基于常染色体STR单体型推断亲缘关系的研究

Relatedness inference is very important for forensic theory and practic. It can be applied to deducing the individual origin,distinguishing authentic relationship from fake and predicting unknown relatedness.Approaches based on pedigree hypothesis and unlinked STR markers are the major technologies in the present determination of relationship. However,these approadches are considerably affected by allele freqencies，unable to identify distant degree of relationship and impossilbe to predict relatedness because of the low STR polymorphism and nonuniform distribution of allele frequencies. Some forensic DNA analysists have tried to use linkage genetic markers to solve these issues. But validated genetic markes have not been found. Such research is rare in China. In our prior experiment, we found that linked STRs can improve the precision of kinship analysis. Also, we have screened several linkage STR pairs from the genome. A multiplex PCR system have been developed to amplified these linked STR markers. Pedigree analysis showed that these linkage STR pairs inherited by haplotypes stbaly. Therefore, the aim of our project is to attempt to screen more linkage STR pairs from the human genome. Then, genetic similarity will be estimated using haplotypes of the linkage STR pairs. On this basis, we will explore how to infer the family relationship using the genetic similarity. We hope that we can find a new relationship analysis method that can infer relatedness accurately from the STR profiles independent of pedigree-based hypothesis and allele frequencies. Distant family relationship could be identified.

亲缘关系推断对推测个体身源、鉴别真假亲属和推断未知个体间亲缘关系具有重要的法医学理论价值和实践意义。目前以非连锁STR为主要遗传标记基于家系假设的亲缘分析方法因STR的多态性低、等位基因频率分布不均匀而存在结果受等位基因频率影响、鉴识的亲缘级别低和无法推测亲缘关系等缺陷。国外同行尝试以连锁遗传标记来解决，但迄今尚无可应用于实践的遗传标记，国内则鲜有研究。项目组在前期预试验中发现连锁STR可以提高亲缘鉴定的准确性；并从基因组中筛查到了数组连锁STR，建立了复合扩增检测体系，家系分析表明这些连锁STR均以单体型遗传。本项目拟在前期工作的基础上，进一步从基因组中筛选出更多紧密连锁STR组，探索根据连锁STR单体型计算遗传相似度来推断亲缘关系的新鉴定方法，以达到不依赖家系假设，免受或少受等位基因频率的影响，基于STR单体型即可准确推断未知个体间、鉴识出亲缘较疏远个体间的亲缘关系的研究目标。

由于多态性有限，常用的非连锁遗传标记无法解决远亲的鉴定问题，本课题研究连锁STR基因在亲缘鉴定的应用问题。首先我们比较了一些松驰连锁STR基因座家系分析得到和基因组作图推算的重组率差异，结果表明家系数据所得的重组率往往高于基因组作图估算值。同时连锁不平衡检验表明松驰连锁STR并不会导致连锁不平衡。为了研究紧密连锁STR对亲缘鉴定的影响，课题建立了扩增位于3、4、17号染色体的7组STR连锁群的复合扩增体系，调查了这些STR基因座的等位基因频率、单倍型频率和重组率等参数。结果发现，家系调查的重组率在3号染色体的STR连锁群上要明显高于基因组作图所得值，说明在亲缘鉴定中使用基因组作图推算的重组率会影响亲缘关系的统计分析。连锁还表明尽管3号染色体的STR在遗传作图的距离虽短于1cM但与连锁不平衡无关，相反4和7号染色体的连锁STR之间则存在连锁不平衡。说明基因组的STR之间是存在重组热点的，亲缘鉴定中应用家系计算所得的重组率进行亲缘评估。.当基于紧密连锁STR的等位基因频率、单倍型频和重组率，采用摸拟方法分析紧密连锁STR半同胞关系的鉴定时，结果表明使用紧密连锁STR，真正的半同胞比不考连锁时更容易得到高的半同胞指数，尤其是有已知亲属参与检验的情形。说明连锁STR。当用连锁STR推断一个三代家系的个体间亲缘系数时，父子、全同胞、半同胞和祖孙关系都可以与无关个体相区别开来，但是半同胞和祖孙关系的亲缘关系系数易发生高估的现象，提示使用连锁STR有利于亲缘关系的推断。同时，比较了不同的亲缘关系推断方法，发现最大似然法推算的亲缘系数最接近实际值，而比较等位基因的方法得到的亲缘系数则易偏离实际值。这些结果反映出仅用一组连锁STR复合扩增体体系推断亲缘关系仍存在不足，需要发掘更多的应用于亲缘分析的连锁STR。本研究对开发和利用连锁STR分析和解决复杂和远亲亲缘关系的鉴定作了初步的探索，为更进一步研究提供了重要参考。