RESUMEN
Objective The purpose of this study was to detect the degradation degree of long-term formalin fixed tissue and to compare the detection rate of STR with SNP. Methods DNA was extracted from 24 formalin-fixed tissues stored at -20 ℃ for five years, and the concentration and degradation index of DNA was quantified with Quantifiler? Trio DNA Kit. A 55-SNP multiplex SNaPshot assay and PowerPlex? 21 system were used to amplify SNP and STR loci, respectively. Results The results showed that the degradation indexes of 24 specimens were ranged from 1~8. The SNP genotypes of the 24 specimens were completely consistent with the non-degraded DNA from the same individuals and the successful genotyping rate was 100%. However, 33 allele dropouts were observed with STR genotyping in 8 samples, of which the degradation index was higher than 2.6, and the fragment size of the 75.8% allele was longer than 300bp. The likelihood ratio based on 16 typable STR loci in the sample was close to that onthe basis of 54 SNPs. However, likelihood ratio based on more than 17 STR loci was over that accord to 54 SNPs. There was a negative correlation between the fragment size of STR and the allele detection rate, and a negative correlation also observed between the degradation index of samples and the allele detection rate except for two samples with mild degradation. Conclusion This study validated that the long-term formalin-fixed tissues were susceptible to degradation, and the SNP was more suitable for detecting these tissues than STR typing system. However more SNP loci are needed to test in order to increase the discrimination power.