Development of a 20-locus fluorescent multiplex system as a valuable tool for national DNA database

The multiplex system allows the detection of 19 autosomal short tandem repeat (STR) loci [including all Combined DNA Index System (CODIS) STR loci as well as D2S1338, D6S1043, D12S391, D19S433, Penta D and Penta E] plus the sex-determining locus Amelogenin in a single reaction, comprising all STR loci in various commercial kits used in the China national DNA database (NDNAD). Primers are designed so that the amplicons are distributed ranging from 90 base pairs (bp) to 450 bp within a five-dye fluorescent design with the fifth dye reserved for the internal size standard. With 30 cycles, 125 pg to 2 ng DNA template showed optimal profiling result, while robust profiles could also be achieved by adjusting the cycle numbers for the DNA template beyond that optimal DNA input range. Mixture studies showed that 83% and 87% of minor alleles were detected at 9:1 and 1:9 ratios, respectively. When 4 ng of degraded DNA was digested by 2-min DNase and 1 ng undegraded DNA was added to 400 μM haematin, the complete profiles were still observed. Polymerase chain reaction (PCR)-based procedures were examined and optimized including the concentrations of primer set, magnesium and the Taq polymerase as well as volume, cycle number and annealing temperature. In addition, the system has been validated by 3000 bloodstain samples and 35 common case samples in line with the Chinese National Standards and Scientific Working Group on DNA Analysis Methods (SWGDAM) guidelines. The total probability of identity (TPI) can reach to 8 × 10−24, where DNA database can be improved at the level of 10 million DNA profiles or more because the number of expected match is far from one person (4 × 10−10) and can be negligible. Further, our system also demonstrates its good performance in case samples and it will be an ideal tool for forensic DNA typing and databasing with potential application.