Quantitative PCR of Small Nucleic Acids: Size Matters

Area of Science:

• Biomarker Discovery
• Molecular Diagnostics
• Quantitative Biology

Background:

• Small nucleic acids (NA), including microRNA (miRNA), are promising biomarkers found in minimally invasive samples.
• Quantitative PCR (qPCR) is the gold standard for measuring nucleic acid copy numbers.

Purpose of the Study:

• To investigate the impact of poly(A) ligation and small NA size on the limit of quantification (LOQ) in qPCR.
• To identify factors limiting the accurate quantification of small NA using qPCR.

Main Methods:

• Quantitative PCR (qPCR) was used to measure small nucleic acids.
• The effect of poly(A) ligation during cDNA synthesis was evaluated.
• The relationship between cycle threshold (Ct) and nucleic acid concentration ([c]) was analyzed.

Main Results:

• Small NA exhibit a sharp transition in cycle threshold (Ct) values.
• While the limit of detection (LOD) can be in the femtomolar range, the LOQ is reduced by over three orders of magnitude, reaching the picomolar range.
• PCR product length was identified as the primary factor limiting the LOQ for small NA.

Conclusions:

• The LOQ of qPCR is significantly reduced for small NA compared to its LOD.
• PCR product length is a critical consideration for optimizing primer design and improving qPCR accuracy for small NA quantification.
• Accurate quantification of small NA biomarkers requires careful attention to assay design limitations.