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Question 15 of 20 Intermediate Interpretation

Two primer sets report opposite transcript changes. How would you resolve the discrepancy?

Short answer

Opposite directions usually mean the two assays are measuring different things, not that one is simply wrong. Check which transcript isoforms, exons and related genes each amplicon covers, then verify each assay's specificity and efficiency before concluding anything about biology.

Why

A gene often produces several transcripts. An amplicon in a shared exon measures their sum, while one spanning a junction unique to a long isoform measures only that variant. If a treatment switches the cell from one isoform to another, the two assays genuinely move in opposite directions and both are right.

Other explanations are technical. One primer pair may also amplify a paralogue, a processed pseudogene or genomic DNA, which the ExonSurfer authors note can happen even with junction-spanning design. Different efficiencies distort fold changes in different ways.

Degraded RNA biases assays near the 5' end more than those near the 3' end, because reverse transcription often primes from the poly-A tail. A melt curve or sequencing of each product tells you what was amplified. Running both assays on a dilution series of the same sample shows whether efficiency differs.

Blotting or a protein measurement can break a tie. The answer to report is which transcripts were measured, not simply which primer set was trusted.

How to work it through

  1. Map both amplicons onto the gene's annotated transcripts and note which isoforms each can detect.
  2. Check each primer pair against the transcriptome and genome for paralogues and pseudogenes.
  3. Run a dilution series for each assay in this sample type and compare efficiencies.
  4. Confirm product identity by melt curve and by sequencing at least once.
  5. Repeat on fresh RNA with matched integrity and include a no-RT control for each assay.
  6. If the isoform hypothesis stands, measure the isoforms directly with junction-specific assays or RNA sequencing.

An example

Assay A, in exon 2, shows a twofold decrease after treatment. Assay B, spanning the exon 7 to 8 junction, shows a threefold increase. Mapping shows exon 2 is unique to a short isoform and exons 7 to 8 are present only in the long one.

RNA sequencing of the same samples confirms that total gene expression is unchanged while the long isoform rises. The discrepancy was an isoform switch, and both assays were correct.

Where the answer stops holding

Isoform-level qPCR can only measure annotated variants, so an unannotated transcript stays invisible. Short-read sequencing assigns isoforms probabilistically and can be wrong for similar transcripts. Even a resolved RNA story may not predict protein, since isoforms differ in translation and stability.

A strong answer covers

  • Map each amplicon to transcripts and exons
  • Check for paralogues, pseudogenes and genomic amplification
  • Compare efficiencies on a dilution series
  • Confirm product identity by melt curve or sequencing
  • Resolve with isoform-specific assays or sequencing

Common mistakes

  • Picking the assay that matches the expected result
  • Assuming one primer pair represents the whole gene
  • Averaging two contradictory assays into a single fold change