親子鑑定中短片段重複多型性STR突變之分析

Variation of Short Tandem Repeat (STR) Loci Mutations in Paternity Cases

人類基因遺傳中存在某些重複且多型性的短鏈鹼基片段，稱為短片段重複序列 DNA（Short Tandem Repeat, STR），可做為人類多型性分析之標記。藉由DNA 片段重複次數之相同或相異情形，辨識樣本檢體之同源性或其血緣遺傳關係。刑事鑑定上常利用 STR 多型性當作個人身分鑑定工具；在民事案件上，STR多型性亦為重要之血緣關係鑑定方法。有關鑑定上的重要統計參數，包括鑑別率（Power of Discrimination），同型接合性的數值（Homozygosity Values），異型接合性的數值（Heterozygosity Values），吻合機率（Matching Probability），排除率（Power of Exclusion），以及親權指數（Paternity Index, PI），這些參數可以做為刑事與民事血緣鑑定之重要參考依據，其中親權指數（Paternity Index, PI）的高低及親子關係之確定率（Wahrscheinlichkeit, W）標準，實為認定雙方是否具親子血緣關係之重要數據。本研究搜集 27 組具直系血親關係之家庭，共 64人 DNA 樣本，利用 AmpF1STR® IdentifilerTM 套組及 ABI Gene Amp® PCR System 9700 系統，鑑定 DNA 樣本之 15 組 STR 基因位，發現有二組子女之 DNA型產生突變，分別為第 4 組家庭之 D19S433 基因位與第 24 組家庭之 FGA 基因位發生突變。短片段重複序列基因位突變機率並不罕見，且容易導致親緣鑑定判斷變得更為複雜，本研究認為親權關係排除（Paternity Excluded）確定，要特別留意 DNA 型別突變之可能性，選擇突變率較低之 STR 基因位及增加 STR DNA 基因型別鑑定，或再提供其他家屬檢體進行 STR DNA 型別分析，以提昇親緣關係確定率。

In the human genome, there are some repetitive and polymorphic short DNA fragments called Short Tandem Repeats (STRs), which can be used as markers for parentage analyses. The versatile nature of polymorphism in the STRs has enabled it to be extensively used in both forensic identifications and parental testing. A number of statistical parameters, including Power of Discrimination, Homozygosis Values, Matching Probability, Power of Exclusion and Paternity Index, are considered as important references. The value of PI (Paternity Index, PI) and W (Wahrscheinlichkeit) are of great importance as inferences in confirming whether there is a parental relationship between the two tested parties. However, it is important for paternity tests to account for the possibility of mutation in gene transmission. We have tested 64 people cases from 27 family groups and found 2 cases with STR loci mutations. There were one case (family 4) with mutation at D19S433 locus and another case (family 24) with mutation at FGA locus. Mutations of STRs are relatively common and often make parentage testing more complicated. We conclude that selecting stable STR locus with low mutation rate or applying more loci to increase the power of discrimination in parentage testing as well as the correct interpretation of resulting genetic profiles are crucial.