From Relative Quantification to Copy Number in qPCR CNV Analysis
Why Is This Important?
qPCR enables rapid and relatively narrow quantification of specific DNA segments, facilitating the identification of deletion/duplication candidates or the estimation of copy numbers at known loci. However, equipment does not directly count copy numbers. Instead, it performs relative quantification by determining whether amplification is detected several cycles earlier or later than a reference sample, interpreting this difference as copy number when multiple assumptions hold true.
qPCR and RT-qPCR must also be distinguished. qPCR refers to the measurement method and can utilize DNA templates. RT-qPCR measures cDNA generated via reverse transcription of RNA. The questions and controls differ entirely depending on whether CNVs are assessed from genomic DNA or gene expression is evaluated from RNA.
What Cq Represents
As PCR cycles repeat, the target product accumulates, and the cycle at which fluorescence exceeds the threshold is recorded as the Cq. Samples with higher initial template amounts typically reach the threshold in earlier cycles. Under ideal efficiency conditions, a one-cycle difference corresponds to approximately a two-fold change; however, actual efficiency varies depending on the primers, amplicon, and specimen.
Comparing target Cq values alone may lead to misinterpreting differences in DNA input amount or quality as copy number variations (CNVs). To address this, calculate the ฮCq by comparing the target Cq to a stable reference locus within the same sample, and then compute the ฮฮCq by comparing this value to a known diploid calibrator. This calculation assumes that the amplification efficiencies of the target and reference are sufficiently similar.
Design of CNV Estimation
Assay locations must be situated within the CNV to ensure accurate detection. If primers designed to detect a deletion in a single exon are positioned in an unaffected exon, the sample may erroneously appear normal. Furthermore, quantitative PCR (qPCR) at a single locus cannot determine the boundaries or orientation of large duplications. Therefore, assays should be distributed across multiple exons or boundary candidates to confirm the pattern.
The reference locus must exhibit stable copy number in the study subjects and amplify with quality comparable to that of the target. It is essential to include diploid controls, positive controls, and no-template controls, while also assessing variability among technical replicates. Validating efficiency and dynamic range via standard curves is also critical.
Small Example
If the relative abundance of one exon is approximately halved in repeated measurements compared to a normal control, while adjacent exons remain normal, a heterozygous exon deletion hypothesis can be proposed. However, an SNV at a primer-binding site may inhibit amplification of one allele, producing an identical pattern. Confirmation via independent methods, such as alternative primer sets or MLPA/sequencing, is required.
Even when a two-fold signal increase is observed, qPCR alone cannot determine whether this results from a tandem duplication or an insertion copy at another locus, nor can it identify which allele is affected.
MIQE and Information to Record
Reproducible qPCR requires documentation of sample extraction methods, DNA quality and quantity, primer sequences or assay IDs, amplicon characteristics, efficiency, threshold settings, replicates, exclusion criteria, and calculation methods. Retaining only result values makes it difficult for other laboratories to reproduce the same copy-number calls.
Calibrators and controls must be included in each batch, and plate position and operator effects should be evaluated. Do not overinterpret fractional copy-number estimates as if they were integer copy numbers of actual cells.
Common Misconceptions
- The ฮฮCq method is not a universally applicable formula under all conditions.
- Relative quantities of 0.5 or 1.5 do not definitively indicate deletion or duplication events.
- Genomic CNV qPCR and RNA expression RT-qPCR are not interchangeable assays.
- High replicate concordance does not exclude systematic primer bias.
Interpretive Caveats
qPCR narrowly interrogates the relative copy number of a target locus but does not independently resolve breakpoints, complex rearrangements, tumor purity, and mosaicism. Clinical calls require assay-specific validation and independent confirmation methods.
Reading in Context
The selection of assay methods and blind spots is connected to the concepts of WGSยทWESยทpanel, structural variant annotation is connected to AnnotSV, and allele fraction and mosaicism are connected to the concept of Germline and somatic VAF.