Multiplex STR Genotyping Facilitates Rapid Screening for Chromosomal Abnormalities: A Technical Analysis of Fluorescence PCR Capillary Electrophoresis

Technical Principles and Design

The multiplex STR genotyping kit for chromosomes (13/18/21/X/Y) is based on fluorescent PCR and capillary electrophoresis technology, utilizing multiplex PCR to simultaneously amplify multiple short tandem repeat (STR) loci on target chromosomes. The primers for each locus are labeled with different fluorescent groups, and the amplification products are separated by capillary electrophoresis. STR genotypes are determined based on fragment length and fluorescence signal intensity. By analyzing the heterozygous/homozygous status or dosage ratios of multiple STR loci on target chromosomes, chromosomal copy number abnormalities can be inferred. The design incorporates internal reference genes or normal reference samples for correction to reduce the impact of sample quality on results. The primer combinations in the kit have been optimized to ensure that amplification products do not overlap in fragment size range while maintaining similar amplification efficiencies, avoiding result bias caused by amplification preference.

During kit development, the population polymorphism and linkage disequilibrium of STR loci must be evaluated, selecting loci with high heterozygosity and stable inheritance in the target population. Additionally, primer design must avoid non-specific amplification and primer dimer formation, with pre-validation against common mutation regions. The selection of fluorescent dyes must match the detection channels of the capillary electrophoresis instrument, with commonly used dye combinations including FAM, VIC, NED, PET, etc. The entire design process requires multiple rounds of optimization and validation to ensure consistency and reproducibility of the kit across different laboratories and batches.

Detection Workflow and Data Analysis

The detection workflow generally includes genomic DNA extraction, multiplex PCR amplification, capillary electrophoresis, and data analysis. First, high-quality DNA is extracted from samples to be tested (such as amniotic fluid, chorionic villi, peripheral blood, etc.), and quantification and quality control are performed to ensure DNA purity and concentration meet kit requirements. Subsequently, the PCR reaction system is prepared according to the kit instructions for thermal cycling amplification. The amplification products are purified or directly diluted, then mixed with internal standards (GeneScan or other fragment length standards) and formamide for denaturation, and separated on a capillary electrophoresis instrument. After electrophoresis, software is used to automatically read the peak height and peak area of each fragment, and STR genotypes are determined based on allele peak ratios or heterozygous patterns.

The core of data analysis is determining whether the target chromosome has aneuploidy. For autosomes 13/18/21, the heterozygous peak ratios of STR loci on the target chromosome are typically compared: if two different peaks appear with a peak area ratio close to 1:1, it indicates normal diploidy; if three peaks appear, or two peaks appear but the peak area ratio significantly deviates from 1:1 (such as 2:1), it suggests possible trisomy. For sex chromosomes X/Y, by detecting the copy number of STR loci on the X chromosome and the presence or absence of Y chromosome-specific STR loci, gender and X chromosome numerical abnormalities can be determined. The analysis software provides quality control parameters such as allele peak height thresholds and internal standard migration deviation, and operators need to perform manual review in conjunction with process requirements. For borderline or abnormal results, repeat testing or validation using other techniques (such as fluorescence in situ hybridization, chromosomal microarray analysis) is recommended.

Clinical Value and Application Scenarios

This multiplex STR genotyping kit has important application value in the field of prenatal diagnosis. After amniocentesis or chorionic villus sampling, this kit can complete rapid screening for common chromosomal aneuploidies (Down syndrome trisomy 21, Edwards syndrome trisomy 18, Patau syndrome trisomy 13, and X/Y chromosome numerical abnormalities) within 24 hours, significantly shortening waiting time. Compared with traditional chromosomal karyotype analysis, this method also has certain sensitivity for detecting some mosaic samples, but attention must be paid to validating the lower detection limit of mosaic proportions. In postpartum peripheral blood samples, it can be used for auxiliary diagnosis of genetic causes such as unexplained intellectual developmental delay and disorders of sex development. Additionally, this kit is suitable for chromosomal analysis of miscarriage tissue samples, helping to identify genetic factors in recurrent miscarriage.

In practical applications, the limitations of the kit must be noted: it cannot detect structural abnormalities such as balanced translocations and inversions, nor can it detect complex karyotypes such as polyploidy and marker chromosomes. Therefore, it should be used in combination with chromosomal karyotype analysis or chromosomal microarray analysis to form a complementary testing strategy. Meanwhile, laboratories should establish standard operating procedures and regularly participate in external quality assessment to ensure the accuracy and reliability of test results. With the increasing coverage of prenatal screening, such rapid, high-throughput STR genotyping kits will play an increasingly important role in clinical genetics laboratories.