Tailed PCR Primer Extends Tiny miRNA Amplicon for Clearer Sanger Sequencing

Short pieces of RNA that regulate genes are notoriously difficult to read with conventional sequencing tools, and a team of Vietnamese researchers has now published a practical fix for one of the most common bottlenecks. In a study published in the Egyptian Journal of Medical Human Genetics, the group developed and validated a tail-extension PCR protocol that stretches a 58-base-pair amplicon derived from microRNA-16-5p to 115 base pairs, long enough to produce reliable Sanger sequencing chromatograms. The work, led by Nga Thi Nguyen, Luan Huu Huynh, Thuy Thi Chung Duong, Thanh Thi Ngoc Nguyen and Hue Thi Nguyen, addresses a technical problem that quietly undermines many microRNA assays: the very molecules being measured are so small that the DNA copies made from them are often too short to sequence cleanly.

MicroRNAs are small non-coding RNAs of roughly 21 to 23 nucleotides that regulate gene expression after transcription by binding to target messenger RNAs, which can lead to translational repression or degradation of those messages. When their expression goes awry, the consequences can be far-reaching; aberrant microRNA expression has been associated with various cancers, influencing tumor initiation, progression and metastasis. Because microRNAs are highly stable in body fluids and tissues, circulating versions have emerged as promising biomarker candidates for diagnosis, prognosis and therapeutic monitoring in oncology. Among them, miR-16-5p plays a crucial role in regulating the cell cycle and apoptosis and acts as a tumor suppressor in several cancer types. Its stable and consistent expression across a wide range of biological samples has also made it a widely used normalization reference in microRNA quantification assays, which means that problems in verifying its sequence ripple through many downstream experiments.

The difficulty lies in the arithmetic of primer design. The mature form of miR-16-5p is only about 22 nucleotides long, so complementary DNA fragments generated during reverse transcription and PCR amplification typically come out shorter than 60 base pairs. Amplicons of that size often yield noisy chromatograms during Sanger sequencing, complicating accurate base calling and hindering the identification of single-nucleotide polymorphisms or isomiRs, the subtle sequence variants that microRNAs can carry. Primer design is similarly constrained, because the primers can approach the length of the target itself, increasing the likelihood of primer-dimer formation and nonspecific amplification. In the stem-loop reverse transcription approach used in this study, the initial product was a 58-base-pair amplicon, right in the problematic range.

The researchers’ solution builds on an established trick: extending the reverse primer with an artificial 5-prime tail sequence to increase the effective amplicon length. Tail-extension strategies have previously been reported to improve primer annealing specificity, reduce secondary structure formation and enhance sequencing chromatogram quality, and earlier work showed that adding 40 to 60 nucleotide tail sequences can improve sequencing quality. What has been lacking, the authors note, is systematic optimization and validation of tail-extended primer systems specifically for microRNA sequencing. Their contribution was to apply thermodynamic principles to the design of the tail itself, treating the primer not just as a longer sequence but as a structure whose stability could be engineered and minimized.

That engineering revolved around Gibbs free energy, or delta-G, the thermodynamic parameter governing secondary structure formation in PCR primers through nearest-neighbor nucleotide interactions. Structures with more negative delta-G values are more thermodynamically stable and therefore more likely to form during amplification, where they can sabotage PCR efficiency. While reaction conditions such as salt concentration and annealing temperature can be tuned to reduce these structures, the intrinsic nucleotide sequence of the primer remains a critical determinant. The team used OligoAnalyzer software with acceptance thresholds of delta-G greater than minus 9 kilocalories per mole for self- and heterodimers and greater than minus 3.5 kilocalories per mole for hairpins, drawing on thermodynamic parameters reported by Santa Lucia Jr. and Hicks. Each variable position in the tail was iteratively adjusted in silico, replacing nearest-neighbor base pairs with ones generating less negative delta-G values, while the 3-prime target-complementary region was kept unchanged.

Five universal sequencing primers were evaluated as candidate tails: M13(-21), M13(-40), M13(-49), T3 and T7. Two of them, M13(-40) and T7, met all the initial criteria, including no complementary binding to the 3-prime end of the target DNA and no self-complementarity, with a low heterodimer delta-G of minus 6.14 kilocalories per mole. To ensure sufficient length for Sanger sequencing, the longer T7 primer, at 20 nucleotides, was selected for the extension region. The initial tailed reverse primer, R_T*, was 75 nucleotides long with a high melting temperature of 77.2 degrees Celsius and relatively strong secondary structure potential, including a hairpin delta-G of minus 4.7 kilocalories per mole and a self-dimer delta-G of minus 6.59 kilocalories per mole. Six nucleotides within the tail region were altered to destabilize these structures. After optimization, the hairpin delta-G rose to minus 2.8 kilocalories per mole, the self-dimer delta-G to minus 6.14, and the heterodimer delta-G to minus 4.59, while the melting temperature dropped slightly to 76.6 degrees Celsius and GC content fell from 48 to 42.7 percent, minimizing the melting-temperature difference with the unmodified forward primer and promoting uniform annealing.

The experimental workflow began with peripheral blood samples from nine healthy volunteers at Ho Chi Minh City Oncology Hospital, collected under ethics committee approval with written informed consent. Plasma was separated, screened to exclude overt hemolysis using a reference chart from the Centers for Disease Control and Prevention, and stored at minus 80 degrees Celsius. Small RNA was extracted using a TRIzol-based method optimized for small RNA recovery, and 100 nanograms of small RNA were used for stem-loop reverse transcription, followed by a 12-cycle pre-amplification step to increase template availability. A synthetic single-stranded miR-16-5p cDNA fragment, custom-synthesized as a positive control, served for primer testing and optimization. In the second PCR, the 5-prime-tailed reverse primer extended the 58-base-pair amplicon to 115 base pairs.

Optimization of the reaction itself proved equally important. Annealing temperatures ranging from 57 to 67 degrees Celsius, and later 63 to 71 degrees Celsius, were tested by agarose gel electrophoresis. Specific products were amplified at all temperatures, but nonspecific amplification appeared at lower temperatures and faded as temperature increased, becoming virtually eliminated at 67 degrees Celsius, which was determined to be optimal. Faint bands below 100 base pairs were observed in negative controls at 67, 69 and 71 degrees Celsius, consistent with primer-dimer formation, a common artifact with short targets, but these did not interfere with amplification of the target amplicon or subsequent sequencing. Three PCR replicates at 67 degrees Celsius confirmed the tailed primer’s performance, and the optimization workflow was repeated three independent times with consistent results.

Sequencing delivered the validation the design promised. Three replicate Sanger reactions on the synthetic template produced identical sequences showing 100 percent concordance with the expected miR-16-5p-derived target region within the tail-extended amplicon, along with the engineered 5-prime tail. The protocol was then applied to biological cDNA from human plasma, where a specific 115-base-pair product was obtained and the sequence matched the predicted 57-base-pair tail plus 58-base-pair target with 100 percent identity. Eight additional biological samples were tested for reproducibility, and all showed specific amplification of the expected 115-base-pair product with no evidence of nonspecific amplification. The authors report that the extended product length contributed to improved signal clarity and reduced sequencing noise, and that because the tail sits outside the target-derived region, readability improved without altering the biological sequence of interest.

The team frames the method as a simple, reliable and economical alternative for laboratories that need targeted sequence verification without the high-throughput workflows, complex library preparation and bioinformatics burden of next-generation sequencing, and as a route to sequence-validated positive controls in microRNA assays. The limitations are candidly acknowledged: only one microRNA target was evaluated, so applicability to other microRNAs with different GC content or structural characteristics remains untested; only one tail design was experimentally validated; no direct quantitative comparison of chromatogram quality between non-tailed and tail-extended amplicons was performed; and the number of biological samples was relatively limited. Future studies incorporating objective metrics such as read length, base-calling quality scores and peak resolution, along with larger sample sets, will be needed to confirm how broadly the strategy generalizes. For now, the study offers a worked, validated example of turning a thermodynamic design principle into a bench-ready protocol for one of molecular biology’s smallest sequencing targets.

Subject of Research: A tail-extension PCR protocol for improving Sanger sequencing of a short miR-16-5p-derived amplicon

Article Title: Development and validation of a tail-extension PCR protocol for improved sanger sequencing of a short miR-16-5p-derived amplicon

Article References: Nguyen, N. T., Huynh, L. H., Duong, T. T. C., Nguyen, T. T. N., & Nguyen, H. T. (2026). Development and validation of a tail-extension PCR protocol for improved sanger sequencing of a short miR-16-5p-derived amplicon. Egyptian Journal of Medical Human Genetics, 27(1), Article 78. https://doi.org/10.1186/s43042-026-00898-6

Image Credits: AI Generated

DOI: 10.1186/s43042-026-00898-6

Keywords: miR-16-5p, microRNA, Sanger sequencing, tail-extension PCR, stem-loop RT-PCR, primer design, Gibbs free energy, secondary structure, amplicon, sequence verification, positive controls, plasma