The polymerase chain reaction (PCR) has spawned numerous specialized variants and applications that extend its utility beyond simple DNA amplification. These techniques enable mutation detection, methylation analysis, molecular diagnostics, and forensic identification.
Allele-Specific PCR
Allele-specific PCR discriminates between DNA sequences differing by a single nucleotide. The method uses primers with the 3-prime terminal base positioned at the polymorphic site. When the terminal base matches the template, amplification proceeds efficiently. When it mismatches, extension is blocked or greatly reduced. By using two primers specific for each allele in separate reactions, the genotype can be determined. The technique is widely used for genotyping single nucleotide polymorphisms and detecting disease-associated mutations such as factor V Leiden and prothrombin G20210A.
Methylation-Specific PCR
Methylation-specific PCR detects DNA methylation patterns, which are important epigenetic marks in cancer and development. Genomic DNA is treated with sodium bisulfite, which converts unmethylated cytosines to uracil while leaving methylated cytosines unchanged. PCR with primers designed to distinguish the converted and unconverted sequences then determines the methylation status of CpG sites. The technique is used for detecting aberrant promoter methylation in tumor suppressor genes and for imprinting analysis.
Inverse PCR
Inverse PCR amplifies DNA regions flanking a known sequence, useful for identifying insertion sites or characterizing unknown flanking regions. Genomic DNA is digested with a restriction enzyme that cuts outside the known region, and the fragments are circularized by ligation. PCR with primers oriented outward from the known sequence then amplifies the unknown flanking DNA across the ligation junction. Inverse PCR has been used to characterize transposon insertion sites and viral integration sites in the genome.
Degenerate PCR
Degenerate PCR uses pools of primers containing mixed bases at specific positions to amplify related genes from different species or members of a gene family. Primers are designed from conserved amino acid sequences, with degeneracy introduced at codon wobble positions. The technique is used to clone homologous genes from new species, identify gene family members, and detect pathogens with variable sequences. Touchdown PCR, where the annealing temperature is progressively decreased, improves specificity in degenerate PCR.
Quantitative PCR Applications
Quantitative PCR (qPCR) extends beyond measuring gene expression. Copy number variation analysis uses qPCR to detect gene deletions or amplifications by comparing the amplification of a target gene to a reference gene. The technique is used in HER2 testing for breast cancer and for detecting chromosomal abnormalities. Pathogen detection uses qPCR to quantify viral load in HIV, hepatitis B, and hepatitis C patients, guiding treatment decisions. GMO detection quantifies the amount of genetically modified material in food products.
Digital PCR Applications
Digital PCR provides absolute quantification without standard curves. It is used for detecting rare mutations in circulating tumor DNA, enabling liquid biopsy for cancer monitoring. Digital PCR also detects residual disease after treatment, identifies fetal aneuploidies from maternal blood, and characterizes copy number variations with high precision. The high sensitivity of digital PCR allows detection of mutant alleles present at frequencies below 0.1%.
Forensic PCR
Short tandem repeat analysis using multiplex PCR is the standard method for human identification in forensics. Commercial kits amplify 15 to 20 STR loci plus the amelogenin sex-determining marker in a single reaction. The amplified fragments are separated by capillary gel electrophoresis, a mode of capillary electrophoresis that uses a polymer sieving matrix to resolve DNA fragments with single-base resolution, and the allele sizes are determined. The probability of two unrelated individuals sharing the same STR profile is typically less than one in a billion. Touch DNA analysis allows profiling from minute samples containing only a few cells.
Single-Cell PCR
Single-cell PCR amplifies the genome or transcriptome of individual cells, revealing heterogeneity that is masked in bulk analysis. The technique begins with cell isolation by micromanipulation, flow sorting, or microfluidics. Whole-genome amplification using multiple displacement amplification or PCR-based methods increases the DNA quantity for downstream analysis. Single-cell RNA sequencing uses reverse transcription and PCR amplification to profile gene expression in individual cells, revealing cell types, states, and developmental trajectories.
