 Section CDE 2025 Final- Section E

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