Question 17 of 20 Advanced Scenario
A promoter reporter is strongly activated but endogenous expression is unchanged. What would you investigate?
Short answer
A plasmid reporter sits outside chromatin and lacks the distal regulatory elements and repressive marks of the real locus, so it can respond when the gene does not. Check the chromatin context first, then rule out artifacts of the reporter protein itself, such as a compound that stabilizes luciferase instead of inducing transcription.
Why
Inoue and colleagues compared the same 2236 candidate enhancers on episomes and integrated into chromosomes, and found that activity differed substantially between the two settings, correlating with different genomic annotations. A plasmid carries only the fragment that was cloned. The endogenous gene also answers to distant enhancers, insulators, DNA methylation and nucleosome positioning.
A reporter can therefore respond to a signal that the closed endogenous locus ignores. There is also a purely chemical trap. Auld and colleagues profiled more than 70,000 compounds and found about 3 percent inhibited firefly luciferase.
Inhibitors can stabilize the enzyme inside cells, so the signal rises and looks like transcriptional activation. Thorne and colleagues describe this stabilization as the main mechanism behind such counter-intuitive gains. Other explanations include copy number differences between wells, transfection variability and a reporter protein whose half-life differs from the endogenous messenger.
The resolution is to measure the endogenous locus directly, by nascent transcription or a knock-in reporter at the native site, and to test the compound on purified luciferase.
A strong answer covers
- Episomal reporters lack chromatin and distal elements
- Endogenous locus may be closed to the same signal
- Luciferase inhibition can stabilize the enzyme and raise signal
- Normalize for transfection and copy number
- Confirm at the native locus with nascent transcription or a knock-in
Common mistakes
- Treating a transfected reporter as a faithful copy of the endogenous promoter
- Reading increased luminescence as increased transcription without a counter-assay
- Comparing wells without normalizing for transfection efficiency
The interviewer might ask next
How would you test whether a compound acts directly on the luciferase enzyme?